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Abstract:

Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but they suffer from light-induced halide segregation, which compromises their operational stability. Here, we directly probe the impact of halide segregation on charge-carrier dynamics at the interface between a mixed-halide perovskite and charge transport layers by using a free-space synchronous multimodal spectroscopy approach, combining time-resolved microwave conductivity, time-resolved photoluminescence (PL) and steady-state PL. We present a method to distinguish directly between charge-carrier dynamics dominated by either majority or minority carriers, enabling us to isolate effects arising from charge-selective extraction from the perovskite to commonly used hole- or electron transport layers, i.e. poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and SnO2, respectively. We show that halide segregation creates iodide-rich phases that capture charge carriers within sub-nanoseconds, which slightly reduces their mobilities at microwave frequencies. We reveal that charge extraction from such iodide-rich domains is still surprisingly feasible, but competes with enhanced radiative recombination resulting from higher charge concentrations caused by funnelling into these minority phases. We demonstrate that together such effects reduce charge diffusion lengths and can account for the widely observed reduction in open-circuit voltages and short-circuit currents in solar cells under operational conditions. Our findings unravel the causes underpinning the adverse impact of halide segregation and provide guidelines to improve device performance.

Abstract:

Halide perovskite solar cells have demonstrated a rapid increase in power conversion efficiencies. Understanding and mitigating remaining carrier losses in halide perovskites is now crucial to enable further increases to approach their practical efficiency limits. Recent observations in halide perovskites have revealed processes such as shallow carrier trapping, which give rise to an apparent non-radiative bimolecular channel that is difficult to distinguish from intrinsic radiative recombination. Here, we quantify this shallow-trap manifestation by jointly analyzing time-resolved photoluminescence and quantum efficiency to separate the total second-order term into radiative (ηesck2r) and shallow-trap-mediated non-radiative contributions (k2non), and evaluate their device impact. We show that k2non is strongly modulated by temperature and surface chemistry and thus depends on extrinsic factors and its origin is independent from deep traps, whereas the intrinsic radiative coefficient and intrinsic second-order recombination follow detailed-balance expectations and align with theoretical evaluations through van Roosbroeck–Shockley relations. Based on density functional theory simulations and Quasi-Fermi level calculations, we propose that surface states are the primary origin of this shallow-trap-related second-order component, contributing up to ∼80 mV of the overall reduction in Voc at room temperature. This work reveals that the origin of carrier losses from two non-radiative recombination types (first and second order) are not linked, emphasizing the need for distinctive mitigation strategies targeting each type to unlock the full efficiency potential of perovskite solar cells.

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:

New compounds are discovered in the under-explored d 10 –s 2 (Cu–Sn) family using exploratory synthesis guided by computational tools. Band-gaps in the visible region with moderate charge-carrier mobilities make these potential solar absorbers. We explore multiple-cation chalco–halide phase fields evaluated by their synthetic accessibility using machine learning models. Exploratory synthesis guided by computational tools leads to the discovery of two new compounds; CuSn 2 SI 3 and Cu 0.35 Sn 5.29 S 2 I 7 , their structures, and electronic and optical properties are reported herein. This is the first report of a stable quaternary compound in the Cu–Sn–S–I phase field. The two new compounds show related crystal structures where Sn 4 S 2 I 4 layers are a common structural motif in both. These Sn 4 S 2 I 4 layers are connected by Cu 2 I 2 layers and disordered Cu–Sn–I layers, forming the three-dimensional structures of CuSn 2 SI 3 and Cu 0.35 Sn 5.29 S 2 I 7 respectively. Electronic band structure calculations using density functional theory show the presence of a direct band gap in CuSn 2 SI 3 and suggest anisotropic transport, in line with the layered structure of the compound. A mixture of the two compounds with ∼86% CuSn 2 SI 3 , shows a band gap in the visible region, close to 2.1 eV and a significant photo-induced charge carrier mobility of ∼1.3 cm 2 V −1 s −1 . This demonstrates Cu–Sn chalco–halides can form a promising phase space to explore for solar absorber materials, with further design and tuning of band gap.