Prof Henry Snaith FRS

Abstract:

Metal halide perovskites have demonstrated extraordinary potential as materials for next-generation optoelectronics including photovoltaics and light-emitting diodes. Nevertheless, our understanding of this material is still far from complete. One remaining puzzle is the phenomenon of perovskite “photo-brightening”: the increase in photoluminescence during exposure to light in an ambient atmosphere. Here, we propose a comprehensive mechanism for the reactivity of the archetypal perovskite, MAPbI3, in ambient conditions. We establish the formation of lead-oxygen bonds by hydrogen peroxide as the key factor leading to perovskite photo-brightening. We demonstrate that hydrogen peroxide can be applied directly as an effective “post-treatment” to emulate the process and substantially improve photoluminescence quantum efficiencies. Finally, we show that the treatment can be incorporated into photovoltaic devices to give a 50 mV increase in open-circuit voltage, delivering high 19.2% steady-state power conversion efficiencies for inverted perovskite solar cells of the mixed halide, mixed cation perovskite FA0.83Cs0.17Pb(I0.9Br0.1)3.

Abstract:

A crosslinkable acrylate random copolymer with both hole-transporting bis(triarylamine) and photocrosslinkable cinnamate side chains is compared to the widely used poly(4-butyl-triphenylamine-4′,4′′-diyl) (PolyTPD) as a hole-transport material (HTM) in positive–intrinsic–negative (p–i–n) perovskite solar cells (PSCs). The crosslinked films of this HTM exhibit improved wettability by precursor solutions of the perovskite relative to PolyTPD; this facilitates high-quality full film coverage by the subsequently deposited perovskite layer on smooth substrates, which is difficult to achieve with PolyTPD without the use of additional interlayers. PSCs fabricated using undoped and crosslinked copolymer achieve steady-state power outputs that are comparable to those of cells incorporating p-doped PolyTPD (with interlayers) as the HTM. The devices made with this material also exhibited improved initial stability under high-intensity ultraviolet LED irradiation, in comparison to those with the PolyTPD analogue. Remarkably, after 3000 h of aging in an oven at 85 °C in a nitrogen-filled glovebox, device efficiency showed no degradation; the SPO was comparable to the initial performance.

Abstract:

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ~200μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure observed in perovskite nanocrystals.