Cooperative Assembly Donor–Acceptor System Induced by

ARTICLE pubs.acs.org/JPCC

Cooperative Assembly Donor
Acceptor System Induced by Intermolecular Hydrogen Bonds Leading to Oriented Nanomorphology for Optimized Photovoltaic Performance Kai Yao,† Lie Chen,*,† Fan Li,† Peishan Wang,† and Yiwang Chen*,†,‡ †

Institute of Polymers, and ‡Institute of Advanced Study, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

bS Supporting Information ABSTRACT: A regioregular poly{[3-(60 -bromohexyl)thiophene]co-[[3-(60 -(1-imidazole) hexyl)thiophene]} (P3HTM) is synthesized via the Grignard metathesis route for the purpose of constructing higher-order supramolecules with self-assembly nanoscale morphology and stabilizing the film morphology in polymer photovoltaic cells. By designing the donor molecules containing imidazole rings and acceptor including carboxylic acids properly for the intermolecular interaction, one can control the complexes stacking induced by the intermolecular hydrogen bonds. The results from red-shifted absorption and enhanced quenching photoluminescence of the P3HTM:PCBA in solvents with polar additives indicate the building blocks through hydrogen-bonding interactions, which is consistent with the nanofibers domains in atomic force microscopy images. In strong contrast, processing of P3HTM and PCBA complexes with heatannealing, constructed from cooperative self-assembly, shows optimized photovoltaic performance, with a Jsc of 9.11 mA cm
2, a Voc of 0.67 V, and a FF of 51.6%; the PCE thus reached 3.2%. Besides, the achieved optimum nanomorphology after annealing can be frozen using the photo-cross-linking method to preserve long-term performance.

’ INTRODUCTION Polymer solar cells (PSCs) have attracted strong interest in recent years due to the prospect of low cost, solution-based processing and the capability to fabricate flexible devices.1 PSCs based on the concept of bulk heterojunction (BHJ) configuration where an active layer comprises a composite of a p-type (donor) and an n-type (acceptor) material represent the most useful strategy to maximize the internal donor
acceptor interfacial area allowing for efficient charge separation.2 However, current device efficiencies and stabilities require further improvement before they can become truly competitive with their inorganic counterparts. As the main factor heavily limits the efficiency and stability of PSC, nanomorphology of the photoactive layer is one of the important issues that has to be solved before going through the stages of industrial production and commercialization. Because the lifetime of the exciton is short, its diffusion length in organic materials is only about 10
20 nm.3 This means that the exciton must reach the D/A interface to give the charge transfer without undergoing radiative or nonradiative decay. Through appropriate control of the morphology, all excitons can be created within a diffusion length of a donor/acceptor interface, and hence be harvested. Therefore, control of the nanoscale morphology of the blend is critical to ensuring that all excitons are collected and dissociated. Once the exciton has dissociated, the free holes and electrons must then be transported through the donor and acceptor phases to their respective electrodes. Consequently, continuous percolation pathways are required through each phase.4 r 2011 American Chemical Society

The self-organization of organic molecules is an attractive approach for nanostructure fabrication, especially the desirable layered structures. For example, thiophene-based polymers, particular poly(3-hexylthiophene) (P3HT), have been extensively studied in the context of PSC because of their dual advantages of extended spectral sensitivity in the long wavelength part of the spectrum and their good charge carrier mobilities, which can be related to their good backbone planarity and high tendency to crystallize.5 This high tendency to crystallize adds to their ability to phase separate into defined heterojunction morphology when blended with electron acceptors like [6,6]phenyl C61-butyric acid methyl ester (PCBM). This crystallization induces phase separation between P3HT and PCBM molecules, pushing PCBM molecules further away from the film air interface, leading to a vertical structure.6 This interaction makes it possible to construct complicated higher-order structures and to realize multifunctional organic materials. In particular, the variety of the hydrogen bonds plays an essential role in controlling the complexity in both chemistry and biology and has also contributed to maximizing the potential of organic materials in chemical and bioengineering.7 Particularly, most polymer solar cells are not thermally stable as subsequent exposure to heat drives further development of the Received: August 11, 2011 Revised: November 20, 2011 Published: November 24, 2011 714

