Cooperative Assembly Donor–Acceptor System Induced by Intermolecular Hydrogen Bonds Leading to Oriented Nanomorphology for Optimized Photovoltaic Performance

Published: November 24, 2011

r 2011 American Chemical Society 714 dx.doi.org/10.1021/jp207704u | J. Phys. Chem. C 2012, 116, 714–721

ARTICLE

pubs.acs.org/JPCC

Cooperative Assembly Donor�Acceptor System Induced byIntermolecular Hydrogen Bonds Leading to OrientedNanomorphology for Optimized Photovoltaic PerformanceKai 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

’ INTRODUCTION

Polymer solar cells (PSCs) have attracted strong interest inrecent years due to the prospect of low cost, solution-based pro-cessing and the capability to fabricate flexible devices.1 PSCsbased on the concept of bulk heterojunction (BHJ) configurationwhere an active layer comprises a composite of a p-type (donor)and an n-type (acceptor) material represent themost useful strat-egy to maximize the internal donor�acceptor interfacial areaallowing for efficient charge separation.2 However, current deviceefficiencies and stabilities require further improvement beforethey can become truly competitive with their inorganic counterparts.

As the main factor heavily limits the efficiency and stability ofPSC, nanomorphology of the photoactive layer is one of theimportant issues that has to be solved before going through thestages of industrial production and commercialization. Becausethe lifetime of the exciton is short, its diffusion length in organicmaterials is only about 10�20 nm.3 This means that the excitonmust reach the D/A interface to give the charge transfer withoutundergoing radiative or nonradiative decay. Through appropriatecontrol of the morphology, all excitons can be created within adiffusion length of a donor/acceptor interface, and hence beharvested. Therefore, control of the nanoscale morphology of theblend is critical to ensuring that all excitons are collected anddissociated. Once the exciton has dissociated, the free holes andelectrons must then be transported through the donor and acce-ptor phases to their respective electrodes. Consequently, con-tinuous percolation pathways are required through each phase.4

The self-organization of organic molecules is an attractiveapproach for nanostructure fabrication, especially the desirablelayered structures. For example, thiophene-based polymers,particular poly(3-hexylthiophene) (P3HT), have been exten-sively studied in the context of PSC because of their dualadvantages of extended spectral sensitivity in the long wavelengthpart of the spectrum and their good charge carrier mobilities,which can be related to their good backbone planarity and hightendency to crystallize.5 This high tendency to crystallize addsto their ability to phase separate into defined heterojunctionmorphology when blended with electron acceptors like [6,6]-phenyl C61-butyric acid methyl ester (PCBM). This crystalliza-tion induces phase separation between P3HT and PCBM mole-cules, pushing PCBM molecules further away from the film airinterface, leading to a vertical structure.6 This interaction makesit possible to construct complicated higher-order structures andto realize multifunctional organic materials. In particular, thevariety of the hydrogen bonds plays an essential role in control-ling the complexity in both chemistry and biology and has alsocontributed to maximizing the potential of organic materials inchemical and bioengineering.7

Particularly, most polymer solar cells are not thermally stableas subsequent exposure to heat drives further development of the

Received: August 11, 2011Revised: November 20, 2011

ABSTRACT:Aregioregular poly{[3-(60-bromohexyl)thiophene]-co-[[3-(60-(1-imidazole) hexyl)thiophene]} (P3HTM) is syn-thesized via the Grignard metathesis route for the purpose ofconstructing higher-order supramolecules with self-assemblynanoscale morphology and stabilizing the film morphology inpolymer photovoltaic cells. By designing the donor moleculescontaining imidazole rings and acceptor including carboxylicacids properly for the intermolecular interaction, one cancontrol the complexes stacking induced by the intermolecularhydrogen bonds. The results from red-shifted absorption and enhanced quenching photoluminescence of the P3HTM:PCBA insolvents with polar additives indicate the building blocks through hydrogen-bonding interactions, which is consistent with thenanofibers domains in atomic force microscopy images. In strong contrast, processing of P3HTM and PCBA complexes with heat-annealing, constructed from cooperative self-assembly, shows optimized photovoltaic performance, with a Jsc of 9.11mA cm�2, aVoc

of 0.67 V, and a FF of 51.6%; the PCE thus reached 3.2%. Besides, the achieved optimum nanomorphology after annealing can befrozen using the photo-cross-linking method to preserve long-term performance.

