Surface Restructuring of Hybrid Perovskite Crystals

Abstract

Hybrid perovskite crystals have emerged as an important class of semiconductors because of their remarkable performance in optoelectronics devices. The interface structure and chemistry of these crystals are key determinants of the device’s performance. Unfortunately, little is known about the intrinsic properties of the surfaces of perovskite materials because extrinsic effects, such as complex microstructures, processing conditions, and hydration under ambient conditions, are thought to cause resistive losses and high leakage current in solar cells. We reveal the intrinsic structural and optoelectronic properties of both pristinely cleaved and aged surfaces of single crystals. We identify surface restructuring on the aged surfaces (visualized on the atomic-scale by scanning tunneling microscopy) that lead to compositional and optical bandgap changes as well as degradation of carrier dynamics, photocurrent, and solar cell device performance. The insights reported herein clarify the key variables involved in the performance of perovskite-based solar cells and fabrication of high-quality surface single crystals, thus paving the way toward their future exploitation in highly efficient solar cells.

Temperature-dependent optical spectra of single-crystal (CH3NH3)PbBr3 cleaved in ultrahigh vacuum

“…2. (CH 3 NH 3 )PbBr 3 is extremely sensitive to gas adsorption [35,45], which can change surface band bending and trap state density, thus altering carrier diffusion and scattering. We point out the necessity to avoid exposure of OIPS to air and to prevent hydration of the surface prior to contacting, if energetic carriers are to be harvested in future experiments.…”

“…These variations can be expected to be reduced by studying single crystals [43,44], opening the opportunity to approach the intrinsic photophysics of OIPS. However, band gaps of (CH 3 NH 3 )PbBr 3 single crystals determined from reflection measurements vary from 2.22 to 2.35 eV [38,39,[45][46][47]. Exposure of crystal surfaces to air results in fast hydration, increasing the room-temperature band gap from 2.22 eV to 2.27 eV [45].…”

“…However, band gaps of (CH 3 NH 3 )PbBr 3 single crystals determined from reflection measurements vary from 2.22 to 2.35 eV [38,39,[45][46][47]. Exposure of crystal surfaces to air results in fast hydration, increasing the room-temperature band gap from 2.22 eV to 2.27 eV [45]. Recent experimental findings made on * Electronic address: daniel.niesner@fau.de lead-bromide single-crystals also include the observation of photoluminescence (PL) from high-energy carriers [28] with the potential to enhance solar cell performance, possibly beyond the Shockley-Queisser limit [48].…”

“…A possible direct-indirect character of the band gap of related (CH 3 NH 3 )PbBr 3 remains to be investigated. Photoluminescence spectra of (CH 3 NH 3 )PbBr 3 showed a doublepeak structure [45,47,51], and further studies are needed to identify its origin.…”

“…Perovskite single-crystals cleaved in ultrahigh vacuum (UHV) show optical properties at room-temperature which are distinctly different from those of thin films and as-grown single crystals [45]. Temperature-dependent measurements can help to develop a more complete picture of these optical properties, which needs to include identification of exciton binding energies, free carriers, as well as direct and possible indirect transitions.…”