Synthesis and Use for Efficient Bulk Heterojunction ... - ACS Publications

Jan 6, 2010 - Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece, Physics Department, Molecular El...
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Novel Low Band Gap Phenylenevinylene Copolymer with BF2-Azopyrrole Complex Units: Synthesis and Use for Efficient Bulk Heterojunction Solar Cells John A. Mikroyannidis,*,† G. D. Sharma,*,‡,§ S. S. Sharma,| and Y. K. Vijay| Chemical Technology Laboratory, Department of Chemistry, UniVersity of Patras, GR-26500 Patras, Greece, Physics Department, Molecular Electronic and Optoelectronic DeVice Laboratory, JNV UniVersity, Jodhpur (Raj.) 342005, India, Jaipur Engineering College, Kukas, Jaipur (Raj.), India, and Thin Film and Membrane Science Laboratory, Department of Physics, Rajsthan UniVersity, Jaipur, India ReceiVed: NoVember 2, 2009; ReVised Manuscript ReceiVed: December 17, 2009

An alternating phenylenevinylene copolymer (P) was synthesized by Heck coupling of 2,5-bis[2-(4bromophenyl)diazeny]-1H-pyrrole with 1,4-divinyl-2,5-bis(hexyloxy)-benzene. The subsequent reaction of P with boron trifluoride diethyl etherate afforded the corresponding BF2-azopyrrole complex (PB) which was used for bulk heterojunction solar cells. A thin film of PB showed a broad absorption band with a longwave absorption maximum at 511 nm and an optical band gap of 1.63 eV. We have used a solvent mixture consisting of THF with various contents of acetone, in order to prepare the PB:PCBM blend films for polymer bulk heterojunction (BHJ) photovoltaic (PV) devices. Since the vapor pressure of the solvent mixture is lower compared to neat THF, the blend films dried slowly and nanoparticles of PB are formed, as indicated by the XRD pattern. The correlation of PB nanoparticles with PV properties of the PB:PCBM BHJ devices was investigated. It was found that the optical absorption and hole transport in the resulting PB:PCBM blend films increase with increasing content of acetone in the THF solution of PB, which is responsible for the enhancement in the power conversion efficiency (PCE). The effect of thermal annealing on the PV response was also investigated. The overall PCE for the BHJ PV devices was 3.54%. This was achieved for the thermally annealed device which was fabricated with a mixture of 2.5% acetone in the THF solution of PB. Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been emerging as one of the attractive renewable energy sources for the possibility of chemically manipulating the material properties of the polymers combined with easy and cheap processing techniques.1 In the past few years, continuous development of new polymers together with optimizing the morphology of the active layer and device structure has led to dramatic improvement of the device performance. Single-layer and tandem PSCs with power conversion efficiencies (PCEs) beyond 5%2 and up to 6.5%,3 respectively, have been reported. Moreover, the lifetime of the devices has been significantly improved, and an appropriate industrial processing technique has also been developed.4 In typical bulk heterojunction PSCs, conjugated polymers and fullerene derivatives (e.g, 1-(3methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PCBM)) function as the light-absorbing/electron-donating and electron-accepting materials, respectively.5 Therefore, the energy gap and film absorption coefficient of conjugated polymers are two of the most important parameters to be optimized for efficient harvesting of solar energy, and hence realizing high performance PSCs. The solar spectrum covers a very broad wavelength from ultraviolet to near-infrared (NIR) regions. Theoretically, the ideal band gap of conjugated polymers for PSCs should be around * Corresponding authors. Phone: +30 2610 997115 (J.A.M.); 91-02912720857 (G.D.S.). Fax: +30 2610 997118 (J.A.M.); 91-0291-2720856 (G.D.S.). E-mail: [email protected] (J.A.M.); sharmagd_in@ yahoo.com (G.D.S.). † University of Patras. ‡ JNV University. § Jaipur Engineering College. | Rajsthan University.

