Journal ofMaterials Chemistry A

PAPER

Publ

ishe

d on

04

Febr

uary

201

3. D

ownl

oade

d by

Pur

due

Uni

vers

ity o

n 23

/05/

2013

12:

56:0

9.

View Article OnlineView Journal | View Issue

aNational Center for Nanoscience and Tec

OPV.CHINA@yahoo.com; xiaoz@nanoctr.cnbUniversity of Chinese Academy of Sciences,

† Electronic supplementary informationincluding synthesis, measurements10.1039/c3ta01525d

‡ D. He and X. Du contributed equally to

Cite this: J. Mater. Chem. A, 2013, 1,4589

Received 17th December 2012Accepted 4th February 2013

DOI: 10.1039/c3ta01525d

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Improving the stability of P3HT/PC61BM solar cells by athermal crosslinker†

Dan He,‡ab Xiaoyan Du,‡ab Wei Zhang,a Zuo Xiao*a and Liming Ding*a

Stabilizing the optimal film morphology of bulk heterojunction polymer solar cells is key for realizing long-

term stable devices. A crosslinker, octane-1,8-diyl bis(1,4-dihydrobenzo[d][1,2]oxathiine-6-carboxylate

3-oxide) (OBOCO), was designed to crosslink PC61BM through a heat-triggered Diels–Alder reaction to

suppress the aggregation tendency of fullerene molecules, thus helping to stabilize the film

morphology and to improve device stability. Devices containing OBOCO (0–25% wt) in the active layer

were fabricated and studied. Open-circuit voltage (Voc) shows an interesting increasing tendency as the

OBOCO content increases from 0% to 20%. Short-circuit current (Jsc) continuously decreases as the

crosslinker content increases. Fill factor (FF) remains stable (>60%) at low OBOCO content (0–15%) but

decreases quickly at high OBOCO content (20–25%). The power conversion efficiency (PCE) shows a

decreasing tendency with OBOCO content increasing. Atomic force microscopy (AFM) indicates that the

nanoscale polymer/fullerene phase separation is retained with 5% addition of OBOCO but deteriorates

when more crosslinker is added. Space Charge Limited Current (SCLC) measurements indicate that the

electron mobility of fullerene continuously decreases while the hole mobility of P3HT remains stable as

the OBOCO content increases, suggesting that the crosslinking took place at the fullerene domains.

After the devices were heated at 150 �C for 4 days, the PCE of the devices without OBOCO decreased

significantly from 2.74% to 0.78%, while that of the devices with 5% OBOCO decreased from 2.77% to

1.72%. Morphological studies indicate that the OBOCO crosslinker can effectively impede heat-

promoted fullerene aggregation, thus leading to a stable morphology and stable device performance.

Introduction

Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on apolymer donor and a fullerene acceptor have received consid-erable attention for the advantages of low-cost, light-weight,mechanical exibility, and roll-to-roll fabrication.1 In recentyears, many studies have focused on pursuing high powerconversion efficiency (PCE) of PSCs by developing new low-bandgap conjugated polymers,2 high LUMO fullerene accep-tors,3 and interfacialmaterials.4 The PCEs of PSCs have exceeded8%.5 A key feature for an efficient polymer solar cell is thebicontinuous architecture in its active layer, in which polymerand fullerene present phase separation with domain size at 10–20 nm.6 This bicontinuous nano-structure not only maximizesthe donor–acceptor interfaces, but also offers fast pathways forcharge carrier transport and collection. However, such optimalmorphology is metastable and will degrade gradually even at

hnology, Beijing 100190, China. E-mail:

Beijing 100049, China

(ESI) available: Experimental details, and instruments. See DOI:

this work.

