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Investigation of High-Performance Air-Processed Poly(3-hexylthiophene)/Methanofullerene Bulk-Heterojunction Solar Cells Sujuan Wu, Jinhua Li, Qidong Tai, and Feng Yan* Department of Applied Physics, The Hong Kong Polytechnic UniVersity, Kowloon, Hong Kong ReceiVed: September 17, 2010; ReVised Manuscript ReceiVed: October 8, 2010
High-performance bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61butyric acid methyl ester (PCBM) have been fabricated in ambient air. The devices treated by two-step thermal annealing exhibit power conversion efficiencies of about 3.40%, which are comparable to the devices prepared in inert atmosphere. The effect of film annealing and postannealing on the device performance has been systematically studied. The microstructure of the composite films was characterized by transmission electron microscopy. Ambipolar field-effect transistors based on P3HT:PCBM have been fabricated to characterize the electron and hole mobilities in the composite film. An impedance analyzer has been used to investigate carrier lifetime in the solar cells. The results indicate that thermal annealing processes can control balanced electron and hole mobilities, reduce traps, and increase carrier lifetime in the organic solar cells and thus optimize the performance of the devices fabricated in air. 1. Introduction Recently, polymer solar cells have attracted much attention due to their possibility of low cost, mechanical flexibility, and easy manufacturing by solution process.1-4 The bulk-heterojunction (BHJ) poly(3-hexythiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cell has been intensively investigated since it had achieved a reproducible power conversion efficiency (PCE) up to 5%.5-11 The critical issue of the organic solar cell is the microstructure of the BHJ active layer, which affects the photon absorption, exciton dissociation, carrier mobility, and thus PCE.9-14 Many factors can influence the morphology of a P3HT:PCBM composite film, including the regioregularity of P3HT,7,15,16 organic solvent,5,17 thermal annealing,5,6,12,18 and solvent-annealing conditions.5,9,10,19 It has been found that the thermal annealing process is the most important step in realizing high-performance devices, which can optimize the microstructure of the active layer and induce good contact between the active layer and the electron-collecting electrode.20-26 Organic photovoltaic devices are normally unstable in air. It has been reported that oxygen can induce traps in the P3HT: PCBM active layer, oxidize metal electrode, and deteriorate the performance and stability of the solar cells.27-29 Therefore almost all of the P3HT:PCBM solar cells with high performance are fabricated entirely in inert atmosphere.5-8 However, it is difficult for most laboratories to maintain the oxygen free environment in the whole fabricating process of devices. On the other hand, fabricating devices in air may decrease the cost for mass production of organic solar cells. Therefore the study of an airprocessed organic solar cell is of practical importance. Until now, there have been few reports about the air-processed P3HT: PCBM solar cells. In this paper, we investigated the effect of a different annealing process on the current-voltage (J-V) characteristics of air-processed solar cells, and intimately analyze the carrier lifetime, mobilities, and microstructure of the active layers in the devices under different fabricating conditions.30-33 * To whom correspondence should be addressed. Phone: (852)-27664054. Fax: (852)-2333-7629. E-mail:
[email protected].
