Impact of the Buffer/Absorber Interface on the Metastability of Fill Factor Temperature Coefficients in CIGSSe Solar Cells

The metastable behavior of the fill factor temperature coefficient (beta(FF,rel)) in Cu(In,Ga)(S,Se)(2) solar cells is investigated for absorbers with a different Ga content in the front surface, and for different buffer materials. The buffer/absorber interface region is suggested to be the location which causes the metastability in beta(FF,rel). Temperature dependent current-voltage curves, light-biased external quantum efficiency, and low temperature capacitance-voltage measurements are performed to characterize the electrical properties and metastable defects in the investigated solar cell devices. The presence of Ga in the absorber front surface, together with the buffer layer material plays a significant role in the metastable behavior. Additionally, energy dispersive X-ray spectroscopy indicates that the InxSy buffer layers allow for elemental interdiffusion from absorbers with a Ga-richer front surface, whereas a Zn(O,S) buffer shows to be rather resilient to atomic diffusion. The elemental interdiffusion between the buffer and absorber layers is proposed to be the cause of the defects causing the observed beta(FF,rel) metastability.