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

The Journal of Physical Chemistry C ARTICLE

morphology toward a state of macrophase separation in themicrometer scale. In such case, improving the thermal stability ofBHJ solar cells is important for the future application of thesedevices because any heat generated by solar irradiation could bedetrimental to the performance of these devices.8 We constructsupramolecules by attaching acidulated fullerene molecules tothe polythiophene with imidazole side chains via hydrogen bond-ing and investigate critical parameters governing their assemblyand photovoltaic properties. The supramolecular approachprovides a new avenue to solution process organic semiconduc-tors as well as to assemble them into stable nanoscopic structuresin thin films.

’RESULTS AND DISCUSSION

On the basis of the idea to utilize this phenomenon, we desig-ned a novel polythiophene derivative with imidazole moieties, asshown 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 compound3-bromohexylthiophene was prepared according to a previouslypublished process.9 The copolymer postfunctionalization strategywas selected because this process allows control of the molecularweight in the final macromolecule, because it starts from a

well-known and defined precursor poly(3-bromohexylthiophenes)(P3HTBr). Regioregular polymer P3HTBr has been synthesizedvia the Grignard metathesis (GRIM) method with high mole-cular weight (Mn = 26.4 kg/mol) and optimizing crystallinity.Moreover, the final macromolecule was purified simply byprecipitation, but not column chromatography as the synthesisof imidazole-containing monomers reported in the literature.10

In addition, the imidazole group included in the monomer couldaffect the homo coupling, due to the functional group incompat-ibility with nickel complexation.11 Attempts to homopolymerizethe 3-(60-(1-imidazole)hexyl)thiophene by the GRIMmethod togive a head-to-tail product failed in our hands with no product.Besides, the Yamamoto condition with Ni(0) catalysis alsoproved 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 ofthe signals corresponding to methylene groups for bromide(Br�CH2�) and imidazole (Im�CH2�) moieties is used tocalculate their composition/ratio. The molar ratio obtained foreach group is (a:b) 20:80 (see Experimental Section in theSupporting Information for detailed analysis data). Moreover, itis observed that the synthesized copolymer P3HTM is soluble in

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



the common organic solvents as the precursor P3HTBr (CHCl3,THF, CH2Cl2), and, interestingly, it is soluble in alcohols, likemethanol.Whenwe prolong the reaction time from 12 to 24 h, allof the bromine atoms are converted to imidazole moieties duringthe substitution reaction completely. However, this homopoly-mer is not like the case of copolymer and cannot dissolve in thecommon organic solvents except alcohols, the poor solvent forP3HTBr, indicating the strong interaction between imidazolemoieties and alcohol molecules. After characterization of thesynthesis process, it is checked whether the imidazole moieties inthe polymer P3HTM kept the properties of thermal stability. Ascompared to polymer P3HTBr, the imidazole side chain slightlydecreases the starting decomposition temperature of copolymeras shown in TGA thermograms (Figure S4), but the Td (5%weight loss temperature) remained as high as 342 �C, implyingthe good thermal stability of P3HTM .

