www.advmat.de COMMUNICATION Efficient Solar Cells with Thin Active Layers Based on Alternating Polyfluorene Copolymer/ Bulk Heterojunctions

By Mei-Hsin Chen, Jianhui Hou,* Ziruo Hong, Guanwen Yang, Srinivas Sista, Li-Min Chen, and Yang Yang*

Polymer solar cells have evolved as a promising cost- and energy levels of the donor material by modifying the [1,2] [13,14] effective alternative to inorganic-based solar cells due to their chemical structure to achieve a high Voc. potential to be low-cost, -weight, and flexible. Since the Amongst various , poly{[2,7-(9-(20-ethylhexyl)-9-hexyl- discovery of ultrafast photoinduced charge transfer from a fluorene])-alt-[5,50-(40, 70-di-2-thienyl-20,10,30-benzothiadiazole)]} conjugated polymer to fullerene molecules, followed by the (PFDTBT) has a deep HOMO level, which leads to a large Voc [3] [15] introduction of the bulk heterojunction (BHJ) concept, when blended with PC61BM. Svensson et al. have reported intensive research with potential materials has been carried polymer PV cells with a Voc of 1 V based on alternating copolymer [4–8] [16] out as future photovoltaic (PV) technology. Two organic PFDTBT blended with PC61BM. Moreover, Ingana¨setal. reported materials with distinct donor and acceptor properties are required a systematic study of PV cells using four different fluorene to form a heterojunction in the bulk film, which is often achieved copolymers by varying the length of the side chain and by solution processing. In such a case, the BHJ not only provides chemical structure, exhibiting power conversion efficiencies above abundant donor/acceptor interfaces for charge separation, but 2–3%. Unfortunately, in their case, the low photocurrent becomes a also forms an interpenetrating network for charge transport.[8,9] major limiting factor in achieving higher efficiencies, suggesting Highly efficient polymer solar cells based on poly(3- low carrier mobilities. hexylthiosphene) (P3HT) and [6,6]-phenyl C61 butyric acid In this study, poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt- 0 methyl ester (PC61BM) have been reported with power conversion [5,5-(4,7-di-2 -thienyl-2,1,3-benzothiadiazole)]} (BisEH-PFDTBT) efficiencies of 4–5%.[6,10–12] and poly{[2,7-(9,9-bis-(3,7-dimethyl-octyl)-fluorene)]-alt-[5,5-(4,7- The two most decisive parameters regarding polymer-solar-cell di-20-thienyl-2,1,3-benzothiadiazole)]} (BisDMO-PFDTBT), which efficiencies are the open-circuit voltage (Voc) and the short-circuit have the same polymer backbone as PFDTBT but different side current (Jsc). Jsc is mostly determined by the light absorption chains, were studied in order to achieve higher efficiency values, as ability of the material, the charge-separation efficiency, and the well as to investigate the side-chain effects. BisDMO-PFDTBT has high and balanced carrier mobilities. On the other hand, Voc is proven to be a promising candidate as a donor material for high limited by the difference in the highest occupied molecular efficiency polymer BHJ solar cells. Under simulated solar illumina- orbital (HOMO) of the donor and the lowest unoccupied tion of AM 1.5G (100 mWcm2), the BisDMO-PFDTBT blended molecular orbital (LUMO) of the acceptor, where a small Voc with (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) achieved a (as compared to the energy) represents a smaller driving maximum power conversion efficiency (PCE) of up to 4.5% with a force for the PV process. For the P3HT:PC61BM system, the Voc is thin active-layer thickness of only 47 nm. The device exhibited an around 0.6 V, which significantly limits the overall device open-circuit voltage (Voc) of 1 V, a short-circuit current (Jsc)of 2 efficiency. An effective method to improve the Voc of polymer 9.1 mA cm , and a reasonably high external quantum efficiency solar cells is to manipulate the HOMO level of the donor and/or (EQE) exceeding 50% over the entire visible range, with an EQE LUMO level of the acceptor.