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

Applications of Chalcogenides as Electron Transport Layers and Doping Materials in Perovskite Solar Cells

Access full text - https://doi.org/10.1007/978-3-030-31866-6_35The work contains the experimental results obtained by applying of ZnS and ZnSe thin films in perovskite solar cells. The techniques of preparing and researching the electrical properties of the obtained devices have been described in details

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

Understanding how excess lead iodide precursor improves halide perovskite solar cell performance

The presence of excess lead iodide in halide perovskites has been key for surpassing 20% photon-to-power conversion efficiency. To achieve even higher power conversion efficiencies, it is important to understand the role of remnant lead iodide in these perovskites. To that end, we explored the mechanism facilitating this effect by identifying the impact of excess lead iodide within the perovskite film on charge diffusion length, using electron-beam-induced current measurements, and on film formation properties, from grazing-incidence wide-angle X-ray scattering and high-resolution transmission electron microscopy. Based on our results, we propose that excess lead iodide in the perovskite precursors can reduce the halide vacancy concentration and lead to formation of azimuthal angle-oriented cubic alpha-perovskite crystals in-between 0 degrees and 90 degrees. We further identify a higher perovskite carrier concentration inside the nanostructured titanium dioxide layer than in the capping layer. These effects are consistent with enhanced lead iodide-rich perovskite solar cell performance and illustrate the role of lead iodide

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

In-gap states of an amorphous In-Ga-Zn-O thin film studied via high-sensitivity ultraviolet photoemission spectroscopy using low-energy photons

Low-density electronic states in the energy gap of an amorphous In-Ga-Zn-O (a-IGZO) film control device performance. Herein, density of states (DOS) distribution from valence band to the in-gap states of 1014 cm−3eV−1 level was determined using high-sensitivity UV photoemission spectroscopy (HS-UPS). Exponential tail states accompanying two energetically-localized states were directly observed as reported previously. The observed slope of the exponential tail state was different from the Urbach energy derived using photothermal deflection spectroscopy, indicating the importance of directly observing the DOS of in-gap states

Conjugated polyelectrolyte hole transport layer for inverted-type perovskite solar cells

Organic-inorganic hybrid perovskite materials offer the potential for realization of low-cost and flexible next-generation solar cells fabricated by low-temperature solution processing. Although efficiencies of perovskite solar cells have dramatically improved up to 19% within the past 5 years, there is still considerable room for further improvement in device efficiency and stability through development of novel materials and device architectures. Here we demonstrate that inverted-type perovskite solar cells with pH-neutral and low-temperature solution-processable conjugated polyelectrolyte as the hole transport layer (instead of acidic PEDOT:PSS) exhibit a device efficiency of over 12% and improved device stability in air. As an alternative to PEDOT: PSS, this work is the first report on the use of an organic hole transport material that enables the formation of uniform perovskite films with complete surface coverage and the demonstration of efficient, stable perovskite/fullerene planar heterojunction solar cellsopen4