Nonradiative Losses in Metal Halide Perovskites

Metal halide perovskites are generating enormous interest for their use in solar cells and light-emission applications. One property linking the high performance of these devices is a high radiative efficiency of the materials; indeed, a prerequisite for these devices to reach their theoretical efficiency limits is the elimination of all nonradiative decay. Despite remarkable progress, there exists substantial parasitic nonradiative recombination in thin films of the materials and when interfaced into devices, and the origin of these processes is still poorly understood. In this Perspective, I will highlight key observations of these parasitic pathways on both the macro- and microscale in thin films and full devices. I will summarize our current understanding of the origin of nonradiative decay, as well as existing solutions that hint at facile ways to remove these processes. I will also show how these nonradiative decay pathways are intimately related to ionic migration, leading to the tantalizing conclusion that eliminating one phenomenon could in turn remove the other, ultimately pushing devices to their theoretical limits.This work has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number PIOF-GA-2013-622630

Heterogeneity at multiple length scales in halide perovskite semiconductors

Materials with highly crystalline lattice structures and low defect concentrations have classically been considered essential for high-performance optoelectronic devices. However, the emergence of high-efficiency devices based on halide perovskites is provoking researchers to rethink this traditional picture, as the heterogeneity in several properties within these materials occurs on a series of length scales. Perovskites are typically fabricated crudely through simple processing techniques, which leads to large local fluctuations in defect density, lattice structure, chemistry and bandgap that appear on short length scales (10 μm). Despite these variable and complex non-uniformities, perovskites maintain exceptional device efficiencies, and are, as of 2018, the best-performing polycrystalline thin-film solar cell material. In this Review, we highlight the multiple layers of heterogeneity ascertained using high-spatial-resolution methods that provide access to the required length scales. We discuss the impact that the optoelectronic variations have on halide perovskite devices, including the prospect that it is this very disorder that leads to their remarkable power-conversion efficiencies

Photobrightening in Lead Halide Perovskites: Observations, Mechanisms, and Future Potential

There has been a meteoric rise in commercial potential of lead halide perovskite optoelectronic devices since photovoltaic cells (2009) and light emitting diodes (2014) based on these materials were first demonstrated. One key challenge common to each of these optoelectronic devices is the need to suppress non-radiative recombination, a process that limits the maximum achievable efficiency in photovoltaic cells and light emitting diodes. In this Progress Report, we dissect recent studies that seek to minimise this loss pathway in perovskites through a photobrightening effect, whereby the luminescence efficiency is enhanced through a light illumination passivation treatment. We highlight the sensitivity of this effect to experimental considerations such as atmosphere, photon energy, photon dose, and also the role of perovskite composition and morphology; under certain conditions there can even be photodarkening effects. Consideration of these factors is critical to resolve seemingly conflicting literature reports. We scrutinise proposed mechanisms, concluding that there is some consensus but further work is needed to identify the specific defects being passivated and elucidate universal mechanisms. Finally, we discuss the prospects for these treatments to minimise halide migration and push the properties of polycrystalline films towards those of their single-crystal counterparts

Impact of Mesoporous Silicon Template Pore Dimension and Surface Chemistry on Methylammonium Lead Trihalide Perovskite Photophysics

© 2020 Wiley-VCH GmbH In influencing fundamental properties—and ultimately device performance—of lead halide perovskites, interfacial interactions play a major role, notably with regard to carrier diffusion and recombination. Here anodized porous Si (pSi) as well as porous silica particles are employed as templates for formation of methylammonium lead trihalide nanostructures. This allows synthesis of relatively small perovskite domains and comparison of associated interfacial chemistry between as-prepared hydrophobic hydrideterminated functionalities and hydrophilic oxide-terminated surfaces. While physical confinement of MAPbBr3 has a uniform effect on carrier lifetime, pore size (7–18 nm) of the silicon-containing template has a sensitive influence on perovskite photoluminescence (PL) wavelength maximum. Furthermore, identity of the surface functionality of the template significantly alters the PL quantum efficiency, with lowest PL intensity associated with the H-terminated pSi and the most intense PL affiliated with the oxideterminated pSi surface. These effects are explored for green-emitting MAPbBr3 as well as infrared-emitting MAPbI3. In addition, the role of silicon surface chemistry on the time-dependent stability of these perovskites packaged within a given mesoporous template is also evaluated, specifically, a lack of miscibility between MAPbI3 and the H-terminated pSi template results in a diffusion of this specific perovskite composition eluting from this porous matrix over time