[13] Until now, fullerene derivatives maxima of 67% at 380 nm. have proved to be one of the best and most commonly used When considering the polymer design on the molecular level, electron acceptors. Fortunately, it is convenient to change the the two main factors relating to the ease of material processibility and PV performance are the polymer solubility in common organic and the hole mobility, respectively. In order to obtain good solubility, it is important to have long side chains [*] Dr. J. Hou, Prof. Y. Yang, Dr. Z. Hong, G. Yang, S. Sista, L. M. Chen attached on the polymer backbone. Note that in our experiment, Department of Materials Science and Engineering University of California the attached side chains are saturated alkyl groups that have little Los Angeles, CA 90095 (USA) influence on the molecular energy levels of the donor material. E-mail: [email protected]; [email protected] Therefore, it allows us to explore the interchain interaction, M. H. Chen particularly the charge-hopping effect. In addition, bulky side Graduate Institute of Photonic and chains have a negative effect on the carrier mobility, since National Taiwan University interchain hopping of charge carriers requires a favorable Taipei 10617 (Taiwan) overlapping of the electron wave function of adjacent conjugated DOI: 10.1002/adma.200900510 units on the polymer main chains. Apparently, if the non-

Adv. Mater. 2009, 21, 1–5 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 2 COMMUNICATION iwon fdsgigcnuae oyes hoigan issue. critical choosing a is polymers, chain side the conjugated for the size/shape designing from appropriate Therefore, of processibility. the viewpoint and solubility the limits ojgtdsd hi smr uk,temblt fthe of probability mobility reduced the the bulky, to due more of decreases is polymer chain conjugated side conjugated niiuly ugsigefiin hretase rmthe from BisDMO-PFDTBT transfer PC to charge and donor efficient BisEH-PFDTBT polymer suggesting with individually, blended was ih bopini h iil region. visible the in absorption light unhn fbt oye lnswsosre hnPC when observed was blends polymer both of quenching 2%. only to limited was euto fte‘sei fet’o h ocnuae side nonconjugated the the of to effect’’ attributed ‘‘steric is the which chains. chains, of be conjugated reduction can adjacent performance of two PV probability the chains The increasing be side via synthesized. improved can different and with designed cells PFDTBTs were solar two PFDTBT-based Therefore, expected. of performance improved addt o oye oa el,wt adgpo . Vada and eV 1.8 of gap band a with resulting cells, solar polymer for candidate ohplmrbed,idctn h rsneo nextremely an of presence for the nm 0.3 indicating of blends, roughness polymer (rms.) both 2 root-mean-square a a of has layer sample active the that show images urn eiaie hc o nydcesstesei effect, steric the decreases better only groups in not resulting methyl BisDMO-PFDTBT which the unbulky of derivative, two fluorene seventh and are third there the on contrary, located and the chain, On main of conjugated effect. probability the the of ethyl ball-and-stick decrease proximity both thus the BisEH-PFDTBT, the in of in group are derivative shown groups ethyl fluorene As the an carbon. of of