Comprehensive Study of Light-Soaking Effect in ZnO/Cu(InGa)Se 2 Solar Cells with Zn-Based Buffer Layers
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COMPREHENSIVE STUDY OF LIGHT-SOAKING EFFECT IN ZnO/Cu(InGa)Se2 SOLAR CELLS WITH Zn-BASED BUFFER LAYERS Sutichai Chaisitsak*, Akira Yamada and Makoto Konagai Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8552, Japan * [email protected] ABSTRACT The light-soaking effect in ZnO/ Cu(InGa)Se2 (CIGS) based solar cells has been studied. A CIGS thin film with Cu(InGa)(SeS)2 surface layer was obtained by selenization (H2Se)/sulfurization (H2S). A high resistively ZnO buffer layer deposited by the atomic layer deposition technique was used as a buffer layer. We found that the light-soaking effect mainly correlates with the properties of the CIGS surface, rather than with the properties of the ZnO buffer/window layer. This phenomenon can be eliminated by surface etching or doping CIGS surface with Zinc. Zinc diffusion using diethylzinc gas has been proposed in this work. To date, we have achieved efficiency of 13.9% (Voc: 560 mV, Jsc: 35.0 mA/cm2, FF: 0.71) without light soaking effect. INTRODUCTION In these few years, several Zinc-based materials have been investigated as an alternative buffer layers to chemical bath deposited (CBD) CdS in Cu(InGa)Se2 thin film solar cells. Some of them, such as ALD (atomic layer deposition)-ZnSe [1] and evaporated ZnInxSey [2], ALD-ZnO [3], CBD-ZnS [4], and CBD-Zn(O,S,OH)x [5], have demonstrated well in efficiency. However, it is commonly observed that the performance of the devices with these buffer layers significantly improves after illumination or after electrical bias, compared to that of the devices with CBD-CdS buffer layers. This phenomenon is known as light-soaking effect or metastable effect. To realize further commercialization of CIGS thin film modules with Zn-based buffer layers, it is strongly required to improve metastability of these devices [6]. The objective of this study is to improve the metastability of the solar cells based on CIGS thin films deposited by selenization/sulfurization. A highly resistive ALD-ZnO was used as a buffer layer. We found that the surface of CIGS absorbers plays an important key for improving device metastability. By modifying the CIGS surface, a high efficient ZnO/CIGS solar cell without metastable effect has been achieved. EXPERIMENTAL Solar cells with structure of ZnO:B/i-ZnO/CIGS/Mo/SLG were investigated. A CIGS thin film with a Cu(InGa)Se2 surface layer was obtained by selenization (H2Se)/sulfurization (H2S) at Showa Shell Sekiyu K.K [7]. A highly resistive ZnO buffer layer was deposited by the ALD technique, using a modified MOCVD system [3]. Diethylzinc (DEZn), H2O and argon were used as zinc, oxygen sources and carrier gas, respectively. The solar cells were completed with MOCVD-ZnO as a window layer and an Al-grid collector. Zinc diffusion in CIGS was carried out by DEZn gas. The gaseous reactant was diluted with a carrier gas (Ar) and transported to the H9.10.1
sample in a way similar to MOCVD. This method enables a precise and flexible control
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