Prof Henry Snaith FRS

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.

Abstract:

Mixed lead-tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite-perovskite tandem solar cells. Previous reports on lead-tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.