Chemistry 2013

ambient temperature. It has been demonstrated that nano-scalefullerene domains tend to grow intomicro-scale domains due tothe aggregation tendency of fullerene molecules.7 When solarcells are working, such an aggregation process acceleratesbecause of sunlight heating, leading to reduction of donor–acceptor interface and device performance deterioration.Therefore, to realize long-term stability of PSCs for practicalapplication, it is essential to stabilize their optimal lmmorphology.8 An effective strategy is to “freeze” the initial nano-structure of the active layer by crosslinking reactions. A numberof crosslinkable groups such as halide, azide, oxirane, oxetane,and acrylates have been installed into the side chains of thepolymer or fullerene.9 The crosslinking reactions can be acti-vated by external stimuli such as heat, UV light, or chemicals. Asa result, very stablelmmorphology and long device lifetime canbe realized for these “crosslinked” devices. However, drawbacksaccompanied these crosslinkable materials. First, the introduc-tion of crosslinkable groups leads to tedious and expensivesynthesis of the polymer and fullerene materials. Second, themodication of the side chains of polymer and fullerene maysignicantly change their original phase behavior, leading tounfavored phase separation and low device performance.9a–g Tocircumvent these problems, we propose a new strategy by addinga crosslinkable additive into a polymer/fullerene blend. Such

J. Mater. Chem. A, 2013, 1, 4589–4594 | 4589

Journal of Materials Chemistry A Paper

Publ

ishe

d on

04

Febr

uary

201

3. D

ownl

oade

d by

Pur

due

Uni

vers

ity o

n 23

/05/

2013

12:

56:0

9.

View Article Online

facilely prepared crosslinkers are designed to react with thepolymer or fullerene and help to stabilize the lm morphologywithout interrupting polymer/fullerene phase separation.10

Herein, we report a prototype of such crosslinkers, octane-1,8-diyl bis(1,4-dihydrobenzo[d][1,2]oxathiine-6-carboxylate 3-oxide)(OBOCO), which can crosslink fullerene through a heat-trig-gered Diels–Alder reaction. By adding 5% (wt) OBOCO into aP3HT/PC61BM blend, the devices show signicantly improvedthermal stability compared with the untreated cells and show nodecrease in PCE. Variation of device performance parameters,active layer morphology and charge carrier mobilities with theOBOCO content will be discussed.

Results and discussion

Fullerenes show similar reactivity as electron-decient olensand can be easily functionalized through Diels–Alder (D–A)reaction with o-quinodimethane type dienes.11 OBOCO wasdesigned to be a dumbbell-like crosslinker with two sultinefragments, the o-quinodimethane precursor, at the ends and aexible alkyl chain as the spacer to connect them (Scheme 1).Over 80 �C, the sultine moiety can release SO2 to produceo-quinodimethane diene, which can react with fullerene.12

Therefore, the crosslinking can take place during the thermalannealing process. The synthetic route for OBOCO is shown inScheme 1. Condensation of 3,4-dimethylbenzoic acid 1 with 1,8-octanediol afforded compound 2 in 74% yield. Bromination ofthe four methyl groups of compound 2 with N-bromosuccini-mide (NBS) and further annulation reaction with rongaliteafforded the target molecule OBOCO in 36% yield.13,14 MALDI-TOF mass spectra show the expected molecular ion peak forOBOCO. Both 1H and 13C NMR spectra indicate the existence ofOBOCO regioisomers resulting from the switching of the

Scheme 1 Synthesis of octane-1,8-diyl bis(1,4-dihydrobenzo[d][1,2]oxathiine-6-carboxylate 3-oxide) (OBOCO) and the scheme for the crosslinking reaction.

4590 | J. Mater. Chem. A, 2013, 1, 4589–4594

locations of S and O atoms on the oxathiane ring (Fig. S5 andS6†). The regioisomerism was further conrmed by synthesis ofa simple methyl ester analogue (see ESI†).

