Improved carrier collection efficiency in CZTS solar cells by Li-enhanced liquid-phase-assisted grain growth

Abstract

The liquid-phase-assisted grain growth (LGG) process is a promising strategy to fabricate large-grain pure sulfide Cu₂ZnSnS₄ (CZTS) layers that span the absorber thickness and improve the carrier collection efficiency in photovoltaic devices. Li doping is an effective route to promote such LGG process of Cu₂ZnSn(S,Se)₄ (CZTSSe) as it can provide liquid Li-Se phase facilitating the growth of large-grain CZTSSe. However, the detailed function of the added Li in grain growth has rarely been investigated in both CZTS and CZTSSe, as the reported in situ, and pre-deposition doping strategies usually suffer from substantial Li losses during the spin-coating process and/or the high-temperature sulfurization process. Herein, by monitoring the temperature-dependent Li loss during the sulfurization process, we demonstrate that a small proportion of the added Li can remain at the CZTS film from the early sulfurization stage and provide Li-S flux to promote the LGG process. An encouraging efficiency of 10.53%, with a remarkably high short-circuit current density of 22.6 mA/cm² and open-circuit voltage of 0.744 V, is achieved by a significantly enlarged grain size of 3 μm with Li addition. This work could enhance the knowledge of employing Li-S as flux for growing large-grain chalcogenide absorbers for high performance devices with better carrier transport.