In situ Current Voltage Measurements for Optimization of a Novel Fullerene Acceptor in Bulk Heterojunction Photovoltaics

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1 FULL PAPER JOURNAL OF POLYMER SCIENCE In situ Current Voltage Measurements for Optimization of a Novel Fullerene Acceptor in Bulk Heterojunction Photovoltaics Christopher G. Shuttle, 1 Neil D. Treat, 1,2 Jian Fan, 3 Alessandro Varotto, 3 Craig J. Hawker, 1,2,3 Fred Wudl, 2,3 Michael L. Chabinyc 1,2 1 Department of Materials, University of California Santa Barbara, California Materials Research Laboratory, University of California Santa Barbara, California Department of Chemistry and Biochemistry, University of California Santa Barbara, California Correspondence to: M. L. Chabinyc ( mchabinyc@engineering.ucsb.edu) Received 16 August 2011; revised 5 October 2011; accepted 6 October 2011; published online 31 October 2011 DOI: /polb ABSTRACT: The evaluation of the power conversion efficiency (PCE) of new materials for organic bulk heterojunction (BHJ) photovoltaics is difficult due to the large number of processing parameters possible. An efficient procedure to determine the optimum conditions for thermal treatment of polymer-based bulk heterojunction photovoltaic devices using in situ currentvoltage measurements is presented. The performance of a new fullerene derivative, 1,9-dihydro-64,65-dihexyloxy-1,9-(methano[1,2] benzomethano)fullerene[60], in BHJ photovolatics with poly(3-hexylthiophene) (P3HT) was evaluated using this methodology. The device characteristics of BHJs obtained from the in situ method were found to be in good agreement with those from BHJs annealed using a conventional process. This fullerene has similar performance to 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methano fullerene in BHJs with P3HT after thermal annealing. For devices with thickness of 70 nm, the short circuit current was 6.24 ma/cm 2 with a fill factor of 0.53 and open circuit voltage of 0.65 V. The changes in the current-voltage measurements during thermal annealing suggest that the ordering process in P3HT dominates the improvement in power conversion efficiency. VC 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: , 2012 KEYWORDS: bulk heterojunction; conducting polymers; diodes; fullerenes; photovoltaics; polymer thin films; semiconducting polymers; thin films INTRODUCTION Organic devices based on polymer: fullerene bulk heterojunctions (BHJs) are attracting interest as low cost, thin film solar cells, with power conversion efficiencies over 8%. 1,2 Bulk-heterojunctions of semiconducting polymers and fullerenes have the highest power conversion efficiencies of organic solar cells and comprise a nanostructured mixture of electron donating and electron accepting semiconductors. 3 Because of limited control of the phase separation of the components in the BHJ during the film formation process, the power conversion efficiency (PCE) is strongly dependent on a wide number of parameters, such as solvent(s), molecular weight, blend ratio, thickness, and post treatment. 4 9 Optimization of the optoelectronic performance of any new BHJ therefore requires significant effort. In addition, as new semiconductor materials are continually being synthesized, it is essential to develop more efficient practices to screen their optoelectronic performance. Processing conditions and post-treatments (e.g., thermal annealing) have a significant impact on the PCE of BHJs. 8,10,11 To optimize organic solar cells, researchers typically vary a number of different parameters including the polymer to fullerene weight ratio and film thicknesses using large numbers of samples. Recent work using roll-to-roll processing to form gradients of concentration and thickness has demonstrated a simpler means to optimize these parameters. 12 The influence of thermal treatment (or thermal annealing) on the PCE of BHJs of poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methano fullerene (PCBM) 6,13,14 and other polymers 15,16 has been widely studied. In particular, the PCE of P3HT:PCBM BHJs can be improved from 1 to 5% by simple heating to 150 C. 10 This widely used thermal treatment was determined empirically based on the performance of large numbers of samples made to study changes due to the temperature and the length of thermal treatment. This approach is inefficient because it necessitates a large number of samples and potentially may miss subtle details of changes in the PCE with temperature. Despite significant effort, PCBM remains the most commonly used acceptor in BHJs 3 although alternate fullerenes are emerging. 17,18 The dominance of PCBM has been correlated with its solubility and morphological characteristics relative Additional Supporting Information may be found in the online version of this article. VC 2011 Wiley Periodicals, Inc. 174 JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2012, 50,

