Solvent Annealing Effect in Polymer Solar Cells Based on Poly(3-hexylthiophene) and Methanofullerenes**

Transcription

1 DOI: 1.12/adfm Solvent Annealing Effect in Polymer Solar Cells Based on Poly(3-hexylthiophene) and Methanofullerenes** By Gang Li, Yan Yao, Hoichang Yang, Vishal Shrotriya, Guanwen Yang, and Yang Yang* The self-organization of the polymer in solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) is studied systematically as a function of the spin-coating time (varied from 2 8 s), which controls the solvent annealing time t a, the time taken by the solvent to dry after the spin-coating process. These blend films are characterized by photoluminescence spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, and grazing incidence X-ray diffraction (GIXRD) measurements. The results indicate that the p-conjugated structure of RR-P3HT in the films is optimally developed when t a is greater than 1 min ( 5 s). For < 5 s, both the short-circuit current (J SC ) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 5 < < 55 s, the J SC and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6 % for a film with a highly ordered p-conjugated structure of RR-P3HT to 1.2 % for a less-ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon-to-electron conversion efficiency spectrum of the less-ordered solar cell shows a clear loss in both the overall magnitude and the long-wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt %). Under solvent annealing conditions, the polymer is seen to be ordered even at 67 wt % PCBM loading. The open-circuit voltage (V OC ) is also affected by the ordering of the polymer and the PCBM loading in the active layer. 1. Introduction Since the discovery of efficient electron transfer between fullerenes and conjugated polymers in bulk-heterojunction (BHJ) polymer solar cells, [1 3] a lot of attention has been focused on engineering the properties of these systems. Especially, over the last five years, major improvements have been achieved in the performance of polymer solar cells. [4 12] The promising performance of these improved polymer solar cells, in conjunction [*] Prof. Y. Yang, Dr. G. Li, Y. Yao, G. Yang Department of Materials Science and Engineering University of California, Los Angeles Los Angeles, CA 995 (USA) yangy@ucla.edu Dr. H. Yang Rensselaer Nanotechnology Center Rensselaer Polytechnic Institute Troy, NY 1218 (USA) Dr. V. Shrotriya Solarmer Energy, Inc. El Monte, California (USA) [**] G. Li and Y. Yao contributed equally to this work. The authors acknowledge the financial support provided by the Office of Naval Research (ONR) (Grant number N ), Solarmer Energy Inc., University of California Discovery Grant, and the Nanoscale Science and Engineering Institute of the National Science Foundation (NSF) (Grant number DMR ). Technical discussions with Dr. Brian Gregg of NREL are greatly appreciated. with their low cost and ease of large-area processing, has lead to much interest from both academic and industrial researchers for the development of inexpensive solar energy harvesting systems. Of all the polymeric systems reported in the literature for the fabrication of BHJ cells, the regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) system currently represents the state-of-art in polymer solar cells. [7 12] New device structures [13 15] and novel materials based on polythiophenes (PTs) [16] have also been devised to further enhance the performance of polymeric solar cells. Several approaches have been used to enhance the performance of RR-P3HT:PCBM solar cells, including thermal and electrical annealing [5,7,9,17] and slow growth. [8] In the slow growth approach, the film is allowed to stand in the liquid phase 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 method significantly improves the efficiency of RR-P3HT:PCBMbased solar cells, similar to the thermal annealing approach, we use the term solvent annealing to describe this process here. In combination with a thermal annealing step, a power conversion efficiency (PCE) of over 4 % has been achieved for RR-P3HT:PCBM solar cells under AM1.5G standard reference conditions (SRC); a very impressive PCE of 3.5 % has been achieved by using only the solvent annealing approach. [8] In this Full Paper, we present a systematic study of the effect of solvent annealing on RR-P3HT:PCBM polymer solar cells with the only variable being the film spin-coating time ( ). We further define a term called the solvent annealing time (t a ), WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 27, 17,

