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Enhanced High-Temperature Long-Term Stability of Polymer Solar Cells with a Thermally Stable TiOx Interlayer Dong Hwan Wang,†,| Sang Hyuk Im,‡,| Hang Ken Lee,† O Ok Park,*,† and Jong Hyeok Park*,§ Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology (BK 21 Graduate Program), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea, Research Park, Korea Research Institute of Chemical Technology, 107, Sinseongno, Yuseong-gu, Daejeon 305-600, Republic of Korea, and Department of Chemical Engineering, Sungkyunkwan UniVersity, Suwon 440-746, Republic of Korea ReceiVed: June 29, 2009; ReVised Manuscript ReceiVed: August 13, 2009
The short lifetime of polymer-based solar cells is an obstacle to their commercialization. Since solar cells would be operated at elevated temperature, it is necessary to improve their high-temperature long-term stability. Insertion of a TiOx interlayer between an Al electrode and the active layer could result in enhanced performance and long-term stability. However, the operational lifetime at elevated temperature becomes worse because of its significant morphological change during high-temperature operation, which possibly deteriorates the interface between Al and the active layer. In this paper, the role of a unique type of a TiOx interlayer from a newly designed polymeric precursor is presented. The operational lifetime at elevated temperature has been much improved by introducing such TiOx interlayer. It is attributed to an improved interfacial stability, owing to relatively reduced morphology change at high-temperature operation. The effectiveness of this unique feature makes it possible to fabricate more efficient organic solar cells by adopting a post-annealing process. 1. Introduction Polymer-material-based solar cells used for renewable electrical power and sustainable energy have been the target of research worldwide in recent years, and substantial progress has been reported in numerous areas.1-7 Since the discovery of efficient electron transfer between [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and conjugated polymers in bulkheterojunction polymer-based solar cells, considerable attention has been directed toward increasing the performance of these systems.8-16 While the reported cell efficiencies are already close to fulfilling some of the requirements for commercial applications, the short lifetime of polymer-based solar cells is still an obstacle to their commercialization.17 In order to achieve a better device lifetime in photovoltaic devices, good encapsulation has to be used, but in most cases this requires glass substrates or expensive multilayer barrier films.18 Despite the remarkable improvements in the long-term stability obtained with encapsulation technology, most of these devices exhibit poor flexibility and a significantly increased fabrication cost. It has previously been demonstrated that the degradation behavior of polymer solar cells involves a number of photochemical mechanisms, including the direct photooxidation of conjugated solids leading to a loss of conjugation and irreversible deterioration of the light-absorbing properties, the photochemical reduction of the organic constituents by the Al electrode and subsequent chemical reaction between the organoaluminum species and oxygen atoms.19 With respect to the first degradation mechanism, numerous research groups have recently reported on polymer-based photovoltaic devices with a TiOx * To whom correspondence should be addressed. E-mail: ookpark@ kaist.ac.kr (O.O.P.);
[email protected] (J.H.P.). | Authors contributed equally to this work. † Korea Advanced Institute of Science and Technology. ‡ Korea Research Institute of Chemical Technology. § Sungkyunkwan University.
interlayer between the active layer and Al electrode. The K. Lee and Alan J. Heeger groups reported air-stable polymer solar cells fabricated by an all solution processing technique using a TiOx interlayer as a shielding and scavenging layer, which prevents the intrusion of oxygen and humidity into the electronically active polymers.20-22 However, the preparation process of the TiOx layer is complicated and involved exposure to air for hydrolysis. Hayakawa et al. produced a solar cell having a similar structure by introducing a functional TiOx layer as a oxygen blocking layer in a bulk-heterojunction solar cell.23 Thus, the device lifetime was enhanced without interfering with the principal ‘flexible solar cell’ concept. Moreover, the use of a new architecture using solution-based titanium oxide as an optical spacer leads to reduced recombination and alters the charge-carrier concentration. The overall efficiency was increased by ∼50% compared to a device without the TiOx interlayer optical spacer.22 Our group recently found that a TiOx interlayer plays an important role in devices with an Al electrode deposited at low vacuum pressure.24 In the case of a device with a TiOx interlayer prepared from titanium isopropoxide (TIP, the most widely used precursor), however, the high-temperature long-term stability was worse than that of the neat device (without TiOx interlayer). This indicates that devices with a TiOx interlayer are not stable when operated under high-temperature conditions. For this reason, other recently introduced devices with a TiOx interlayer were not annealed at high temperature after coating the Ti precursor on the active layer during cell preparation.23-25 These negative effects can cause two important bottlenecks for general use of the TiOx layer in polymer solar cells. Because the temperature of solar cells under irradiation of solar light during midday might go up to 80 °C the stability of solar cell with the TiOx layer would be so bad. Second, recent research papers showed that it is possible to fabricate more efficient organic solar cells through the use of a postannealing
10.1021/jp9060939 CCC: $40.75 2009 American Chemical Society Published on Web 09/02/2009
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Figure 1. Polymer solar cells employing a TiOx interlayer prepared from a newly designed precursor between the active layer and an Al electrode.
