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Effect of Annealing Solvent Solubility on the Performance of Poly(3-hexylthiophene)/ Methanofullerene Solar Cells Jong Hwan Park, Jong Soo Kim, Ji Hwang Lee, Wi Hyoung Lee, and Kilwon Cho* Department of Chemical Engineering/School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology, Pohang, 790-784, Korea ReceiVed: April 1, 2009; ReVised Manuscript ReceiVed: July 16, 2009
The effect of the solubility of the annealing solvent on the performance of poly(3-hexylthiophene) (P3HT): C61-butyric acid methyl ester (PCBM) solar cells is studied. The short-circuit current (Jsc) and the fill factor (FF) increase remarkably, regardless of the type of annealing solvent, whereas a reduction of the open-circuit voltage (Voc) (of 0.1-0.2 V) is observed after solvent annealing. Interestingly, both the value of Jsc and the power conversion efficiency (PCE) are higher for the poor-solvent-annealed devices than for the good-solventannealed ones. A good solvent vapor induces better self-organization of P3HT than a poor solvent vapor. However, the exciton loss increases due to excessive phase separation. A study of the space-charge-limited current (SCLC) reveals no significant differences between the carrier mobilities of good- and poor-solventannealed devices. Furthermore, the measured photocurrent suggests that the space charges no longer limit the values of Jsc and FF for all the solvent-annealed devices. These results indicate that the higher Jsc and PCE values obtained for the poor-solvent-annealed devices can be attributed to the optimized phase separation of the active layers, which induces balanced carrier mobility and minimum exciton loss. 1. Introduction Photovoltaic devices based on organic materials are among the most promising renewable and alternative energy sources because of their many advantages, such as low cost, easy fabrication, lightweight, and mechanical flexibility.1 Since the discovery of efficient photo induced power generation between a polymer donor and a fullerene acceptor in bulk heterojunctions,2 there have been significant efforts to enhance the photovoltaic performance of the devices by using new materials,3-8 modifying the electrodes,9-12 enhancing the lightabsorption properties,13,14 and engineering the fabrication process.15-19 Recently, a bulk-heterojunction solar cell consisting of a blend of regioregular poly(3-hexylthiophene) (RR-P3HT) and [6:6]-phenyl-C61 butyric acid methyl ester (PCBM) was found to exhibit a power conversion efficiency (PCE) of over 3% under AM 1.5G standard reference conditions.18,19 In bulkheterojunction structures, the donor-acceptor interface area, where excitons dissociate, is much larger than that in typical inorganic bilayer solar cells. Simultaneously, separated charges travel to the electrodes through the three-dimensional interpenetrating network of donors and acceptors. However, for “asprepared” P3HT:PCBM films, the hole mobility is much lower than the electron mobility, and the resulting space charges deteriorate the photocurrents of the solar-cell device.20-22 Thermal annealing of the active layer can enhance the lightabsorption properties and the hole mobility by inducing the selforganization of P3HT.18,23,24 Unfortunately, exciton loss also increases when the domain size of the donor or acceptor phase exceeds the exciton diffusion length of the conjugated polymers.25 Therefore, it is reasonable to speculate that there is an optimum thermal-annealing time and an appropriate temperature for maximizing the PCE.18,26 * To whom correspondence should be addressed. E-mail: kwcho@ postech.ac.kr.
Similarly, solvent annealing of P3HT:PCBM films can also enhance the performance of solar cells.19,27 In the case of solvent annealing, however, little is known about the correlations between the solvent-annealing conditions and the phase separation of the active films that determine the photovoltaic performance. To obtain a high performance of the P3HT:PCBM solar cells, an appropriate solvent-annealing process, which leads to a desirable nanophase separation, is needed. Here, we demonstrate a systematic way to achieve optimum solvent-annealing conditions by controlling the solubility and the boiling point of the annealing solvent. The effect of solvent annealing on P3HT/ PCBM blends using several different common small molecules is studied. The choice of solvents gives two separate tests solvents with near identical boiling point but different solvation properties, and solvents with near identical solvation properties but different boiling points. These two parameters of solvents are key parameters determine self-organization and phase separation of blend materials. Good solvents can make the P3HT and PCBM molecules more mobile than poor or nonsolvents. It has previously been proposed that the molecular orientation, the crystallinity, and the resulting field-effect mobility of P3HT thin films can be controlled by controlling the solubility of the blending solvents.28 In addition, the penetration rate of solvent molecules can be controlled by changing the boiling point of the annealing solvent. To explain the performance variation according to the type of annealing solvent, we investigated the efficiencies of four main processes that determine the generation of photocurrent, namely, light absorption, exciton diffusion, charge transfer, and charge collection.29 The light absorption and exciton diffusion efficiencies of the active layers were investigated and the results correlated with the degree of phase separation. Both charge transfer and charge collection efficiency were estimated using space-charge-limited-current (SCLC) measurements.
