Bs

MIo

a

ARRAA

KOBSO

1

raemceabpacebaiab

pbpop

0d

Synthetic Metals 159 (2009) 556–560

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

enzothiadiazole-sandwiched quarter thiophene-based oligomer for organicolar cells

ingliang Sun, Li Wang, Yong Cao ∗

nstitute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials, South China University of Technology, Guangzhou 510640, People’s Republicf China

r t i c l e i n f o

rticle history:eceived 13 July 2008eceived in revised form 25 October 2008

a b s t r a c t

A novel diphenylaminofluorenyl-capped benzothiadiazole-sandwiched quarter thiophene-based nar-row band-gap oligomer 4,7-bis-{[5-(9,9-bis(octyl)-7-diphenylaminofluorene-2-yl)-thiophen-2-yl]-5-thiophen-2-yl}-[2,1,3] benzothiadiazole (DPAFL–DBBT–DPAFL which is further abbreviated to DDBBTD)

ccepted 24 November 2008vailable online 13 January 2009

eywords:rganic solar cells

has been synthesized and characterized. The obtained molecule absorbs light from 300 to 800 nm in solidfilm. PL peak of DDBBTD in solid film is centered at 731 nm. Photovoltaic cell by using the blend of theobtained oligomer and PCBM as active layer was fabricated by spin-coating method. Open-circuit voltagereached 0.85 V with the maximum energy conversion efficiency 0.15% and photocurrent response was

ulk heterojunctionolution processingligomer

extended over 700 nm.

. Introduction

Organic semi-conducting materials have been widelyesearched to match the different demand for photo-electronicpplications, such as light emitting diodes, solar cells and fieldffect transistors. Among a wide range of organic conjugatedolecular [2,1,3]-benzothiadiazole (BTZ) derivatives have received

onsiderable attentions for their exceptional optoelectronic prop-rties [1]. BTZ having a high electron affinity [1] is widely useds an acceptor in donor–acceptor–donor (D–A–D) type narrowand-gap oligomers [2], comonomer [3] for narrow band-gapolymer and dopant [4] for light emitting diodes. There arelso reports about BTZ-based D–A–D oligomers which are end-apped with different group for solution processing organiclectronic applications [5–7]. Very recently, there is report aboutenzothiadiazole-sandwiched dihexyl quarter thiophene used asctive semiconductor in P-channel OFETS, which shows high mobil-ty [8]. There are several groups working on oligothiophenes, suchs Hartmann group reported the synthesis and characterization ofisdiarylamino-capped oligothiophenes [9].

In the recent research, energy conversion efficiency (ECE) ofolymer bulk heterojunction (BHJ) solar cell has achieved 5%

y controlling the morphology of the active layer composed ofoly(3-hexyl-thiophene) (P3HT) and PCBM [10–12]. Specific meth-ds include thermal annealing of devices, control of solidificationrocess, etc. [13,14]. Before polymer BHJ solar can have large

∗ Corresponding author. Tel.: +86 20 87114609.E-mail address: poycao@scut.edu.cn (Y. Cao).

379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2008.11.026

© 2008 Elsevier B.V. All rights reserved.

scale commercial applications, its performance should be furtherimproved. One drawback of P3HT and PCBM-based BHJ solar cell isthe relatively low open-circuit voltage (0.5–0.6 V). Since the open-circuit voltage is ultimately limited by the difference between thehighest occupied molecular orbital (HOMO) of the donor and theLUMO of the acceptor [15]. There are two ways to increase theopen-circuit voltage, either by lowering HOMO of donor side orby enhancing LUMO of acceptor side. Recently there is report aboutsynthesis of new C60 derivative with higher LUMO level comparedwith PCBM to improve the Voc of P3HT and PCBM solar cell [16].

In this paper we report about synthesis of benzothiadiazole-sandwiched quarter thiophene end-capped with diphenyl-aminofluorene group and its application as donor phase blendingwith PCBM as active layer for organic solar cell by spin-coatingmethod. The solubility of benzothiadiazole-sandwiched quarterthiophene improves significantly by end capping the oligomerwith diphenylaminofluorene group. The oligomer shows widerange UV–vis absorption (300–800 nm) and low optical band-gap(1.75 eV). The HOMO level of the oligomer −5.25 eV is a little lowerthan the HOMO of P3HT (−5.1 eV). This may contribute to therelatively high open-circuit voltage 0.85 V of the solar cell based onDDBBTD and PCBM compared to the P3HT and PCBM-based solarcell.

