Oxide route for production of Cu₂ZnSnS₄ solar cells by pulsed laser deposition

In this work, we have investigated Cu2ZnSnS4 (CZTS) solar cells made from oxide, oxy-sulfide and sulfide precursors produced by pulsed laser deposition (PLD). Although sulfide precursors are widely used to fabricate CZTS solar cells, Sn loss is commonly observed due to the high volatility of SnxSy species during high temperature sulfurization. This can lead to a non-ideal absorber composition and a high density of detrimental Sn-related defects that severely affect the performance of the device. By using oxide precursors, we have shown that the Sn loss can be significantly reduced due to the higher thermal stability of SnxOy species when compared to their sulfide counterparts. However, the reaction mechanism for the oxide route results in rough CZTS films. We hypothesize that the SO2 gas that forms during the conversion from oxide to sulfide is trapped in the film during sulfurization, and can lead to grains with hollow cavities and thus increase the surface roughness. Therefore, we have developed an annealing route for the oxide precursors at lower annealing pressures, which leads to improved film morphology and device performance. As a result, we report a power conversion efficiency of 5.4% for solar cells made from oxide precursors. This is the highest value reported for a CZTS absorber produced by PLD.