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Scheme 1. Synthesis and Representative Structure of Copolymer P3HTM by Postfunctionalization and PCBA

morphology toward a state of macrophase separation in the micrometer scale. In such case, improving the thermal stability of BHJ solar cells is important for the future application of these devices because any heat generated by solar irradiation could be detrimental to the performance of these devices.8 We construct supramolecules by attaching acidulated fullerene molecules to the polythiophene with imidazole side chains via hydrogen bonding and investigate critical parameters governing their assembly and photovoltaic properties. The supramolecular approach provides a new avenue to solution process organic semiconductors as well as to assemble them into stable nanoscopic structures in thin films.

well-known and defined precursor poly(3-bromohexylthiophenes) (P3HTBr). Regioregular polymer P3HTBr has been synthesized via the Grignard metathesis (GRIM) method with high molecular weight (Mn = 26.4 kg/mol) and optimizing crystallinity. Moreover, the final macromolecule was purified simply by precipitation, but not column chromatography as the synthesis of imidazole-containing monomers reported in the literature.10 In addition, the imidazole group included in the monomer could affect the homo coupling, due to the functional group incompatibility with nickel complexation.11 Attempts to homopolymerize the 3-(60 -(1-imidazole)hexyl)thiophene by the GRIM method to give a head-to-tail product failed in our hands with no product. Besides, the Yamamoto condition with Ni(0) catalysis also proved unsuccessful, and only a few oligomers were obtained (see Scheme S1 in the Supporting Information). The structure of copolymer P3HTM is confirmed by FT-IR, NMR (1H and 13C), and EA analysis. The integration area of the signals corresponding to methylene groups for bromide (Br
CH2
) and imidazole (Im
CH2
) moieties is used to calculate their composition/ratio. The molar ratio obtained for each group is (a:b) 20:80 (see Experimental Section in the Supporting Information for detailed analysis data). Moreover, it is observed that the synthesized copolymer P3HTM is soluble in

’ RESULTS AND DISCUSSION On the basis of the idea to utilize this phenomenon, we designed a novel polythiophene derivative with imidazole moieties, as shown in Scheme 1. The synthesis of head-to-tail HT-poly{[3-(60 -bromohexyl)thiophene]-co-[[3-(60 -(1-imidazole) hexyl)thiophene]} (P3HTM) was shown in Scheme 1. The compound 3-bromohexylthiophene was prepared according to a previously published process.9 The copolymer postfunctionalization strategy was selected because this process allows control of the molecular weight in the final macromolecule, because it starts from a 715

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Figure 2. Absorption maximum in UV
vis of P3HTM and the increasing factor of PL intensity (as compared to the intensity in CB) as a function of MeOH/CB ratio.

