Prof Henry Snaith FRS

Abstract:

All‐inorganic perovskite materials are attractive alternatives to organic‐inorganic perovskites because of their potential for higher thermal stability. While CsPbI3 is compositionally stable under elevated temperatures, the cubic perovskite α‐phase is thermodynamically stable only at >330°C and the low‐temperature perovskite γ−phase is metastable and highly susceptible to non‐perovskite δ‐phase conversion in moisture. Many methods have been reported which show that incorporation of acid (aqueous HI) or “HPbI3” – recently shown to be dimethylammonium lead iodide (DMAPbI3) – lower the annealing temperature required to produce the black, perovskite phase of CsPbI3. The optical and crystallographic data presented here show that DMA can successfully incorporate as an A‐site cation to replace Cs in the CsPbI3 perovskite material. This describes the stabilization and lower phase transition temperature reported in the literature when HI or HPbI3 are used as precursors for CsPbI3. The Cs‐DMA alloy only forms a pure‐phase material up to ∽25% DMA; at higher concentrations the CsPbI3 and DMAPbI3 begin to phase segregate. These alloyed materials are more stable to moisture than neat CsPbI3, but do not represent a fully inorganic perovskite material.

Abstract:

Despite their impressive performance as a solar absorber, much remains unknown on the fundamental properties of metal halide perovskites (MHPs). Their polar nature in particular is an intense area of study, and the relative permittivity (ε_r) is a parameter widely used to quantify polarization over a range of different time scales. In this report, we have exploited frequency-dependent time-resolved microwave conductivity (TRMC) to study how ε_r values of a range of MHPs change as a function of time, upon optical illumination. Further characterization of charge carriers and polarizability are conducted by femtosecond transient absorption and stimulated Raman spectroscopy. We find that changes in ε_r are roughly proportional to photogenerated carrier density but decay with a shorter time constant than conductance, suggesting that the presence of charge carriers alone does not determine polarization.

Abstract:

Hybrid organic-inorganic perovskites are exciting materials for solar-energy applications whose microscopic properties are still not well understood. Atomic-resolution (scanning) transmission electron microscopy, (S)TEM, has provided invaluable insights for many crystalline solar-cell materials, and is used here to successfully image CH(NH2)2PbI3 thin films with low electron-radiation dose. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. We demonstrate that beaminduced degradation of the perovskite leads to an initial loss of CH(NH2)2 + ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observe aligned point defects and climbdissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead-halide perovskites.

Abstract:

The fabrication of perovskite film is crucial for achieving efficient perovskite photoelectric device. Herein, a simple and novel encapsulation growth method was applied to prepare high-quality quasi-2D perovskite films with advantages of compact and uniform morphology, high crystallinity with lower defect density, enhanced photoluminescence quantum yield (PLQY) and optimized multidimensional domain distribution and crystallite orientation for perovskite light-emitting diodes (PeLEDs). The encapsulation growth method was found to decrease the proportion of the low-dimensional (n = 1,2,3) domains while increasing the high-dimensional domains content with randomly-oriented crystals, which simultaneously enhanced the overall energy landscape effect and charges transport within the quasi-2D perovskite films, and the PLQY of the quasi-2D perovskites significantly improved from 9.2% to 60.0%. Finally, an efficient flexible green PeLEDs was obtained with a high luminous efficiency (LE) of 47.1 cd/A, and a luminance brightness of 8300 cd/m , and an efficient sky-blue PeLEDs was also achieved with record EQE of 12.8% by using encapsulation growth method. This encapsulation growth method provides a promising strategy for boosting the efficiency of quasi-2D PeLEDs. 2

Abstract:

Although metal halide perovskite photovoltaics have shown an unprecedented rise in power conversion efficiency (PCE), they remain far from their theoretical PCE limit. Among the highest efficiencies to date are delivered when polycrystalline films are enhanced via “molecular passivation”, but this can introduce new instabilities, in particular under severe accelerated aging conditions (e.g., at 85 °C in the dark or under full spectrum simulated sunlight). Here, we utilize a benzylammonium bromide passivation treatment to improve device performance, achieving the champion stabilized power output (SPO) of 19.5 % in a p-i-n device architecture. We correlate the improved device performance with a significant increase in charge carrier diffusion lengths, mobilities, and lifetimes. Furthermore, treated devices maintain an increased performance during 120 h combined stressing under simulated full spectrum sunlight at 85 °C, indicating that enhancement from this passivation treatment is sustained under harsh accelerated aging conditions. This is a crucial step toward real-world operation-relevant passivation treatments.