second model consists the BisEH-PFDTBT on of located chain side calculation. molecular-geometries The general a is which calculation, were polymers double-u two a and split-valence the method, Hartree–Fock quantum-chemical in the by units calculated of fluorene geometry optimum alkyl-substituted The 1. the Scheme in fluorene depicted are different derivatives, with BisDMO-PFDTBT, and BisEH-PFDTBT iul eotdwt orslblt,ada eut the result, a as and solubility, poor ( weight with molecular reported viously iE-FTTadb BisDMO-PFDTBT. b) and BisEH-PFDTBT 1. Scheme PC h i-cy-usiue FTT(ioPDB)wspre- was (Biso-PFDTBT) PFDTBT bis-octyl-substituted The p [15] – p 71 [17] oee,PDB-ae oyesaesilapromising a still are polymers PFDTBT-based However, Mwscoe nta fPC of instead chosen was BM tcig oee,asd hi ihisfcetlength insufficient with chain side a However, stacking. V h hmclsrcueadtesikwt-almdlof model stick-with-ball the and structure chemical The hmclsrcueadbl-n-tc oe fa) of model ball-and-stick and structure Chemical oc rud1V yatcigfvrbesd his an chains, side favorable attaching By V. 1 around p M – n p ´ fteslto-rcsal Biso-PFDTBT solution-processable the of ) ,weetehgetPEotie was obtained PCE highest the where K, 5 71 tcig u loicesstesolubility. the increases also but stacking, ai e,63G a dpe nthe in adopted was 6-31G, set, basis M tmcfremcocp (AFM) microscopy force Atomic BM. p – p 61 tcigdet h steric the to due stacking [18,19] M edn oenhanced to leading BM, m m p – ß p 2 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 tcigbetween stacking m ufc area surface m 71 [16] BM n l atro 1 a civdbsdo BisD- on based achieved was 51% of factor fill MO-PFDTBT:PC a and n P3HT:PC and fidu i xd (ITO)/poly(3,4-ethylenedioxythiophene): oxide nm)/BisDMO-PFDTBT: (40 (PEDOT:PSS) tin poly(styrenesulfonate) indium the of films polymer-blend most of the shows in 3 Figure 50% in inset than The region. intense visible more are spectra transmission niaigavr mohadaopossurface. amorphous and smooth very a indicating eitne( resistance ela h hcns auso h cielyr r itdin listed are layer, the active of BisEH-PFDTBT:PC the slopes voltage, the of open-circuit Comparing values 1. thickness Table the as well otg ( voltage ntePEvleadsre eitneo h w polymers two the obtained BisEH-PFDTBT:PC of were spectra resistance transmission series The from later. and discussed be value will PCE the in MO-PFDTBT:PC sdsrbdi h xeietlscin o BisEH- For section. Experimental the in PFDTBT:PC described as fol 7n Fg ) h pia ausotie from obtained values optimal The 2). PFDTBT:PC (Fig. nm 47 the only of iue1. Figure mohsraeadfiepaesprto,a hw in shown 1.5G AM as illumination solar separation, simulated phase With mWcm b. (100 fine and 1a and Figure surface smooth J–V V F mgso )BisEH-PFDTBT:PC a) of images AFM hrceitc fteP eie ae nBisEH- on based devices PV the of characteristics oc f09 ,asotcrutcret( current short-circuit a V, 0.97 of ) R 71 71 s 2 hnBisDMO-PFDTBT:PC than ) 61 MadBisDMO-PFDTBT:PC and BM BisDMO-PFDTBT:PC and BM 71 ,amxmmPEo .% ihopen-circuit high a 4.5%, of PCE maximum a ), Mfim ne hi pia odtos(i.3), (Fig. conditions optimal their under films BM M ohrsruhesvle r rud03nm, 0.3 around are values roughness rms Both BM. 71 Mbedwt hnatv-ae thickness active-layer thin a with blend BM 71 M BisDMO-PFDTBT:PC BM, J–V 71 71 Mhslre series larger has BM M h differences The BM. d.Mater. Adv. J 71 uvsaon the around curves sc 71 71 Madb BisD- b) and BM f91m cm mA 9.1 of ) Mbed,as blends, BM Mfim,the films, BM www.advmat.de 2009 , 71 21, BM, 1–5 2 , COMMUNICATION 3 [24] (1) BM BM 71 71 BM (1:3, 71 BM (circle) 61 BM (200 nm) BM device is 61 71 BM devices. 