10.1021/jp9029562 CCC: $40.75 2009 American Chemical Society Published on Web 09/11/2009
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2. Experimental Methods Regioregular P3HT (Mn ) 37k, PDI ) 2.0, RR ) 90-93%) and PCBM (99.5%) used in this study were obtained from Rieke Metals, Inc. and Nano-C, Inc. respectively. The P3HT:PCBM blend solutions (weight ratio, 1:1) were prepared in chlorobenzene at a concentration of 10 mg/mL. The devices were fabricated on top of ITO-coated substrates. After cleaning the ITO surface, PEDOT:PSS (Baytron P TP AI 4083, Bayer AG) was spin-coated to a layer with a thickness of 30-50 nm and baked at 120 °C (for 60 min) in a convection oven. The 80-100 nm thick photoactive layer was spin-coated on the PEDOT: PSS layer for 120 s to minimize the residual solvent. Before LiF/Al deposition, the devices were transferred to a glass Petri dish (d ) 65 mm) containing various solvents and covered with a glass cap. After solvent annealing, the samples were dried overnight (>12 h) at room temperature in an N2-filled glovebox. Finally, LiF (0.6 nm)/Al (150 nm) cathodes were thermally deposited. The current density-voltage (J-V) characteristics were measured using Keithley 4200 source/measure units in the dark and under AM 1.5 solar illumination (Oriel 1 kW solar simulator) with respect to the reference cell PVM 132 (calibrated at the National Renewable Energy Laboratory, NREL, at an intensity of 100 mW/cm2). GI-XRD measurements were performed using the 10C1 (1D) and 4C2 (2D) beamlines (wavelength ) 1.54 Å) at the Pohang Accelerator Laboratory (PAL). The angle between the film surface and the incident beam was fixed at 0.18°. The measurements were obtained at scanning intervals of 2θ between 3 and 27°. 3. Results and Discussion Our experimental system was based on the “external solvent vapor treatment”.30 The as-prepared devices were placed in a glass Petri dish containing the solvents and then covered with a glass cap for the desired annealing time. We used three solvents with similar boiling temperatures but different P3HT solubilities, namely, methylene chloride (Tb ) 39.8 °C), acetone (Tb ) 56.3 °C), and chloroform (Tb ) 61.2 °C), and also introduced two more “good” solvents with higher boiling points [i.e., chlorobenzene (Tb ) 131 °C) and 1,2-dichlorobenzene (Tb ) 180.5 °C)] to investigate the effect of solvent volatility. By applying the solvent extraction method, P3HT can be fractionated according to its solubility in extracting solvents.31,32 From solvent-extraction and gel-permeation-chromatography (GPC) results for raw P3HT, we found that the solubility of selected j n ) 33k) solvents follows the following order: chloroform (M jn ) j ≈ chlorobenzene (Mn ) 34k) ≈ 1,2-dichlorobenzene (M j j 36k) > methylene chloride (Mn ) 15k) . acetone (Mn ) 4k) j n of P3HT that solvent can dissolve). Acetone exhibits almost (M ”nonsolvent” behavior for high-Mw P3HT and can only dissolve low-Mw P3HT. Methylene chloride is a poor solvent for P3HT, whereas chloroform, chlorobenzene, and 1,2-dichlorobenzene can dissolve whole raw P3HT quite easily. The solubility of PCBM is similar to the case of P3HT. There are a few literature data about solubility of PCBM33 but we can compare the solubility of PCBM by dissolving in the five solvents. Acetone is almost nonsolvent (10 mg/ mL). Figure 1 and Table 2 show the J-V characteristics of the as-prepared and solvent-annealed devices under AM 1.5 illumination with an intensity of 100 mW/cm2. The as-prepared devices show a poor performance with an open-circuit voltage Voc ) 0.69 V, a short-circuit current Jsc ) 3.85 mA/cm2, a fill
Figure 1. J-V characteristics (AM 1.5, 100 mW/cm2) of P3HT:PCBM Photovoltaic (PV) cells before annealing (9), and after solvent annealing with acetone (0), methylene chloride (b), chloroform (O), chlorobenzene (2), and 1,2-dichlorobenzene (4). The annealing time was 60 min.
