Impact of additives on the chemical bath deposition of Zn(O,S) used as buffer layer in high-efficiency Cu(In,Ga)Se2-based solar cells

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Cu(In,Ga)Se2-based solar cells achieve highest efficiencies up to 23.4% by using solution-grown Zn(O,S) as the buffer layer. State-of-the-art procedures for Zn(O,S) deposition based on the ammonia-thiourea approach are still characterized by a low growth rate and a high material consumption of reactants. One of the approaches to improve the Zn(O,S) deposition rate is the implementation of compounds leading to a much faster thiourea decomposition and thus to a higher availability of sulfide anions. In this work, we test the impact of oxidative and reductive additives, namely sodium peroxydisulfate Na2S2O8 and hydroxylamine NH2OH, respectively. The Zn(O,S) growth rate could be significantly increased and the consumption of reactants drastically reduced without compromising the performance of fabricated solar cell devices. The composition of the layers was quantified by time-of-flight secondary ion mass spectrometry measurements and the [S]/([S]+[O]) molar ratio was found to vary between 0.65 and 0.85 depending on the thiourea and additive concentration. For cell preparation, in-line co-evaporated Cu(In,Ga)Se2 with RbF post-deposition treatment was used and conversion efficiencies up to 19%, comparable to reference cells with a CdS buffer, could be realized. We discuss the reaction mechanism and conclude for both additives that the resulting sulfide concentration increase evolves via formation of N-hydroxyguanidine intermediates.