To prove the reaction between OBOCO and fullerene(PC61BM) during the thermal annealing process, we studied theabsorption spectra changes for PC61BM/OBOCO (1.7 : 1, w/w,equimolar) and PC61BM-only lms aer annealing the lms at150 �C for 20 min (Fig. 1). As shown in Fig. 1a, the characteristicabsorption peak of PC61BM at �338 nm disappeared for thePC61BM/OBOCO lm but remained for the PC61BM-only lmaer the thermal treatment. The heat-triggered D–A reactionbetween OBOCO and PC61BM alters the fullerene p-conjugationsystem, leading to the disappearance of the PC61BM charac-teristic absorption. The same absorption spectra were obtainedfor the lm before and aer rinsing with o-dichlorobenzene(ODCB), indicating the crosslinked PC61BM/OBOCO lmexhibits good solvent-resistant behavior. However, the thermaltreatment of the P3HT/OBOCO (1.7 : 1, w/w) lm neitherchanged the characteristic absorption of P3HT nor made thelm solvent-resistant (Fig. 1b). The above results demonstratethat OBOCO can selectively crosslink fullerenes in the activelayer.

To evaluate the effect of OBOCO on device performance,solar cells with a conventional structure, ITO/PEDOT:PSS/

Fig. 1 Effects of OBOCO crosslinker on the characteristic absorption and solvent-resistant behavior of (a) PC61BM and (b) P3HT films. NA: not annealed; A:annealed at 150 �C for 20 min.

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

Publ

ishe

d on

04

Febr

uary

201

3. D

ownl

oade

d by

Pur

due

Uni

vers

ity o

n 23

/05/

2013

12:

56:0

9.

View Article Online

P3HT:PC61BM:OBOCO/Ca/Al, were fabricated. The weight ratiobetween OBOCO and the P3HT/PC61BM blend varied from 0%to 25%. The P3HT/PC61BM weight ratio was 1 : 1. The blendsolution was spin-coated onto the PEDOT:PSS layer and wasannealed at 150 �C for 20 min to ensure complete crosslinking.Fig. 2 shows the changes of device performance parameterswith OBOCO content. In Fig. 2a, Voc continuously increasesfrom 0.636 V to 0.684 V as OBOCO content increases from 0% to20%. When the OBOCO content reaches 25%, Voc drops to0.643 V. Voc is proportional to the energy level differencebetween the donor HOMO and the acceptor LUMO.15 The initialincrease of Voc with OBOCO content can be attributed to theraising of the fullerene LUMO level caused by crosslinking,since the D–A reaction on PC61BM shrinks fullerene p-conju-gation and decreases the fullerene electron affinity, thus raisingthe fullerene LUMO level.16 However, the insulating additivecould also affect the polymer/fullerene phase separation andcharge carrier transport. Such negative effects may becomesignicant when a large amount of OBOCO is added. Low Voc at25% OBOCO content might result from the reduction of chargecarrier concentration in the active layer. The decrease of Jsc withOBOCO content reects the negative effects of OBOCO ondevice performance (Fig. 2b). Jsc decreases from 6.92 mA cm�2

to 1.10 mA cm�2 as OBOCO content increases from 0% to 25%.FF reaches 65.5% at 5% OBOCO content and remains over 60%till the OBOCO content reaches 15% (Fig. 2c). FF drops quicklywhen more OBOCO is added and is only 43.5% at 25% OBOCOcontent. The low Jsc and FF at high OBOCO content suggest thattoo much insulating additive severely disrupts charge carriertransport. The PCE of the solar cells decreases with increasingOBOCO content (Fig. 2d). By adding 5% OBOCO, the PCE(2.77%) is almost identical to that of the solar cells withoutcrosslinker (2.74%) indicating that the negative effect of thecrosslinker is negligible at this content.

To gure out the effect of OBOCO on lm morphology andcharge carrier transport, we investigated the changes of lmmorphology and electron/hole mobility of the active layer with

Fig. 2 Variation of device performance parameters with OBOCO content. (a) Voc,(b) Jsc, (c) FF, (d) PCE.