Figure 2. TEM images of P3HT:PCBM films before and after solvent annealing: (a) image of the as-prepared film. (b-f) Images of the annealed films: (b) acetone, (c) methylene chloride, (d) chloroform, (e) chlorobenzene, and (f) 1,2-dichlorobenzene.
factor FF ) 0.30, and a PCE ) 0.8%. After solvent annealing, the values of Jsc and FF increased dramatically whereas a reduction of Voc of 0.05-0.17 V is observed according to the solvent type. When annealing with a poor solvent, such as acetone or methylene chloride, the increase in Jsc compensates for the decrease of Voc and leads to an enhancement of PCE up to 3.3%. The photovoltaic parameters of solvent-annealed devices were investigated as a function of the annealing time. In the case of methylene chloride, chloroform, and chlorobenzene, the photovoltaic parameters changed slightly after five minutes, which implies that these solvent molecules need a very short time to penetrate the films so that complete selforganization of the P3HT and PCBM molecules can occur in a relatively fast manner. On the other hand, in the case of acetone and 1,2-dichlorobenzene, the photovoltaic parameters changed gradually and the maximum PCE was reached after 60 min. This behavior can be attributed to the low solubility of acetone and the high boiling point of 1,2-dichlorobenzene. The nanoscale phase separation of P3HT:PCBM blend films was examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments. As shown in Figure 2a, the bright P3HT-rich and dark PCBM-rich domains18,34 are well distributed within less than 30 nm in the as-prepared film. The poor-solvent-treated films, such as acetone- [Figure 2b] and methylene-chloride-treated [Figure 2c] films show almost the same morphology as the as-prepared films. However, for those films annealed with good solvents, P3HT nanofibrils and dark PCBM aggregates are well developed. Moreover, AFM height images (see Figure 3) support the increase of the phase
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Figure 4. (a) Out-of-plane and (b) in-plane grazing-incident-angle X-ray diffraction (GI-XRD) intensities for P3HT:PCBM films before and after solvent annealing: 9 As-prepared, 0 acetone, b methylene chloride, O chloroform, 2 chlorobenzene, and ∆ 1,2-dichlorobenzene.
TABLE 1: Summary of the Photovoltaic Properties of Organic Photovoltaic (OPV) Devices Annealed with Various Solventsa Figure 3. AFM height images of P3HT:PCBM films before and after solvent annealing: (a) as-prepared, (b) acetone, (c) methylene chloride, (d) chloroform, (e) chlorobenzene, (f) 1,2-dichlorobenzene. The annealing time was 60 min and the height scale is 20 nm for all the images.
