Solar Energy Materials and Solar Cells

Reaction pathways during formation of Ag(In0.75Ga0.25)Se-2 films were investigated by ex-situ characterization of time-progressive reactions using rapid thermal processing at 450 degrees C under 5% Ar/H2Se atmosphere. Composition measurements using energy dispersive x-ray spectroscopy (EDS) and X-ray fluorescence (XRF) show that with Ag/(Ag + Cu) = 1.0, Ga grading and Se uptake occur gradually up to 20 min. With 7 min reaction, there was no sign of a AgInSe2 phase and only a Ag(ln,Ga)(5)Se-8 chalcogenide phase formed. The reaction was completed with formation of Ag(In,Ga)Se-2 and Ag(In,Ga)(5)Se-8 within 20 min. The addition of Cu to the precursor so that Ag/(Ag + Cu) = 0.75 speeds up the reaction and reduces the formation of the Ag(In,Ga)(5)Se-8 phase. EDS and XRF analyses indicated that Ga grading and Se uptake were completed after 10 min. XRD analysis also shows formation of CuInSe2 on the surface of the sample with Cu after 3.5 min reaction prior to other chalcogenide formation. On the other hand, a strong Ag(In,Ga)Se-2 (112) XRD peak appeared after 5 min reaction compared to 10 min reaction for 100% Ag films. EDS and Raman scattering measurements on the back-side of the films after delamination from the substrate indicated that Ga accumulated in AgGaSe2 and Ga-Se phases at the back-side of films after 45 min reaction. XRD analysis showed that Ag tends to stay in the stable zeta-Ag(3)A(In,Ga) phase during the reaction with Ag/(Ag + Cu) = 1.0, but not with 0.75, and this causes the longer reaction time and the non uniformity.