The UV�vis absorption spectra of polymers in solutions areshown in Figure 1a; the maximum absorption wavelength atabout 440 nm of P3HTBr is similar to that of P3HT, with anunobvious difference in the case of the P3HTM copolymer,which is associated with the π�π* transitions of the polythio-phene main chain. Significant changes in the absorption spectra

in polar solvents, such as methanol and ethanol, are observed. Inchlorobenzene (CB) solution, the molecular orientations in thesolutions are completely random. However, when the methanolis added and the proportion of polar solvents increases, theP3HTM forms hydrogen bonded with MeOH. These resultsreflect the differences in molecular orientation between nonpolarand polar solvents, indicated by the red-shifted absorption maxi-mum. To demonstrate direct evidence of the formation of theintermolecular O�H 3 3 3N hydrogen bonds in the depositedfilms, wemeasured the IR absorption spectra of the polymer trea-ted with MeOH. The formation of the intermolecular hydrogenbonds as shown in Figure S1 causes the blue shift of the mainpeaks around 3000 cm�1, attributed to the C�H stretchingmodes, which are affected by the intermolecular hydrogenbonds.12 The same phenomenon is observed in the optical digitalimages and photoluminescence spectra (Figure 1b). Other polarsolvents like 1-propyl alcohol (PrOH) and tetrahydrofuran (THF)are chosen to elaborate the effect of intermolecular bonds in thecomplexes. However, for the UV�vis absorption spectra ofpolymers (Figure S5), when we replace the CB:MeOH (1:27)with CB:PrOH or CB:THF, the significantly red-shift featuresare devastated, especially for the CB:THF with only a 3 nm red-shift. It indicates that the more intensive polar solvent, MeOH,favors the molecular orientations. The fluorescence enhance-ment is observed due to transformation of the initially formedpolymer aggregates into new species within which polymer seg-ments are possibly separated by hydrogen bonds with the polarsolvents for enhanced molecular orientation, confirmed by theenhanced diffraction peaks in XRD pattern (Figure S6). In termsof polarity parameter for the P3HTM, the absorption maximumin UV�vis and the increasing factor of PL intensity as a functionof MeOH/CB ratio are summarized in Figure 2, by using thevalues of single CB solvent as the reference. Therefore, the ten-dency has been built by taking more values of MeOH/CB mix-tures, and the optical properties are saturated at a ratio of 50:1(MeOH/CB).

With the red-shifted and enhanced absorption after annealingtreatment, the imidazole rings attached at the end of the hexylchain of P3HT do not appear to significantly disturb the π�πstacking of the polythiophene backbone. Considering the self-assembly properties of the P3HTM, we explore the supramolecular

Figure 1. Self-assembly in solution. (a) UV/vis spectra of copolymerP3HTM in chlorobenzene (CB) solution, chlorobenzene solutionblending methanol (MeOH) as 1:3, 1:9, and 1:27, respectively (1 �10�6 M), and its pure film. The inset shows their corresponding opticaldigital 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).

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



organization between P3HTM and PCBA under the formationof the intermolecular O�H 3 3 3N hydrogen bonds in the film.Blending the P3HTM:PCBA (1:0.8 w/w) in chlorobenzeneoriginates the pristine film absorption spectra (Figure 3a) thatare summations of the spectra of the single species in this solvent,a strong indication that both compounds are molecularly dis-solved and no direct interaction exists. However, when themethanol (MeOH, 2% v/v) is added as processing additive,13 thered-shifted absorption maximum with enhanced intensity revealsthe function of intermolecular bonds in the complexes. Mean-while, when the content of MeOH is increased, the PCBA clus-ters begin to accumulate and then precipitate, which can bedirectly demonstrated by the TEM images (Figure S7 in theSupporting Information). Particular, the coassembly effect ofP3HTM:PCBA has been confirmed using thermal annealingtreatment. The red-shift as well as the increased absorption band,which repeats in the donor�acceptor complexes, features thestronger π�π stacking. This result is consistent with thefluorescence quenching (Figure 3b), indicating the formationof a self-assembly system with a more effective photoinducedelectron-transfer process.

