Abstract
A new family of polythienothiophene-co-benzodithiophene copolymers with different amounts of fluorine decoration (PBFx) had been successfully synthesized. Detailed structure–property investigations covering physical properties, morphology, and solar cell performance with respect to the fluorine content in the polymers were performed by a series of structural characterization techniques. A PCE of 8.75% was obtained with the highest fluorinated polymer; the morphological trends, as well as the crystalline structure, were also investigated, shedding light on this important material modification.
Introduction
Polymer solar cells (PSCs) have attracted much attention due to their potential in flexible, lightweight, and low-cost solar energy harvesting devices.(1, 2) One of the key strategies to improve PSCs is to develop better light-harvesting conjugated polymers with desired optical, electronic, and electrical properties. Therefore, different classes of conjugated polymers such as homopolymers,(3) donor–acceptor (D–A) conjugated polymers,(4-9) quinoid polymers,(10, 11) two-dimensional polymers(12, 13) and other structural polymers have been synthesized and studied for PSC applications. By fine-tuning the chemical structures, the energy levels and bandgaps of the polymers can be effectively tuned to optimize the PSC performance. For example, introduction of electron-withdrawing fluorine atoms on the backbone of the conjugated copolymers can simultaneously increase both the electron affinity (EA) and ionization potential (IP) of the polymer with a deepening of both the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) without affecting the bandgap of the polymer significantly.(14-28) As a result, the desired light harvesting properties can be retained while the deepened HOMO level leads to increased open-circuit voltage (Voc) of the polymer:fullerene bulk-heterojunction(BHJ) cells.(29) Recent studies also suggested that the introduction of fluorine atoms in appropriate locations of the polymer can enhance the internal dipole within the polymer, thus providing extra driving forces to improve the exciton dissociation efficiency in the BHJs, leading to improved short-circuit currents (Jsc) and fill factors (FF).(30-32) These collective effects result in polymer solar cells with very high power conversion efficiencies (PCE) of 7–9%.(33-36)
In conjunction with the development of more efficient conjugated polymers through the above-mentioned chemical modification strategies, optimization of the morphology of the BHJ is also required in order to achieve the anticipated improvement in PSC performance.(37) Unfortunately, achieving the desired nanoscale phase separation of the polymer and fullerene in the BHJ film is not trivial and it depends on many parameters, including processing conditions and solvents, substrate wetting properties, compositions of the mixture comprising the active layer, and annealing (thermal and solvent) conditions. Subtle modifications of the chemical structure of the polymers can also markedly alter the surface energies, segmental interactions, and segment rigidity, all of which can influence the morphology of the resultant active layer and the distribution of components within the active layer and at interfaces.(14, 17, 28, 38) As a result, the current approach to exploit the property of new materials for PSC applications is largely proceeded by painstaking device optimization. There is a strong need to systematically study the structure–property-performance relationship of new polymers and establish rational design criteria for developing even better organic photovoltaic materials.
In this study, we describe the synthesis of a series of polythienothiophene-co-benzodithiophene based polymers with different fluorine content (PBFx), a systematic study of their optical, electronic, and electrical properties and the characteristics of PSCs based on these polymers. A detailed structural analysis of these polymers in the BHJ films was performed using transmission electron microscopy (TEM), grazing incidence X-ray diffraction (GIXD), and resonant soft X-ray scattering (RSoXS) to elucidate the influence of fluorine content on the morphology of the active layer. From these studies, a structure–property-performance relationship was established, providing materials design criteria for new conjugated polymers for photovoltaic applications.
The polythienothiophene-co-benzodithiophene-based polymers were chosen for this study due to their high PSC performance with PCE of 7–9%.(19, 35, 36, 39-41) By replacing the commonly used alkyl- or alkoxyl-based side chain with the alkylthiophene one on the benzodithiophene unit, the resulting polymers showed narrower bandgaps and improved charge transport properties owing to the extended conjugation of the polymer. It is interesting to note that this such modification also deepened the HOMO level of the polymer.(35, 42, 43) Therefore, a new series of polymers (PBFx) were synthesized based on the copolymerization of the alkylthiophene-substituted benzodithiophene (BDT-T) unit with different feed-ratios of the thienothiophene (TT) and fluorinated thienothiophene (TT-F) unit,(44-46) so as to control the fluorine content in the polymer in a systematic manner (Figure 1). It was found that the morphologies of the polymer:fullerene BHJ films strongly depend on the fluorine content of the polymers with a reduction in the size scale of phase separation with increasing fluorine content. The best device performance was obtained from the polymer with the highest fluorine content where a PCE of 8.75% was found. The improved performance not only arises from the decreased size scale of the BHJ morphology but also to the enhanced Voc and FF arising from the increased HOMO level of the polymer and more balanced charge transport properties in the BHJ film, respectively.
Experimental Section
Materials 
2-(2-ethyl-hexyl)thiophene (1) and 4,8-dehydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione (2) were synthesized according to a previously reported method.(47, 48) 2-ethylhexyl-4,6-dibromothieno[3,4-b]thiophene-2-carboxylate (TT) was purchased from 1-Material Inc.; 2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT-F) was purchased from Suna Tech Inc. All of these chemicals were used as received. Tetrahydrofuran (THF) was dried over Na/benzophenoneketyl and freshly distilled prior to use. The other materials were commercially available and used as received.
Figure 1. Synthesis of PBFx series polymers. (a) n-BuLi, THF, 0 °C, then 50 °C, 1 h; then compound 2, 50 °C, 1 h; then SnCl2/HCl/H2O, ambient temperature, 1.5 h; (b) n-BuLi, THF, ambient temperature, 2 h; then Me3SnCl, ambient temperature, overnight; (c) [Pd(PPh3)4], toluene/DMF (10:1), reflux, 42 h.
4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene (3)
Using a 150 mL argon-purged flask, n-butyllithium (2.5 M in hexane, 13.2 mL, 33 mmol) was added dropwise to a solution of compound 1 (5.88 g, 30 mmol) in THF (30 mL) at 0 °C. The mixture was then warmed to 50 °C and stirred for 1 h. Subsequently, 4,8-dehydrobenzo[1,2-b:4,5- b′]dithiophene-4,8-dione (2.16 g, 10 mmol) was added to the reaction mixture, which was then stirred for 1 h at 50 °C. After cooling the reaction mixture toambient temperature, a mixture of SnCl2·2H2O (18 g, 80 mmol) in 10% HCl (32 mL) was added and the mixture was stirred for additional 1.5 h, after which it was poured into ice water. The mixture was extracted with petroleum ether twice and the combined organic phases were concentrated to obtain the crude compound 3. Further purification was carried out by column chromatography on silica gel using petroleum ether as the eluent to obtain pure compound 3 as pale yellow viscous liquid (4.1 g, yield 71%). 1H NMR (CDCl3, 300 MHz), d: 7.66(d, 2 H), 7.46 (d, 2 H), 7.30 (d,2 H), 6.90 (d, 2 H), 2.88 (d, 4 H), 1.75 (m, 2 H), 1.53–1.18 (br, 16 H), 0.92–0.83 (m, 12 H) ppm. 13C NMR (CDCl3, 75 MHz), d : 145.20, 139.06, 137.49, 136.43, 127.61, 127.31, 125.32, 124.01, 123.29, 41.54, 34.33, 32.53, 29.41, 25.83, 23.04, 15.66, 11.01 ppm. MS (APCI, m/z): calcd. for C34H42S4 [M + 1]+, 579.2; found, 579.6.
2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T)
A solution of compound 3 (4.23 g, 7.32 mmol) in THF (30 mL) at 0 °C was placed in a 150 mL argon purged flask, and then n-butyl lithium (1.6 M in hexane, 16 mL, 25.6 mmol) was added. The reaction mixture was then stirred for 2 h at ambient temperature. Subsequently, chlorotrimethylstannane (1.0 M in hexane, 33 mL, 33 mmol) was added and the mixture was stirred overnight at ambient temperature. Then, the mixture was extracted by dichloromethane and the combined organic phase was concentrated to obtain compound BDT-T. Further purification was carried out by recrystallization using hexane/ethanol to obtain the pure compound BDT-T as a light-yellow solid (3.28 g, yield 49%). 1H NMR (CDCl3, 300 MHz), d: 7.68 (s, 2 H), 7.31 (d, 2 H), 6.90 (d, 2 H), 2.87 (d, 4 H), 1.73 (m, 2 H), 1.53–1.35 (br, 16 H), 0.95 (m, 12 H), 0.39 ppm (s, 18 H).13C NMR (CDCl3, 75 MHz), d: 145.40, 143.28, 142.25, 138.00, 137.33, 131.19, 127.54, 125.31, 122.41, 41.58, 34.26, 32.52, 28.98, 25.81, 23.05, 15.43, 10.99, 8.33 ppm. MS (APCI, m/z): calcd. for C40H58S4Sn2 [M + 1]+, 904.4; found, 904.0.
Synthesis of Polymers
The comonomer BDT-T (0.25 mmol), TT, TT-F were mixed in 5 mL of toluene and 0.5 mL of DMF. After being purged with argon for 15 min, [Pd(PPh3)4] (10 mg) was added as the catalyst, and the mixture was then purged with argon for 5 min. The reaction mixture was stirred and heated to reflux for 42 h. Then, 2-(tributylstannyl)thiophene and 2-bromothiophene were added to end-cap the polymer chain. The reaction mixture was cooled to room temperature and dropped into methanol. The polymer was precipitated and then collected by filtration. The polymer was subjected to Soxhlet extraction with methanol, acetone, and hexane. The solid was dried under vacuum. After drying in the vacuum drying oven, the polymer was dissolved in toluene, then the Pd-thiol gel (Silicycle Inc.) (30 mg) was added and the solution was stirred for 2 h, followed by silica-gel chromatography with toluene eluent. The polymer was reprecipitated using methanol and dried. Finally, PBF series polymers were obtained as dark purple solid.
The comonomer feed ratios of TT to TT-F are 100:0, 75:25, 50:50, 25:75, and 0:100, and the corresponding copolymers were named PBF0, PBF25, PBF50, PBF75, and PBF100 respectively.
PBF0: Yield (75%). GPC: Mn = 34.1 kDa, Mw = 75.1 kDa, PDI = 2.2. Td = 365 °C.
PBF25: Yield (72%). GPC: Mn = 51.6 kDa, Mw = 123.0 kDa, PDI = 2.4. Td = 377 °C.
PBF50: Yield (68%). GPC: Mn = 52.6 kDa, Mw = 133.1 kDa, PDI = 2.5. Td = 373 °C.
PBF75: Yield (73%). GPC: Mn = 40.2 kDa, Mw = 114.8 kDa, PDI = 2.9. Td = 373 °C.
PBF100: Yield (80%). GPC: Mn = 32.4 kDa, Mw = 86.1 kDa, PDI = 2.7. Td = 384 °C.
Measurements and Characterization
The 13C NMR and 1H NMR data were collected on a Bruker AVANCE Digital NMR workstation operating at 75 and 300 MHz, respectively. Chemical shifts were reported as δ value (ppm) relative to an internal tetramethylsilane(TMS) standard.Gel permeation chromatography (150 °C in 1,2,4-trichlorobenzene) was performed on a Polymer Laboratories PL220 Chromatograph with linear polystyrene as references. Mass spectrometry (MS) data were obtained on a WatersTQD with atmospheric pressure chemical ionization resource (APCI). UV–vis absorption spectra were measured using a HP 8453 spectrophotometer. Thermogravimetric analyzer (TGA) measurements were performed on a Netzsch TG 209 in nitrogen, with a heating rate of 20 °C min–1. Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 204. External quantum efficiency (EQE) measurements were taken using a monochromator (Newport, Cornerstone 130) joined to the same xenon lamp and a lock-in amplifier (Stanford Research Systems, SR 830) coupled to a light chopper. The electrochemical cyclic voltammetry experiments were conducted on a CHI800 electrochemical workstation equipped with a graphite electrode coated with thin copolymer film as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a Pt sheet counter electrode in a 0.1 M tetrabutylammoniumhexafluorophosphate (Bu4NPF6) acetonitrile solution, at a scan rate of 50 mVs–1 at room temperature. The potential of the saturated calomel reference electrode was internally calibrated using the ferrocene/ferrocenium redox couple (Fc/Fc+), which has a known reduction potential of −4.8 eV. Thus, the IP values of the copolymers were calculated according to the following empirical formulas:(49)
The Eox is the onset oxidation potential vs SCE. It should be pointed out that the reductive curves of the copolymers could hardly be obtained. Therefore, the EA values of the copolymers were estimated from their optical band gaps and IP values using the equation
where Egopt denotes the optical band gaps of the copolymers and EA calculated here from the Egopt was approximate values because it ignores the exciton binding energy.(50)
where Egopt denotes the optical band gaps of the copolymers and EA calculated here from the Egopt was approximate values because it ignores the exciton binding energy.(50)Characterization of PSCs
Power conversion efficiencies (PCEs) were measured under a computer-controlled Keithley 2400 sourcemeter under 1 sun, AM 1.5 G solar simulator (Japan, SAN-EI, XES-40S1). The light intensity of the sun simulation was calibrated by a standard silicon solar cell(certified by NREL) before the testing, giving a value of 100 mW cm–2 during the test. The current density–voltage (J–V) characteristics of hole-only devices were recorded with a Keithley 236 sourcemeter under dark. The external quantum efficiency (EQE) spectra was performed on a commercial EQE measurement system (Beijing, Zolix, DSR 100UV–B).