2 JOURNAL OF POLYMER SCIENCE FULL PAPER to other fullerenes. 19 When comparing novel fullerene acceptors to PCBM using the same donor polymer, the processing conditions are frequently held the same despite the possibility that alternate conditions may be better for novel materials. 19,20 The large number of samples typically required to optimize the PCE of any new BHJ may be particularly troublesome when the initial quantity of material available may be limited. Thus, without improved methods to screen new materials it is possible that otherwise promising materials may be abandoned prematurely. Herein, we report a new fullerene derivative, 1,9-dihydro- 64,65-dihexyloxy-1,9-(methano[1,2] benzomethano)fullerene[60] (DHOM) and its performance in BHJs with P3HT. Because the optimal processing conditions are unknown for DHOM, we utilized in situ current-voltage (J-V) measurements during thermal treatment to determine appropriate conditions for improving the PCE in this BHJ. We first determine a maximum temperature for annealing by heating a device using a controlled temperature ramp to find the highest temperature at which the device performance does not degrade. Then, by annealing at the determined critical annealing temperature, we determine how long a device should be annealed for by monitoring the real-time changes in J-V characteristics as a function of time. The results from the in situ method were found to be in good agreement with samples made using conventional annealing as well. This method enabled us to compare the behavior of DHOM and PCBM in BHJs with P3HT and we find similar processing conditions for each lead to similar PCE despite the structural differences in the two fullerene derivatives. box on the prepared substrates at 700 rpm for 40 s and 2000 rpm for 5 s yielding an active layer thickness of 70 nm. A 100 nm thick Al cathode (0.06 cm 2 ) was then deposited by thermal evaporation under vacuum (<10 6 torr). The in situ J-V curves were measured at elevated temperatures in a home-built set-up in a nitrogen atmosphere glovebox. An array of 14 green 1W LEDs was utilized as the light source. A P3HT:DHOM device was attached to a metal mount on a hotplate and was electrically connected using a dip-clip. The temperature of the device was continually measured using a thermocouple. For the controlled temperature ramp studies, the device mount was placed on the hotplate set at room temperature, with the ITO side facing upwards. The hotplate was set with a temperature ramp of 4 CperminandJ-V curves were measured in a light then dark sequence using a Keithley 2400 sourcemeter. For the constant temperature measurements, the device mount was first brought up to the intended annealing temperature. The mount was then removed, and then the solar cell was quickly connected before placing it back on the hotplate. J-V curves were then continually measured. It took 2 min for the device to rise to the annealing temperature. RESULTS AND DISCUSSION Characterization of DHOM Dihydronaphthyl derivatives of C 60 have drawn attention as alternates to PCBM. 21 We report here a novel derivative, DHOM (synthetic procedure in the experimental section) [Fig. 1(a)]. This fullerene was found to have good solubility in EXPERIMENTAL Materials P3HT and other chemicals were used as received from Merck Chemicals and Sigma-Aldrich, respectively. Synthesis of DHOM A mixture of C 60 (100 mg, 0.14 mmol), Bu 4 NI(167 mg, 0.45 mmol), and 1,2-bisbromomethyl-4,5-bishexyloxybenzene (97 mg, 0.21 mmol) in dry toluene (150 ml) was refluxed in the dark for 27 h. After cooling down to room temperature, the solution washed with H 2 O, and purified by flash column (silica gel, toluene/hexane 1:1) to afford the target compound (55 mg, 38%). 1 H NMR (CDCl 3, 200 MHz): d ppm 7.22 (s, 2H), 4.72 (br, 2H), 4.36 (br, 2H), 4.17 (t, J ¼ 6.6 Hz, 2H), 1.92 (m, 4H), 1.56 (m, 4H), 1.39 (m, 8H), 0.94 (t, J ¼ 6.6 Hz, 3H). NMR data is presented in the supporting information. Device Fabrication and Testing ITO-coated glass substrates were ultrasonicated in acetone, 2% soap in water, deionized water, and 2-propanol for 20 min and dried with a stream of nitrogen. PEDOT:PSS (Clevios PH500) was deposited by spin coating at 4000 rpm for 40 s and dried at 165 C for 10 min yielding a film thickness of 40 nm.the P3HT:DHOM solutions (1:0.7 by weight) were prepared in a N 2 filled glove box with a total concentration of 20 mg ml 1 in chlorobenzene and stirred overnight at 70 C. The BHJ solution was deposited in a N 2 filled glove FIGURE 1 (a) Chemical structure for 1,9-dihydro-64,65-dihexyloxy-1,9-(methano[1,2] benzomethano)fullerene[60] (DHOM) and (b) cyclic voltammetry in ODCB. Tetrabutylammoniumtetrafluoroborate, ca. 0.1 M, was the supporting electrolyte, with a scan rate of 100 mv S 1. Platinum was used as working electrode. After each measurement, ferrocene was used as an internal standard. JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2012, 50,