This journal is ª The Royal Society of Chemistry 2013

OBOCO content by AFM and SCLC, respectively. From AFMheight images, the root-mean-square (RMS) roughnesses of thelms with 0%, 5%, 10%, 15%, 20%, and 25% OBOCO contentare 0.66 nm, 1.03 nm, 4.56 nm, 6.22 nm, 10.50 nm, and 11.30nm, respectively, indicating that the addition of OBOCO dras-tically increases the roughness of the lm (Fig. 3). Phase imagesindicate the lms with 0% and 5% OBOCO show ne nanoscalemorphology with well-developed bicontinuous interpenetratingnetworks, suggesting that the polymer–fullerene phase separa-tion is not interrupted by 5%OBOCO. HighOBOCO content (10–25%) induces signicant aggregation of PC61BM in the activelayer and deteriorates device performance. To nd out whetherthe morphology changes are caused by the additive itself or bycrosslinking reactions, we tried compound 2 (0–25%), which hasa similar structure as OBOCO but can not crosslink PC61BM.AFM indicates that compound 2 slightly affects the lmmorphology (Fig. S11†). RMS roughnesses are 0.59 nm, 0.61 nm,1.33 nm, 1.43 nm, 1.22 nm, and 2.10 nm, for thelms containing0%, 5%, 10%, 15%, 20%, and 25% compound 2, respectively.Phase images show small changes with increasing additivecontent. These results are in sharp contrast to OBOCO addition,indicating that the morphology changes did result from thecrosslinking between OBOCO and PC61BM. Electron and holemobilities were obtained by fabricating the electron-only andhole-only devices and measuring the SCLC. It is interesting tond that the electron mobility of PC61BM continuouslydecreases with OBOCO content, from 6.6 � 10�4 cm2 V�1 s�1 at0% to 1.6 � 10�6 cm2 V�1 s�1 at 25%, while the hole mobilityremains stable (�10�4 cm2 V�1 s�1) with changing OBOCO

Fig. 3 AFM height and phase images for the active layers with different OBOCOcontents: (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20%, (f) 25%.

J. Mater. Chem. A, 2013, 1, 4589–4594 | 4591

Fig. 4 Variation of electron and hole mobilities of the active layer with OBOCOcontent.

Journal of Materials Chemistry A Paper

Publ

ishe

d on

04

Febr

uary

201

3. D

ownl

oade

d by

Pur

due

Uni

vers

ity o

n 23

/05/

2013

12:

56:0

9.

View Article Online

content (Fig. 4). This result proves our hypothesis that thecrosslinking takes place in the fullerene domains and lowers theelectron mobility of fullerene because the insulating additivecan hinder fullerene packing and block electron transport.16b,17

The mobility change can be used to explain Jsc and FF changeswith crosslinker content in Fig. 2. The continuous decrease infullerene electron mobility leads to continuous decrease inconductivity of the active layer, thus a continuous decrease in Jsc.FF relates to the balance of charge carrier transport in devices. Asshown in Fig. 4, the electron and hole mobilities at low OBOCOcontent are relatively close to each other, suggesting that theelectron and hole transport are more balanced at low additivecontent. So, FF values are higher at low OBOCO content.

The thermal stability of the devices without and with 5%OBOCO was investigated by continuously heating the devices at150 �C for 4 days and tracking their performance (Fig. 5). It isquite clear that the devices with OBOCO show signicantlyimproved thermal stability compared with the untreated

Fig. 5 Variation of device performance parameters (a) Voc, (b) Jsc, (c) FF, (d) PCEwith continuous heating at 150 �C. OBOCO content: 0% (-); 5% (:).

4592 | J. Mater. Chem. A, 2013, 1, 4589–4594

devices. In Fig. 5a, the devices with 5%OBOCO show quite stableVoc, which remains at �0.64 V aer 4 days' thermal treatment,while Voc for the devices without OBOCO decreases from 0.636 Vto 0.506 V (by 20%) aer the thermal treatment. Jsc and FF forboth devices decrease with continuous heating but the decreaseis smaller for the devices with OBOCO. The PCE for the devicewith 5% OBOCO decreases from 2.77% to 1.72% (by 38%) whilethat for the device without OBOCO decreases from 2.74% to0.78% (by 72%) (Fig. 5d). These results indicate that the OBOCOcrosslinker can effectively enhance solar cell stability, probablyby stabilizing the morphology of the active layer.

To clarify the effect of the crosslinker on the morphologystability of the active layer, AFM and optical microscopy were

Fig. 6 Morphology changes for the active layers before (left) and after (right)heating at 150 �C for 100 h. AFM height images: (a) without OBOCO; (b) with 5%OBOCO; optical microscopy images: (c) without OBOCO; (d) with 5% OBOCO.

