Prof Henry Snaith FRS

Abstract:

Organic-inorganic halide perovskites are attracting extraordinary attention in the field of energy materials. The reaction of hybrid lead halide perovskites with Li metal has been recently proposed for a number of potential applications. However, the mechanisms for Li uptake in such materials, such as intercalation and conversion, are still unknown. Using a combination of density functional theory, electrochemical and diffraction techniques, we consider Li intercalation and conversion reactions in CH3NH3PbI3, CH3NH3PbBr3 and CH3NH3PbCl3. Our simulations suggest that conversion reactions with Li are far more energetically preferable in these materials than Li intercalation. Calculations confirm the formation of Pb metal as a result of Li conversion in all three materials, and this is supported by an X-ray diffraction analysis of CH3NH3PbBr3. The results of this study provide fresh insights into lithium and halide perovskite reactions that will hopefully drive further exploration of these materials for a wider variety of energy applications.

Abstract:

Perovskite solar cells with record power conversion efficiency are fabricated by alloying both hybrid and fully inorganic compounds. While the basic electronic properties of the hybrid perovskites are now well understood, key electronic parameters for solar cell performance, such as the exciton binding energy of fully inorganic perovskites, are still unknown. By performing magneto-transmission measurements, we determine with high accuracy the exciton binding energy and reduced mass of fully inorganic CsPbX3 perovskites (X = I, Br, and an alloy of these). The well-behaved (continuous) evolution of the band gap with temperature in the range of 4–270 K suggests that fully inorganic perovskites do not undergo structural phase transitions like their hybrid counterparts. The experimentally determined dielectric constants indicate that at low temperature, when the motion of the organic cation is frozen, the dielectric screening mechanism is essentially the same for both hybrid and inorganic perovskites and is dominated by the relative motion of atoms within the lead halide cage.

Abstract:

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)2PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose bandstructure picture is moderated by a modest degree of energetic disorder.