Snaith group

Abstract:

As perovskite photovoltaics stride towards commercialization, reverse bias degradation in shaded cells that must current match illuminated cells is a serious challenge. Previous research has emphasized the role of iodide and silver oxidation, and the role of hole tunnelling from the electron-transport layer into the perovskite to enable the flow of current under reverse bias in causing degradation. Here we show that device architecture engineering has a significant impact on the reverse bias behaviour of perovskite solar cells. By implementing both a ~35-nm-thick conjugated polymer hole transport layer and a more electrochemically stable back electrode, we demonstrate average breakdown voltages exceeding −15 V, comparable to those of silicon cells. Our strategy for increasing the breakdown voltage reduces the number of bypass diodes needed to protect a solar module that is partially shaded, which has been proven to be an effective strategy for silicon solar panels.

Abstract:

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3–x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and “liquid-like” Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose–Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

Abstract:

The deposition of large ammonium cations onto perovskite surfaces to passivate defects and reduce contact recombination has enabled exceptional efficiency and stability in perovskite solar cells. These ammonium cations can either assemble as a thin molecular layer at the perovskite surface or induce the formation of a low-dimensional (usually two-dimensional) perovskite capping layer on top of the three-dimensional perovskite. The formation of these two different structures is often overlooked by researchers, although they impact differently on device operation. In this Review, we seek to distinguish between these two passivation layers. We consider the conditions needed for the formation of low-dimensional perovskite and the electronic properties of the two structures. We discuss the mechanisms by which each method improves photovoltaic efficiency and stability. Finally, we summarize the knowledge gaps that need to be addressed to better understand and optimize ammonium cation-based passivation strategies.