Joshua Lilly

Abstract:

Bulk formamidinium lead triiodide (FAPbI3) films host spontaneously formed quantum-confined (QC) domains, but their structural origin remains unclear. Using controlled material degradation in humid air as a dynamic lattice perturbation, we track the evolution of QC features in thin-film absorption of FAPbI3. With aging, above-bandgap QC features redshift and diminish, indicating weakened electronic confinement. Concurrently, X-ray diffraction reveals that breakdown of α-phase connectivity coincides with the loss of short-range higher-order hexagonal (nH, n > 2) polytypes as the material converts to the 2H δ-phase. Such polytypic nanodomains may generate peaked absorption features by forming higher-energy barriers confining charge carriers within α-FAPbI3 or by introducing distinct electronic states associated with mixed octahedral connectivity. Progressive degradation dismantles this framework, causing the disappearance of the QC features. Our results identify the structural motifs underpinning QC effects and propose that controlling higher-order (n > 2) hexagonal polytypes offers a route to tuning quantum confinement in FAPbI3 films.

Abstract:

Mixed-halide perovskites exhibit ideal band gaps for use in perovskite-based multijunction photovoltaics, but stable performance is compromised by light-induced halide segregation. Photoluminescence (PL) tracking is universally used to monitor such photoinstability; however, here we reveal that such data do not accurately quantify halide segregation. We utilize a combination of simultaneously recorded PL and X-ray diffraction (XRD) measurements to explore CH3NH3Pb(I1–x Br x )3 films across 18 different halide ratios. While PL data suggests that segregation rates increase exponentially with bromide fraction x, XRD patterns reveal that they are actually unchanged. We demonstrate that PL cannot accurately reflect the rate and extent of halide segregation because it is governed by charge funneling to iodide-rich minority domains, which is strongly influenced by additional factors, including luminescence efficiency, band energetics, and charge extraction. To assess the efficacy of treatments to suppress such photoinstabilities, it is therefore essential to probe changes across the full material volume, e.g. by monitoring XRD or absorption spectra.

Abstract:

A novel class of semiconducting compounds, metal‐halide perovskites (MHPs), has emerged as a versatile platform for advanced optoelectronic device architectures, offering a unique combination of exceptional physical properties and facile processing. In this study, we present a monolithic high‐speed photodetector capable of directly sensing the time delay between two light pulses with a temporal resolution of at least 170 ps, corresponding to a light propagation distance of ~5 cm—making it well suited for Light Detection and Ranging (LiDAR) applications. This outstanding time resolution is achieved through a signal‐balancing detection scheme that effectively overcomes the limitations of conventional photodetectors, whose response speed is inherently limited by charge‐carrier lifetime and transit time. The device exhibits an exceptionally low noise spectral density, comparable to that of state‐of‐the‐art silicon photodiodes. The fully symmetric device stack comprises a crystalline CsPbBr3 absorber layer tens of microns thick, fabricated via a confined melt process. Comprehensive electro‐optical characterization reveals charge‐carrier lifetimes and mobilities on both microscopic and macroscopic length scales, using transient photoluminescence, time‐resolved photocurrent, time of flight, and terahertz pump–probe spectroscopy. The CsPbBr3 layer exhibits charge‐carrier lifetimes exceeding 100 ns, a microscopic electron–hole mobility of 15 ± 1 cm2 V−1 s−1, and a macroscopic non‐dispersive hole mobility of 8.5 cm2 V−1 s−1. image

Abstract:

Mixed‐halide perovskites are ideal mid‐ and wide‐gap absorbers for multijunction solar cells, but stable photovoltaic performance is severely hampered by halide segregation. This study reveals that crystalline film quality and halide segregation are critically affected by bromide fraction x in CH3NH3Pb(I1−xBr x )3 because of macrostrain and ordered‐phase formation. X‐ray diffractometry across stoichiometries spanning 22 bromide fractions demonstrates that central compositions near x = 0.5 form two macrostrained phases, which exhibit halide segregation under light at different rates. While the overall amplitude of phase segregation follows a broadly symmetric distribution in compositional space, maximized near x = 0.5, the potentially ordered compositions of CH3NH3PbIBr2 and CH3NH3PbI2Br diverge sharply, presenting particularly stable and unstable scenarios, respectively. Notably, halide segregation is shown to occur even below the widely quoted perceived threshold of x = 0.2. Such analysis highlights promising approaches to mitigate halide segregation, through engineering of macrostrained phases and local atomistic ordering. Together, these observations provide crucial benchmarks for proposed models of halide segregation and establish new routes toward segregation‐resistant materials for multijunction perovskite‐based photovoltaics.

Abstract:

The copper–silver–bismuth–iodide compound Cu2AgBiI6 has emerged as a promising lead-free and environmentally friendly alternative to wide-bandgap lead-halide perovskites for applications in multijunction solar cells. Despite its promising optoelectronic properties, the efficiency of Cu2AgBiI6 is still severely limited by poor charge collection. Here, we investigate the impact of commonly used charge transport layers (CTLs), including poly­[bis­(4-phenyl)­(2,4,6-trimethylphenyl)­amine] (PTAA), CuI, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and SnO2, on the structural and optoelectronic properties of coevaporated Cu2AgBiI6 thin films. We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO2, induce the formation of unintended impurity phases within the CuI–AgI–BiI3 solid solution space, significantly influencing structural and optoelectronic properties. We demonstrate that identification of these impurity phases requires careful cross-validation combining absorption, X-ray diffraction and THz photoconductivity spectroscopy because their structural and optoelectronic properties are very similar to those of Cu2AgBiI6. Our findings highlight the critical role of CTLs in determining the structural and optoelectronic properties of coevaporated copper–silver–bismuth–iodide thin films and underscore the need for advanced interface engineering to optimize device efficiency and reproducibility.