All-Solution-Processed Cu₂ZnSnS₄ Solar Cells with Self-Depleted Na₂S Back Contact Modification Layer

The thin-film photovoltaic material Cu₂ZnSnS₄ (CZTS) has drawn worldwide attention in recent years due to its earth-abundant, nontoxic element constitution, and remarkable photovoltaic performance. Although state-of-the-art power conversion efficiency is achieved by hydrazine-based methods, effort to fabricate such devices in a high throughput, environmental-friendly way is still highlydesired. Here a hydrazine-free all-solution-processed CZTS solar cell with Na₂S self-depleted back contact modification layer for the first time is demonstrated, using a ball-milled CZTS as light absorber, low-temperature solution-processed ZnO electron-transport layer as well as silver-nanowire transparent electrode. The inserting of Na₂S self-depleted layer is proven to effectively stabilize the CZTS/Mo interface by eliminating a detrimental phase segregation reaction between CZTS and Mo-coated soda lime glass, thus leading to a better crystallinity of CZTS light absorbing layer, enhanced carrier transportation at CZTS/Mo interface as well as a smaller series resistance. Furthermore, the self-depletion feature of the Na₂S modification layer also averts hole-transportation barrier within the devices. The results show the vital importance of interfacial engineering for these CZST devices and the Na₂S interface layer can be extended to other optoelectronic devices using Mo contact.