61 ] Thickness (nm) 2 cm ) is the absorbance of the V was then obtained based on l [23] Þ l The electron and hole mobilities ð [6] BM (triangle), and P3HT:PC IQE 71 h BM (1:3, w/w) and BisDMO-PFDTBT:PC 71 Þ Þ The IQE l l ð is the EQE, and abs( ð [22] Þ l EQE abs ð h Transmission spectrum obtained from BisEH-PFDTBT: PC EQE [25,26] h Þ¼ l ð Furthermore, the transport of electrons/holes in the polymer ). IQE 2 the device, the 100%standard reflectance silver mirror baseline with a was reflectivityvisible higher collected region. than using 97% over a the h where Accordingly, thecontributing high to IQE thelayer high (Fig. is 2). performance one for such of ablend the is thin an important active crucial parameterbalanced that charge-carrier must factors transport be well issing controlled. a the A space-charge prerequisite build-up, forrate which suppres- the will device significantly performance. deterio- Figure 3. can be(SCLC) calculated from the space-charge-limited current above 70% throughout themaximum majority of value the of visibleconversion 87%, region properties with which a of validatesdevices this the exhibit higher efficient system. IQE values photon Interestingly,compared over a these to much the broader two region values as reported for P3HT:PC active layer. The IQE of the BisDMO-PFDTBT:PC (right), spin-coated at their optimal conditions. (square), BisDMO-PFDTBT:PC the following equation: polymer-blend filmspolymer-blend under films of (40 ITO/PEDOT:PSS nm)/BisDMO-PFDTBT:PC optimal conditions. The inset shows the (47 nm) (left) and ITO/PEDOT:PSS (40 nm)/P3HT:PC For BM 61 BM (1:3, [21] 71 ] PCE [%] FF [%] Series resistance [ 2 value, which oc 5.5 eV for both V measured for this [mA cm 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim sc sc J J ß a deeper donor HOMO characteristics of PV devices based on BisEH-PFDTBT:PC [13,20] J–V is limited by the energy-level ) characteristics of PVdevices based [V] oc J–V oc V V BM films are highly semi-transparent, BM results in a higher BM, the EQE maxima is about 67%, BM provide the absorption region of each 71 71 71 71 characteristics (Fig. 2). BM (1:3, w/w) and BisDMO-PFDTBT:PC 71 BM 0.97 9.1 4.5 51 2.8 47 J–V 1–5 71 BM 0.95 8.4 3.5 44 7.6 49 71 21, ). , 2 2009 Current density–voltage ( Detailed values obtained from BM (47 nm) and ITO/PEDOT:PSS (40 nm)/P3HT:PC 71 Both EQE and IQE (internal quantum efficiency) were The absolute absorption spectra of BisEH-PFDTBT, BisD- BisDMO-PFDTBT:PC Adv. Mater. Table 1. on BisEH-PFDTBT:PC PC Figure 2. www.advmat.de measured for the best-performing devices (Fig. 5). BisEH-PFDTBT:PC demonstrating their high potential for tandem cells applications. MO-PFDTBT, and PC while exceeding 50% for over halfa of the high visible photon-conversion region, indicating efficiency.the Integrating the EQE product valueconsistent of result, and matching closely the to global the reference spectrum yields a w/w) polymer(100 blends, mW cm under AM 1.5G simulated solar illumination polymers, with a band gap of 1.9S1). eV (Supporting Information, It Fig. issolely thus determined confirmed by that theside polymer the chains. main molecular The chain, deeper energy regardlessare HOMO of levels stable level the against are implies oxidization, thatexpected. thus the good polymers In device addition, lifetimes since can be material (Fig. 4).electrochemical The cyclic voltammetry HOMO (CV) energy to level be was measured by particular device. Furthermore,converted changes IQE, versus the which absorbed , is isthe determined absorbance by the values based fraction