As Electron Donor in Efficient Bulk Heterojunction ... - ACS Publications

ARTICLE pubs.acs.org/JPCC

Synthesis of a Broadly Absorbing Modified PCBM and Application As Electron Acceptor with Poly(3-Hexylthiophene) As Electron Donor in Efficient Bulk Heterojunction Solar Cells J. A. Mikroyannidis,*,† D. V. Tsagkournos,† S. S. Sharma,‡ and G. D. Sharma*,§,|| †

Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece Department of Physics, Government Engineering College for Women, Ajmer, Rajasthan, India § Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur (Raj.) 342005, India R& D Center for Science and Engineering, Jaipur Engineering College, Kukas, Jaipur (Raj.), India

)

‡

ABSTRACT: A simple and effective modification of PCBM was carried out by a two-step reaction. The modified PCBM, i.e., A was obtained as a mixture of monoadduct and multiadducts arising from the Prato reaction and was soluble in THF. It showed broader and stronger absorption in the visible region of the solar spectrum than PCBM due to the presence of a cyanovinylene 4-nitrophenyl segment. The lowest occupied molecular energy level (LUMO) of A was measured and found that it is higher than that of PCBM by 0.15 eV. The polymer solar cell (PSC) based on the P3HT as the electron donor with A as the electron acceptor shows an open circuit voltage (Voc) of 0.80 V, a short-circuit current (Jsc) of 9.0 mA/cm2, and a power conversion efficiency (PCE) of 3.88%, which are higher than those for the PSC based on P3HT:PCBM. The higher PCE for the PSC based on P3HT:A accounts for the increase in both Jsc and Voc, due to the increased absorption of A in the visible region and its higher LUMO level. The PCE of the PSC has been further increased to 4.50% and 5.32% when the P3HT:A blend film is cast from mixed solvents and subsequent thermal annealing, respectively. The improvement in PCE is mainly due to the increase in Jsc, which has been attributed to the increase in the crystallinity of the film leading to an increase in the hole mobility, which causes a balanced charge transport in the devices.

’ INTRODUCTION Solution-processable organic photovoltaics (OPVs) based on either polymers or small molecules, or their mixtures, are becoming progressively more attractive.1
5 Their advantage over thin film photovoltaics based on inorganic semiconductors arises from the possibility of fabricating organic photovoltaics at low cost, using a process which is simple and easy to scale-up. These devices also offer the added advantages of being lightweight and mechanically flexible, as well as the ability to tailor the physical, optical, and chemical properties of the active components through synthesis and processing. Regioregular poly(3-alkylthiophene)s (P3ATs), particularly poly(3-hexylthiophene) (P3HT), is one of the most widely studied donor materials for OPV due to their favorable optoelectrical properties, good processability, and good self-organization capability. Bulk heterojunction (BHJ) OPV devices consisting of binary blends of regioregular P3HT donor and fullerene derivative acceptor, such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have shown promising power conversion efficiency (PCE) values. In particular, the PCE of the polymer solar cells (PSCs) based on P3HT:PCBM reached over 5%2,4,6
8 by thermal treatment,4 r 2011 American Chemical Society

solvent2 and vapor7 annealing, as well as mixture solvent treatment.8 In order to improve the PCE of the PSCs, much work has been devoted to finding new conjugated polymers aimed at broader absorption, lower bandgap, higher hole mobility, and suitable electronic energy levels. During the last two years, several low bandgap polymers with enhanced absorption abilities have been reported and so far the highest overall PCE achieved in PSCs based on these polymers is in the range 5
7.5%.9a
g A PCE of 8.3% has been reported by Konarka, which is the current world record.9h However, further improvement of the photovoltaic (PV) performance of the PSCs based on P3HT:PCBM is limited by the relatively large band gap of P3HT (∼1.9 eV), which limits the harvest of solar light, and the relatively small energy difference between the lowest unoccupied molecular orbital (LUMO) of PCBM and the highest occupied molecular orbital (HOMO) of P3HT, which results in a lower open circuit voltage (Voc) of the P3HT:PCBM based PSCs to Received: February 10, 2011 Revised: March 5, 2011 Published: March 30, 2011 7806

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The Journal of Physical Chemistry C ∼0.6 V. To further improve the P3HT-based device performance, different C60 derivatives10
16 have been synthesized for PV electron acceptor materials. Particularly, much effort has been devoted to modifying the substituent of PCBM by introducing additional substituents on its phenyl ring,10,11 or replacing the phenyl ring with other groups.12 However, among these fullerene derivatives, most of them show poorer or just comparable PV properties to that of PCBM.12c,d,16b In recent years, PCBM bisadduct17 or PCBM multiadduct,18 and endohedral fullerenes19 were reported for application as PV acceptor materials. These fullerene derivatives possess higher LUMO energy levels which result in higher Voc as well as higher PCE17,19 of the P3HTbased OSCs. A significant bis-adduct fullerene derivative, ICBA, formed by two indene units covalently connected to the fullerene sphere of C60 has recently been reported by Hou and Li.20 Interestingly, the presence of two aryl groups improves the visible absorption compared to the parent PCBM, as well as its solubility and the LUMO energy level, which is 0.17 eV higher than PCBM. The PV devices formed with P3HT as the semiconducting polymer revealed PCE values of 5.44% under illumination of AM1.5, 100 mW cm
2, while PCBM afforded an efficiency of 3.88% under the same experimental conditions. More recently, device optimization of BHJ PSCs based on P3HT as donor and ICBA as acceptor has been performed by the same research group. In particular, the optimized PSC with the P3HT:ICBA weight ratio of 1:1, solvent annealing and prethermal annealing at 150 °C for 10 min, has exhibited a high PCE of 6.48% with a Voc of 0.84 V, Jsc of 10.61 mA/cm2, and FF of 72.7%, under the illumination of AM 1.5G, 100 mW/cm2. These values are the highest reported in the literature so far for P3HT-based PSCs.21 Recently, our group reported the synthesis of a modified PCBM derivative, F, which contains cyanovinylene 4-nitrophenyl segments.22 It was soluble in common organic solvents and showed stronger absorption than PCBM in the range of 250
900 nm. BHJ solar cells based on P3HT as electron donor with F as electron acceptor displayed a PCE of 4.23%, while the device based on P3HT:PCBM displayed a PCE of 2.93% under the same conditions. The maximum overall PCE of 5.25% has been achieved with the PSC based on the P3HT:F blend deposited from a mixture of solvents and subsequent thermal annealing.22 Herein, we describe a simple and effective modification of PCBM, which was carried out in only two steps. At the first step, PCBM reacted with terephthaldehyde in the presence of Nmethylglycine (Prato reaction23) to afford a formyl derivative as a mixture of monoadduct and multiadducts. The latter was subsequently condensed with 4-nitrobenzyl cyanide to afford the modified PCBM, i.e., A. It is soluble in THF. The modified PCBM (A) showed stronger absorption in the visible region of solar spectrum than PCBM in both solution and thin film due to the presence of the cyanovinylene 4-nitrophenyl segment. It has been well established that the incorporation of this segment to various polymers24,25 or small molecules25
29 broadened their absorption spectra and extended them into the near-infrared (NIR) region. Moreover, the LUMO level of A is by 0.15 eV higher than that of PCBM. We have investigated the PV response of the devices based on a P3HT:A BHJ active layer, where A is used as electron acceptor, sandwiched between ITO/PEDOT:PSS and Al electrodes. The PSCs based on P3HT:A blend cast from THF solvent exhibit Voc of 0.80 V and Jsc of 9.0 mA/cm2 and PCE of ∼3.88%, which are

