“solvent annealing” effect in polymer solar cells based on poly(3-hexylthiophene) and...

DOI: 10.1002/adfm.200600624

“Solvent Annealing” Effect in Polymer Solar Cells Basedon Poly(3-hexylthiophene) and Methanofullerenes**

By Gang Li, Yan Yao, Hoichang Yang, Vishal Shrotriya, Guanwen Yang, and Yang Yang*

1. Introduction

Since the discovery of efficient electron transfer between ful-lerenes and conjugated polymers in bulk-heterojunction (BHJ)polymer solar cells,[1–3] a lot of attention has been focused onengineering the properties of these systems. Especially, overthe last five years, major improvements have been achieved inthe performance of polymer solar cells.[4–12] The promising per-formance of these improved polymer solar cells, in conjunction

with their low cost and ease of large-area processing, has leadto much interest from both academic and industrial researchersfor the development of inexpensive solar energy harvestingsystems. Of all the polymeric systems reported in the literaturefor the fabrication of BHJ cells, the regioregular poly(3-hexyl-thiophene) (RR-P3HT):[6,6]-phenyl C61-butyric acid methylester (PCBM) system currently represents the state-of-art inpolymer solar cells.[7–12] New device structures[13–15] and novelmaterials based on polythiophenes (PTs)[16] have also been de-vised to further enhance the performance of polymeric solarcells. Several approaches have been used to enhance the per-formance of RR-P3HT:PCBM solar cells, including thermaland electrical annealing[5,7,9,17] and slow growth.[8] In the slowgrowth approach, the film is allowed to stand in the liquidphase after spin-coating, by storing it in a confined volume(such as a glass petri dish) to allow the solvent to dry slowly(see Experimental for details). Since the slow growth methodsignificantly improves the efficiency of RR-P3HT:PCBM-based solar cells, similar to the thermal annealing approach, weuse the term “solvent annealing” to describe this process here.In combination with a thermal annealing step, a power conver-sion efficiency (PCE) of over 4 % has been achieved forRR-P3HT:PCBM solar cells under AM1.5G standard refer-ence conditions (SRC); a very impressive PCE of 3.5 % hasbeen achieved by using only the solvent annealing approach.[8]

In this Full Paper, we present a systematic study of the effectof solvent annealing on RR-P3HT:PCBM polymer solar cellswith the only variable being the film spin-coating time (ts). Wefurther define a term called the solvent annealing time (ta),

1636 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 1636–1644

–[*] Prof. Y. Yang, Dr. G. Li, Y. Yao, G. Yang

Department of Materials Science and EngineeringUniversity of California, Los AngelesLos Angeles, CA 90095 (USA)E-mail: [email protected]. H. YangRensselaer Nanotechnology CenterRensselaer Polytechnic InstituteTroy, NY 12180 (USA)Dr. V. ShrotriyaSolarmer Energy, Inc.El Monte, California 91731 (USA)

[**] G. Li and Y. Yao contributed equally to this work. The authorsacknowledge the financial support provided by the Office of NavalResearch (ONR) (Grant number N00 014-04-1-0434), Solarmer Ener-gy Inc., University of California Discovery Grant, and the NanoscaleScience and Engineering Institute of the National Science Founda-tion (NSF) (Grant number DMR 0 117 792). Technical discussionswith Dr. Brian Gregg of NREL are greatly appreciated.

The self-organization of the polymer in solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenylC61-butyric acid methyl ester (PCBM) is studied systematically as a function of the spin-coating time ts (varied from 20–80 s),which controls the solvent annealing time ta, the time taken by the solvent to dry after the spin-coating process. These blendfilms are characterized by photoluminescence spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, andgrazing incidence X-ray diffraction (GIXRD) measurements. The results indicate that the p-conjugated structure of RR-P3HTin the films is optimally developed when ta is greater than 1 min (ts ∼ 50 s). For ts < 50 s, both the short-circuit current (JSC) andthe power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for50 < ts < 55 s, the JSC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of thepolymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered p-conjugated structure ofRR-P3HT to 1.2 % for a less-ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, theincident photon-to-electron conversion efficiency spectrum of the less-ordered solar cell shows a clear loss in both the overallmagnitude and the long-wavelength response. The solvent annealing effect is also studied for devices with different concentra-tions of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under “solvent annealing” conditions, the polymer is seento be ordered even at 67 wt % PCBM loading. The open-circuit voltage (VOC) is also affected by the ordering of the polymerand the PCBM loading in the active layer.