onInformation, the of Fig. S2). reflection When mode measuring (Supporting the reflectance spectra of difference between the HOMO level oflevel the of donor and the the acceptor LUMO components, level blended with PC correlates to the BisDMO-PFDTBT:PC (200 nm), spin-coated atBisDMO-PFDTBT: their PC optimal conditions. Notably, the w/w), respectively, under AM 1.5G simulated solar illumination (100 mW cm 4 COMMUNICATION h lcrnadhl oiiis( mobilities hole and electron curves, SCLC the fitting by the obtained results the on Based voltage. epciey n 7 and respectively, PFDTBT:PC r o ls oteplmrbcbn.Hwvr o the for However, backbone. interchains polymer the the since effect to is steric close which the mobility, in too electron increase the are the than to lower order due one is blend BisEH-PFDTBT:PC imn)adBisDMO-PFDTBT:PC and diamond) hc seuvln otefimtikes and thickness, film the to equivalent is which space, free of permittivity iE-FTT iDOPDB,adPC and BisDMO-PFDTBT, BisEH-PFDTBT, respectively. h Q ausaecluae rmteasrac ae ntereflectivity the on based absorbance the mode. from calculated are values IQE The i.S) h oemblt fteBisEH-PFDTBT:PC the of mobility hole The S3). Fig. where J 5. Figure PC and (triangle), MO-PFDTBT 4. Figure SCLC ¼ e r 8 9 Q n Q r lte o BisEH-PFDTBT:PC for plotted are EQE and IQE BisD- (circle), BisEH-PFDTBT of spectra absorption Absolute stedeeti osato h material, the of constant dielectric the is " r 71 " 0 M epciey(uprigInformation, (Supporting respectively BM, m V L 3 2 71 10 Mae5 are BM 5 L n 3 and stedsac ewe h electrodes, the between distance the is 71 M(qae ls h hcns of thickness The films. (square) BM 71 10 M(rageadsur)P cells. PV square) and (triangle BM 10 5 5 71 n 6 and cm Mae1,1,ad3 nm, 30 and 15, 15, are BM 2 V m ß e 1 10 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 s V and 1 71 steapplied the is 6 o BisDMO- for M(iceand (circle BM cm 2 e m 0 V h )for sthe is 71 1 s BM (2) 1 , pia odto fP3HT/PC of condition optimal (thickness iDOPDB eebeddwt PC with blended were BisDMO-PFDTBT cielyr fBisEH-PFDTBT:PC of 110 layers at rpm annealing 3000 active at thermal solution and blend s, the 30 spin-coating for by obtained were layers active otdwt EO:S IOPDTPS.TeP3HT:PC The substrate ITO (ITO/PEDOT:PSS). an PEDOT:PSS from taken with was coated spectrum reference blended)-coated The substrates. (ITO/PEDOT:PSS/polymer ITO PEDOT:PSS on spin-coated oye a civdwe oho h ie ouin a a had solutions mixed the of both when achieved polymer:PC each of was performance device best polymer The (CB)[19]. chlorobenzene in than DCB a 1,wt D f15 and 1.5, of PDI a with 21K, was oa el.Orrslsidct htBsM-FTTblended BisDMO-PFDTBT that PC indicate with results Our cells. solar lentn oyurn ooye n sda nelectron an as used PC and with blended donor copolymer polyfluorene alternating 5. and 2 Figures in our shown to data corresponds experimental to strongly leads which electrons performance, and device holes superior of photo- transport device. balanced the more the the within Therefore, maintained diminish is is not transport neutrality electrical will charge and current, effect When space-charge layer. the active balanced, carrier the balanced in in resulting unity, transport to close is mobilities hole and hog h s fasao akt en natv rao .2cm 0.12 of area active film an polymer define the to mask of of shadow top Dektek nm a 100 on of a and evaporated use with calcium the thermally of measured through was nm 20 as and of respectively, aluminum, consists nm, 47 cathode and The profilometer. 