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aJiangsu Key Laboratory of Advanced Func

Department of Polymer Science and Engin

Engineering and Materials Science, Sooch

China. E-mail: ywli@suda.edu.cn; tuy

65882130; Tel: +86 512 65882130bi-Lab Suzhou Institute of Nano-Tech and Na

Sciences, Suzhou, 215123, P. R China. E-ma

† Electronic supplementary informationcharacterizations of SMs; TGA, electrocsurfaces of SMs; AFM images of SM:PC7

electrochemical properties and ph10.1039/c3nr03048b

Cite this: DOI: 10.1039/c3nr03048b

Received 13th June 2013Accepted 5th August 2013

DOI: 10.1039/c3nr03048b

www.rsc.org/nanoscale

This journal is ª The Royal Society of

Acceptor–donor–acceptor-based small molecules withvaried crystallinity: processing additive-inducednanofibril in blend film for photovoltaic applications†

Chao Li,a Yujin Chen,a Yue Zhao,a Huifang Wang,a Wei Zhang,a Yaowen Li,*ab

Xiaoming Yang,a Changqi Ma,b Liwei Chen,*b Xiulin Zhua and Yingfeng Tu*a

A series of acceptor–donor–acceptor-based small molecules (SMs)

with varied crystallinity were successfully synthesized. The processing

additive can induce the SMs to self-organize as nanofibrils with

higher crystallinity and controlled scales of nanofibrils, which have

significant influence on the photovoltaic performance.

The self-assembly of semiconducting p-conjugated materialshas been intensively studied during the past 20 years.1,2 Specicinterest has been placed on polythiophenes, especially onpoly(3-hexylthiophene) (P3HT), due to good solubility in organicsolvents and the relatively higher charge mobility.3 Recently, theself-assembled 1D nanostructures of P3HT have been known tolargely enhance the crystallinity, mechanical exibility, and thespeed of charge transport, compared with the 2D thin lmcounterparts.4,5 The directional transport of the holes along theP3HT nanobrils led to increased carrier mobility in thin lmtransistors. When employed in bulk-heterojunction (BHJ)organic solar cells (OSCs), the P3HT nanobrils led to remark-able improvement in the power conversion efficiency (PCE).6,7

Because the nanobrils with nanoscale phase separation canprompt excitons to reach donor–acceptor interfaces and becollected, where the photogenerated excitons can diffuse intheir diffusion lengths (5–20 nm),8 and charge carriers canreach the electrodes and internal order by continuous path-ways. In these previous studies, p-conjugated polymers were

tional Polymer Design and Application,

eering, College of Chemistry, Chemical

ow University, Suzhou, 215123, P. R.

ingfeng@suda.edu.cn; Fax: +86 512

no-Bionics (SINANO), Chinese Academy of

il: lwchen2008@sinano.ac.cn

(ESI) available: Synthetic process andhemical properties, molecular orbital

1BM blend lms; EQE curves; optical,otovoltaic parameters. See DOI:

Chemistry 2013

normally used to obtain nanobrils. A limited number ofstudies have been reported on small molecule (SM)-basednanobrils in blend lms,9 although SMs exhibit the advan-tages of high purity, favourable solution processability, well-dened molecular structures and denite molecular weight.Furthermore, one-step formation of nanobrils in blend lmshas been accomplished with only polymer components.10 Thedifficulty of controlling the aggregation and packing structuresof SMs accounts for their limited use as nanobrils in blendlms, and spontaneous in situ formation of nanobrils of SMsin blend lms is a challenging task.