Inverted PSCs Fabrication 
The ITO-coated glass substrates were cleaned by sonication in acetone, detergent, deionized water, and isopropyl alcohol and dried in oven at 80 °C for 12 h before used. Then, the ITO substrates were transferred into a N2 protected glovebox where the H2O concentration is ⩽0.5 ppm and O2 concentration is ⩽20 ppm. The cross-linkable polymer PF3N-OX (0.5 mg mL–1) was spin-casted from its methanol:acetic acid (99:1 v/v) solution onto the precleaned ITO substrates to form ∼5 nm thin film, and the cross-linking process was carried out by heating the half-dried films under 150 °C and fluorescence lamp illumination (no UV lamps required) for 20 min. The solutions for the active layer were prepared by dissolving polymer:PC71BM (1:1.5, wt/wt) blend into chlorobenzene:1,8-diiodoctane (97:3 v/v) with polymer concentration at 10 mg/mL. These solutions were heated at 60 °C overnight to completely dissolve the polymer and fullerene. After cooling down to RT and without any filtering, the active layer solutions were spin-coated onto PF3N-OX to yield 110 ± 10 nm thick active layers by adjusting the spin-coating speed. After that, about 10 nm molybdenum oxide (MoO3) and 100 nm aluminum were thermally deposited on top of the active layer through a shadow mask in a vacuum chamber with base pressure of 2 × 10–6 mbar. The effective area of a device was defined to be 0.16 cm2 by the shadow mask.
Figure 2. UV–vis absorption spectra of PBFx polymers in chlorobenzene solution (a) and solid films (b). (c) Cyclic voltammetry curves of the PBFx polymers (Potential vs Fc/Fc+).
Hole Mobility Measurement
Hole-only devices were fabricated with the device structure ITO/PEDOT:PSS(45 nm)/polymer(150 nm)/MoO3(10 nm)/Al. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, which is described by the equation
where J is the current, μh is the zero-field mobility, ε0 is the permittivity of free space, εr is the relative permittivity of the material, d is the thickness of the active layer, and V is the effective voltage. The effective voltage can be obtained by subtracting the built-in voltage (Vbi) and the voltage drop (Vs) from the substrate’s series resistance from the applied voltage (Vappl), V = Vappl – Vbi – Vs. The hole-mobility can be calculated from the slope of the J1/2 ∼ V curves.
where J is the current, μh is the zero-field mobility, ε0 is the permittivity of free space, εr is the relative permittivity of the material, d is the thickness of the active layer, and V is the effective voltage. The effective voltage can be obtained by subtracting the built-in voltage (Vbi) and the voltage drop (Vs) from the substrate’s series resistance from the applied voltage (Vappl), V = Vappl – Vbi – Vs. The hole-mobility can be calculated from the slope of the J1/2 ∼ V curves.GIXD Characterization
Grazing incidence X-ray scattering characterization of the thin films was performed at the Stanford Synchrotron Radiation Lightsource (SSRL) on beam 11-3. The scattering signal was recorded on a 2-D image plate (MAR-345) with a pixel size of 150 μm. The samples were ∼15 mm long in the direction of the beam path, and the detector was located at a distance of 450 mm from the sample center (distance calibrated using a lanthanum hexaboride standard). The incidence angle of 0.12° was chosen, which gave the optimized signal-to-background ratio. The beam size was 50 by 150 μm, which resulted in a beam exposure on the sample of 150 μm wide over the entire length of the sample. Typically, a 300 s exposure time was used to collect the diffraction signals. The data was processed and analyzed using WxDiff software package. The (010) peak information is obtained by using Gaussian peaks to fit the diffraction profile (two Gaussians were used with one simulate PCBM peak and another one simulate π–π stacking peak).
RSoXS Characterization
RSoXS was performed at beamline 11.0.1.2 Lawrence Berkeley National Lab. Thin film samples were spin-casted on to PF3N-OX covered SiO2 wafers. This film preparation process is the same with device fabrication. Then dilute HF solution was used to float the BHJ layer, which was then transferred onto Si3N4 substrate (NorcadaInc.), and the experiment was done in transition mode. The scattering signals were collected in a vacuum using a Princeton Instrument PI-MTE CCD camera.
TEM Characterization
Bright-field transmission electron microscopy studies were conducted with a JEOL 2000 FX TEM operating at an accelerating voltage of 200 kV without any defocus in experiment. The same sample preparation procedure as in RSoXS was used to float the BHJ thin film, which was then collected by copper grid.
Results and Discussion
Synthesis and Characterization
Figure 1 shows the synthetic procedure used in this study (details in Supporting Information). All the polymers were prepared by Stille polymerization. The two monomers TT and TT-F only differ by the fluorine atom, enabling us to investigate the influence of only the fluorine content on the properties of PBFx. Different feed-ratios of TT to TT-F of 100:0, 75:25, 50:50, 25:75, and 0:100 were used. The corresponding copolymers are referred to as PBF0, PBF25, PBF50, PBF75, and PBF100, respectively. All the polymers showed good solubility in common organic solvents and good thermal stability up to 350 °C before degradation (TGA study shown in Supporting Information). The series of PBFx polymers had molecular weights varing between 30 and 60 kg/mol, which are considered to be quite high for low bandgap polymers. Li et al. reported that polymers with molecular weight within this range showed similar performance in the BHJ solar cells, whereas polymers with molecular weight less than 20 kg/mol showed lower performance due to poor packing of the polymers.(51)
Optical and Electrochemical Properties
The absorption spectra of the PBFx polymers in chlorobenzene (CB) solution and solid films are shown in Figure 2a and b, respectively. In solution all the polymers show a π–π transition peak at ∼700 nm and a slightly weaker resonance structure peak at ∼600 nm. In thin films, the polymers also exhibit similar absorption characteristics with almost no spectral shift of the main peak at 700 nm. The relative intensity of the absorption peak at 600 nm became stronger in thin films, indicating an H-type aggregation of the main chain.(52) The relative intensities of the 600 and 700 nm peaks varied slightly between the different polymers, suggesting that the packing of the polymers was affected by the fluorine content. The onset of the thin film absorption blue-shifted slightly as the fluorine content increased, indicating a small increase in the optical band gaps (Table 1). Cyclic voltammetry was used to study the energy levels of the polymers (Figure 2c). The IP values were estimated from the onset of the oxidation peak vs SCE in the voltammogram and calculated by the equation of IP = e[Eox – Eox(Fc/Fc+) + 4.80]. The fluorine substitution slightly increases the IP of the polymers, with a difference of 0.06 eV obtained between the highest fluorinated and the nonfluorinated polymers. The approximate EA values of these copolymers were estimated from the difference between the IP values and Egopt and are almost identical at 3.66 eV.(50)
Table 1. Physical Properties of PBFx Polymers
| polymer | Mn/PDI [kDa]a | Film Eg [eV]b | absolute coefficient [cm–1]c | IP [eV]d | EA [eV]e |
|---|---|---|---|---|---|
| PBF0 | 34/2.2 | 1.57 | 6.2 × 104 | 5.22 | 3.65 |
| PBF25 | 51/2.4 | 1.59 | 6.6 × 104 | 5.26 | 3.67 |
| PBF50 | 52/2.5 | 1.60 | 6.4 × 104 | 5.26 | 3.66 |
| PBF75 | 40/2.9 | 1.60 | 6.0 × 104 | 5.28 | 3.68 |
| PBF100 | 32/2.7 | 1.61 | 5.6 × 104 | 5.28 | 3.67 |
a
Determined by GPC (1,2,4-trichlorobenzene) against PS standards.
b
Band gap calculated from the onset of the absorption of the solid film.
c
Measured from film absorption spectra at λmax.
d
Measured by cyclicvoltammetry.
e
Determined from IP – Eg.
Photovoltaic Properties 
Bulk heterojunction (BHJ) solar cells using these polymers as the electron donors and PC71BM as the electron acceptor were fabricated in an inverted geometry by using our newly developed poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-bis(3-ethyl(oxetane-3-ethyloxy)hexyl)fluorene)] (PF3N-OX) as the cathode buffer layer. The cross-linked PF3N-OX interfacial layer could effectively modify the electronic properties of ITO and improve electron extraction property, with the additional benefit of enhanced device stability.(53-58) MoO3/Al was used as the anode for the inverted cells. The active layer blends (polymer:PC71BM = 1:1.5 w/w after optimization) were processed from either pure chlorobenzene solution or chlorobenzene solution with 3 vol % of 1,8-diiodoctane (DIO) as processing additive to optimize the morphology.
Figure 3. (a) Current–voltage characteristics of polymer solar cells for each polymer under simulated AM1.5G illumination (100 mW cm–2). (b) External quantum efficiency curves of the devices for each polymer.
The current density–voltage (J–V) curves and the performance of the devices processed from CB:DIO are shown in Figure 3a and Table 2, respectively. The J–V curves and the device data processed from pure chlorobenzene are summarized in the Supporting Information. The Voc of the BHJ solar cells increased with fluorine con tent in the polymers due to the deepening of the HOMO levels,(59, 60) which is in agreement with our materials design principle. PBF100 showed a Voc of 0.79 eV, which is 0.07 eV larger than that obtained from the nonfluorinated PBF0. This value is among the highest for this class of polymers. The Jsc of all the BHJ devices are close to 17 mA/cm2, which indicates that the light harvesting properties of the polymers are quite similar. The hole mobility of the copolymers was examined by the space charge limited current (SCLC) method in single carrier devices. As shown in Table 2, increasing fluorine content enhances the hole mobility of the BHJ films from 9.27 × 10–6 cm2 V–1 s–1 (PBF0) to 2.75 × 10–5 cm2 V–1 s–1 (PBF100). Figure 3b shows the external quantum efficiency (EQE) curves of the polymer:PC71BM blends; the Jsc calculated from the integrated value of the EQE spectrum was in good agreement with the device results. All the PBFx polymers exhibited a broad response range, whereas PBF0 showed more effective solar energy harvesting capabilities in the short-wavelength range. PBF100 showed a slightly higher EQE compared to other fluorinated materials, thus yielding a slightly larger Jsc value. The PBF0-based devices showed a moderate FF of 56.7% due to the relatively low hole mobility in the BHJ. For the fluorine-containing polymers, an obvious trend of increasing FF with the fluorine content was observed from 54.2% for PBF25 to 65.2% for PBF100. The increase in the FF can be attributed to the gradual increase in hole mobility of the BHJ films with increasing fluorine content of the polymers. Another factor that will affect the FF is the film morphology, nanostructured film with optimized morphology is desirable to promote exciton dissociation and reduce charge recombination, leading to higher FF. By controlling the morphology with the fluorine content of the polymers, a significant improvement of both the Voc and FF was achieved, increasing the PCE from 6.99% for PBF0-based devices to 8.75% for the PBF100-based devices.
Figure 4. (a) Typical GIXD scattering images of PBFx polymers and blends. (b) Analyzed data from (010) peak of GIXD results of blend films.
Table 2. Photovoltaic Performances of Devices with PBFx:PC71BM (Weight Ratio of 1:1.5) under the Illumination of AM 1.5 G, 100 mW cm–2a
| polymer | Jsc (mA/cm2) | Voc (V) | FF (%) | PCE (%) (max) | mobility (cm2 V–1 s–1) |
|---|---|---|---|---|---|
| PBF0 | 16.69 ± 0.37 | 0.72 ± 0.01 | 56 ± 2 | 6.69 ± 0.30(6.99) | 9.27 × 10–6 |
| PBF25 | 16.19 ± 0.23 | 0.73 ± 0.01 | 54 ± 1 | 6.38 ± 0.10(6.48) | 1.29 × 10–5 |
| PBF50 | 16.63 ± 0.28 | 0.75 ± 0.01 | 57 ± 1 | 7.01 ± 0.30(7.31) | 1.36 × 10–5 |
| PBF75 | 16.32 ± 0.31 | 0.76 ± 0.01 | 59 ± 3 | 7.28 ± 0.25(7.53) | 1.48 × 10–5 |
| PBF100 | 16.74 ± 0.14 | 0.79 ± 0.01 | 63 ± 2 | 8.36 ± 0.39(8.75) | 2.75 × 10–5 |
a
Above were average values and the corresponding standard deviations were calculated over eight devices.