Significantly, air-processed P3HT:PCBM solar cells under optimum thermal annealing conditions can show a PCE comparable to that of the cells fabricated in an inert environment. 2. Experimental Section Device Fabrication. All materials for the fabrication of P3HT:PCBM solar cells were used as received. P3HT (4002EE, regioregularity: 90-93%) and PCBM were purchased from Rieke Metals and Nano-C, respectively. The poly(3,4-ethylenedioxy thiophene):polystyrene sulfonic acid (PEDOT:PSS) was bought from Baytron P, Bayer AG.. The sheet resistance of indium tin oxide (ITO) coated glass substrate is 15 Ω/0. Blend solutions with a 1:0.8 weight ratio of P3HT to PCBM were prepared in chlorobenzene (CB). The polymer solar cells were fabricated with a typical sandwich structure of ITO/PEDOT:PSS/active layer/Al. Each step of the fabrication process was normally carried out in air unless specified otherwise. PEDOT:PSS solution was spincoated at 3000 rpm for 30 s on the cleaned ITO substrates and baked at 150 °C for 30 min. The P3HT:PCBM blend solution in chlorobenzene (20 mg/mL and 16 mg/mL for P3HT and PCBM, respectively) was then spin-coated on PEDOT:PSS at 800 rpm for 30 s and dried in a covered glass Petri dish for 30 min. The thickness of the coated P3HT:PCBM active layer is about 150 nm. Then the films were transferred into a glovebox filled with high-purity N2 and annealed on a hot plate (film annealing). The purpose of the film annealing is to remove residual solvent and improve the crystallinity of P3HT and PCBM in the active layer. Then the devices were taken out of the glovebox and kept in air until the deposition of the top Al electrode. An Al electrode with a thickness of about 80 nm was deposited on the active layer by thermal evaporation through a shadow mask. The active area of each device is 0.2 cm2. The device was then taken out to air and transferred to the glovebox for postannealing. The purpose of the postannealing is to remove oxygen in the active layer and optimize the contact between the active layer and Al cathode. After the postannealing, devices were encapsulated by glass caps and epoxy in the glovebox.
10.1021/jp108886p 2010 American Chemical Society Published on Web 11/22/2010
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Figure 1. (a) Current-voltage (J-V) curves of P3HT:PCBM solar cells with active layer annealed at various temperatures for 10 min (film annealing); (b, c) Voc, fill factor (FF), Jsc, and PCE of P3HT: PCBM devices as a function of annealing temperature before the deposition of Al electrodes.
For each condition, three devices are fabricated and the average results are presented. Field-effect transistors (FETs) based on P3HT:PCBM were fabricated on SiO2/n-Si substrates to study the effect of film annealing on the mobility of carriers. Source and drain Au electrodes were evaporated on the P3HT:PCBM active layer through a shadow mask. The device was then transferred to the glovebox and annealed at different conditions. The channel width and length of the transistors are 2 mm and 100 µm, respectively. Device Characterization. Current-voltage (J-V) characteristics of these solar cells were measured by Keithley 2400 source meter under 1-sun illumination (AM 1.5 G, 100 mW/ cm2) from a solar simulator (Newport 91160, 150 W). The illumination intensity was calibrated by a standard solar cell, which was traced to the National Renewable Energy Laboratory (NREL) of USA. Transmission electron microscopy (TEM, JEOL JEM-2010) was used to investigate the micrograph of the P3HT:PCBM active layers annealed at different temperatures. The transistors based on P3HT:PCBM were measured in the glovebox by using a semiconductor parameter analyzer (Agilent 4156 C). The mobility of both holes and electrons was calculated from the transfer characteristics of the P3HT:PCBM ambipolar FETs. The impedance of solar cells in the dark was
Figure 2. TEM images for P3HT:PCBM films annealed at different temperatures for 10 min: (a)110, (b) 130, and (c) 150 °C. Scale bar for each figure is 20 nm.
recorded by an impedance analyzer (HP 4294) in a frequency range from 40 Hz to 1 MHz with an oscillating voltage of 50 mV. 3. Results and Discussion Thermal annealing processes are very important for the performance of organic solar cells. We find that the best airprocessed device was fabricated by two-step thermal annealing, i.e. film annealing and postannealing, which correspond to
Poly(3-hexylthiophene)/Methanofullerene Solar Cells
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Figure 3. (a) Transfer characteristics of organic field effect transistors based on P3HT:PCBM annealed at different temperatures. VDS ) 100 V. (b) Electron and hole mobility of P3HT:PCBM thin film annealed at various annealing temperatures for 10 min.