1.5 eV.6 However, most conjugated polymers reported so far, e.g., polythiophenes and poly(phenylene vinylene)s (PPVs),7,8 exhibit optical band gaps larger than or around 2.0 eV, and therefore can only harvest visible light. This mismatch of the absorption to the solar spectrum significantly limits the device performance of PSCs. Consequently, development of NIRabsorbing or low band gap polymers is very important. Constructing polymers consisting of alternating electron-rich and electron-deficient aromatic units is the most successful approach to lower the optical band gap of the conjugated polymers.9 Various electron-deficient units, e.g., electron-deficient heterocycles,10,11 perylene diimides,12 and cyanovinylenes,13 and several electronrich units, e.g., thiophene, pyrrole, fluorene, and carbazole, have been used to construct low band gap polymers. However, of those with NIR absorption,10d,e,i,14 only a few exhibit PCE beyond 1%.10e,14e,f A literature survey revealed that various pyrrole derivatives have been used for BHJ PV cells very recently.15-21 Azo dyes are a class of compounds containing a NdN double bond and, due to their ability to absorb visible light, they have been extensively used. Among the known azo dyes, fivemembered heterocyclic azo dyes, such as azothiazole, azothiophene, azopyrrole, and azofuran, are important, since they have pronounced bathochromic absorptions compared to azobenzene dyes.22 NIR-absorbing azo dyes, based on five-membered heterocyclic rings, have been made effectively by extending the conjugated NdN bond from monoazo to bisazo dyes to enhance molecular π-resonance effects.23 Various azo dyes have been used as sensitizers for dye-sensitized solar cells (DSSCs).24 Very recently, a series of symmetrical 2,5-bisazopyrroles were synthesized by a one-step reaction of substituted phenyl diazonium salts with pyrrole under basic conditions.25 The reaction of 2,5-bis(4-dimethylaminophenylazo)-pyrrole with

10.1021/jp910467c  2010 American Chemical Society Published on Web 01/06/2010

Novel Low Band Gap Phenylenevinylene Copolymer boron trifluoride provided a BF2-azopyrrole complex with NIR absorption spectrum.25 On the basis of this investigation, herein we prepared a novel alternating phenylenevinylene copolymer (P) with 2,5-bisazopyrrole segments along the main chain. Copolymer P reacted subsequently with BF3Et2O to afford the corresponding BF2-azopyrrole complex (PB) which was used for BHJ PSCs. Copolymer PB has the donor-acceptor (D-A) structure with the dihexyloxyphenylenevinylene as the D unit and the BF2-azopyrrole complex as the A unit. The hexyloxy side chains of PB enhanced the solubility of the copolymer. PB is expected to afford efficient PSCs due to its relatively low band gap. The BF2 moieties are stable in nonprotic solvents but unstable in protic solvents. This feature may be a serious limitation concerning their use for solar cells, since they are humidity sensitive. This can be overcome by using the nanoparticles of PB in the blend. We introduce PB nanoparticles in the PB:PCBM blend films via a solvent mixture, aiming to improve the crystallinity of PB and hole transport. The techniques of absorption spectroscopy, X-ray diffraction (XRD) pattern, optical microstructure image, and hole only devices are used to investigate the correlation between the morphology and PV properties of the resulting PV devices. Experimental Section Characterization Methods. IR spectra were recorded on a Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were obtained using a Brucker spectrometer. Chemical shifts (δ values) are given in parts per million with tetramethylsilane as an internal standard. UV-vis spectra were recorded on a Beckman DU-640 spectrometer with spectrograde THF. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer. Gel permeation chromatography (GPC) analysis was conducted with a Waters Breeze 1515 apparatus equipped with a 2410 differential refractometer as a detector (Waters Associate) and Styragel HR columns with polystyrene as a standard and tetrahydrofuran (THF) as an eluent. The crystallinity of the blends was studied using the X-ray diffraction (XRD) technique. Thin film XRD spectra were recorded using a panalytical (XRD) system (USA) having Cu KR acting as the radiation source of wavelength λ ) 1.5405 Å. The optical topographical images of the films were obtained from an optical micrograph system (Labomed optical microscope of 1 µm resolution) and were done under ambient conditions. Reagents and Solvents. 1,4-Divinyl-2,5-bis(hexyloxy)benzene (3) was prepared by Stille coupling reaction26 of 1,4dibromo-2,5-bis(hexyloxy)-benzene with tributylvinyltin.27 N,Ndimethylformamide (DMF) and tetrahydrofuran (THF) were dried by distillation over CaH2. Triethylamine was purified by distillation over KOH. All other reagents and solvents were commercially purchased and were used as supplied. Synthesis. 2,5-Bis[2-(4-bromophenyl)diazeny]-1H-pyrrole (2). A flask was charged with a suspension of 4-bromoaniline (1.37 g, 7.96 mmol) in water (10 mL). Hydrochloric acid (2 mL) was added to the suspension until the mixture was homogeneous. The solution was cooled and kept at 0-5 °C in an ice bath and diazotized by addition of a solution of NaNO2 (0.56 g, 8.11 mmol) in water (5 mL) followed by stirring for 0.5 h at 0-5 °C. The solution of 4-bromophynyl diazonium salt (1) thus prepared was immediately used for the next coupling reaction.