In the P3HT:PCBM system, the crystallinity of the P3HTupon thermal annealing more than likely holds the key to themorphology behavior and the unusual diffusion behavior of theactive layer, in which the PCBMmust diffuse where the tie chainsbetween the crystals limit the extent to which the PCBM canswell the P3HT.14 However, the introduction of the O�H 3 3 3N

hydrogen bonds in the P3HTM:PCBA system may possess asignificant change to the diffusion of the PCBM. To investigatehow the morphologies of the P3HTM:PCBA (1:0.8 w/w) filmswith hydrogen bonds evolved over annealing treatment, we employAFM 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 spincoating chlorobenzene solutions reveal a much rougher surfacewith root-mean-square (rms) roughness of 6.47 nm. In contrast,AFM height images of films spun from CB:MeOH (49:1) showfibers textures, suggesting that a small fraction of aggregates arepresent at this composition. The assembly structure formed inchlorobenzene and methanol solvent mixtures is quite similar tothefibrillar formedbyhydrogen-bonded supramolecular aggregates.15

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

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

Figure 5. (a,b) TEM images of P3HTM:PCBA (1:0.8 w/w) films usingthe CB as solution under different treatments (a) as-cast and (b) afterannealing 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 thezoom-in image of the annealed sample with the corresponding SAEDpattern.



The oriented coassembly morphologies with fibers of the annea-led films are supported by the studies using top views scanningelectron microscopy (Figure S8).

Furthermore, the morphology of the active layer is verified bytransmission electron microscopy (TEM) in Figure 5. For theblend casted in CB solution, the interpenetrating networks arenot well developed, and the PCBM aggregation regions are veryobvious. The additive MeOH induces the formation of networkwithin the composite film and hinders the formation of exces-sively large PCBM aggregates. The strong interactions betweenthe donor�acceptor provide sufficient driving force to keep thePCBA in P3HTMmicrodomains and prevent macrophase separa-tion. Upon annealing, the random network tends to crystallize inone direction, which is significantly different with P3HT fibrils inP3HT/PCBM blends.16 Therefore, hydrogen-bond interactionbetween P3HTM and PCBA is proposed to be the essentialdriving force that induces the donor�acceptor cooperativeassembly. The supramolecular organization between the com-plexes can achieve the orientated aggregated PCBA regions,resulting in two independent pathways for the respective chargecarriers.6a The layer distance of P3HTM(about 2.1 nm) is consistentwith the d-spacing value of the low-angle Bragg reflections (at 2θ =4.05�) in the XRD diffractogram (Figure S9) of P3HTM:PCBAafter annealing.17

The bulk heterojunction PSCs are fabricated with the devicestructure (Figure 6a) according to themethod similar to previousreports (details in the Supporting Information). To accuratelyevaluate the PCEs of the photovoltaic devices, it is essential to

determine the energy levels of the materials correctly. Figure 6bdisplays the electronic energy level diagram of the device com-ponents, as well as the P3HTM:PCBA (1:0.8 w/w) photoac-tive layer of the device. The highest occupied molecular orbital(HOMO) energy level of P3HTM is determined by electro-chemical cyclic voltammetry (Figure S10), and the LUMO isdeduced from the UV�vis absorption onset.18 The P3HTMdisplays a HOMO energy level over 0.1 eV lower than thecurrently favored polymer, P3HT (�4.9 eV), implying that ahigher Voc could be obtained than that of the P3HT-baseddevices (about 0.6 V). Different device fabrication conditionsare tested, and the device performance data are summarized inTable 1. Figure 6c shows the typical current density�voltage( J�V) characteristics under one sun of simulated AM 1.5G solarirradiation (100 mW cm�2) of the devices. The large phase sepa-ration of the untreated devices prepared using CB solvents withunfavorable extended pathways for the charge transport results ina low fill factor (FF) of 0.27 with a poor PCE of 0.57%. Opti-mized photovoltaic devices, which give a PCE of 3.2% with a Vocof 0.67 V, a Jsc of 9.11 mA cm�2, and a fill factor (FF) of 0.52, areobtained by spin-casting the blends in CB with 2% MeOHadditive, and then allowing the devices to anneal at 130 �C forone-half an hour. This demonstrates that morphology and orien-ted packing are subtle and important to achieve a better photo-voltaic performance with high FF, which increases the electron�hole charge separation and transfer and suppresses recombina-tion loss. Comparatively, corresponding results of the device withthe P3HT:PCBA and P3HTM:PCBM active layer prepared in

Figure 6. (a) The device structure of polymer solar cells. (b) The energy level diagram of the corresponding components in the devices. (c) J�Vcharacteristics 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.



