Tailoring Cu ₂ CdSnS ₄ Phase Evolution for High‐Efficiency Solar Cells via Precursor Engineering

Abstract Copper‐based sulfide Cu 2 CdSnS 4 solar cells exhibit excellent electronic band properties with the substitution of Cd with Zn in Cu 2 ZnSnS 4 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‐x S 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 Cu 2 SnS 3 (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 Cd 2− ] 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.