Photovoltaic & Optoelectronic Device Group

Abstract:

2D halide perovskites are among intensely studied materials platforms profiting from solution-based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical, as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase transitions in 2D perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in 2D perovskites.

Abstract:

Constructing stable electrodes which function over long timescales at large current density is essential for the industrial realization and implementation of water electrolysis. However, rapid gas bubble detachment at large current density usually results in peeling-off of electrocatalysts and performance degradation, especially for long term operations. Here we construct a mechanically-stable, all-metal, and highly active CuMo₆S₈/Cu electrode by in-situ reaction between MoS₂ and Cu. The Chevrel phase electrode exhibits strong binding at the electrocatalyst-support interface with weak adhesion at electrocatalyst-bubble interface, in addition to fast hydrogen evolution and charge transfer kinetics. These features facilitate the achievement of large current density of 2500 mA cm^-2 at a small overpotential of 334 mV which operate stably at 2500 mA cm^-2 for over 100 h. In-situ total internal reflection imaging at micrometer level and mechanical tests disclose the relationships of two interfacial forces and performance of electrocatalysts. This dual interfacial engineering strategy can be extended to construct stable and high-performance electrodes for other gas-involving reactions.

Abstract:

Mixed-halide perovskites offer ideal bandgaps for tandem solar cells, but photoinduced halide segregation compromises photovoltaic device performance. This study explores the influence of a hole-transport layer, necessary for a full device, by monitoring halide segregation through in situ, concurrent X-ray diffraction and photoluminescence measurements to disentangle compositional and optoelectronic changes. This work demonstrates that top coating FA0.83Cs0.17Pb(Br0.4I0.6)3 perovskite films with a poly(triaryl)amine (PTAA) hole-extraction layer surprisingly leads to suppression of halide segregation because photogenerated charge carriers are rapidly trapped at interfacial defects that do not drive halide segregation. However, the generation of iodide-enriched regions near the perovskite/PTAA interface enhances hole back-transfer from the PTAA layer through improved energy level offsets, increasing radiative recombination losses. It is further found that while passivation with a piperidinium salt slows halide segregation in perovskite films, the addition of a PTAA top-coating accelerates such effects, elucidating the specific nature of trap types that are able to drive the halide segregation process. This work highlights the importance of selective passivation techniques for achieving efficient and stable wide-bandgap perovskite photovoltaic devices.