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which is the time taken by the solvent to dry after the spin-coating process. These two parameters, ts and ta, are correlated:the shorter the spin-coating time, the longer is the solvent an-nealing time required for the film to dry. Spin-coating is a fastdrying process, whereas solvent annealing is a slow growth pro-cess. By systematically varying ts (and thus ta), we have con-ducted a comprehensive investigation of the evolution of themorphology of the RR-P3HT:PCBM polymer films and theperformance of the polymer solar cells. At a spinning speed of1000 rpm, the long-wavelength absorption of RR-P3HT isobserved to be well preserved for ta longer than 1 min in aRR-P3HT:PCBM system with a 1:1 ratio by weight of the com-ponents. A major loss in absorption over the long-wavelengthrange is observed for a specific ts window (50–55 s), as detectedby UV-vis spectroscopy and corroborated by measurements ofthe corresponding devices. This indicates a transition from ahighly ordered active layer to a less ordered film. Devices withhighly ordered films show much higher PCE values and photo-luminescence (PL) intensities as compared to devices with lessordered films, indicating that maximum PL quenching does notalways lead to the best device performance, even within thesame system. Atomic force microscopy (AFM) and grazing in-cidence X-ray diffraction (GIXRD) results provide furtherevidence for the influence of ts and ta. Studies on devices withvarious PCBM concentrations (25–67 wt %) unambiguouslyshow that for higher PCBM loading fractions, the RR-P3HTpolymer tends to be less ordered in films without solventannealing. However, after solvent annealing, ordering of theRR-P3HT component has been observed for up to 67 %PCBM loading.

2. Results and Discussion

The experiments have been designed based on several ax-ioms. Firstly, both conventional spin-coating (fast drying) andsolvent annealing (slow growth) processes transform the poly-mer solution into a solid polymer film. Secondly, during thespin-coating process, the effective solvent evaporation rate ismuch higher than that in an isolated glass petri dish. Therefore,the length of ts effectively determines the time required to drythe film, i.e., the solvent annealing time ta. Dichlorobenzene(DCB) has been chosen as the solvent because its high boilingpoint allows a wide window of spin-coating times to be used.At a spin-coating speed of 1000 rpm, the solvent evaporatesmuch faster than for static samples (such as in a glass petridish). We notice that with an increase in ts from 20 to 50 s, tadecreases dramatically from ca. 20 to 1 min, as detected by thetime required for a dramatic change in the color of the filmwhen it is transformed from the liquid (orange) to the solid(dark purple) phase. As described above, during the spin-coat-ing process, the majority of the solution is removed from thesubstrate by the exerted centrifugal force and the film thick-ness decreases very rapidly. After longer spinning times, anequilibrium state is established and the film thickness becomesconstant. At a spinning speed of 1000 rpm, profilometry(Dektek) results indicate that the film is ca. 165 nm thick for

ts = 20 s; the rest of the samples (ts = 30–80 s) exhibit a filmthickness of 150 ± 5 nm. All the experiments have beenconducted at room temperature. At elevated temperatures, thesolvent will be removed more rapidly and the blend film willlose its crystallinity. As illustrated in a previous report, under aN2 environment, ta is reduced from 3 min at room temperatureto 20 s at 70 °C; this is accompanied by a concomitant decreasein the long-wavelength absorption and a significant degrada-tion in the performance of the device.[8] The same trend is ex-pected if the temperature is increased during the spin-coatingprocess, although we have not tested this hypothesis due to ex-perimental limitations. The UV-vis spectra shown in Figure 1aare quite similar for solvent annealed films with ta varying from20 to 1 min (corresponding to ts varying from 20 to 50 s). In allthe cases, the vibronic features are clearly observed, indicatingthat a 1 min solvent annealing step is enough to maintain theordering of RR-P3HT in a 1:1 (by weight) RR-P3HT:PCBMblend film. The film properties start to change for ts of 52 and55 s. The ts = 52 s film starts to solidify immediately after thespin-coating process, and it takes only ca. 20 s to observe a fullcolor change. When the ts is increased to 55 s, the films solidify

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from 20 s for # 1 to 80 s for # 7). As ts becomes longer, the absorption inthe long-wavelength region and the PL intensity are decreased.

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G. Li et al./“Solvent Annealing” Effect in Polymer Solar Cells

immediately after spin-coating. However, when these films areleft in glass petri dish, a further darkening of the color is ob-served, indicating that a small amount of the DCB residue isstill present in the films. The UV-vis spectra of the film fabri-cated with ts = 55 s show a significant decrease in absorption inthe red region of the electromagnetic spectrum and the origi-nally strong vibronic shoulders are significantly diminished. Ats of 80 s represents an extreme case of minimal solvent anneal-ing, and the film shows a further decrease in absorption in thered region and a diminution of the vibronic features. ForRR-P3HT:PCBM films, the solvent annealed films have themost prominent vibronic features reported in the literature, in-dicating strong interchain–interlayer interactions[18–21] amongthe RR-P3HT chains as well as good polymer ordering in theblend films. Apart from the film with ts = 20 s, the absorptionspectra below 450 nm are very similar for all the films, indicat-ing that all the films have a similar thickness (i.e., similaramounts of RR-P3HT and PCBM). The PCBM absorption isclearly unaffected by ta, which indicates that the driving forcefor better device properties is the self-organization of the poly-mer through a solvent annealing process.