process (at ∼150 °C).26-28 Therefore, it is impossible to improve the cell performance with the TiOx layer from the TIP precursor by the postannealing process, owing to their low high-temperature stability. This is the focus of the present work. To address this drawback in polymer solar cells with a TiOx interlayer, a thermally stable polymeric precursor was synthesized from TIP through hydrolysis and a condensation sol-gel reaction in the presence of acetic acid. The polymer solar cell with the TiOx interlayer prepared from the polymeric precursor (Figure 1) showed greatly enhanced long-term, high-temperature stability without any sacrifice of the cell efficiency. 2. Experimental Section Synthesis of Polymeric TiOx. Acetic acid exothermically reacts with TIP and can form metal alkoxo-acetates. While the isopropoxide groups in TIP are preferentially hydrolyzed and then condensated, the bridged acetate ligands remain bonded to titanium throughout the condensation reaction. Since they are not hydrolyzed, the bridging CH3COO- ligands significantly change the condensation pathway, likely promoting the formation of linear polymers composed of edge-shaped octahedra.29 In a typical synthesis, 80 g of TIP was dropped into 80 g of methanol in a round flask and 16 g of glacial acetic acid was then added to the solution at room temperature. After 30 min, 5 g of deionized water (18 MΩ) was dropped into the solution and the reaction allowed to proceed for 24 h. Through the entire synthesis, the reaction mixture was vigorously stirred by a magnetic bar. The TiOx solution was subsequently diluted with methanol for further use. Fabrication of Polymer Solar Cells. The polymer solar cell with the thermally stable TiOx interlayer was fabricated as follows. Indium tin oxide (ITO) glass was cleaned with chloroform, isopropanol, and acetone and then treated with an oxygen plasma before use. A conducting polymer poly(3,4ethylene dioxythiophene):poly(styrene sulfonate) (Baytron P) was spin-coated to a thickness of ∼35 nm. P3HT and PCBM dissolved in chlorobenzene were then spin-coated to a thickness of 220 nm at 900 rpm for 5 s to serve as an active layer. The TiOx interlayer was then spin-coated to various thicknesses and the metal Al electrode was thermally deposited to a thickness of 150 nm under low (1.1 × 10-5 torr) and high (1.1 × 10-6 torr) vacuum pressures. The deposited Al electrode area defines the active area of the devices which was 4-6 mm2. Finally, the solar cell devices were postannealed at 150 °C for 30 min on a digital controllable hot plate in a glovebox filled with inert gas. For the preannealed device, thermal annealing was conducted at 140 °C for 4 min before TiOx spin-casting. The J-V curves were measured with a Keithley 2400 source measure unit at AM 1.5 illumination. A 1000W xenon lamp (Oriel, 91193) was used as a light source and its intensity was calibrated by a reference cell of Si (Fraunhofer ISE, certificate no. C-ISE269).
Figure 2. FTIR spectra of the polymeric TiOx. The inset figure shows the proposed chemical structure of the newly designed TiOx precursor.