2 which is the time taken by the solvent to dry after the spincoating process. These two parameters, and t a, are correlated: the shorter the spin-coating time, the longer is the solvent annealing time required for the film to dry. Spin-coating is a fast drying process, whereas solvent annealing is a slow growth process. By systematically varying (and thus t a ), we have conducted a comprehensive investigation of the evolution of the morphology of the RR-P3HT:PCBM polymer films and the performance of the polymer solar cells. At a spinning speed of 1 rpm, the long-wavelength absorption of RR-P3HT is observed to be well preserved for t a longer than 1 min in a RR-P3HT:PCBM system with a 1:1 ratio by weight of the components. A major loss in absorption over the long-wavelength range is observed for a specific window (5 55 s), as detected by UV-vis spectroscopy and corroborated by measurements of the corresponding devices. This indicates a transition from a highly ordered active layer to a less ordered film. Devices with highly ordered films show much higher PCE values and photoluminescence (PL) intensities as compared to devices with less ordered films, indicating that maximum PL quenching does not always lead to the best device performance, even within the same system. Atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD) results provide further evidence for the influence of and t a. Studies on devices with various PCBM concentrations (25 67 wt %) unambiguously show that for higher PCBM loading fractions, the RR-P3HT polymer tends to be less ordered in films withouolvent annealing. However, after solvent annealing, ordering of the RR-P3HT component has been observed for up to 67 % PCBM loading. 2. Results and Discussion The experiments have been designed based on several axioms. Firstly, both conventional spin-coating (fast drying) and solvent annealing (slow growth) processes transform the polymer solution into a solid polymer film. Secondly, during the spin-coating process, the effective solvent evaporation rate is much higher than that in an isolated glass petri dish. Therefore, the length of effectively determines the time required to dry the film, i.e., the solvent annealing time t a. Dichlorobenzene (DCB) has been chosen as the solvent because its high boiling point allows a wide window of spin-coating times to be used. At a spin-coating speed of 1 rpm, the solvent evaporates much faster than for static samples (such as in a glass petri dish). We notice that with an increase in from 2 to 5 s, t a decreases dramatically from ca. 2 to 1 min, as detected by the time required for a dramatic change in the color of the film when it is transformed from the liquid (orange) to the solid (dark purple) phase. As described above, during the spin-coating process, the majority of the solution is removed from the substrate by the exerted centrifugal force and the film thickness decreases very rapidly. After longer spinning times, an equilibrium state is established and the film thickness becomes constant. At a spinning speed of 1 rpm, profilometry (Dektek) results indicate that the film is ca. 165 nm thick for = 2 s; the rest of the samples ( = 3 8 s) exhibit a film thickness of 15 ± 5 nm. All the experiments have been conducted at room temperature. At elevated temperatures, the solvent will be removed more rapidly and the blend film will lose its crystallinity. As illustrated in a previous report, under a N 2 environment, t a is reduced from 3 min at room temperature to 2 s at 7 C; this is accompanied by a concomitant decrease in the long-wavelength absorption and a significant degradation in the performance of the device. [8] The same trend is expected if the temperature is increased during the spin-coating process, although we have not tested this hypothesis due to experimental limitations. The UV-vis spectra shown in Figure 1a are quite similar for solvent annealed films with t a varying from 2 to 1 min (corresponding to varying from 2 to 5 s). In all the cases, the vibronic features are clearly observed, indicating that a 1 min solvent annealing step is enough to maintain the ordering of RR-P3HT in a 1:1 (by weight) RR-P3HT:PCBM blend film. The film properties start to change for of 52 and 55 s. The = 52 s film starts to solidify immediately after the spin-coating process, and it takes only ca. 2 s to observe a full color change. When the is increased to 55 s, the films solidify PL (c.p.s) O.D (a) (b) λ (nm) (s) t a (s) # 2 ~ ~ ~ ~ ~ N/A 6 8 N/A λ (nm) (s) # Figure 1. a) Absorption and b) PL spectra of nm thick 1:1 RR-P3HT:PCBM films fabricated by spin-coating for different times ( from 2 s for # 1 to 8 s for # 7). As becomes longer, the absorption in the long-wavelength region and the PL intensity are decreased. Adv. Funct. Mater. 27, 17, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 immediately after spin-coating. However, when these films are left in glass petri dish, a further darkening of the color is observed, indicating that a small amount of the DCB residue is still present in the films. The UV-vis spectra of the film fabricated with = 55 s show a significant decrease in absorption in the red region of the electromagnetic spectrum and the originally strong vibronic shoulders are significantly diminished. A of 8 s represents an extreme case of minimal solvent annealing, and the film shows a further decrease in absorption in the red region and a diminution of the vibronic features. For RR-P3HT:PCBM films, the solvent annealed films have the most prominent vibronic features reported in the