In situ electrochemical doping enhances the efficiency of polymer photovoltaic devices† Ming-Shin Su, a Hai-Ching Su, b Chih-Yin Kuo, a Yi-Ren Zhou a and Kung-Hwa Wei * a Received 20th October 2010, Accepted 9th February 2011 DOI: 10.1039/c0jm03550e In this study, we found that the formation of a p–i–n junction through in situ electrochemical doping is a promising way to enhance the performance of polymer photovoltaic devices. We applied a pre-bias to metal triflate [LiOTf, KOTf, Ca(OTf) 2 , Zn(OTf) 2 ]/poly(ethylene oxide) (PEO)–incorporated poly[5-(2 0 -ethylhexyloxy)-2-methoxy-1,4-phenylenevinylene] (MEH-PPV)/{6}-1-(3-(methoxycarbonyl) propyl)-{5}-1-phenyl-[5,6]-C 61 (PCBM) photovoltaic devices to form p–i–n junctions in their active layers. Auger depth profile analyses and alternating-current capacitance analyses of these doped devices revealed that the positive and negative ions were distributed unequally to form an asymmetrical p–i–n structure in a thin layer of ca. 100 nm of the polymer, and the intrinsic layer became thinner when formed under a higher pre-bias voltage. Atomic force microscopy and transmission electron microscopy revealed that the addition of metal triflate/PEO to MEH-PPV/PCBM improved the morphology of the composite films. Among the various doped devices, the MEH-PPV/PCBM device incorporating a LiOTf/PEO mixture exhibited the highest power conversion efficiency, a 40% increase relative to that of the reference device (MEH-PPV/PCBM). Introduction Organic photovoltaic devices are attracting much attention because of their potential for use as cheap, large-area, flexible devices. Such devices featuring a bulk heterojunction (BHJ) structure exhibit much higher power conversion efficiencies (PCEs) than corresponding non-heterojunction devices because of their larger interfacial areas for exciton dissociation. 1–6 Much effort has been made to optimize the internal structures of polymer solar cells to increase their lifetimes and PCEs; for example, modifying the work functions of the electrodes 7,8 by incorporating metal oxides such as TiO x , MoO 3 , WO 3 and V 2 O 5 between the active layers and electrodes 9,10 and taking advantage of nano-scale effects, have been carried out. 11,12 Phase compati- bility and overall uniformity of the film roughness are critically important properties for the optimal performance of polymer solar cells. 13,14 Nevertheless, the built-in potential across the disordered active layer, with a thickness of ca. 100 nm, which is essential for efficient light absorption, leads to a relatively lower built-in electric field and thus, lower carrier collection efficiency. 15–19 These obstacles, which are also encountered in small-molecule solar cells, can be partially overcome by fabri- cating organic solar cells with a p–i–n architecture. 20–23 Recently, a small-organic-molecule p–i–n structure was developed by doping small-molecule organic layers to increase the conductivity and, therefore, ensure efficient charge transport and the forma- tion of ohmic contacts at organic–metal interfaces. 24 As a result, the built-in potential is predominant and strong across the resistive and thin intrinsic regions, rendering a higher built-in electric field and, consequently, a higher carrier collection efficiency. Polymer photovoltaic devices can be conveniently fabricated using cost-effective solution processing techniques, such as spin- coating or inkjet printing, making them attractive for large-area applications. Nevertheless, multilayered polymer p–i–n struc- tures are not readily fabricated through solution processing because of interfacial mixing problems. One promising approach toward polymer p–i–n photovoltaic devices is the use of in situ electrochemical doping. 25 The active layer of such photovoltaic devices contains salts with mobile ions, which can drift towards electrodes under an applied bias. For light-emitting electro- chemical cells (LECs), the spatial separation of ions that induce doping (oxidation and reduction) of the active materials near the electrodes (i.e., p-type doping near the anode and n-type doping near the cathode) has been demonstrated previously. 26,27 The p–i–n structure is, therefore, formed in situ under bias in a single-layered thin film, which is readily fabricated through solution processing. Two types of polymer-based chemically fixed p–i–n photovoltaic devices have been prepared using such a Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, 30050, Taiwan, ROC. E-mail: khwei@mail.nctu.edu.tw; Fax: +886-3-5724727; Tel: +886-3- 5731771 b Institute of Lighting and Energy Photonics, National Chiao Tung University, 301 Gaofa 3rd. Road, Tainan, 71150, Taiwan, ROC † Electronic supplementary information (ESI) available: current density–voltage and EQE curves of p–i–n MEH-PPV/PCBM photovoltaic devices, enlarged Auger depth profile consequence. See DOI: 10.1039/c0jm03550e This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 6217–6224 | 6217 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 6217 www.rsc.org/materials PAPER Published on 23 March 2011. Downloaded by National Chiao Tung University on 25/04/2014 02:48:39. View Article Online / Journal Homepage / Table of Contents for this issue