in polar solvents, such as methanol and ethanol, are observed. In chlorobenzene (CB) solution, the molecular orientations in the solutions are completely random. However, when the methanol is added and the proportion of polar solvents increases, the P3HTM forms hydrogen bonded with MeOH. These results reflect the differences in molecular orientation between nonpolar and polar solvents, indicated by the red-shifted absorption maximum. To demonstrate direct evidence of the formation of the intermolecular O
H 3 3 3 N hydrogen bonds in the deposited films, we measured the IR absorption spectra of the polymer treated with MeOH. The formation of the intermolecular hydrogen bonds as shown in Figure S1 causes the blue shift of the main peaks around 3000 cm
1, attributed to the C
H stretching modes, which are affected by the intermolecular hydrogen bonds.12 The same phenomenon is observed in the optical digital images and photoluminescence spectra (Figure 1b). Other polar solvents like 1-propyl alcohol (PrOH) and tetrahydrofuran (THF) are chosen to elaborate the effect of intermolecular bonds in the complexes. However, for the UV
vis absorption spectra of polymers (Figure S5), when we replace the CB:MeOH (1:27) with CB:PrOH or CB:THF, the significantly red-shift features are devastated, especially for the CB:THF with only a 3 nm redshift. It indicates that the more intensive polar solvent, MeOH, favors the molecular orientations. The fluorescence enhancement is observed due to transformation of the initially formed polymer aggregates into new species within which polymer segments are possibly separated by hydrogen bonds with the polar solvents for enhanced molecular orientation, confirmed by the enhanced diffraction peaks in XRD pattern (Figure S6). In terms of polarity parameter for the P3HTM, the absorption maximum in UV
vis and the increasing factor of PL intensity as a function of MeOH/CB ratio are summarized in Figure 2, by using the values of single CB solvent as the reference. Therefore, the tendency has been built by taking more values of MeOH/CB mixtures, and the optical properties are saturated at a ratio of 50:1 (MeOH/CB). With the red-shifted and enhanced absorption after annealing treatment, the imidazole rings attached at the end of the hexyl chain of P3HT do not appear to significantly disturb the π
π stacking of the polythiophene backbone. Considering the selfassembly properties of the P3HTM, we explore the supramolecular

Figure 1. Self-assembly in solution. (a) UV/vis spectra of copolymer P3HTM in chlorobenzene (CB) solution, chlorobenzene solution blending methanol (MeOH) as 1:3, 1:9, and 1:27, respectively (1  10
6 M), and its pure film. The inset shows their corresponding optical digital images of the P3HTM in different solutions (1  10
5 M). (b) Fluorescence spectra of polymer P3HTM in chlorobenzene solution, and chlorobenzene solution blending methanol as 1:3, 1:9, and 1:27, respectively (1  10
6 M).

the common organic solvents as the precursor P3HTBr (CHCl3, THF, CH2Cl2), and, interestingly, it is soluble in alcohols, like methanol. When we prolong the reaction time from 12 to 24 h, all of the bromine atoms are converted to imidazole moieties during the substitution reaction completely. However, this homopolymer is not like the case of copolymer and cannot dissolve in the common organic solvents except alcohols, the poor solvent for P3HTBr, indicating the strong interaction between imidazole moieties and alcohol molecules. After characterization of the synthesis process, it is checked whether the imidazole moieties in the polymer P3HTM kept the properties of thermal stability. As compared to polymer P3HTBr, the imidazole side chain slightly decreases the starting decomposition temperature of copolymer as shown in TGA thermograms (Figure S4), but the Td (5% weight loss temperature) remained as high as 342
C, implying the good thermal stability of P3HTM . The UV
vis absorption spectra of polymers in solutions are shown in Figure 1a; the maximum absorption wavelength at about 440 nm of P3HTBr is similar to that of P3HT, with an unobvious difference in the case of the P3HTM copolymer, which is associated with the π
π* transitions of the polythiophene main chain. Significant changes in the absorption spectra 716

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Figure 4. Tapping-mode AFM topographic images obtained from the surfaces of P3HTM:PCBA (1:0.8 w/w) films prepared: (a) as-spun using the CB as solvent and (b) as-spun using CB:MeOH (49:1) as solvent.

Figure 3. Mixed molecular assemblies. (a) UV/vis spectra and (b) fluorescence spectra of active layer P3HTM:PCBA (1:0.8 w/w) using the CB and CB/MeOH (49:1) as solvents under different treatments as-spun and after annealing at 130
C. Inset in (a) shows the UV/vis absorption of pure P3HTM film before and after annealing treatment.