3. Results and Discussion To characterize the chemical structures of the produced TiOx, the Fourier transform infrared (FTIR) spectrum (see Figure 2) was measured by the KBr pellet method using dried TiOx powder. The absorption band at 620 cm-1, assigned to the stretching vibration υ(Ti-O-iPr), shows that unreacted isopropoxide groups partially remained. The absorption bands at 550, 660, 1030, 1425, and 1542 cm-1 are attributed to the stretching and vibrations of the Ti-O-Ti bond, confirming the polymerization of TIP into TiOx.29-31 The symmetric and asymmetric vibrations of the carboxylic groups corresponded to υs (CO2-) ) 1450 cm-1 and υa(CO2-) ) 1580 cm-1, respectively.29 The difference in frequency (∆υ ) 130 cm-1) between these two bands is typical of an acetate ion acting as a bidentate (bifunctional) ligand indicating that two acetate ions are bridging two titanium atoms (see the inset of Figure 2).29-31 Through the characterization via FT-IR, it can be predicted that TiOx develops into a linear polymer bridged by a bidentate acetate ligand. Figure 3a shows the J-V curves of the polymer solar cell devices with TiOx interlayers of different thicknesses. The thickness of the TiOx interlayer was simply adjusted by changing the concentration of the TiOx solution; 0, 1.5, 3, and 6 nm thick TiOx interlayers were evaluated in this study. These devices were fabricated by pre- and postannealing treatment. As indicated in the figure, the performance of the solar cell is dependent on the thickness of the TiOx interlayer and the efficiency of the device can be increased from 2.59% to 3.0%. When the thickness of the TiOx interlayer was 0, 1.5, 3, and 6 nm, the efficiency of the device was 2.57%, 2.93%, 3.02%, and 2.69%, respectively. The increase in the device efficiency brought about by the introduction of the TiOx interlayer is mainly attributed to the increase of the electron conductivity, because the open circuit voltage (Voc) and fill factor are mostly maintained at the same level but the short circuit current (Jsc) is significantly increased from 8.72 to 9.40 mA/cm2. The capacity of the TiOx interlayer to serve as a hole-blocking layer also increases the device efficiency. The polymeric TiOx interlayer with a thickness of 1.5 nm showed a relatively high Jsc and good device efficiency. However, the efficiency was inconsistent, because the layer was too thin to homogeneously cover the entire device. The optimum thickness of the TiOx interlayer was ∼3 nm, which provided the best efficiency consistently and reproducibly (more than 10 samples were tested). On the other hand, the efficiency was decreased in the case of the 6 nm thick interlayer, possibly due
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Figure 4. Efficiencies of the polymer solar cells with polymeric TiOx and TIP-based TiOx interlayers depending on the annealing temperature.
Figure 3. (A) Photocurrent-voltage curves of polymer solar cells with TiOx layers of different thicknesses. (B) Photocurrent-voltage curves of polymer solar cells prepared by different annealing processes.
to the increased series resistance, which can be explained by the decrease of Jsc. Figure 3b shows the J-V curves of polymer solar cells prepared with the polymeric TiOx and TIP-based TiOx, respectively. After preparing the cells via a preannealing process, some of them were annealed again at high temperature (∼150 °C for 30 min). It is noteworthy that the cells with the synthesized polymeric TiOx interlayer were sufficiently thermally stable that the device efficiency could be increased through the post annealing process. In contrast, the device incorporating the TiOx interlayer prepared from TIP showed seriously deteriorated performance. As can be seen in Figure 3b, the efficiency of the device with the polymeric TiOx interlayer was dramatically enhanced by the postannealing process. Figure 4 shows the efficiency of the polymer solar cells with the polymeric precursor and TIP-based TiOx interlayers at different operating temperatures. The two types of devices with TiOx layer were fabricated at room temperature and then annealed at 50, 100, and 150 °C for 10 min. The cell efficiency of the devices with the polymeric TiOx interlayer increased continuously as the annealing temperature was increased from room temperature to 150 °C, with efficiencies of 2.30%, 2.70%, 2.95%, and 3.02% being obtained, respectively. In contrast, in the same temperature range, the efficiency values of the devices with the TIP-based TiOx interlayer decreased with increasing annealing temperature, as follows, 2.76%, 2.92%, 1.52%, to 0.93%. Therefore, the difference in the efficiency between the devices with the polymeric precursor and TIP-based TiOx