3 FULL PAPER JOURNAL OF POLYMER SCIENCE revealed a peak in the PCE as the temperature increased [Fig. 2(a,b)]. An as-cast device was heated using a controlled temperature ramp of 4 C per minute and illuminated with green LED s (wavelength 520 nm). The absorption of P3HT blue-shifts and decreases with increasing temperature. 22 Thus, to minimize the impact of change in absorption for these measurements it is important to use a monochromatic light source with output intensity near the peak absorption of P3HT rather than a white light source. If this spectral shift were the only factor influencing charge generation, we would expect to observe a decrease in short-circuit current due to a reduction in absorption at elevated temperatures in contrast to the observed results. The J-V characteristics under illumination as function of temperature reveal a strong increase in the short circuit current with heating before decreasing at 150 C. We find that, as the device temperature is increased above 50 C, the dark current, short-circuit current, J sc, and fill factor (FF) strongly increase, whilst the series resistance and the open-circuit voltage, V oc, decrease [Fig. 3(a)]. At approximately 150 C, the J sc, FF, and dark current (J dark ) reach a maximum, and the series resistance (estimated from the slope of the dark current at 1.5 V) reaches a minimum. As the temperature is FIGURE 2 (a) In situ J-V curve measurements under incident light and (b) in the dark as a function of device temperature. chlorobenzene, chloroform, and tetrahydrofuran. Its solubility in chlorobenzene was 40 mg/ml, which is similar to PCBM (50 mg/ml). 19 Cyclic voltammetry revealed two reduction peaks [Fig. 1(b)] and its LUMO of 3.67 ev was estimated by measuring the difference between the onset of first reduction and the half-wave potential of the ferrocene standard. Optimization of the PCE To determine the performance of DHOM in BHJs with P3HT, a number of initial blend compositions were examined. An as-cast film with 1.0:0.7 blend of regio-regular P3HT and DHOM of thickness 70 nm gave PCE of 1% under simulated AM1.5 irradiation. Devices were also made for 1.0:1.0 and 1.0:1.3 blend ratios, but were found to give lower efficiencies as-cast, = 0.6%. Based on these results, we chose to focus on the 1.0:0.7 ratio blend and use an in situ annealing study to improve its PCE. The P3HT:DHOM devices were heated using a temperature ramp and measurements of J-V curves during this ramp FIGURE 3 (a) In situ short-circuit current (J sc, red circles), open-circuit voltage (V oc, blue asterisk), FF (green open squares) measured under incident light and (b) series resistance (Rs, open blue squares) and value for the dark current at 1.5V (J dark, red circles). 176 JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2012, 50,

4 JOURNAL OF POLYMER SCIENCE FULL PAPER FIGURE 4 Plots of the short-circuit current, open-circuit voltage and FF, whilst a P3HT:DHOM device is annealed at 145 C. increased above 150 C, the J sc, FF, and J dark all start to decrease. Assuming that the reduction in J-V parameters at temperatures above 150 C reflect irreversible changes in blend morphology, this result suggests that the highest temperature at which the device should be annealed without degradation in device performance is 150 C. Annealing at temperatures much below or much above this temperature will limit the achievable PCE for these devices. At this point in the characterization, the time required for the device to be annealed is not known (see below) therefore we chose a temperature slightly lower than 150 C for the maximum annealing temperature, herein selected as 145 C. The J-V characteristics represent both the effects of temperature dependent changes on carrier transport and injection, as well as morphological changes. By considering the predicted changes due to heating, we can attempt to differentiate between both effects. In general, the carrier mobility of organic semiconductors is thermally activated and tends to increase with temperature. 23 Thus, we expect an increase in the forward bias current, and, if either geminate or nongeminate recombination losses are present at room temperature, 24 an increase in both the J sc and FF. Any increase in carrier mobility will cause a reduction in series resistance. We also expect to observe an increase in forward current due to increased carrier injection if there are simple injection barriers present. As consequence, all of these mechanisms should increase J dark, leading to an increase in the FF and a decrease in V oc. 25,26 All these features are observed in the results presented here. There are several features however, which cannot obviously be explained due to the effects of heating alone. As the temperature is increased from room temperature, the J sc increases until approximately 35 C whereupon it decreases until 45 C, subsequently followed by a sharp increase with further increases in temperature. The same behavior, while less significant, is also observed for the dark current. We have also observed this effect in other P3HT-based