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higher than those for P3HT:PCBM blend.22 We have also fabricated PSCs with the BHJ active layer (P3HT:A) deposited from mixed solvents 1,2-dichlorobenzene/tetrahydrofuran (DCB/ THF) and subsequent thermal annealing at 120 °C for 2 min and achieved PCE of 4.50% and 5.32%, respectively. The improvement of PCE is mainly due to the increase in the Jsc, which has been ascribed to the enhanced crystallinity of P3HT, when the blend was cast from mixed solvents and was thermally annealed. The enhanced crystallinity leads to an increase in the hole mobility, causing a balanced charge transport in the device, which results in high photocurrent.

’ EXPERIMENTAL SECTION Synthesis of PCBM-1. A mixture of PCBM (0.0200 g, 0.0219 mmol), terephthaldehyde (0.0146 g, 0.1089 mmol), N-methylglycine (0.0097 g, 0.1089 mmol), and chlorobenzene (15 mL) was refluxed for 17 h under nitrogen atmosphere. The mixture was concentrated under reduced pressure to remove all solvent. Methanol was added to the concentrate and the mixture was sonicated and heated at ∼40 °C to dissolve the unreactive terephthaldehyde. Then, it was centrifuged to isolate the crude product. This process was repeated once more. The crude reaction product was purified by silica gel column chromatography with toluene/THF (1:3) to afford PCBM-1 as a mixture of monoadduct and multiadducts (0.0152 g, yield 65%). 1 H NMR (THF-d8) ppm: 9.95 (s, formyl); 7.83
7.32 (m, phenylene); 4.94 (s, NCH2); 4.29 (m, NCHPh); 3.57 (s, COOCH3); 2.84 (s, NCH3); 2.83 (t, CH2(CH2)2COO); 2.32 (t, (CH2)2CH2COO); 2.02 (m, CH2CH2CH2COO). FT-IR (KBr, cm
1): 526, 572, 1186, 1436 (fullerene); 1734 (ester carbonyl); 1703 (formyl). Synthesis of A. A flask was charged with a solution of PCBM-1 (0.0235 g, 0.0219 mmol on the basis of monoadduct) and 4-nitrobenzyl cyanide (0.0178 g, 0.1098 mmol) in THF (10 mL). tert-BuOK (0.0100 g, 0.09 mmol) dissolved in THF (5 mL) was added to the solution. The reaction mixture was stirred for 1 h at room temperature under nitrogen and then was concentrated under reduced pressure. Water was added to the concentrate and A precipitated as a brown-green solid. It was centrifuged, dried, and washed thoroughly with chloroform. The crude product was purified by silica gel column chromatography with toluene/THF (1:4) to afford A as a mixture of monoadduct and multiadducts (0.0145 g, yield 55%). 1 H NMR (THF-d8) ppm: 8.17 (m, phenylene ortho to nitro); 7.83
7.32 (m, other phenylene and cyanovinylene); 4.94 (s, NCH2); 4.29 (m, NCHPh); 3.57 (s, COOCH3); 2.84 (s, NCH3); 2.83 (t, CH2(CH2)2COO); 2.32 (t, (CH2)2CH2COO); 2.02 (m, CH2CH2CH2COO). FT-IR (KBr, cm
1): 526, 572, 1186, 1436 (fullerene); 1734 (ester carbonyl); 2170 (cyano); 1522, 1348 cm
1 (nitro). 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 Bruker 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. The electrochemical properties of A were studied by cyclic voltammetry (CV) (CH Instruments). A glassy carbon coated with A, a Pt plate, and an Ag/Agþ electrode were used as working, counter, and reference electrode, respectively. The electrolyte solution was 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in 7807

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Scheme 1. Synthesis of A

anhydrous acetonitrile. The potential scan rate was 100 mV/s. The film of A was coated on the glassy carbon electrode using THF solution. Device Fabrication and Characterization. The solutions of the blend P3HT:A (1:1w/w) were prepared in concentration of 10 mg/mL using THF as solvent and stirred for 2 h. For the mixed solvents, 2% volume of DCB was added to the P3HT:A solution in THF and then stirred for 2 h. All the BHJ PV devices were prepared using the following device fabrication procedure. Glass/ITO substrates were cleaned with detergent, ultrasonicated with distilled water, and isopropyl alcohol and finally dried on a hot plate at 80 °C for 20 min. A thin layer (80 nm) of PEDOT:PSS (BAYTRON, conductive grade) was spin coated onto a cleaned ITO and subsequently dried at 80 °C for 20 min. The photoactive layer of the blend P3HT:A was deposited by spin coating from the prepared solution (THF or mixed solvents) on the top of the PEDOT:PSS layer. The device was completed by depositing a thin layer (100 nm) of Al on the top of active layer, at pressure of less than 10
5 Torr. The active area of the devices was 10 mm2. For thermal annealing, the blend films were placed on a hot plate and heated at 120 °C for 2 min and then cooled up to room temperature, before the deposition of the Al electrode. We have designated the devices as follows ITO=PEDOT:PSS=P3HT:AðTHFÞ=Al ðdevice AÞ ITO=PEDOT:PSS=P3HT:AðDCB=THFÞ=Al ðdevice BÞ ITO=PEDOT:PSS=P3HT:AðDCB=THF, thermally annealedÞ=Al ðdevice CÞ The hole only devices, i.e., ITO/PEDOT:PSS/blend/Au, were used to estimate the hole mobility in the blend films and were fabricated as described above, with the exception that the top electrode was replaced with Au. Electron only devices having structure Al/blend/Al were also fabricated by spin coating the active layer on glass/Al substrates, followed by the deposition of

Al electrode. We have also fabricated the devices based on only A to get information about the type of conductivity and charge carrier mobility. The current
voltage (J-V) characteristics of the devices were carried out on a computer controlled Keithley 238 source meter. A Xenon lamp (500 W) was used as the white light source and the optical power at the sample was 100 mW/cm2 (the equivalent of one sun at 1.5 AM).

’ RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 outlines the twostep reaction for the modification of PCBM. In particular, PCBM reacted with terephthaldehyde (mol ratio 1:5) in the presence of N-methylglycine in chlorobenzene to afford the formyl derivative PCBM-1 (Prato reaction23). The product was washed with methanol to dissolve the unreactive terephthaldehyde. The crude product was purified by column chromatography eluting with toluene/THF (1:3) to afford PCBM-1 as a mixture of monoadduct and multiadducts arising from the Prato reaction.30,31 The composition of PCBM-1 depends upon the reactant ratio in the mixture and the experimental conditions. PCBM-1 was subsequently condensed with 4-nitrobenzyl cyanide in the presence of tert-BuOK in THF to afford the target modified PCBM, i.e., A. The crude product was purified by column chromatography. A was obtained also as a mixture of monoadduct and multiadducts, which were not isolated because of the experimental difficulties. This fact, however, is not expected to have a strong influence on the PV parameters and PCEs.32 A was soluble in THF (∼20 mg mL
1), while it was insoluble in chloroform. In addition, A dissolved in acetonitrile, dioxane, and 1,2-dichlorobenzene (DCB). The structure of A was confirmed by IR and 1H NMR spectroscopy. The FT-IR spectrum showed absorptions at 526, 572, 1186, and 1436 cm
1 associated with the fullerene structure. Moreover, it displayed characteristic absorption bands at 1734 (ester carbonyl); 2170 (cyano) and 1522, 1348 cm
1 (nitro). 7808

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Figure 1. UV
vis absorption spectra of PCBM and A in THF solution (a) and thin film (b).

Figure 2. Cyclic voltammogram of A at scan rate 100 mV/s.

Moreover, the 1H NMR spectrum showed upfield signals at 8.17 (phenylene ortho to nitro) and 7.83
7.32 ppm (other

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Figure 3. (a) Current
voltage (J-V) characteristics of the ITO/A/Al device in dark at room temperature. (b) Current
voltage characteristics of the Al/A/Al device in log
log scale at room temperature.

phenylene and cyanovinylene), while the aliphatic moieties resonated at 4.94
2.02 ppm. Photophysical and Electrochemical Properties of A. Figure 1 presents the UV
vis absorption spectra of both PCBM and A in dilute (10
5 M) THF solution and thin film. The thin film was prepared from THF solution by spin coating on quartz substrate. Interestingly, A showed much stronger absorption than PCBM for the region of 400
800 nm in solution and 400
1000 nm in thin film. This feature bodes well for the PV properties of A. The absorption peaks of A at ∼470 and 610 nm in solution and thin film are attributed to the cyanovinylene 4-nitrophenyl segment, because it has been well established in our previous investigations.24
29 However, the absorption of A was slightly weaker than that of PCBM for the region of 300
400 nm. The electrochemical properties of A were studied by cyclic voltammetry (Figure 2). The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital 7809

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(HOMO) were estimated from their onset reduction (Eonset red ) and oxidation potentials (Eonset ox ) obtained from the cyclic voltammogram, using the following expressions.33 EHOMO ¼
qðEonset þ 4:7ÞeV ox ELUMO ¼
qðEonset red þ 4:7ÞeV Eonset red

Eonset ox

ð1Þ

þ

and are in volt versus Ag /Ag. The LUMO where, energy level of A is
3.80 eV, which is raised by 0.15 eV in comparison to that of PCBM (
3.95 eV). The higher LUMO energy level of A is desirable for its application in PSCs to get higher Voc. The HOMO energy levels are
6.30 eV and
5.70 eV for PCBM22 and A, respectively. This indicates that the HOMO energy level of A also moved upward in comparison to PCBM. The shift in the LUMO level toward the vacuum level is due to the presence of the strong electron withdrawing cyanovinylene 4-nitrophenyl group in the molecule of A. Electrical Properties of A. We have investigated the electrical properties of the single layer device ITO/A/Al in dark to get information about the type of semiconductivity of A. Figure 3a shows the J-V characteristics of the device based on pristine A thin film in dark. The J-V curve of this device shows a rectification effect when negative potential is applied to Al with respect to the ITO electrode. Since the LUMO (
3.80 eV) of A is very close to the work function of Al (4.1 eV) electrode, the interface between A and Al forms a nearly Ohmic contact (barrier of about 0.3 eV) for electron injection from Al to the LUMO level of A. However, the HOMO (
5.7 eV) of A is very far from the work function of ITO (
4.8 eV) and the interface between ITO and A forms the Schottky barrier (barrier of 0.9 eV) for hole injection into the HOMO of A. Consequently, the rectification effect is due to the formation of the Schottky barrier, formed at the ITO-A interface. This indicates that material A behaves as n-type organic semiconductor (electron acceptor). The charge carrier mobility of the materials either electron donor or acceptor used in the organic PV device is also an important factor, which influences the Jsc and the PCE of the device. We have fabricated the device having configuration Al/A/ Al. In this device, both Al electrodes form the nearly Ohmic contact with A and the electrons are injected from the bottom electrode into the LUMO level of A. The electrons are collected by the top Al electrode and the device behaves as electron only device. The J-V curve of the device was plotted in log
log scale, and is shown in Figure 3b. It can be seen from this figure that in the low voltage region (up to 0.3 V), J is linearly dependent on V with a slope of unity, which corresponds to Ohmic region. In the relatively high voltage (beyond 0.3 V), the current density can also be fitted to be linearly dependent on voltage with slope two. Therefore, the J-V characteristics in the dark, in the higher voltage region, correspond to the SCLC behavior with trap free limit. The electron mobility of A was estimated from the following equation:34 J ¼ ð9=8Þεo εr μe ðV 2 =d3 Þ

ð2Þ

where J is the current density, V is the applied voltage, εr and εo are the relative dielectric constant of the organic layer and permittivity of the free space (8.85  10
12 F/m), respectively, μe is the electron mobility, and d is the thickness of the organic layer. Using εr = 3.5, d = 80 nm, the estimated electron mobility fitting the eq 2 with the J-V characteristics in SCLC region (Figure 3b) is in the order of 3.4  10
4 cm2/(V s). This value is

