Development of Selenization/Sulfurization Process for High Quality Cu(In, Ga)(S, Se) 2 Solar Cells on High Strain Point
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Development of Selenization/Sulfurization Process for High Quality Cu(In, Ga)(S, Se)2 Solar Cells on High Strain Point Glass Substrates Takeshi Tomizawa, Reo Usui, Takeshi Okato and Hidefumi Odaka Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan ABSTRACT This study provides a recipe of a 2-step selenization and sulfurization method for high strain point (HSP) glass to improve the quality of Cu(In, Ga)(S, Se)2 (CIGSSe). The recipe is distinguished by slow selenization growth before sulfurization growth at the high temperature of 580 °C. We used proto-type HSP glass instead of standard soda lime glass (SLG) to tolerate this higher temperature process. The provided slow selenization recipe improved an averaged relative efficiency by 14 percent compared to a rapid selenization recipe. We confirmed the improvement of the quality of CIGSSe which was characterized by the high crystal quality, the smooth surface, the uniform depletion layer and reduced defects as measured by XRD, SEM, EBIC and Admittance spectroscopy. INTRODUCTION Owing to its high conversion efficiency, CIGS solar cell has been intensively investigated in many institutes and companies. The highest efficiency of 20.3 % in the small cell has been reported using the evaporation method [1]. The efficiency of 17.8% has been achieved on a 30 × 30 cm2 module using the selenization method [2]. Although CIGS and its family are the most successful among thin film solar cells, the present conversion efficiencies are lower than that expected by the theory because the open circuit voltage doses not increase along with the band gap due to the presence of the DX centers. One of methods to overcome this issue is to fabricate the CIGS absorber layer at high temperature. A recent research has suggested that the open circuit voltage in wide band gap, larger than 1.2 eV, was improved by fabricating at the high temperature of 600°C ~650 °C [3]. High conversion efficiencies including the world record are also achieved by the high temperature processes exceeding 600 °C [1, 3-5]. At the temperature of around 600 °C, it is necessary to use the high strain point (HSP) glass which was designed to tolerate much higher temperatures than standard soda lime glass (SLG). For this reason, we have developed HSP glass substrates for CIGS solar cells. To evaluate the substrates, we have also developed a 2-step selenization and sulfurization method. This method is widely used by many module makers because it is suitable to apply to large size modules. Our previous works have revealed that the combination of high temperature processes at 580 °C and HSP glass substrates are necessary for the high efficiency of selenized and sulfulized Cu(In, Ga)(S, Se)2 (CIGSSe) [6, 7]. The high growth temperature reduced defects in CIGSSe to be compensated by sodium. In this high quality CIGSSe, the sodium diffusion from the HSP glass is enough to improve the efficiency although it contains less sodium than the SLG. In this study, we provide a 2-step selenization
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