organization between P3HTM and PCBA under the formation of the intermolecular O
H 3 3 3 N hydrogen bonds in the film. Blending the P3HTM:PCBA (1:0.8 w/w) in chlorobenzene originates the pristine film absorption spectra (Figure 3a) that are summations of the spectra of the single species in this solvent, a strong indication that both compounds are molecularly dissolved and no direct interaction exists. However, when the methanol (MeOH, 2% v/v) is added as processing additive,13 the red-shifted absorption maximum with enhanced intensity reveals the function of intermolecular bonds in the complexes. Meanwhile, when the content of MeOH is increased, the PCBA clusters begin to accumulate and then precipitate, which can be directly demonstrated by the TEM images (Figure S7 in the Supporting Information). Particular, the coassembly effect of P3HTM:PCBA has been confirmed using thermal annealing treatment. The red-shift as well as the increased absorption band, which repeats in the donor
acceptor complexes, features the stronger π
π stacking. This result is consistent with the fluorescence quenching (Figure 3b), indicating the formation of a self-assembly system with a more effective photoinduced electron-transfer process. In the P3HT:PCBM system, the crystallinity of the P3HT upon thermal annealing more than likely holds the key to the morphology behavior and the unusual diffusion behavior of the active layer, in which the PCBM must diffuse where the tie chains between the crystals limit the extent to which the PCBM can swell the P3HT.14 However, the introduction of the O
H 3 3 3 N

Figure 5. (a,b) TEM images of P3HTM:PCBA (1:0.8 w/w) films using the CB as solution under different treatments (a) as-cast and (b) after annealing at 130
C; and (c,d) TEM images of P3HTM:PCBA (1:0.8) films using the CB:MeOH (49:1) as solution under different treatments (c) as-cast and (d) after annealing at 130
C. The insets in (d) show the zoom-in image of the annealed sample with the corresponding SAED pattern.

hydrogen bonds in the P3HTM:PCBA system may possess a significant change to the diffusion of the PCBM. To investigate how the morphologies of the P3HTM:PCBA (1:0.8 w/w) films with hydrogen bonds evolved over annealing treatment, we employ AFM in the tapping mode to characterize their topographies. For direct comparison, we prepared the films for AFM analysis (Figure 4) under the same conditions used for device fabrication (ITO/PEDOT:PSS/active layer). Thin films fabricated by spin coating chlorobenzene solutions reveal a much rougher surface with root-mean-square (rms) roughness of 6.47 nm. In contrast, AFM height images of films spun from CB:MeOH (49:1) show fibers textures, suggesting that a small fraction of aggregates are present at this composition. The assembly structure formed in chlorobenzene and methanol solvent mixtures is quite similar to the fibrillar formed by hydrogen-bonded supramolecular aggregates.15 717

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Figure 6. (a) The device structure of polymer solar cells. (b) The energy level diagram of the corresponding components in the devices. (c) J
V characteristics of device (ITO/PEDOT:PSS/P3HTM:PCBA/LiF/Al) before and after annealing treatment with active layer using various solvents. (d) Incident photon-to-current efficiency (IPCE) of photovoltaic cells based on P3HTM:PCBA at different treatments.

The oriented coassembly morphologies with fibers of the annealed films are supported by the studies using top views scanning electron microscopy (Figure S8). Furthermore, the morphology of the active layer is verified by transmission electron microscopy (TEM) in Figure 5. For the blend casted in CB solution, the interpenetrating networks are not well developed, and the PCBM aggregation regions are very obvious. The additive MeOH induces the formation of network within the composite film and hinders the formation of excessively large PCBM aggregates. The strong interactions between the donor
acceptor provide sufficient driving force to keep the PCBA in P3HTM microdomains and prevent macrophase separation. Upon annealing, the random network tends to crystallize in one direction, which is significantly different with P3HT fibrils in P3HT/PCBM blends.16 Therefore, hydrogen-bond interaction between P3HTM and PCBA is proposed to be the essential driving force that induces the donor
acceptor cooperative assembly. The supramolecular organization between the complexes can achieve the orientated aggregated PCBA regions, resulting in two independent pathways for the respective charge carriers.6a The layer distance of P3HTM (about 2.1 nm) is consistent with the d-spacing value of the low-angle Bragg reflections (at 2θ = 4.05
) in the XRD diffractogram (Figure S9) of P3HTM:PCBA after annealing.17 The bulk heterojunction PSCs are fabricated with the device structure (Figure 6a) according to the method similar to previous reports (details in the Supporting Information). To accurately evaluate the PCEs of the photovoltaic devices, it is essential to

