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Solution-processed bulk heterojunction organic solar cells based onan oligothiophene derivativeBin Yin , Liying Yang , Yongsheng Liu , Yongsheng Chen , Qingjin Qi et al.Citation Appl. Phys. Lett. 97, 023303 2010; doi 10.1063/1.3460911View online http//dx.doi.org/10.1063/1.3460911View Table of Contents http//apl.aip.org/resource/1/APPLAB/v97/i2Published by the American Institute of Physics.Related ArticlesTuning open-circuit voltage in organic solar cells by magnesium modified Alq3J. Appl. Phys. 110, 083104 2011Enhanced charge collection in confined bulk heterojunction organic solar cellsAPL Org. Electron. Photonics 4, 224 2011Enhanced charge collection in confined bulk heterojunction organic solar cellsAppl. Phys. Lett. 99, 163301 2011Origin of the dark-current ideality factor in polymerfullerene bulk heterojunction solar cellsAppl. Phys. Lett. 99, 153506 2011Analysis of interface carrier accumulation and relaxation in pentacene/C60 double-layer organic solar cell byimpedance spectroscopy and electric-field-induced optical second harmonic generationJ. Appl. Phys. 110, 074509 2011Additional information on Appl. Phys. Lett.Journal Homepage http//apl.aip.org/Journal Information http//apl.aip.org/about/about_the_journalTop downloads http//apl.aip.org/features/most_downloadedInformation for Authors http//apl.aip.org/authorsDownloaded 31 Oct 2011 to 60.28.33.220. Redistribution subject to AIP license or copyright; see http//apl.aip.org/about/rights_and_permissionsSolution-processed bulk heterojunction organic solar cells basedon an oligothiophene derivativeBin Yin, 1, a Liying Yang, 1 Yongsheng Liu, 2 Yongsheng Chen, 2 Qingjin Qi, 1Fengling Zhang, 1,3 and Shougen Yin 1, b1Key Laboratory of Display Materials and Photoelectric Devices, Ministry of Education,Institute of Material Physics, and Tianjin Key Laboratory for Photoelectric Materials and Devices,Tianjin University of Technology, Tianjin 300384, People ’s Republic of China2Key Laboratory for Functional Polymer Materials and Center for Nanoscale Science and Technology,Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071,People ’s Republic of China3Department of Physics, Chemistry, and Biology IFM, Biomolecular and Organic Electronics,Center of Organic Electronics, Linkping University, SE-58183 Linkping, SwedenReceived 5 January 2010; accepted 10 June 2010; published online 12 July 2010Organic bulk heterojunction BHJ solar cells based on a dicyanovinyl-substituted oligothiopheneas a donor and 6,6-phenyl C61 butyric acid methyl ester PCBM as an acceptor were fabricatedand characterized. The oligothiophene derivative can absorb long wavelength photons of the solarradiation, which makes the solar cells with an optimized weight ratio of 11.4 have a decentshort-circuit current density 12.4 mA / cm2 and open-circuit voltage 0.88 V under AM 1.5Gillumination with an intensity of 100 mW / cm2. A power conversion efciency PCE of 3.7 isachieved, which is among the best PCEs of solution processed small molecule BHJ solar cells. 2010 American Institute of Physics. doi 10.1063/1.3460911 Bulk heterojunction BHJ solar cells based on conju-gated polymer/fullerene systems are evolving into a promis-ing phase for low-cost photovoltaic applications due to solu-tion processing, light weight, and large scale exibledevices.1,2 The power conversion efciency PCE of the de-vices basedon these materials are predicted to reach 15 bymodeling. 3 At present, the solar cells based on poly 3-hexylthiophene P3HT 6,6-phenyl C61 butyric acid methyl es-ter PCBM attract a great deal of attention and have dem-onstrated efciencies as high as 5– 6.4 In recent years,solution processable BHJ organic solar cells based on conju-gated small molecules and fullerenes have been extensivelyinvestigated. 5– 11 Solution processable small molecules havenumerous merits, such as, high chemical stability, high car-rier mobility, well dened structures, easy synthesis and pu-rication, intrinsically monodisperse, and few deep electrontraps in solid lms. 12– 14 Oligothiophenes possess the aboveadvantages, which makes them promising donors for solu-tion processed BHJ solar cells when blend them with PCBM.PCE up to 2.3 and 3.0 were demonstrated using an oli-gothiophene with a dialkylated diketopyrrolopyrrole chro-mophore as a donor and soluble fullerenes PCBM or 6,6-phenyl C71 butyric acid methyl ester as an acceptor,respectively. 