Acceptor–donor–acceptor (A–D–A) molecules, which are atype of donor–acceptor (D–A) structure molecule, have attractedmore and more interest due to their outstanding photovoltaicperformance11–14 because the A–D–A molecules possessextended conjugated length, good tunable energy levels andrelatively higher crystallization through delicate chemicalstructure design and modication. Additionally, the processingadditive plays an important role in controlling the morphologyof the BHJ active layer due to its selective (differential) solubilityof the fullerene component and higher boiling point than thehost solvent. Such a combination of properties leads tocontrollable aggregation of donor materials in solution, whichtranslates to an increase in the sizes and enhanced crystallinityof the BHJ domains in the lm.15

Herein we report a series of A–D–A-based SMs (T(TDPPT)2,TT(TDPPT)2 and BDT(TDPPT)2) by designing their coplanarconformation and unambiguously demonstrate that theypossess varied crystallinity and morphologies when blendedwith PC71BM. The self-organization and crystallization of SMsin SM:PC71BM blend lm can be further regulated with theprocessing additive 1,8-diiodooctane (DIO), which givesenhanced crystallization and controllable scales of nanobrilscompared to lms spun from pristine solvent. This method wasused to fabricate the active layer of BHJ OSCs, where both theshort circuit current density (Jsc) and ll factor (FF) were obvi-ously improved, suggesting that these self-organized highlyordered nanobril blend lms can facilitate charge separation

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Fig. 2 UV-vis absorption spectra of SMs in chloroform solution (a) and in film (b).

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and transportation, which has a good perspective for thefabrication of nanobril-based active layers.

T(TDPPT)2, TT(TDPPT)2 and BDT(TDPPT)2 were synthesizedby the palladium(0)-catalyzed Stille coupling reactions betweena central electron-donating moiety and different electron-accepting moieties. The chemical structures are shown in Fig. 1,and the detailed synthetic procedure and characterizations ofcompounds are described in the ESI (Scheme S1†). This series ofsmall molecules possess varied molecular conformations,which exhibit different dihedral angles (Fig. 1). The order ofcoplanarity is BDT(TDPPT)2 > T(TDPPT)2 > TT(TDPPT)2. Theyalso show good solubility in common organic solvents, andexhibit good thermal stability with 5% weight loss temperatures(Td) higher than 350 �C under N2, as revealed by thermogravi-metric analysis (TGA) (Fig. S1†). The UV-vis absorptions of thethree SMs are compared, as shown in Fig. 2a, and the maximumabsorption wavelength and the absorption edge are almost thesame in solution. This indicates that the intensities of theintramolecular charge transfer (ICT) interaction between D andA moieties are similar,16 which is not affected by the variedconformation of molecules. The red-shi in the lm state of allSMs (Fig. 2b) as compared to the solution state (Fig. 2a) suggeststhat strong p–p stacking due to the intermolecular interactionin the solid state of the SMs is more favorable than in thesolution. The low optical band gaps (Eg,opt) of the SMs, whichare estimated from the absorption edges of the thin lms,further conrm their potential applications in solar cells. TheBDT(TDPPT)2 shows an increase in ionization potential ofapproximately 0.1 eV, in comparison to that of T(TDPPT)2 andTT(TDPPT)2, probably as a result of larger resonance stabiliza-tion energy of the fused ring over one or two thiophene rings.17

The cyclic voltammetry (CV) diagrams of the SMs wereobtained by using n-Bu4NPF6 as supporting electrolyte inacetonitrile solution (Fig. S2†). It reveals the similar highestoccupied molecular orbital (HOMO) and lowest unoccupiedmolecular orbital (LUMO) of the SMs are �5.20–5.30 eV and�3.30–3.44 eV vs. ferrocene/ferricinium, respectively (see TableS1†). The tendency of energy levels is in good agreement withthose calculated at the B3LYP/6-31G* level (Fig. S3†).18 Thissuggests that the varied conformation of molecules does notcontribute to the voltammogram.

Differential scanning calorimetry (DSC) experiments werecarried out to test the crystallization of three SMs, and the

Fig. 1 Chemical structures and optimized geometries of T(TDPPT)2, TT(TDPPT)2and BDT(TDPPT)2.

