|Narrow bandgap CuIn(Sex,Te1-x)2 – based solar cells for chalcopyrite tandem photovoltaic devices|
|Zacharie Jehl Li Kao, Taizo Kobayashi, Tokio Nakada
Department of Electrical Engineering and Electronics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa, 252-5258, Japan, Sagamihara, Japan
We propose CuIn(Sex,Te1-x)2 as a quaternary chalcopyrite material for narrow-bandgap thin films solar cells, and focus on the purpose of realizing tandem photovoltaic devices by using this type of solar cell combined with a semi-transparent CIGSe solar cell. Numerical simulations are realized to evaluate the potential of such device, and high quality CIST films are grown on Mo-coated SLG substrates by 4 sources co-evaporation and a modified three stages process. The films are characterized by surface and in-depth X-ray Photoelectron Spectroscopy and by Transmission-Reflection Spectroscopy. A very narrow bandgap of less than 0.9 eV is achieved, much narrower than ternary compounds such as CuInSe2, CuIn3Te5 or CuInTe2. As a result, both J-V and EQE characterizations show a significant increase in the absorption of low energy photons, leading to a short circuit current of 41.5mA.cm-2 and an efficiency of 7% under standard AM 1.5 illumination. By using a CIGSe layer deposited on glass, we also perform multijunction-like J-V and EQE measurements to demonstrate the superiority of the narrow bandgap CuIn(Sex,Te1-x)2 over wider bandgap ternary compounds in such configuration. A filtered efficiency of eff=2% is achieved, more than 3 times higher than that of ternary compounds.