10,11 The best performance of solution processedsmall molecule solar cells with a PCE of 4.4 was reportedby Nguyen et al.15In this letter, the donor used for the BHJ solar cells wasan oligothiophene derivative 5,5 -bidicyanovinyl -3,3 ,3 ,3 ,3 ,3 - sexioctyl - 2,5 2 ,5 2 ,2 5 ,2 5 ,2 5 ,2 -septithiophene DCN7T , which is com-posed of seven thiophene rings substituted with two terminalelectron-withdrawing dicyanovinyl-substituent end groups.The chemical structure of DCN7T is shown in Fig. 1a andthe synthesis of DCN7T was described elsewhere.16The photovoltaic devices were made by using a commonfabrication process. The active layer was prepared by spincoating the blends of DCN7T 8 mg ml -1 and PCBM inchloroform in different weight ratios wt/wt 1x, x0.6,1.0, 1.4, and 1.8 onto a precleaned indium tin oxideITO coated glass substrate, which was modied by spin-coated a thin layer of poly- 3,4-ethylenedioxythiophene polystyrenesulfonate PEDOTPSS Baytron as a holeextraction/electron blocking layer and then dried in nitrogenwithout further treatment. Finally, LiF and Al were thermallyevaporated through a shadow mask on the active layer as acathode. The schematic diagram of the solar cells based onDCN7TPCBM is shown in Fig. 1b, which has a structureof ITO 17 / sq /PEDOTPSS 40 nm/DCN7TPCBMaElectronic mail yinbinmail.nankai.edu.cn.bAuthor to whom correspondence should be addressed. Electronic mailsgyintjut.edu.cn. FAX 86 226021-4010. Tel. 86 22 6021-4019.FIG. 1. a Chemical structure of DCN7T. bSchematic diagram of photo-voltaic device based on DCN7TPCBM. c Approximate HOMO/LUMOlevels for DCN7T/PCBM based photovoltaic device.APPLIED PHYSICS LETTERS 97, 023303 20100003-6951/2010/97 2/023303/3/30.00 2010 American Institute of Physics97 , 023303-1Downloaded 31 Oct 2011 to 60.28.33.220. Redistribution subject to AIP license or copyright; see http//apl.aip.org/about/rights_and_permissions110 nm/LiF 1 nm/Al 70 nm. The effective area of eachcell is 9 mm 2. The current density versus voltage J-Vcurves of the photovoltaic devices are recorded undersimulated AM 1.5G with an illumination intensity of100 mW / cm2. A xenon lamp with a lter broadpassGRB-3, Beijing Changtuo Scientic limited company tosimulate AM 1.5G conditions was used as the excitationsource with a power of 100 mW / cm2 white light illumina-tion from the ITO side. Light source illumination intensitywas measured using a calibrated broadband optical powermeter FZ-A, wavelength range 400– 1000nm, PhotoelectricInstrument Co, Beijing Normal University, China.17 Theelectrochemical band gap of DCN7T is about 1.7 eV as de-termined by cyclic voltammetry, while the optical band gapis about 1.68 eV,16 indicating that the band gap of DCN7T issmaller than that of P3HT, which is approximately 2.0 eV.18The highest occupied molecular orbital HOMO and lowestunoccupied molecular orbital LUMO of DCN7T andPCBM, as well as other relevant energy levels are shown inFig. 1c.The absorption spectra of the thin lms of DCN7T,P3HT, and DCN7T/PCBM are presented in Fig. 2a. Theabsorption spectrum of DCN7T lm spreads from 400 to800 nm, with a peak at 614 nm, indicating that DCN7Tabsorbs solar energy more effectively than P3HT, whichabsorbs photons in the range of 400– 650nm with a peak at522 nm. In addition, it is important to note that a shoulder at670 nm suggests a vibronic progression enforced by strongintermolecular interactions among the thiophene rings andthe cyanovinyl-substituents in DCN7T molecule, 7,19 which isredshifted compare with that of P3HT at 600 nm. It isobvious that the blend lms of DCN7T/PCBM exhibitbroader absorption spectra than that of the pure DCN7T dueto the complementary absorption spectra between theDCN7T and PCBM, implying an effective solar photon har-vest.The photoluminescence PL spectra of DCN7T andDCN7TPCBM with different weight ratios 10.6, 11, 11.4,and 11.8 are shown in Fig. 2b. The lms were preparedby spin-casting from chloroform solutions onto quartzsubstrates. The pure DCN7T lm displays a strong peak at375 nm with excitation at 250 nm. The PL intensity of thelm is remarkably reduced after doping with PCBM. The PLemission of DCN7T is more effectively quenched by increas-ing the weight of PCBM in the blends. Efcient PL quench-ing of the lms with the weight ratios of 11.4 and 11.8suggests that efcient exciton dissociation occurs at the in-terface between DCN7T and PCBM. Furthermore, the uo-rescence decays of the DCN7TPCBM lms were measuredby the JY FluoroLog 3 spectrophotometer using a 265 nmpulsed light-emitting diode, where the solid curves are biex-ponential t to the experimental data and can be seen in Fig.2c. The uorescence decay of DCN7TPCBM blend 11.4is signicantly faster than those of the other two ratios, im-plying that in the lm of 11.4 wt/wt the photogeneratedexcitons have a high possibility to diffuse to the interfacebetween DCN7T and PCBM where they are dissociated tofree charge carriers becausethe lm has a better morphology.The J-V curves of the photovoltaic devices based onDCN7TPCBM, with two weight ratios of 10.6 and 11.4, inthe dark and under illumination without annealing treatmentare plotted in Fig. 3a, which shows that under illuminationof 100 mW / cm2, the devices with a blended ratio of 11.4wt/wt display obvious photoresponse. A PCE of 