The reaction pathways to form Cu(InGa)Se2 or Cu(InGa)S2 films at 450°C from metallic precursors were evaluated by reacting Cu–In–Ga films in H2Se or H2S for 10, 30, or 90min and characterizing the phase composition of the resulting films. A starting composition comprising Cu9(In0.64Ga0.36)4 and In phases was detected by x-ray diffraction in Cu–Ga–In precursors annealed at 450°C in an Ar atmosphere. When the precursors were reacted in H2Se, a graded Cu(InGa)Se2 film was formed with a Ga-rich composition and residual Cu–Ga intermetallics at the interface with the Mo back contact. The intermetallic compounds were observed to evolve from Cu9(In0.64Ga0.36)4 to Cu9Ga4 with increasing selenization time. Reaction in H2S formed inhomogeneous Cu(InGa)S2 with Cu–In intermetallics. The results are consistent with thermochemical predictions of the preferential reaction of In with Se, and Ga with S. These reaction preferences can explain the formation of a graded Cu(InGa)Se2 film during reaction in H2Se and provide a refined understanding of the reaction sequence in two-step H2Se∕H2S processes.

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Control of the through-film composition and adhesion are critical issues for Cu(In,Ga)Se2 (CIGS) and/or Cu(In,Ga)(Se,S)2 (CIGSS) films formed by the reaction of Cu–In–Ga metal precursor films in H2Se or H2S. In this work, CIGSS films with homogenous Ga distribution and good adhesion were formed using a three-step reaction involving: (1) selenization in H2Se at 400 °C for 60 min, (2) temperature ramp-up to 550 °C and annealing in Ar for 20 min, and (3) sulfization in H2S at 550 °C for 10 min. The 1st selenization step led to fine grain microstructure with Ga accumulation near the Mo back contact, primarily in a Cu9(In1−xGax)4 phase. The 2nd Ar anneal step produces significant grain growth with homogenous through-film Ga distribution and the formation of an InSe binary compound near the Mo back contact. The 3rd sulfization step did not result in any additional change in Ga distribution or film microstructure but a small S incorporation near the CIGSS film surface and complete reaction of InSe to form CIGSS were observed. The three-step process facilitates good control of the film properties by separating different effects of the reaction process and a film growth model is proposed. Finally, CIGSS solar cells with the three-step reaction were fabricated and devices with efficiency = 14.2% and VOC = 599 mV were obtained.

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