Nanoscale

crystallization temperature of T(TDPPT)2, TT(TDPPT)2 andBDT(TDPPT)2 is 152.9 �C, 160.8 �C, and 171.2 �C, respectively(Fig. 3a). The corresponding enthalpies of crystallization are28.83 J mol�1, 41.29 J mol�1 and 30.73 J mol�1, and reveal thatTT(TDPPT)2 has the best crystallization ability due to the variedmolecular conformations. Blend lms containing SMs andPC71BM component were examined by out-of-plane X-raydiffraction (XRD), which was used to further determine thecrystallization and stacking mode of SMs when blended withPC71BM.

As shown in Fig. 3b, in comparison with the broad and weakdiffraction peak of T(TDPPT)2:PC71BM and BDT(TDPPT)2:PC71BMblend lms, the TT(TDPPT)2:PC71BM blend lm shows a sharpand strong diffraction peak at q ¼ 0.42 A corresponding to the(100) orientation for the TT(TDPPT)2 lamellar organized crystal-lite, which indicates that TT(TDPPT)2 possesses higher crystal-linity and ordered arrangement. These results may be caused bythe disruption of the formation of the low energy component withthe presence of PC71BM

19 and thus suppression of SM crystalli-zation. Addition of 1–2 vol% DIO in chlorobenzene to the solventwas used to cast the SM:PC71BM blend lms. It is interesting tond that all the SM:PC71BM blend lms show obvious diffractionpeaks and increased diffraction intensity, which indicates that theprocessing additive can signicantly enhance crystallinity ofSM:PC71BM blend lms and thus increase their degree of order.Particularly in the TT(TDPPT)2:PC71BM blend lms, the (100)diffraction peak appears at the same diffraction angle and is moresharp for the highly ordered lamellar orientation compared withthat before adding DIO. More importantly, the lamellar d-spacing(1.5 nm) of the TT(TDPPT)2:PC71BM blend lm is more closelypacked than the other blend lms and that of P3HT:PCBM, whichis benecial for charge transport between components.20 This canbe explained that the processing additive allows the SMs to

Fig. 3 (a) The first-cooling and second-heating DSC scans of pure SMs at the rateof 10 �C min�1; (b) the out-of-plane XRD spectra of SM:PC71BM blend films andSM:PC71BM with DIO blend films.

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crystallize more completely by providing increased drying timeduring the kinetically limited spin-coating process.19 Thisphenomenon is consistent with the P3HT:PCBM system for highperformance photovoltaic devices.21,22

Due to the additive having different inuences on the SMs,with varied crystallization and further self-organization,atomic force microscopy (AFM) and transmission electronmicroscopy (TEM) were used to further investigate the micro-and nanostructures of the SM:PC71BM blend lms with andwithout processing additive. The SM:PC71BM blend lmswithout additive generally show relatively low roughness,which indicates that the SMs have relatively good miscibilitywith PC71BM (Fig. S4d–f†). The gradually increasing roughnessfrom 0.38 nm, 0.52 nm to 2.35 nm may be caused by thedifferent molecular conformations and crystallization of SMsconsequently affected their miscibility and interactions withPC71BM. This result can be further conrmed by the increasedgrain sizes in the TEM images (Fig. 4a–c), resulting in low holemobility across the entire lm due to multiple crosses overgrain boundaries.20 Interestingly, the blend lm microstruc-tures have been affected by the processing additive (DIO).SM:PC71BM blend lms with DIO processing show obviouschanges in the varied scales of the nanobrils of SM crystal-lites compared with that without DIO processing (Fig. 4a–c),which are broadly distributed in this image with an optimummorphology for high-performance BHJ PSCs.10 The formationof crystallites can be further conrmed by the increasedroughnesses from the AFM images (Fig. S4g–i†). In this study,the T(TDPPT)2:PC71BM blend lm with DIO processing showsrelatively smaller T(TDPPT)2 crystal nanobrils (in the range of4 nm width), good phase separation and some continuousT(TDPPT)2-rich islands in the TEM image (Fig. 4d). In contrast,TT(TDPPT)2:PC71BM and BDT(TDPPT)2:PC71BM blend lmswith DIO processing show crystal nanobrils with respectivewidths of 10–15 nm and 15–20 nm and lengths of 30–100 nmand 70–180 nm. It is noted that pure SM lms and SM:PC71BMblend lms (Fig. S4a–f†) do not show any bril-like