Conformational chain defects (twists or disruptions of pla-narity) can interrupt polymer conjugation and lead to a smallerp-conjugation length and blue-shifting of the p–p* absorptionband in linear conjugated polymers like P3HT.[22–24] Highly re-gioregular P3HT can self-organize to form semicrystalline la-mellar morphologies with higher order architectures,[18–21]

thereby yielding high hole mobilities, which are important forboth field effect transistor (FET)[21] and solar cell applications.The RR-P3HT absorption at longer wavelengths as well as thevibronic shoulders are significantly reduced in intensity inRR-P3HT:PCBM films with high loading fractions of PCBMfabricated in the traditional way without any solvent annealing(yellow phase). This indicates a partial loss of crystallinityand/or a smaller conjugation length in RR-P3HT due to disrup-tions introduced by PCBM. The quick removal of the solventfavors a uniform P3HT:PCBM distribution and prevents theformation of highly ordered lamellae by disrupting the inter-chain interactions between RR-P3HT chains. Upon solvent an-nealing, the RR-P3HT absorption and the prominent vibronicfeatures are restored for the blend films.[25] The absorptionspectra of the blend films represent a simple addition of thespectra of PCBM and RR-P3HT films, which strongly supportsthe occurrence of phase segregation and the formation of crys-talline RR-P3HT domains in the blend films.

PL quenching provides direct evidence for exciton dissocia-tion, and thus efficient PL quenching is necessary to obtain ef-ficient organic solar cells. However, this does not necessarilymean that the stronger the PL quenching, the better the perfor-mance of the solar cells. Figure 1b shows PL spectra for thesame set of films shown in Figure 1a with different degrees ofsolvent annealing. The PL spectra of films # 1 to 3 with ts of 20to 40 s are characterized by similar PL intensities of 20 500 to21 900 counts per second (cps) (as compared to 280 000 cps forpure P3HT films). For films with a ts of 50 s (# 4), the PL inten-sity drops slightly to 19 200 cps. However, further increasing tsdrastically reduces the PL intensity to 6800 cps in film # 7 (ts of

80 s), which is less than 30 % of the corresponding value forsolvent annealed films. Since the system is polycrystalline innature, the angle dependence of the PL is weak. The scaling ofthe PL intensity with film thickness has a minimal effect anddoes not alter the conclusions presented here. Earlier studieson PTs have shown that regiorandom P3HT (absorption isblue-shifted with respect to RR-P3HT) has a more than one or-der of magnitude higher PL quantum efficiency (g = 8 %) thanRR-P3HT (92 % RR) (g < 0.5 %).[26] This is explained by thefact that self-assembled RR-P3HT lamellae are characterizedby strong interchain–interlayer interactions, which split thehighest occupied molecular orbital (HOMO) and lowest unoc-cupied molecular orbital (LUMO) levels, with the loweredLUMO level becoming optically forbidden,[27] thus resulting inweaker radiative transitions. The apparent discrepancy be-tween the absorption and PL measurements is resolved by con-sidering the ultrafast (ca. 40–100 fs) photoinduced chargetransfer process at the donor/acceptor (D/A) interface,[2] whichclearly leads to a difference in the PL quantum efficiencies. Indisordered blend films, the D/A interface is larger since the do-nor and acceptor distribution is frozen during the spin-coatingprocess from homogeneous blend solutions. In addition, in thelocally self-organized RR-P3HT domains, defect sites intro-duced by PCBM as well as non-radiative RR-P3HT decaychannels are eliminated, which might also contribute to PL en-hancement.

Eight devices have been fabricated for each type of film andthe measured device parameters are plotted in Figure 2.Figure 2a shows the current–voltage (J–V) curves for the bestdevices of each type. Figure 2b shows the statistical data for de-vice parameters derived from eight devices of each type. Themaximum short-circuit current (JSC) is 9.6 mA cm–2 in de-vice # 1, corresponding to the longest ta (ca. 20 min, or a shortts). However, reducing ta to as short as 1 min (ts is 50 s) onlyleads to a small decrease in JSC (JSC = 9.1 mA cm–2). The in-creased thickness of the films (# 1) may also result in slightlylarger values of JSC. Further increasing the spinning time leadsto a rapid decrease in JSC to 4.0 mA cm–2 for ts of 55 s and2.8 mA cm–2 for ts of 80 s. This trend—an initial plateau for theJSC and PCE values, followed by a quick decrease—agrees wellwith both the UV-vis and PL results. A similar trend has beenobserved for the open-circuit voltage VOC. The VOC isca. 0.58 V for ts of 20 to 50 s, and dramatically increases to0.67 V for the device fabricated with a ts of 80 s. However, thefill factor (FF) does not significantly change even with drasticvariations in JSC. Apart for the 55 s ts device (FF = 58.3 %), allthe other devices exhibit high FF values ranging from 62.2 to64.8 %. We conjecture that this indicates that the deteriorationof the devices due to the loss of polymer ordering starts with adecrease in the JSC values, followed by a lowering of the FFvalues. At a relatively low spinning speed (1000 rpm), even thedevice with ts = 80 s might represent a film with an intermedi-ate degree of order, i.e., an intermediate state between a devicewith a highly ordered film (expected to exhibit high JSC andhigh FF) and one with a highly disordered film (expected to ex-hibit low JSC and low FF). The AFM morphology data andGIXRD measurements discussed below further corroborate

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this hypothesis. The combined effect is that the PCE is reducedslightly from 3.64 % (ts = 20 s device) to 3.38 % (ts = 50 s de-vice) and then significantly drops to 1.17 % for the device withts = 80 s.

