Perovskite solar cells have shown unprecedent performance increase up to 22% efficiency. However, their photovoltaic performance has shown fast deterioration under light illumination in the presence of humid air even with encapulation. The stability of perovskite materials has been unsolved and its mechanism has been elusive. Here we uncover a mechanism for irreversible degradation of perovskite materials in which trapped charges, regardless of the polarity, play a decisive role. An experimental setup using different polarity ions revealed that the moisture-induced irreversible dissociation of perovskite materials is triggered by charges trapped along grain boundaries. We also identified the synergetic effect of oxygen on the process of moisture-induced degradation. The deprotonation of organic cations by trapped charge-induced local electric field would be attributed to the initiation of irreversible decomposition.

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Trapped charge-driven degradation of perovskite solar cells

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

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A piperidinium salt stabilizes efficient metal-halide perovskite solar cells.

Long-term device stability is one of the most critical issues that impede perovskite solar cell commercialization. Here we show that a thin layer of a functional hygroscopic polymer, poly(ethylene oxide), PEO, on top of the perovskite thin film, can make perovskite-based solar cells highly stable during operation and in a humid atmosphere. We prove that PEO chemically interacts with lead ions on the perovskite surface, and thus passivates undercoordinated defect sites. Importantly, defect healing by PEO not only results in an improvement of the photo-voltage but also makes the perovskite thin film stable. We demonstrate that the hygroscopic PEO thin film can prevent water inclusion into the perovskite film by screening water molecules, thus having a multi-functional role. Overall, such interface engineering leads to highly durable perovskite solar cells, which, in the presence of PEO passivation, retained more than 95% of their initial power conversion efficiency over 15 h illumination, under load, in ambient atmosphere without encapsulation. Our findings experimentally reveal the role of interface engineering in mastering the instability of perovskite materials and propose a general approach for improving the reliability of perovskite-based optoelectronic devices.

Enhanced solar cell stability by hygroscopic polymer passivation of metal halide perovskite thin film

Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives

Thermal instability of organic–inorganic hybrid perovskites will be an inevitable hurdle for commercialization. Recently, all-inorganic cesium lead halide perovskites, in particular, CsPbI2Br, have emerged as thermally stable and efficient photovoltaic light absorbers. However, the fundamental properties of this material have not been studied in detail. The crystal formation behavior of CsPbI2Br is investigated by examining the surface morphology, crystal structure, and chemical state of the perovskite films. We discover a previously uncharacterized feature that the formation of black polymorph through optimal annealing temperature proves to be critical to both solar cell efficiency and phase stability. Our optimized planar heterojunction solar cell exhibits a J–V scan efficiency of 10.7% and open-circuit voltage of 1.23 V, which far outperforms the preceding literature.

Unveiling the Crystal Formation of Cesium Lead Mixed-Halide Perovskites for Efficient and Stable Solar Cells

We have presented a detailed analysis of the phase transition kinetics and binding energy states of solution processed methylammonium lead iodide (MAPbI3) thin films prepared at ambient conditions and annealed at different elevated temperatures. It is the processing temperature and environmental conditions that predominantly control the crystal structure and surface morphology of MAPbI3 thin films. The structural transformation from tetragonal to cubic occurs at 60 °C with a 30 minute annealing time while the 10 minute annealed films posses a tetragonal crystal structure. The transformed phase is greatly intact even at the higher annealing temperature of 150 °C and after a time of 2 hours. The charge transfer interaction between the Pb 4f and I 3d oxidation states is quantified using XPS.

Phase transition kinetics and surface binding states of methylammonium lead iodide perovskite.

We have demonstrated a robust protocol to prepare Cu2−xS thin films with a controlled crystal phase and size which exhibit localized surface plasmon resonance (LSPR) coupled exciton effects by a simple template free single step wet chemical method without any surfactant. The LSPR frequency can be tuned in the Cu2−xS thin films by the growth temperature and time which controls the free carrier density. These selectively grown Cu2−xS thin films possess a tunable band gap (2.6–1.4 eV) due to the quantum size effect. The origin of the LSPR coupled exciton effects are discussed.

M. Vijaykumar

Papers

Phase transition kinetics and surface binding states of methylammonium lead iodide perovskite.

Growth mechanisms and origin of localized surface plasmon resonance coupled exciton effects in Cu2−xS thin films