Revealing the role of tin fluoride additive in narrow bandgap Pb-Sn perovskites for highly efficient flexible all-perovskite tandem cells

TL;DR

This study investigates the spatial distribution and chemical transformations of SnF2 additive in Pb-Sn perovskites, revealing its crucial role in enhancing the efficiency of flexible all-perovskite tandem solar cells.

Contribution

It provides the first detailed visualization of SnF2 distribution and its chemical transformation in Pb-Sn perovskites, linking these to photovoltaic performance.

Findings

SnF2 transforms into a fluorinated oxy-phase on the surface.

F- ions accumulate at the buried interface.

Achieved 18.5% efficiency in flexible Pb-Sn perovskite solar cells.

Abstract

Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface, due to its rapid oxidation. In addition, at the buried perovskite interface, we detected and visualized the accumulation of F- ions. We found that the photoluminescence quantum yield of Pb-Sn perovskite reached the highest value with 10 mol% SnF2 in the precursor solution. When integrating the optimized absorber in flexible devices, we obtained the flexible Pb-Sn perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and demonstrated 23.1%-efficient flexible 4-terminal all-perovskite tandem cells.