The ordering of the RR-P3HT domains is crucial for light ab-sorption, whereas the formation of interpenetrating networksof RR-P3HT and PCBM is important for facilitating chargetransport. This leads to a reduction of the interface area be-tween RR-P3HT and PCBM. The high-performance solar cellsfabricated from solvent annealed films indicate that the inter-face area is sufficiently large (with significant PL quenching)and provides sufficient exciton dissociation. In the trade-off be-tween a) efficient charge separation and b) absorption pluscharge transport for maximizing the efficiency of polymer solarcells, the benefits of the latter clearly dominate that of the for-mer in the RR-P3HT:PCBM system. The size of the polymercrystalline domains is also estimated to be comparable to theexciton diffusion length (i.e., a few nanometers).

The VOC in polymer solar cells has been widely discussedover the years. With the metal–insulator–metal (MIM)[28] pic-

ture not sufficient to explain the phenomena observed, themost widely propagated belief is that for systems with ohmiccontact at both electrodes, VOC varies linearly with the energydifference between the donor HOMO and the acceptorLUMO.[29] There is also some evidence for the influence of sur-face dipoles in these devices.[15,30] However, here, with all theparameters being the same except ts, it is interesting to see avariation of ca. 0.1 V in the VOC. Two mechanisms are conjec-tured to contribute to this reduction in VOC. The first possibili-ty is the formation of a band structure[31] instead of molecularenergy levels (or alternatively the splitting of the HOMO andLUMO levels)[27] due the strong interchain–interlayer interac-tions originating from the relatively high ordering of RR-P3HTin the solvent annealed films, which thus leads to a reduced ef-fective “bandgap” relative to the HOMO–LUMO difference.A second possibility is that in solvent annealed films,RR-P3HT has a considerably longer conjugation length onaverage than in films without solvent annealing,[25] as indicatedby the much stronger absorption intensity in the longer wave-length region, as discussed above. Therefore, the reduction inVOC is reasonable given that the difference between the effec-tive HOMO of the polymer and the LUMO of the acceptor de-creases. Chen et al.[19] have observed that the band edge of re-gioregular head–tail (HT) P3HT is 0.4 eV lower than that ofregiorandom P3HT. It is reasonable to expect that P3HT in asolvent annealed 1:1 blend film is RR-P3HT, whereas P3HT ina blend film that has not been annealed is expected to be simi-lar to regiorandom P3HT. Similar changes in VOC have beenreported previously in the literature for thermally annealedP3HT:PCBM systems;[5,7,9–12] while the decrease in VOC uponthermal annealing has been ascribed to vertical phase segrega-tion,[17] the improved crystalline ordering of RR-P3HT couldplay a dominant role in the process.

The incident photon-to-electron conversion efficiency(IPCE) results of four devices with ts of 20, 50, 55, and 80 s areshown in Figure 3. Consistent with the trend in JSC, the IPCEmagnitude is first slightly reduced from 63.3 % (#1, ts = 20 s) to59.1 % (#4, ts = 50 s), followed by a rapid decrease to 44.3 %


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for a device with a ts of 55 s (#6), and finally to 23.8 % for a de-vice with a ts of 80 s (#7). In addition to the observed change inmagnitude, the lineshape of the IPCE curve is also significantlyaltered. Device 1 shows a plateau in the visible region, with theIPCE being greater than 58 % from 460 to 600 nm. The photo-response of Device 4 is diminished in the red region of theelectromagnetic spectrum close to 600 nm. The IPCE curvesfor Devices 6 and 7, in which the polymer ordering is signifi-cantly lower, clearly show a drastic reduction in intensity in thered region after a peak at 510 nm. Quantitatively, the ratios ofthe IPCE values at 610 nm to the peak IPCE values (i.e.,IPCEk = 610 nm/IPCEmax) in Devices 1, 4, 6, and 7 are 90, 83, 58,and 50 %, respectively. This trend in the IPCE characteristicsclosely follows the trend observed by UV-vis spectroscopy indi-cating a loss of polymer ordering in the blend films. The re-duced absorption in the red region of the electromagnetic spec-trum leads to a quick drop in the IPCE of the devices.