Figure 4. Absorption spectra of P3HT:A thin films cast from THF, DCB/THF and DCB/THF (thermally annealed).

fairly high electron mobility, comparable to that of both PC60BM and PC71BM. This indicates that A can be used as an electron acceptor for BHJ organic solar cells. Optical Properties of P3HT:A Blend. The absorption spectra of P3HT:A thin films deposited from THF, DCB/THF, and DCB/THF (thermally annealed) blend are shown in Figure 4. It can be seen that the absorption spectra of the blend show the combination of the individual components. In particular, the absorption peak around 525 nm corresponds to P3HT, which is associated with the interchain π
π* transition, whereas the peak around 630 nm corresponds mainly to A. The P3HT:A blend also shows a broad absorption from 350 to 750 nm, which closely matches with the solar spectrum. Therefore, we expect more photons to be absorbed by the P3HT:A blend as compared to P3HT:PCBM.22 The blend cast from the mixed solvents, i.e., DCB/THF shows not only a broader band absorption, but also an enhanced intensity of the absorption, which is further increased when the blend cast from the mixed solvents is thermally annealed at 120 °C for 2 min. As it has been reported earlier, the thin film absorption of P3HT also red shifts and displays more distinct vibronic structures, when a solvent with high boiling point is added to the solution before spin-casting. Upon the addition of DCB to the blend solution of THF, the absorption band corresponding to P3HT shifts toward longer wavelength (red shift) and becomes broader. A further red shift in the absorption band has also been observed for the thermally annealed blend. The red shift in absorption of P3HT indicates an increased intermolecular ordering and planarity in the polymer backbone. We have measured the photoluminescence (PL) spectra of P3HT and P3HT:A blends cast from THF and DCB/THF solvents (Figure 5). The PL is quenched by an electron transfer from the excited state of polymer to A molecules. The calculated grain size of P3HT:A from the XRD data is comparable to the exciton diffusion length (10
20 nm) and therefore the PL intensity is quenched efficiently. The degree of the PL quenching is more distinct for the P3HT:A film cast from DCB/THF solvents, in spite of better crystalline nature of the film cast from DCB/THF mixed solvents. Hence, we speculate that the film cast from the mixed solvents may be denser as compared to the 7810

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Table 1. Calculated d Spacing and Grain Size for the P3HT:A Films Cast from Different Solvents d spacing (nm)

grain size (nm)

THF

1.56

12.3

DCB/THF DCB/THF (Thermally annealed)

1.62 1.67

13.4 14.3

solvent

Figure 5. Photoluminescence (PL) spectra of (a) P3HT and (b),(c) P3HT:A film cast from THF and DCB/THF solvents, respectively.

Figure 6. XRD profiles of P3HT:A thin films cast from THF, DCB/ THF solvents, DCB/THF (thermally annealed).

film cast from the THF solvent due to the different boiling points of DCB and THF. It has been reported that the PL is quenched effectively in densely packaged layer because of shorter intermolecular distances.35 XRD Profile of P3HT:A Blend. We have recorded the XRD pattern of the P3HT:A thin films cast from THF, DCB/THF, and DCB/THF (thermally annealed). We have observed that regardless of the casting conditions, all P3HT:A films showed the well ordered P3HT (100) (2θ = 5.56°) peak, which is related with the interchain of P3HT.36 We have also observed that for only P3HT film, this peak has stronger intensity than that of P3HT:A films. It is suggested that although the peak intensity of P3HT is slightly reduced by blending A with P3HT, the P3HT is well crystallized in all films prepared from different solvents. Both the as-cast and thermally-annealed films prepared from DCB/ THF solvents show stronger and sharper peaks in comparison to the film cast from THF solvent, suggesting that the degree of P3HT crystallization is better in the film cast from mixed solvents. In order to analyze the peaks in detail, XRD was performed with a narrow range (2θ = 3
8°) in which the only P3HT (100) peak is observed (Figure 6). The peak position observed in

Figure 7. (a) Current
voltage characteristics under illumination intensity of 100 mW/cm2 and (b) IPCE spectra of devices A, B, and C.

DCB/THF cast film is slightly shifted to low angle as compared to the THF cast film. This peak is further shifted toward the low angle, when the DCB/THF cast film is thermally annealed. This indicates that the d spacing of P3HT (100) becomes larger during the crystallization of P3HT. We have calculated the d-spacing parameter and grain size from the information of (100) peak using the Bragg’s and Scherrer’s equation,37 respectively (Table 1). It can be seen from this table that the d spacing and grain size is higher for the film cast from mixed solvents as compared to that from THF solvent, which is due to the increase in the crystallization of P3HT.38 The thermal annealing further 7811

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Table 2. Photovoltaic Parameters of the Devices fill short circuit current open circuit factor power conversion device (Jsc) (mA/cm2) voltage (Voc) (V) (FF) efficiency (PCE) (%) A

9.0

0.80

0.54

3.88

B

9.9

0.76

0.60

4.50

C

10.95

0.76

0.64

5.32

improves the crystallization, as observed from the XRD data (diffraction peak corresponding to (100) further intensify). The increase in the crystallite size also provides further insight onto effective charge transport and also provides high internal effective field for charge separation, resulting in higher value of PCE. Photovoltaic Properties of P3HT:A Blend. In the BHJ PV devices, the weight ratio of donor/acceptor (D/A) has a large effect on the PCE of the device, because there should be a balance between the absorbance and the charge transporting network of the active layer used in the device. When the acceptor content is too low, the electron transporting ability will be limited. Moreover, when the acceptor content is too high, the absorbance and hole transporting ability of the active layer will be decreased. Therefore, we have fabricated BHJ PV devices with different weight ratios for P3HT:A (1:0.5, 1:1, 1:1.5, and 1:2) using THF and mixed solvents and we found the optimized PV device is for the weight ratio of 1:1. The J-V characteristics of devices A, B, and C fabricated from different solvents, under illumination intensity of 100 mW/cm2, are shown in Figure 7(a). The PV parameters, i.e., short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of the devices are listed in Table 2. It can be seen from this table that the Voc is increased from 0.68 V for PCBM22 based device to 0.80 V for the A based device. The higher Voc value for the PSC device based on P3HT: A is attributed to the higher LUMO energy level of A as compared to PCBM, because it is well-known that the Voc of BHJ PSCs is proportional to the difference between the HOMO energy level of the donor and the LUMO energy level of the acceptor. It can be seen from the optical absorption spectra of P3HT:A that this blend shows a broader absorption band as compared to the P3HT:PCBM blend, which is attributed to the improvement in the Jsc for the PSC based on the P3HT:A blend. The illumination light is absorbed by both P3HT and A, which causes an enhancement in the photogenerated excitons in the blend, affording higher photocurrent. The increased values of both Jsc (9.0 mA/cm2) and Voc (0.80 V) for P3HT:A blend, with respect to P3HT:PCBM based device, give an overall PCE of ∼3.88% for the device based on P3HT:A (device A) blend cast from THF solvent. The values of incident photon to current efficiency (IPCE) have been estimated using the following expression: IPCEð%Þ ¼ 1240Jsc =λPin

