Mechanism and Timescales of Reversible p-Doping of Methylammonium Lead Triiodide by Oxygen

Improved electronic uniformity and nanoscale homogeneity in template-grown CsPbBr3 nanorods

One-dimensional metal halide perovskites are among the most promising candidate materials for optoelectronic devices. However, the heterogeneity and fast degradation of perovskite nanowires (NWs) and nanorods (NRs) synthesized using conventional approaches impose a bottleneck for their optoelectronic applications. Recently, all-inorganic perovskite CsPbBr3 NRs with tailored dimensions, crafted using an amphiphilic bottlebrush-like block copolymer (BBCP) as nanoreactors, have demonstrated enhanced stabilities. Herein, we report the electronic investigation into these template-grown CsPbBr3 NRs using dielectric force microscopy (DFM), a contactless, nondestructive imaging technique. All freshly prepared CsPbBr3 NRs exhibited ambipolar behaviors for up to two months after sample synthesis. A transition from ambipolar to p-type behaviors occurred after two months, and nearly all NRs completed the transition within two weeks. Moreover, template-grown CsPbBr3 NRs displayed better nanoscale electronic homogeneity compared to their conventional counterparts. The improved electronic uniformity and nanoscale homogeneity place the template-grown CsPbBr3 NRs in a unique advantageous position for optoelectronic applications.

Visualizing the Low-Energy Electronic Structure of Prototypical Hybrid Halide Perovskite through Clear Band Measurements

Organic-inorganic hybrid perovskites (OIHPs) are a promising class of materials that rival conventional semiconductors in various optoelectronic applications. However, unraveling the precise nature of their low-energy electronic structures continues to pose a significant challenge, primarily due to the absence of clear band measurements. Here, we investigate the low-energy electronic structure of CH3NH3PbI3 (MAPI3) using angle-resolved photoelectron spectroscopy combined with ab initio density functional theory. We successfully visualize the electronic structure of MAPI3 near the bulk valence band maximum by using a laboratory photon source (He Iα, 21.2 eV) at low temperature and explore its fundamental properties. The observed valence band exhibits a highly isotropic and parabolic band characterized by small effective masses of 0.20-0.21 me, without notable spectral signatures associated with a large polaron or the Rashba effect, subjects that are intensely debated in the literature. Concurrently, our spin-resolved measurements directly disprove the giant Rashba scenario previously suggested in a similar perovskite compound by establishing an upper limit for the Rashba parameter (αR) of 0.28 eV Å. Our results unveil the unusually complex nature of the low-energy electronic structure of OIHPs, thereby advancing our fundamental understanding of this important class of materials.

Electrochemical Doping of Halide Perovskites by Noble Metal Interstitial Cations

Metal halide perovskites are an attractive class of semiconductors, but it has proven difficult to control their electronic doping by conventional strategies due to screening and compensation by mobile ions or ionic defects. Noble-metal interstitials represent an under-studied class of extrinsic defects that plausibly influence many perovskite-based devices. In this work, doping of metal halide perovskites is studied by electrochemically formed Au⁺ interstitial ions, combining experimental data on devices with a computational analysis of Au⁺ interstitial defects based on density functional theory (DFT). Analysis suggests that Au⁺ cations can be easily formed and migrate through the perovskite bulk via the same sites as iodine interstitials (I_i ⁺ ). However, whereas I_i ⁺ compensates n-type doping by electron capture, the noble-metal interstitials act as quasi-stable n-dopants. Experimentally, voltage-dependent, dynamic doping by current density-time (J-t), electrochemical impedance, and photoluminescence measurements are characterized. These results provide deeper insight into the potential beneficial and detrimental impacts of metal electrode reactions on long-term performance of perovskite photovoltaic and light-emitting diodes, as well as offer an alternative doping explanation for the valence switching mechanism of halide-perovskite-based neuromorphic and memristive devices.

Electronic properties of metal halide perovskites and their interfaces: the basics

We have witnessed tremendous progress of metal halide perovskite (MHP)-based optoelectronic devices, especially in the field of photovoltaics. Despite intensive research in the past few years, questions still remain regarding their fundamental optoelectronic properties, among which the electronic properties exhibit an interplay of numerous phenomena that deserve serious scrutiny. In this Focus article, we aim to provide a contemporary understanding of the unique electronic properties that has been resolved by the community. First introducing some of the basic concepts established in semiconductor physics, the intrinsic and extrinsic electronic properties of the MHPs are disentangled and explained. With this, the complex interplay of interface-, dopant-, and surface state-induced electronic states in determining the electrostatic landscape in the material can be comprehended, and the energy level alignment in device architectures more reliably assessed.

X-Ray-Induced Modification of the Photophysical Properties of MAPbBr3 Single Crystals

Methylammonium lead tribromide (MAPbBr₃) perovskite single crystals demonstrate to be excellent direct X-ray and gamma-ray detectors with outstanding sensitivity and low limit of detection. Despite this, thorough studies on the photophysical effects of exposure to high doses of ionizing radiation on this material are still lacking. In this work, we present our findings regarding the effects of controlled X-ray irradiation on the optoelectronic properties of MAPbBr₃ single crystals. Irradiation is carried out in air with an imaging X-ray tube, simulating real-life application in a medical facility. By means of surface photovoltage spectroscopy, we find that X-ray exposure quenches free excitons in the material and introduces new bound excitonic species. Despite this drastic effect, the crystals recover after 1 week of storage in dark and low humidity conditions. By means of X-ray photoelectron spectroscopy, we find that the origin of the new bound excitonic species is the formation of bromine vacancies, leading to local changes in the dielectric response of the material. The recovery effect is attributed to vacancy filling by atmospheric oxygen and water.