devices with PCBM. 27 The temperature at which this transition occurs may be related to the glass transition of the blend. This result is consistent with work showing that increasing the acceptor content in P3HT blends leads to an increase in the glass transition with respect to that of pure P3HT, believed to be near 12 C. 28 In addition, we also observe a second maximum for J sc at 90 C, where it again decreases, before again increasing above 110 C. This second maximum is also likely to be a change induced due to thermal annealing for the blend, although its origin is less clear. We determined the required annealing time by performing in situ J-V curvemeasurementswhilstannealingadeviceatthe predetermined optimum 145 C annealing temperature (Fig. 4). It takes 2 min for the device to reach the annealing temperature, and thus, we only present the J-V characteristics measured after this time period. We find that J sc and FF increase, and the V oc decreases until around 20 min where each value stays constant. This indicates that the effects of annealingarecompletedin20min.furtherannealingatthis temperature, up to 70 min, does not cause degradation in these values. This suggests that the optimum annealing conditions for these P3HT:DHOM devices is 145 C for 20 min. It also is apparent that the increase in J sc after 2 min. is relatively small, when compared with the controlled ramp study given in (Fig. 2). This observation suggests that the effects of annealing on the short-circuit current occur rapidly at this temperature. The relatively fast change in short circuit current is consistent with our recent diffusion studies, which show that changes in morphological structure can occur on rapid timescales. 29 To examine whether devices annealed at lower temperature from our peak temperature, a device was annealed at 90 C as a function of time [Fig. 5(a)]. It was found that the J sc and FF increased by 41% and 27% while the V oc decreased by 4% within the first 20 mins. After the first 20 min, the J sc, FF, and V oc continued to change by less than 2.5% over the next 50 min. The same device was then cooled to room temperature and heated back to 90 C to ensure that all relevant changes in the device characteristics are irreversible [Fig. 5(b)]. It was found that the J sc, FF, and V oc changed very little (less than 2%) up to 45 min of further annealing. Thus, these results signify that annealing at temperatures below the peak annealing temperature determined by the in situ method do not result in similar performance at least at experimentally convenient timescales. These changes are dominated by improvement of the FF rather than the J sc suggesting that simply following the peak current is not enough to determine an appropriate annealing temperature. Comparison of In situ Annealing and Direct Annealing It is assumed that samples directly heated to a particular temperature behave similarly to those annealed using a temperature ramp with frequent illumination. However, it is important to verify that the illumination does not influence the outcome of the thermal annealing sequence. Figure 6 shows the AM1.5 light and dark J-V curves for an as-cast device and post-annealing at 145 C for 20 min. We observe a significant improvement in device performance upon annealing, with an increase in efficiency by more than a factor of 2, from 1 to 2.1%. The J sc density and FF have increased to 6.24 ma cm 2 and 53%, respectively, whilst the V oc has slightly decreased to 0.65 V. As comparison, similar thin P3HT:PCBM devices (70 nm) fabricated in the same device structure yield efficiencies JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2012, 50,

5 FULL PAPER JOURNAL OF POLYMER SCIENCE FIGURE 5 Isothermal in situ short-circuit current (J sc, red circles), open-circuit voltage (V oc, blue stars), FF (green open squares) measured under incident light at 90 C. of 2.5% (J sc ¼ 7.0 ma/cm 2, V oc ¼ 0.62 V, and FF ¼ 0.58) only slightly larger than obtained here. We observed no change in device performance upon annealing for a further 10 min at 145 C, and 5 min at a higher temperature 155 C, consistent with the temperature and time dependent studies. However, as predicted in the controlled temperature ramp study, annealing at a higher temperature of 165 C for a further 20 min. leads to a 10% reduction in device efficiency (also shown in Fig. 3). This further supports that using our simple in situ approach we are able to quickly arrive at the optimum annealing conditions. We note that for the device that was used in the in situ current voltage measurements annealed as a function of time at 145 C, gave the same device performance as the device annealed on the hotplate, also shown in Figure 3. This result indicates that the changes in device performance during the in situ measurements are due to the effect of thermal annealing and not due to the effects of continually measuring current voltage curves, which potentially could cause increases in device performance