literature, indicating strong interchain interlayer interactions [18 21] among the RR-P3HT chains as well as good polymer ordering in the blend films. Apart from the film with = 2 s, the absorption spectra below 45 nm are very similar for all the films, indicating that all the films have a similar thickness (i.e., similar amounts of RR-P3HT and PCBM). The PCBM absorption is clearly unaffected by t a, which indicates that the driving force for better device properties is the self-organization of the polymer through a solvent annealing process. Conformational chain defects (twists or disruptions of planarity) can interrupt polymer conjugation and lead to a smaller p-conjugation length and blue-shifting of the p p* absorption band in linear conjugated polymers like P3HT. [22 24] Highly regioregular P3HT can self-organize to form semicrystalline lamellar morphologies with higher order architectures, [18 21] thereby yielding high hole mobilities, which are important for both field effect transistor (FET) [21] and solar cell applications. The RR-P3HT absorption at longer wavelengths as well as the vibronic shoulders are significantly reduced in intensity in RR-P3HT:PCBM films with high loading fractions of PCBM fabricated in the traditional way without any solvent annealing (yellow phase). This indicates a partial loss of crystallinity and/or a smaller conjugation length in RR-P3HT due to disruptions introduced by PCBM. The quick removal of the solvent favors a uniform P3HT:PCBM distribution and prevents the formation of highly ordered lamellae by disrupting the interchain interactions between RR-P3HT chains. Upon solvent annealing, the RR-P3HT absorption and the prominent vibronic features are restored for the blend films. [25] The absorption spectra of the blend films represent a simple addition of the spectra of PCBM and RR-P3HT films, which strongly supports the occurrence of phase segregation and the formation of crystalline RR-P3HT domains in the blend films. PL quenching provides direct evidence for exciton dissociation, and thus efficient PL quenching is necessary to obtain efficient organic solar cells. However, this does not necessarily mean that the stronger the PL quenching, the better the performance of the solar cells. Figure 1b shows PL spectra for the same set of films shown in Figure 1a with different degrees of solvent annealing. The PL spectra of films # 1 to 3 with of 2 to 4 s are characterized by similar PL intensities of 2 5 to 21 9 counts per second (cps) (as compared to 28 cps for pure P3HT films). For films with a of 5 s (# 4), the PL intensity drops slightly to 19 2 cps. However, further increasing drastically reduces the PL intensity to 68 cps in film # 7 ( of G. Li et al./ Solvent Annealing Effect in Polymer Solar Cells 8 s), which is less than 3 % of the corresponding value for solvent annealed films. Since the system is polycrystalline in nature, the angle dependence of the PL is weak. The scaling of the PL intensity with film thickness has a minimal effect and does not alter the conclusions presented here. Earlier studies on PTs have shown that regiorandom P3HT (absorption is blue-shifted with respect to RR-P3HT) has a more than one order of magnitude higher PL quantum efficiency (g = 8 %) than RR-P3HT (92 % RR) (g <.5 %). [26] This is explained by the fact thaelf-assembled RR-P3HT lamellae are characterized by strong interchain interlayer interactions, which split the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, with the lowered LUMO level becoming optically forbidden, [27] thus resulting in weaker radiative transitions. The apparent discrepancy between the absorption and PL measurements is resolved by considering the ultrafast (ca. 4 1 fs) photoinduced charge transfer process at the donor/acceptor (D/A) interface, [2] which clearly leads to a difference in the PL quantum efficiencies. In disordered blend films, the D/A interface is larger since the donor and acceptor distribution is frozen during the spin-coating process from homogeneous blend solutions. In addition, in the locally self-organized RR-P3HT domains, defecites introduced by PCBM as well as non-radiative RR-P3HT decay channels are eliminated, which might also contribute to PL enhancement. Eight devices have been fabricated for each type of film and the measured device parameters are plotted in Figure 2. Figure 2a shows the current voltage (J V) curves for the best devices of each type. Figure 2b shows the statistical data for device parameters derived from eight devices of each type. The maximum short-circuit current (J SC ) is 9.6 ma cm 2 in device # 1, corresponding to the longest t a (ca. 2 min, or a short ). However, reducing t a to as short as 1 min ( is 5 s) only leads to a small decrease in J SC (J SC = 9.1 ma cm 2 ). The increased thickness of the films (# 1) may also result in slightly larger values of J SC. Further increasing the spinning time leads to a rapid decrease in J SC to 4. ma cm 2 for of 55 s and 2.8 ma cm 2 for of 8 s. This trend an initial plateau for the J SC and PCE