Incongruent reaction of Cu–(InGa) intermetallic precursors in H2Se and H2S

Abstract

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.

A study on composition, structure and optical properties of copper-poor CIGS thin film deposited by sequential sputtering of CuGa/In and In/(CuGa+In) precursors

“…This difference between In/(CuGaþIn) and CuGa/In is related to Ga content in CIG precursors. This mixed peak is believed to be a Cu 9 In 4 -Cu 9 Ga 4 alloy with Ga/(InþGa) ratio [22]. From the XRD results, we can confirm the texture formation of CIG precursor layers as determined from composition of CIG precursors [23].…”

“…Also, it can consider that the Ga atoms are diffused toward interface between Mo film and CIGS absorption layer. Se preferentially reacts with In during the selenization process, because reaction energy of In is lower than Ga for Se [22]. The elemental composition distributions onto the surface and into the depth of the surface demonstrate the diffusion behavior of Ga atoms from composition profile by XPS, EDS and SIMS analysis.…”

Cu(In,Ga)Se<inf>2</inf> film formation from selenization of mixed metal/metal-selenide precursors

“…The nodules are Cu and Ga rich compared to the overall composition, while the background is predominantly In. The similar precursor structure was previously reported [4].…”

“…However, it is widely observed that Ga migrates away from the junction to the Mo back contact [2,3,4]. This backcontact Ga accumulation prevents the increase in bandgap near the junction that is needed for the achievement of high voltage solar cells.…”

“…An additional reaction step with sulfur has been implemented to increase the voltage [1] because the sulfur incorporation also increases the bandgap. The back-contact Ga accumulation has been attributed to the formation of Cu-In and Cu-Ga intermetallics and their incongruent reaction rates [4]. Furthermore, the decomposition of Cu9Ga4 intermetallic remaining at the backside of the film is the rate-limiting step in the selenization process.…”