Amongst several processes which have been developed for the production of reliable chalcopyrite Cu(InGa)Se2 photovoltaic absorbers, the 2–step metallization–selenization process is widely accepted as being suitable for industrial–scale application. Here we visualize the detailed thermal behavior and reaction pathways of constituent elements during commercially attractive rapid thermal processing of glass/Mo/CuGa/In/Se precursors on the basis of the results of systematic characterization of samples obtained from a series of quenching experiments with set-temperatures between 25 and 550 °C. It was confirmed that the Se layer crystallized and then melted between 250 and 350 °C, completely disappearing at 500 °C. The formation of CuInSe2 and Cu(InGa)Se2 was initiated at around 450 and 550 °C, respectively. It is suggested that pre-heat treatment to control crystallization of Se layer should be designed at 250–350 °C and Cu(InGa)Se2 formation from CuGa/In/Se precursors can be completed within a timeframe of 6 min.
The selenization of Mo/CuGa/In/Se (Se layer thickness: 1 microm) precursors followed by sulfurization was investigated. Particular emphasis was placed on the effect of the variation of the selenization temperature and sulfurization time on the morphology and compositional depth profiles of the resulting Cu(InGa)(SeS)2 (CIGSS) absorber; in addition, the currentvoltage characteristics of the corresponding devices were investigated. The selenization of the precursors was achieved by using a tube-type rapid thermal annealing system at various temperatures (500, 550, and 600 °C). Post-sulfurization of Cu(InGa)Se2 (CIGS) was performed in the same system by flowing H2S/He gas at 600 °C for different periods of time (5, 10, and 15 min). Post-sulfurization can improve the open-circuit voltage of a solar cell by attracting Ga toward the surface region of the light absorber and incorporating S into the absorber to yield quinary CIGSS compounds. In addition, the voids at the Mo/CIGS interface, produced during the selenization of the CuGa/In/Se precursors, were effectively removed after post-sulfurization. Among the process conditions explored in this study, the selenization of Mo/CuGa/In/Se at 550 °C for 7 min followed by sulfurization at 600 °C for 10 min produced the device with the best performance, providing also good material properties in terms of morphology and compositional homogeneity.
Sputter-deposited bilayer CuGa/In precursors were coated by a Se layer with a different thickness from 0.5 to 1.5 μm to yield glass/Mo/CuGa/In/Se structure. Selenization of the precursors with a 0.5 μm-thick Se layer resulted in partial selenization with a relatively uniform distribution of Ga, whereas Cu(InGa)Se2 formed from 1.0 and 1.5 μm-thick Se layers showed complete selenization but with Ga accumulation at the bottom. Partial selenization of the Se-coated metal precursors by a 0.5 μm-thick Se layer was confirmed to yield better incorporation of S and effective re-distribution of Ga to form Cu(InGa)(Se,S)2 films with homogenous depth profiles of Ga and S.
Binary bilayer glass/Mo/In2Se3/Cu2Se precursors have been used to rapidly form CuInSe2. Considering their possible application to large-area deposition processes, bilayer precursors were deposited by sequential radiofrequency sputtering of In2Se3 and direct current sputtering of Cu2Se onto unheated, Mo-coated glass substrates. High-temperature X-ray diffraction analysis of the glass/Mo/Cu2Se sample confirmed that the as-deposited polycrystalline Cu2−xSe phase is likely transformed to CuSe at approximately 210 °C, and then to CuSe2 at 260 °C. Further increase in temperature resulted in the peritectic decomposition of CuSe2 to CuSe (+liquid) at approximately 330 °C, and then to Cu2−xSe (+liquid) at around 380 °C with the release of Se. Pre-annealing of In2Se3/Cu2Se precursors in Se environment resulted in the formation of a liquid phase, which is in equilibrium with CuSe. Rapid, thermal annealing of pre-annealed samples between 500 and 550 °C apparently enhanced grain growth and reduced the reaction time to about 3 min; this can be explained by a liquid phase-assisted grain growth mechanism.