CB/MeOH(49:1) are added (details in Figure S11), where theeffect of the intermolecular interaction between donor and acce-ptor has been shielded with normal materials, P3HT and PCBM,respectively. However, both devices show very poor perfor-mances (in Table 1). Thus, the hydrogen bonds involved inthe systemconstruct a delicate balance betweenPCBA self-assemblyand crystallization of the P3HTM to develop the desired mor-phology. To further calibrate the Jsc data, incident photon-to-current efficiency (IPCE) spectra of the devices are measured(Figure 6d). The enhanced absorption response and the red-shiftmaxima after annealing are associated with the UV�vis curves.

The trend indicates the dependence of the performance ofBHJ solar cells on the intermolecular interaction between P3HTMand 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 thinfilms absorption spectra of P3HTM:PCBA prepared in CB andCB/MeOH (49:1) with or without annealing, which gives thebest power conversion efficiency, are shown in Figure 7a. Theabsorption maxima and absorption coefficient vary with the pre-paration conditions of the active layer. Intermolecular O�H 3 3 3N hydrogen-bond interaction between P3HTM and PCBA isproposed to promote donor�acceptor cooperative assemblyupon annealed treatment, and therefore better interchain stack-ing is achieved in the mixture solvents by the fact of the enhancedmaximum absorption coefficient and red-shift lineshapes, whichagrees well with the enhanced photocurrent and the results of theIPCE curves of the devices. To gain further insight into the orien-tational control of the nanophase separation by cooperativeassembly, the structure changes of the thin films are investigatedby polarized UV�vis absorption spectroscopy. Through thepolarization absorption of the annealed films casted in CB andCB/MeOH (49:1) at their maximum absorption peaks, respec-tively, we can conclude the long axis of the molecule arrange-ment.19 As shown in Figure 7b, the reduced absorbance of theπ�π* band peaking at A^ direction and the increasing absorp-tion at A ) direction indicate that the alignment of the bulk struc-tures prepared in mixture solvents adopts a more oriented longaxis alignment than those in CB solvents, whereA^ andA ) are theabsorbances perpendicular and parallel to the long axis direction,respectively. Additionally, time-resolved photoluminescence(TRPL) is conducted to analyze photophysics in the P3HTM:

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

Figure 7. (a) The absorption spectra of P3HTM:PCBA films measureddirectly from solar cells with the annealed P3HT:PCBM film asreference. (b) UV�vis absorption spectra of the P3HTM:PCBA filmannealed prepared in CB and CB/MeOH (49:1) with linearly polarizedincident light parallel or perpendicular to the long axis of the moleculedirection, which is decided by the polarized absorption of correspondingfilms. The inset in (b) shows TRPL spectra, pumped at 400 nm andprobed at 620 nm.

Table 1. Device Performance of P3HTM/PCBA (w/w 1:0.8)BHJ Solar Cells before and after Annealing Using VariousSolvents (under AM 1.5, 100 mW/cm2 Irradiation)a

device 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 1.71 0.503 31.0 0.27

P3HTM:PCBMf 2.32 0.694 38.7 0.63aAll values represent averages from six 0.04 cm2 devices on a single chip.b Jsc is the short-circuit current.

c Voc is the open-circuit voltage.dThe

fill factor (FF) is a graphic measure of the squareness of the I�V curve.eThe results for comparison are the device annealed at 130 �Cfor 30mim. fThe corresponding results of the device with the P3HT:PCBA (w/w 1:0.8) and P3HTM:PCBM (w/w 1:0.8) active layersprepared in CB/MeOH(49:1) are added as references.