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

Publ

ishe

d on

04

Febr

uary

201

3. D

ownl

oade

d by

Pur

due

Uni

vers

ity o

n 23

/05/

2013

12:

56:0

9.

View Article Online

performed for both devices without and with crosslinkingtreatment (Fig. 6). Aer thermal treatment at 150 �C for 100 h,the RMS roughness of the lm without OBOCO signicantlyincreases from 0.66 nm to 2.05 nm (Fig. 6a), while that of thelm with 5% OBOCO shows a slight increment from 1.03 nm to1.31 nm (Fig. 6b). Optical microscopy indicates that distinctfullerene micro-crystals appear in the lms without OBOCO(Fig. 6c and S15†). This is due to the heat-promoted fullereneaggregation.18 In contrast, no fullerene aggregates wereobserved for the lms with 5% OBOCO aer thermal treatment(Fig. 6d). OBOCO crosslinker can enhance the morphologystability of the active layer by impeding fullerene aggregation.

Conclusions

In conclusion, an innovative crosslinker OBOCO has beensuccessfully developed to enhance the stability of P3HT/PC61BMsolar cells. The crosslinking was realized by a heat-triggered D–A reaction between sultine groups on OBOCO and fullerenedouble bonds. By adding 5% OBOCO into the active layer, thepolymer/fullerene phase separation was not interrupted whilethe heat-promoted fullerene aggregation was dramaticallyinhibited, leading to signicantly enhanced thermal stability ofthe optimal lmmorphology. Consequently, a very stable devicecan be obtained. Further efforts on developing crosslinkers thatcan simultaneously improve device performance and stabilityare currently ongoing.

ExperimentalSolar cell fabrication and characterization

Patterned ITO glass with a sheet resistance of 15 U sq�1 wasultrasonically cleaned using detergent, distilled water, acetone,isopropanol sequentially and then given UV–ozone treatment. A30 nm thick poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS, Clevios� P VP Al 4083) layer wasformed on ITO substrates by spin coating an aqueous disper-sion onto ITO glass. PEDOT:PSS coated substrates were dried at140 �C for 10 min. P3HT/PC61BM (1 : 1) blend in ODCB (20 mgmL�1) with OBOCO (0–25%) was spin-coated (800 rpm for 60 s)onto the PEDOT:PSS layer. Then, the lms were annealed at150 �C for 20 min. Finally, Ca (�10 nm) and Al (�100 nm) werethermally evaporated under a shadow mask (pressure ca. 10�4

Pa). The effective area for the devices is 4 mm2. J–V curves weremeasured on a computerized Keithley 2420 SourceMeter. Devicecharacterization was done in air using a xenon-lamp-basedsolar simulator (Newport, AM1.5G, 100 mW cm�2). Thermalstability was investigated by putting the cells on a hotplate(150 �C) and tracking their performance. AFM was carried outon a Dimension 3100 microscope (Veeco). Optical microscopyimages were obtained by using a DM4000 microscope (Leica).

Electron mobility measurement

The structure for electron-only devices was Al/active layer/Ca/Al.5b Al (�80 nm) was rstly evaporated onto a glass substrate.The P3HT:PC61BM blends in ODCB with different amounts ofOBOCO were spin-coated onto the Al layer. The lms were

This journal is ª The Royal Society of Chemistry 2013

annealed at 150 �C for 20 min. Finally, Ca (�5 nm) and Al (�100nm) were thermally evaporated under a shadow mask (pressureca. 10�4 Pa). J–V curves were measured on a computerizedKeithley 2420 SourceMeter in the dark. The thicknesses of theactive layers were measured by a KLA Tencor D-120prolometer.

