Increase in Open-Circuit Voltage and Improved Stability of Organic Solar Cells by Inserting a Molybdenum Trioxide Buffer
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1154-B10-73
Increase in Open-Circuit Voltage and Improved Stability of Organic Solar Cells by Inserting a Molybdenum Trioxide Buffer Layer
Hideyuki Murata, Yoshiki Kinoshita, Yoshihiro Kanai, Toshinori Matsushima and Yuya Ishii School of Materials Science, Japan Advanced Institute of Technology (JAIST) 1-1 Asahidai, Nomi, Ishikawa 923-129, Japan ABSTRACT We report an increase in open-circuit voltage (Voc) by inserting an MoO3 layer on ITO substrate to improve built-in potential of organic solar cells (OSCs). In the OSCs using 5,10,15,20-tetraphenylporphyrine (H2TPP) as p-type material and C60 as n-type material, the Voc effectively increased from 0.57 to 0.97 V with increasing MoO3 thickness. The obtained highest Voc (0.97 V) is consistent with the theoretical value estimated from the energy difference between the LUMO (-4.50 eV) of C60 and the HOMO (-5.50 eV) of H2TPP layer. Importantly, the enhancement in the Voc was achieved without affecting the short-circuit current density (Jsc) and the fill-factor (FF). Thus, the power conversion efficiency of the device increased linearly from 1.24% to 1.88%. We also demonstrated that a MoO3 buffer layer enhances the stability of OSCs after photo-irradiation. We have investigated the stability of OSCs using H2TPP and N,N’di(1-naphthyl)-N,N’-diphenylbenzidine as p-type layer. Both devices with MoO3 layer showed improved stability. These results clearly suggest that the interface between ITO and p-type layer affects device stability. INTRODUCTION In recent years, organic solar cells have attracted much attention as a new inexpensive renewable energy source. The increase in power conversion efficiency (ηP) and the improvement of stability of organic solar cells are key issues in the development of organic solar cells. The ηP of the solar cells depends on three device parameters, such as the open-circuit voltage (Voc), the short-circuit current density (Jsc), and the fill factor (FF). Among those parameters, the Jsc can be improved by the use of bulk heterojunctions (e.g., the composite of p-type and n-type materials) as an active layer in both polymer [1] and small molecule-based solar cells. However, there is no enhancement effect on the Voc by the use of bulk heterojunction structure. For the further improvement of ηP, it is essential to enhance Voc, while keeping the corresponding Jsc. It has been shown that the Voc depends on the energy difference between the lowest unoccupied molecular orbital (LUMO) of the electron acceptor material and the highest occupied molecular orbital (HOMO) of the electron donor material [2–4]. Recently, Mutolo et al. reported that Voc of the solar cells increased in double heterojunction solar cells composed of boron subphthalocyanine chloride (HOMO level= -5.60 eV) and C60 (LUMO level= -4.50 eV) [5]. We have reported that Voc incresed by inserting a thin layer of CuPc and Zn-phthalocyanine (ZnPc) with higher HOMO level (-5.1 eV) at the interface of pentacene (HOMO= -5.0 eV) and C60 [6]. In this study, we found that a modification of ITO surfac
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