separation of P3HT:PCBM layers by solvent annealing. In a previous study,27 it was also found that the surface roughness of photoactive films increases after thermal annealing. In our experiment, a relatively smooth surface [with a root-mean-square (rms) roughness of 0.5 nm] was observed for the as-prepared device. After solvent annealing, however, the rms roughness increased to 1.0, 1.6, 2.7, 2.5, and 3.0 nm for acetone-, methylene chloride-, chloroform-, chlorobenzene-, and 1,2dichlorobenzene-annealed films, respectively. To examine the crystal structure of the active layer, grazingincident X-ray diffraction (GI-XRD) measurements were carried out. Figure 4a shows the out-of-plane diffraction patterns of the P3HT:PCBM blend films before and after solvent annealing. The (100) reflections (2θ ) 5.3°) due to the lamella layer structure of P3HT are intense for the good-solvent-annealed films. In these films, the hexyl side chains of P3HT are highly oriented with an edge-on structure, which indicates that good solvents lead to a higher degree of edge-on structure. In the cases of acetone and methylene chloride, the (100) reflections also increased relative to the as-prepared films. However, this increase was small compared to that obtained with good solvents. As shown in the in-plane XRD patterns (see Figure 4b), good solvent molecules almost completely disorganize the pre-existing P3HT face-on crystal domains. On the other hand, for poor and nonsolvents, both face-on and edge-on structures are developed during the annealing process. It was previously reported that face-on P3HT crystals disappeared and edge-on crystals developed well after thermal treatment at the order-todisorder transition temperature.35 However, thermal annealing of blend films at moderate temperature cannot deteriorate the P3HT face-on structures.11,36 In regioregular P3HT thin films, the edge-on crystal structure is thermodynamically favored whereas the face-on crystal structure is kinetically favored.28
device
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
as-prepared acetone methylene chloride chloroform chlorobenzene 1,2-dichlorobenzene
0.71 0.57 0.52 0.59 0.64 0.63
3.74 10.72 11.44 8.22 8.35 7.76
0.35 0.54 0.55 0.55 0.54 0.59
0.94 3.29 3.27 2.66 2.88 2.86
a
Annealing time: 60 min.
To change the P3HT crystal orientation from a face-on to an edge-on phase, an external energy that is larger than the activation barrier must be applied to the polymer chains. Based on our previous morphology studies, we can conclude that the phase separations are mainly determined by the crystallization of P3HT. It may be possible that the movement of PCBM drives the morphology change but it occurs when annealed by good solvent for long time (Figure 2). However, P3HT can selforganize even at the nonsolvent (acetone) annealing case (Figure 4). Recently, it was reported that room-temperature solvent annealing is a relatively mild condition compared to the thermal annealing, and induces the self-organization of only P3HT.37 We can also conclude that good solvent molecules can make P3HT chains mobile enough to overcome the activation barrier for the phase transition of the crystal structure, thus inducing a large phase separation. In addition, we also found that the phase separation of active films can be kinetically controlled by changing the boiling point of the annealing solvent. There were no significant differences in the morphology and performance of the three good-solvent-annealed devices. The Voc values of all the devices drop after solvent annealing. Especially, the methylene-chloride-treated device displays a poor Voc, which is about 0.2 V lower than that of the as-prepared device (see Table 1). Although some arguments still remain concerning the origin of Voc, it is widely accepted that the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor determines the Voc value of the
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Figure 5. (a) UV/vis absorption and (b) photoluminescence spectra of P3HT:PCBM films before annealing (9) and after solvent annealing with acetone (0), methylene chloride (b), chloroform (O), chlorobenzene (2), and 1,2-dichlorobenzene (∆). The annealing time was 60 min for all the films.
P3HT:PCBM bulk-heterojunction solar cell.38 To investigate the optical band gap of the solvent-annealed P3HT devices, we took UV/vis absorption spectra of the blend films before and after solvent annealing (see Figure 5a). Except for the case of the acetone-vapor-treated films, three absorption peaks clearly appeared and the overall absorption spectra were red shifted after solvent annealing. This result can be interpreted assuming that the red shifted spectrum results from a lower energy state of delocalized excitons in the highly π-conjugated P3HT domains.39,40 Therefore, the lowering of Voc for solvent-annealed devices can be explained by the reduced optical band gap of the P3HT donor molecules. When comparing the degree of Voc drop after solvent annealing, we observe that the good-solventannealed devices show a less significant drop in Voc than the poor-solvent-annealed ones (Table 1). However, the reason for this small Voc drop for good-solvent-annealed devices is still not clear. We speculate that the higher Voc of the good-solventannealed devices could be due to a decrease in the contact area between the PCBM and poly(3,4-ethylenedioxythiophene) (PEDOT) layers.41 As shown in XRD results, the crystalline structure of P3HT in good solvent annealed films are most edgeon which directly contact with PEDOT electrode. The contact between the PCBM and PEDOT layers may cause undesired charge carrier recombination at PCBM/PEDOT interface, and reduction of Voc. Similar high Voc values after thermal annealing at high temperature have been reported previously.42,43 The short-circuit current (Jsc) is directly related to the external quantum efficiency (EQE), that is, to the product of light absorption, exciton diffusion, charge transfer, and chargecollection efficiency.29 Hence, the increase in light absorption may not be the main factor explaining the different degrees of Jsc increase according to the annealing solvents. Although the light-harvesting characteristics of an acetone-annealed device are lower than those of the other solvent-annealed devices, its Jsc value is over 10 mA/cm2 and therefore larger than the values usually obtained for good-solvent-treated devices. Furthermore, no significant differences in light absorption are observed between methylene-chloride-annealed and good-solvent-annealed films.