GIXD Characterization 
To study the effect of fluorination of the polymers on the morphology of the BHJ blends, a combination of the structural characterization techniques were employed. First, grazing incidence X-ray diffraction (GIXD) was used to study the structural order of these polymers in pristine thin films and blended films (processed from CB and CB:DIO mixture). The experiments were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) on beamline 11-3 using a 2-D image plate (MAR-345) at an incidence angle of 0.12°. The data was processed and analyzed using WxDiff software package.(61) As seen from Figure 4a and Supporting Information Figure S5, a face-on orientation was found for all the pure PBFx thin films, with evident (010) π–π stacking reflection in the out-of-plane direction and the (100) lamellar d-spacing reflection in the in-plane direction. With the polymer:PCBM blends, the diffraction arising from the PCBM overlaps the (010) reflection; consequently, peak fitting routines had to be used to extract detailed structural information, the broad peak distribution and the weak or even absence of the higher reflections indicate that the polymers are not highly ordered. For all the polymers and their blends, the (100) reflection fixed at q ∼ 0.25 A–1 corresponding to a d-spacing of 2.5 nm, which indicated addition of fluorine does not alter d-spacing significantly. This d-spacing is determined by the alkyl-thienyl group on the BDT unit, and correlates well with previous publications.(40) Detailed analyses of the (010) π–π stacking peak in the blends are summarized in Figure 4b (line-cut profiles in the out-of-plane direction are shown in Supporting Information Figure S6.) to show the influence of fluorine modification on structural changes in the films. As can be seen, a π–π stacking distance of ∼3.89 Å and a (010) “crystal size” between 28 to 33 Å (determined from the Scherrer equation) are seen for all the blends, both showing almost no changes with fluorine content variation. Compared to the crystal size of ∼20 Å found in the commonly studied PTB7:PCBM blend,(62) the (010) crystal size of ∼30 Å for our polymers is slightly larger, corresponding to a stack of 7–8 layers. The increased crystal size of the polymers can be attributed to the enhanced π–π interaction induced by the alkylthienyl side chain.(40) The integrated area under the (010) peak was used to determine a relative crystallinity of the PBFx polymers in the blends. The relative crystallinity was found to increase with increasing fluorine content up to PBF50 and then decrease with further increase in the fluorine content. This is probably due to the enhanced π-stacking interactions between the polymers, as quadrupolar interactions can be formed between adjacent fluorine-substituted thienothiophene moieties, which contains an electron-deficient fluorinated aromatic ring and an electron-rich nonfluorinated ring,(38) and the exception for PBF100 may arise from the relatively lower molecular weight.(51, 63) The thin films processed from CB and CB:DIO yielded similar results, indicating that the solvent additive did not affect the crystalline structure of these materials. Although the PBF50 and PBF75 show the highest relative crystallinity with more π–π stacking in the out-of-plane direction of the blend film, the corresponding hole mobilities are lower than that of the PBF100. This indicates that crystallinity cannot be the sole factor influencing the charge transport properties and performance of the devices. Other factors, like connectivity between the crystallites, may also be important because the charges must be transported across the entire thickness of the film before they can be collected by the electrodes. The higher hole mobilities in the case of PBF100 blends, despite the lower crystallinity, may be attributed to the formation of continuous charge transport pathways in the BHJ film. On the whole, the fluorine substitution does not significantly change the lamellar d-spacing, π–π stacking distance and (010) crystal size of the polymers but increases the relative crystallinity of the polymers in the blends; together with a better connectivity between the crystallites, all these make fully fluorinated PBF100 show the highest hole mobility.
Figure 5. TEM images of PBFx:PCBM blends processed from chlorobenzene and chlorobenzne:DIO mixture. (M1 denotes chlorobenzene processed thin films; M2 denotes chlorobenzene:DIO processed thin films.)
TEM Characterization 
Transmission electron microscopy (TEM) was used to provide real-space images of the morphologies of the PBFx:PCBM blends cast from CB and CB:DIO. Thin films processed from chlorobenzene contained large aggregates of PCBM (Figure 5). In the PBF0:PCBM blends, some PCBM aggregates were even ∼300 nm in size. The average center-to-center spacing between the aggregates was ∼300 nm. In the case of fluorinated polymer blends, the PCBM aggregates became slightly smaller but had a higher areal density. When the blend films were cast from CB:DIO, the films became smoother with phase separation on the tens of nanometers size scale. Consequently, the additive is regulating the size of the morphology by solubilizing the PCBM, allowing the polymer to come out of solution initially and then allowing the PCBM to come out of solution in a matrix defined the polymer, in agreement with other reports.(64-66)
Figure 6. (a) RSoXS profiles of chlorobenzene processed thin film blends. (b) RSoXS profiles of chlorobenzene:DIO processed thin film blends. (c) Feature size of phase separation and scattering power of blended thin films.
RSoXS Characterization
Scattering methods provide a statistical view of the phase separation that complements results from TEM.(67) Resonant soft X-ray scattering (RSoXS) utilizing the unique optical contrast between polymer and fullerene at the carbon edge (284.2 eV) for the thin films are summarized in Figure 6. For the CB-cast films, the interferences observed clearly indicate a characteristic length scale of several hundred nanometers (2π/q). PBF0 blends showed a length scale of 285 nm, which correlates well with TEM characterization. PBF100 blends showed a feature size of 185 nm. In between these two polymers, the size of the phase separated domains decreased in proportion to the fluorine content of the polymer (Figure 6a). When the films were cast from CB:DIO, the characteristic length scale was reduced to tens of nanometers and also kept a decreasing trend with the increasing fluorine content of the polymers (Figure 6b). In these blends, the morphology consists of PCBM aggregates imbedded in a polymer rich matrix and can be treated as a two-phase system, so the scattering actually originates from optical contrast between polymer-rich and fullerene-rich domains. In view of this, I(q)q2 as a integrating function of q in Figure 6a and b, a measure of the scattering power of the system, is shown in Figures 6c. For both the CB and CB:DIO cases, a reduction in the total scattering power along with the increasing fluorine content was seen. However, the origins of this reduction are different in the two cases. For the CB cast films, a large size scale structure is seen, consistent with the electron microscopy results, due to the formation of PCBM domains. The relative solubilities of the fluorinated polymers and PCBM in CB can describe the reduction in the scattering power. With increasing concentration of fluorine in the polymer, the solubility decreases and, as such, will come out of solution earlier during the solvent evaporation and compete with the PCBM, which leads to the size scale of these domain decreasing. The integrated scattering is independent of the size and shape of the domains, and the continuous decrease with increasing fluorine concentration indicates that the contrast between polymer-rich and fullerene-rich domains in the system, arising from the average electron densities of the domains, is decreasing. This is consistent with the relative rates at which the PCBM and polymer come out of solution. As for CB:DIO cast films, the total scattering decreases along with the characteristic length scale of the morphology, but the average crystal size, as reflected by the width of the (010) reflection, remains relatively constant. This indicates that the polymer crystallizes from solution, forming a network structure, with a mesh-size reflected in the interference maximum, where crystallinity, that is, the total amount of the fluorinated polymer in the crystalline phase, increases. The remaining PCBM and polymer fill in the space between the fibrillar network causing the electron density of the mixture to increase, giving rise to a reduction in the total integrated scattering. As the concentration of fluorine in the polymer exceeds ∼50%, this crystal size remains relatively constant, the effective network size continues to decrease, from ∼90 nm for PBF0 to ∼60 nm for PBF25 to ∼40 nm for PBF50 to ∼30 nm for PBF75 and PBF100, which indicates more sufficient interpenetration of polymer and PCBM in the film with increasing fluorine content. As with the blends cast from only CB, the reduction in the characteristic length scale of the morphology leads to an increase in the interfacial area, which usually leads to a larger Jsc. This was not the case here, though, since the difference in Jsc between the devices was quite small. Consequently, as with the mixtures cast from only CB, the size scale of the morphology is only one parameter that is varying and one must, also, consider the total crystallinity in the system and the distribution of the components within the domains. With increasing fluorine content of the polymer, the gradually improved solar cell performance can, therefore, be attributed to the enhanced Voc and more balanced charge transport due to the reduced size scale and more effective crystalline network for component distribution within the domains, which yielded a better FF.
Conclusions
In conclusion, we have synthesized a series of benzodithiophene-thienothiophene copolymers with different amounts of fluorine decoration. We observed that incorporating fluorine into the thienothiophene units could deepen the HOMO energy levels without significantly influencing the bandgap and the light harvesting properties of the polymers. As a result, an improved Voc in OPV devices was achieved while the high Jsc was retained. The FFs of the devices were found to be strongly depending on the morphology and charge transport properties in the BHJ films. Improved morphology with phase separation length scale in the tens of nanometers can be achieved by casting the films with solvent additive. It was also found that the size of phase separation further decreases with increasing fluorine content possibly due to the interactions of the polymer and PCBM with the casting solvent and additive. Although the reduction in the size scale of the phase separated morphology with increasing fluorine content in the polymers is in good agreement with the improved PCE of the corresponding devices, the in-depth structural and electrical analysis performed in our study indicate that other factors, such as the polymer crystallinity, distribution of components within the domains, and charge transport property, also affect the ultimate device efficiency. These findings provide new insights to evaluate the structure–property-performance relationship in polymer solar cells and lead toward better design criteria for photovoltaic materials.
Supporting Information
TGA plots of copolymers, DSC scan curves, the J1/2 vs V plots of mobility measurement of polymers, GIXD images, line-cut profiles, and so forth. This material is available free of charge via the Internet at http://pubs.acs.org.
†Author Contributions
These authors contributed equally.
The authors declare no competing financial interest.
Acknowledgment
This work was financially supported by the Ministry of Science and Technology (No. 2014CB643501), the Natural Science Foundation of China (No. 21125419, 50990065 and 51010003), and the Guangdong Natural Science Foundation (No. S2012030006232). Structural characterization of the materials was supported by the Department of Energy-supported Energy Frontier Research Center at the University of Massachusetts (DE-SC0001087). GIWAXS was performed at the Stanford Synchrotron Radiation Light Source (SSRL) on beam 11-3, R-SoXS was performed at beamline 11.0.1.2 at the Advanced Light Source, Berkeley National Laboratory, which are supported by the DOE, Office of Science, and Office of Basic Energy Sciences.
