Pre-incorporation of Na into flexible Cu(In,Ga)Se2 thin film solar cells

Incorporation of sodium into the Cu(In,Ga)Se2 (CIGS) absorber layer is a key step for the growth of high conversion efficiency CIGS solar cells. However, the doping of Na into CIGS layer on a flexible substrate, i.e. Titanium foil, is still a challenge. In this paper, we report our results on comparing two different Na pre-incorporation methods in constructing flexible CIGS solar cells on Ti substrates: i) by sputtering a soda-lime glass thin film (SLGTF) onto Ti substrates prior to the growth of CIGS and Mo back contact layers and ii) by evaporating a NaF precursor layer on the Mo layer before the CIGS growth. By carefully optimizing the growth conditions, the conversion efficiencies of 13.47% and 15.02% are achieved for the SLGTF sample and the NaF sample, respectively. The NaF precursor method shows a better effect. The diffusion of Na and the gradient of Ga play important roles in the performance of the solar cells.