Spray pyrolysis of CuIn(S,Se)₂ solar cells with 5.9% efficiency: A method to prevent Mo oxidation in ambient atmosphere

Direct spray pyrolysis to form CuInS₂ (CIS) on molybdenum substrate in ambient environment has been a challenge because of the ease of Mo oxidation at low temperatures. MoO₂ formation affects the wettability of precursor solution during spray pyrolysis, which degrades the uniformity of CIS film and acts as a resistive layer for carrier transport. In this paper, Mo oxidation was prevented by using excess sulfur in the precursor solution under a gradual heating and spray process. A thin precursor layer was initially deposited as a barrier layer to prevent oxygen adsorption on Mo surface before the temperature was increased further to form polycrystalline CuInS₂. The CuIn(S,Se)₂ (CISSe) device fabricated from selenization of the spray-pyrolyzed CIS film exhibited a power conversion efficiency (PCE) of 5.9%. The simple spray method proposed here can be used to deposit a variety of Cu-based chalcopyrite precursor to produce high-quality thin film solar cells.