Texture and morphology variations in (In,Ga)₂Se₃ and Cu(In,Ga)Se₂ thin films grown with various Se source conditions

Texture and morphology variations in co-evaporated (In,Ga) ₂Se₃ and Cu(In,Ga)Se₂ (CIGS) films grown with various Se source conditions during growth were studied. The Se species of simply evaporated, large molecular Se (E-Se, low-sticking coefficient), and RF-plasma cracked atomic Se (R-Se, high sticking coefficient) were used in the present work. (In,Ga)₂Se₃ precursor films, which were prepared during the first stage of CIGS film growth by the three-stage process, showed systematic variations in texture and Na distribution profile with varying evaporative Se (E-Se) flux. The properties of CIGS films and solar cells also showed systematic variations, and the open-circuit voltage (V_oc) and fill factor were found to be especially sensitive to the E-Se flux. R-Se grown (In,Ga)₂Se₃ precursor films featured granular morphology with strong (105) and (301) peaks in the diffraction pattern, and the texture was very similar to an E-Se grown film fabricated with a Se to group III metal (In + Ga) flux ratio (P_{[Se]/[In + Ga]}) of about 6, although the nominal P_{[Se]/[In + Ga]} used for an R-Se source was very small and less than 0.5. The R-Se grown CIGS films displayed, however, highly dense surfaces and larger grain sizes than E-Se grown CIGS films. The controllability of film morphology and the Na diffusion profile in (In,Ga)₂Se ₃ and CIGS films with various Se source conditions are discussed.