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Summary The most important obstacle to widespread use of perovskite solar cells is their poor stability under operational stressors. Here, we systematically monitor the evolution of the photovoltaic performance of perovskite solar cells based on formamidinium-cesium lead iodide (FA0.9Cs0.1PbI3) for 600 h, under a series of controlled operational stressors. Although these devices exhibit reasonable thermal stability, their stability under illumination or stabilized power output (SPO) is far from commercial demands. Synchrotron-based nanoprobe X-ray fluorescence and X-ray-beam-induced current measurements reveal that current-blocking Cs-rich phases segregate during stress tests. The decrease in performance is in line with the increasing density of the Cs-rich clusters in area upon illumination. Theoretical calculations indicate that light-generated carriers provide the thermodynamic driving force for that phase segregation. Our findings correlate device performance to microscopic behavior and atomistic mechanisms and shed light on inhibiting the cation-dependent phase segregation during device operation.

Understanding the optoelectronic properties of optically active materials at the nanoscale often proves challenging due to the diffraction-limited resolution of visible light probes and the dose sensitivity of many optically active materials to high-energy electron probes. In this study, we demonstrate correlative synchrotron-based scanning x-ray excited optical luminescence (XEOL) and x-ray fluorescence (XRF) to simultaneously probe local composition and optoelectronic properties of halide perovskite thin films of interest for photovoltaic and optoelectronic devices. We find that perovskite XEOL stability, emission redshifting, and peak broadening under hard x-ray irradiation correlates with trends seen in photoluminescence measurements under continuous visible light laser irradiation. The XEOL stability is sufficient under the intense x-ray probe irradiation to permit proof-of-concept correlative mapping. Typical synchrotron XRF and nano-diffraction measurements use acquisition times 10–100x shorter than the 5-second acquisition employed for XEOL scans in this study, suggesting that improving luminescence detection should allow correlative XEOL measurements to be performed successfully with minimal material degradation. Analysis of the XEOL emission from the quartz substrate beneath the perovskite reveals its promise for use as a real-time in-situ x-ray dosimeter, which could provide quantitative metrics for future optimization of XEOL data collection for perovskites and other beam-sensitive materials. Overall, the data suggest that XEOL represents a promising route towards improved resolution in the characterization of nanoscale heterogeneities and defects in optically active materials that may be implemented into x-ray nanoprobes to complement existing x-ray modalities.

A short-wave infrared (SWIR) reflectometry method is developed to quantify the water content in solar modules in a noncontact approach. Water content has been implicated as a key factor in many module degradation pathways, including contact corrosion and yellowing. Despite these established issues, to date no robust method for quantitative in situ determination of water content in standard modules is available. The water reflectometry detection (WaRD) method provides this capability by leveraging the SWIR optical spectrum, in which water exhibits vibrational absorption bands, module components are highly transparent, and solar cell back reflectors are reflective. Here, we describe the operating principle and demonstrate measurement of moisture in encapsulated aluminum back surface field and passivated-emitter rear contact architectures over the full range of temperatures and relative humidity relevant to field operation and damp heat testing.

An outstanding issue in the longevity of photovoltaic (PV) modules is the accelerated degradation caused by the presence of moisture. Moisture leads to interfacial instability, de-adhesion, encapsulant decomposition, and contact corrosion. However, experimental characterization of moisture in PV modules is not trivial and its impacts can take years or decades to establish in the field, presenting a major obstacle to designing high-reliability modules. First principles calculations provide an alternative way to study the ingress of water and its detrimental effect on the structure and decomposition of the polymer encapsulant and interfaces between the encapsulant and the semiconductor, the metal contacts, or the dielectric layer. In this work, we use density functional theory (DFT) computations to model single chain, crystalline and cross-linked structures, infrared (IR) signatures, and degradation mechanisms of ethylene vinyl acetate (EVA), the most common polymer encapsulant used in Si PV modules. IR-active modes computed for low energy EVA structures and possible decomposition products match well with reported experiments. The EVA decomposition energy barriers computed using the Nudged Elastic Band (NEB) method show a preference for acetic acid formation as compared to acetaldehyde, are lowered in the presence of a water solvent or hydroxyl ion catalyst, and match well with reported experimental activation energies. This systematic study leads to a clear picture of the hydrolysis-driven decomposition of EVA in terms of energetically favorable mechanisms, possible intermediate structures, and IR signatures of reactants and products.

Lead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high-quality crystals with tunable bandgaps in the visible and near-infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin-film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.

Humidity is widely accepted as a risk factor to the durability of photovoltaic modules. The ingress of moisture from humid environments into the module is linked to a host of module degradation modes, including contact corrosion, encapsulant yellowing, and delamination. While these degradation pathways are studied extensively, the exact moisture content within a module is typically a hidden variable, making it difficult to establish a quantitative relationship between module moisture content and degradation. We leverage our recently developed water reflectomery detection (WaRD) technique along with biased photoluminescence imaging to spatially quantify the moisture content and cell parameters of silicon modules subjected to various damp heat conditions for 2000 hours at a 1 millimeter resolution. This unique dataset reveals two modes of series resistance increase - finger interruptions and cell-wide “background” resistance - each showing varied sensitivity to moisture exposure depending on module architecture.

Improved understanding of the origins of instability during photovoltaic operation of perovskite solar cell materials must be established to overcome barriers to commercialization. In this study, we analyze the microscopic mechanisms of degradation in high-performing methylammonium free (FA0.9Cs0.1PbI3) perovskite solar cells (PSC) over 600 hours of operation under stressors inherent to PV operation, including heat, illumination, and a load while excluding atmospheric effects by testing in a water-and oxygen-free atmosphere. While the PSCs exhibit reasonable thermal stability, they show considerable performance loss under constant illumination or stable power output. Synchrotron-based nanoprobe X-ray fluorescence and X-ray beam induced current (XRF/XBIC) measurements reveal segregation of current-blocking Cs-rich phases during stress testing. The decrease in performance correlates with the resulting number density of the Cs-rich clusters, which varies by stress condition. These findings unveil cation-dependent instability in FA0.9Cs0.1PbI3 perovskites and provide a framework for understanding the energy landscape in alloy perovskites to guide the engineering of long-lived halide perovskite devices.