Park et al. Next, we examined the exciton diffusion efficiency via photoluminescence (PL) spectroscopy. The PL of blend films is quenched by a photoinduced charge transfer between the donor and acceptor molecules. Thus, efficient PL quenching indicates a phase separation between P3HT and PCBM within the exciton diffusion length scale.25 As shown in Figure 5b, the degree of PL quenching of good-solvent-annealed devices decreases whereas the acetone-treated films show almost the same PL intensity as the as-prepared films. When the active layers are annealed with good solvents, the domain size of the P3HT nanofibrils sufficiently exceeds the typical exciton diffusion length (LD ≈ 10 nm) of semiconducting polymers.44 Furthermore, relatively large, dark PCBM clusters (with a size of approximately 100 nm) are mainly observed in the goodsolvent-annealed films. On the other hand, the P3HT and PCBM domain sizes of the poor solvents (i.e., the acetone- and methylene-chloride-treated films) did not exceed their exciton diffusion length. Therefore, we can conclude that the large phase separation induced by good solvents may limit the Jsc enhancement whereas the interpenetrating networks accomplished by the P3HT nanofibrils and the PCBM clusters may boost up the charge-carrier mobilities. It has been reported that the photocurrent can be determined by the field- and temperature-dependent dissociation probability, P(E,T), of an electron-hole pair at the donor/acceptor interface.22,45 The electron-hole pair, that is, the precursor of free charge carriers, is created when an exciton reaches the donor/acceptor interface. This may then decay to the ground state or dissociate into free carriers. Since the exciton dissociation efficiency is conceptually similar to the probability of electron-hole-pair dissociation, we compared the P(E,T) values of all the solvent-annealed devices. Figure 6a shows a doublelogarithmic plot of the photocurrent (Jph ) Jlight - Jdark) originated from the as-prepared and solvent-annealed P3HT: PCBM devices as a function of the effective applied voltage (V0 - V). In the saturation regime for (V0 - V) > 0.2-0.3 V, the photocurrent can be approximated by Jph ) eG(E,T)L, where L is the film thickness and G(E,T) is the product of P(E,T) and the maximum generation rate of a bound electron-hole pair (Gmax). At high voltages (V0 - V > 1 V), all the electron-hole pairs are dissociated and the photocurrent is saturated at Jsat ) eGmaxL. The calculated charge-transfer probabilities under shortcircuit conditions (Psc ) Jsc/Jsat) are over 0.8 and not very different for annealing solvents with different solubilities (Psc,acetone ) 0.9, Psc,chlorobenzene ) 0.84). Therefore, the chargetransfer efficiency is not the limiting factor that determines the photocurrent. However, the saturation current, Jsat (or Gmax), of the good-solvent-annealed devices is only 70% that of the poorsolvent-annealed devices (see Figure 6a). Similar to the PL experiments, the saturation currents of the annealed devices show us that more excitons decay to the ground state during the exciton-diffusion process in the case of active films annealed with good solvents. Finally, we investigated the charge-collection efficiency by means of SCLC measurements. In general, the charge-transfer and charge-collection efficiencies at organic donor-acceptor interfaces approach 100%.29 In the case of the as-prepared P3HT:PCBM films, however, the space charge limits the chargecollection efficiency because the hole mobility is lower than the electron mobility. According to the SCLC criteria, a halfpower dependence of the photocurrent on the applied voltage is observed, and the FF cannot exceed about 40%.46 Thus, to obtain a high charge-collection efficiency, it is necessary to balance the carrier mobilities by enhancing the hole mobility.