Hole mobility measurement

The structure for hole-only devices was ITO/PEDOT:PSS/activelayer/MoO3/Al.19 Patterned ITO glass with a sheet resistance of15 U sq�1 was ultrasonically cleaned using detergent, distilledwater, acetone, isopropanol sequentially, and then given UV–ozone treatment. A 30 nm thick PEDOT:PSS layer was formed onthe ITO substrates by spin coating an aqueous dispersion ontoITO glass. PEDOT:PSS coated substrates were dried at 140 �C for10 min. The P3HT:PC61BM blends in ODCB with differentamounts of OBOCOwere spin-coated onto the PEDOT:PSS layer.The lms were annealed at 150 �C for 20 min. Finally, MoO3

(�5 nm) and Al (�100 nm) were successively evaporated ontothe active layer under a shadow mask (pressure ca. 10�4 Pa). J–Vcurves were measured on a computerized Keithley 2420 Sour-ceMeter in the dark. The thicknesses of the active layers weremeasured by a KLA Tencor D-120 prolometer.

Acknowledgements

This work was supported by the “100 Talents Program” ofChinese Academy of Sciences, and National Natural ScienceFoundation of China (21102028).

Notes and references

1 (a) B. C. Thompson and J. M. J. Frechet, Angew. Chem., Int.Ed., 2008, 47, 58; (b) G. Dennler, M. C. Scharber andC. J. Brabec, Adv. Mater., 2009, 21, 1323; (c) M. Helgesen,R. Søndergaard and F. C. Krebs, J. Mater. Chem., 2010, 20,36; (d) F. G. Brunetti, R. Kumar and F. Wudl, J. Mater.Chem., 2010, 20, 2934.

2 (a) Y. Cheng, S. Yang and C. Hsu, Chem. Rev., 2009, 109, 5868;(b) C. Duan, F. Huang and Y. Cao, J. Mater. Chem., 2012, 22,10416; (c) Y. Liang and L. Yu, Acc. Chem. Res., 2010, 43, 1227;(d) H. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang,L. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649; (e)H. Zhou, L. Yang, A. C. Stuart, S. C. Price, S. Liu andW. You, Angew. Chem., Int. Ed., 2011, 50, 2995; (f) J. Cao,W. Zhang, Z. Xiao, L. Liao, W. Zhu, Q. Zuo and L. Ding,Macromolecules, 2012, 45, 1710; (g) W. Zhang, J. Cao,Y. Liu, Z. Xiao, W. Zhu, Q. Zuo and L. Ding, Macromol.Rapid Commun., 2012, 33, 1574; (h) J. Gao, D. He,W. Zhang, Z. Xiao, Q. Zuo, Z. Shi and L. Ding, Synth. Met.,2012, 162, 1694; (i) X. Shen, S. Chen, Z. Xiao, Q. Zuo,Y. Chen and L. Ding, Polym. J., 2012, 44, 978; (j) L. Liao,W. Zhang, Z. Xiao, J. Cao, Z. Liu, Q. Zuo and L. Ding,Polym. J., 2012, DOI: 10.1038/pj.2012.172.

3 (a) Y. He and Y. Li, Phys. Chem. Chem. Phys., 2011, 13, 1970;(b) C. Li, H. Yip and A. K. Y. Jen, J. Mater. Chem., 2012, 22,4161; (c) Y. Li, Acc. Chem. Res., 2012, 45, 723.

J. Mater. Chem. A, 2013, 1, 4589–4594 | 4593

Journal of Materials Chemistry A Paper

Publ

ishe

d on

04

Febr

uary

201

3. D

ownl

oade

d by

Pur

due

Uni

vers

ity o

n 23

/05/

2013

12:

56:0

9.

View Article Online

4 (a) L. Chen, Z. Xu, Z. Hong and Y. Yang, J. Mater. Chem., 2010,20, 2575; (b) H. Yip and A. K. Y. Jen, Energy Environ. Sci., 2012,5, 5994; (c) A. Duarte, K. Pu, B. Liu and G. C. Bazan, Chem.Mater., 2011, 23, 501; (d) R. Steim, F. R. Kogler andC. J. Brabec, J. Mater. Chem., 2010, 20, 2499; (e) F. Huang,H. Wu and Y. Cao, Chem. Soc. Rev., 2010, 39, 2500.