thermal annealing processes before and after the deposition of an Al electrode, respectively. Devices annealed only one time have shown inferior performance. Film annealing is critical to the evolution of the microstructure of the P3HT:PCBM active layer.5 To investigate the effects of film annealing, P3HT:PCBM films were annealed at various temperature with the annealing time of 10 min. The postannealing condition of all of the devices is controlled to be 120 °C for 20 min, which is the optimum condition for the postannealing process that will be addressed later. Figure 1a shows the J-V characterizations of the P3HT: PCBM solar cells annealed at different temperatures (film annealing). Panels b and c of Figure 1 summarize detailed photovoltaic parameters including short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of the devices as a function of film annealing temperature. As shown in Figure 1, the optimum temperature for film annealing is 130 °C, which results in a PCE of 3.40%. The PCE increases with the increase of annealing temperature below 130 °C. Further increase of the annealing temperature results in a rapid decrease in PCE to 2.15% for 140 °C and to 1.91% for 150 °C. A similar trend has been observed for Jsc, Voc, and FF. To better understand the film annealing effect, some composite films after thermal annealing were peeled off and observed under a TEM. Figure 2 shows the TEM micrographs of P3HT:PCBM films annealed at 110, 130, and 150 °C, respectively. It can be found that the higher annealing temperature induces the larger PCBM grains, resulting in the smaller interface area between P3HT and PCBM, which is not beneficial for exciton dissociation in the composite film. These results can explain the deterioration of the J-V characteristics of the devices annealed at too high temperatures. On the other hand, hole conduction between PCBM crystallites is greatly dependent on the grain size and, obviously, too small grain size is unfavorable for hole transportation, which has been confirmed by the characterization of FETs in the following. It is worth noting that the average grain size of PCBM crystallites
Figure 4. (a) J-V curves of P3HT:PCBM solar cells treated at various postannealing temperatures for 20 min (RT: room temperature); (b, c) Voc, FF, Jsc, and PCE of P3HT:PCBM solar cells as a function of postannealing temperature.
is about 10 nm when the annealing temperature is 130 °C, which is an ideal length scale for exciton dissociation.6 Then we investigated the ambipolar FETs based on P3HT: PCBM to study the effect of film annealing. Figure 3 shows the transfer characteristics (IDS-VG) of ambipolar FETs annealed at different temperatures. The electron mobility increases whereas the hole mobility decreases with the increase of annealing temperature. When the annealing temperature is about 135 °C, the FET demonstrates hole and electron mobilities with an equivalent value. It has been reported that balanced carrier mobilities can reduce charge recombination and improve carrier transport in P3HT:PCBM BHJ solar cells.28 Therefore the best performance of the solar cells annealed at about 130 °C can be attributed to the balanced electron and hole mobilities in the bicontinuous networks between P3HT and PCBM as well as the optimum grain size of PCBM crystallites for exciton dissociation. Next we studied the postannealing effect on the devices which had been annealed at the same condition (film annealing at 130 °C for 10 min) before the deposition of Al electrodes. The postannealing process can reduce series resistance, increase fill factor, and short circuit current of P3HT:PCBM solar cells.20,25 More importantly, P3HT:PCBM composite films are exposed to air after the film annealing process and thus they are normally doped with oxygen, which can be removed by the postannealing process in inert atmosphere.34-36 Figure 4a shows the J-V curves of P3HT:PCBM solar cells postannealed at various
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Figure 5. (a) Nyquist plots of P3HT:PCBM devices postannealed at 120 °C for 20 min measured at different bias voltages. (b) The lifetime of carriers in the devices postannealed at three different temperatures at various bias voltages. (c) Nyquist plots of devices treated at various postannealing temperatures measured at the bias voltage of 0.6 V (close to Voc), (d) the lifetime of carriers in devices postannealed at various temperatures and characterized at the bias voltage of 0.6 V.