values, followed by a quick decrease agrees well with both the UV-vis and PL results. A similar trend has been observed for the open-circuit voltage V OC. The V OC is ca..58 V for of 2 to 5 s, and dramatically increases to.67 V for the device fabricated with a of 8 s. However, the fill factor (FF) does noignificantly change even with drastic variations in J SC. Apart for the 55 s device (FF = 58.3 %), all the other devices exhibit high FF values ranging from 62.2 to 64.8 %. We conjecture that this indicates that the deterioration of the devices due to the loss of polymer ordering starts with a decrease in the J SC values, followed by a lowering of the FF values. At a relatively low spinning speed (1 rpm), even the device with = 8 s might represent a film with an intermediate degree of order, i.e., an intermediate state between a device with a highly ordered film (expected to exhibit high J SC and high FF) and one with a highly disordered film (expected to exhibit low J SC and low FF). The AFM morphology data and GIXRD measurements discussed below further corroborate WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 27, 17,

4 J sc (ma/cm 2 ) PCE (%) J sc (ma/cm 2 ) (a) (b) Bias (V) (s) # (s) (s) Figure 2. a) The current voltage characteristics of polymer solar cells with seven different films spin-coated for different times as the active layers (the films are nm thick). b) Device parameters derived from eight devices of each type. V oc (V) Fill Factor this hypothesis. The combined effect is that the PCE is reduced slightly from 3.64 % ( = 2 s device) to 3.38 % ( = 5 s device) and then significantly drops to 1.17 % for the device with =8 s. The ordering of the RR-P3HT domains is crucial for light absorption, whereas the formation of interpenetrating networks of RR-P3HT and PCBM is important for facilitating charge transport. This leads to a reduction of the interface area between RR-P3HT and PCBM. The high-performance solar cells fabricated from solvent annealed films indicate that the interface area is sufficiently large (with significant PL quenching) and provides sufficient exciton dissociation. In the trade-off between a) efficient charge separation and b) absorption plus charge transport for maximizing the efficiency of polymer solar cells, the benefits of the latter clearly dominate that of the former in the RR-P3HT:PCBM system. The size of the polymer crystalline domains is also estimated to be comparable to the exciton diffusion length (i.e., a few nanometers). The V OC in polymer solar cells has been widely discussed over the years. With the metal insulator metal (MIM) [28] picture noufficient to explain the phenomena observed, the most widely propagated belief is that for systems with ohmic contact at both electrodes, V OC varies linearly with the energy difference between the donor HOMO and the acceptor LUMO. [29] There is also some evidence for the influence of surface dipoles in these devices. [15,3] However, here, with all the parameters being the same except, it is interesting to see a variation of ca..1 V in the V OC. Two mechanisms are conjectured to contribute to this reduction in V OC. The first possibility is the formation of a band structure [31] instead of molecular energy levels (or alternatively the splitting of the HOMO and LUMO levels) [27] due the strong interchain interlayer interactions originating from the relatively high ordering of RR-P3HT in the solvent annealed films, which thus leads to a reduced effective bandgap relative to the HOMO LUMO difference. A second possibility is that in solvent annealed films, RR-P3HT has a considerably longer conjugation length on average than in films withouolvent annealing, [25] as indicated by the much stronger absorption intensity in the longer wavelength region, as discussed above. Therefore, the reduction in V OC is reasonable given that the difference between the effective HOMO of the polymer and the LUMO of the acceptor decreases. Chen et al. [19] have observed that the band edge of regioregular head tail (HT) P3HT is.4 ev lower than that of regiorandom P3HT. It is reasonable to expect that P3HT in a solvent annealed 1:1 blend film is RR-P3HT, whereas P3HT in a blend film that has not been annealed is expected to be similar to regiorandom P3HT. Similar changes in V OC have been reported previously in the literature for thermally annealed P3HT:PCBM systems; [5,7,9 12] while the decrease in V OC upon thermal annealing has been ascribed to vertical phase segregation, [17] the improved crystalline ordering of RR-P3HT could play a dominant role in the process. The incident photon-to-electron conversion efficiency (IPCE) results of four devices with of 2, 5, 55, and 8 s are shown in Figure 3. Consistent with the trend in J SC, the IPCE magnitude is firslightly reduced from 63.3 % (#1, =2 s)to 59.1 % (#4, = 5 s), followed by a rapid decrease to 44.3 % IPCE (%) λ (nm) (s) # Figure 3. IPCE spectra of polymer solar cells # 1, 4, 6, and 7 with spincoating times of 2, 5, 55, and 8 s, respectively. Both the IPCE magnitude and the photoresponse in the red region of the electromagnetic spectrum are reduced as increases. Adv. Funct. Mater. 