Thermal Stability of CuInSe2 solar Cell with Sputter-Zn(O,S) Buffer Layer As a Bottom Cell

Abstract

The power conversion efficiency of solar cells with multiple sub-cells in a tandem device can be extended beyond the Shockley-Queisser limit for a single-junction device, shifting the absorption onset to longer wavelengths and reducing thermalization losses. Theoretical calculations have been reported that the optimum band-gaps of absorber materials for a series-connected two-junction are 1.72 eV for the top cell and 1.14 eV for the bottom cell, respectively. The CuInSe2 (CIS) solar cells are well suited to be used as bottom cell in tandem solar cells. In case of monolithic tandem solar cell, Cu(In,Ga)Se2 based-cells with CdS buffer layer are quite vulnerable to high temperatures above 200oC, destroying the p-n junction by the diffusion of Cd atoms. Thus, the thermal stability of buffer layer prepared on the absorber of bottom cell should be important factor in solar cell performance.In this study, we fabricated CIS solar cell with sputtered-Zn(O,S) buffer layer as a bottom cell because Zn-based buffer layer has a higher thermal stability as compared with CdS buffer layer. The completed solar cells were heated up to temperatures of 200 - 400oC for 60 min under vacuum in order to trace the degradation of cell performances. To investigate the interface quality and degradation mechanism after post-annealing process, the drive-level capacitance profiling and admittance spectroscopy have being applied as an analytic tool, which gives an information on bulk defect states, carrier density and defect energy at p-n junction. The temperature dependent current density-voltage measurements were also performed in the dark, red and white light from a solar simulator. These results clearly present that there is clear correlation between thermal stability and solar cell performance.