5 (a) X. Li, W. C. H. Choy, L. Huo, F. Xie, W. E. I. Sha, B. Ding,X. Guo, Y. Li, J. Hou, J. You and Y. Yang, Adv. Mater., 2012, 24,3046; (b) Z. He, C. Zhong, X. Huang, W. Wong, H. Wu,L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636; (c)L. Dou, J. Gao, E. Richard, J. You, C. Chen, K. C. Cha,Y. He, G. Li and Y. Yang, J. Am. Chem. Soc., 2012, 134,10071; (d) Z. He, C. Zhong, S. Su, M. Xu, H. Wu andY. Cao, Nat. Photonics, 2012, 6, 591; (e) Y. Zhou,C. F. Hernandez, J. W. Shim, T. M. Khan and B. Kippelen,Energy Environ. Sci., 2012, 5, 9827.

6 G. Yu, J. Cao, J. C. Hummelen, F. Wudl and A. J. Heeger,Science, 1995, 270, 1789.

7 (a) X. Yang, J. K. J. van Duren, R. A. J. Janssen, M. A. Michelsand J. Loos, Macromolecules, 2004, 37, 2151; (b) X. Yang,J. Loos, S. C. Veenstra, W. H. J. Verhees, M. M. Wienk,J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, NanoLett., 2005, 5, 579.

8 M. Jørgensen, K. Norrman, S. A. Gevorgyan, T. Tromholt,B. Andreasen and F. C. Krebs, Adv. Mater., 2012, 24, 580.

9 (a) B. J. Kim, Y. Miyamoto, B. Ma and J. M. J. Frechet, Adv.Funct. Mater., 2009, 19, 2273; (b) A. Charas, Q. Ferreira,J. Farinhas, M. Matos, L. Alcacer and J. Morgado,Macromolecules, 2009, 42, 7903; (c) B. Gholamkhass andS. Holdcro, Chem. Mater., 2010, 22, 5371; (d) H. J. Kim,A. Han, C. Cho, H. Kang, H. Cho, M. Y. Lee,J. M. J. Frechet, J. H. Oh and B. J. Kim, Chem. Mater., 2012,24, 215; (e) G. Griffini, J. D. Douglas, C. Piliego,T. W. Holcombe, S. Turri, J. M. J. Frechet and J. L. Mynar,Adv. Mater., 2011, 23, 1660; (f) C. Nam, Y. Qin, Y. S. Park,H. Hlaing, X. Lu, B. M. Ocko, C. T. Black and R. B. Grubbs,Macromolecules, 2012, 45, 2338; (g) M. Drees, H. Hoppe,C. Winder, H. Neugebauer, N. S. Saricici, W. Schwinger,F. Schaffler, C. Topf, M. C. Scharber, Z. Zhu andR. Gaudiana, J. Mater. Chem., 2005, 15, 5158; (h)A. E. A. Contoret, S. R. Farrar, M. O'Neill and J. E. Nicholls,Chem. Mater., 2002, 14, 1477; (i) Y. Cheng, C. Hsieh, Y. He,C. Hsu and Y. Li, J. Am. Chem. Soc., 2010, 132, 17381; (j)Y. Cheng, F. Cao, W. Lin, C. Chen and C. Hsieh, Chem.Mater., 2011, 23, 1512; (k) N. Cho, H. Yip, S. K. Hau,K. Chen, T. Kim, J. A. Davies, D. F. Zeigler and A. K. Y. Jen,J. Mater. Chem., 2011, 21, 6956.

10 (a) R. Png, P. Chia, J. Tang, B. Liu, S. Sivaramakrishnan,M. Zhou, S. Khong, H. S. O. Chan, J. H. Burroughes,

4594 | J. Mater. Chem. A, 2013, 1, 4589–4594

L. Chua, R. H. Friend and P. K. H. Ho, Nat. Mater., 2010, 9,152; (b) C. Tao, M. Aljada, P. E. Shaw, K. H. Lee, H. Cavaye,M. N. Balfour, R. J. Borthwick, M. James, P. L. Burn,I. R. Gentle and P. Meredith, Adv. Energy Mater., 2013, 3, 105.

