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The successful development of optoelectronic devices is contingent on a detailed understanding of interactions between light and excited energy states in photoactive materials. In 2D perovskites, excitons are the dominant photogenerated species and their energetic structure plays a pivotal role, governing photon absorption and emission processes. In these materials, dark exciton states can undergo photoluminescence due to the relaxation of selection rules and this process can be modulated by an external magnetic field, enabling unambiguous identification of the exciton fine structure. Previous reports of magneto-optical spectroscopy on 2D perovskites have been restricted to the macroscopic response, where key information is lost regarding the microscopic heterogeneity of the photoluminescence. Here, we use magneto-optical microscopy for the first time on perovskite materials to elucidate the spatial variation of exciton emission processes. In 2D perovskite thin films, we distinguish between regions of localised bright and dark exciton populations, correlated to the film morphology. In single crystals, we show that dark excitons become localised at the edges, where excitons can be trapped in two distinct types of sub-gap states. This work represents significant progress in understanding the properties of exciton emission in 2D perovskites, which is crucial for the development of optoelectronic technology., Comment: 24 pages, 4 figures

2D metal halide perovskites have enabled significant stability improvements in perovskite devices, particularly in resistance to moisture. However, some 2D perovskites are even more susceptible to photooxidation compared to 3D perovskites. This is particularly true for more commonly investigated Ruddlesden-Popper (RP) perovskites that exhibit increased susceptibility to photoinduced degradation compared to Dion-Jacobson (DJ) perovskites. Comparisons between different RP and DJ perovskites reveal that this phenomenon cannot be explained by commonly proposed differences in superoxide ion generation, interlayer distance and lattice structural rigidity differences. Instead, the resistance to photooxidation of DJ perovskites can be attributed to decreased likelihood of double deprotonation events (compared to single deprotonation events in RP perovskites) required for the loss of organic cations and the perovskite decomposition. Consequently, DJ perovskites are less susceptible to oxidative degradation (both photo- and electrochemically induced), which leads to improved operational stability of solar cells based on these materials., Comment: Main text: 19 pages, 6 figures, supplementary information: 62 pages, 47 figures

We present a simple, yet powerful analysis of Suns-photoluminescence quantum yield measurements that can be used to determine the surface saturation current densities of thin film semiconductors. We apply the method to state-of-the-art polycrystalline perovskite thin films of varying absorber thickness. We show that the non-radiative bimolecular recombination in these samples originates from the surfaces. To the best of our knowledge, this is the first study to demonstrate and quantify non-linear (bimolecular) surface recombination in perovskite thin films., Comment: arXiv admin note: text overlap with arXiv:2306.02222

We present a novel analytical model for analysing the spectral photoluminescence quantum yield of non-planar semiconductor thin films. This model considers the escape probability of luminescence and is applied to triple-cation perovskite thin films with a 1-Sun photoluminescence quantum yield approaching 25%. By using our model, we can decouple the internal radiative, external radiative, and non-radiative bi-molecular recombination coefficients. Unlike other techniques that measure these coefficients separately, our proposed method circumvents experimental uncertainties by avoiding the need for multiple photoluminescence measurement techniques. We validate our model by comparing the extracted implied open-circuit voltage, effective luminescence escape probabilities, absorptivity, and absorption coefficient with values obtained using established methods and found that our results are consistent with previous findings. Next, we compare the implied 1-Sun radiative open-circuit voltage and radiative recombination current obtained from our method with literature values. We then convert the implied open-circuit voltage and implied radiative open-circuit voltage to the injection-dependent apparent-effective and apparent-radiative carrier lifetimes, which allow us to decouple the different recombination coefficients. Using this lifetime analysis, we predict the efficiency losses due to each recombination mechanism. Our proposed analytical model provides a reliable method for analysing the spectral photoluminescence quantum yield of semiconductor thin films, which will facilitate further research into the photovoltaic properties of these materials., Comment: Main text: 10 figures, 6 tables

A direct, camera-based implied open-circuit voltage (iVOC) imaging method via the novel use of a single bandpass filter (s-BPF) is developed for large-area photovoltaic solar cells and solar cell precursors. This method images the photoluminescence (PL) emission using a narrow BPF with centre energy in the high-energy tail of the PL emission taking advantage of the close-to-unity absorptivity of typical photovoltaic devices with low variability in this energy range. As a result, the exact value of the sample's absorptivity within the BPF transmission band is not required. The use of a s-BPF enables the adaptation of a fully contactless approach to calibrate the absolute PL photon flux for camera-based spectrally-integrated imaging tools. The method eliminates the need for knowledge of the imaging system spectral response and the use of the emission and excitation spectral shapes. Through an appropriate choice of the BPF centre energy, a range of absorber compositions or a single absorber with different surface morphologies (e.g., planar vs textured) can be imaged, all without the need for additional detection optics. The feasibility of this s-BPF method is first assessed using a high-quality Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3 perovskite neat film. The error in iVOC is determined to be less than 1.5%. The efficacy of the method is then demonstrated on device stacks with two different perovskite compositions commonly used in single-junction and monolithic tandem solar cells., Comment: 23 pages, 4 figures

Solar energy is anticipated to be the most viable source of sustainable green energy. Perovskites have gained significant research attention in recent years as a solar energy harvesting material due to their desirable photovoltaic enabling properties. The potential strategies for a more effective use of these materials can involve multiple energy conversion mechanisms through a single device or employing materials where a solar or thermal input provides multiple electrical outputs to enhance the overall energy harvesting capability. In this context, the present review focuses on perovskites, including both organic halide perovskites and inorganic oxide perovskites, due to their proven properties as photovoltaic materials and their intriguing potential for additional functionality, such as ferroelectricity. Ferroelectrics are a special class of perovskites that have been studied in detail for photoferroic, pyroelectric and thermoelectric effects and energy storage, which we briefly review here. Furthermore, the possibilities of simultaneously tuning these mechanisms in perovskite materials for multiple energy conversion mechanisms and storage for ultra-high density capacitor and battery applications is also examined in order to attain a better understanding and to present novel opportunities. An understanding of all these mechanisms and device prospects will inspire and inform the selection of appropriate materials and potential novel designs so that the available solar and thermal resource could be utilized in a more effective manner. This review will not only help in selecting an appropriate material from the existing pool of perovskite materials, but will also provide an outlook and assistance to researchers in developing new material systems., Comment: 56 Pages, 10 Figures, 2 Tables