ð3Þ

where Pin (W/m2) and λ (nm) are the illumination intensity and wavelength of the monochromatic light, respectively. The IPCE spectra of devices A, B, and C are shown in Figure 7(b). It can be seen from this figure that the IPCE spectra of the devices closely resemble the absorption profile of the blend, indicating that both components (donor and acceptor) used in the active layer of the device contribute to the photocurrent. The resemblance of the IPCE spectra of the device with the optical absorption spectra of

the blend also indicates the formation of BHJ photoactive layer and increased D/A interfacial area in the device. The performance of the solar cells based on P3HT:A films is further enhanced by using mixed solvents for preparing the films. A mixed solvents approach has been shown to be effective in several polymer solar cell devices, including those based on low band gap polymers.39 The BHJ morphology can also be modified by incorporating a small amount of processing additives to the solution during the device fabrication.40 We have also investigated the effect of additive in BHJ solution before spin-casting the film, on the PV response of the device based on P3HT:A. We have added DCB as additive, which has a higher boiling point than THF. We have fabricated devices with mixed solvents (DCB/THF having DCB content of 0.5%, 1.0% and 1.5%) and found that the optimized PV parameters for the BHJ device, in which the blend was cast from mixed solvents, was obtained for 1% DCB content (device B). Figure 7(a) also shows the effect of mixed solvents on the PV performance of PSCs based on P3HT:A blend (devices B and C) and the PV data are summarized in Table 2. The values of both Jsc and FF have been enhanced, when DCB is added to the blend solution prepared from THF solvent. However, the Voc remains almost the same. The overall PCE has been enhanced from 3.88% to 4.50%, for DCB/THF based device. The PCE has been further improved to 5.32%, when the film prepared from DCB/ THF solvents is thermally annealed. The IPCE spectra of devices B and C are also shown in Figure 7(b). The IPCE values are higher for these devices as compared to those for the device prepared from THF solvent (device A). The values of IPCE are consistent with the values of Jsc observed in the J-V characteristics under illumination. The increased values of IPCE and Jsc have been attributed to the enhanced value of the absorption coefficient of the blend, the broad absorption band (400
750 nm), and the improved crystallinity of the films cast from mixed solvents as compared to those cast from THF solvent. The Jsc value calculated from the integration of the IPCE spectrum of devices B and C is 10.2 mA/ cm2 and 11.4 mA/cm2, respectively, which closely matches the Jsc value obtained from the J-V measurement under white light illumination. The thin film absorption of P3HT:A also red shifts and becomes broader, when DCB is added to the solution before spin coating. This increases the intermolecular ordering and crystalline nature of the P3HT (as supported from XRD profile) and can be a factor for the improved PV performance of the devices. A similar phenomenon has been observed by Yang et al. in P3HT:PCBM blend41 and other BHJ polymer solar cells.42 Since an increase in Jsc and FF mainly affect the PCE, to precisely assess the correlation between the solvent used for casting the blend film and device PV performance, we have examined the electron and hole mobilities of the fabricated devices prepared under different conditions. For high efficiency polymer BHJ solar cells, the mobility of electrons and holes in the BHJ active layers is an important parameter that must be well controlled, because a balanced charge carrier transport in the device is an essential prerequisite for increasing Jsc and FF.43 The electron and hole mobilities of P3HT:A blend film were investigated using the devices prepared under different conditions, using SCLC measurements with electron only/hole only devices. Figure 8(a) shows the variation of dark current of ITO/PEDOT:PSS/P3HT:A/Au devices with the corrected bias voltage, which is determined by the difference between the work function of Au and the HOMO level of P3HT. Electron only devices Al/ 7812

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Figure 9. Variation of photocurrent (Jlight
Jdark) as a function of effective applied voltage (Vo
Vapp) for devices A, B, and C.

Figure 8. Current
voltage characteristics of the (a) ITO/PEDOT: PSS/P3HT:A/Au hole only (b) Al/P3HT:A/Al electron only devices prepared from THF, DCB/THF, and DCB/THF (thermally annealed) blends.

P3HT:A/Al were fabricated, and the dark J-V characteristics were investigated, as shown in Figure 8(b). In the trap-free region, where all of the traps are filled, the SCLC behavior can be characterized by the Mott
Gurney square law (eq 2). The hole mobility calculated from the currents in the square law region is higher for the BHJ active layer cast from DCB/THF solvents and it is further improved when a thermally annealed film is used for device fabrication. In the P3HT:A device, the holes were separated from the excitons and moved toward ITO through the P3HT domains and the electrons passed through the A domains. Therefore, the hole mobility enhancement (3.24  10
6 cm2/ (V s), 2.65  10
5 cm2/(V s), 1.8 x10
4 cm2/(V s) for THF, DCB/THF, and thermally annealed DCB/THF, respectively) for the blend prepared from mixed solvents is caused by the

enhanced π
π stacking and the chain ordering, as supported by the optical absorption and XRD data. However, whereas the hole mobility (μh) increases by 1 order of magnitude, the electron mobility (μe) does not increase significantly (2.86  10
4 cm2/(V s), 3.0  10
4 cm2/(V s) and 3.06  10
4 cm2/(V s) for THF, DCB/THF, and thermally annealed DCB/THF). The value of the μe in the blend is almost the same (slightly lower) with that of pure A, which is due to the increased solubility of A in common solvent as compared to PCBM. This also can be attributed to the fact that the A phase in the blend does not crystallize during the spin-casting process, as it has been confirmed from the XRD data. This observation shows that the A domain and cluster regions remain the same, regardless of solvent, to produce similar electron mobilities. With the hole mobility enhancement and a similar electron mobility, a carrier mobility balance between holes and electrons could be achieved, which would reduce the space charges resulting in the improvement in the Jsc and overall PCE. The ratio of electron and hole mobility is reduced, reaching toward unity (1.7 for thermally annealed film). When the charge transport in the device was unbalanced (in the THF cast P3HT:A film, where μh is significantly lower than μe), hole accumulation occurred in the device and photocurrent was space charge limited.44 A smaller value of μe/μh ratio, indicates a more balanced charge transport in the device, which contributes to higher values of both Jsc and PCE. The exciton dissociation efficiency of these devices was also studied. The generation rate of free carriers (G) is described by the following expression: GðT, EÞ ¼ Gmax PðT, EÞ where Gmax is the maximum generation rate of bound electron
hole pairs, P(T,E) is the probability of charge separation at the donor
acceptor interface,45 limited by temperature (T) and the electric field (E). Because the photocurrent can be determined by the rate of generation of free carriers, which is governed by the probability of charge separation,45,46 the exciton dissociation efficiency, similar to the probability of charge separation, is an 7813

