Tailoring Cu2CdSnS4 Phase Evolution for High-Efficiency Solar Cells via Precursor Engineering

Copper-based sulfide Cu₂CdSnS₄ solar cells exhibit excellent electronic band properties with the substitution of Cd with Zn in Cu₂ZnSnS₄ due to the reduction of sub-band-gap states. However, their performance remains inferior compared to other thin-film solar cells, and the fundamental material characteristics that are responsible for this inferior performance are not elucidated. In this paper, the performance-limiting factors of complicated chemical reactions involved in the sulfurization process are revealed by an in-depth investigation of phase evolution and grain growth. It is shown that the Cu_2-xS in a Cl-based precursor involved a multi-step phase fusion reaction with the CdS and SnS_x intermediate phases, leading to a severe V_OC deficit. Conversely, it is observed that a rapid phase transition with the formation of Cu₂SnS₃ (CTS) at the initial stage for the Ac-dominated sample generates numerous nucleation centers, resulting in poor crystallization. Hence, when a favorable ratio of Cl⁻/Ac⁻ anion is employed, the substantial deficit in V_OC of the CCTS solar cells primarily originated from [2Cu_Cd⁺+Sn_Cd²⁻] defect cluster is alleviated, which is believed to result from the modified multi-phase fusion and grain growth mechanism. The noteworthy champion efficiency of 11.89% with a V_OC/V_OC,SQ of 65.0% for the CCTS solar cells is achieved.