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J. Phys. Chem. C, Vol. 113, No. 40, 2009 17583 TABLE 2: Hole and Electron Mobility of P3HT:PCBM Films Evaluated Using the Mott-Gurney Law (ε ) 3ε0) film as-prepared acetone methylene chloride chloroform chlorobenzene 1,2-dichlorobenzene
Figure 6. (a) Double-logarithmic plot of the experimental photocurrent (Jph) of as-prepared and solvent-annealed P3HT:PCBM devices as a function of the effective applied voltage (V0 - V), where JL(V0) ) JD(V0). The films are 80-90 nm thick. The solid line indicates the position of the short-circuit current (JSC). Dark J-V characteristics for (b) hole-only devices (ITO/PEDOT:PSS/P3HT:PCBM/Pd) with a thickness L ) 90 ( 5 nm and (c) electron-only devices (Al/P3HT: PCBM/Al) with a thickness L ) 200 ( 5 nm. The slopes of the dotted and dashed lines are one and two, respectively: 9 As-prepared, 0 acetone, b methylene chloride, O chloroform, 2 chlorobenzene, and ∆ 1,2-dichlorobenzene.
To investigate the space-charge effects, we extracted the hole and electron mobilities from the SCLC J-V characteristics obtained in the dark for hole- and electron-only devices.22,30 Figure 6b shows the dark-current characteristics of ITO/PEDOT: PSS/P3HT:PCBM/Pd devices as a function of the bias corrected by the built-in voltage determined from the 0.2 V difference in work function between Pd and the HOMO level of P3HT.22 In the same manner, we also fabricated electron-only devices (Al/ P3HT:PCBM/Al) and investigated their J-V characteristics in the dark (see Figure 6c). We can expect that Ohm’s law will be observed at low voltages as an effect of thermal free carriers. For the presence of carrier traps in the active layer, there is a trap-filled-limit (TFL) region between the ohmic and the trapfree SCLC regions. The SCLC behavior in the trap-free region can be characterized using the Mott-Gurney square law47
J ) (9/8)εµ(V2 /L3) where ε is the static dielectric constant of the medium and µ is carrier mobility. The hole mobility extracted from the currents in the squarelaw region increases about 50 times after solvent annealing (see Figure 6b). Although the hole mobility of the good-solvent-
µh (cm2/(V s)) -6
1 × 10 2 × 10-5 4 × 10-5 5 × 10-5 6 × 10-5 4 × 10-5
µe (cm2/(V s)) -5
7 × 10 8 × 10-5 7 × 10-5 7 × 10-5 1 × 10-4 2 × 10-4
µe/µh 70 4 1.8 1.4 1.7 5
annealed devices is higher, this improved mobility is less than two times compared to the poor-solvent-annealed devices. Moreover, the electron mobilities of acetone- and methylenechloride-annealed films are quite similar to those of the as-prepared films but about ten times smaller than that of a film treated with 1,2-dichlorobenzene (see Figure 6b). These results can be attributed to the formation of P3HT nanofibrils and PCBM clusters which enhance the carrier mobility.30 In summary, the mobility difference between holes and electrons decreased remarkably, and the space-charge effect diminished, for all the solvent-annealed devices (see Table 2). Goodman and Rose categorized the J-V characteristics of photovoltaic devices into four regimes, namely, linear, transition, half-power, and saturation, as the applied voltage increases.20 However, only the half-power and saturation regimes are possible when the hole mobility is considerably lower than the electron mobility. On the other hand, only the linear and saturation regimes are possible when the carrier mobility is perfectly balanced. As shown in Figure 6a, for the as-prepared devices, a half-power dependence of the photocurrent is observed within the full range of applied voltages, except for V0 - V < 0.1 V. This means that the photocurrent of the asprepared devices is limited by the space charge. However, all the annealed devices show J-V characteristics exhibiting the four typical regimes of photodiodes as well as narrow halfpower regimes. Moreover, the saturation region starts before reaching the short-circuit-current conditions. Therefore, regardless of the solvent solubility, a high FF value can be achieved for all the devices by solvent annealing. From these results, we can conclude that the space charges cannot limit the chargecollection efficiency even in the case of poor-solvent-annealed devices. 