2. Experimental

2.1. Materials

All reagents, unless otherwise specified, were obtainedfrom Aldrich, Acros, and TCI Chemical Co., and used as

Meta

rntydN

2

2b

5NA4w

7CH

2(

yt1q2fdse

41C5111H

2

isdtitssAwtnoatmpon

M. Sun et al. / Synthetic

eceived. All the solvents were further purified under aitrogen flow. 9,9-Bis(octyl)-2-diphenylamino-7-fluorenyl-4,4,5,5-etramethyl-1,3,2-dioxaborolan-2-yl and 4,7-bis-(2,2′-bithien-5-l)-[2,1,3]-benzothiadiazole were prepared following the proce-ure described in reference [17,18], and characterized by the 1HMR and 13C NMR spectra.

.2. Synthesis of oligomer

.2.1. 4,7-Bis(5-(5-bromothiophen-2-yl)-thiophen-2-yl)-[2,1,3]--enzothiadiazole (1)

4,7-Bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole (2.32 g,mmol) was dissolved in CHCl3 in nitrogen atmosphere, and-bromosuccinimide (1.78 g, 10 mmol) was immediately added.fter the reaction mixture being stirring at room temperature forh the solvent was removed at a reduced pressure and the residueas recrystallized from CHCl3 (yield: 60%).

1H NMR (300 MHz, CDCl3) ı 8.06 (d, J = 3.75, 2H), 7.88 (s, 2H),.24 (d, J = 3.99, 2H) 7.07 (m, 4H); elemental analyses calcd. for22H10Br2N2S5: C, 42.45; H, 1.61; S, 25.72; N, 4.50. Found: C, 42.52;, 1.75; S, 25.15; N, 4.72.

.2.2. 4,7-[Bis-5-(9,9-bis(octyl)-7-diphenylaminofluorene-2-yl)-2,2′-bithien-5-yl)]-[2,1,3]-benzothiadiazole (DDBBTD)

9,9-Bis(octyl)-2-diphenylamino-7-fluorenyl-4,4,5,5-tetrameth-l-1,3,2-dioxaborolan-2-yl (1.43 g, 2.1 mmol), 4,7-bis(5-(5-bromo-hiophen-2-yl)thiophen-2-yl)-[2,1,3]-benzothiadiazole (619 mg,mmol), (PPh3)4Pd(0) (0.5–2.0 mol%) and several drops of Ali-uat336 were dissolved in a mixture of toluene and aqueousM Na2CO3. The solution was refluxed with vigorous stirring

or 36 h in argon atmosphere. The solution was concentrated toryness. The crude product was purified by chromatography onilica gel to afford the desired dark purple product (petroleumther:CH2Cl2 = 3:1, yield 62%).

1H NMR (300 MHz, CDCl3) ı 0.87 (t, J = 6.54, 12H), 1.12–1.17 (m,8H), 1.96 (m, 8H), 7.04 (t, J = 7.11, 6H), 7.17 (d, J = 7.5, 10H), 7.28 (m,4H), 7.58 (m, 8H) 7.83 (s, 2H), 8.04 (s, 2H); 13C NMR (100 MHz,DCl3) ı 14.12, 22.65, 23.93, 29.27, 29.31, 30.03, 31.85, 40.30,5.20, 119.27, 119.59, 120.50, 122.60, 123.50, 123.92, 124.24, 124.54,24.65, 124.80, 124.96, 125.36, 127.98, 128.32, 129.20, 132.00,35.71, 136.00, 137.87, 138.05, 139.00, 140.83, 144.54, 147.34, 147.99,51.55, 152.39; elemental analyses calcd. for C104H110N4S5: C, 79.24;, 7.03; S, 10.17; N, 3.55. Found: C, 78.71; H, 7.12; S, 10.42; N, 3.81.