27, 17, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 for a device with a of 55 s (#6), and finally to 23.8 % for a device with a of 8 s (#7). In addition to the observed change in magnitude, the lineshape of the IPCE curve is also significantly altered. Device 1 shows a plateau in the visible region, with the IPCE being greater than 58 % from 46 to 6 nm. The photoresponse of Device 4 is diminished in the red region of the electromagnetic spectrum close to 6 nm. The IPCE curves for Devices 6 and 7, in which the polymer ordering is significantly lower, clearly show a drastic reduction in intensity in the red region after a peak at 51 nm. Quantitatively, the ratios of the IPCE values at 61 nm to the peak IPCE values (i.e., IPCE k = 61 nm /IPCE max ) in Devices 1, 4, 6, and 7 are 9, 83, 58, and 5 %, respectively. This trend in the IPCE characteristics closely follows the trend observed by UV-vis spectroscopy indicating a loss of polymer ordering in the blend films. The reduced absorption in the red region of the electromagnetic spectrum leads to a quick drop in the IPCE of the devices. Previous studies indicate that polymer crystallization from solution into a thin film is a complex exothermic process, which can be affected by the solution evaporation rate. [32] The mesoscale film morphology in the lateral direction of the RR-P3HT:PCBM (1:1 weight ratio) films has been visualized using tapping mode atomic force microscopy (TM-AFM). Figure 4 shows typical TM-AFM topographic and phase images of RR-P3HT:PCBM blend films spin-coated at 1 rpm with different (3 and 8 s). It has been found that the surface topography of the film spin-coated for 3 s is significantly rougher than the film cast for 8 s; the former films have a root mean square (rms) roughness of 3. nm (Fig. 4a), as compared to.8 nm (Fig. 4c) for the latter films. The fibrillar crystalline Figure 4. TM-AFM topography (left) and phase (right) images for RR-P3HT:PCBM films fabricated using different processing conditions. The spin-coating times are a,b) 3 s and c,d) 8 s. domains of RR-P3HT are clearly visible in both cases, but the widths of the nanofibrillar domains are clearly different. Recently, Brinkmann et al. investigated the semicrystalline structure of RR-P3HT (molecular weight (MW) = 35 ) and found a lamellar periodicity of approximately 28 nm. [33] This 28 nm long-range order includes the crystalline region as well as disordered zones which harbor structural defects like chain ends and folds and tie segments. [33] Our observations of these solar cell systems (RR-P3HT, MW of 3 ) agree very well with the above findings; Figure 4b and d indicate the 28 nm periodicity of the polymeric structures. However, different processing conditions (3 and 8 s) can result in different combinations of crystalline and disordered regions in the periodic structure. It has been found that the films cast for 8 s show a longer disordered zone (crystalline grain boundaries (GBs) are visible even in the TM-AFM topography image shown in Fig. 4c) as compared to films cast for 3 s, resulting in a higher energy barrier for inter-fibrillar hopping and therefore a lower carrier mobility. [34] This agrees well with previous studies by Kline et al. [35] and Zhang et al. [36] that have demonstrated the correlation between FET mobility and the GBs/nanofibril-widths of RR-P3HT in the FETs. Since AFM observations are limited only to the surface of the film, the overall crystalline structure of RR-P3HT in the blend films has been further studied by GIXRD. As shown in Figure 5, 2D GIXRD patterns of films cast for 3 (Fig. 5a) and 8 s (Fig. 5b) clearly show the intense reflections of the (1) layer and the (1) crystals along the q z (substrate normal) and q xy (substrate parallel) axes, respectively, implying that these films have very highly ordered edge-on hexyl sidechains and that the p-conjugated planes of RR-P3HT are parallel with respect to the substrate. [32,37] However, based on 1D outof-plane X-ray (Fig. 5c) and azimuthal angle scan (Fig. 5d) profiles extracted from the 2D GIXRD data, we have determined that the film cast for 3 s has a much higher crystallinity than the film cast for 8 s with longer t a. In addition, from the broadening of the azimuthal peaks, it appears that a longer solvent drying process possibly leads to a broader distribution of the molecular orientations of RR-P3HT in the films. This result is consistent with the relatively higher film roughness observed for films cast for 3 s as compared to films cast for 8 s (Fig. 4). Based on the TM-AFM and GIXRD data, we also conclude that the loss of P3HT long-wavelength absorption in films cast for 8 s is mainly due to the smaller conjugation length of the polymer rather than because of a decrease in crystallinity. As discussed above, the film cast for 8 s represents an intermediate situation, and the FF is less sensitive to carrier mobility than the photocurrent. [38] This partially WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 27, 17,

