Chlorine Incorporation in 1.8 eV Wide Bandgap Perovskite Solar Cells to Enhance Efficiency and Photostability

Monolithic all-perovskite tandem solar cells show the potential of high efficiency at low manufacturing cost. In the 2-terminal device configuration, the current matching condition requires an optical bandgap of 1.8 eV for the wide bandgap top cell, to optimally pair with the 1.25 eV narrow bandgap bottom cell in the stack. To develop 1.8 eV bandgap perovskite, 40 mol% of Br is generally alloyed with I in the perovskite. However, solar cells with such high Br fractions in perovskite absorber suffer from low open-circuit voltages and exhibit poor photostability. Adding Cl to form triple halide (I, Br, Cl) perovskite compositions is a promising approach to obtain 1.67 eV perovskite materials without using high Br fractions.<br/>In this contribution, we present our results on Cl incorporation to achieve 1.8 eV wide bandgap perovskite compositions with much lower Br fractions. We investigate Cl-containing precursors with respect to their effectivity to incorporate Cl into the perovskite lattice. Furthermore, we probe the limits of Cl incorporation for given Br fractions and perform steady-state and time-resolved photoluminescence measurements to check the photostability of the triple-halide perovskite films as well as the charge carrier lifetimes. Additionally, we investigate the uniformity and spatial distribution of Cl in the perovskite film by XPS and ToF-SIMS depth profiling. Finally, we implement the most promising films in solar cells to evaluate the PV performance as well as the thermal and the light-soaking stability.