Previous studies indicate that polymer crystallization fromsolution into a thin film is a complex exothermic process, whichcan be affected by the solution evaporation rate.[32] Themesoscale film morphology in the lateral direction of theRR-P3HT:PCBM (1:1 weight ratio) films has been visualizedusing tapping mode atomic force microscopy (TM-AFM).Figure 4 shows typical TM-AFM topographic and phase im-ages of RR-P3HT:PCBM blend films spin-coated at 1000 rpmwith different ts (30 and 80 s). It has been found that the sur-face topography of the film spin-coated for 30 s is significantlyrougher than the film cast for 80 s; the former films have a rootmean square (rms) roughness of 3.0 nm (Fig. 4a), as comparedto 0.8 nm (Fig. 4c) for the latter films. The fibrillar crystalline

domains of RR-P3HT are clearly visible in both cases, but thewidths of the nanofibrillar domains are clearly different. Re-cently, Brinkmann et al. investigated the semicrystalline struc-ture of RR-P3HT (molecular weight (MW) = 35 000) and founda lamellar periodicity of approximately 28 nm.[33] This 28 nmlong-range order includes the crystalline region as well as disor-dered zones which harbor structural defects like chain endsand folds and tie segments.[33] Our observations of these solarcell systems (RR-P3HT, MW of 30 000) agree very well withthe above findings; Figure 4b and d indicate the 28 nm period-icity of the polymeric structures. However, different processingconditions (30 and 80 s) can result in different combinations ofcrystalline and disordered regions in the periodic structure. Ithas been found that the films cast for 80 s show a longer disor-dered zone (crystalline grain boundaries (GBs) are visible evenin the TM-AFM topography image shown in Fig. 4c) as com-pared to films cast for 30 s, resulting in a higher energy barrierfor inter-fibrillar hopping and therefore a lower carrier mobil-ity.[34] This agrees well with previous studies by Kline et al.[35]

and Zhang et al.[36] that have demonstrated the correlation be-tween FET mobility and the GBs/nanofibril-widths ofRR-P3HT in the FETs.

Since AFM observations are limited only to the surface ofthe film, the overall crystalline structure of RR-P3HT in theblend films has been further studied by GIXRD. As shown inFigure 5, 2D GIXRD patterns of films cast for 30 (Fig. 5a) and80 s (Fig. 5b) clearly show the intense reflections of the(100) layer and the (010) crystals along the qz (substrate nor-mal) and qxy (substrate parallel) axes, respectively, implyingthat these films have very highly ordered edge-on hexyl side-

chains and that the p-conjugated planes ofRR-P3HT are parallel with respect to thesubstrate.[32,37] However, based on 1D out-of-plane X-ray (Fig. 5c) and azimuthal an-gle scan (Fig. 5d) profiles extracted fromthe 2D GIXRD data, we have determinedthat the film cast for 30 s has a much high-er crystallinity than the film cast for 80 swith longer ta. In addition, from thebroadening of the azimuthal peaks, it ap-pears that a longer solvent drying processpossibly leads to a broader distribution ofthe molecular orientations of RR-P3HTin the films. This result is consistent withthe relatively higher film roughness ob-served for films cast for 30 s as comparedto films cast for 80 s (Fig. 4).

Based on the TM-AFM and GIXRDdata, we also conclude that the loss ofP3HT long-wavelength absorption in filmscast for 80 s is mainly due to the smallerconjugation length of the polymer ratherthan because of a decrease in crystallinity.As discussed above, the film cast for 80 srepresents an intermediate situation, andthe FF is less sensitive to carrier mobilitythan the photocurrent.[38] This partially


Figure 4. TM-AFM topography (left) and phase (right) images for RR-P3HT:PCBM films fabricatedusing different processing conditions. The spin-coating times are a,b) 30 s and c,d) 80 s.

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explains the high FF of these devices although JSC is reducedfrom 9.6 to 2.8 mA cm–2.

GIXRD results indicate that in these blend films, the P3HTchains have the same edge-on orientation on the substrate as inpure P3HT films.[32,37] For films cast for 80 s, the peak maxi-mum of the scattering vector qz is at 0.378 Å–1, correspondingto an interlayer spacing of 16.6 Å, whereas for the films castfor 30 s (highly crystalline), the interlayer spacing is deter-mined to be 16.1 Å. These results support the hypothesis thatfilms cast for 30 s have stronger interlayer interactions withinthe RR-P3HT domains as compared to films cast for 80 s. Sincecarrier transport occurs perpendicular to the polymer layers,hole transport in devices based on films cast for 30 s is easier,which is consistent with the J–V characteristics. The TM-AFMand GIXRD results provide strong evidence that the highlypreferred morphology of interpenetrating BHJ solar cells is re-alized by the solvent-annealing approach.