Excitonic Properties of Low-Band-Gap Lead-Tin Halide Perovskites

The MAPb1–xSnxI3 (x = 0–1) (MA = methylammonium) perovskite family comprises a range of ideal absorber band gaps for single- and multijunction perovskite solar cells. Here, we use spectroscopic measurements to reveal a range of hitherto unknown fundamental properties of this materials family. Temperature-dependent transmission results show that the temperature of the tetragonal to orthorhombic structural transition decreases with increasing tin content. Through low-temperature magnetospectroscopy, we show that the exciton binding energy is lower than 16 meV, revealing that the dominant photogenerated species at typical operational conditions of optoelectronic devices are free charges rather than excitons. The reduced mass increases approximately proportionally to the band gap, and the mass values (0.075–0.090me) can be described with a two-band k·p perturbation model extended across the broad band gap range of 1.2–2.4 eV. Our findings can be generalized to predict values for the effective mass and binding energy for other members of this family of materials

Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites

Fundmental comprehension of light-induced processes in perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on charge carrier recombination kinetics by simultaneously acquiring the photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450–780 nm). The presence of PbI2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI2/perovskite interface, which is only active when charge carriers are generated in PbI2, and therefore provide deeper insight into how excess PbI2 may improve the properties of perovskite-based devices

Correlated Electrical and Chemical Nanoscale Properties in Potassium-Passivated, Triple-Cation Perovskite Solar Cells

Perovskite semiconductors are an exciting class of materials due to their promising performance outputs in photovoltaic devices. To boost their efficiency further, researchers introduce additives during sample synthesis, such as KI. However, it is not well understood how KI changes the material and, often, leaves precipitants. To fully resolve the role of KI, multiple microscopy techniques are applied and the electrical and chemical behavior of a Reference (untreated) and a KI‐treated perovskite are compared. Upon correlation between electrical and chemical nanoimaging techniques, it is discovered that these local properties are linked to the macroscopic voltage enhancement of the KI‐treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses indicates that the additive partially migrates to the surface, yet surprisingly does not deteriorate the performance locally, rather, the voltage response homogeneously increases. The research presented within provides a diagnostic methodology, which connects the nanoscale electrical and chemical properties of materials, relevant to other perovskites, including multication and Pb‐free alternatives

Correlated Electrical and Chemical Nanoscale Properties in Potassium-Passivated, Triple-Cation Perovskite Solar Cells

Perovskite semiconductors are an exciting class of materials due to their promising performance outputs in optoelectronic devices. To boost their efficiency further, researchers introduce additives during sample synthesis, such as KI. However, it is not well understood how KI changes the material and, often, leaves precipitants. To fully resolve the role of KI, a multiple microscopy techniques is applied and the electrical and chemical behavior of a Reference (untreated) and a KI-treated perovskite are compared. Upon correlation between electrical and chemical nanoimaging techniques, it is discovered that these local properties are linked to the macroscopic voltage enhancement of the KI-treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses indicates that the additive partially migrates to the surface, yet surprisingly; does not deteriorate the performance locally, rather, the voltage response homogeneously increases. The research presented within provides a diagnostic methodology, which connects the nanoscale electrical and chemical properties of materials, relevant to other perovskites, including multication and Pb-free alternatives.University of Maryland All-S.T.A.R. Fellowship Hulka Energy Research Fellowship National Science Foundation US Department of Energy The Royal Society Office of Naval Researc

Enhanced visible light absorption in layered Cs_{3}Bi_{2}Br_+{9} through mixed-valence Sn(II)/Sn(IV) doping

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs_{3}Bi_{2}Br_{9}, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr2 to the solution-phase synthesis of Cs_{3}Bi_{2}Br_{9} leads to substitution of up to 7% of the Bi(III) ions by equal quantities of Sn(II) and Sn(IV). The nature of the substitutional defects was studied by X-ray diffraction, {133}^Cs and {119}^Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(II) and Sn(IV) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(II) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(II) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure–property relationships of lead-free vacancy-ordered perovskite structures