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Figure 10. AFM images of P3HT:A blend spin-cast from (a) THF, (b) DCB/THF and DCB/THF (thermally annealed). The scan size of all images is 5  5 μm.

important process for enhancing the device performance. Therefore, the P(T,E) of each P3HT:A blend device was compared to get information about the increase in the Jsc for the device prepared from the spin coating blend from mixed solvents. In reverse bias analysis, the photocurrent is often plotted as a function of the effective applied voltage. The photocurrent is defined as Jphoto = Jlight
Jdark, where Jlight and Jdark are the current densities of the device measured under illumination and in the dark, respectively. The effective applied voltage is defined as Veff = Vo
Vappl, where Vo is the compensation defined as the voltage, where Jphoto = 0 and Vappl is the applied voltage. Figure 9 shows a log
log plot of the Jphoto arising from the J-V curves of the P3HT:A based devices cast from different solvent conditions, as a function of Veff. The photocurrent increases linearly in the low effective voltage region and subsequently tends to saturate in the higher effective voltage region. The photocurrent could be estimated by Jphoto = qG (T,E)d, in the saturation regime for Veff > 0.4
0.5 V, where q is the electrical charge and d is the blend film thickness. In the high voltage region (>1 V), most of the bound electron
hole pairs were separated into free carriers, and photocurrent was saturated to Jsat = qGmax d.45b Under short circuit conditions, the value of Jsc/Jsat gives information about the charge separation efficiency. This value is 0.84, 0.91, and 0.94 for devices A, B, and C, respectively. Recombination is also an important process to charge separation efficiency and has strong influence on the charge collection. It has been reported that when Jsc/Jsat > 0.9, the charge extraction is much more efficient than charge recombination.47 The value of Jsc/Jsat for devices A, B, and C indicates that the charge extraction process is more efficient than the recombination for devices B and C as compared to device A. The value of saturation current, Jsat, of the device A (10.86 mA/ cm2) is less than that for the devices B and C, as shown in Figure 9. As mentioned earlier, because most of the bound electron
hole pairs are separated into free carriers in the saturation regime, Gmax is mainly governed by the quantity of absorbed photons.45a The values of Gmax for devices A, B, and C are of the order C > B > A. The increase in the value of Gmax for device C, results entirely from the enhanced absorption, implying that almost all of the excitons generated in the blend dissociated at the D
A interfaces and formed electron
hole pairs. This conclusion is supported by the optical absorption data, which demonstrate that increased light absorption intensity enhances the photocurrent. Although a slight

broad absorption band is observed with the blend cast from mixed solvents and subsequent thermal annealing, the net effect of broad absorption band and the increase in intensity is an improvement of the spectral overlap with the solar emission, which contributes to the increase of the light absorption.48 Morphology of the photoactive layer employed for the device fabrication is also very important for the PV performance of organic solar cells.49 We have used atomic force microscopy (AFM) to investigate the morphology of the P3HT:A thin films spin-cast from different solvents, i.e., THF, DCB/THF, and DCB/THF (thermally annealed) (Figure 10). The P3HT:A blend film cast from THF solvent exhibits rougher morphology with large domains as compared to the film cast from DCB/THF solvent. This feature could lead to a decrease in the D/A interfacial area for efficient charge separation, resulting in lower Jsc in organic solar cells. However, the film cast from DCB/THF solvent and subsequent thermal annealing, shows smoother morphology, indicating the increase in the D/A interfacial area and resulting in higher Jsc. The P3HT, cast from mixed solvents and subsequent thermal annealing, increases the conjugation length and crystallinity of self-assembled P3HT. Since the crystallinity of P3HT is a critical factor for enhancement of its hole mobility, the increased crystallinity of P3HT forms a percolative pathway for hole transport. As a consequence, the enhanced hole mobility balances the electron mobility and reduces the charge accumulation in the photoactive layer, resulting in improved PCE.

’ CONCLUSIONS A modified PCBM (A), which contains a cyanovinylene 4-nitrophenyl segment, was synthesized as a mixture of monoadduct and multiadducts. It was soluble in THF, while it was insoluble in chloroform. A showed much stronger visible absorption than PCBM, which is desirable for PV applications. We have fabricated PSCs with the P3HT:A blend, sandwiched between ITO/PEDOT:PSS and Al electrodes. The Voc and Jsc values of the BHJ device based on P3HT:A blend cast from THF solvent reached 0.80 V and 9.0 mA/cm2, respectively, leading to an overall PCE of ∼3.88%, which is significantly improved as compared to a BHJ device based on P3HT:PCBM (2.93%). The increase in Jsc has been attributed to the stronger absorption of A in the visible region, which is missing for PCBM. However, the increase in the Voc has been ascribed to the higher LUMO 7814

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The Journal of Physical Chemistry C level of A as compared to PCBM. The PCE of the PSCs based on P3HT:A blend deposited from mixed solvents (DCB/THF) has been improved up to 4.50%. Moreover, this has been further increased to 5.32%, when the P3HT:A blend cast from mixed solvents was thermally annealed before the deposition of the Al electrode. We have observed that the improvement in the PCE has been mainly attributed to the increase in the Jsc. It can be seen from the XRD data that the crystallinity of P3HT has been increased when the blend is cast from mixed solvents, and further increases upon subsequent thermal treatment. The improvement in the PCE has been attributed to the increase in the hole mobility using mixed solvents for deposition of the thin film, leading to more balanced charge transport in the device.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ30 2610 997115; Fax: þ30 2610 997118; E-mail: [email protected] (J.A.M.). Tel: 91-0291-2720857; Fax: 91-0291-2720856. E-mail: [email protected] (G.D.S.).