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1521 To a solution of pyrrole (0.27 g, 4.02 mmol) and pyridine (2 mL) in methanol (100 mL) was slowly added the solution of 4-bromophynyl diazonium salt (1) at 0-5 °C. The resulting mixture was stirred for 10 h and then concentrated under reduced pressure. The precipitate was filtered, washed with water, and dried to afford 2. It was purified by column chromatography, eluting with a mixture of dichloromethane and hexane (1:1). Yield 80% (1.39 g). FT-IR (KBr, cm-1): 3442, 1654, 1578, 1484, 1400, 1362, 1246, 1068, 1004, 828, 778, 500. 1 H NMR (CDCl3) ppm: 9.80 (s, 1H, NH of pyrrole); 7.80 (m, 4H, aromatic ortho to azo); 7.40 (m, 4H, aromatic ortho to bromine); 6.88 (s, 2H, arromatic of pyrrole). Anal. Calcd for C16H11Br2N5: C, 44.37; H, 2.56; N, 16.17. Found: C, 43.86; H, 2.43; N, 16.02. Copolymer P. A flask was charged with a mixture of 2 (0.3108 g, 0.719 mmol), 3 (0.2377 g, 0.719 mmol), Pd(OAc)2 (0.0067 g, 0.030 mmol), P(o-tolyl)3 (0.0503 g, 0.165 mmol), DMF (5 mL), and triethylamine (3 mL). The flask was degassed and purged with N2. The mixture was heated at 90 °C for 24 h under N2. Then, it was filtered and the filtrate was poured into methanol. The precipitate was filtered and washed with methanol. The crude product was purified by dissolving in THF and precipitating into methanol (0.3154 g, yield 73%). FT-IR (KBr, cm-1): 3442, 2952, 2928, 2856, 1584, 1486, 1468, 1376, 1256, 1204, 1066, 1032, 1006, 828. 1 H NMR (CDCl3) ppm: 9.82 (s, 1H, NH); 7.81 (m, 4H, aromatic ortho to azo); 7.39 (m, 4H, aromatic meta to azo); 7.10 (m, 4H, olefinic); 6.93 (m, 2H, aromatic ortho to oxygen); 6.87 (s, 2H, aromatic of pyrrole); 4.02 (m, 4H, OCH2(CH2)4CH3); 1.79 (m, 4H, OCH2CH2(CH2)3CH3); 1.34 (m, 12H, O(CH2)2(CH2)3CH3); 0.91 (t, J ) 5.4 Hz, 6H, O(CH2)5CH3). Anal. Calcd for (C38H43N5O2)n: C, 75.84; H, 7.20; N, 11.64. Found: C, 75.06; H, 7.34; N, 11.52. Copolymer PB. To a solution of P (0.40 g) and triethylamine (2 mL) in dry chloroform (20 mL) was slowly added boron trifluoride diethyl etherate (1 mL) under N2. The resulting solution was refluxed for 2 h and then evaporated to dryness. The residue was triturated with ether, filtered, and dried to afford PB as a dark purple product (0.35 g, 81%). FT-IR (KBr, cm-1): 3194, 2928, 2856, 1586, 1484, 1194, 1066, 1028, 824. 1 H NMR (CDCl3) ppm: 7.89 (m, 4H, aromatic ortho to azo); 7.38 (m, 4H, aromatic meta to azo); 7.09 (m, 4H, olefinic); 6.93 (m, 2H, aromatic ortho to oxygen); 6.90 (s, 2H, aromatic of pyrrole); 4.01 (m, 4H, OCH2(CH2)4CH3); 1.80 (m, 4H, OCH2CH2(CH2)3CH3); 1.35 (m, 12H, O(CH2)2(CH2)3CH3); 0.93 (t, J ) 5.4 Hz, 6H, O(CH2)5CH3). Anal. Calcd for (C38H42BF2N5O2)n: C, 70.26; H, 6.51; N, 10.78. Found: C, 69.58; H, 6.65; N, 10.49. DeWice Fabrication and Characterization. First, the copolymer PB was well dissolved in THF solution with a concentration of 10 mg/mL. Then, acetone in various contents (2.5, 5.0, and 7.5%) was added into the copolymer solution which was stirred at room temperature for 2 h. Finally, this copolymer solution was mixed with the PCBM solution in THF (10 mg/mL), and the resulting mixture was used to prepare the copolymer PB: PCBM blend film. The devices were fabricated on the top of the indium tin oxide (ITO) coated glass substrates. After cleaning the ITO substrate, the PEDOT:PSS layer was spin coated with a thickness of 50-60 nm and baked at 80 °C for 1 h in an oven. The 80-90 nm thick photoactive layer was spin coated from the above prepared blend solutions on the top