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, B. C.; Ashraf, R. S.; Thomas, S.; White, A. J. P.; Biniek, L.; Nielsen, C. B.; Zhang, W. M.; Huang, Z. G.; Tuladhar, P. S.; Watkins, S. E.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I. Chem.Commun. 2012, 48, 7699[CrossRef], [PubMed], [CAS]15.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XpvV2nu74%253D&md5=d0aa9eb53ae65a0ef2482a09d0faa74bSchroeder, Bob C.; Ashraf, Raja Shahid; Thomas, Stuart; White, Andrew J. P.; Biniek, Laure; Nielsen, Christian B.; Zhang, Weimin; Huang, Zhenggang; Tuladhar, Pabitra Shakya; Watkins, Scott E.; Anthopoulos, Thomas D.; Durrant, James R.; McCulloch, IainChemical Communications (Cambridge, United Kingdom) (2012),Thieno[3,2-b]thienobis(silolothiophene), a new electron rich hexacyclic monomer has been synthesized and incorporated into three novel donor-acceptor low-bandgap polymers. By carefully choosing the acceptor co-monomer, the energy levels of the polymers could be modulated and high power conversion efficiencies of 5.52% were reached in OPV devices. - 16.Fei
, Z. P.; Shahid, M.; Yaacobi-Gross, N.; Rossbauer, S.; Zhong, H. L.; Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Chem. Commun. 2012, 48, 11130[CrossRef], [PubMed], [CAS]16.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsFCgt7fI&md5=6d51bf83d5f37d027c69deb97471445eFei, Zhuping; Shahid, Munazza; Yaacobi-Gross, Nir; Rossbauer, Stephan; Zhong, Hongliang; Watkins, Scott E.; Anthopoulos, Thomas D.; Heeney, MartinChemical Communications (Cambridge, United Kingdom) (2012),We report the first synthesis of a tetrafluorinated 4,7-bis(3,4-difluorothiophen-2-yl)-2,1,3-benzothiadiazole monomer and its polymn. with dithieno[3,2-b:2',3'-d]germole by Stille coupling to afford a low band gap polymer with a high ionization potential. Direct comparison to the non-fluorinated analog demonstrates that fluorination results in an increase in ionization potential with no change in optical band gap, and enhanced aggregation over the non-fluorinated polymer. These desirable properties result in a significant enhancement in OPV device performance in blends with PC71BM. - 17.Albrecht
, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D. J. Am. Chem. Soc. 2012, 134, 14932[ACS Full Text], [PubMed], [CAS]17.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtFClu77E&md5=7ce2fd2e205982bd2e4e62b04bebf41cAlbrecht, Steve; Janietz, Silvia; Schindler, Wolfram; Frisch, Johannes; Kurpiers, Jona; Kniepert, Juliane; Inal, Sahika; Pingel, Patrick; Fostiropoulos, Konstantinos; Koch, Norbert; Neher, DieterJournal of the American Chemical Society (2012),A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC70BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphol. studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amt. of additives needed to achieve optimum device performance. Time-delayed collection field and charge extn. by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amt. of free charges, meaning that geminate recombination is greatly reduced. Addnl., a 3-fold redn. in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC70BM is largely detd. by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit c.d. of 14 mA/cm2, an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a high energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells. - 18.Price
, S. C.; Stuart, A. C.; Yang, L. Q.; Zhou, H. X.; You, W. J. Am. Chem. Soc. 2011, 133, 4625[ACS Full Text], [PubMed], [CAS]18.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXivVShu7c%253D&md5=a68c5d43de2fe6f1879db95e587221d7Price, Samuel C.; Stuart, Andrew C.; Yang, Liqiang; Zhou, Huaxing; You, WeiJournal of the American Chemical Society (2011),Recent research advances on conjugated polymers for photovoltaic devices have focused on creating low band gap materials, but a suitable band gap is only one of many performance criteria required for a successful conjugated polymer. This work focuses on the design of two medium band gap (∼2.0 eV) copolymers for use in photovoltaic cells which are designed to possess a high hole mobility and low HOMO and LUMO energy levels. The resulting fluorinated polymer PBnDT-FTAZ exhibits efficiencies above 7% when blended with [6,6]-Ph C61-butyric acid Me ester in a typical bulk heterojunction, and efficiencies above 6% are still maintained at an active layer thicknesses of 1 μm. PBnDT-FTAZ outperforms poly(3-hexylthiophene), the current medium band gap polymer of choice, and thus is a viable candidate for use in highly efficient tandem cells. PBnDT-FTAZ also highlights other performance criteria which contribute to high photovoltaic efficiency, besides a low band gap. - 19.Liang
, Y. Y.; Xu, Z.; Xia, J. B.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. P. Adv. Mater. 2010, 22, E135[CrossRef], [PubMed], [CAS]19.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXmvVGnu74%253D&md5=ec934d24dae469f3d5c43081f33079d6Liang, Yongye; Xu, Zheng; Xia, Jiangbin; Tsai, Szu-Ting; Wu, Yue; Li, Gang; Ray, Claire; Yu, LupingAdvanced Materials (Weinheim, Germany) (2010),A new semiconducting polymer, PTB7, for BHJ solar cells was developed and a cell based on the blend films of PTB7 with PC71BM exhibited an efficiency up to 7.4 %, the first polymer solar cell system showing a PCE >7 %. Both EQE and IQE obtained from such solar cells are high, implying that the harvest of solar energy is efficient. The results confirm that the thieno[3,4-b]thiophene-benzodithiophene polymers are a promising class of materials for high-performance polymer solar cells. - 20.Zhou
, H. X.; Yang, L. Q.; Stuart, A. C.; Price, S. C.; Liu, S. B.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995[CrossRef], [PubMed], [CAS]20.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXjt12jsbo%253D&md5=e19d8aeb3555eb0656e57d731cadeb19Zhou, Huaxing; Yang, Liqiang; Stuart, Andrew C.; Price, Samuel C.; Liu, Shubin; You, WeiAngewandte Chemie, International Edition (2011),The authors synthesized 5,6-difluoro-4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTffBT) and applied it to form a new low-bandgap polymer contg. fluorinated benzothiadiazole structural unit, PBnDT-DTffBT, having both a decreased HOMO and LUMO energy level. With a noticeably high VOC of 0.91 V, a fairly high Jsc of 12.9 mA cm-2, and an enhanced FF of 0.61, the overall efficiency of the PBnDT-DTffBT BHJ device achieved 7.2 % in initial trials. This efficiency is among the highest obtained for polymer/PC61BM BHJ solar cells, and shows a good potential for the DTffBT unit and the incorporation of fluorine atoms in creating high performance materials for BHJ solar cells. - 21.Li
, Z.; Lu, J. P.; Tse, S. C.; Zhou, J. Y.; Du, X. M.; Tao, Y.; Ding, J. F. J. Mater. Chem. 2011, 21, 3226[CrossRef], [CAS]21.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhvFChsrg%253D&md5=fb8c3621f1ccaeec05071364fadc176aLi, Zhao; Lu, Jianping; Tse, Shing-Chi; Zhou, Jiayun; Du, Xiaomei; Tao, Ye; Ding, JianfuJournal of Materials Chemistry (2011),A new electron deficient building block, difluorobenzothiadiazole (FBT) was designed and synthesized for the 1st time. Two low band gap polymers contg. this unit or benzothiadiazole (BT) and an electron rich unit, benzo[1,2-b:4,5-b']dithiophene (BDT) were synthesized by Stille coupling reaction. Fluorine substitution significantly lowered the energy levels but did not affect the UV absorption properties of the polymer. Comparatively, the fluorinated polymer showed a higher m.p. and a reduced soly. The elec. and photovoltaic properties of these polymers were studied by fabricating thin film transistors and polymer solar cells. The solar cell based on FBT polymer and PC71BM gives power conversion efficiency up to 3.4% under AM 1.5 G illumination (100 mW cm-2). - 22.Zhang
, Y.; Chien, S. C.; Chen, K. S.; Yip, H. L.; Sun, Y.; Davies, J. A.; Chen, F. C.; Jen, A. K.Y. Chem. Commun. 2011, 47, 11026[CrossRef], [PubMed], [CAS]22.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXht1ehsrbF&md5=fd167621768066dc036a35d4683a283eZhang, Yong; Chien, Shang-Chieh; Chen, Kung-Shih; Yip, Hin-Lap; Sun, Ying; Davies, Joshua A.; Chen, Fang-Chung; Jen, Alex K.-Y.Chemical Communications (Cambridge, United Kingdom) (2011),Small band-gap conjugated polymers based on monofluoro- and difluoro-substituted benzothiadiazole were developed. Highly efficient polymer solar cells (PCE ≤5.40%) could be achieved for devices made from these polymers. - 23.Zhang
, Y.; Zou, J. Y.; Cheuh, C. C.; Yip, H. L.; Jen, A. K. Y. Macromolecules 2012, 45, 5427[ACS Full Text], [CAS]23.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XptFWjtL4%253D&md5=c4d964c879404ce8af3b23abe9d0157cZhang, Yong; Zou, Jingyu; Cheuh, Chu-Chen; Yip, Hin-Lap; Jen, Alex K.-Y.Macromolecules (Washington, DC, United States) (2012),A partially fluorinated low bandgap polymer, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7-(5-fluoro-[2,1,3]-benzothiadiazole)] (PCPDTFBT) was synthesized through a microwave-assisted Stille polymn. It was found that PCPDTFBT has better π-π stacking in soln. than its nonfluorinated analog, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-([2,1,3]-benzothiadiazole)] (PCPDTBT), resulting in 2 times higher hole mobility. Power conversion efficiency (PCE) of the device using PCPDTFBT/PC71BM as active layer (5.51%) is much higher than the device using PCPDTBT/PC71BM (2.75%) that was fabricated under the same condition without using any solvent additive to modify the morphol. The significantly enhanced PCE is the result of improved open circuit voltage and short circuit current coming from the lower lying HOMO energy level and the appropriate morphol. of PCPDTFBT. In addn., the device with PCPDTFBT/PC71BM could also be processed from nonchlorinated org. solvents such as o-xylene to obtain high PCE of 5.32% (which is the highest value for PCPDTBT type polymers processed without using chlorinated solvents). Further device optimization by inserting a thin layer of fullerene-contg. surfactant between the active layer and Ag cathode resulted in even higher PCE of 5.81%. These encouraging results showed that PCPDTFBT has the potential to be used as a low bandgap polymer to provide complementary absorption in tandem solar cells. - 24.Peng
, Q.; Liu, X. J.; Su, D.; Fu, G. W.; Xu, J.; Dai, L. M. Adv. Mater. 2011, 23, 4554[CrossRef], [PubMed], [CAS]24.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtFers7jM&md5=9278a775e0a14d6464326d8a9a440881Peng, Qiang; Liu, Xiangju; Su, Dan; Fu, Guanwen; Xu, Jun; Dai, LimingAdvanced Materials (Weinheim, Germany) (2011),The concept of bulk heterojunction (BHJ) has improved the power conversion efficiencies (PCE5) of the PSCs significantly by blending electron-donating conjugated polymers and electron-accepting fullerene derivs. However, the improvement of PCEs using poly(3-hexylthiophene) and [6,6]PCBM has limited in these devices by their low open circuit voltages (Voc) and narrow absorption spectra. Aiming at achieving broader absorption, conjugated polymers were developed rapidly to be the alternatives of P3HT with efficiency values over 7%. In order to obtain high PCEs, the ideal bandgaps of electron-donating polymers should be 1.2-1.8 eV with HOMOs of -5.2 to -5.8 eV and LUMOs of -3.7 to -4.0 eV. The authors here report the synthesis of two Donor-Acceptor alternating copolymers, PBDT-OBT and PBDT-FBT, by polymg. modified benzo[1,2-b:4,5-b'] dithiophene-BT and a modified benzothiadiazole. The crude polymers were purified by solvent extn. and the polymers repptd. Mol. wts. were high and decompn. temps. 342 °C and 367°C, resp., showing good thermal stability. Pi-Pi stacking in the solid state polymers extends the red and IR absorbance, with optical bandgaps 1.67 and 1.63, well within the ideal design range. Cyclic voltammetry show the HOMO levels to be -5.32 and -5.41 eV, resp. (also withing the ideal design range) and LUMO levels -3.58 (a bit small) and -3.78 eV (within ideal design target). Heterojunction solar cells using ITO/PEDOT:PSS/copolymer:PC71BM (1:2 wt./wt. blend)/Ca/Al layer configuration with these copolymers attained an av. 5.50% and 6.08% conversion, resp., with Voc over 0.8 V and short circuit currents over 12 mA/cm2. - 25.Schroeder
, B. C.; Huang, Z. G.; Ashraf, R. S.; Smith, J.; D’Angelo, P.; Watkins, S. E.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I. Adv. Funct. Mater. 2012, 22, 1663[CrossRef], [CAS]25.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslCnurg%253D&md5=6f328110e59e97e2caa5c87e7d9a9880Schroeder, Bob C.; Huang, Zhenggang; Ashraf, Raja Shahid; Smith, Jeremy; D'Angelo, Pasquale; Watkins, Scott E.; Anthopoulos, Thomas D.; Durrant, James R.; McCulloch, IainAdvanced Functional Materials (2012),Silaindacenodithiophene is copolymd. with benzo[c][1,2,5]thiadiazole (BT) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT), resp. their fluorinated counter parts 5,6-difluorobenzo[c][1,2,5]thiadiazole (2FBT) and 5,6-difluoro-4,7-di(thiophen-2-yl) benzo[c][1,2,5]thiadiazole (2FDTBT). The influence of the thienyl spacers and fluorine atoms on mol. packing and active layer morphol. is studied with regard to device performances. bulk heterojunction (BHJ) solar cells based on silaindacenodithiophene donor-acceptor polymers achieved PCE's of 4.5% and hole mobilities of ≤0.28 cm2/(V s) are achieved in an org. field-effect transistor (OFET). - 26.Zhou
