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Abstract:

FAPbI3 has emerged as a promising semiconductor for photovoltaic applications offering a suitable bandgap for single-junction cells and high chemical stability. However, device efficiency is negatively affected by intrinsic quantum confinement (QC) effects that manifest as additional peaks in the absorption spectra. Here, we show that aerosol-assisted crystallization is an effective method to improve crystallinity and suppresses regions exhibiting QC in FAPbI3. We demonstrate that films with minimized QC effects exhibit markedly enhanced optoelectronic properties, such as higher charge-carrier mobilities and recombination lifetimes. Films crystallized under an aerosol solvent flow of either a mixture of N, N-dimethylformamide and dimethyl sulfoxide or methylammonium thiocyanate vapor displayed reduced charge-carrier recombination losses and improved diffusion lengths compared to those of thermally annealed control films. Our study indicates clear correlations between suppression of QC features in absorption spectra with optimization of crystallinity and mitigation of internal strain, highlighting pathways toward high-performance solar cells.

Abstract:

Metal-halide semiconductors have emerged as promising materials for solar cells, with lead-based perovskites demonstrating remarkable efficiencies in tandem architectures. Yet, their performance still falls short of the theoretical limit, especially for wide-bandgap semiconductors, primarily due to interfacial losses at the semiconductor/charge-transport layer interface. This thesis investigates the photophysics at these interfaces through various spectroscopic techniques, providing insights into the underlying loss mechanisms and guiding mitigating strategies to achieve higher efficiencies.

Unfavourable energy-level alignment at the interface with charge-transport layers results in substantial open-circuit voltage losses. A systematic increase in the valence band maximum of FA0.83Cs0.17Pb (I1−𝑥Br𝑥 )3 with increasing bromide content 𝑥 from 0 to 1, when interfaced with the commonly employed hole transport layer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), provides an ideal platform to study photophysical losses arising from energy-level misalignment. The combination of time-resolved photoluminescence and numerical modeling reveals that increasing energy-level misalignment leads to increasing accumulation of holes in PTAA, which then subsequently recombine non-radiatively across the interface via interfacial defects, thereby reducing the open-circuit voltage and overall device efficiency.

Wide-bandgap mixed-halide perovskites often suffer from halide segregation where prolonged illumination drives a spatial separation of the mixed-phase perovskite into both iodide-rich (I-rich) and bromide-rich domains. By using a synchronous multimodal spectroscopy that combines timeresolved photoluminescence, time-resolved microwave conductivity and steady-state photoluminescence spectroscopy, the effect of halide segregation on the interfacial processes at FA0.83Cs0.17Pb(I0.6Br0.4 )3 interfaced with commonly used charge-transport layers such as PTAA and SnO2 is investigated. In neat perovskite films, halide segregation enhances radiative bimolecular recombination as charge-carrier funnelling increases the local carrier density within the narrow bandgap I-rich domains. Nevertheless, the charge-carrier mobility remains largely preserved after segregation. In the presence of charge-transport layers, charge extraction occurs predominantly via the I-rich phase following segregation. Although mobility retention is reduced in these heterostructures, the transport layers facilitate charge back transfer, mitigating the reduction in carrier lifetime at later times. The combined decrease in lifetime owing to enhanced radiative recombination and reduction in mobilities limits the diffusion length and therefore charge-carrier collection efficiency after halide segregation.

Concerns over the lead toxicity and instability of metal-halide perovskites have motivated the development of lead-free, all-inorganic Cu2AgBiI6 within the CuI–AgI–BiI3 phase space. However, it suffers from lower device efficiencies compared to its lead-perovskite counterparts, primarily due to poor charge-collection efficiency. Optoelectronic studies of coevaporated Cu2AgBiI6 interfaced with various charge-transport layers such as PTAA, CuI, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and SnO2 revealed that inorganic transport layers such as CuI and SnO2 induce the formation of unintended impurity phases within the CuI–AgI–BiI3 phase space, significantly altering structural and optoelectronic properties. These impurities reduce charge-carrier mobilities and diffusion lengths, thereby limiting its device efficiency.