.3. Fabrications of solar cell devices

Photovoltaic solar cells were fabricated on pre-patternedndium–tin oxide (ITO) with a sheet resistance 10–20 �/�. Theubstrate was ultrasonically cleaned with acetone, detergent,eionized water and 2-propanol subsequently. Oxygen plasmareatment was made for 10 min as the final step of substrate clean-ng to improve the contact angle just before film coating. Ontohe ITO glass a layer of polyethylenedioxythiophene–polystyreneulfonic acid (PEDOT:PSS) film with a thickness of 50 nm waspin-coated from its aqueous dispersion (Baytron P 4083, BayerG), aiming at avoiding the possibility of leakage. PEDOT:PSS filmas dried at 80 ◦C for 2 h in the vacuum oven. The solution of

he DDBBTD and fullerenes in chlorobenzene was prepared in aitrogen-filled dry box (Vacuum Atmosphere Co.) and spin-coatedn the top of the ITO/PEDOT:PSS film. The typical thickness of thective layer was 70–80 nm. Then a thin layer of 40 nm barium and

he subsequent 150 nm thick aluminum capping layers were ther-

ally deposited by vacuum evaporation through a mask at a baseressure below 3 × 10−4 Pa. The deposition speed and thicknessf the barium and aluminum layers were monitored by a thick-ess/rate meter (model STM-100, Sycon). The cathode area defines

ls 159 (2009) 556–560 557

the active area of the device. The typical active area of the devicesin this study is 0.15 cm2.

2.4. Measurement

1H NMR spectra were recorded on a Bruker DRX 300 spectrom-eter operating at 300 MHz and were referred to tetramethylsilane.Elemental analyses were performed on a Vario EL ElementalAnalysis Instrument (Elementar Co.). UV–visible absorption spec-tra were measured on a HP 8453 spectrophotometer. Cyclicvoltammetry was measured on a Potentiostat/Galvanostat model283 electrochemical workstation (Princeton Applied Research)at a scan rate of 50 mV/s with a nitrogen-saturated solutionof 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) inacetonitrile (CH3CN), respectively with platinum and saturatedcalomel electrodes (SCE) as the working and reference elec-trodes. Current–voltage (I–V) characteristics were measured by asource meter (Keithley 236), silicon photodiode and a computer-controlled light source-monochromator-Lock-in system (Merlin,Oriel). Energy conversion efficiency was measured under illu-mination of AM 1.5 solar simulator with 100 mW/cm2 power.The ECE and fill factor (FF) were calculated by the followingequations:

ECE = FF × Jsc × Voc

Pin(1)

FF = Jm × Vm

Jsc × Voc(2)

where Pin is the incident radiation flux, Jsc and Voc are, respectivelythe short-circuit current density and open-circuit voltage, and Jmand Vm are, respectively the current density and voltage at themaximum power output.

3. Results and discussions

3.1. Synthesis and chemical characterization

The synthetic route for the DDBBTD is summarized inScheme 1. The synthesis of 9,9-bis(octyl)-2-diphenylamino-7-fluorenyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl (2) was car-ried out according to reference [17]. Suzuki coupling of (1) and (2)afforded dark purple DDBBTD in 62% yield under typical reactionconditions. The target oligomer is characterized by 1H NMR, 13CNMR and elemental analyses.

3.2. Optical properties

The normalized UV–vis absorption spectrum of DDBBTD inthe solution of THF (Fig. 1(a)) shows a broad peak consisting oftwo absorption features, one is peaked at 399 nm attributableto �–�* transition for diphenylaminofluorene unit; the otherone is peaked at 538 nm which is attributable to absorptionof 4,7-bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole center part.The normalized UV–vis absorption spectrum of DDBBTD in thinsolid film (Fig. 1(b)) shows similar two band absorption fea-tures as in THF solution but with significantly red shift, one ispeaked at 413 nm attributable to �–�* transition for dipheny-laminofluorene unit; the other one is peaked at 559 nm withabsorption edge extended up to 709 nm which is attributable toabsorption of 4,7-bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole

center part. Optical band-gap as 1.75 eV is estimated from theabsorption onset (709 nm) in thin solid film. By end capping the4,7-bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole with dipheny-laminofluorene, the band-gap of the oligomer is significantlyreduced by 0.2 eV, compared with the dihexyl end-capped 4,7-

558 M. Sun et al. / Synthetic Metals 159 (2009) 556–560

ic rou

b[obP

ti4beepnec

Fb

Scheme 1. Synthet

is-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole (band-gap 1.95 eV)8]. Fig. 1(c) shows the absorption spectrum of the blend filmsf DDBBTD and PCBM (1:2 by weight) which is featured byroad band from superposition of absorptions of DDBBTD andCBM.