Fig. 4 TEM images obtained from the blend films spin-coated from 10 mg mL�1

chlorobenzene solution: (a) T(TDPPT)2:PC71BM; (b) TT(TDPPT)2:PC71BM; (c)BDT(TDPPT)2:PC71BM; (d) T(TDPPT)2:PC71BM with DIO; (e) TT(TDPPT)2:PC71BMwith DIO; (f) BDT(TDPPT)2:PC71BM with DIO.

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morphology, which indicates that the PC71BM is not thedriving force for formation of nanobrils. The nanobrilsappeared only when DIO was added to the mixture ofSM:PC71BM. Meanwhile, XRD data shows that the crystallinityof the SMs has been greatly increased aer adding DIO. Theformation of the crystal nanobrils and their growth in thethree blend lms aer adding DIO reveal that the self-orga-nization of SMs are very sensitive to varied molecular confor-mation, crystallization, evaporation rate and solubilityparameter of the solvent. The processing additive can providea ne adjustment of the crystal nanobrils according to themolecular conformation and crystallization. Considering theprevious results and the mechanism of molecular crystalliza-tion and the processing additive, a speculative self-organizingprocess for crystal nanobrils is suggested as follows: Theformation of nanobrils are mainly caused by the crystalliza-tion of the SM and self-organization between SMs andPC71BM. Therefore, a free region and sufficient time for crys-tallization and self-organization are necessary. DIO dissolvesPC71BM more readily than SMs, and is a solvent with highboiling point, which can extend the evaporation time of thesolvent and hence provides sufficient time for self-organiza-tion. Thus, processing additives provide a medium that ismore uid and allows the chains to nd optimal conforma-tions. A molecule with higher crystallization and coplanaritypossesses a stronger driving force for p–p stacking and self-organization ability.

The photovoltaic properties of the SM nanobrils wereevaluated by fabricating BHJ OSCs with a conventional devicestructure (ITO/PEDOT:PSS/SM:PC71BM/LiF/Al). The detaileddevice fabrication process is described in the ESI.† Fig. 5 showsthe current density–voltage (J–V) characteristics of the devices,and the corresponding photovoltaic parameters are given inTable S2.†

The optimized BHJ solar cells based on SM:PC71BM withoutDIO processing show similar poor PCE and larger series resis-tance (Rs) and lower shunt resistance (Rsh), which may becaused by the lower phase separation of T(TDPPT)2:PC71BM andTT(TDPPT)2:PC71BM, and large “grain” of BDT(TDPPT)2 withdisrupted boundaries. With DIO processing, the devices showsignicantly improved photovoltaic performance due to theformation of crystal nanobrils and good phase separation,23

which have been demonstrated by our previous results. It isinteresting to nd that the Jsc of device based on SM:PC71BMblend lm with DIO processing is signicantly affected (in

Fig. 5 Current–voltage characteristics of photovoltaic cells based on SM:PC71BMin (a) chlorobenzene without DIO and (b) chlorobenzene with DIO.

