本研究主要探討加入碳酸銫(Cs2CO3)作為介面層之後的元件特性(Blend/Cs2CO3/Al)，電子注入的機制，以及元件後熱退火對元件特性的影響。結果發現：在陰極鋁與主動層間加入一極薄的Cs2CO3作為介面層，可以有效地提升元件的開路電壓和填充係數，進而提升有機太陽能電池的功率轉換效率，元件的開路電壓為0.57 V，短路電流密度為9.45 mA/cm2，填充係數可達0.60，而功率轉換效率可達到3.05%。從光電子能譜儀的測量結果可發現，加入Cs2CO3作為介面層之後，會有電子轉移至主動層，使有機物產生N-型攙雜，幫助電子的注入，進而降低元件的串聯電阻。同時，我們由開路電壓對溫度的關係圖可以求得元件開路電壓的最大值，結果發現加入Cs2CO3作為介面層後，元件開路電壓的最大值為1.02 V，此值相當接近donor的HOMO與acceptor的LUMO之能階差，因而可證實Cs2CO3/Al電極可形成較好的歐姆接觸。此外，在ITO/PEDOT: PPS/P3HT: PCBM/Cs2CO3/Al元件做元件後熱退火處理後，從AFM圖可得知，陰極與主動層間的接合度獲得改善，避免在主動層與陰極間介面附近形成缺陷，進而減少漏電路徑並提升元件的並聯電阻，當元件後熱退火處理的條件為140 ˚C，為時四分鐘，元件的功率轉換效率可以提升至3.70%。

In this study, we mainly investigated the cell characteristics of Blend/Cs2CO3/Al devices, the electron-injection mechanism of the Cs2CO3/Al cathode, as well as the post-annealing effect on the device performance. We found that an insertion of Cs2CO3 improves the device performances including Voc, FF, and PCE. In the device of Blend/Cs2CO3/Al, a Voc of 0.57 V, a Jsc of 9.45 mA/cm2, a FF of 0.60, and a PCE of 3.05% were achieved. The result of XPS measurement revealed that a portion of electrons would be transferred into the active blend after inserting an interlayer of Cs2CO3, inducing the N-type dopping of the active layer. The N-type dopping process enhances the electron injection, being responsible for the reduced series resistance of the device. Meanwhile, the maximum Voc can be determined form the temperature dependence of Voc. The (Voc)max after inserting Cs2CO3 was found be 1.02 V which is extremely close to the corresponding value of the energy difference between the HOMO of donor and the LUMO of acceptor, suggesting that Cs2CO3/Al electrode can form a better ohmic contact. From the AFM images, the adhesion can be improved after the ITO/PEDOT: PPS/P3HT: PCBM/Cs2CO3/Al device was post-annealed. This prevents the forming of defects near the interface between the active layer and the cathode and therefore reduces leakage paths, resulting in the increase of the calculated shunt resistance. Under the optimum post-annealing condition (140 ˚C, 4 min), the power conversion efficiency of the device can be improved to 3.7%.

[1]. C. W. Tang, “Two-layer organic photovoltaic cell,” Appl. Phys. Lett. 48, 183 (1986).
[2]. G. Yu, K. Pakbaz, and A. J. Heeger, “Semiconducting polymer diodes: Large size, low cost photodetectors with excellent visible-ultraviolet sensitivity,” Appl. Phys. Lett. 64, 3422 (1994).
[3]. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, “Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions,” Science 270, 1789 (1995).
[4]. P. Peumans, V. Bulovic´, and S. R. Forrest, “Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes,” Appl. Phys. Lett. 76, 2650 (2000).
[5]. P. Peumans and S. R. Forrest, “Very-high-efficiency double-heterojunction copper phthalocyanine/C60 photovoltaic cells,” Appl. Phys. Lett. 79, 126 (2001).
[6]. J. Xue, B. P. Rand, S. Uchida, and S. R. Forrest, “A Hybrid Planar-Mixed Molecular Heterojunction Photovoltaic Cell,” Adv. Funct. Mater. 17, 66 (2005).
[7]. G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, “High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends,” Nat. Mater. 4, 864 (2005).
[8]. J. Y. Kim, S. H. Kim, H. H. Lee, K. Lee, W. Ma, X. Gong, and A. J. Heeger, “New Architecture for High-Efficiency Polymer Photovoltaic Cells Using Solution-Based Titanium Oxide as an Optical Spacer,” Adv. Mater. 18, 572 (2006).