interlayer was not large at room temperature. However, the performance difference gradually increased from room temperature to 150 °C due to the high thermal stability of the polymeric TiOx interlayer. During operation at high temperature, the device with the TIP-based TiOx was thermally degraded, whereas the device with polymeric TiOx showed very high thermal stability. Device stability is a serious problem for polymer solar cells such as those made of semiconducting polymer materials. This problem is due to the simultaneous exposure to oxygen or water vapor, rapid photo oxidation, the occurrence of device degradation, and light illumination.32 The polymeric TiOx interlayer plays a protective role against air and humidity and preserves the device performance over a long lifetime. A test for measuring the lifetime of a reference device and a device with a spin-coated polymeric TiOx layer was conducted at room temperature. Figure 5 shows the device stabilities of the reference device and the device with the polymeric TiOx interlayer in air. As can be seen in Table 1, the efficiency of the device with polymeric TiOx interlayer decreased from 2.93% to 2.17% after 160 h, whereas that of the reference device decreased from 2.66% to 1.36%. It is anticipated that by inserting a solutionprocessed polymeric titanium oxide layer between the active layer and Al electrode in polymer solar cells, devices with strong air stability and enhanced efficiencies can be realized. The polymeric TiOx interlayer acts as a barrier and shielding layer, as previously reported.23 It thus prevents the intrusion of humidity and oxygen into the active polymer materials, thereby enhancing the lifetime of uncoated polymeric TiOx devices exposed to air. Figure 6 shows the long-term stabilities of the reference device and the devices with the TiOx interlayer prepared from the polymeric precursor and TIP in air at 80 °C. The cell efficiency of the device containing TiOx prepared from TIP decreases to ∼0.5 after 50 h, whereas the efficiency of the device without TiOx decreases to ∼0.76 after 160 h. This provides good evidence for the negative effect of the TIP-based TiOx interlayer on the long-term, high-temperature stability. However, consistent with the device stability behavior under ambient conditions, the device with the polymeric TiOx interlayer again exhibited excellent air-stability. Figure 7a,b shows the AFM images of the TIP-based TiOx films before and after thermal annealing at 150 °C for 30 min, respectively, which demonstrate the morphological changes of the TiOx interlayer in the device after the thermal annealing
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Figure 5. Normalized efficiencies of the devices without and with a polymeric TiOx interlayer as a function of the storage time in air.
TABLE 1: Efficiencies of the Devices with and without the Polymeric TiOx Interlayer device efficiencies (%)
0h
24 h
48 h
72 h
120 h
160 h
reference device (in air) device with polymeric TiOx (in air)
2.66 2.93
2.42 2.68
2.09 2.45
1.92 2.33
1.67 2.24
1.36 2.17
process. The rms and Ra values of the TIP-based TiOx films without annealing are 12.288 and 9.794 nm, respectively. On the other hand, the annealed films display rms and Ra values of 16.235 and 12.685 nm, respectively. Figure 7c,d shows the AFM images of the polymeric precursor-based TiOx films without and with thermal annealing at 150 °C for 30 min. When no annealing process was applied, the rms and Ra values are 12.064 and 9.564 nm, respectively. The rms and Ra values in the case with the annealing process were 12.236 and 9.741 nm, respectively, which are similar to those before thermal annealing. Furthermore, aggregated nanoparticles were observed in the TiOx layer prepared from TIP after the thermal annealing process. Hence, the performance of the device with the TIP-based TiOx interlayer
Figure 6. Normalized efficiencies of the devices without TiOx interlayer, with a polymeric TiOx interlayer and with TIP-based TiOx interlayer as a function of the storage time in air at 80 °C.
