The role of Na incorporation in the low-temperature processed CIGS thin film solar cells using post deposition treatment

Na incorporation is widely considered to play an important role in low-temperature processed Cu(In,Ga)Se2 (CIGS) solar cells. In this work, we used post deposition treatment (PDT) method for Na incorporation in CIGS films, which involved the deposition of a thin NaF layer accompanied by thermal annealing. It was found that thermal annealing drove the diffusion of Na into CIGS absorbers without causing loss of elements and change of crystallinity. The dependence of the photovoltaic performance and recombination of CIGS solar cells on the Na PDT temperature was investigated. Impedance spectroscopy and admittance spectroscopy were used to characterize the recombination and defects of CIGS solar cells. The results indicated that the CIGS solar cells with Na PDT exhibited significant improvement of photovoltaic performance compared to those with Na-free. The enhanced transport performances of Na-treated cells were attributed to the decreased space-charge region (SCR) recombination. Both Na-free and Na-treated at 300 °C CIGS devices showed a donor defect with activation energy of about 295 meV, while the Na-treated devices demonstrated lower density of states, which resulted in an improved Voc. A maximum conversion efficiency of 11.9% was achieved for CIGS solar cells with Na PDT temperature at 300 °C.