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the range of 5.22–7.48 mA cm�2) by the varied scales ofcrystal nanobrils, which did not have an impact on the open-circuit voltage (Voc). The gradually improved Jsc of the devicesbased on BDT(TDPPT)2:PC71BM, TT(TDPPT)2:PC71BM andT(TDPPT)2:PC71BM indicate that the nanobrils with smallerscale and moderate crystallinity provided larger interfacialcontact areas between donor and acceptor materials, which canfacilitate the charge separation and transportation,24 althoughthe TT(TDPPT)2:PC71BM blend lm possesses the nanobrilswith highest degree of crystallinity. The varied Jsc and ll factor(FF) can be further conrmed by the mobility of electrons andholes in the blends, because they are important parameters forboth the ability and balance of charge carrier transportation.The T(TDPPT)2:PC71BM and TT(TDPPT)2:PC71BM blend lmswith DIO processing show relatively higher hole mobility of2.4 � 10�5 cm2 V�1 s�1 and 7.4 � 10�5 cm2 V�1 s�1 (Fig. S5 andTable S3†), respectively, which can be attributed to the highercrystallinity of T(TDPPT)2 and TT(TDPPT)2 in the blend lms asindicated from the XRD data. In the case of electron mobility ofthe blend lms, the T(TDPPT)2:PC71BM blend lm with largerinterfacial contact areas shows the highest electron mobility.The generally higher hole and electron mobilities of theT(TDPPT)2:PC71BM blend lm give better charge transportationability and hence the highest Jsc. The ratios between the electronand hole mobilities are away from unity, i.e., 2.0, 0.2 and1.9 respectively for T(TDPPT)2:PC71BM, TT(TDPPT)2:PC71BMand BDT(TDPPT)2:PC71BM blend lms with DIO processing,resulting in unbalanced charge transport in the BHJ OSCs basedon these blend lms, and nally giving low FF. Meanwhile,T(TDPPT)2:PC71BM and BDT(TDPPT)2:PC71BM blend lmsexhibit higher FF compared with that of TT(TDPPT)2:PC71BMblend lm, whichmay be caused by their better balanced chargetransport properties. Therefore, these results rmly conrm ourspeculation that the processing additive can induce SMswith varied crystallinity to controllably self-organize with goodphase separation, higher nanoscale crystallinity and improvednanobrils, and give a general guideline for optimizingactive layer morphology of BHJ OSCs. The external quantumefficiency (EQE) curves of SM:PC71BM with DIO processingunder monochromatic light are shown in Fig. S6.† The EQEcurves exhibit a broad response covering 350–800 nm, and thecalculated Jsc values from the integrals of EQE curved areconsistent with those of J–V measurement. It can be seen fromthis gure that the EQE spectra of the devices resemble theabsorption spectra of the blend lms (Fig. S7†) used in thedevice, indicating both components used in the blendcontribute to the photocurrent. It is noted that the EQE in thewavelength region 450–700 nm is broader than the absorptionspectra of the blend lms. This can be ascribed to the contri-bution of crystallized SM nanobrils, which are more efficientphotogenerated charge carriers, especially in the wavelengthregion of 450–550 nm. Furthermore, the varied nanostructuresand crystallinities of the nanobrils also inuence the photo-current. T(TDPPT)2:PC71BM blend lm with a smaller scale andmoderate crystallinity, providing larger interfacial contactareas, shows a larger value compared to the other two blendlms across a broad spectral range.

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Conclusions

In summary, we successfully synthesized a series of A–D–A-based SMs with varied crystallinity. DIO processing can providean efficient method for the self-organization of crystallized SMswithout being disrupted by the appearance of PC71BM. DIO canalso induce the SMs with varied crystallinity to form nanobrilswith higher crystallinity and controllable length. The self-organized ordered smaller nanobrils of T(TDPPT)2:PC71BMblend lm give efficient charge separation and transportation,therefore leading to higher Jsc and PCE.

Acknowledgements

We acknowledge the nancial support from the National NatureScience Foundation of China (grant no. 21204057), JiangsuProvincial Natural Science Foundation of China (grant no.BK2012213), Research Fund for the Doctoral Program of HigherEducation of China (grant no. 20113201120004), Open Projectof State Key Laboratory of Supramolecular Structure andMaterials (grant no. sklssm201333), the Project of InternationalCooperation of the Ministry of Science and Technology of China(2011DFA50530) and Priority Academic Program Developmentof Jiangsu Higher Education Institutions.