11 (a) C. Thilgen and F. Diederich, Chem. Rev., 2006, 106, 5049;(b) P. Hudhomme, C. R. Chim., 2006, 9, 881; (c) Z. Xiao, G. Ye,Y. Liu, S. Chen, Q. Peng, Q. Zuo and L. Ding, Angew. Chem.,Int. Ed., 2012, 51, 9038.

12 X. Meng, W. Zhang, Z. Tan, C. Du, C. Li, Z. Bo, Y. Li, X. Yang,M. Zhen, F. Jiang, J. Zheng, T. Wang, L. Jiang, C. Shu andC. Wang, Chem. Commun., 2012, 48, 425.

13 P. Belik, A. Giigel, A. Kraus, M. Walter and K. Mullen, J. Org.Chem., 1995, 60, 3307.

14 M. D. Hoey and D. C. Dittmer, J. Org. Chem., 1991, 56, 1947.15 (a) C. J. Brabec, A. Cravino, D. Meissner, N. S. Saricici,

T. Fromherz, M. T. Rispens, L. Sanchez andJ. C. Hummelen, Adv. Funct. Mater., 2001, 11, 374; (b)B. P. Rand, D. P. Burk and S. R. Forrest, Phys. Rev. B:Condens. Matter Mater. Phys., 2007, 75, 115327; (c)M. C. Scharber, D. Muhlbacher, M. Koppe, P. Denk,C. Waldauf, A. J. Heeger and C. J. Brabec, Adv. Mater.,2006, 18, 789.

16 (a) Y. He, H. Chen, J. Hou and Y. Li, J. Am. Chem. Soc., 2010,132, 1377; (b) C. Zhang, S. Chen, Z. Xiao, Q. Zuo and L. Ding,Org. Lett., 2012, 14, 1508; (c) G. Ye, S. Chen, Z. Xiao, Q. Zuo,Q. Wei and L. Ding, J. Mater. Chem., 2012, 22, 22374; (d)Z. Xiao, Y. Matsuo, I. Soga and E. Nakamura, Chem. Mater.,2012, 24, 2572; (e) K. H. Kim, H. Kang, S. Y. Nam, J. Jung,P. S. Kim, C. H. Cho, C. Lee, S. C. Yoon and B. J. Kim,Chem. Mater., 2011, 23, 5090; (f) M. Lenes,G. J. A. H. Wetzelaer, F. B. Kooistra, S. C. Veenstra,J. C. Hummelen and P. W. M. Blom, Adv. Mater., 2008, 20,2116; (g) Y. Cheng, M. Liao, C. Chang, W. Kao, C. Wu andC. Hsu, Chem. Mater., 2011, 23, 4056; (h) Y. He, G. Zhao,B. Peng and Y. Li, Adv. Funct. Mater., 2010, 20, 3383; (i)X. Meng, W. Zhang, Z. Tan, Y. Li, Y. Ma, T. Wang, L. Jiang,C. Shu and C. Wang, Adv. Funct. Mater., 2012, 22, 2187; (j)E. Voroshazi, K. Vasseur, T. Aernouts, P. Heremans,A. Baumann, C. Deibel, X. Xue, A. J. Herring, A. J. Athans,T. A. Lada, H. Richtere and B. P. Randa, J. Mater. Chem.,2011, 21, 17345.

17 R. C. I. MacKenzie, J. M. Frost and J. Nelson, J. Chem. Phys.,2010, 132, 064904.

18 (a) Y. Zhang, H. Yip, O. Acton, S. K. Hau, F. Huang andA. K. Y. Jen, Chem. Mater., 2009, 21, 2598; (b) C. Li, S. Chien,H. Yip, C. Chueh, F. Chen, Y. Matsuo, E. Nakamura andA. K. Y. Jen, Chem. Commun., 2011, 47, 10082.

19 M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang and Y. Cao,J. Am. Chem. Soc., 2011, 133, 9638.

This journal is ª The Royal Society of Chemistry 2013