49 around were aus ute mrvmn ntecrirmblt eg,to (e.g., mobility carrier the in 10 improvement further values, ymncrmtclgtfo eo appsigtruha through passing lamp xenon a [21]. Corporation) Research from Research Acton Stanford light (SpectraPro-2150i, monochromator (SR830, illuminated monochromatic were amplifier devices the by lock-in when condition EQEs a short-circuit temperature. under room Systems) using at box measured glove filled were nitrogen a measurements in electrical performed the were All spectrophotometer. solar UV-vis 50 a W using Cary 150 obtained Varian were spectra [ThermoOriel Newport absorption a The using detector. simulator calibrated thermopile measured 818T-10 was intensity solar light was the a and photocurrent (AM1.5G)] simulator The using unit. illumination source-measure under 2400 Keithley a [6]. ref. in reported ,-ihooezn DB u otebte ouiiyo PC of solubility better the to due (DCB) 1,2-dichlorobenzene BisDMO-PFDTBT:PC eotdmto 1] h oeua egt( weight molecular The [15]. method reported eut o elzn ihpromneBJplmrslrcells. solar polymer BHJ high-performance realizing for results C f45.TeEEadPEas hwhg conversion high show the also from As extraction PCE cell. charge-carrier the and effective EQE suggesting The efficiencies, 4.5%. of PCE Experimental ne aepesr f1 of pressure base a under ocnrto fBsM-FTT( t)pu PC plus wt%) (1 BisDMO-PFDTBT of concentration EO:S ByrnPV 148)wt hcns faon 0nm, 40 around of thickness conducting a spin-coating 150 with by at 4083) baking A1 modified by The followed VP was anode. P substrate the (Baytron ITO as PEDOT:PSS the substrates glass of ITO-coated surface patterned on fabricated 1.3. of PDI apePeaainfrTasiso Spectra Transmission for Preparation Sample ncnlso,BsM-FTTwssnhszda an as synthesized was BisDMO-PFDTBT conclusion, In Instrumentation Material eieFabrication Device 3 cmV 71 h w FTTplmr eesnhszdfloigthe following synthesized were polymers PFDTBT two The : Mi rmsn oye-ln ytm ihahigh a with system, polymer-blend promising a is BM 71 1 0 m hw nFgr a pncae codn othe to according spin-coated was 3 Figure in shown nm) 200 Mrtoo : freape t nDBwt a with DCB in wt% 1 example, [for 1:3 of ratio BM s 1 V fti ls fplmr hudyedpromising yield should polymers of class this of ) h urn–otg ( current–voltage The : oc rai oo/cetrBJP el were cells PV BHJ donor/acceptor Organic : nteetoplmr aesfcetyhigh sufficiently have polymers two these in 71 8 71 o 5mni min i.BsHPDB and BisEH-PFDTBT air. ambient in min 15 for C Mbed h ai ewe h electron the between ratio the blend, BM Mt arct ihyefiin polymer efficient highly fabricate to BM 10 61 M Mi : / ai,dsovdi C,as DCB, in dissolved ratio, w/w 1:1 in BM n 6 fteBsM-FTTws2K iha with 20K, was BisDMO-PFDTBT the of 71 or( Torr (1 Torr MadBisDMO-PFDTBT:PC and BM 8 o 0mn h hcns fthe of thickness The min. 10 for C J–V uvswr bandusing obtained were curves ) h oye lnswere blends polymer The : 71 M ¼ Maddsovdin dissolved and BM n fteBisEH-PFDTBT the of ) 3.2Pa). 133.32 d.Mater. 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Dyakonov, J. Parisi, N. S. Sariciftci, in: Adv. Mater. Acknowledgements The authors would like to acknowledge theInc. financial support from Solarmer andN00014-04-1-0434, Program from Manageralso Dr. P. the like Armistead).NSC-0962917-I-002-113 M.H.C. to Office would for thank financialwith the of support. W. L. National The Kwan, NavalElectronic technical M. Science Laboratory H. discussion Council Park, of Research H.appreciated. University of Y. of Chen, Supporting (grant Taiwan, California, andInterScience Los T. Information Project L. or Angeles, number Chen from is is the at deeply the author. available Organic online from Wiley www.advmat.de