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SCHEME 1: Synthesis of Copolymers P and PB

of the PEDOT:PSS layer for 1 min. The thickness of the active layer was adjusted by tuning the spin coating rate. After the drying process, the polymer films were thermally annealed at 120 °C for 3 min on a hot plate. Finally, an Al electrode was deposited by the vacuum thermal evaporation method on the top of the photoactive layer to form the ITO/PEDOT:PSS/PB: PCBM(1:1)/Al device structure. The current-voltage (J-V) characteristics of the PV devices were determined using a computer controlled Keithley source meter under an illumination intensity of 100 mW/cm2. The optical absorption spectra of the PB:PCBM blended films were measured using a Perkin-Elmer UV-visible spectrophotometer on quartz substrates. Results and Discussion Synthesis and Characterization. Scheme 1 outlines the synthesis of copolymers P and PB. Specifically, following the general procedure to prepare azo dyes, 4-bromophenyl diazonium salt (1) salt was formed by reacting 4-bromoaniline with sodium nitrite/aqueous HCl at temperatures of 0-5 °C. Neutralization with pyridine and treatment with a half equivalent of pyrrole in methanol gave the symmetrical bisazopyrrole 2. The latter was reacted with an equimolar amount of 3 in DMF, utilizing triethylamin as an acid scavenger and Pd(OAc)2 as a catalyst to afford by Heck polymerization28 the alternating phenylenevinylene copolymer P. When P was mixed with boron trifluoride diethyl ether and triethylamine and refluxed in chloroform, the corresponding BF2-azopyrrole complex PB was formed.25 The complex PB was found to be stable and soluble in nonprotic solvents such as acetone, THF, acetonitrile, dichloromethane, etc., but unstable in protic solvents such as methanol. This behavior of PB conforms to literature data concerning the boron-azopyrrole complex.25 Since the complexation reaction of P took place under mild conditions, no cleavage of the copolymer backbone was observed. Evidence for it was obtained from the average molecular weights (Mn) of P and PB, by GPC, which were comparable. In particular, the Mn was 8800 and 8200 with a polydispersity of 2.3 and 2.1 for P and PB, respectively. It has been well established that the Heck polymerization affords polymers with relatively low molecular weights. When P reacted with BF3Et2O to afford PB, the color of the reaction solution changed considerably from red to dark purple. Figure 1 depicts the FT-IR spectra of P and PB. Copolymer P showed characteristic absorption bands at 2952, 2928, and 2856 cm-1 (C-H stretching of hexyloxy chains); 1584, 1486, and 1376 cm-1 (aromatic); and 1256, 1204 (ether bond), and 960 cm-1 (trans olefinic bond). Moreover, P displayed a weak absorption at 3442 cm-1 assigned to the NH stretching of the pyrrole ring. The spectrum of PB as compared to P was broader and showed new strong absorptions at 3194, 1066, and 1028 cm-1 associated with the BF2-azopyrrole complex. In addition,