3.7,open-circuit voltages Voc of 0.88 V, ll factor FFof 0.34,and the short-circuit current densities Jsc of 12.4 mA cm-2are achieved. The J-V curves of the photovoltaic deviceshaving different blend weight ratios 10.6, 11, 11.4, and11.8 wt/wt under AM 1.5G illumination with an intensity of100 mW / cm2 are shown in Fig. 3b. Among the devicesinvestigated so far, the PCE are 0.04, 1.1, 3.7, and3.2 for the devices with the ratios of 10.6, 11, 11.4, and11.8, respectively. The efciency of the solar cells increasewith increasing PCBM weight and then decrease.The devicewith the ratio of 11.4 demonstrates the best performance. Itis possible that in the case of small PCBM content, such asfor 10.6 wt/wt , the PCBM is too little to form an interpen-etrating network morphology for exciton diffusing to anddissociating at the interface between two components. Be-FIG. 2. a Absorption spectra of DCN7T, P3HT, andDCN7TPCBM lms in various weight ratios of 10.6,11, 11.4, and 11.8. b PL spectra of DCN7T andDCN7TPCBM in lms with different ratios wt/wt ex-citated with the wavelength of 250 nm. c Fluores-cence decays of DCN7TPCBM lms with three differ-ent ratios as follows 11 hollow squares, 11.4hollow circles , and 11.8 hollow triangles .FIG. 3. a Logarithmic J- V characteristics of the pho-tovoltaic devices based on different blends ofDCN7TPCBM 10.6, 11.4 wt/wt in dark and underAM 1.5G illumination 100 mW /cm 2. b J-V curvesof photovoltaic devices based on four blends ofDCN7TPCBM 10.6, 11, 11.4, and 11.8 wt/wt un-der AM 1.5G illumination.023303-2 Yin et al. Appl. Phys. Lett. 97 , 023303 2010 Downloaded 31 Oct 2011 to 60.28.33.220. Redistribution subject to AIP license or copyright; see http//apl.aip.org/about/rights_and_permissionssides that, with that ratio, PCBM is too little to form a con-tinuous path for electron transport through the active layer tocollection electrode. When the PCBM content increases inthe blend, as in 11.4 wt/wt , it is enough to produce aneffective donor/acceptor interface for exciton dissociationand to form a percolation pathway for charge transport to thecollection electrodes. The strong dependence of PCE on theblend ratios of the devices shows that the morphology inu-ence on the performance of solar cells. As the PCBM contentincreased, as for 11.8 wt/wt , aggregation may be occur-ring, which could deter exciton generation, separation andtransport in the active layer. This is partly veried by themorphology variation among the blended lms, as shown intopography images of atomic force microscopy AFM Fig.4. The topography image of pristine DCN7T lm is smoothand the root-mean-square roughness is about 7.7 nm. Afterintroducing PCBM into DCN7T, the three different donor/acceptor ratios show some phase segregation. The lm withblend ratio of 11.4 exhibits a more continuous and roughmorphology compared to the other two blend ratios and thecorresponding device demonstrates the highest PCE.The summary of the photovoltaic properties for thesedevices is given in Table I . Under AM 1.5G illumination, theVoc 0.60 – 0.88V slightly depends on the blend ratios. TheVoc is based upon the difference in the LUMO level of theacceptor and the HOMO level of the donor because of theFermi level pinning mechanism. In addition, the Voc can beinuenced by other factors such as, the ratios of the donorand the acceptor, processing conditions, etc.20,21 The maxi-mum Voc of the devices basedon DCN7T/PCBM is 1.1 V,governed by the HOMO of DCN7T 5.1 eVand the LUMOof PCBM 4.0 eV. The slightly lower Voc of the devices maybe caused by the loss of the carriers collected at the elec-trodes. In contrast, the dependence of Jsc on the weight ratiosof the two components is pronounced. For example, the Jscincreases from 0.29 to 12.4 mA cm-2 , when the blend ratiochanges from 10.6 to 11.4, resulting in more than one orderof magnitude improvement.In conclusion, solution processed BHJ solar cells basedon an oligothiophene derivative DCN7T was investigated.The oligothiophene derivative having two terminal electron-withdrawing groups exhibits an intermolecular donor-acceptor structure, which increases the absorption in the vis-ible range.22 The BHJ solar cell with DCN7T and PCBM atthe ratio of 11.4 exhibits a PCE as high as 3.7 under AM1.5G illumination with an intensity of 100 mW / cm2 in air,which is among the highest efciency to date for solutionprocessed small molecule BHJ solar cells. The PCE of theBHJ solar cells can be further improved by tuning the mo-lecular system and optimizing the device fabrication, how-ever the FF of these devices are still low.The authors gratefully acknowledge the nancial supportfrom NSFC of China Grant Nos. 60876046 and 60976048,Key Project of Chinese Ministry of Education Grant No.209007, Key Project of Tianjin Science and TechnologyGrant No. 10ZCKFGX01900 , and the Tianjin Key Disci-pline of Material