determine the energy levels of the materials correctly. Figure 6b displays the electronic energy level diagram of the device components, as well as the P3HTM:PCBA (1:0.8 w/w) photoactive layer of the device. The highest occupied molecular orbital (HOMO) energy level of P3HTM is determined by electrochemical cyclic voltammetry (Figure S10), and the LUMO is deduced from the UV
vis absorption onset.18 The P3HTM displays a HOMO energy level over 0.1 eV lower than the currently favored polymer, P3HT (
4.9 eV), implying that a higher Voc could be obtained than that of the P3HT-based devices (about 0.6 V). Different device fabrication conditions are tested, and the device performance data are summarized in Table 1. Figure 6c shows the typical current density
voltage ( J
V) characteristics under one sun of simulated AM 1.5G solar irradiation (100 mW cm
2) of the devices. The large phase separation of the untreated devices prepared using CB solvents with unfavorable extended pathways for the charge transport results in a low fill factor (FF) of 0.27 with a poor PCE of 0.57%. Optimized photovoltaic devices, which give a PCE of 3.2% with a Voc of 0.67 V, a Jsc of 9.11 mA cm
2, and a fill factor (FF) of 0.52, are obtained by spin-casting the blends in CB with 2% MeOH additive, and then allowing the devices to anneal at 130
C for one-half an hour. This demonstrates that morphology and oriented packing are subtle and important to achieve a better photovoltaic performance with high FF, which increases the electron
hole charge separation and transfer and suppresses recombination loss. Comparatively, corresponding results of the device with the P3HT:PCBA and P3HTM:PCBM active layer prepared in 718

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Table 1. Device Performance of P3HTM/PCBA (w/w 1:0.8) BHJ Solar Cells before and after Annealing Using Various Solvents (under AM 1.5, 100 mW/cm2 Irradiation)a Jsc (mA cm
2)b

Voc (V)c

FF (%)d

η (%)

CB

3.06

0.691

26.8

0.57

CB/MeOH (49:1)

5.86

0.683

41.3

1.65

CBe

6.96

0.685

45.3

2.16

CB/MeOH (49:1) e

9.11

0.672

51.6

3.16

P3HT:PCBAf P3HTM:PCBMf

1.71 2.32

0.503 0.694

31.0 38.7

0.27 0.63

device

a

All values represent averages from six 0.04 cm2 devices on a single chip. Jsc is the short-circuit current. c Voc is the open-circuit voltage. d The fill factor (FF) is a graphic measure of the squareness of the I
V curve. e The results for comparison are the device annealed at 130
C for 30mim. f The corresponding results of the device with the P3HT: PCBA (w/w 1:0.8) and P3HTM:PCBM (w/w 1:0.8) active layers prepared in CB/MeOH(49:1) are added as references. b