PB lacked the NH absorption at 3442 cm-1 due to complexation of the pyrrole nitrogen. These features supported the complexation which shifted bathochomically the absorption spectrum of the copolymer.25 This presumably results from significant π-electron delocalization in the planar structure dominated by complexation of the electron-withdrawing BF2 group.25 Compound 2 as well as copolymers P and PB were characterized by FT-IR and 1H NMR spectroscopy (see the Experimental Section). Figure 2 shows the 1H NMR spectrum of PB. Photophysical and Electrochemical Properties. The photophysical properties of PB were investigated by absorption both in dilute (10-5 M) THF solution and thin film (Figure 3). All of the photophysical characteristics of PB are summarized in Table 1. The absorption spectrum of PB exhibited a maximum (λa,max) at 434 nm in solution and 511 nm in thin film which corresponds to the π-π* transition along the copolymer backbone. A considerable red-shift by 77 nm of λa,max was observed upon going from solution to the thin film owing to the aggregate formation. The complex has been found to be most planar due to a rigid trans-azo configuration25 which favors the aggregation especially in thin film where the intrermolecular

Figure 1. FT-IR spectra of copolymers P (top) and PB (bottom).

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Figure 2. 1H NMR spectrum of copolymer PB in CDCl3 solution. The solvent peak is denoted by an asterisk.

Figure 4. Normalized UV-visible spectra of the PB:PCBM blend films prepared from THF solution containing different percentages of acetone.

Figure 3. Normalized UV-vis absorption spectra of copolymer PB in THF solution and thin film.

TABLE 1: Optical and Electrochemical Properties of PB λa,maxa in solution (nm) λa,maxa in thin film (nm) Egopt b (eV) HOMO (eV) LUMO (eV) Egele c(eV)

434 511 1.63 -5.1 -3.45 1.65

a λa,max: the absorption maxima from the UV-vis spectra in THF solution or in thin film. b Egopt: optical band gap determined from the absorption onset in thin film. c Egele: electrochemical band gap determined from cyclic voltammetry.

interactions are strong. The thin film absorption spectrum of PB was broad with an onset at 763 nm corresponding to an optical band gap (Egopt) of 1.63 eV. This relatively low Egopt value indicates that PB is an attractive copolymer for PV applications. The broad infrared absorption of PB is dominated by the π-resonance effects which were achieved by extending the conjugation around the NdN bond and forming a rigid azo configuration.25 Furthermore, the two electron-donating hexyloxy groups red-shifted the absorption spectrum of PB. Here, we prepared copolymer PB solution containing PB nanoparticles by adding a small volume of acetone into the THF solution. The thickness of the PB:PCBM blend layer spin coated at a fixed rate increases with the increase of acetone content. The spin coating rate for different solutions is adjusted to fabricate the PB:PCBM blend film with almost the same thickness. The optical absorption spectra of PB:PCBM blend film from different solutions are shown in Figure 4. For PB:

PCBM blend film fabricated from neat THF solution, the PB absorption peak is located at 511 nm with a shoulder around 610-615 nm and the absorption around 370 nm assigned to PCBM. With the addition of acetone, the absorption peak which corresponds to PB shifts to the red and one vibronic absorption shoulder is observed due to the enhanced conjugation length and more ordered structure of PB, as has been observed in P3HT conjugated polymer.29,30 The absorption of PB in the red region increases with increasing content of acetone, indicating that more copolymer PB nanoparticles are formed. Cyclic voltammetry is employed to calculate the oxidation and reduction potential, the highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) energy levels of copolymer PB. It shows reversible oxidation (p-doping/rereduction) and irreversible reduction (n-doping/ reoxidation) waves. Potentials have been measured with respect to a Ag/Ag+ electrode. HOMO and LUMO levels were calculated from the following equation EHOMO ) -e(Eox + 4.7) and ELUMO ) -e(Ered + 4.7). Electro-chemical band gaps have been calculated from Egele ) EHOMO - ELUMO. PB has an onset reduction potential and oxidation potential at -1.26 and 0.4 V vs Ag/Ag+. The calculated values of HOMO and LUMO energy levels of PB are shown in Table 1. The band gap estimated from cyclic voltammetry (Egele) is in good agreement with the optical band gap (Egopt) estimated from the optical absorption data. The values of the LUMO and HOMO of PCBM were also estimated from the cyclic voltammetry and were found to be approximately -3.9 and -6.3 eV, respectively. The difference in the LUMO level of PB and PCBM is about 0.45 eV, which indicates that the combination of copolymer PB as a donor with PCBM as an acceptor can be used for efficient BHJ polymer PV devices. Structural Characterization. Figure 5 shows the XRD patterns of PB:PCBM blend films prepared from different solutions. The P:PCBM blend film prepared from neat THF solution is almost amorphous. When the crystalline PB nanoparticles are introduced with the addition of acetone, the PB: PCBM blend films shows three different peaks from (h00) (h ) 1-3) reflections corresponding to the interchain spacing in PB associated with the hexyloxy chains. No significant π-stacking was observed, since the PB nanoparticles are parallel to substrate. With the addition of acetone, the intensity of the peak located at 2θ ) 5.8°, which corresponds to (100), increases by

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Figure 5. XRD pattern of PB:PCBM blend films prepared from the THF solution containing different percentages of acetone.

Figure 7. Current-voltage curves for the hole only devices based on PB:PCBM blends, prepared from THF solution containing different percentages of acetone.

Figure 6. Optical images of the PB:PCBM thin film blends prepared from different solutions (40 µm × µm scale).

increasing the acetone content in the solution. This indicates that higher crystalline PB nanoparticles are introduced by adding acetone into the solution. The optical microstructure images of the as cast and thermally annealed PB:PCBM blend films prepared from different solutions are shown in Figure 6. It can be seen from these optical images that the surfaces of the films prepared from the solution containing acetone were rougher than that prepared from neat THF. This indicates that the rough surface is probably attributed to the polymer organization, as already has been reported for the P3HT:PCBM blend.2c,31 Since we do not have a TEM/AFM facility, we are not able to estimate the exact dimension of the nanoparticles. The rough surface blend films (Figure 6) made from the acetone containing THF solutions suggests also a higher degree of ordering. We assume that this ordered structure reduces the series resistance of the device, thus increasing the photocurrent as described in the last part of the discussion. The rough surface of blend films made from the acetone containing THF solutions suggest also a higher degree of ordering. This ordered structure reduces the series resistance of the device, thus increasing the photocurrent as described in the last part of the discussion. The thermally annealed blend film further shows more rough surfaces, as shown in Figure 6. This indicates that

the thermal annealing of the blend films further increases the degree of ordering, resulting in an additional increase in photocurrent. Photovoltaic Properties. The electron and hole mobilities in the blend film can be determined from the J-V measurements using electrodes which suppress either the injection of electrons or holes resulting electron and hole only devices, respectively. To investigate the effect of copolymer PB nanoparticles in the PB:PCBM blend film on hole mobility, we have fabricated the hole only ITO/PEDOT:PSS/PB:PCBM/Au devices. The work function of PEDOT:PSS matches the HOMO of PB, forming an Ohmic contact for hole injection, where the Au strongly suppresses the electron injection into the LUMO of PCBM, owing to the large mismatch between its work function and the LUMO of PCBM. Figure 7 shows the J-V characteristics of the hole only devices, with PB:PCBM blend films prepared from different solutions. The hole mobilities of PB:PCBM blends were extracted from the J-V characteristics in the dark, using the conventional model of space charge limited current.32 The estimated value of hole mobility for the device prepared from neat THF is about 4.6 × 10-5 cm2/(V s). The hole mobility increased with increasing concentration of acetone in the solvent mixture and is about 9.2 × 10-5 and 9.8 × 10-5 cm2/(V s), when the concentration of acetone is 5 and 7.5%, respectively. This indicates that the formation of copolymer PB nanoparticles in the blend film enhances largely the hole transport. With increasing acetone concentration in the solution, the amount of crystalline copolymer PB nanoparticles increases, as evidenced by the XRD patterns. The increase in hole mobility and transport is not only due to the enhancement of the copolymer PB crystallinity but also due to the good connectivity of crystalline copolymer nanoparticles, as has been observed in P3HT.33,34 Figure 8 shows the J-V characteristics of ITO/PEDOT:PSS/ copolymer PB:PCBM/Al devices with the copolymer PB:PCBM film fabricated from different solutions under an illumination intensity of 100 mW/cm2. The PV parameters are summarized in Table 2. It can be seen that the PV device based on a blend film prepared from neat THF exhibits an open circuit voltage (Voc) of 0.86 V, a short circuit current (Jsc) of 3.8 mA/cm2, and an overall PCE of 1.57%. When acetone is added to THF solution of copolymer PB, the Jsc value of the resulting PV device reaches ∼6.65 mA/cm2. This may be attributed not only