at high temperature was degraded, due to the unwanted change in the morphology, which possibly deteriorated the interface between the Al electrode and the active layer. However, for the polymeric precursor-based TiOx films, the rms and Ra values for the cases with and without thermal annealing are very similar. Hence, the functions of the TiOx interlayer, such as electron transporting and hole blocking are not affected by the operating temperature of the devices. Another advantage of using the polymeric precursor-based TiOx film is that the polymeric precursor has relatively fewer reactive sites, thus resulting in reduced unwanted oxidation reactions with the active layer. Pacios et al. showed that the absorbance is decreased in the wavelength region near to the maximum in the absorption of the pristine conjugated polymer after photodegradation.18 Figure 8 shows the optical absorbance of an P3HT:PCBM blend film (∼20 nm thickness) overlayered with different TiOx films (∼3 nm) before and after being annealed at 150 °C. Initially, the bilayer films show almost the same intensities. After thermal annealing, the absorbance of the bilayer film with the polymeric precursor-based TiOx increases. This increase is well correlated with the J-V behavior, as shown in Figure 3. However, the absorbance of the bilayer film with the TIP-based TiOx after the thermal annealing process is almost constant. This might be due to the fact that the increment of the absorbance of P3HT is offset by the reduction in absorbance of P3HT, owing to its oxidation.18 The optical absorbance of P3HT seems to be reduced by the TIP-based TiOx film after the thermal annealing process, but this effect alone is insufficient to explain the dramatic decrease of the cell efficiency as shown in Figure 3b. Recently, our group reported a low-vacuum processable polymer solar cell realized by the unique effect of a TiOx interlayer.24 To investigate the effects of the polymeric TiOx interlayer on the performance of the cells with Al deposited under a low-vacuum condition, we fabricated a device having a thermally stable TiOx interlayer under a low vacuum pressure and compared its properties with those of cells without an interlayer. Figure 9 shows the J-V curves of the devices
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Figure 7. (A) AFM images of TIP-based TiOx films (3 nm) without thermal annealing. (B) AFM images of TIP-based TiOx films (3 nm) with thermal annealing at 150 °C for 30 min. (C) AFM images of polymeric precursor-based TiOx films (3 nm) without thermal annealing. (D) AFM images of polymeric precursor-based TiOx films (3 nm) with thermal annealing at 150 °C for 30 min.
Figure 8. UV-vis absorption spectra of P3HT:PCBM/TiOx bilayer prepared using different Ti precursors. The black and green colors correspond to the as-produced films and red and blue to the films thermally annealed at 150 °C.
fabricated under a low vacuum pressure (1.1 × 10-5 torr.). The beneficial effects of the TiOx interlayer were observed when the polymeric TiOx was introduced into the devices. 4. Conclusions It was found that the polymer solar cells with the newly designed TiOx layer between the active layer and Al electrode have excellent long-term, high-temperature stability compared to the devices with a TIP-based TiOx interlayer. From the AFM observation, obvious morphological changes of the TIP-based TiOx interlayer were observed after annealing at high temperature, which possibly deteriorated the interface between Al and
Figure 9. Photocurrent-voltage curves of devices with and without a TiOx layer fabricated under low vacuum pressure.
the active layer. Reduced optical absorbance after thermal annealing was also observed in the P3HT:PCBM/TIP-based TiOx bilayer. The integration of the polymeric TiOx layer between the active layer and the Al electrode of the device makes it possible to fabricate more efficient organic solar cells through the use of a postannealing process. This technology is expected to play an important role in the practical application of TiOx layers in polymer solar cells. Acknowledgment. This work was partially supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea and This work was also partially supported by the grant from the ERC program of the Korea Science and Engineering Foundation (KOSEF) funded by the Korea Ministry
Polymer Solar Cells with a Stable TiOx Interlayer of Education, Science and Technology (MEST) (No. R11-2009045-01002-0(2009). Supporting Information Available: FTIR spectra of the polymeric TiOx and TiOx interlayer prepared from titanium isopropoxide thermally annealed at different temperatures. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1997, 587. (2) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (3) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (4) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077. (5) Liu, R. C. J. Phys. Chem. C 2009, 113, 9368. (6) Kim, H.; Shin, M; Kim, Y. J. Phys. Chem. C 2009, 113, 1620. (7) Thompson, B. C.; Frechet, J. M. J. Angew. Chem. ReV. 2008, 47, 58. (8) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. AdV. Func. Mater. 2007, 17, 1636. (9) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (10) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Func. Mater. 2005, 15, 1617. (11) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (12) Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Chem. Mater. 2005, 17, 2175. (13) Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D. L. Appl. Phys. Lett. 2007, 90, 163511. (14) Kim, K. C.; Park, J. H.; Park, O. O. Sol. Energy Mater. Sol. Cells 2008, 92, 1188.
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