Fig. 2(a) shows the normalized PL spectrum of the oligomer inhe solution of THF at 1 × 10−3 mol/L. The PL spectrum of oligomern solution shows a peak at 440 nm with clear vibronic shoulder at67 nm, which can be attributed to diphenylaminofluorene unit. Aroad 681 nm peak can be assigned to narrow band-gap center partmission. Fig. 2(b) shows the PL spectrum of DDBBTD in solid filmxcited by 325 nm line of HeCd laser which is dominated by a PLeak centered at 731 nm which can be attributed to the emission of

arrow band-gap center part. This fact indicates that the excitationnergy is almost completely transferred from DPAFL unit to DBBTenter part in solid film.

ig. 1. UV–vis absorption spectra of DDBBTD in THF solution (a), in solid film (b),lend films of DDBBTD and PCBM 1:2 by weight (c).

te of the oligomer.

3.3. Electrochemical characteristics

Cyclic voltammetry measurement of DDBBTD shows tworeversible oxidation wave at 0.85 and 1.30 V versus SCE stan-dard electrode (Fig. 3). The fluorene oxidation process is atabout 0.9 V estimated with ferrocene as an external standard(Eox(Fc/Fc+) = 0.45 V versus SCE) [19]. The oxidation peak of 4,7-bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole is at 0.92 estimatedwith a platinum wire working electrode, a gold counter electrode,and an Ag/AgCl in 3 M KCl reference electrode [8]. So the oxidationprocess of DDBBTD at 1.3 V can be attributed to fluorene units [7,19],and the oxidation process of DDBBTD at 0.85 V might be the oxida-tion process of 4,7-bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole

[8]. The HOMO level EHOMO = −5.25 eV is estimated according to theempirical equation EHOMO = −(Eox + 4.4) eV [20]. Unfortunately, nocathodic reduction waves for DDBBTD could be detected. The LUMOlevel can be estimated from its HOMO level and optical energy gapwith ELUMO = −3.50 eV.

Fig. 2. PL spectra of DDBBTD in THF solution (a) and in solid film (b).

M. Sun et al. / Synthetic Metals 159 (2009) 556–560 559

Fa(e

3

ccoc(Fii(DSfHaPow

FD

Fig. 5. AFM images (1 �m × 1 �m) of the oligomer/PCBM blend films.

ig. 3. The electrochemical characteristic of the DDBBTD (scan rate is 50 mV/s withnitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate

Bu4NPF6) in acetonitrile (CH3CN), respectively with platinum and saturated calomellectrodes (SCE) as the working and reference electrodes).

.4. Solar cell characteristics

Bulk heterojunction solar cells were fabricated in the deviceonfiguration: ITO/PEDOT/DDBBTD:PCBM/Ba/Al by solution pro-essing. Weight ratios of DDBBTD:PCBM were 1:2. The device showspen-circuit voltage (Voc) of 0.85 V with energy conversion effi-iencies 0.15%, fill factor 30.37% and short-circuit current densityJsc) of 0.65 mA/cm2 under AM 1.5 solar simulator (100 mW/cm2).ig. 4 compares J–V characteristics of devices in dark and under thellumination. We note that Voc (0.85 V) of DDBBTD:PCBM devicess slightly higher than that from devices of P3HT with PCBMVoc = 0.5–0.6 V). Considering more red-shifted photoresponse ofDBBTD:PCBM-based device this is a good number. According tocharber et al. [15], Voc is linearly correlated with the energy dif-erence of the HOMO of the donor and the LUMO of the acceptor.igher Voc of DDBBTD:PCBM-based solar cells can be partially

ttributed to the lowering HOMO level of DDBBTD (−5.25 eV) over3HT donor polymers (−5.1 eV) [16]. Fig. 5 shows the AFM imagesf oligomer and PCBM composite film. From the uneven surface,e can see clear phase separation which might come from aggre-

ig. 4. J–V characteristic of the PVC with configuration ITO/PEDOT:PSS/DBBTD:PCBM (1:2)/Ba/Al in dark and under the illumination (photo).