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Figure 8. Current-voltage characteristics of ITO/PEDOT:PSS/PB: PCBM/Al devices prepared from THF solution with different percentages of acetone.

TABLE 2: Photovoltaic Parameters of the As Cast and Annealed PB:PCBM Bulk Heterojunction PV Cells under an Illumination Intensity of 100 mW/cm2 with the PB:PCBM Blend Films, Fabricated from THF Solution Containing Various Percentages of Acetonea solvent THF only

as cast annealed 2.5% acetone as cast annealed 5% acetone as cast annealed 7.5% acetone as cast annealed

Jsc (mA/cm2) Voc (V) 3.8 7.4 4.7 7.9 6.65 7.0 6.22 6.7

0.86 0.83 0.82 0.80 0.81 0.81 0.80 0.81

FF

PCE (η) (%)

0.48 0.53 0.51 0.56 0.52 0.54 0.54 0.56

1.57 3.25 1.96 3.54 2.80 3.06 2.68 3.04

a Jsc is the short circuit current, Voc is the open circuit voltage, FF is the fill factor and PCE (η) is the power conversion efficiency.

to the enhanced optical absorption and hole transport, due to the good connectivity of formed copolymer nanoparticles, but also to induced PCBM aggregation that also enhances the electron collection. We have observed that the hole mobility and optical absorption of the blend increases gradually, by increasing the content of acetone in THF solution, but the best PV performance with Jsc ) 6.65 mA/cm2 and PCE of 2.80% is achieved for the PV device with copolymer PB:PCBM film fabricated from the solution containing 5% acetone. Further increase of the acetone concentration up to 7.5% decreases both Jsc and PCE. This feature may be attributed to the presence of some PCBM molecules trapped between crystalline copolymer PB nanoparticles forming isolated islands at a high content of copolymer PB nanoparticles. These isolated PCBM islands may trap the photogenerated electrons and decrease the Jsc value, as has been observed for thermally annealed P3HT:PCBM films.35,36 We have also investigated the effect of thermal annealing of the PV devices based on the PB:PCBM blend films prepared from different solutions, and their J-V characteristics under an illumination intensity of 100 mW/cm2 are shown in Figure 9. The PV parameters are also summarized in Table 2. It can be seen that the PV device based on the PB:PCBM prepared from the THF solution was significantly enhanced with a Jsc value of 7.4 mA/cm2 and PCE of 3.25% after post thermal annealing. This increase in both Jsc and PCE was attributed to the demixing of copolymer PB and PCBM and enhanced ordering of PB. It is observed that, for the PV device prepared from acetone mixed THF solution of copolymer PB, the Jsc is

Figure 9. Current-voltage characteristics of thermally annealed ITO /PEDOT:PSS/PB:PCBM/Al devices prepared from THF solution with different concentrations of acetone.