’ ACKNOWLEDGMENT We are thankful to Prof. Y.K. Vijay, Thin film and Membrane Science Laboratory, Department of Physics, University of Rajasthan, Jaipur for allowing us to undertake the device fabrication and characterization in his laboratory. ’ REFERENCES (1) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323. (2) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (3) Moule, A. J.; Meerholz, K. Adv. Funct. Mater. 2009, 19, 3028. (4) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617. (5) Peet, J.; Soci, C.; Coffin, R. C.; Nguyen, T. Q.; Mikhailovsky, A.; Moses, D.; Bazan, G. C. Appl. Phys. Lett. 2006, 89, 252105. (6) Kim, Y. Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197. (7) Zhao, Y.; Xie, Z. Y.; Qu, Y.; Geng, Y. H.; Wang, L. X. Appl. Phys. Lett. 2007, 90, 043504. (8) (a) Moule, A. J.; Meerholz, K. Adv. Mater. 2008, 20, 240. (b) Li, L.; Lu, G. H.; Yang, X. J. Mater. Chem. 2008, 18, 1984. (c) Chang, Y.-M.; Wang, L. J. Phys. Chem. C 2008, 112, 17716. (d) Yao, Y.; Hou, J. H.; Xu, Z.; Li, G.; Yang, Y. Adv. Funct. Mater. 2008, 1, 1783. (e) Wang, W. L.; Wu, H. B.; Yang, C. Y.; Luo, C.; Zhang, Y.; Chen, J. W.; Cao, Y. Appl. Phys. Lett. 2007, 90, 183512. (9) (a) Cho, S.; Coates, N.; Moon, J. S.; Park, S. H.; Roy, A.; Beaupre, S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photon. 2009, 3, 297. (b) Chen, H. Y.; Hour, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Li, G. Nat. Photon. 2009, 3, 649. (c) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135. (d) Huo, L.; Hou, J.; Zhang, S.; Chen, H. Y.; Yang, Y. Angew. Chem., Int. Ed. 2010, 49, 1500. (e) Hou, J.; Chen, H. Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130, 16144. (f) Zou, Y.; Naiari, A.; Berrouard, P.; Beaupre, S.; Arich, B. R.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2010, 132, 5330. (g) Wang, E.; Wang, L.; Luo, L.; Zhang, W.; Feng, J.; Cao, Y. Appl. Phys. Lett. 2008, 92, 033307.(h) www.konarka.com. (10) Kooistra, F. B.; Knol, J.; Kastenberg, F.; Popescu, L. M.; Verhees, W. J. H.; Kroon, J. M.; Hummelene, J. C. Org. Lett. 2007, 9, 551. (11) (a) Xu, Z.; Chen, L.-M.; Yang, G. W.; Huang, C.-H.; Hou, J. H.; Wu, Y.; Li, G.; Hsu, C.-S.; Yang, Y. Adv. Funct. Mater. 2009, 19, 1227. (b)

ARTICLE

Yang, C. D.; Kim, J. Y.; Cho, S.; Lee, J. K.; Heeger, A. J.; Wudl, F. J. Am. Chem. Soc. 2008, 130, 6444. (12) (a) Zhang, Y.; Yip, H.-L.; Acton, O.; Hau, S. K.; Huang, F.; Jen, A. K.-J. Chem. Mater. 2009, 21, 2598. (b) Popescu, L. M.; Hof, P. V.; Sieval, A. B.; Jonkman, H. T.; Hummelen, J. C. Appl. Phys. Lett. 2006, 89, 213507. (c) Troshin, P. A.; Hoppe, H.; Renz, J.; Egginger, M.; Yu, J. L.; Goryachev, A. E.; Peregudov, A. S.; Lyubovskaya, R. N.; Gobsch, G.; Sariciftci, N. S.; Razumov, V. F. Adv. Funct. Mater. 2009, 19, 779. (d) Choi, J. H.; Son, K.-I.; Kim, T.; Kim, K.; Ohkubo, K.; Fukuzumi, S. J. Mater. Chem. 2010, 20, 475. (e) Zhao, H. Y.; Guo, X. Y.; Tian, H. K.; Li, C. Y.; Xie, Z. Y.; Geng, Y. H.; Wang, F. S. J. Mater. Chem. 2010, 20, 3092. (13) Niinomi, T.; Matsuo, Y.; Hashiguchi, M.; Sato, Y.; Nakamura, E. J. Mater. Chem. 2009, 19, 5804. (14) Matsuo, Y.; Sato, Y.; Niinomi, T.; Soga, I.; Tanaka, H.; Nakamura, E. J. Am. Chem. Soc. 2009, 131, 16048. (15) (a) Renz, J. A.; Troshin, P. A.; Gobsch, G.; Razumov, V. F.; Hoppe, H. Phys. Status Solidi 2008, 2, 263. (b) Backer, S. A.; Sivula, K.; Kavulak, D. F.; Frechet, J. M. J. Chem. Mater. 2007, 19, 2927. (c) Park, S. H.; Yang, C.; Cowan, S.; Lee, J. K.; Wudl, F.; Lee, K.; Heeger, A. J. J. Mater. Chem. 2009, 19, 5624. (16) (a) Riedel, I.; Hauff, E. V.; Parisi, J.; Martín, N.; Giacalone, F.; Dyakonov, V. Adv. Funct. Mater. 2005, 15, 1979. (b) Zhao, G. J.; He, Y. J.; Xu, Z.; Hou, J. H.; Zhang, M. J.; Min, J.; Chen, H.-Y.; Ye, M. F.; Hong, Z. R.; Yang, Y.; Li, Y. F. Adv. Funct. Mater. 2010, 20, 1480. (17) Lenes, M.; Wetzelaer, G.-J. A. H.; Kooistra, F. B.; Veenstra, S. C.; Hummelen, K. J.; Blom, P. W. M. Adv. Mater. 2008, 20, 2116. (18) Lenes, M.; Shelton, S. W.; Sieval, A. B.; Kronholm, D. F.; Hummelen, J. C. K.; Blom, P. W. M. Adv. Funct. Mater. 2009, 19, 3002. (19) (a) Ross, R. B.; Cardona, C. M.; Guldi, D. M.; Sankkaranarayanan, S. G.; Reese, M. O.; Kopidakis, N.; Peet, J.; Walker, B.; Bazan, G.; Keuren, E. V.; Holloway, B. C.; Drees, M. Nat. Mater. 2009, 8, 208. (b) Ross, R. B.; Cardona, C. M.; Swain, F. B.; Guldi, D. M.; Sankaranarayanan, S. G.; Keuren, E. V.; Holloway, B. C.; Drees, M. Adv. Funct. Mater. 2009, 19, 2332. (20) He, Y.; Chen, H.-Y.; Hou, J.; Li, Y. J. Am. Chem. Soc. 2010, 132, 1377. (21) Zhao, G.; He, Y.; Li, Y. Adv. Mater. 2010, DOI: 10.1002/ adma.201001339. (22) Mikroyannidis, J. A.; Kabanakis, A. N.; Sharma, S. S.; Sharma, G. D. Adv. Funct. Mater. 2011, 21, 746. (23) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798. (24) Mikroyannidis, J. A.; Kabanakis, A. N.; Balraju, P.; Sharma, G. D. Macromolecules 2010, 43, 5544. (25) Mikroyannidis, J. A.; Sharma, S. S.; Vijay, Y. K.; Sharma, G. D. ACS Appl. Mater. Interfaces 2010, 2, 270. (26) Mikroyannidis, J. A.; Stylianakis, M. M.; Balraju, P.; Suresh, P.; Sharma, G. D. ACS Appl. Mater. Interfaces 2009, 1, 1711. (27) Mikroyannidis, J. A.; Stylianakis, M. M.; Suresh, P.; Balraju, P.; Sharma, G. D. Org. Electron. 2009, 10, 1320. (28) Mikroyannidis, J. A.; Kabanakis, A. N.; Kumar, A.; Sharma, S. S.; Vijay, Y. K.; Sharma, G. D. Langmuir 2010, 26, 12909. (29) Mikroyannidis, J. A.; Suresh, P.; Sharma, G. D. Org. Electron. 2010, 11, 311. (30) Lu, Q.; Schuster, D. I.; Wilson, S. R. J. Org. Chem. 1996, 61, 4764. (31) Kordatos, K.; Bosi, S.; Da Ros, T.; Zambon, A.; Lucchini, V.; Prato, M. J. Org. Chem. 2001, 66, 2802.  .; (32) Delgado, J. L.; Bouit, P.-A.; Filippone, S.; Herranza, M. A Martín, N. Chem. Commun. 2010, 46, 4853. (33) (a) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F. J. Am. Chem. Soc. 2006, 128, 4911. (b) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem. 2003, 13, 800. (34) Goodman, A. M.; Rose, A. J. Appl. Phys. 1971, 41, 2823. (35) Mikhnenko, O. V.; Cordella, F.; Sieval, A. B.; Hummelen, J. C.; Blom, P. W. M.; Loi, M. A. J. Phys. Chem. B 2009, 113, 9104. (36) Sakurai, T.; Toyoshima, S.; Kubota, M.; Yamanari, T.; Taima, T.; Saito, K.; Akimoto, K. Photon Factory Activity Rep. 2008, 25, 146. 7815

