Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Abstract

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Reversible Phase Segregation and Amorphization of Mixed-Halide Perovskite Nanocrystals in Glass Matrices

“…The inflection point (800 min) of phase segregation time (Figure 2f) is significantly longer than that of single crystals and thin films. 40,41 All of these results demonstrate that the mixed-halide perovskite NCs in glass matrices have excellent photostability and a slower phase segregation process than other types of perovskite. 45,46 Interestingly, the photothermal effect has a much greater influence on phase segregation than that of light illumination itself for mixed-halide perovskite NCs.…”

“…There is still no observable phase segregation of mixed-halide perovskite NCs in glass matrices at 350 K (Figure S2b), even though the excitation power density we used is much higher than the previous report. 40,41 Notably, the PL intensity of GC-ClBr underwent a slight increase after laser irradiation at 350 K, while the PL intensity of GC-BrI did not show any change (Figure 2, parts b and c). The PL enhancement of GC-ClBr indicates that there is a possibility of temperature/irradiationinduced growth of mixed-halide perovskite NCs in the glass matrix.…”

Probing longitudinal carrier transport in perovskite thin films via modified transient reflection spectroscopy

“…Complementary to the electrical methods, THz spectroscopy (steady state or time-resolved) is also a widely used technique to probe the charge mobility in perovskite materials. 14,15 However, in perovskite polycrystalline films, THz spectroscopy actually measures the inner-grain (or intrinsic) charge mobility, and is thus inappropriate to be directly applied for the estimation of L D, as the transport of carriers is also limited by the inter-grain structure. 16…”

Molecular Design and Cost-Effective Synthesis of Tetraphenylethene-Based Hole-Transporting Materials for Hybrid Solar Cell Application

“…Hybrid perovskite solar cells (PSCs) have become an intriguing area of photovoltaic research due to their high charge carrier mobility, tunable band gap energy, low exciton binding energy, low cost, and high power conversion efficiency. − Conventionally, a PSC consists of light harvesting materials, an electron transport layer (ETL), a hole transport layer (HTL), and electrodes. Perovskites are excited by solar light, and the formed excitons are transferred down to the ETL along with the regeneration of the perovskite by the HTL.…”