We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

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Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

Recently, perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency (25.2%) and low fabrication cost. However, the short lifetime under operation is the major obstacle for their commercialization. With efforts from the entire PSC research community, significant advances have been witnessed to improve the device operational stability, and a timely summary on the progress is urgently needed. In this review, we first clarify the definition of operational stability and its significance in the context of practical use. By analyzing the mechanisms in established approaches for operational stability improvement, we summarize several effective strategies to extend device lifetime in a layer-by-layer sequence across the entire PSC. These mechanisms are discussed in the contexts of chemical reactions, photo-physical management, technological modification, etc., which may inspire future R&D for stable PSCs. Finally, emerging operational stability standards with respect to testing and reporting device operational stability are summarized and discussed, which may help reliable device stability data circulate in the research community. The main target of this review is gaining insight into the operational stability of PSCs, as well as providing useful guidance to further improve their operational lifetime by rational materials processing and device fabrication, which would finally promote the commercialization of perovskite solar cells.

Towards commercialization: the operational stability of perovskite solar cells

Multifunctional Enhancement for Highly Stable and Efficient Perovskite Solar Cells

Formamidinium–caesium mixed-cation perovskites have shown better thermal stability than their methylammonium-containing counterparts but they suffer from photoinstability induced by iodide migration and phase segregation. Here we improve their photostability by adding slightly excessive AX (at a molar percentage of 0.25% to Pb2+ ions), where A is formamidinium or caesium and X is iodine. The excessive AX does not improve the initial solar cell efficiency. It compensates iodide vacancies and suppresses ion migration and defects generation during long-term illumination by around tenfold compared with AX-deficient devices. Consequently, generation of hole traps and phase segregation is impeded, with the former limiting solar cell efficiency after degradation. The perovskite mini-modules reached a certified stabilized efficiency of 18.6% with an aperture area of ~30 cm2, corresponding to an active area efficiency of 20.2%. The mini-module maintains 93.6% of the initial efficiency after continuous operation under 1 sun illumination for >1,000 h at 50 ± 5 °C in air. Methylammonium-free perovskite solar cells have achieved promising efficiency and thermal stability yet iodide migration limits their operational stability. Deng and colleagues show that an excess of formamidinium or caesium iodide precursor suppresses iodide vacancies preventing ion migration and eventually the generation of hole traps during device operation.

Defect compensation in formamidinium–caesium perovskites for highly efficient solar mini-modules with improved photostability

Solution processing of semiconductors is highly promising for the high-throughput production of cost-effective electronics and optoelectronics. Although hybrid perovskites have potential in various device applications, challenges remain in the development of high-quality materials with simultaneously improved processing reproducibility and scalability. Here, we report a liquid medium annealing (LMA) technology that creates a robust chemical environment and constant heating field to modulate crystal growth over the entire film. Our method produces films with high crystallinity, fewer defects, desired stoichiometry, and overall film homogeneity. The resulting perovskite solar cells (PSCs) yield a stabilized power output of 24.04% (certified 23.7%, 0.08 cm2) and maintain 95% of their initial power conversion efficiency (PCE) after 2000 hours of operation. In addition, the 1-cm2 PSCs exhibit a stabilized power output of 23.15% (certified PCE 22.3%) and keep 90% of their initial PCE after 1120 hours of operation, which illustrates their feasibility for scalable fabrication. LMA is less climate dependent and produces devices in-house with negligible performance variance year round. This method thus opens a new and effective avenue to improving the quality of perovskite films and photovoltaic devices in a scalable and reproducible manner.

Liquid medium annealing for fabricating durable perovskite solar cells with improved reproducibility.

Microscopic Degradation in Formamidinium-Cesium Lead Iodide Perovskite Solar Cells under Operational Stressors

X‐Ray Microscopy of Halide Perovskites: Techniques, Applications, and Prospects

How Strain Alters CO2 Electroreduction on Model Cu(001) Surfaces

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.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202010330

Passivation Properties and Formation Mechanism of Amorphous Halide Perovskite Thin Films

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.

Rishi E. Kumar

Papers

Microscopic Degradation in Formamidinium-Cesium Lead Iodide Perovskite Solar Cells under Operational Stressors

X‐Ray Microscopy of Halide Perovskites: Techniques, Applications, and Prospects

How Strain Alters CO2 Electroreduction on Model Cu(001) Surfaces

Passivation Properties and Formation Mechanism of Amorphous Halide Perovskite Thin Films

Quantitative Determination of Moisture Content in Solar Modules by Short-Wave Infrared Reflectometry