Siyu Yan

Abstract:

Thermal evaporation is an industrially compatible technique for fabricating metal halide perovskite thin films, without the requirement for hazardous solvents. It offers precise control over film thickness and is a good candidate for large-scale production of commercial optoelectronic metal halide perovskite devices, such as solar cells. The use of additives to passivate electronic defects in solution-processed metal halide perovskite has led to dramatic increases in device performance. However, there are a few reports of vapor-deposited films with coevaporated passivating agents. Triphenylphosphine oxide (TPPO) has been used as an effective surface passivating agent in solution-processed metal halide perovskite films. It is a promising candidate passivating agent for coevaporation, where it is beginning to be used with encouraging results. However, here we report that triphenylphosphine oxide is incompatible with thermal deposition in the same deposition chamber. Such TPPO remnants are found to result in severe suppression of the perovskite phase, long-range crystalline ordering, and optical absorption of lead halide perovskite films subsequently deposited in the same chamber. TPPO contamination persists even through repeated baking cycles, with the reduction of the contaminant to acceptable levels requiring vacuum chamber dismantling and manual cleaning. We conclude that TPPO should not be coevaporated in order to prevent the contamination of future batches.

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:

2D lead halide perovskites (2DPs) offer chemical compatibility with 3D perovskites and enhanced stability, which are attractive for applications in photovoltaic and light‐emitting devices. However, such lowered structural dimensionality causes increased excitonic effects and highly anisotropic charge‐carrier transport. Determining the diffusivity of excitations, in particular for out‐of‐plane or inter‐layer transport, is therefore crucial, yet challenging to achieve. Here, an effective method is demonstrated for monitoring inter‐layer diffusion of photoexcitations in (PEA)2PbI4 thin films by tracking time‐dependent changes in photoluminescence spectra induced by photon reabsorption effects. Selective photoexcitation from either substrate‐ or air‐side of the films reveals differences in diffusion dynamics encountered through the film profile. Time‐dependent diffusion coefficients are extracted from spectral dynamics through a 1D diffusion model coupled with an interference correction for refractive index variations arising from the strong excitonic resonance of 2DPs. Such analysis, together with structural probes, shows that minute misalignment of 2DPs planes occurs at distances far from the substrate, where efficient in‐plane transport consequently overshadows the less efficient out‐of‐plane transport in the direction perpendicular to the substrate. Through detailed analysis, a low out‐of‐plane excitation diffusion coefficient of (0.26 ± 0.03) ×10−4 cm2 s−1 is determined, consistent with a diffusion anisotropy of ≈4 orders of magnitude.

Abstract:

Deformation of metal halide perovskite can induce many interesting properties. This study focuses on a manganese-based organic-inorganic perovskite with a unique structure in which tetrahedral and octahedral coordination coexist in single crystal unit cell. This perovskite emits at 519 and 615 nm at room temperature. In contrast to conventional perovskites, this perovskite regulates the Dexter energy transfer between the two coordination modes through asymmetric deformation without phase transition, producing a reversible and tunable photoluminescence. Notably, under atmospheric pressure, as temperature increases from liquid nitrogen temperature to 135 °C, the luminescence color shifts progressively from red with a CIE coordinate of (0.59, 0.27) to yellow green with a CIE coordinate of (0.33, 0.56), with excellent reversibility. Additionally, at room temperature, the luminescence color shifts progressively from orange with a CIE coordinate of (0.54, 0.42) to red with a CIE coordinate of (0.61, 0.27) as pressure increases from 1 atm to 7.5 GPa. This novel tetrahedral and octahedral coexisting perovskite has a temperature-pressure equivalence effect in modulating luminescent color changes. It tunes emission by forming asymmetric deformations through the contraction (or expansion) of tetrahedra and expansion (or contraction) of octahedra upon stimulation, providing a new pathway to tune the emission of perovskites.