, H. X.; Yang, L. Q.; You, W. Macromolecules 2012, 45, 607[ACS Full Text], [CAS]26.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XlvVSlsA%253D%253D&md5=c8788f8a0439d2fcaf55c7f472ba0327Zhou, Huaxing; Yang, Liqiang; You, WeiMacromolecules (Washington, DC, United States) (2012),Review. The research on the polymer-based solar cells (PSCs) has attracted an increasing amt. of attention in recent years because PSCs pose potential advantages over mainstream inorg.-based solar cells, such as significantly reduced material/fabrication costs, flexible substrates, and light wt. of finished solar cells. The research community has made great progress in the field of bulk heterojunction (BHJ) polymer solar cells since its inception in 1995. The power conversion efficiency (PCE), a key parameter to assess the performance of solar cells, has increased from 1% in the 1990s to over 8% just recently. These great advances are mainly fueled by the development of conjugated polymers used as the electron-donating materials in BHJ solar cells. In this Perspective, we first briefly review the progress on the design of conjugated polymers for polymer solar cells in the past 16 years. Since a conjugated polymer can be arbitrarily divided into three constituting components-the conjugated backbone, the side chains, and the substituents-we then focus on the rational design of conjugated polymers by sep. discussing the influence of each component on the phys. and photovoltaic (PV) properties of these polymers. Special attention is paid to the design of donor-acceptor type low-band-gap polymers because this approach is prevailing in the literature with its unique features. In doing so, we strive to ext. useful rules for the rational design of conjugated polymers with predictable properties. We conclude by proposing future research opportunities to achieve even higher PCEs for PSCs. - 27.Bronstein
, H.; Frost, J. M.; Hadipour, A.; Kim, Y. J.; Nielsen, C. B.; Ashraf, R. S.; Rand, B. P.; Watkins, S.; McCulloch, I. Chem. Mater. 2013, 25, 277[ACS Full Text], [CAS]27.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXnvVGktw%253D%253D&md5=e1e47be686264c529c33a34bde725ed2Bronstein, Hugo; Frost, Jarvist M.; Hadipour, Afshin; Kim, Youngju; Nielsen, Christian B.; Ashraf, Raja Shahid; Rand, Barry P.; Watkins, Scott; McCulloch, IainChemistry of Materials (2013),Two novel indacenodithiophene (IDT) based donor-acceptor conjugated polymers for use in org. field effect transistors and photovoltaic devices were synthesized and characterized. The effect of inclusion of two fluorine atoms on the acceptor portion of the polymer is thoroughly studied via a range of techniques. The inductively withdrawing and mesomerically donating properties of the fluorine atoms result in a decrease of the HOMO, with little effect on the LUMO as demonstrated through d. functional theory (DFT) anal. Inclusion of fluorine atoms also leads to a potentially more planar backbone through inter and intrachain interactions. Use of the novel materials in org. field effect transistor (OFET) and org. photovoltaic (OPV) devices leads to high mobilities around 0.1 cm2/(V s) and solar cell efficiencies around 4.5%. - 28.Stuart
, A. C.; Tumbleston, J. R.; Zhou, H. X.; Li, W. T.; Liu, S. B.; Ade, H.; You, W. J. Am. Chem. Soc. 2013, 135, 1806[ACS Full Text], [PubMed], [CAS]28.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXktV2rsA%253D%253D&md5=3540ce287971e79097674bb0351fa2dfStuart, Andrew C.; Tumbleston, John R.; Zhou, Huaxing; Li, Wentao; Liu, Shubin; Ade, Harald; You, WeiJournal of the American Chemical Society (2013),Three structurally identical polymers, except for the no. of fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT), are investigated in detail, to further understand the impact of these fluorine atoms on open circuit voltage (Voc), short circuit current (Jsc), and fill factor (FF) of related solar cells. While the enhanced Voc can be ascribed to a lower HOMO level of the polymer by adding more fluorine substituents, the improvement in Jsc and FF are likely due to suppressed charge recombination. While the reduced bimol. recombination with raising fluorine concn. is confirmed by variable light intensity studies, a plausibly suppressed geminate recombination is implied by the significantly increased change of dipole moment between the ground and excited states for these polymers as the no. of fluorine substituents increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly more scattering in the in-plane lamellar stacking and out-of-plane π-π stacking directions, obsd. with GIWAXS. This indicates that the addn. of fluorine leads to a more face-on polymer crystallite orientation with respect to the substrate, which could contribute to the suppressed charge recombination. R-SoXS also reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains. The higher domain purity is correlated with an obsd. decrease in PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to 12% (PBnDT-DTffBT). The disclosed fluorine impact not only explains the efficiency increase from 4% of PBnDT-DTBT (0F) to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution should be generally considered in the future design of new polymers. - 29.Yu
, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789[CrossRef], [CAS]29.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXhtVSiurjF&md5=cb6bac39a1bea41cca0f86ad28f20b3cYu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J.Science (Washington, D. C.) (1995),The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivs. Composite films of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge sepn. results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bi-continuous network of internal donor-acceptor heterojunctions. - 30.Carsten
, B.; Szarko, J. M.; Son, H. J.; Wang, W.; Lu, L. Y.; He, F.; Rolczynski, B. S.; Lou, S. J.; Chen, L. X.; Yu, L. P. J. Am. Chem. Soc. 2011, 133, 20468[ACS Full Text], [PubMed], [CAS]30.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsVGkt7vE&md5=450010672d50d1dee849684258f3d4f7Carsten, Bridget; Szarko, Jodi M.; Son, Hae Jung; Wang, Wei; Lu, Luyao; He, Feng; Rolczynski, Brian S.; Lou, Sylvia J.; Chen, Lin X.; Yu, LupingJournal of the American Chemical Society (2011),A new low band gap copolymer PBB3 contg. [6,6']bi[thieno[3,4-b]thiophenyl]-2,2'-dicarboxylic acid bis-(2-butyloctyl) ester (BTT) and 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b']dithiophene (BDT) units was synthesized and tested for solar cell efficiency. PBB3 showed a broad absorbance in the near-IR region with a substantially red-shifted (by more than 100 nm) λmax at 790 nm as compared to the PTB series of polymers, which have been previously reported. The PBB3 polymer also showed both a favorable energy level match with PCBM (with a LUMO energy level of -3.29 eV) and a favorable film domain morphol. as evidenced by TEM images. Despite these seemingly optimal parameters, a bulk heterojunction (BHJ) photovoltaic device fabricated from a blend of PBB3 and PC71BM showed an overall power conversion efficiency (PCE) of only 2.04% under AM 1.5G/100 mW/cm-2. The transient absorption spectra of PBB3 showed the absence of cationic and pseudo charge transfer states that were obsd. previously in the PTB series polymers, which were also composed of alternating thienothiophene (TT) and BDT units. We compared the spectral features and electronic d. distribution of PBB3 with those of PTB2, PTB7, and PTBF2. While PTB2 and PTB7 have substantial charge transfer characteristics and also relatively large local internal dipoles through BDT to TT moieties, PTBF2 and PBB3 have minimized internal dipole moments due to the presence of two adjacent TT units (or two opposing fluorine atoms in PTBF2) with opposite orientations or internal dipoles. PBB3 showed a long-lived excitonic state and the slowest electron transfer dynamics of the series of polymers, as well as the fastest recombination rate of the charge-sepd. (CS) species, indicating that electrons and holes are more tightly bound in these species. Consequently, substantially lower degrees of charge sepn. were obsd. in both PBB3 and PTBF2. These results show that not only the energetics but also the internal dipole moment along the polymer chain may be crit. in maintaining the pseudocharge transfer characteristics of these systems, which were shown to be partially responsible for the high PCE device made from the PTB series of low band gap copolymers. - 31.Rolczynski
, B. S.; Szarko, J. M.; Son, H. J.; Liang, Y. Y.; Yu, L. P.; Chen, L. X. J. Am. Chem. Soc. 2012, 134, 4142[ACS Full Text], [PubMed], [CAS]31.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslOntLY%253D&md5=28faf580c57d4f510f2dcee938d46935Rolczynski, Brian S.; Szarko, Jodi M.; Son, Hae Jung; Liang, Yongye; Yu, Luping; Chen, Lin X.Journal of the American Chemical Society (2012),Record-setting org. photovoltaic cells with PTB polymers have recently achieved ∼8% power conversion efficiencies. A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the no. and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device power conversion efficiency variations of 2-8%. Using near-IR, ultrafast optical transient absorption spectroscopy combined with steady-state electrochem. methods we were able to obtain transient absorption signatures not only for the exciton and charge-sepd. states but also for an intramol. (pseudo) charge-transfer state in isolated PTBF polymers in soln., in the absence of the acceptor phenyl-C61-butyric acid Me ester (PCBM) mols. This led to the discovery of branched pathways for intramol., ultrafast exciton splitting to populate (a) the charge-sepd. states or (b) the intramol. charge-transfer states on the sub-picosecond time scale. Depending on the no. and position of the fluorine pendant atoms, the charge-sepn./transfer kinetics and their branching ratios vary according to the trend for the electron d. distribution in favor of the local charge-sepn. direction. More importantly, a linear correlation is found between the branching ratio of intramol. charge transfer and the charge sepn. of hole-electron pairs in isolated polymers vs. the device fill factor and power conversion efficiencies. The origin of this correlation and its implications in materials design and device performance are discussed. - 32.Zang
, H. D.; Liang, Y. Y.; Yu, L. P.; Hu, B. Adv. Energy Mater. 2011, 1, 923[CrossRef], [CAS]32.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtlSht7%252FF&md5=3c196c5644d8361a590cd2beeb1e7b5fZang, Huidong; Liang, Yongye; Yu, Luping; Hu, BinAdvanced Energy Materials (2011),This article reports exptl. studies on internal charge dissocn., transport, and collection by using magnetic field effects of photocurrent (MFEPC) and light-assisted dielec. response (LADR) in highly-efficient org. solar cells based on photovoltaic polymer PTB2 and PTB4 with intra-mol. "donor-acceptor" interaction. The MFEPC at low-field (< 150 mT) indicates that intra-mol. "donor-acceptor" interaction generates charge dissocn. in un-doped PTB2 and PTB4 films, which is similar to that in lightly doped P3HT (Poly(3-hexylthiophene)) with 5 wt% PCBM (1-(3-methyloxycarbonyl)-propyl-1-Ph (6,6) C61). After PTB2 and PTB4 are mixed with PCBM to form bulk-heterojunctions, the MFEPC at high-field (> 150 mT) reveals that the charge-transfer complexes formed at PTB2:PCBM and PTB4:PCBM interfaces have much lower binding energies due to stronger electron-withdrawing abilities, as compared to the P3HT:PCBM device, towards the generation of photocurrent. Furthermore, the light-assisted dielec. response: LADR indicates that the PTB2:PCBM and PTB4:PCBM solar cells exhibit larger capacitances relative to P3HT:PCBM device under photoexcitation. This reflects that the PTB2:PCBM and PTB4:PCBM bulk heterojunctions have more effective charge transport and collection than the P3HT:PCBM counterpart. As a result, our exptl. results indicate that intra-mol. "donor-acceptor" interaction plays an important role to enhance charge dissocn., transport, and collection in bulk-heterojunction org. solar cells. - 33.Dou
, L. T.; Chen, C. C.; Yoshimura, K.; Ohya, K.; Chang, W. H.; Gao, J.; Liu, Y. S.; Richard, E.; Yang, Y. Macromolecules 2013, 46, 3384[ACS Full Text], [CAS]33.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXmsFGrsbw%253D&md5=a67d19b2b89f07f44639ca9add9601deDou, Letian; Chen, Chun-Chao; Yoshimura, Ken; Ohya, Kenichiro; Chang, Wei-Hsuan; Gao, Jing; Liu, Yongsheng; Richard, Eric; Yang, YangMacromolecules (Washington, DC, United States) (2013),We describe the detailed synthesis and characterization of an electron-rich building block, dithienopyran (DTP), and its application as a donor unit in low-bandgap conjugated polymers. The electron-donating property of the DTP unit was found to be the strongest among the most frequently used donor units such as benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units. When the DTP unit was polymd. with the strongly electron-deficient difluorobenzothiadiazole (DFBT) unit, a regio-random polymer (PDTP-DFBT, bandgap = 1.38 eV) was obtained. For comparison with the DTP unit, polymers contg. alternating benzodithiophene (BDT) or cyclopentadithiophene (CPDT) units and the DFBT unit were synthesized (PBDT-DFBT and PCPDT-DFBT). We found that the DTP based polymer PDTP-DFBT shows significantly improved soly. and processability compared to the BDT or CPDT based polymers. Consequently, very high mol. wt. and sol. PDTP-DFBT can be obtained with less bulky side chains. Interestingly, PDTP-DFBT shows excellent performance in bulk-heterojunction solar cells with power conversion efficiencies reaching 8.0%, which is significantly higher than PBDT-DFBT and PCPDT-DFBT based devices. This study demonstrates that DTP is a promising building block for high-performance solar cell materials. - 34.Chen