CB/MeOH(49:1) are added (details in Figure S11), where the effect of the intermolecular interaction between donor and acceptor has been shielded with normal materials, P3HT and PCBM, respectively. However, both devices show very poor performances (in Table 1). Thus, the hydrogen bonds involved in the system construct a delicate balance between PCBA self-assembly and crystallization of the P3HTM to develop the desired morphology. To further calibrate the Jsc data, incident photon-tocurrent efficiency (IPCE) spectra of the devices are measured (Figure 6d). The enhanced absorption response and the red-shift maxima after annealing are associated with the UV
vis curves. The trend indicates the dependence of the performance of BHJ solar cells on the intermolecular interaction between P3HTM and PCBA; then we investigated the absorption coefficient, molecular orientation, and photophysics of the same P3HTM: PCBA (1:0.8) BHJ films under different conditions. The thin films absorption spectra of P3HTM:PCBA prepared in CB and CB/MeOH (49:1) with or without annealing, which gives the best power conversion efficiency, are shown in Figure 7a. The absorption maxima and absorption coefficient vary with the preparation conditions of the active layer. Intermolecular O
H 3 3 3 N hydrogen-bond interaction between P3HTM and PCBA is proposed to promote donor
acceptor cooperative assembly upon annealed treatment, and therefore better interchain stacking is achieved in the mixture solvents by the fact of the enhanced maximum absorption coefficient and red-shift lineshapes, which agrees well with the enhanced photocurrent and the results of the IPCE curves of the devices. To gain further insight into the orientational control of the nanophase separation by cooperative assembly, the structure changes of the thin films are investigated by polarized UV
vis absorption spectroscopy. Through the polarization absorption of the annealed films casted in CB and CB/MeOH (49:1) at their maximum absorption peaks, respectively, we can conclude the long axis of the molecule arrangement.19 As shown in Figure 7b, the reduced absorbance of the π
π* band peaking at A^ direction and the increasing absorption at A direction indicate that the alignment of the bulk structures prepared in mixture solvents adopts a more oriented long axis alignment than those in CB solvents, where A^ and A are the absorbances perpendicular and parallel to the long axis direction, respectively. Additionally, time-resolved photoluminescence (TRPL) is conducted to analyze photophysics in the P3HTM:

Figure 7. (a) The absorption spectra of P3HTM:PCBA films measured directly from solar cells with the annealed P3HT:PCBM film as reference. (b) UV
vis absorption spectra of the P3HTM:PCBA film annealed prepared in CB and CB/MeOH (49:1) with linearly polarized incident light parallel or perpendicular to the long axis of the molecule direction, which is decided by the polarized absorption of corresponding films. The inset in (b) shows TRPL spectra, pumped at 400 nm and probed at 620 nm.

Table 2. Effect of Intermolecular Hydrogen Bonds on Molecular Orientation and the Parameters of P3HTM/PCBA Active Layer Films before and after Annealing Using Various Solvents prepared

order

extinction coefficient

PL

conditions

parameter Sa

(104 cm
1)b

lifetimec

CB

0.057

1.47

1.39 ns

CB annealedd

0.119

3.10

1.17 ns

CB/MeOH (49:1) annealed

0.328

4.12

0.89 ns )

)

An order parameter S is calculated by S = (A
A^)/(A + 2A^). b Measured from film absorption spectra at λmax. c The average lifetimes yielded from the curve fitting of TRPL spectra. d The film annealed at 130
C for 30 min. a

)

)

PCBA system. The exciton lifetime decreases (from 1.17 to 0.89 ns) with polar additive in the solvents. The improved charge separation at the P3HTM:PCBA interfaces may be attributed to more efficient electronic coupling between the polymer and fullerene and confirms the well-defined heterojunction morphology revealed by SEM and TEM images.20 We therefore infer that, under annealing conditions with cooperative effect, the optimizing active layer morphology enables the formation of higher-order 719

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and 26.8% to 9.11 mA cm
2 and 51.6%, respectively. By associating all of the morphology data in this study with the results of the bulk orientation measurement, the molecular alignment induced by the cooperative effect of intermolecular interaction strongly affects the PV performance of the organic devices with nanoscale microstructure. In addition, the deterioration of the photoconversion performance is suppressed in the polymer photovoltaic cells as compared to cells with noncross-linkable P3HT:PCBM.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text giving the experimental details, instrumentation, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Efficiencies of P3HT:PCBM and P3HTM:PCBA devices annealed at 130
C for different times. The P3HTM:PCBA blends experience with or without exposure to UV. The different devices are prepared under identical conditions except for the solvents.