The PCBM loading in the blend system is an important pa-rameter controlling the performance of the polymer solarcells.[39–41] We now study the effect of PCBM loading on the or-dering of RR-P3HT in blend systems with and without solventannealing; our results vary significantly from previous reportsin the literature.[39] 2 wt % RR-P3HT (in DCB) is used as thestarting material, and is added into PCBM to obtain

RR-P3HT:PCBM solutions with ratios of 4:1, 2:1,and 1:2 (by weight). The spinning speed is 1000 rpmand ts of 30 and 80 s have been used to produce filmswith and without solvent annealing. The UV-vis spec-tra shown in Figure 6a for 4:1 RR-P3HT:PCBM filmsspin-coated for different times are very similar. Thefilm without solvent annealing shows a slight reduc-tion in the two long-wavelength vibronic featuresand a very small blue-shift in the absorption. The ef-fect of solvent annealing is thus much less than forthe 1:1 films. On the other hand, upon increasing thePCBM loading in the blend to 67 wt %, the solventannealed films demonstrate significant absorption inthe red region of the electromagnetic spectrum, aswell as distinct vibronic features, whereas the vibron-ic features are almost completely lost when no sol-vent annealing is performed.

Figure 6a also shows the PL spectra of these fourfilms. For 4:1 RR-P3HT:PCBM films, the PL inten-sity of the solvent annealed film is 43 800 cps at639 nm. The PL peak decreases in intensity to31 800 cps without solvent annealing—a 27 % reduc-tion in the PL intensity. This is much smaller than the70 % reduction in PL intensity (21 900 to 6800 cps)observed for 1:1 RR-P3HT:PCBM films. The differ-ences in the PL intensity become much larger whenthe PCBM loading is increased to 67 wt %. The filmwith solvent annealing has a PL peak intensity of18 100 cps (at 658 nm), whereas the PL of the filmwithout solvent annealing is almost completely sup-pressed to 2200 cps, almost into the backgroundnoise. For films with different ratios of RR-P3HTand PCBM, the PL intensities of films without sol-

vent annealing are decreased as compared to solvent annealedfilms by the following amounts: from 280 000 to 220 000 cps (by21 %) in pure RR-P3HT films, from 43 300 to 31 800 cps (by27 %) for 4:1 RR-P3HT:PCBM films, from 27 000 to 18 000 cps(by 33 %) for 2:1 RR-P3HT:PCBM films, from 22 000 to6800 cps (by 70 %) for 1:1 RR-P3HT:PCBM films, and from18 100 to 2200 cps (by > 88 %) for 1:2 RR-P3HT:PCBM films.

Several conclusions can be derived from these observations.1) The PCBM molecules act as defect sites and destroy the or-dering of RR-P3HT by disrupting the packing of the polymerchains during the solvent removal process and also by reducingthe polymer conjugation length. The greater the amount ofPCBM used, the more severe is the disruption. 2) TheRR-P3HT polymer has strong “self-healing” abilities due to in-terchain interactions. Solvent annealing helps to maintain theordered structure of RR-P3HT with up to 67 wt % PCBMloading in the blend.

The J–V characteristics of the corresponding solar cells areshown in Figure 6b. The differences observed between deviceswith and without solvent annealing follow the same trend asexpected from UV-vis and PL measurements. For devices fabri-cated using 4:1 RR-P3HT:PCBM films, the J–V characteristicswith and without annealing are quite similar. JSC in the solventannealed device is 5.32 mA cm–2, and is reduced by only about


Figure 5. 2D GIXRD patterns of 1:1 RR-P3HT:PCBM films (spin-coated at 1000 rpm)fabricated with different ts: a) 30 s and b) 80 s. 1D out-of-plane c) X-ray and d) azi-muthal scan (at q(100)) profiles extracted from (a) and (b). The results show that thefilm spin-coated for 30 s is more crystalline than the film spin-coated for 80 s. How-ever, even the latter film is clearly crystalline, indicating that it is intermediate betweenthe highly ordered and disordered phases. Longer ta (as shorter ts) tends to lead to awider variety of RR-P3HT molecular orientations in the films, which is also consistentwith the higher film roughness.

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3 % to 5.16 mA cm–2 in devices fabricated without solvent an-nealing. Both the VOC and FF are very close (0.636 vs. 0.633 Vand 49.7 vs. 47.5 % with and without solvent annealing) andthe combined effect is a slight decrease (by 7.7 %) in the PCEfrom 1.68 to 1.55 %. Solvent annealing has a greater influenceon 2:1 P3HT:PCBM devices. Without solvent annealing, JSC

decreases from 7.92 to 6.94 mA cm–1 (by 7 %); the FF is re-duced from 67.9 to 59.1 % (by 13 %). The VOC increasesslightly from 0.61 to 0.63 V, and the PCE decreases from 3.29to 2.58 % (by 21 %). When a PCBM loading of 67 wt % is used,along with significant reductions in the PL intensity and the ab-sorption in the red region of the electromagnetic spectrum, JSC

is drastically reduced from 8.29 to 1.33 mA cm–2 (by 84 %).The FF also decreases sharply from 62.8 to 37.4 %. The VOC in-creases to 0.70 V as a result of poor ordering in the polymer,and the PCE is reduced by 88 %, from 2.94 % for the annealedfilm to merely 0.35 % for the device without solvent annealing.Again, these results indicate that solvent annealing is critical to

heal the disruption in the ordering of the polymer induced byincorporating PCBM molecules in the blend system.