, H. C.; Chen, Y. H.; Liu, C. C.; Chien, Y. C.; Chou, S. W.; Chou, P. T. Chem. Mater. 2012, 24, 4766[ACS Full Text], [CAS]34.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslOmu7rP&md5=4f33086cae53f7c417aa52844682cbd0Chen, Hsieh-Chih; Chen, Ying-Hsiao; Liu, Chi-Chang; Chien, Yun-Chen; Chou, Shang-Wei; Chou, Pi-TaiChemistry of Materials (2012),A tailor-made medium-band gap fluorinated quinoxaline-based conjugated polymer of PBDT-TFQ was designed and synthesized as a donor material for bulk-heterojunction (BHJ) solar cells. This polymer is possessed of an intrachain donor-acceptor architecture and exhibits a broad and strong absorption spectrum across the entire UV-vis region. The introduction of F atoms with high electron affinity to the quinoxaline moiety is effective in further lowering both the HOMO and the LUMO energy levels of PBDT-TFQ to attain higher open-circuit voltage (Voc). With an optimized blend ratio of PBDT-TFQ:PC71BM (1:1, wt./wt.), a high power conversion efficiency (PCE) of 8.0% was obtained, with a Voc of 0.76 V, a short-circuit c.d. (Jsc) of 18.2 mA cm-2, and a fill factor (FF) of 58.1% under AM 1.5G irradn. The resulting copolymer reveals an outstanding Jsc value, arising from the higher hole mobility of PBDT-TFQ, together with the better continuous percolation pathways within the polymer blend for efficient exciton dissocn. and charge transport. - 35.Liao
, S. H.; Jhuo, H. J.; Cheng, Y. S.; Chen, S. A. Adv. Mater. 2013, 25, 4766[CrossRef], [PubMed], [CAS]35.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXht1Crtb3O&md5=3b150968c8be6f7652948f1fc24ded30Liao, Sih-Hao; Jhuo, Hong-Jyun; Cheng, Yu-Shan; Chen, Show-AnAdvanced Materials (Weinheim, Germany) (2013),Novel low-bandgap polymer PTB7-Th is presented as a donor, combining the advantage of incorporating the 2-ethylhexyl-thienyl group into the BDT unit (BDT-T) to improve the coplanarity of the main chain to extend the absorption band to longer wavelengths and enhance the absorption coeff. relative to PTB7. The fluorine-substituted TT unit with the 2-ethylhexyl carboxylate group (F-TT) is also retained for a higher HOMO level. This PTB7-Th composite with PC71BM as the acceptor is used as the active layer for i-PSCs, doping the ZnO cathode with a fullerene deriv. (PCBE-OH, 0.5 wt% of the ZnO precursor) to give a ZnO-C60 nanofilm (40 nm) on ITO as the cathode. This ZnO-C60 cathode provides dual functionalities for enhanced electron collection, including producing a fullerene- deriv.-rich cathode surface and promotion of electron cond. at the interface and bulk. For the device with ZnO-C60 as the cathode the PCE is improved for PTB7-Th:PC71BM from 7.64% to 9.35%, for PTB7:PC71BM from 6.65% to' 8.21%, and for P3HT:ICBA from 5.26% to 6.60%, compared to that with ZnO cathode. - 37.Chen
, W.; Nikiforov, M. P.; Darling, S. B. Energy Environ. Sci. 2012, 5, 8045[CrossRef], [CAS]37.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVyksr7P&md5=9961ad9c1f918f6ac189ae057eb774a1Chen, Wei; Nikiforov, Maxim P.; Darling, Seth B.Energy & Environmental Science (2012),A review. Org. and hybrid org.-inorg. photovoltaics are among the most promising options for low-cost and highly scalable renewable energy. In order to fully realize the potential of these technologies, power conversion efficiencies and stability will both have to be improved beyond the current state-of-the-art. The morphol. of the active layer is of paramount importance in the photon to electron conversion process in org. and hybrid solar cells, with all length scales, from mol. ordering to intradevice compn. variability, playing key roles. Given the central influence of morphol., characterizing the structure of these surprisingly complex material systems at multiple length scales is one of the grand challenges in the field. This review addresses the techniques, some of which have only recently been applied to org. and hybrid photovoltaics, available to scientists and engineers working to understand-and ultimately improve-the operation of these fascinating devices. - 38.Liang
, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J. Am. Chem. Soc. 2009, 131, 7792[ACS Full Text], [PubMed], [CAS]38.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXmtVGnt7g%253D&md5=785ccdf49d57dc516238464e29822880Liang, Yongye; Feng, Danqin; Wu, Yue; Tsai, Szu-Ting; Li, Gang; Ray, Claire; Yu, LupingJournal of the American Chemical Society (2009),This paper describes synthesis and photovoltaic studies of a series of new semiconducting polymers with alternating thieno[3,4-b]thiophene and benzodithiophene units. The phys. properties of these polymers were finely tuned to optimize their photovoltaic effect. The substitution of alkoxy side chains to the less electron-donating alkyl chains or introduction of electron-withdrawing fluorine into the polymer backbone reduced the HOMO energy levels of polymers. The structural modifications optimized polymers' spectral coverage of absorption and their hole mobility, as well as miscibility with fulleride, and enhanced polymer solar cell performances. The open circuit voltage, Voc, for polymer solar cells was increased by adjusting polymer energy levels. It was found that films with finely distributed polymer/fulleride interpenetrating network exhibited improved solar cell conversion efficiency. Efficiency over 6% has been achieved in simple solar cells based on fluorinated PTB4/PC61BM films prepd. from mixed solvents. The results proved that polymer solar cells have a bright future. - 39.Chen
, W.; Xu, T.; He, F.; Wang, W.; Wang, C.; Strzalka, J.; Liu, Y.; Wen, J. G.; Miller, D. J.; Chen, J. H.; Hong, K. L.; Yu, L. P.; Darling, S. B. Nano Lett. 2011, 11, 3707[ACS Full Text], [PubMed], [CAS]39.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXpsVOhtbk%253D&md5=6940fd75e50c3e04183cd327b8f3876bChen, Feng; You, Hongjun; Yang, Shengchun; Yang, Zhimao; Song, Xiaoping; Lagally, Max G.; Ding, BingjunCrystal Growth & Design (2011),The authors have developed a periodic nucleation and growth mechanism for nanoparticle-aggregated structure based on electron tunneling process that occurs in chem. replacement reactions. This electron tunneling process, which is a fundamental phys. phenomenon, successfully explains the repeated formation of nanoparticles that was obsd. in the expt. Using the nanoparticle-mediated progress, the authors have interpreted the origin of layer-by-layer nanoparticle stacking polycryst. nanostructures and mesocrystal in chem. redns. - 40.Huang
, Y.; Guo, X.; Liu, F.; Huo, L. J.; Chen, Y. N.; Russell, T. P.; Han, C. C.; Li, Y. F.; Hou, J. H. Adv. Mater. 2012, 24, 3383[CrossRef], [PubMed], [CAS]40.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XnvVyrt7Y%253D&md5=536e920196cc8a40f464b1c3579d2645Huang, Ye; Guo, Xia; Liu, Feng; Huo, Lijun; Chen, Yuning; Russell, Thomas P.; Han, Charles C.; Li, Yongfang; Hou, JianhuiAdvanced Materials (Weinheim, Germany) (2012),Understanding the correlation between the mol. structure,morphol. and photovoltaic properties of conjugated polymers used in bulk heterojunction (BHJ) polymer-based, org. photovoltaics (OPV5) is of crit. importance in designing high-efficiency org. photovoltaic polymers. With these criteria in mind, we have taken a systematic and effective mol. design route to control and tune the structural ordering and morphol. of new low band gap polymers and their blends with [6,6]-Ph C71-butyric acid Me ester(PC70BM). Here, we focus on using mol. modifications to achieve optimized thin film morphologies and, therefore, high efficiencies. Poly[(2-((2-ethylhexyl)sulfonyl)thieno[3- -b]thiophen-2,6-diyl)-alt-(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl] (PBDTTT-S), a polymer based on benzo[1,2-b:4,5-b']dithiophene (BDT) and thieno[3,4-b]thiophene (TT), was chosen, since it has bulky sulfonyl side group that results in a disordered packing of the chains. Based on the strategy of achieving conjugated polymers with large π-overlap area and good ordering structure, here we report one new polymer, poly[(((2-hexyldecyl)sulfonyl)-4,6-di(thiophen-2-yl)thieno[3,4-b]thiophene-2,6-diyl)-alt-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)] (PBDTDTTT-S-T), where T represents the thiophene side group on the BDT unit. Introducing the thiophene unit onto the BDTside position and between the BDT-T and TT-S units can enlarge the π-overlap area parallel to and normal to the chain axis and reduce the steric hindrance between side chains. The photovoltaic efficiencies of PBDTDTTT-S-T-based solar cells were as high as 7.81 %, showing nearly a 25 % increase after the simple modification. This effective and simple design motif can be widely used in other existing and new conjugated systems to optimize their efficiencies. It is shown that extending the conjugation area in the monomer and, therefore, face-to-face packing between the polymer chains is an effective way to significantly increase photovoltaic efficiencies. A correlation between the mol. structure, thin film morphol., and photovoltaic properties of these conjugated polymers is established that provides guidance for the mol. design of new photovoltaic polymers. - 41.Li
, X. H.; Choy, W. C. H.; Huo, L. J.; Xie, F. X.; Sha, W. E. I.; Ding, B. F.; Guo, X.; Li, Y. F.; Hou, J. H.; You, J. B.; Yang, Y. Adv. Mater. 2012, 24, 3046[CrossRef], [PubMed], [CAS]41.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xms1eru7g%253D&md5=31df9001438c3680284f50587878b8d9Li, Xuanhua; Choy, Wallace C. H.; Huo, Lijun; Xie, Fengxian; Sha, Wei E. I.; Ding, Baofu; Guo, Xia; Li, Yongfang; Hou, Jianhui; You, Jingbi; Yang, YangAdvanced Materials (Weinheim, Germany) (2012),There is no expanded citation for this reference. - 42.Huo
, L. J.; Zhang, S. Q.; Guo, X.; Xu, F.; Li, Y. F.; Hou, J. H. Angew. Chem., Int. Ed. 2011, 50, 9697[CrossRef], [PubMed], [CAS]42.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtVOmsLjL&md5=f70b7a47600da98be8652721ff8b650dHuo, Lijun; Zhang, Shaoqing; Guo, Xia; Xu, Feng; Li, Yongfang; Hou, JianhuiAngewandte Chemie, International Edition (2011),Two conjugated polymers based on benzo[1,2-b:4,5-b']dithiophene (BDT), with conjugated 5-alkylthiophen-2-yl substituents (alkyl = 2-ethylhexyl) at the 4 and 8 positions on the BDT ring system, were synthesized and compared with the corresponding 4,8-bis(5-alkoxythiophen-2-yl) BDT monomers. The BDT-based polymers were prepd. by Stille coupling polymn. with the corresponding bis(trimethylstannyl) derivs.; the co-monomers were (1) 4,6-dibromothieno[3,4-b]thiophene-2-carboxylic acid, 2-ethylhexyl ester, and (2) 1-(4,6-dibromothieno[3,4-b]thien-2-yl)-2-ethyl-1-hexanone. The two polymers were evaluated in fully fabricated polymer solar cells. The polymers exhibited better thermal stabilities, red-shifted UV absorption spectra, lower HOMO and LUMO energy levels, significantly higher hole mobility, and greatly improved photovoltaic properties, in comparison with their corresponding alkoxy-substituted analogs. - 43.Dou
, L. T.; Gao, J.; Richard, E.; You, J. B.; Chen, C. C.; Cha, K. C.; He, Y. J.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2012, 134, 10071[ACS Full Text], [PubMed], [CAS]43.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XnsF2gurw%253D&md5=39ac483f85b1fa4d9b29c423b0e20fc3Dou, Letian; Gao, Jing; Richard, Eric; You, Jingbi; Chen, Chun-Chao; Cha, Kitty C.; He, Youjun; Li, Gang; Yang, YangJournal of the American Chemical Society (2012),The design, synthesis, and characterization are presented of a series of new low bandgap polymers specifically for tandem polymer solar cells. These polymers have a backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated onto the BDT unit to form BDTT and BDTP units, resp.; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the FDPP unit. Low bandgap polymers (bandgap = 1.4-1.5 eV) were prepd. using BDTT, BDTP, FDPP, and DPP units via Stille-coupling polymn. These structural modifications lead to polymers with different optical, electrochem., and electronic properties. Single junction solar cells were fabricated, and the polymer:PC71BM active layer morphol. was optimized by adding 1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 3-6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential for high efficiency tandem polymer solar cells. - 44.Lim
, D. C.; Kim, K. D.; Park, S. Y.; Hong, E. M.; Seo, H. O.; Lim, J. H.; Lee, K. H.; Jeong, Y.; Song, C.; Lee, E.; Kim, Y. D.; Cho, S. Energy Environ. Sci. 2012, 5, 9803[CrossRef], [CAS]44.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhvVams7zJ&md5=7b36173d4746a2add4a4974679bd0d02Lim, Dong Chan; Kim, Kwang-Dae; Park, Sun-Young; Hong, Eun Mi; Seo, Hyun Ook; Lim, Jae Hong; Lee, Kyu Hwan; Jeong, Yongsoo; Song, Changsik; Lee, Eunji; Kim, Young Dok; Cho, ShinukEnergy & Environmental Science (2012),Using a novel polymer (polythienothiophene-co-benzodithiophenes 7 F-20) as a donor and phenyl-C71-butyric acid Me ester as an acceptor of bulk heterojunction, inverted org. photovoltaics (OPVs) were fabricated. Wet-chem. prepd. ZnO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were used as buffer layers. Particularly, for PEDOT:PSS deposition, no annealing step was employed. This inverted OPV showed a power conversion efficiency (PCE) of ∼7.0%, which is comparable to the hitherto reported highest efficiency of the inverted OPV with vacuum-deposited MoO3 as hole-collecting buffer layers without plasmonic enhancements. Incorporation of Au nanoparticles into PEDOT:PSS was performed for plasmonic enhancement of the electromagnetic field, whereas ZnO thin layers were deposited on ZnO using at. layer deposition for quenching electron-hole recombination at surface defects of ZnO ripples. These addnl. treatments could be used for improving the performance of OPV, which ultimately resulted in a PCE of 7.9%. - 45.Park