further increased after the post thermal annealing. It can be seen from Table 2 that the degree of improvement in the value of Jsc after thermal annealing decreases, as the concentration of acetone in the copolymer solution increases. This is attributed to the fact that some PCBM aggregates are trapped among the crystalline copolymer PB nanoparticles and the demixing of PCBM is restrained by these crystalline copolymer PB nanoparticles. The PV device with PB:PCBM blend layer prepared from a solution containing 2.5% acetone exhibits the best PV performance among these devices with a Jsc value of 7.9 mA/cm2, Voc value of 0.80 V, and fill factor (FF) of 0.56. These values raise the PCE up to 3.54%, which is higher than the PCE of the PV device prepared from neat THF solution of copolymer. These results show that the small amount of copolymer nanoparticles, resulting from the addition of acetone in the solution of copolymer PB, can enhance the hole mobility via good connectivity of copolymer PB nanoparticles and lead to an enhancement in the PV performance of the device. We have estimated the maximum generation rate (Gmax) for producing free carriers from the bound electron-hole pairs at the donor-acceptor interfaces according to the literature.37-39 Figure 10 shows the variation of experimental photocurrent (Jph) of the as cast and thermally annealed PV devices with PB:PCBM blend films prepared for THF solution containing different percentages of acetone, as a function of the effective voltage (Vo - V); Vo is the compensation voltage at which Jph is zero. The value of Jph begins to saturate at large reverse bias, as observed in Figure 10. The saturation photocurrent (Jsat) is given by Jsat ) qGmaxL, where q is the electronic charge and L is the thickness of the photoactive layer used in the device. Figure 11 shows the variation of Gmax as a function of acetone concentration in THF solution. For PV devices based on the as cast PB:PCBM blend film, Gmax increases with increasing the concentration of acetone in THF solution of PB, and reaches a maximum at a percentage of 5%. This indicates that the preaggregated PB nanoparticles in the solution induce the PCBM demixing in PB:PCBM blend film to form the separate electron and hole pathways which are efficiently collected at the respective electrodes. However, the value of Gmax decreases when the acetone percentage in THF solution of PB is 7.5%. This may be attributed to the fact that some PCBM molecules are trapped between the crystalline PB nanoparticles and subsequently reduce the charge collection efficiency.

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Figure 10. Photocurrent as a function of the effective applied voltage (Vo - V) for the as cast (a) and thermally annealed (b) PB:PCBM bulk heterojunction PV cells with the PB:PCBM blend films made from THF solution containing various percentages of acetone.

soluble in nonprotic solvents such as acetone, THF, dichloromethane, and chloroform but unstable in protic solvents such as methanol. The thin film absorption spectrum of PB was broad with an onset at 763 nm corresponding to an optical band gap of 1.63 eV. Crystalline copolymer PB nanoparticles are introduced into PB:PCBM blend films via adding a small amount of acetone into a THF solution of PB, and its effect on PV performance is investigated. It is found that the hole transport in the resulting PB:PCBM blend films increases with increasing concentration of acetone due to the enhanced PB crystallinity and highly interconnected nanoparticle networks. The small contents of PB nanoparticles not only improve the demixing between PB and PCBM but also enhance the hole transport via the crystalline nanoparticles network, resulting in efficient charge collection. The power conversion efficiency has been further improved with the thermally annealed PV devices. References and Notes

Figure 11. Variation of Gmax of electron-hole pairs for the as cast and thermally annealed PB:PCBM BHJ photovoltaic devices with PB: PCBM blend films prepared from THF solution containing different percentages of acetone.

The value of Gmax for the annealed device based on PB:PCBM (without acetone) is higher than the one for the as cast film. In this case, the PCBM demixing into aggregates can easily occur for the PV device with PB:PCBM blend film prepared from a THF solution of PB thermal annealing, resulting in an improvement of Gmax. The PB:PCBM blend prepared from neat THF solution is almost amorphous. The small content of acetone (2.5%) in the THF solution of PB does not restrain PCBM molecules from demixing, and the Gmax is further enhanced. However, the high content of acetone in the THF solution of PB increases the concentration of PB nanoparticles in the blend and restrains PCBM from further demixing and the Gmax is slightly improved as compared to the device based on the as cast blend. Conclusions A phenylenevinylene copolymer P which contains bisazapyrrole segments along the main chain was synthesized by Heck coupling. The reaction of P with BF3Et2O gave the corresponding BF2-azopyrrole complex PB. The latter was stable and

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