Solvent engineering enables tin-lead perovskite films with long carrier diffusion lengths and reduced tin segregation
Sheng Li, Xiaotian Yang, Siyang Cheng, Yujie Yang, Hao Li, Zhuo Zheng, Mubai Li, Qiuhan Yu, Shengjun Yuan, Qianqian Lin, Zhiping Wang
Abstract
All-perovskite tandem solar cells offer great promise for achieving low levelized cost of electricity, but their performance remains limited by insufficient near-infrared photon absorption in narrow bandgap tin-lead (Sn-Pb) subcells. Micron-thick Sn-Pb layers are essential for maximizing absorption, yet high-concentration precursor solutions often cause non-uniform crystallization, stoichiometric imbalance and limited carrier diffusion lengths. Here we identify the root cause of these limitations as the insufficient coordination of tin(II) iodide (SnI2) in conventional dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) binary solvent system at high precursor concentrations, resulting in Sn-rich colloids that nucleate detrimental Sn-rich phases in final films. To address this, we develop a ternary solvent system that fully coordinates with SnI2, suppressing Sn-rich phases and enabling stoichiometric, micron-thick Sn-Pb films with carrier diffusion lengths of ~11 μm. The enhanced Sn-Pb absorber achieves efficiencies of 24.2% in single-junction cells and 29.3% in tandem devices, along with significantly improved long-term operational stability. The performance of all-perovskite tandem solar cells remains limited by the insufficient infrared photon absorption in the narrow bandgap subcells. Here, the authors develop a ternary solvent system to suppress tin-rich phases and achieve an efficiency of 29.3% in operationally stable devices.