6 Figure 5. 2D GIXRD patterns of 1:1 RR-P3HT:PCBM films (spin-coated at 1 rpm) fabricated with different : a) 3 s and b) 8 s. 1D out-of-plane c) X-ray and d) azimuthal scan (at q (1) ) profiles extracted from (a) and (b). The results show that the film spin-coated for 3 s is more crystalline than the film spin-coated for 8 s. However, even the latter film is clearly crystalline, indicating that it is intermediate between the highly ordered and disordered phases. Longer t a (as shorter ) tends to lead to a wider variety of RR-P3HT molecular orientations in the films, which is also consistent with the higher film roughness. explains the high FF of these devices although J SC is reduced from 9.6 to 2.8 ma cm 2. GIXRD results indicate that in these blend films, the P3HT chains have the same edge-on orientation on the substrate as in pure P3HT films. [32,37] For films cast for 8 s, the peak maximum of the scattering vector q z is at.378 Å 1, corresponding to an interlayer spacing of 16.6 Å, whereas for the films cast for 3 s (highly crystalline), the interlayer spacing is determined to be 16.1 Å. These results support the hypothesis that films cast for 3 s have stronger interlayer interactions within the RR-P3HT domains as compared to films cast for 8 s. Since carrier transport occurs perpendicular to the polymer layers, hole transport in devices based on films cast for 3 s is easier, which is consistent with the J V characteristics. The TM-AFM and GIXRD results provide strong evidence that the highly preferred morphology of interpenetrating BHJ solar cells is realized by the solvent-annealing approach. The PCBM loading in the blend system is an important parameter controlling the performance of the polymer solar cells. [39 41] We now study the effect of PCBM loading on the ordering of RR-P3HT in blend systems with and withouolvent annealing; our results vary significantly from previous reports in the literature. [39] 2 wt % RR-P3HT (in DCB) is used as the starting 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 1 rpm and of 3 and 8 s have been used to produce films with and withouolvent annealing. The UV-vis spectra shown in Figure 6a for 4:1 RR-P3HT:PCBM films spin-coated for different times are very similar. The film withouolvent annealing shows a slight reduction in the two long-wavelength vibronic features and a very small blue-shift in the absorption. The effect of solvent annealing is thus much less than for the 1:1 films. On the other hand, upon increasing the PCBM loading in the blend to 67 wt %, the solvent annealed films demonstrate significant absorption in the red region of the electromagnetic spectrum, as well as distinct vibronic features, whereas the vibronic features are almost completely lost when no solvent annealing is performed. Figure 6a also shows the PL spectra of these four films. For 4:1 RR-P3HT:PCBM films, the PL intensity of the solvent annealed film is 43 8 cps at 639 nm. The PL peak decreases in intensity to 31 8 cps withouolvent annealing a 27 % reduction in the PL intensity. This is much smaller than the 7 % reduction in PL intensity (21 9 to 68 cps) observed for 1:1 RR-P3HT:PCBM films. The differences in the PL intensity become much larger when the PCBM loading is increased to 67 wt %. The film with solvent annealing has a PL peak intensity of 18 1 cps (at 658 nm), whereas the PL of the film withouolvent annealing is almost completely suppressed to 22 cps, almost into the background noise. For films with different ratios of RR-P3HT and PCBM, the PL intensities of films withouolvent annealing are decreased as compared to solvent annealed films by the following amounts: from 28 to 22 cps (by 21 %) in pure RR-P3HT films, from 43 3 to 31 8 cps (by 27 %) for 4:1 RR-P3HT:PCBM films, from 27 to 18 cps (by 33 %) for 2:1 RR-P3HT:PCBM films, from 22 to 68 cps (by 7 %) for 1:1 RR-P3HT:PCBM films, and from 18 1 to 22 cps (by > 88 %) for 1:2 RR-P3HT:PCBM films. Several conclusions can be derived from these observations. 1) The PCBM molecules act as defecites and destroy the ordering of RR-P3HT by disrupting the packing of the polymer chains during the solvent removal process and also by reducing the polymer conjugation length. The greater the amount of PCBM used, the more severe is the disruption. 2) The RR-P3HT polymer has strong self-healing abilities due to interchain interactions. Solvent annealing helps to maintain the ordered structure of RR-P3HT with up to 67 wt % PCBM loading in the blend. The J V characteristics of the corresponding solar cells are shown in Figure 6b. The differences observed between devices with and withouolvent annealing follow the same trend as expected from UV-vis and PL measurements. For devices fabricated using 4:1 RR-P3HT:PCBM films, the J V characteristics with and without annealing are quite similar. J SC in the solvent annealed device is 5.32 ma cm 2, and is reduced by only about Adv. Funct. Mater. 27, 17, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