, H. Y.; Lim, D. G.; Kim, K. D.; Jang, S. Y. J. Mater. Chem. A 2013, 1, 6327[CrossRef], [CAS]45.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXntlWqtL8%253D&md5=b9007b9f91f10c77d6cc17e9530a1a93Park, Hye-Yun; Lim, Dongchan; Kim, Kwang-Dae; Jang, Sung-YeonJournal of Materials Chemistry A: Materials for Energy and Sustainability (2013),High-performance soln.-processable ZnO thin films for use as electron-transporting layers (ETLs) of inverted-structured polymer solar cells (I-PSCs) are developed via a low-temp. annealing (<200°) sol-gel process. The properties of the low-temp.-annealed ZnO (L-ZnO) thin films (used as ETLs) are optimized based on the evaluation of the roles of the internal nanocrystal (NC) orientation and film-surface morphol. in charge transport/transfer in I-PSCs. The low-temp. annealing conditions (dynamic annealing or static annealing) could be successfully manipulated to alter the NC orientation of the L-ZnO films, whereas tactical control of the precursor-coating conditions enabled the embedding of nanoripples on the film surfaces. Suppression of the preferential (002) plane NC orientation of the L-ZnO layers is beneficial for charge transport in I-PSCs; these devices should be evaluated in a manner different from field-effect transistors (FETs). The performance of ETLs is further enhanced by the development of nanoripple-embedded L-ZnO film surfaces, which furnish an increased area for contact with the active layers. The I-PSCs fabricated using the optimized L-ZnO films display a >20% higher power-conversion efficiency (PCE) than those employing the conventional L-ZnO films for a range of active materials including poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid Me ester (PC60BM) and poly(thienothiophene-co-benzodithiophenes) (PTB7-F20)/phenyl-C71-butyric acid Me ester (PC71BM) blends. A PCE of 6.42% is achieved for the I-PSCs using the optimized L-ZnO films and PTB7-F20/PC71BM blends as the ETL and active materials, resp. This study presents a universal method for optimizing sol-gel-driven ZnO-based ETLs, while the low-temp. processability and long-term stability of the developed ETLs are beneficial for the commercialization of I-PSCs. - 46.Wang
, H. X.; Yu, X. F.; Yi, C.; Ren, H.; Liu, C.; Yang, Y. L.; Xiao, S.; Zheng, J.; Karim, A.; Cheng, S. Z. D.; Gong, X. J. Phys. Chem. C 2013, 117, 4358[ACS Full Text], [CAS]46.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXitlChs7s%253D&md5=71c9504d7078b7bd63433db9a26d32c0Wang, Hangxing; Yu, Xinfei; Yi, Chao; Ren, He; Liu, Chang; Yang, Yali; Xiao, Steven; Zheng, Jie; Karim, Alamgir; Cheng, Stephen Z. D.; Gong, XiongJournal of Physical Chemistry C (2013),Novel low band gap fluorinated copolymers were synthesized and characterized. The performance of inverted polymer solar cells made by these fluorinated copolymers blended with fullerene derivs. was also investigated. The studies of the effect of fluorine units on the electronic properties and the thin film morphologies of fluorinated copolymers demonstrated that an optimal window of 20-40% fluorine units coupled with thieno[3,4-b]thiopehene exists for a max. hole mobility that correlated well with a dispersed interconnected morphol. of the fluorinated copolymer, thus enhancing the performance of polymer solar cells. More fluorine units coupled with thieno[3,4-b]thiopehene adversely affected the phase morphol. to coarsen that in turn reduced polymer solar cells performance. Nevertheless, the polymer solar cell performance of the highest fluorinated content polymer was twice as much as the nonfluorinated copolymer. These results indicate that the electronic properties as well as the thin film morphologies, not only in nanoscale but also in microscale, of electroactive and photoactive polymers are collectively very important for org. photovoltaic devices. - 47.He
, F.; Wang, W.; Chen, W.; Xu, T.; Darling, S. B.; Strzalka, J.; Liu, Y.; Yu, L. P. J. Am. Chem. Soc. 2011, 133, 3284[ACS Full Text], [PubMed], [CAS]47.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXitFWrur4%253D&md5=f5eb44b7ada291a842063d2df809d5d9He, Feng; Wang, Wei; Chen, Wei; Xu, Tao; Darling, Seth B.; Strzalka, Joseph; Liu, Yun; Yu, LupingJournal of the American Chemical Society (2011),A series of semiconducting copolymers (PTAT-x) contg. extended π-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the π-π stacking in the polymer/PC61BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC61BM. - 48.Huo
, L. J.; Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Jiang, Y.; Chen, T. L.; Yang, Y. Macromolecules 2009, 42, 6564[ACS Full Text], [CAS]48.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXpsFarsrY%253D&md5=0d28ffc96508af6f08513c7146f0859dHuo, Lijun; Hou, Jianhui; Chen, Hsiang-Yu; Zhang, Shaoqing; Jiang, Yang; Chen, Teresa L.; Yang, YangMacromolecules (Washington, DC, United States) (2009),A series of low-bandgap polymers based on a sol. chromophore of 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit were synthesized by introducing of different electron-rich building blocks copolymd. with DPP unit. Four new DPP-based polymers, PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP, were characterized by gel permeation chromatog., thermogravimetric anal., NMR, UV-visible absorption, and electrochem. cyclic voltammetry. The results indicate that their bandgaps as well as their mol. energy levels are readily tuned by copolymg. with different conjugated electron-donating units. In order to investigate their photovoltaic properties, polymer solar cell devices based on PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP were fabricated with a structure of ITO/PEDOT:PSS/polymers:PC70BM (1:2 wt. ratio)/Ca/Al under the illumination of air-mass 1.5G, 100 mW/cm2. The power conversion efficiencies of the four DPP-based polymer solar cell devices were measured and shown in this paper. The best performance of the polymer solar cell device was obtained by using PDPP-BDP as the electron donor material, and a power conversion efficiency of 4.45% with an open-circuit voltage of 0.72 V, a short-circuit c.d. of 10.0 mA/cm2, and a fill factor of 61.8% was achieved, which is the best result among the DPP-based polymer materials. It is apparent that the PDPP-BDP-based device exhibits a very broad response range, covering from 300 to 850 nm. The results of the solar cells indicate that these types of materials are very promising candidates for highly efficient polymer solar cells. - 49.Li
, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J. Synth. Met. 1999, 99, 243[CrossRef], [CAS]49.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXhsVWksbg%253D&md5=f49bc98afffb05f6d24e74e6eba67bc7Li, Yongfang; Cao, Yong; Gao, Jun; Wang, Deli; Yu, Gang; Heeger, Alan J.Synthetic Metals (1999),The electrochem. p-doping potentials (φp) and n-doping potentials (φn) of 10 sol. poly(1,4-phenylenevinylene) (PPV) derivs. and of the conjugated polymer blend in a light-emitting electrochem. cell (LEC) were measured by cyclic voltammetry. The energy levels corresponding to the HOMO and the LUMO of the polymers were detd. from the onset potentials for n-doping (φn') and p-doping (φp'), resp. The electrochem. energy gap, Eg'=Δφ=φp'-φn', agrees well with the optical energy gap (Eg). For copolymers of poly(2-methoxy, 5-(2'-ethyl-hexyloxy) para-phenylene vinylene) (MEH-PPV) and poly(2-Bu, 5-(2'-ethyl-hexyl) para-phenylene vinylene) (BuEH-PPV), the n-doping potentials are nearly independent of the ratio of MEH-PPV to BuEH-PPV. However, p-doping potentials depend strongly on the ratio, the higher the MEH-PPV fraction, the lower the p-doping potential of the copolymer. For cyano-PPV (CN-PPV), both p-doping and n-doping potentials were shifted by ca. 0.6 V (more electroneg.) as a result of the electron withdrawing effect of cyano side group. The electrochem. p-doping and n-doping processes of the MEH-PPV polymer blend in the LEC device were confirmed from linear sweep voltammetry of the LEC; the doping onset potentials were the same as for an MEH-PPV film measured in a liq. electrolyte. - 50.Bredas
, J.- L. Mater. Horiz. 2014, 1, 17[CrossRef], [CAS]50.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvVShsbzI&md5=aff7852ce88cd8123e075f4e9915a381Bredas, Jean-LucMaterials Horizons (2014),A review. The energy gap between the highest occupied and lowest unoccupied electronic levels is a crit. parameter detg. the electronic, optical, redox, and transport (elec.) properties of a material. However, the energy gap comes in many flavors, such as the band gap, HOMO-LUMO gap, fundamental gap, optical gap, or transport gap, with each of these terms carrying a specific meaning. Failure to appreciate the distinctions among these different energy gaps has caused much confusion in the literature, which is manifested by the frequent use of improper terminol., in particular, in the case of org. mol. or macromol. materials. Thus, it is our goal here to clarify the meaning of the various energy gaps that can be measured exptl. or evaluated computationally, with a focus on π-conjugated materials of interest for org. electronics and photonics applications. - 52.Liu
, F.; Gu, Y.; Wang, C.; Zhao, W.; Chen, D.; Briseno, A. L.; Russell, T. P. Adv. Mater. 2012, 24, 3947[CrossRef], [PubMed], [CAS]52.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XosVejsLg%253D&md5=7892e4b40b540f44d2ada01025b72c51Liu, Feng; Gu, Yu; Wang, Cheng; Zhao, Wei; Chen, Dian; Briseno, Alejandro L.; Russell, Thomas P.Advanced Materials (Weinheim, Germany) (2012),We have demonstrated that a copolymer of diketopyrrolopyrrole and quaterthiophene (pDPP) is a promising low bandgap polymer for org. solar cells. The polymer shows good crystallinity and the morphol. of pDPP:PC71BM blends cast from mixed solvents is shown to be a multi-length scale morphol. comprised of fibrils of pDPP that form during the early stages of solvent evapn. These fibrils are seen to form a network within which a further deposition of the pDPP and PC71BM occurs and where an ordering of the pDPP occurs, segregating the PC71BM into a two-phase system with a characteristic length scale smaller than the mesh size of the cryst. network. Upon thermal annealing, the overall spatial characteristics of the morphol. do not change, but the pDPP in the interfibrillar regions crystallizes further. While one would expect that an increase in the ordering of the pDPP would enhance the performance of the device, the opposite is seen. This might come from the anode interface inter-mixing and PC71BM depletion at cathode interface. - 53.Zhang
, K.; Guan, X.; Huang, F.; Cao, Y. Acta Chim. Sin. 2012, 70, 2489There is no corresponding record for this reference. - 54.Liu
, S. J.; Zhang, Z. P.; Chen, D. C.; Duan, C. H.; Lu, J. M.; Zhang, J.; Huang, F.; Su, S. J.; Chen, J. W.; Cao, Y. Sci. China: Chem. 2013, 43, 437There is no corresponding record for this reference. - 55.Hsieh
, C. H.; Cheng, Y. J.; Li, P. J.; Chen, C. H.; Dubosc, M.; Liang, R. M.; Hsu, C. S. J. Am. Chem. Soc. 2010, 132, 4887[ACS Full Text], [PubMed], [CAS]55.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXjtVyms7o%253D&md5=8f92888d3364c08ae402174df6e0aec0Hsieh, Chao-Hsiang; Cheng, Yen-Ju; Li, Pei-Jung; Chen, Chiu-Hsiang; Dubosc, Martin; Liang, Ru-Meng; Hsu, Chain-ShuJournal of the American Chemical Society (2010),A novel PCBM-based n-type material, [6,6]-phenyl-C61-butyric styryl dendron ester (PCBSD), functionalized with a dendron contg. two styryl groups as thermal cross-linkers, has been rationally designed and easily synthesized. In situ crosslinking of PCBSD was carried out by heating at a low temp. of 160 °C for 30 min to generate a robust, adhesive, and solvent-resistant thin film. This cross-linked network enables a sequential active layer to be successfully deposited on top of this interlayer to overcome the problem of interfacial erosion and realize a multilayer inverted device by all-soln. processing. An inverted solar cell device based on an ITO/ZnO/C-PCBSD/P3HT:PCBM/PEDOT:PSS/Ag configuration not only achieves enhanced device characteristics, with an impressive PCE of 4.4%, but also exhibits an exceptional device lifetime without encapsulation; it greatly outperforms a ref. device (PCE = 3.5%) based on an ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag configuration without the interlayer. This C-PCBSD interlayer exerts multiple pos. effects on both P3HT/C-PCBSD and PCBM/C-PCBSD localized heterojunctions at the interface of the active layer, including improved exciton dissocn. efficiency, reduced charge recombination, decreased interface contact resistance, and induction of vertical phase sepn. to reduce the bulk resistance of the active layer as well as passivation of the local shunts at the ZnO interface. Moreover, this promising approach can be applied to another inverted solar cell, ITO/ZnO/C-PCBSD/PCPDTBT:PC71BM/PEDOT:PSS/Ag, using PCPDTBT as the p-type low-band-gap conjugated polymer to achieve an improved PCE of 3.4%. Incorporation of this cross-linked C60 interlayer could become a std. procedure in the fabrication of highly efficient and stable multilayer inverted solar cells. - 56.Liu