structures for efficient exciton and charge transport, thereby improving the FF and PCE values. Parameters of the maximum absorption coefficient, order parameter S, and exciton lifetime are summarized in Table 2. Particularly, the remaining bromine-functionalized P3HTM not only maintains the solubility but also contains cross-linkable brominated unit.21 This can lead to a more stable PCE and smaller deterioration of the device performance in comparison to those of P3HT:PCBM devices. Photo-cross-linking is carried out under inert argon atmosphere using UV light (λ = 254 nm) from a low power hand-held lamp with an exposure time of about 30 min. The photo-cross-linking behavior of Br content is clearly confirmed by the insolubility of the film in CB, which is speculated via a radical mechanism initiated by the photochemical cleavage of the C
Br bonds under deep UV irradiation. To examine the use of photo-cross-linkable P3HTM for enhancing the stability of BHJ devices, devices made from P3HT and P3HTM are compared, and the results are shown in Figure 8. When devices are prepared without any exposure to UV light as a control experiment, both P3HT:PCBM and P3HTM:PCBA devices show similar initial performances. However, the P3HTM:PCBA blends treated by UV irradiation for 30 min show a completely different tendency with stable device performance (75% initial device efficiency after 40 h of annealing at 130
C).

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86 791 83969562. Fax: +86 791 83969561. E-mail: [email protected] (Y.C.); [email protected] (L.C.).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51073076, 51003045, 51172103, and 50902067). ’ REFERENCES (1) (a) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222–225. (b) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011, 133, 4625–4631. (c) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649–653. (2) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791. (b) Zhang, Y.; Yip, H.-L.; Acton, O.; Hau, S. K.; Huang, F.; Jen, A. K.-Y. Chem. Mater. 2009, 21, 2598–2600. (c) Huo, L.; Guo, X.; Zhang, S.; Li, Y.; Hou, J. Macromolecules 2011, 44, 4035–4037. (3) (a) Nunzi, J. M. C. R. Phys. 2002, 3, 523–542. (b) Shaw, P. E.; Ruseckas, A.; Samuel, I. D. W. Adv. Mater. 2008, 20, 3516–3520. (4) (a) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc. Rev. 2011, 40, 1185–1199. (b) Po, R.; Maggini, M.; Camaioni, N. J. Phys. Chem. C 2010, 114, 695–706. (5) (a) Ren, G.; Wu, P.-T.; Jenekhe, S. A. ACS Nano 2011, 5, 376–384. (b) Higashihara, T.; Ohshimizu, K.; Hirao, A.; Ueday, M. Macromolecules 2008, 41, 9505–9507. (c) Peet, J.; Heeger, A. J.; Bazan, G. C. Acc. Chem. Res. 2009, 42, 1700–1708. (d) Tada, A.; Geng, Y.; Wei, Q.; Hashimoto, K.; Tajima, K. Nat. Mater. 2011, 10, 450–455. (6) (a) Van Bavel, S. S.; Sourty, E.; De With, G.; Loos, J. Nano Lett. 2009, 9, 507–513. (b) Tsoi, W. C.; Spencer, S. J.; Yang, L.; Ballantyne, A. M.; Nicholson, P. G.; Turnbull, A.; Shard, A. G.; Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J.-S. Macromolecules 2011, 44, 2944–2952. (7) (a) Rancatore, B. J.; Mauldin, C. E.; Tung, S.-H.; Wang, C.; Hexemer, A.; Strzalka, J.; Frechet, J. M. J.; Xu, T. ACS Nano 2010, 4, 2721–2729. (b) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210–1250. (c) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38–68. (8) (a) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (b) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77. (9) Zhai, L.; Pilston, R. L.; Zaiger, K. L.; Stokes, K. K.; McCullough, R. D. Macromolecules 2003, 36, 61–64.

