Session F43: Physical Processes in Metal Halide Perovskites for Photovoltaics

Structural Origins of Heterogeneity in Halide Perovskite Photophysics

Photoluminescence spectroscopy, and imaging, are widely used to characterize halide perovskite semiconductors. While heterogeneity and distributed kinetics are widely reported, they are often interpreted differently by different authors.  In this talk we explore recombination dynamics in halide perovskites as probed by time-resolved photoluminescence spectroscopy and photoluminescence imaging.  After a discussion of recombination kinetics and their common interpretations, we show that spatial heterogeneity and surface defects are essential features that account for the photoluminescence dynamics commonly reported in perovskite thin films. Finally, we  explore the structural origins of this heterogeneity from the standpoint of both surface chemistry and strain in the semiconductor using a combination of correlative multimodal microscopy and hyperspectral imaging.   

Halide Ion Migration in Halide Perovskites and Its Impact on the Photovoltaic Stability

The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the metal halide perovskite film during the operation of solar cells. Of particular interest is the halide ion mobility in mixed halide perovskites, which offer an insight into the halide ion migration. The origin of processes leading to halide migration is investigated using mixed halide perovskite films. Both hole trapping at I-site and the defect mediated migration influence the mobility of iodide species under photoirradiation. The iodide species migrating toward the grain boundaries render the mixed halide film phase-segregated under photoirradiation.Ways to suppresses the halide ion mobility through introduction of Cs at the A-site will be discussed.   

A Matter of Mixing: Nanoscale Heterogeneity and Stability in Metal Halide Perovskite Solar Cells

To date, halide perovskite solar cells (PSC) yield the highest efficiency and stability when absorbers comprised of a mixture of ions are utilized. Mixed compositions allow for a fine tuning of the geometric conditions and enables the formation of photoactive perovskite phases. Nevertheless, chemical heterogeneity in the film can initiate phase segregation of the material into pure phases, which prefer their non-perovskite δ-phases, and consequently lead to degradation of the device performance. Spatially resolved analyses have revealed microscopic heterogeneity in PSCs, including compositional heterogeneity. However, characterizing the chemical heterogeneity on small length scales has proven challenging. Here, we use X-ray scattering methods to probe nanoscale heterogeneity in PSC absorber layers and couple these results to device level stability studies to understand the role heterogeneity plays in device performance. We prepare a set of perovskite thin films, choosing a variation of annealing conditions to ultimately vary the heterogeneity in the film. In this talk, we will provide a summary of the approach used to characterize the heterogeneity in these films and comment on the loss in device performance and subsequent phase segregation induced by nanoscale heterogeneity.   

Design of Multidimensional Molybdenum Sulfide Electrocatalysts to Drive CO2 and CO Conversion to Alcohols

The design of materials that address the growing dichotomy of simultaneously increasing energy demands and carbon emissions is an imperative that has progressively affected energy-related research efforts. An emerging technical avenue in this area is the conversion of vastly abundant renewable energy sources that can be harnessed and directed towards synthesis of traditionally fossil fuel-based products from atmospheric feedstocks like CO₂. To this end, our work establishes structure—function relationships for materials within the versatile classes of MX₂ (M = Mo, W; X = S, Se) and Chevrel-Phase (CP) MyMo₆X₈ (M = alkali, alkaline, transition or post-transition metal; y = 0-4; X = S, Se,Te) chalcogenides. The molybdenum sulfide structures from both families exhibit exceptional promise as CO₂R catalysts. Furthermore, we have identified the CP catalyst framework as selective towards the electrochemical reduction of CO₂ and CO to methanol (only major liquid-phase product) under applied potentials as mild as -0.4 V vs RHE. Reactivity toward electrochemical reduction of CO₂ and CO to methanol is correlated with increased population of chalcogen states, as confirmed via X-Ray Absorption Spectroscopy. Overall, this work seeks to unravel optimally reactive novel small-molecule reduction catalyst compositions.