Of the solvent annealed “ordered” devices, the one with a4:1 weight ratio of RR-P3HT:PCBM shows a relatively low JSC

value (5.32 mA cm–2), as well as a low FF (49.7 %). This is mostlikely due to the low PCBM loading, which is just above thepercolation limit (16–17 wt %),[23,42] resulting in a much lowerelectron mobility than hole mobility. This is also corroboratedby our previous time-of-flight (TOF) results where the electronmobility was seen to increase monotonically with increasingPCBM loading.[43] The unbalanced electron and hole mobilitiesare believed to be behind the reduced FF of this device. Therelatively high FF values obtained for solvent annealed deviceswith RR-P3HT:PCBM ratios of 2:1, 1:1, and 1:2 by weight canalso be understood by the relatively balanced and non-disper-sive electron and hole mobilities, as verified by TOF measure-ments. Another interesting phenomenon relates to the changesin VOC observed here: as the PCBM loading increases from 25to 67 wt %, a monotonic decrease of VOC by 0.07 V is observedfor the solvent annealed devices. The VOC decreases from0.63 V for a 4:1 RR-P3HT:PCBM blend film to 0.61 V for a 2:1film, 0.58 V for a 1:1 film, and finally to 0.56 V for a 1:2 ratioof RR-P3HT:PCBM. However, these observations cannot beexplained by changes in the ordering of RR-P3HT (variationsin crystallinity) in the blend film, since there is no reason whyP3HT in the 25 wt % PCBM device should be any less orderedthan in the 67 wt % PCBM device. A similar phenomenon hasbeen observed in poly(2-methoxy-5-(2′-ethylhexoxy)-1,4-phen-ylenevinylene) (MEH-PPV):C60 BHJ solar cells; when the C60

loading fraction is increased from 0 to 20 %, there is a mono-tonic decrease in VOC from 1.60 V for the pure polymer to0.82–1.01 V for 2 wt % C60 devices fabricated using a varietyof organic solvents.[44] The cross-sectional area occupied byC60, as well as a morphology-induced interfacial factor, havebeen invoked to explain this phenomenon. Since the solventand film-development pattern are the same for all the solventannealed devices, changes in morphology are not likely to bevery significant in the present case.

We now further discuss the solvent annealing approach.Arnautov et al. have reported a slow solvent evaporation(SSE)[45] study of pure MEH-PPV; redder PL with clearervibronic structure is observed upon SSE, indicating closerpacking of the emissive long-conjugation-length polymerchains. Sirringhaus et al. have reported enhanced P3HT tran-sistor mobilities by using a high-boiling-point (219 °C) solvent,1,2,4-trichlorobenzene (TCB), instead of chloroform.[37] Forthe pure RR-P3HT films studied here, films without solventannealing have slightly weaker vibronic features in the absorp-tion spectra. The lineshapes of the PL spectra are also verysimilar and there is only a 21 % decrease in the PL intensity forfilms without solvent annealing, which is significantly smallerthan in the case of 50 and 67 wt % PCBM blend films. Solventannealing also has a negligible effect on devices with lowPCBM loadings; a monotonically increasing improvement isobserved at higher PCBM loading fractions. However, no suchimprovement has been observed for 1:4 (by weight) MEH-PPV:PCBM devices using the solvent annealing effect; the


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Figure 6. a) Effect of solvent annealing on the absorption and PL spectraof RR-P3HT:PCBM films with 4:1 and 1:2 weight ratios of the components.b) J–V characteristics of RR-P3HT:PCBM (4:1 squares, 2:1 circles, and 1:2triangles) solar cells with (solid symbols, 30 s) and without (hollow sym-bols, 80 s) solvent annealing. The differences in device performance be-come more pronounced for higher PCBM loading fractions.

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MEH-PPV absorption is retained even at 80 wt % PCBM load-ings. This is analogous to the effect of thermal annealing onRR-P3HT:PCBM and MEH-PPV:PCBM devices (without sol-vent annealing). The former devices are significantly improvedupon annealing, whereas the latter are only negligibly affected.The critical parameter behind the significant improvement indevice performance by solvent annealing is therefore believedto be the unique structure of RR-P3HT. RR-P3HT exhibitshigh crystallinity due to its high regioregularity and strong in-terchain interactions, which are not present in polymers likeMEH-PPV. The rapid removal of the solvent disrupts the pla-nar conformation and ordering of the P3HT chains during thetraditional spin-coating process; however, the intrinsic drivingforce for self-organization in RR-P3HT induces the recoveryof the ordered structure during the quasi-equilibrium solventannealing process. The data presented here indicates that thesolvent annealing process can be as short as one minute toachieve a reasonably high solar cell performance. This informa-tion is of significance for the future large-scale manufacture ofsolar cells with high throughput.