Notes and references

1 A. O. Patil, A. J. Heeger and F. Wudl, Chem. Rev., 1988, 88,183–200.

2 V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier,R. Silbey and J.-L. Bredas, Chem. Rev., 2007, 107, 926–952.

3 H. Xin, O. G. Reid, G. Ren, F. S. Kim, D. S. Ginger andS. A. Jenekhe, ACS Nano, 2010, 4, 1861–1872.

4 H. Yang, T. J. Shin, L. Yang, K. Cho, C. Y. Ryu and Z. Bao, Adv.Funct. Mater., 2005, 15, 671–676.

5 R. Zhang, B. Li, M. C. Iovu, M. Jeffries-El, G. Sauve, J. Cooper,S. Jia, S. Tristram-Nagle, D. M. Smilgies, D. N. Lambeth,R. D. McCullough and T. Kowalewski, J. Am. Chem. Soc.,2006, 128, 3480–3481.

6 B.-G. Kim, M.-S. Kim and J. Kim, ACS Nano, 2010, 4, 2160–2166.

7 F. S. Kim, G. Ren and S. A. Jenekhe, Chem. Mater., 2010, 23,682–732.

8 S. Gunes, H. Neugebauer and N. S. Saricici, Chem. Rev.,2007, 107, 1324–1338.

9 C.-Y. Chang, C.-E. Wu, S.-Y. Chen, C. Cui, Y.-J. Cheng,C.-S. Hsu, Y.-L. Wang and Y. Li, Angew. Chem., Int. Ed.,2011, 50, 9386–9390.

10 X. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees,M. M. Wienk, J. M. Kroon, M. A. J. Michels andR. A. J. Janssen, Nano Lett., 2005, 5, 579–583.

11 Z. Li, G. He, X. Wan, Y. Liu, J. Zhou, G. Long, Y. Zuo,M. Zhang and Y. Chen, Adv. Energy Mater., 2012, 2, 74–77.

12 S. Loser, C. J. Bruns, H. Miyauchi, R. o. P. Ortiz, A. Facchetti,S. I. Stupp and T. J. Marks, J. Am. Chem. Soc., 2011, 133, 8142–8145.

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13 J. Zhou, X. Wan, Y. Liu, G. Long, F. Wang, Z. Li, Y. Zuo, C. Liand Y. Chen, Chem. Mater., 2011, 23, 4666–4668.

14 Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You,Y. Yang and Y. Chen, Adv. Energy Mater., 2011, 1, 771–775.

15 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses,A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497–500.

16 Y. Li, L. Xue, H. Li, Z. Li, B. Xu, S. Wen and W. Tian,Macromolecules, 2009, 42, 4491–4499.

17 I. Mcculloch, M. Heeney, C. Bailey, K. Genevicius,I. Macdonald, M. Shkunov, D. Sparrowe, S. Tierney,R. Wagner, W. M. Zhang, M. L. Chabinyc, R. J. Kline,M. D. Mcgehee and M. F. Toney, Nat. Mater., 2006, 5, 328–333.

This journal is ª The Royal Society of Chemistry 2013

18 Y. Li, B. Xu, H. Li, W. Cheng, L. Xue, F. Chen, H. Lu andW. Tian, J. Phys. Chem. C, 2011, 115, 2386–2397.

19 J. Peet, N. S. Cho, S. K. Lee and G. C. Bazan, Macromolecules,2008, 41, 8655–8659.

20 H. X. Zhou, L. Q. Yang andW. You,Macromolecules, 2012, 45,607–632.

21 A. Pivrikas, P. Stadler, H. Neugebauer and N. S. Saricici,Org. Electron., 2008, 9, 775–782.

22 G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang and Y. Yang, Adv.Funct. Mater., 2007, 17, 1636–1644.

23 L. M. Chen, Z. R. Hong, G. Li and Y. Yang, Adv. Mater., 2009,21, 1434–1449.

24 P. Zhang, C. Li, Y. Li, X. Yang, L. Chen, B. Xu, W. Tian andY. Tu, Chem. Commun., 2013, 49, 4917–4919.

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