Prof Henry Snaith FRS

Abstract:

Hybrid organic-inorganic perovskites (HOIPs) have emerged as a versatile class of semiconductors for numerous optoelectronic applications. Here, we demonstrate light-excitation-dependent two-dimensional (2D) position-sensitive detectors (PSDs) using a mixed-phase perovskite, FA_0.83Cs_0.17Pb(I_0.9Br_0.1)₃, as the active semiconductor, incorporated within a five-terminal device geometry. The light-induced lateral photovoltage, which is initiated by selective charge transfer across the metal-perovskite barrier interface, is utilized to achieve the excitation-position-dependent electric response. The 2D PSD devices exhibit a spatially dependent linear variation of the photosignal with sensitivity >50 μV mm^-1 and a low position detection error (1-2%), making them suitable for applications such as quadrant detectors. Further, it is observed that the device architecture plays a key role in controlling the dynamics and linearity of the HOIP PSDs. The large active area devices (up to ∼2 cm × 2 cm) exhibit a distinct spatial variation of the photosignal. We utilize the functionality of the PSD device for light-tracking applications by implementing a continuous detection scheme.

Abstract:

Silver-bismuth based semiconductors represent a promising new class of materials for optoelectronic applications because of their high stability, all-inorganic composition, and advantageous optoelectronic properties. In this study, charge-carrier dynamics and transport properties are investigated across five compositions along the AgBiI4–CuI solid solution line (stoichiometry Cu4x(AgBi)1−xI4). The presence of a close-packed iodide sublattice is found to provide a good backbone for general semiconducting properties across all of these materials, whose optoelectronic properties are found to improve markedly with increasing copper content, which enhances photoluminescence intensity and charge-carrier transport. Photoluminescence and photoexcitation-energy-dependent terahertz photoconductivity measurements reveal that this enhanced charge-carrier transport derives from reduced cation disorder and improved electronic connectivity owing to the presence of Cu+. Further, increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities. Finally, ultrafast charge-carrier localization is observed upon pulsed photoexcitation across all compositions investigated, lowering the charge-carrier mobility and leading to Langevin-like bimolecular recombination. This process is concluded to be intrinsically linked to the presence of silver and bismuth, and strategies to tailor or mitigate the effect are proposed and discussed.

Abstract:

Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Here we identify lead monovacancy (V_Pb) defects in MAPbI₃ (MA = CH₃NH₃⁺) using positron annihilation lifetime spectroscopy with the aid of density functional theory. Experiments on thin film and single crystal samples all exhibited dominant positron trapping to lead vacancy defects, and a minimum defect density of ~3 × 10¹⁵ cm^-3 was determined. There was also evidence of trapping at the vacancy complex [Formula: see text] in a minority of samples, but no trapping to MA-ion vacancies was observed. Our experimental results support the predictions of other first-principles studies that deep level, hole trapping, [Formula: see text], point defects are one of the most stable defects in MAPbI₃. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms.