Dr Alexandra Ramadan [she/her]

Abstract:

We investigate the impact of employing fullerene self-assembled monolayers (SAMs) in conjunction with SnO2 electron-transport layers (ETLs) in negative–intrinsic–positive (n–i–p) perovskite solar cells. We compare the efficacy of the fullerene-SAM surface-binding group—carboxylic or phosphonic acid—on the passivation, charge transport, and energetics of the modified-SnO2 surface. Planar n–i–p perovskite solar cells with these SAM-modified SnO2 exhibit significantly reduced hysteresis, and the steady-state maximum power point tracked efficiencies (η MPP) of the devices improved from an average of 18.6–20.0% comparing devices with neat SnO2 to the C60-PA SAM inclusion, respectively. While C60-PA SAM improved the initial solar cell efficiency, this benefit was not maintained during light exposure at elevated temperatures as the fullerene SAM weakened adhesion at the perovskite/metal oxide interface in the aged devices. These results highlight the importance of improving the mechanical robustness of perovskite/charge-transport layer interfaces under operational aging conditions.

Abstract:

We investigate BA₂MAPb₂I₇ (BAMA) quasi-2D Ruddlesden-Popper perovskite thin films prepared through single-crystal-derived and conventional polycrystalline routes. Morphological and X-ray diffraction analyses reveal significant differences in film texture, crystallinity, and phase distribution. Steady-state and time-resolved optical spectroscopies show that polycrystalline films are mainly composed of n = 2 and n = 3 phases and exhibit limited interphase energy transfer. In contrast, single-crystal-derived films display a richer excitonic landscape characterized by the presence of higher-(n) domains. Transient photoluminescence and pump-probe measurements demonstrate that the n = 2 exciton acts as the primary donor state and, uniquely in the single-crystal-derived films, undergoes two distinct transfer processes on the same timescale. The correlation between the decay of the n = 2 exciton and the population of lower-energy excitonic states provides direct evidence of hierarchical exciton funneling. These findings highlight the crucial role of phase distribution and crystallinity in governing exciton migration and energy-transfer pathways in low-dimensional perovskite heterostructures.

Abstract:

Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but they suffer from light-induced halide segregation, which compromises their operational stability. Here, we directly probe the impact of halide segregation on charge-carrier dynamics at the interface between a mixed-halide perovskite and charge transport layers by using a free-space synchronous multimodal spectroscopy approach, combining time-resolved microwave conductivity, time-resolved photoluminescence (PL) and steady-state PL. We present a method to distinguish directly between charge-carrier dynamics dominated by either majority or minority carriers, enabling us to isolate effects arising from charge-selective extraction from the perovskite to commonly used hole- or electron transport layers, i.e. poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and SnO2, respectively. We show that halide segregation creates iodide-rich phases that capture charge carriers within sub-nanoseconds, which slightly reduces their mobilities at microwave frequencies. We reveal that charge extraction from such iodide-rich domains is still surprisingly feasible, but competes with enhanced radiative recombination resulting from higher charge concentrations caused by funnelling into these minority phases. We demonstrate that together such effects reduce charge diffusion lengths and can account for the widely observed reduction in open-circuit voltages and short-circuit currents in solar cells under operational conditions. Our findings unravel the causes underpinning the adverse impact of halide segregation and provide guidelines to improve device performance.

Abstract:

Exciton–polaritons in two-dimensional (2D) perovskites offer a promising platform for room-temperature polaritonics; however, their relaxation and scattering mechanisms remain only partially understood. Here, we report, for the first time, strong exciton–photon coupling in pure phase n = 1 butylammonium lead iodide (BA2PbI4) microcavities and employ femtosecond transient absorption (TA) spectroscopy to directly probe the underlying exciton-reservoir-to-polariton relaxation dynamics. Global analysis of the TA spectra reveals three distinct characteristic timescales: t1 (< 1 ps), governed by ultrafast excitonic many-body interactions, including phase-space filling, Coulomb-screening, biexciton formation, and cavity-resonance reshaping; t2 (5–15 ps), arising from exciton-reservoir-to-polariton scattering and exciton–exciton annihilation (EEA), together with bandgap renormalization (BGR); and t3 (> 50 ps), associated with long-lived dark-state or trap-state relaxation. The decay-associated spectra (DAS) further indicate that although transient polariton-associated spectral response emerges on picosecond timescales, efficient population accumulation within the lower polariton branch (LPB) remains strongly limited by ultrashort polariton lifetimes, incomplete reservoir-to-LPB feeding, and nonradiative trapping processes. Taken together, these results provide a detailed ultrafast spectroscopic insight into the nonequilibrium relaxation pathways and kinetic bottlenecks governing 2D perovskite microcavities and offer guidance for future optimization strategies aimed at enabling room-temperature polariton condensation and nonlinear effects in pure phase n = 1 2D perovskite systems.