Joshua Lilly

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.

Abstract:

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, the prevalence of RP defects is increased with increased Cs‐excess in vapor‐deposited CsPbI3 thin films while superior structural stability but inferior photophysical properties are observed. Significantly, using electron microscopy, it is found that the atomic positions at the planar defect are comparable to those of a free surface, revealing their role in phase stabilization. Density functional theory (DFT) calculations reveal the RP planes are electronically benign, however, antisites observed at RP turning points are likely to be malign. Therefore it is proposed that increasing RP planes while reducing RP turning points offers a breakthrough for improving both phase stability and photophysical performance. The formation mechanism revealed here can apply more generally to RP structures in other perovskite systems.