Effect of Sodium on the Properties of Ag(In, Ga)Se2 Thin Films and Solar Cells

Ag(In, Ga)Se-2 (AIGS) thin films were deposited by a modified three-stage method, using a molecular beam epitaxy apparatus. The influence of sodium on the properties of AIGS films was investigated, using Mo-coated soda-lime-glass (SLG) and quartz as substrates. A sodium concentration gradient was observed along the thickness direction of the AIGS film. This resulted from sodium diffusion from the SLG substrate. No sodium was observed in the AIGS film fabricated on the quartz substrate. The grain size and crystallinity of AIGS increased upon sodium doping. Photoluminescence (PL) experiments on the AIGS film showed that exponentially tailed donor states introduced by indium atoms were passivated by sodium. This led to free excitons being observed in the PL spectrum. Potential fluctuation was used to explain the emission spectra of both sodium-containing and sodium-free samples. The donor and acceptor levels included in the quasi-donor-acceptor pair recombination were observed at approximately 35 meV interstitial defect (GaAg) and 100 meV (V-Se), respectively. Solar cell performance was significantly improved by sodium doping, probably because of defect passivation.