[9]. G. Li, V. Shrotriya, Y. Yan, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on Poly(3-hexylthiophene),” Appl. Phys. Lett. 98, 043704 (2005).
[10]. B. W. D Andrade, J. Brooks, V. Adamovich, M. E. Thompson, S. R. Forrest, “White Light Emission Using Triplet Excimers in Electrophosphorescent Organic Light-Emitting Devices,” Adv. Mater. 14, 1032 (2002).
[11]. A. P. Kulkarni, C. J. Tonzola, A. Babel, and S. A. Jenekhe, “Electron transport materials for organic light-emitting diodes,” Chem. Mater. 16, 4556 (2004).
[12]. Y. Z. Lee, X. Chen, S. A. Chen, P. K. Wei, and W. S. Fann, “Efficient red- and orange-light-emitting diodes realized by excitation energy transfer from blue-light-emitting conjugated polymers ,” Phys. Rev. B 56, 4479 (1997).
[13]. Y. Z. Lee, X. Chen, S. A. Chen, P. K. Wei, and W. S. Fann, “Soluble Electroluminescent Poly(phenylene vinylene)s with Balanced Electron- and Hole Injections,” J. Am. Chem. 123, 2296 (2001).
[14]. G. Horowitz, “Organic thin film transistors: From theory to real devices ,” J. Mater. Res. 19, 1946 (2004).
[15]. Chih-Wei Chu, Sheng-Han Li, Chieh-Wei Chen, Vishal Shrotriya, and Yang Yang, “Organic electrical bistable devices and rewritable memory cells,” Appl. Phys. Lett. 87, 193508 (2005).
[16]. L. P. Ma, J. Liu, and Y. Yang, “Organic electrical bistable devices and rewritable memory cells,” Appl. Phys. Lett. 80, 2997 (2002). (Memory-1)
[17]. S. Möller, C. Perlov, W. Jackson, C. Taussig, and S. R. Forrest, “A polymer/semiconductor write-once read-many-times memory,” Nature 426, 166 (2003).
[18]. V. G. Kozlov, V. Bulovi , P. E. Burrows and S. R. Forrest, “Laser action in organic semiconductor waveguide and double-heterostructure devices,” Nature 389, 362 (1997).
[19]. T. Rabe, S. Hamwi, J. Meyer, P. Görrn, T. Riedl, H.-H. Johannes, and W. Kowalsky, “Suitability of lithium doped electron injection layers for organic semiconductor lasers,” Appl. Phys. Lett. 90, 151103 (2007).
[20]. Handbook of Conducting Polymers, edited by T. A. Skotheim, R. L.Elsenbaumer, and J. R. Reynolds (Marcel Dekker, New York, 1998).
[21]. T. Aernouts*, W. Geens, J. Poortmans, P. Heremans, S. Borghs, and R. Mertens, “Extraction of bulk and contact components of the series resistance in organic bulk donor-acceptor-heterojunctions,” Thin Solid Films 403-404, 297 (2002).
[22]. M. R. Reyes, K. Kim, and D. L. Carroll, “High-efficiency photovoltaic devices based on annealed Poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61 blends,” Appl. Phys. Lett. 87, 083506 (2005).
[23]. F. Padinger, R. S. Rittberger, and N. S. Sariciftci, “Effects of Postproduction Treatment on Plastic Solar Cells,” Adv. Funct. Mater. 13, 85 (2003).
[24]. Y. Kim, S. A. Choulis, J. Nelson, and D. D. C. Bradley, “Device annealing effect in organic solar cells with blends of regioregular poly (3-hexylthiophene) and soluble fullerene,” Appl. Phys. Lett. 86, 063502 (2005).
[25]. W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, “Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology,” Adv. Funct. Mater. 15, 1617(2005).
[26]. Y. Zhao, Z. Xie, Y. Qu, Y. Geng, and L. Wang, “Solvent-vapor treatment induced performance enhancement of poly (3-hexylthiophene): methanofullerene bulk-heterojunction,” Appl. Phys. Lett. 90, 043504 (2007).
[27]. C. J. Ko, Y. K. Lin, F. C. Chen, and C. W. Chu, “Modified buffer layers for polymer photovoltaic devices,” Appl. Phys. Lett. 90, 063509 (2007).
[28]. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys. 93, 3693 (2003).
[29]. A. Moliton, and J. M. Nunzi, “How to model the behavior of organic photovoltaic cells,” Polym Int 55, 583 (2006).