due to light soaking or electric field driven changes. DHOM Versus PCBM as an Acceptor in BHJs PCBM is the most widely used acceptor for BHJ solar cells 6,13,14 and is a benchmark for comparison of new fullerene derivatives. In comparison to PCBM, DHOM has similar electronic levels and solubility, but a substantially different molecular structure. Comparing the behavior of P3HT:DHOM solar cells to P3HT:PCBM solar cells, the two systems show a very similar trend upon thermal annealing. For example, we observe a local maximum for the J sc near 35 Cinbothsystems and a sharp increase in the J sc at 50 C [Fig. 3(a) and Supporting Information Fig. S1)]. The same trend continues when examining the change in the V oc,ff,j dark,andr series. Indeed, the only difference between the two systems seems to be that the temperature at which the maximum value for the J sc is observed is 10 C lower (145 C instead of 155 C) in the DHOM system. For P3HT:PCBM, we previously applied similar techniques in conjunction with X-ray diffraction to correlate the change in photoconversion efficiency with the molecular ordering of the active layer. 27 We found that the improvement in the ordering of the P3HT was the dominant morphological factor responsible for the improvement in the PCE with thermal annealing. Because both systems follow similar trends upon thermal annealing, this result suggests that both active layers undergo similar morphological changes upon thermal annealing. With this assumption, the increase in the performance of P3HT:DHOM is dominated by the increase in the P3HT crystallite number and size. Thus, it is surprising that substantially changing the chemical structure of the fullerene has little influence on the factors that govern the evolution of the PCE with thermal annealing. In a broad study of fullerenes, it was concluded that the solubility of the fullerene in organic solvents is correlated to the PCE. 19 While in these studies a range of processing conditions, which were not specified for each fullerene, were used, it determined that 2:3 fullerene:p3ht blends annealed at 160 C for 5 min gave generally reliable results. Our results here with DHOM are in reasonable agreement with the rough peak in PCE found for fullerenes with solubility of mg/ml in FIGURE 6 Comparison of AM1.5 light and dark J-V curves for a P3HT:DHOM device as-cast, post-annealing at 145 C for 20 min, post-annealing at 165 C for a further 20 min, and for the device used in the time dependent in situ studies presented in Figure JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS 2012, 50,

6 JOURNAL OF POLYMER SCIENCE FULL PAPER chlorobenzene. The observation that the thermal processing of both PCBM and DHOM had similar features suggests that part of the success of such broad comparative studies may be that the thermal behavior of P3HT dominates the processing conditions leading to improved PCE more than the fullerene. Whether similar observations would hold for other semiconducting polymers is not known. CONCLUSIONS We have synthesized and characterized a new fullerene, DHOM, for use in BHJ solar cells. This acceptor has similar performance to PCBM in P3HT:fullerene BHJs. A fast and a simple approach was used to find thermal annealing conditions that improve the PCE of BHJs formed from P3HT and DHOM. This approach significantly reduces the number of samples and number of studies that are required to determine the best temperature and time period to anneal for from approximately 20 devices 10 to only 2. Clearly, such studies do not determine a global optimal processing condition due to the freedom of choice of blend ratio and casting solvent, but do allow rational optimization of a given blend. The annealing conditions for improvement in P3HT:DHOM photovoltaics were found to be similar to those for P3HT:PCBM despite the structural differences in the two fullerenes. The similarity in the temperature at which improvements in the short circuit current and FF begin demonstrates the dominant role of P3HT in the morphological changes influencing the PCE in BHJs. This report demonstrates that in situ characterization is a useful technique to illuminate the temperature-dependent changes in the PCE of BHJs, and we suggest that it could potentially be used in the future to screen annealing temperatures in large area roll-toroll systems in combination with other optimization methodologies. 30 ACKNOWLEDGMENTS Portions of this work were carried out using the MRL Central Facilities, which are supported by the MRSEC Program of the NSF under Award No. DMR ; a member of the NSFfunded Materials Research Facilities Network ( NDT acknowledges support from the ConvEne IGERT Program (NSF-DGE ) and NSF Graduate Research Fellowship. Partial support was provided by the Center for Energy Efficient Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC REFERENCES AND NOTES 1 Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. 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