Table 2. Effect of Intermolecular Hydrogen Bonds onMolecular Orientation and the Parameters of P3HTM/PCBAActive Layer Films before and after Annealing Using VariousSolvents

prepared

conditions

order

parameter Saextinction coefficient

(104 cm�1)bPL

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 nsaAn order parameter S is calculated by S = (A ) � A^)/(A ) + 2A^).bMeasured from film absorption spectra at λmax.

cThe average lifetimesyielded from the curve fitting of TRPL spectra. dThe film annealed at130 �C for 30 min.



structures for efficient exciton and charge transport, therebyimproving the FF and PCE values. Parameters of the maximumabsorption coefficient, order parameter S, and exciton lifetime aresummarized in Table 2.

Particularly, the remaining bromine-functionalized P3HTMnot only maintains the solubility but also contains cross-linkablebrominated unit.21 This can lead to a more stable PCE andsmaller deterioration of the device performance in comparison tothose of P3HT:PCBM devices. Photo-cross-linking is carried outunder inert argon atmosphere using UV light (λ = 254 nm) froma low power hand-held lamp with an exposure time of about 30min. The photo-cross-linking behavior of Br content is clearlyconfirmed by the insolubility of the film in CB, which is specu-lated via a radical mechanism initiated by the photochemicalcleavage of the C�Br bonds under deep UV irradiation. To exa-mine the use of photo-cross-linkable P3HTM for enhancing thestability of BHJ devices, devices made from P3HT and P3HTMare compared, and the results are shown in Figure 8. When devi-ces are prepared without any exposure to UV light as a controlexperiment, both P3HT:PCBM and P3HTM:PCBA devicesshow similar initial performances. However, the P3HTM:PCBAblends treated by UV irradiation for 30 min show a completelydifferent tendency with stable device performance (75% initialdevice efficiency after 40 h of annealing at 130 �C).

’CONCLUSION

We have synthesized a new P3HT derivative and have demon-strated a simple approach to control the molecular stacking viathe intermolecular hydrogen bonds by changing the solvents. Byblendingwith PCBA, intermolecularO�H 3 3 3Nhydrogen bondsform between the molecules in the film. By associating theUV�vis and PL data, the coassembly effect of P3HTM:PCBAafter annealing treatment provides a more effective photoin-duced electron-transfer process. More importantly, TEM imagesand AFM images of the morphology show that the supramole-cules can effectively organize semiconductor nanoparticles intoordered arrays and the nanoscale donor/acceptor phase separa-tion is achieved, which provides a viable and effective means totransport electron and hole as needed. As a result, BHJ solar cellsbased on P3HTM:PCBA show remarkably enhanced photovoltaicperformance, with the Jsc and FF increasing from 3.06 mA cm�2

and 26.8% to 9.11 mA cm�2 and 51.6%, respectively. By asso-ciating all of the morphology data in this study with the results ofthe bulk orientation measurement, the molecular alignment in-duced by the cooperative effect of intermolecular interactionstrongly affects the PV performance of the organic devices withnanoscale microstructure. In addition, the deterioration of thephotoconversion performance is suppressed in the polymerphotovoltaic cells as compared to cells with noncross-linkableP3HT:PCBM.

’ASSOCIATED CONTENT

bS Supporting Information. Text giving the experimentaldetails, instrumentation, and characterization. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’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 ScienceFoundation of China (51073076, 51003045, 51172103, and50902067).

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Figure 8. Efficiencies of P3HT:PCBM and P3HTM:PCBA devicesannealed at 130 �C for different times. The P3HTM:PCBA blendsexperience with or without exposure to UV. The different devices areprepared under identical conditions except for the solvents.



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