’ CONCLUSION We have synthesized a new P3HT derivative and have demonstrated a simple approach to control the molecular stacking via the intermolecular hydrogen bonds by changing the solvents. By blending with PCBA, intermolecular O
H 3 3 3 N hydrogen bonds form between the molecules in the film. By associating the UV
vis and PL data, the coassembly effect of P3HTM:PCBA after annealing treatment provides a more effective photoinduced electron-transfer process. More importantly, TEM images and AFM images of the morphology show that the supramolecules can effectively organize semiconductor nanoparticles into ordered arrays and the nanoscale donor/acceptor phase separation is achieved, which provides a viable and effective means to transport electron and hole as needed. As a result, BHJ solar cells based on P3HTM:PCBA show remarkably enhanced photovoltaic performance, with the Jsc and FF increasing from 3.06 mA cm
2 720

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(10) (a) Salinas-Castillo, A.; Camprubí-Robles, M.; Mallavia, R. Chem. Commun. 2010, 46, 1263–1265. (b) Li, Z.; Lou, X.; Yu, H.; Li, Z.; Qin, J. Macromolecules 2008, 41, 7433–7439. (11) McCullough, R. D.; Ewbank, P. C.; Loewe, R. S. J. Am. Chem. Soc. 1997, 119, 633–634. (12) (a) Yokoyama, D.; Sasabe, H.; Furukawa, Y.; Adachi, C.; Kido, J. Adv. Funct. Mater. 2011, 21, 1375–1382. (b) Yokoyama, D.; Setoguchi, Y.; Sakaguchi, A.; Suzuki, M.; Adachi, C. Adv. Funct. Mater. 2010, 20, 386–391. (13) (a) Lee, E.; Hammer, B.; Kim, J.-K.; Page, Z.; Emrick, T.; Hayward, R. C. J. Am. Chem. Soc. 2011, 133, 10390–10393. (b) Merlo, J. A.; Frisbie, C. D. J. Phys. Chem. B 2004, 108, 19169–19179. (14) (a) Chen, D.; Nakahara, A.; Wei, D.; Nordlund, D.; Russell., T. P. Nano Lett. 2011, 11, 561–567. (b) Yang, X. N.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nano Lett. 2005, 5, 579–583. (15) (a) Nozaki, K.; Kawashima, Y.; Oda, T.; Hiyama, T.; Kanie, K.; Kato, T. Macromolecules 2002, 35, 1140–1142. (b) Jonkheijm, P.; Stutzmann, N.; Chen, Z.; de Leeuw, D. M.; Meijer, E. W.; Schenning, A. H. J.; Wurthner, F. J. Am. Chem. Soc. 2006, 128, 9535–9540. (16) Chiu, M.-Y.; Jeng, U.-S.; Su, M.-S.; Wei, K.-H. Macromolecules 2010, 43, 428–432. (17) (a) Zhang, F. Q.; Meng, Y.; Gu, D.; Yan, Y.; Chen, Z. X.; Tu, B.; Zhao, D. Y. Chem. Mater. 2006, 18, 5279–5288. (b) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Adv. Funct. Mater. 2005, 15, 1193–1196. (18) (a) Chen, C.-H.; Hsieh, C.-H.; Dubosc, M.; Cheng, Y.-J.; Hsu, C.-S. Macromolecules 2010, 43, 697–708. (b) Zhou, H.; Yang, L.; Xiao, S.; Liu, S.; You, W. Macromolecules 2010, 43, 811–820. (19) (a) Okano, K.; Mikami, Y.; Hidaka, M.; Yamashita, T. Macromolecules 2011, 44, 5605–5611. (b) Nishizawa, T.; Hadykesuma, L.; Tajima, K.; Hashimoto, K. J. Am. Chem. Soc. 2009, 131, 2464–2465. (20) De, S.; Pascher, T.; Maiti, M.; Jespersen, K. G.; Kesti, T.; Zhang, F. L.; Inganas, O.; Yartsev, A.; Sundstrom, V. J. Am. Chem. Soc. 2007, 129, 8466–8472. (21) (a) Kim, B. J.; Miyamoto, Y.; Ma, B.; Frechet, J. M. J. Adv. Funct. Mater. 2009, 19, 2273–2281. (b) Miyanishi, S.; Tajima, K.; Hashimoto, K. Macromolecules 2009, 42, 1610–1618.

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dx.doi.org/10.1021/jp207704u |J. Phys. Chem. C 2012, 116, 714–721