7 O.D (a) J (ma/m 2 ) P3HT:PCBM 4:1 3s 4:1 8s 1:2 3s 1:2 8s λ (nm) -5 P3HT:PCBM (s) -6 4:1 3 4: : :1 8 1: : Bias (V) (b) 5 x1 4 3 % to 5.16 ma cm 2 in devices fabricated withouolvent annealing. Both the V OC and FF are very close (.636 vs..633 V and 49.7 vs % with and withouolvent annealing) and the combined effect is a slight decrease (by 7.7 %) in the PCE from 1.68 to 1.55 %. Solvent annealing has a greater influence on 2:1 P3HT:PCBM devices. Withouolvent annealing, J SC decreases from 7.92 to 6.94 ma cm 1 (by 7 %); the FF is reduced from 67.9 to 59.1 % (by 13 %). The V OC increases slightly from.61 to.63 V, and the PCE decreases from 3.29 to 2.58 % (by 21 %). When a PCBM loading of 67 wt % is used, along with significant reductions in the PL intensity and the absorption in the red region of the electromagnetic spectrum, J SC 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 V OC increases to.7 V as a result of poor ordering in the polymer, and the PCE is reduced by 88 %, from 2.94 % for the annealed film to merely.35 % for the device withouolvent annealing. Again, these results indicate thaolvent annealing is critical to PL (c.p.s) Figure 6. a) Effect of solvent annealing on the absorption and PL spectra of 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 1:2 triangles) solar cells with (solid symbols, 3 s) and without (hollow symbols, 8 s) solvent annealing. The differences in device performance become more pronounced for higher PCBM loading fractions. G. Li et al./ Solvent Annealing Effect in Polymer Solar Cells heal the disruption in the ordering of the polymer induced by incorporating PCBM molecules in the blend system. Of the solvent annealed ordered devices, the one with a 4:1 weight ratio of RR-P3HT:PCBM shows a relatively low J SC value (5.32 ma cm 2 ), as well as a low FF (49.7 %). This is most likely due to the low PCBM loading, which is just above the percolation limit (16 17 wt %), [23,42] resulting in a much lower electron mobility than hole mobility. This is also corroborated by our previous time-of-flight (TOF) results where the electron mobility was seen to increase monotonically with increasing PCBM loading. [43] The unbalanced electron and hole mobilities are believed to be behind the reduced FF of this device. The relatively high FF values obtained for solvent annealed devices with RR-P3HT:PCBM ratios of 2:1, 1:1, and 1:2 by weight can also be understood by the relatively balanced and non-dispersive electron and hole mobilities, as verified by TOF measurements. Another interesting phenomenon relates to the changes in V OC observed here: as the PCBM loading increases from 25 to 67 wt %, a monotonic decrease of V OC by.7 V is observed for the solvent annealed devices. The V OC decreases from.63 V for a 4:1 RR-P3HT:PCBM blend film to.61 V for a 2:1 film,.58 V for a 1:1 film, and finally to.56 V for a 1:2 ratio of RR-P3HT:PCBM. However, these observations cannot be explained by changes in the ordering of RR-P3HT (variations in crystallinity) in the blend film, since there is no reason why P3HT in the 25 wt % PCBM device should be any less ordered than in the 67 wt % PCBM device. A similar phenomenon has been observed in poly(2-methoxy-5-(2 -ethylhexoxy)-1,4-phenylenevinylene) (MEH-PPV):C 6 BHJ solar cells; when the C 6 loading fraction is increased from to 2 %, there is a monotonic decrease in V OC from 1.6 V for the pure polymer to V for 2 wt % C 6 devices fabricated using a variety of organic solvents. [44] The cross-sectional area occupied by C 6, as well as a morphology-induced interfacial factor, have been invoked to explain this phenomenon. Since the solvent and film-development pattern are the same for all the solvent annealed devices, changes in morphology are not likely to be very 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 clearer vibronic structure is observed upon SSE, indicating closer packing of the emissive long-conjugation-length polymer chains. Sirringhaus et al. have reported enhanced P3HT transistor mobilities by using a high-boiling-point (219 C) solvent, 1,2,4-trichlorobenzene (TCB), instead of chloroform. [37] For the pure RR-P3HT films studied here, films withouolvent annealing have slightly weaker vibronic features in the absorption spectra. The lineshapes of the PL spectra are also very similar and there is only a 21 % decrease in the PL intensity for films withouolvent annealing, which is significantly smaller than in the case of 5 and 67 wt % PCBM blend films. Solvent annealing also has a negligible effect on devices with low PCBM loadings; a monotonically increasing improvement is observed at higher PCBM loading fractions. However, no such improvement has been observed for 1:4 (by weight) MEH- PPV:PCBM devices using the solvent annealing effect; the WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 27, 17,