3. Conclusions

We have systematically varied the spin-coating time inRR-P3HT:PCBM solar cells to study the influence of the self-organization of the polymer on the performance of the solarcells. The beneficial aspects of solvent annealing (or slowgrowth) can be retained with a 1 min solvent annealing step fora 1:1 RR-P3HT:PCBM system. The advantages of the solventannealing approach are more pronounced at higher PCBMloadings. The variation in VOC is partially explained by thelonger conjugation length and formation of a band structure inwell-organized RR-P3HT domains in solvent annealed solarcells.

4. Experimental

The experimental procedures used were similar to that described byus previously [8]. The polymer photovoltaic devices were fabricated byspin-coating blends of RR-P3HT:PCBM in various weight ratios. Theblend films were sandwiched between a transparent anode and a cath-ode. The anode consisted of a glass substrate coated with indium tin ox-ide (ITO) and modified by spin-coating a poly(3,4-ethylenedioxythio-phene):poly(styrene sulfonate) (PEDOT:PSS) layer. The cathodeconsisted of Ca (ca. 25 nm) coated with Al (ca. 80 nm). Before devicefabrication, the ITO (ca. 150 nm) coated glass substrates were cleanedby sequential ultrasonic treatment in detergent, deionized water, ace-tone, and isopropyl alcohol. A thin layer (ca. 30 nm) of PEDOT:PSS(Baytron P VP A1 4083) was spin-coated to modify the ITO surface.After baking at 120 °C for ca. 20 min, the substrates were transferredto a N2-filled glove box (< 0.1 ppm O2 and H2O). RR-P3HT (pur-chased from Rieke Metals and used as received) was first dissolved inDCB to obtain a 20 mg mL–1 (2 wt %) solution, followed by blendingwith PCBM (Solenne, used as received) in various weight ratios. Theblend was stirred for ca. 14 h at 40 °C in a glove box. The films fabri-cated using a 1:1 mixture by weight of P3HT:PCBM were found to beca. 150 nm in thickness by profilometry (Dektek). The film thicknesswas chosen such that the transition from slow growth to fast growthfilms could be achieved by varying ts from ca. 20 to 80 s. The ts was thus

long enough to guarantee a very similar film thickness for all films(devices). The spin-coating was conducted in a glove box with dimen-sions of ca. 60 in. (length) × 30 in. (height) × 30 in. (width), and the filmwas open to the N2-filled atmosphere. The spinning speed was chosenas 1000 rpm and was preceded by two stages, one at 100 rpm and an-other at 300 rpm, both for 1 s each. The spin-coating process wasstopped after the 1000 rpm stage with natural deceleration. The spin-coated films (either liquid or solid, depending on ts) were then left in acovered glass petri dish with an inner radius of 3.5 in. and a height of0.5 in. As an example, spin-coating for 20 s at 1000 rpm resulted in taof ca. 20 min. The active device area was ca. 0.11 cm2. The samples forabsorption measurements were prepared by the same procedure beforethe cathode deposition step, and then measured using a Varian Cary 50UV-vis spectrophotometer. The J–V curves were measured under a N2

atmosphere using a Keithley 2400 source measurement unit. Thephotocurrent was measured under AM1.5G 100 mW cm–2 illuminationfrom a Thermal Oriel 96 000 150 W solar simulator. The light intensitywas determined using a mono-silicon detector (with a KG-5 visible col-or filter) calibrated by the National Renewable Energy Laboratory(NREL). All efficiency values reported in this work were corrected bythe spectral mismatch factor [46]. The IPCE values were measuredusing a lock-in amplifier (SR830, Stanford Research Systems) with acurrent preamplifier (SR570, Stanford Research Systems) under short-circuit conditions after illuminating the devices with monochromaticlight from a xenon lamp passing through a monochromator (Spectra-Pro-2150i, Acton Research). Prior to the IPCE measurements, thespectral response of the mono-silicon solar cell was measured and nor-malized to the NREL standards. The PL measurements were per-formed using a Jobin-Yvon-Spex Fluorolog fluorescence spectropho-tometer. The same set of samples was used for both UV-vis and PLmeasurements. A Digital Instruments Multimode scanning probe mi-croscope was used to obtain the AFM images. A silicon wafer with anatural oxide was used as the substrate for GIXRD measurements. Thesubstrates (glass for AFM and Si for GIXRD) were coated with PED-OT:PSS and RR-P3HT:PCBM films following the same procedures de-scribed above for device fabrication. 2D GIXRD measurements ofthese films were performed at beamline X21 of the National Synchro-tron Light Source (NSLS) at Brookhaven National Laboratory. Thesample was mounted on a two-axis goniometer on top of a x–z stageand the scattered intensity was recorded using a 2D Mar chargecoupled device (CCD) detector. The incident-beam angle was set to0.3° in order to increase the scattering intensity.

Received: July 16, 2006Revised: February 13, 2007

Published online: May 18, 2007

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“solvent annealing” effect in polymer solar cells based on poly(3-hexylthiophene) and...

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