[30]. V. D. Mihailetchi, H. Xie, B. de Boer, L. J. A. Koster, and P. W. M. Blom, “Charge Transport and Photocurrent Generation in Poly(3-hexylthiophene):Methanofullerene Bulk-Heterojunction Solar Cells,” Adv. Funct. Mater. 13, 85 (2003).
[31]. C. Waldauf, M. C. Scharber, P. Schilinsky, J. A. Hauch, and C. J. Brabec, “Physics of organic bulk heterojunction devices for photovoltaic applications,” J. Appl. Phys. 99, 104503 (2006).
[32]. L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom, “Ultimate efficiency of polymer/fullerene bulk heterojunction solar cells,” Appl. Phys. Lett. 88, 093511 (2006)
[33]. V. D. Mihailetchi, P. W. M. Blom, J. C. Hummelen, and M. T. Rispens, “Cathode dependence of the open-circuit voltage of polymer:fullerene bulk heterojunction solar cells,” J. Appl. Phys. 94, 6849 (2003).
[34]. P. Vanlaeke, A. Swinnen, I. Haeldermans, G. Vanhoyland, T. Aernouts, D. Cheyns, C. Deibel, J. D’Haen, P. Heremans, J. Poortmans, and J. V. Manca, “P3HT/PCBM bulk heterojunction solar cells: Relation between morphology and electro-optical characteristics,” Solar Energy Materials & Solar cells, 90, 2150 (2006).
[35]. C. J. Brabec, S. E. Shaheen, C. Winder, and N. S. Sariciftci, “Effect of LiF/metal electrodes on the performance of plastic solar cells,” Appl. Phys. Lett. 80, 1288 (2002).
[36]. G. Li, C. W. Chu, V. Shrotriya, J. Huang, and Y. Yang, “Efficient inverted polymer solar cells,” Appl. Phys. Lett. 88, 253503 (2006).
[37]. C. I. Wu, C. T. Lin, Y. H. Chen, M. H. Chen. Y. J. Lu, and C. C. Wu, “Electronic structures and electron-injection mechanisms of cesium-carbonate-incorporated cathode structures for organic light-emitting devices” Appl. Phys. Lett. 88, 152104 (2005)
[38]. Y. Li, D. Q. Zhang, L. Duan, R. Zhang, L. D. Wang, and Y. Qiu, “Elucidation of the electron injection mechanism of evaporated cesium carbonate cathode interlayer for organic light-emitting diodes,” Appl. Phys. Lett. 90, 012119 (2007).
[39]. N. Koch, A. Elschner, J. P. Rabe, and R. L. Johnson, “Work Function Independent Hole-Injection Barriers Between Pentacene and Conducting Polymers,” Adv. Mater. 17, 300 (2005).
[40]. S. Kirchmeyer and K. Reuter, “Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophene),” J. Mater. Chem. 15, 2077 (2005).
[41]. T. F. Guo, F. S. Yang, Z. J. Tsai, T. C. Wen, and S. N. Hsieh, Y. S. Fu, and C. T. Chung, “Organic oxide/Al composite cathode in efficient polymer light-emitting diodes,” Appl. Phys. Lett. 88, 113501 (2006)
[42]. G. Greczynski, M. Fahlman, and W. R. Salaneck, “Hybrid interfaces of poly(9,9-dioctylfluorene) employing thin insulating layers of CsF: A photoelectron spectroscopy study,” J. Chem. Phys. 114, 8628 (2001).
[43]. Y. Park, J. Lee, S. K. Lee, and D. Y. Kim, “Photoelectron spectroscopy study of the electronic structures of Al/MgF2/tris-(8-hydroxyquinoline) aluminum interfaces,” Appl. Phys. Lett. 80, 1288 (2002).
[44]. J. Ivanco, B. Winter, F. P. Netzer, M. G. Ramsey, L. Gregoratti, and M. Kiskinova, “Oxygen as a surfactant for Al contact metallization of organic layers,” Appl. Phys. Lett. 85, 585 (2004).
[45]. L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, P. W. M. Blom, “Light intensity dependence of open-circuit voltage of polymer:fullerene solar cells,” Appl. Phys. Lett. 86, 123509 (2005).
[46]. E. A. Katz, D. Faiman, S. M. Tuladhar, J. M. Kroon, M. M. Wienk, T. Fromherz, F. Padinger, C. J. Brabec, and N. S. Sariciftci, “Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions,” J. Appl. Phys. 90, 5343 (2001).