Halide Segregation in Mixed Halide Perovskites: Visualization and Mechanisms

Suppressing Halide Segregation in Wide-Bandgap Perovskite Absorbers by Transamination of Formamidinium

All-perovskite tandem solar cells are emerging at a fast rate because of their potential to exceed efficiencies of Si-perovskite tandems, in combination with faster manufacturing, lower cost, and the ability to be processed on flexible substrates. Mixing halides is a key to achieve wide-bandgap absorbers, which however suffer from halide segregation under illumination, resulting in lowering of the bandgap. To tackle this problem, butylamine (BA) has been added to the perovskite precursor solution and is found to react with the formamidinium (FA) cation, producing N-butylformamidinium (BuFA+), which accumulates at the perovskite surface and grain boundaries. The creation of the BuFA cation results in suppressed halide segregation and improved crystallization. Density functional theory calculations propose the reduction of halide defect formation upon the addition of BA, being a key to stabilize mixed-halide perovskites. Lastly, we observe a more stable performance of single junction p-i-n perovskite solar cells with the addition of BA under constant illumination at 65 °C.

Stability of Mixed Lead Halide Perovskite Films Encapsulated in Cyclic Olefin Copolymer at Room and Cryogenic Temperatures

Lead Mixed Halide Perovskites (LMHPs), CsPbBrI2, have attracted significant interest as promising candidates for wide bandgap absorber layers in tandem solar cells due to their relative stability and red-light emission with a bandgap ∼1.7 eV. However, these materials segregate into Br-rich and I-rich domains upon continuous illumination, affecting their optical properties and compromising the operational stability of devices. Herein, we track the microscopic processes occurring during halide segregation by using combined spectroscopic measurements at room and cryogenic temperatures. We also evaluate a passivation strategy to mitigate the halide migration of Br/I ions in the films by overcoating with cyclic olefin copolymer (COC). Our results explain the correlation between grain size, intensity dependencies, phase segregation, activation energy barrier, and their influence on photoinduced carrier lifetimes. Importantly, COC treatment increases the lifetime charge carriers in mixed halide thin films, improving efficient charge transport in perovskite solar cell applications.

Temperature-Dependent Reversal of Phase Segregation in Mixed-Halide Perovskites

Understanding the mechanism of light-induced halide segregation in mixed-halide perovskites is essential for their application in multijunction solar cells. Here, photoluminescence spectroscopy is used to uncover how both increases in temperature and light intensity can counteract the halide segregation process. It is observed that, with increasing temperature, halide segregation in CH₃ NH₃ Pb(Br_0.4 I_0.6 )₃ first accelerates toward ≈290 K, before slowing down again toward higher temperatures. Such reversal is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors. High light intensities meanwhile can also reverse halide segregation; however, this is found to be only a transient process that abates on the time scale of minutes. Overall, these observations pave the way for a more complete model of halide segregation and aid the development of highly efficient and stable perovskite multijunction and concentrator photovoltaics.