, S. J.; Zhong, C. M.; Zhang, J.; Duan, C. H.; Hu, X. W.; Huang, F. Sci. China: Chem. 2011, 54, 1745[CrossRef], [CAS]56.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsVamtbrM&md5=1c08a222d1029a4f625df3f0ca99e17dLiu, Sheng Jian; Zhong, Cheng Mei; Zhang, Jie; Duan, Chun Hui; Wang, Xiao Hui; Huang, FeiScience China: Chemistry (2011),A novel crosslinkable water/alc. sol. conjugated polymer PFN-C contg. oxetane groups and aminoalkyl groups in the side chains has been developed and used as highly efficient electron injection and transporting material for polymer light-emitting diodes (PLEDs). The unique soly. in polar solvents and crosslinkable ability of PFN-C render it a good candidate for soln. processed multilayer PLEDs. It was found that PFN-C can greatly enhance the electron injection from high work-function metal cathode, due to its pendant amino groups. As a result, PLEDs with PFN-C/Al cathode exhibited comparable device performance to the devices with Ba/Al cathode. The resulting green light-emitting device showed promising performance with a max. luminance efficiency of 13.53 cd A-1. - 57.Zhong
, C. M.; Liu, S. J.; Huang, F.; Wu, H. B.; Cao, Y. Chem. Mater. 2011, 23, 4870[ACS Full Text], [CAS]57.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtlalur3J&md5=bf1816f0cd8c82b13c0b5e884b7026aaZhong, Cheng-Mei; Liu, Sheng-Jian; Huang, Fei; Wu, Hong-Bin; Cao, YongChemistry of Materials (2011),Soln. processable and cross-linkable amino-functionalized polyfluorene materials can effectively enhance the electron injection from the In Sn oxide (ITO) electrodes, and highly efficient inverted org. LEDs are fabricated using cross-linkable conjugated polymers as a single electron injection layer without the need for n-type metal oxide layers. The mechanism for the electron injection enhancement effect of the ITO/cross-linkable conjugated polymer interface is studied by XPS, Kelvin probe, and photovoltaic measurement. The amino groups among these polymers can effectively lower the work function of ITO, which will greatly enhance the electron injection in the resulting devices. - 58.Dong
, Y.; Hu, X. W.; Duan, C. H.; Liu, P.; Liu, S. J.; Lan, L. Y.; Chen, D. C.; Ying, L.; Su, S. J.; Gong, X.; Huang, F.; Cao, Y. Adv. Mater. 2013, 25, 3683[CrossRef], [CAS]58.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXosVelu7g%253D&md5=6e99b249937f9cb3089abec2cbdfe980Dong, Yang; Hu, Xiaowen; Duan, Chunhui; Liu, Peng; Liu, Shengjian; Lan, Liuyuan; Chen, Dongcheng; Ying, Lei; Su, Shijian; Gong, Xiong; Huang, Fei; Cao, YongAdvanced Materials (Weinheim, Germany) (2013),A novel electron-deficient TZNT moiety has been developed. A series of donor-acceptor type of conjugated polymers based on this novel acceptor exhibited a medium band gap of ca. 1.9 eV along with low-lying HOMO level of ca. -5.2 eV. - 59.Brabec
, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 374[CrossRef], [CAS]59.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXnsVCgtr4%253D&md5=dfc80f96f933d74b49bdca69fe371f08Brabec, Christoph J.; Cravino, Antonio; Meissner, Dieter; Sariciftci, N. Serdar; Fromherz, Thomas; Rispens, Minze T.; Sanchez, Luis; Hummelen, Jan C.Advanced Functional Materials (2001),A series of highly sol. fullerene derivs. with varying acceptor strengths (i.e., first redn. potentials) was synthesized and used as electron acceptors in plastic solar cells. These fullerene derivs., methanofullerene [6,6]-Ph C61-butyric acid Me ester, a new azafulleroid, and a ketolactam quasifullerene, show a variation of almost 200 mV in their first redn. potential. The open circuit voltage of the corresponding devices correlated directly with the acceptor strength of the fullerenes, whereas it was rather insensitive to variations of the work function of the neg. electrode. These observations are discussed within the concept of Fermi level pinning between fullerenes and metals via surface charges. - 60.Gadisa
, A.; Svensson, M.; Andersson, M. R.; Inganäs, O. Appl. Phys. Lett. 2004, 84, 1609[CrossRef], [CAS]60.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXhslalu7o%253D&md5=d988f45f2f3cee94d6eb621bc6e63f0eGadisa, Abay; Svensson, Mattias; Andersson, Mats R.; Inganas, OlleApplied Physics Letters (2004),The photovoltaic parameters of donor/acceptor blend org. solar cells are highly influenced by several parameters, such as the strength of the acceptor species, the morphol. of the film due to the solvent, and the mobility of the free charge carriers. The open-circuit voltage (Voc) of solar cells based on series of conjugated polythiophene polymers were measured and compared. In every cell, the donor polymer was blended with an electron acceptor fullerene mol. The devices were constructed in a sandwich structure with indium tin oxide (ITO)/metallic polymer (PEDOT:PSS) acting as an anode and Al or LiF/Al acting as a cathode. Comparing the Voc of all the cells shows that this important photovoltaic parameter is systematically varying with the polymer. The variation of photovoltage is attributed to the variation of the oxidn. potential of the donor conjugated polymers after due consideration of the different injection conditions in the varying polymers. - 61.Mannsfeld
, S. C. B.; Virkar, A.; Reese, C.; Toney, M. F.; Bao, Z. Adv. Mater. 2009, 21, 2294[CrossRef], [CAS]61.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXnt12nt7w%253D&md5=eca12391cd0c331e2afa481e6e6c536cMannsfeld, Stefan C. B.; Virkar, Ajay; Reese, Colin; Toney, Michael F.; Bao, ZhenanAdvanced Materials (Weinheim, Germany) (2009),Grazing incidence x-ray diffraction in combination with crystallog. refinement calcns. yielded the mol. packing in pentacene monolayers on silica. The pentacene mol. adopt a completely tilt-free herringbone motif, in contrast to the commonly obsd. for thin films or bulk crystals. The theor. charge transport properties of the monolayer and bulk packing motifs are discussed on the basis of d. functional calcns., and the results offer an explanation for the high performance of pentacene thin film transistors relative to that of pentacene single crystals on the same substrate. - 62.Collins
, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H. Adv. Energy Mater. 2013, 3, 65[CrossRef], [CAS]62.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsFKhsbo%253D&md5=65e8041abbbc5d385c778266605b8c30Collins, Brian A.; Li, Zhe; Tumbleston, John R.; Gann, Eliot; McNeill, Christopher R.; Ade, HaraldAdvanced Energy Materials (2013),The importance of morphol. to org. solar cell performance is well known, but to date, the lack of quant., nanoscale and statistical morphol. information has hindered obtaining direct links to device function. Here resonant X-ray scattering and microscopy are combined to quant. measure the nanoscale domain size, distribution and compn. in high efficiency solar cells based on PTB7 and PC71BM. The results show that the solvent additive diiodooctane dramatically shrinks the domain size of pure fullerene agglomerates that are embedded in a polymer-rich 70/30 wt.% molecularly mixed matrix, while preserving the domain compn. relative to additive-free devices. The fundamental miscibility between the species - measured to be equal to the device's matrix compn. - is likely the dominant factor behind the overall morphol. with the additive affecting the dispersion of excess fullerene. As even the mol. ordering measured by X-ray diffraction is unchanged between the two processing routes the change in the distribution of domain size and therefore increased domain interface is primarily responsible for the dramatic increase in device performance. While fullerene exciton harvesting is clearly one significant cause of the increase owing to smaller domains, a measured increase in harvesting from the polymer species indicates that the mol. mixing is not the reason for the high efficiency in this system. Rather, excitations in the polymer likely require proximity to a pure fullerene phase for efficient charge sepn. and transport. Furthermore, in contrast to previous measurements on a PTB7-based system, a hierarchical morphol. was not obsd., indicating that it is not necessary for high performance. - 63.Chu
, T. Y.; Lu, J. P.; Beaupre, S.; Zhang, Y. G.; Pouliot, J. R.; Zhou, J. Y.; Najari, A.; Leclerc, M.; Tao, Y. Adv. Funct. Mater. 2012, 22, 2345[CrossRef], [CAS]63.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xjt1Okt7g%253D&md5=eb0649b3636bcb8083eec399c8f8efc0Chu, Ta-Ya; Lu, Jianping; Beaupre, Serge; Zhang, Yanguang; Pouliot, Jean-Remi; Zhou, Jiayun; Najari, Ahmed; Leclerc, Mario; Tao, YeAdvanced Functional Materials (2012),A series of low-bandgap alternating copolymers of dithienosilole and thienopyrrolodione (PDTSTPDs) are prepd. to investigate the effects of the polymer mol. wt. and the alkyl chain length of the thienopyrrole-4,6-dione (TPD) unit on the photovoltaic performance. High-mol.-wt. PDTSTPD leads to a higher hole mobility, lower device series resistance, a larger fill factor, and a higher photocurrent in PDTSTPD:[6,6]-Ph C71 butyric acid Me ester (PC71BM) bulk-heterojunction solar cells. Different side-chain lengths show a significant impact on the interchain packing between polymers and affect the blend film morphol. due to different solubilities. A high power conversion efficiency of 7.5% is achieved for a solar cell with a 1.0 cm2 active area, along with a max. external quantum efficiency (EQE) of 63% in the red region. - 64.Liao
, H. C.; Ho, C. C.; Chang, C. Y.; Jao, M. H.; Darling, S. B.; Su, W. F. Mater. Today 2013, 16, 326[CrossRef], [CAS]64.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhs1ynsb7O&md5=a3d1d2cca9d591e7396fa93623ef138dLiao, Hsueh-Chung; Ho, Chun-Chih; Chang, Chun-Yu; Jao, Meng-Huan; Darling, Seth B.; Su, Wei-FangMaterials Today (Oxford, United Kingdom) (2013),Bulk heterojunction (BHJ) photovoltaics represent one of the most promising technologies in low-cost, high-throughput, environmentally friendly energy conversion. Morphol. control is one pillar of the recent remarkable progress in power conversion efficiency. This review focuses on morphol. control by processing with solvent additives, which has been extensively adopted and exhibits promising compatibility with large-scale processing. Recent investigations including material selection, morphol. variations at various length scales, and interpretations of the interaction among additives and BHJ materials will be discussed. Insights into the role of solvent additives represent an important resource for further improvement in materials and processing designs. - 65.Lee
, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619[ACS Full Text], [PubMed], [CAS]65.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXitFehs7Y%253D&md5=d308439e7a9e6f936987d8979c8742ecLee, Jae Kwan; Ma, Wan Li; Brabec, Christoph J.; Yuen, Jonathan; Moon, Ji Sun; Kim, Jin Young; Lee, Kwanghee; Bazan, Guillermo C.; Heeger, Alan J.Journal of the American Chemical Society (2008),Two criteria for processing additives introduced to control the morphol. of bulk heterojunction (BHJ) materials for use in solar cells were identified: (i) selective (differential) soly. of the fullerene component and (ii) higher b.p. than the host solvent. Using these criteria, the authors have studied the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphol. by selective soly. of the fullerene component is demonstrated using these high b.p. processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the ref. device) to 5.1%. - 66.Lou
, S. J.; Szarko, J. M.; Xu, T.; Yu, L. P.; Marks, T. J.; Chen, L. X. J. Am. Chem. Soc. 2011, 133, 20661[ACS Full Text], [PubMed], [CAS]66.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsFCms73L&md5=6cb90d58524bad1a4250bc326a361640Lou, Sylvia J.; Szarko, Jodi M.; Xu, Tao; Yu, Luping; Marks, Tobin J.; Chen, Lin X.Journal of the American Chemical Society (2011),Processing additives are used in org. photovoltaic systems to optimize the active layer film morphol. However, the actual mechanism is not well understood. Using X-ray scattering techniques, the effects are analyzed of an additive diiodooctane (DIO) on the aggregation of a high-efficiency donor polymer PTB7 and an acceptor mol. PC71BM under solar cell processing conditions. It is concluded that DIO selectively dissolves PC71BM aggregates, allowing their intercalation into PTB7 domains, thereby optimizing both the domain size and the PTB7-PC71BM interface. - 67.Gann
, E.; Young, A. T.; Collins, B. A.; Yan, H.; Nasiatka, J.; Padmore, H. A.; Ade, H.; Hexemer, A.; Wang, C. Rev. Sci. Instrum. 2012, 83, 045110[CrossRef], [CAS]67.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XlsVWksL4%253D&md5=d34df052a1d04145f21d0606709c87beGann, E.; Young, A. T.; Collins, B. A.; Yan, H.; Nasiatka, J.; Padmore, H. A.; Ade, H.; Hexemer, A.; Wang, C.Review of Scientific Instruments (2012),The authors present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the C K-edge (∼285 eV). The authors describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface were implemented to allow real-time data processing and preliminary data anal. simultaneous with data acquisition. (c) 2012 American Institute of Physics.
