Current status at solar frontier: From fundamental research to industrial mass production of Cd-free Cu(In,Ga)(Se,S)2 solar cell

A conversion efficiency of 20.9% was achieved on Cu(In,Ga)(Se,S)₂ solar cell prepared through the sputtering-selenization/sulfurization process with Zn-based Cd-free buffer layer last year. In this paper, we will discuss the results of numerous analyses from the champion cell. The temperature and illumination dependences of Voc were measured to extract the Shockley-Read-Hall recombination rates at the buffer/absorber interface (Rⁱ), in the space charge region (R^d), and in the quasi-neutral region (R^b) according to the method recently proposed by Li et al. [1]. The values of Ri, Rd, and Rb were deduced as 1.9×10¹⁵, 2.3×10¹⁷, and 1.1×10¹⁷ cm^-2s^-1, respectively. Comparing these values with those of the NREL´s cell, our cell had similar Rb, better Ri , and worse Rd. A cross sectional image of the electron back scatter diffraction pattern revealed that smaller grains were segregated on the top of the absorber layer up to the depth of 200 nm. As the depth of the space charge region deduced from both the capacitance-voltage measurement and the combined scanning electron microscopy-electron beam induced current mapping was about 200 nm, the layer with small grains was expected to be the main cause of the inferior Rd. In addition to the longitudinal inhomogeneity of grain size, the electroluminescence imaging and the laser beam induced current mapping taken on the cell surface indicate that the lateral non-uniformity may also be deteriorating cell performance.