temperatures. Panels b and c of Figure 4 demonstrate the variation of Jsc, Voc, FF, and PCE as a function of postannealing temperature. Voc, FF, and PCE increase with the increase of postannealing temperature below 120 °C and then slowly decrease when the temperature is higher, indicating that the optimum postannealing temperature is around 120 °C. The improvement of the device performance by postannealing can be attributed to better contact between the active layer and Al electrode as well as oxygen dedoping in the device.20,35,36 It is worth noting that the annealing temperature for effective oxygen dedoping in P3HT reported by Mattis et al. is around 120 °C, which is consistent with the postannealing effect for our solar cells.36 However, higher postannealing temperature may induce bigger PCBM grains and lower carrier mobility in the P3HT matrix and thus degrade the device performance. To further identify the postannealing effect, the carrier lifetime in devices treated at different postannealing temperatures has been characterized by impedance spectroscopy. Figure 5a shows Nyquist plots of P3HT:PCBM devices postannealed at 120 °C for 20 min measured at different bias voltages. Each plot is made up of a large semicircle at low frequencies together with an almost straight line in the high frequency. This type of impedance pattern belongs to the responses usually encountered in systems in which carrier transport is determined by diffusion-recombination between P3HT and PCBM.37 The series resistance, Rs ≈ 5.8 Ω cm2, is relatively low. The lowfrequency semicircle is strongly dependent on the bias voltage, whereas the high-frequency parts show no dependence. Figure 5b shows the lifetime of carriers in the devices postannealed at different temperatures derived from the impedance measurements at various bias voltages.30-33 The lifetime decreases with the increase of forward bias voltage, which is a reasonable result since the recombination of carriers at the p-n junction is easier at higher forward bias voltage due to higher carrier densities.30,32,33 It is interesting to find that the device postannealed at 120 °C shows the longest carrier lifetime at each bias voltage as well as the highest PCE in all of these
devices, indicating that the lifetime of carriers is of paramount importance for the device performance. Figure 5c shows the Nyquist plots of P3HT:PCBM devices postannealed at different temperatures characterized at the bias voltage of 0.6 V (close to Voc). It can be found that the series resistance Rs and the recombination resistance decrease with the increase of annealing temperature.30 Figure 5d shows the lifetime of carriers in devices postannealed at various temperatures at the bias voltage of 0.6 V. The lifetime increases with the increase of postannealing temperatures below 120 °C and then decreases at higher temperatures. The increase of carrier lifetime after postannealing can be attributed to oxygen dedoping in the active layer. Oxygen can create trap states in both P3HT and PCBM, which increase the recombination rate of carriers in the composite film.34-36 Therefore the lifetime of carriers in the solar cell can be substantially increased after the postannealing. In addition, oxygen dedoping also can increase the Voc of a solar cell since Voc is related to the recombination rate of carriers at the interface,38 which is consistent with our experimental results. The postannealing time at the temperature of 120 °C has been carefully studied. Figure 6a shows the J-V curves of the devices postannealed at 120 °C for different periods of time. Panels b and c of Figure 6 demonstrate the dependence of Jsc, Voc, FF, and PCE on the postannealing time. All of these parameters were improved in the first 20 min and then relatively stable when the annealing time is within 50 min. However, the devices were degraded if the annealing time is longer than 60 min, which can be attributed to the thermodynamical instability of PCBM crystallites in the composite films.11 4. Conclusion In summary, air-processed P3HT:PCBM solar cells have been carefully investigated. The performance of the organic devices is very sensitive to the fabrication conditions. A power conversion efficiency of 3.40% can be achieved by two-step thermal
Poly(3-hexylthiophene)/Methanofullerene Solar Cells
Figure 6. (a) J-V curves of P3HT:PCBM solar cells postannealed at 120 °C for different periods of time; (b, c) Voc, FF, Jsc, and PCE of P3HT:PCBM solar cells as a function of postannealing time at 120 °C.
annealing processes, including film annealing and postannealing at the temperatures of 130 and 120 °C, respectively. Film annealing can be used to control the crystallinity of PCBM and balanced electron and hole mobilities in the composite film. Postannealing of the devices is critical to oxygen dedoping in the composite film processed in air. The carrier lifetime in P3HT:PCBM was characterized by impedance spectroscopy, which shows strong dependence on the postannealing conditions. The P3HT:PCBM solar cells with the best performance show the longest carrier lifetime, indicating that the oxygen dedoping is critical to air-processed organic solar cells. Acknowledgment. This work is financially supported by the Hong Kong Polytechnic University (project nos. J-BB9S and A-SA54). References and Notes (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15.
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