arXiv subjects

Qimu Yuan

Publications and source records attributed to Qimu Yuan.

3 recordsLinked to original sources

arXiv2026

Discovery of Ferroelectric Twin Boundaries in a Photoactive Halide Perovskite

Halide perovskites have emerged as promising materials for next-generation photovoltaics, laser sources and X-ray detectors. There is intense debate as to whether some photoactive halide perovskites exhibit ferroelectric behaviour and whether it might be possible to utilise the bulk photovoltaic effect to enhance the performance of halide perovskite solar cells. Here, using low-dose scanning transmission electron microscopy, we discover the existence of ferroelastic twin boundaries in vapor-deposited CsPbI3 thin films, parallel to {110} and {112}. Remarkably, despite photoactive CsPbI3 being centrosymmetric and non-polar, we observe directly that Pb atoms shift at {110} twin boundaries driving a local ferroelectric-like polarisation. These polar twin walls form an intrinsic array of nanoscale functional interfaces, spaced ~30-50 nm apart, embedded within the non-polar perovskite lattice. In contrast, {112} twin boundaries remain non-polar but strongly suppress octahedral tilt and off-centre Cs atom displacements, revealing a different untapped ferroic degree of freedom. These discoveries together uncover previously hidden ferroic functionality in halide perovskite semiconductors, opening opportunities for enhanced conductivity and photovoltaic behaviour through domain wall engineering.

cond-mat.mtrl-sci↗

arXiv2026

Deriving a comprehensive dataset of optical constants for metal halide perovskites

Accurate optical constants are essential for modelling light propagation, absorption, and ultimately photovoltaic performance in state of the art perovskite solar cells and is especially important for multiple junction or tandem cells. However, available datasets for metal halide perovskites remain sparse, inconsistent in quality, and often suffer from unphysical sub bandgap extinction caused by surface roughness and limitations of conventional ellipsometry fits. Here, we present a comprehensive library of complex refractive indices (n,k) for a technologically relevant set of FA based lead perovskites, spanning bromide compositions from 0 to 100 percent, and mixed Pb Sn perovskites with Sn fractions from 0 to 60 %. Using state of the art fabrication protocols that yield high quality films, we combine variable angle spectroscopic ellipsometry measurements with highly sensitive sub bandgap probes, including photothermal deflection spectroscopy for neat lead based perovskites and Fourier transform photocurrent spectroscopy for Pb Sn alloys, to reconstruct fully zeroed dielectric functions across and below the band edge. The measured data are then stitched and recalculated via a Kramers Kronig consistent framework, ensuring physically accurate behaviour across the full spectral range. Finally, we introduce a transformation based interpolation scheme that preserves spectral shape and feature alignment, enabling reliable determination of (n,k) for any intermediate composition or band gap. This complete dataset and interpolation protocol provide a standardized foundation for optical modelling of perovskite and tandem solar cells, addressing longstanding data gaps and supporting accurate simulations of next generation photovoltaic architectures.

cond-mat.mtrl-sci↗

arXiv2025

Ruddlesden-Popper defects act as a free surface: role in formation and photophysical properties of CsPbI3

The perovskite semiconductor, CsPbI3, holds excellent promise for solar cell applications due to its suitable bandgap. However, achieving phase-stable CsPbI3 solar cells with high power conversion efficiency remains a major challenge. Ruddlesden-Popper (RP) defects have been identified in a range of perovskite semiconductors, including CsPbI3. However, there is limited understanding as to why they form or their impact on stability and photophysical properties. Here we increase the prevalence of RP defects with increased Cs-excess in vapour-deposited CsPbI3 thin films and observe superior structural stability but inferior photophysical properties. Significantly, using electron microscopy, we find that the atomic positions at the planar defect are comparable to those of a free surface, revealing their role in phase stabilisation. Density functional theory (DFT) calculations reveal the RP planes are electronically benign, however, antisites observed at RP turning points are likely to be malign. We therefore propose that increasing RP planes while reducing RP turning points could offer a breakthrough for improving both phase stability and photophysical performance. The formation mechanism revealed here may well apply more generally to RP structures in other perovskite systems.

cond-mat.mtrl-sci↗