Abstract
An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH3NH3Pb(Br0.5I0.5)3 is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH3NH3Pb(Br0.5I0.5)3 perovskite cells would require a reduction of the trap-related c...
Highlights
An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology
The most promising hybrid perovskites for twoterminal cells are based on lead iodide−bromide perovskites (stoichiometry APb(BrxI1−x)[3], where A is a monovalent cation, typically CH3NH3+, CH(NH2)2+, Cs+, or a mixture thereof) whose bandgap can be tuned through the ratio of the two halide ions.[4,9−12] photoluminescence (PL) techniques have revealed that the bandgap of these materials typically red shifts under illumination,[10,13] an undesirable trait in particular for a tandem solar cell material.[14−16] This bandgap shift has been attributed to spatial segregation of the different halide ions in the perovskite,[13] resulting in separated regions of iodide-rich and bromide-rich perovskites within the remaining well-mixed phase
Reports have shown that halide segregation is dependent on the perovskite film morphology, with improved material crystallinity correlated with reduced amounts of segregation.[17−19] Halide segregation has been observed in the absence of light, when charge carriers are injected into the film via an electric potential difference,[14,16] and extraction of excited charge carriers from the perovskite film was found to lower the extent of segregation.[16]
Summary
An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. We show that charge carriers recombining through trap-mediated routes are responsible for halide segregation, which allows us to create an empirical model for the growth of iodide-rich perovskite PL over time, as a function of various film and illumination parameters.
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