dx.doi.org/10.1021/jp201337u |J. Phys. Chem. C 2011, 115, 7806–7816

The Journal of Physical Chemistry C

ARTICLE

(37) Cullity, B. D.; Stock, S. R. Elements of X-ray Diffraction, 3rd ed.; Prentice Hall Inc.: New York, 2001, 167. (38) (a) Chen, M. Y.; Jeng, U. S.; Su, C. H.; Linag, K. S.; Wei, K. H. Adv. Mater. 2009, 20, 2573. (b) Venlaeke, P.; Swinnen, A.; Haeldermans, I.; Vanhoyland, G.; Aernouts, T.; Cheyns, D.; Deibel, C.; D’Haen, J.; Heremans, P.; Poortmans, J.; Manca, J. V. Sol. Energy Mater. Sol. Cells 2006, 90, 2150. (39) (a) Yao, Y.; Hou, J. H.; Xu, Z.; Li, G.; Yang, Y. Adv. Funct. Mater. 2008, 18, 1783. (b) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (c) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu., L. Adv. Mater. 2010, 22, E135. (d) Chan, S. H.; Hsiano, Y. S.; Hung, L. I.; Hwang, G. W.; Chen, H. L.; Ting, C.; Chen, C. P. Macromolecules 2010, 43, 3399.(e) Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.; Frechet, J. M. J. J. Am. Chem. Soc. DOI: 10.1021/ja108115y, (f) Jo, J.; Gendron, D.; Najari, A.; Moon, J. S.; Cho, S.; Leclerc, M.; Heeger, A. J. Appl. Phys. Lett. 2010, 97, 203303. (40) (a) 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. (b) Hoven, C. V.; Dang, X. D.; Coffin, R. C.; Peet, J.; Nguyen, T. Q.; Bazan, G. C. Adv. Mater. 2010, 22, E63. (41) Chen, H. Y.; Yang, H. C.; Yang, G. W.; Sista, S.; Zadoyan, R. B.; Li, G.; Yang, Y. J. Phys. Chem. C 2009, 113, 7946. (42) (a) Liu, J. G.; Shao, S. Y.; Wang, H. F.; Zhao, K.; Xue, L. J.; Gao, X.; Xie, Z. Y.; Han, Y. C. Org. Electron. 2010, 11, 775. (b) 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. (c) Hwang, I. W.; Cho, S.; Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2008, 104, 033706. (43) (a) Gupta, D.; Mukhopadhaya, S.; Narayan, K. S. Sol. Energy Mater. Sol. Cells 2008, 94, 1390.(b) Yum, M. H.; Kim, G. H.; Yang, C.; Kim, J. Y. J. Mater. Chem. DOI: 10.1039/c0jm00790k. (44) Moule, A. J.; Meerholz., K. Adv. Mater. 2008, 20, 240. (45) (a) Koster, L. J. A.; Smits, E. C. P.; Mihailetchi, V. D.; Blom, P. W. M. Phys. Rev. B 2005, 72, 085205. (b) Park, J. H.; Kim, J. S.; Lee, J. H.; Lee, W. H.; Cho, K. J. Phys. Chem. C 2009, 40, 17579. (46) (a) Lampert, M. A.; Mark, P. Current Injection in Solids; Academic Press: New York, 1970. (b) Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Phys. Rev. Lett. 2005, 94, 126602. (47) Kim, J. S.; Lee, J. H.; Park, J. H.; Shim, C.; Sim, M.; Cho, K. Adv. Funct. Mater. 2010, DOI: 10.1002/adfm.20100971. (48) Mihailetchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Adv. Funct. Mater. 2006, 16, 699. (49) (a) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617. (b) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864.

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