8 MEH-PPV absorption is retained even at 8 wt % PCBM loadings. This is analogous to the effect of thermal annealing on RR-P3HT:PCBM and MEH-PPV:PCBM devices (withouolvent annealing). The former devices are significantly improved upon annealing, whereas the latter are only negligibly affected. The critical parameter behind the significant improvement in device performance by solvent annealing is therefore believed to be the unique structure of RR-P3HT. RR-P3HT exhibits high crystallinity due to its high regioregularity and strong interchain interactions, which are not present in polymers like MEH-PPV. The rapid removal of the solvent disrupts the planar conformation and ordering of the P3HT chains during the traditional spin-coating process; however, the intrinsic driving force for self-organization in RR-P3HT induces the recovery of the ordered structure during the quasi-equilibrium solvent annealing process. The data presented here indicates that the solvent annealing process can be as short as one minute to achieve a reasonably high solar cell performance. This information is of significance for the future large-scale manufacture of solar cells with high throughput. 3. Conclusions We have systematically varied the spin-coating time in RR-P3HT:PCBM solar cells to study the influence of the selforganization of the polymer on the performance of the solar cells. The beneficial aspects of solvent annealing (or slow growth) can be retained with a 1 min solvent annealing step for a 1:1 RR-P3HT:PCBM system. The advantages of the solvent annealing approach are more pronounced at higher PCBM loadings. The variation in V OC is partially explained by the longer conjugation length and formation of a band structure in well-organized RR-P3HT domains in solvent annealed solar cells. 4. Experimental The experimental procedures used were similar to that described by us previously [8]. The polymer photovoltaic devices were fabricated by spin-coating blends of RR-P3HT:PCBM in various weight ratios. The blend films were sandwiched between a transparent anode and a cathode. The anode consisted of a glass substrate coated with indium tin oxide (ITO) and modified by spin-coating a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer. The cathode consisted of Ca (ca. 25 nm) coated with Al (ca. 8 nm). Before device fabrication, the ITO (ca. 15 nm) coated glass substrates were cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol. A thin layer (ca. 3 nm) of PEDOT:PSS (Baytron P VP A1 483) was spin-coated to modify the ITO surface. After baking at 12 C for ca. 2 min, the substrates were transferred to a N 2 -filled glove box (<.1 ppm O 2 and H 2 O). RR-P3HT (purchased from Rieke Metals and used as received) was first dissolved in DCB to obtain a 2 mg ml 1 (2 wt %) solution, followed by blending with PCBM (Solenne, used as received) in various weight ratios. The blend was stirred for ca. 14 h at 4 C in a glove box. The films fabricated using a 1:1 mixture by weight of P3HT:PCBM were found to be ca. 15 nm in thickness by profilometry (Dektek). The film thickness was chosen such that the transition from slow growth to fast growth films could be achieved by varying from ca. 2 to 8 s. The 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 dimensions of ca. 6 in. (length) 3 in. (height) 3 in. (width), and the film was open to the N 2 -filled atmosphere. The spinning speed was chosen as 1 rpm and was preceded by two stages, one at 1 rpm and another at 3 rpm, both for 1 s each. The spin-coating process was stopped after the 1 rpm stage with natural deceleration. The spincoated films (either liquid or solid, depending on ) were then left in a covered glass petri dish with an inner radius of 3.5 in. and a height of.5 in. As an example, spin-coating for 2 s at 1 rpm resulted in t a of ca. 2 min. The active device area was ca..11 cm 2. The samples for absorption measurements were prepared by the same procedure before the cathode deposition step, and then measured using a Varian Cary 5 UV-vis spectrophotometer. The J V curves were measured under a N 2 atmosphere using a Keithley 24 source measurement unit. The photocurrent was measured under AM1.5G 1 mw cm 2 illumination from a Thermal Oriel W solar simulator. The light intensity was determined using a mono-silicon detector (with a KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). All efficiency values reported in this work were corrected by the spectral mismatch factor [46]. The IPCE values were measured using a lock-in amplifier (SR83, Stanford Research Systems) with a current preamplifier (SR57, Stanford Research Systems) under shortcircuit conditions after illuminating the devices with monochromatic light from a xenon lamp passing through a monochromator (Spectra- Pro-215i, Acton Research). Prior to the IPCE measurements, the spectral response of the mono-silicon solar cell was measured and normalized to the NREL standards. The PL measurements were performed using a Jobin-Yvon-Spex Fluorolog fluorescence spectrophotometer. The same set of samples was used for both UV-vis and PL measurements. A Digital Instruments Multimode scanning probe microscope was used to obtain the AFM images. A silicon wafer with a natural oxide was used as the substrate for GIXRD measurements. 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