Fundamental thermodynamics and experiments in fabricating high efficiency CuInSe2 solar cells by selenization without the use of H2Se

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Abstract
Selenization is the current process by which state‐of‐the‐art CuInSe2 polycrystalline thin‐film photovoltaic modules are industrially fabricated. The distinguishing characteristic of this approach is that material deposition is separate from compound formation. In conventional selenization, In‐Cu layers, often referred to as precursors, are deposited on molybdenum‐coated glass substrates and subsequently transformed into CuInSe2 following exposure to a selenium‐containing environment. Although the highly toxic gas, H2Se, has been considered a necessary component of selenization, recent safety concerns have accelerated the development of Se vapor as a possible substitute for H2Se. In more recent variations of the process, solid selenium is incorporated during the precursor fabrication step, and subsequent thermal annealing is used to form compounds among the three elements. In this paper, we discuss the thermodynamic fundamentals of selenization using elemental Se as an alternative to H2Se. This discussion is augmented by empirical observations drawn from our own efforts in fabricating efficient (≳10%) CdS/CuInSe2 devices by selenization in thermally‐evaporated Se vapors. Indium transport, presumably via the formation of In2Se or InSe gaseous species, dominates the kinetics of selenization using sequentially evaporated (indium on copper) precursors, while lateral phase separation was observed in the case of co‐deposited In+Cu precursors.

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