Fig. 6. Photosensitivity (A/W) of the DDBBTD and PCBM composite.

gation of oligomer or PCBM. This will further affect the surfacearea of P–N junction for charge separation and the efficiency ofcharge separation. This might account for the relatively low perfor-mance of the solar cell. Fig. 6 shows the photosensitivity response(A/W) of PVC based on DDBBTD:PCBM composite film which cover300–700 nm. The maximum photosensitivity is 0.11 A/W at 585 nm.By comparing the relative intensity of the UV–vis absorption spec-trum and the photosensitivity of the DDBBTD:PCBM composite film(1:2) at 413 and 559 nm, we can find that the narrow band-gapcenter part contribute more effectively to the photocurrent thanthe diphenylaminofluorene part. The preliminary result of the bulkheterojunction solar cells based on DDBBTD and PCBM showedthe red-shifted photoresponse and higher photosensitivity than thedevice from DDBTD oligomer-based solar cell reported previously[7]. However, energy conversion efficiency of DDBBTD device isneeded to be further optimized, since FF of the device is quite low.

4. Conclusion

We have synthesized of a novel conjugated small molecule

diphenylaminefluorenyl end-capped 4,7-bis-(2,2′-bithien-5-yl)-[2,1,3]-benzothiadiazole as donor material for realization of bulkheterojunction organic solar cell by spin-coating method. Thedevice shows open-circuit voltage (Voc) of 0.85 V with energy

5 Meta

cc(

A

FSs

R

[

[17] M.L. Sun, L. Wang, X.H. Zhu, B. Du, R.S. Liu, W. Yang, Y. Cao, Sol. Energy Mater.

60 M. Sun et al. / Synthetic

onversion efficiencies 0.15%, fill factor 30.37% and short-circuiturrent density (Jsc) of 0.65 mA/cm2 under AM 1.5 solar simulator100 mW/cm2).

cknowledgments

The authors are deeply grateful to the National Natural Scienceoundation of China (Project No. 50433030) and the Ministry ofcience and Technology Project (No. 2002CB613404) for financialupport.

eferences

[1] M. Akhtaruzzaman, M. Tomura, J. Nishida, Y. Yamashita, J. Org. Chem. 69 (2004)2953.

[2] C. Kitamura, S. Tanaka, Y. Yamashita, Chem. Mater. 8 (1996) 570.

[3] Q. Hou, Y.S. Xu, W. Yang, M. Yuan, J.B. Peng, Y. Cao, J. Mater. Chem. 12 (2002)

2887.[4] S. Fan, M.L. Sun, J. Wang, W. Yang, Y. Cao, Appl. Phys. Lett. 91 (2007) 213502.[5] J. Huang, C. Li, Y.J. Xia, X.H. Zhu, J.B. Peng, Y. Cao, J. Org. Chem. 72 (2007) 8580.[6] C. He, Q.G. He, Y.J. He, Y.F. Li, F.L. Bai, C.H. Yang, Y.Q. Ding, L.X. Wang, J.P. Ye, Sol.

Energy Mater. Sol. Cells 90 (2006) 1815.[

ls 159 (2009) 556–560

[7] M.L. Sun, L. Wang, B. Du, Y. Xiong, R.S. Liu, Y. Cao, Synth. Met. 158 (2008)125.

[8] P. Sonar, S.P. Singh, S. Sudhakar, A. Dodabalapur, A. Sellinger, Chem. Mater. 20(2008) 3184.

[9] A. Tabet, A. Schröder, H. Hartmann, D. Rohde, L. Dunsch, Org. Lett. 5 (2003)1817.

[10] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nat.Mater. 6 (2007) 497.

[11] J.G. Xue, B.P. Rand, S. Uchida, S.R. Forrest, Adv. Mater. 17 (2005) 66.12] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger, Science

317 (2007) 222.[13] W.L. Wang, H.B. Wu, C.Y. Yang, C. Luo, Y. Zhang, J.W. Chen, Y. Cao, Appl. Phys.

Lett. 90 (2007) 183512.[14] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Nat. Mater. 4

(2005) 864.[15] M.C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J.

Brabec, Adv. Mater. 18 (2006) 789.[16] M. Lenes, G.J.A.H. Wetzelaer, F.B. Kooistra, S.C. Veenstra, K.J. Hummelen, P.W.M.

Blom, Adv. Mater. 20 (2008) 2116.

Sol. Cells 91 (2007) 1681.[18] H.A.M. van Mullekom, J.A.J.M. Vekemans, E.W. Meijer, Chem. Eur. J. 4 (1998)

1235.[19] Z.H. Li, M.S. Wong, Y. Tao, J.P. Liu, Chem. Eur. J. 11 (2005) 3285.20] Y.F. Li, Y. Cao, J. Gao, D.L. Wang, G. Yu, A.J. Heeger, Synth. Met. 99 (1999) 243.