4. Conclusion We have carried out a systematic study of the effect of the solvent solubility and boiling point on the degree of nanoscale phase separation of photoactive films by solvent annealing and have studied its influence on the photovoltaic performance. By applying a poor-solvent vapor to as-cast P3HT:PCBM blend films, we were able to balance the carrier mobilities without exciton loss and could improve the photovoltaic performance, thereby achieving levels comparable to those obtained with thermally annealed devices. The charge-transfer and chargecollection characteristics of P3HT:PCBM blend films were significantly improved and did not represent performancelimiting factors for any of the solvent-annealed devices. Although higher light-absorption properties and better carrier mobilities were achieved by applying good-solvent vapors, the increased exciton loss resulting from the large-scale phase separation over the exciton diffusion length was found to limit the Jsc enhancement. We believe that our study will contribute to the fabrication of high-performance P3HT:PCBM organic photovoltaic devices using solvent annealing to develop optimum bulk-heterojunction networks.
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Acknowledgment. This work was supported by Energy R&D program (20093020010040-11-1-000) under the Korea Ministry of Knowledge Economy and Creative Research InitiativeAcceleration Research Program (R17-2008-029-01001-0). The authors thank the Pohang Accelerator Laboratory for providing the synchrotron radiation sources at 4C2, 8C1, and 10C1 beamlines used in this study. References and Notes (1) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30, 10. (2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (3) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (4) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85. (5) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Ingana¨s, O.; Andersson, M. R. AdV. Mater. 2003, 15, 988. (6) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371. (7) Kooistra, F. B.; Mihailetchi, V. D.; Popescu, L. M.; Kronholm, D.; Blom, P. W. M.; Hummelen, J. C. Chem. Mater. 2006, 18, 3068. (8) Kooistra, F. B.; Knol, J.; Kastenberg, F.; Popescu, L. M.; Verhees, W. J. H.; Kroon, J. M.; Hummelen, J. C. Org. Lett. 2007, 9, 551. (9) Zhang, F.; Johansson, M.; Andersson, M. R.; Hummelen, J. C.; Ingana¨s, O. AdV. Mater. 2002, 14, 662. (10) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Appl. Phys. Lett. 2002, 80, 1288. (11) Kim, J. S.; Park, J. H.; Lee, J. H.; Jo, J.; Kim, D. Y.; Cho, K. Appl. Phys. Lett. 2007, 91, 112111. (12) Irwin, M. D.; Buchholz, D. B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2783. (13) Roman, L. S.; Ingana¨s, O.; Granlund, T.; Nyberg, T.; Svensson, M.; Andersson, M. R.; Hemmelen, J. C. AdV. Mater. 2000, 12, 189. (14) Lee, J. H.; Park, J. H.; Kim, J. S.; Lee, D. Y.; Cho, K. Org. Electron. 2009, 10, 416. (15) Moule, A. J.; Meerholz, K. AdV. Mater. 2008, 20, 240. (16) Zhang, F.; Jespersen, K. G.; Bjo¨rnstro¨m, C.; Svensson, M.; Andersson, M. R.; Sundstro¨m, V.; Magnusson, K; Moons, E.; Yartsev, A.; Ingana¨s, O. AdV. Funct. Mater. 2006, 16, 667. (17) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R; Hinsch, A.; Meisser, D.; Sariciftci, N. S. AdV. Funct. Mater. 2004, 14, 1005. (18) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (19) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. AdV. Funct. Mater. 2007, 17, 1636. (20) Goodman, A. M.; Rose, A. J. Appl. Phys. 1971, 42, 2823. (21) Melzer, C.; Koop, E. J.; Mihailetchi, V. D.; Blom, P. W. M. AdV. Funct. Mater. 2004, 14, 865. (22) Mihailetchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. AdV. Funct. Mater. 2006, 16, 699.
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