The electronic structure and related optical properties of an emerging thin-film photovoltaic material CH3NH3PbBr3 are studied. A block-shaped α-phase CH3NH3PbBr3 single crystal with the natural ⟨100⟩ surface is synthesized solvothermally. The room-temperature dielectric function ε = ε1 + iε2 spectrum of CH3NH3PbBr3 is determined by spectroscopic ellipsometry from 0.73 to 6.45 eV. Data are modeled with a series of Tauc-Lorentz oscillators, which show the absorption edge with a strong excitonic transition at ∼2.3 eV and several above-bandgap optical structures associated with the electronic interband transitions. The energy band structure and ε data of CH3NH3PbBr3 for the CH3NH3(+) molecules oriented in the ⟨111⟩ and ⟨100⟩ directions are obtained from first-principles calculations. The overall shape of ε data shows a qualitatively good agreement with experimental results. Electronic origins of major optical structures are discussed.

Electronic Structure and Optical Properties of α-CH 3 NH 3 PbBr 3 Perovskite Single Crystal

Density functional theory screening of the hybrid double perovskites (MA)2BIBiX6 (BI=K,Cu,Ag,Tl; X=Cl,Br,I) shows that systems with band gaps similar to those of the MAPbX3 lead compounds can be expected for BI=Cu,Ag,Tl. Motivated by these findings, (MA)2TlBiBr6, isoelectronic with MAPbBr3, was synthesised and found to have a band gap of ~2.0eV.

Exploring the properties of lead-free hybrid double perovskites using a combined computational-experimental approach

Impedance Spectroscopy for Metal Halide Perovskite Single Crystals: Recent Advances, Challenges, and Solutions

Metal halide perovskites (MHPs) have recently emerged as an ideal semiconductor for photovoltaic applications. Most of the advantageous properties of perovskites are dominated by their large charge carrier mobility (μ), which is a key parameter to gauge the photovoltaic performance and stability of MHPs. Simultaneously, the rich structural variation, form and crystallinity of perovskites underpin their carrier mobility. In this review article, we focus on the quantification of carrier mobility in perovskites with dimensional transformation and crystallinity. Furthermore, we have critically analyzed the variation in mobility values as probed by different analytical techniques. We also discuss the fundamental mobility limiting factors that hinder efficient charge transport in these materials and present the recent advancement with a special focus on the dimensionalities including reduced dimensional perovskites and single crystal systems. In addition, the present challenges and possible pathways for mobility enhancement are also discussed in detail.

The effect of dimensionality on the charge carrier mobility of halide perovskites

Recently, cuprous halide perovskite-type materials have drawn tremendous attention for their intriguing optical properties. Here, a zero-dimensional (0D) Cu(I)-based compound of [(C3H7)4N]2Cu2I4 ([C3H7)4N]+ = tetrapropylammonium cation) was synthesized by a facile solution method, a monoclinic system of P21/n symmetry with a Cu2I42- cluster as the confined structure. The as-synthesized [(C3H7)4N]2Cu2I4 exhibits bright dual-band pure white emission with a photoluminescence quantum yield (PLQY) of 91.9% and CIE color coordinates of (0.33, 0.35). Notably, this compound also exhibits an ultrahigh color rendering index (CRI) of 92.2, which is comparable to the highest value of single-component metal halides reported recently. Its Raman spectra provide a clear spectral profile of strong electron-phonon interaction after [(C3H7)4N]+ incorporation, favoring the self-trapped exciton (STE) formation. [(C3H7)4N]2Cu2I4 can give dual-STE bands at the same time because of the Cu-Cu metal bond in a Cu2I42- cluster, whose populations could be scaled by temperature, together with the local dipole orientation modulation of neighboring STEs and phase transition related emission color coordinate change. Particularly, the outstanding chemical- and antiwater stability of this compound was also demonstrated. This work illustrates the potential of such cuprous halide perovskite-type materials in multifunctional applications, such as lighting in varied environments.

Pure White Emission with 91.9% Photoluminescence Quantum Yield of [(C3H7)4N]2Cu2I4 out of Polaronic States and Ultra-High Color Rendering Index.

Advances in Lead-Free Perovskite Single Crystals: Fundamentals and Applications

Self-Healing Cs3Bi2Br3I6 Perovskite Wafers for X-Ray Detection

CsPbBr3 single crystals have potential for application in ionizing-radiation detection devices due to their optimal optoelectronic properties. Yet, their mixed ionic–electronic conductivity produces instability and hysteretic artifacts hindering the long-term device operation. Herein, we report an electrical characterization of CsPbBr3 single crystals operating up to the time scale of hours. Our fast time-of-flight measurements reveal bulk mobilities of 13–26 cm2 V–1 s–1 with a negative voltage bias dependency. By means of a guard ring (GR) configuration, we separate bulk and surface mobilities showing significant qualitative and quantitative transport differences. Our experiments of current transients and impedance spectroscopy indicate the formation of several regimes of space-charge-limited current (SCLC) associated with mechanisms similar to the Poole–Frenkel ionized-trap-assisted transport. We show that the ionic-SCLC seems to be an operational mode in this lead halide perovskite, despite the fact that experiments can be designed where the contribution of mobile ions to transport is negligible.

Surface versus Bulk Currents and Ionic Space-Charge Effects in CsPbBr3 Single Crystals

Characterization of Aerosol Deposited Cesium Lead Tribromide Perovskite Films on Interdigited ITO Electrodes

Transparent electrodes consisting of silver nanowires (Ag NWs) are a solution‐processed alternative to commonly used indium tin oxide electrodes. Here, Ag NW electrodes protected by a tin oxide (SnOx) are explored and unprecedented thermal stability is found. While unprotected Ag NW electrodes fail at 250 °C, the SnOx Ag NW electrodes remain stable for 40 h at 250 °C and withstand high temperatures up to 500 °C for short times. First, an optimized method of synthesis that provides uniform Ag NWs with high reproducibility is used. Afterward, a SnOx shell is formed in a wet chemical reaction. Fabrication of highly conductive electrodes requires thermal annealing at 300 °C for 5 min under ambient atmosphere. Electrodes with a sheet resistance as low as 20 Ω sq‐1 and visible transmittance of 84% are demonstrated. It is shown that a ≈2 nm thick SnOx shell effectively protects the Ag NWs in a temperature range between 200 and 500 °C, whereas unprotected Ag NWs suddenly fail at temperatures beyond 200 °C. It is strongly anticipated that these improvements in the stability of Ag NWs open a large field of further investigations and applications.

Albert These

Papers

Advances in Lead-Free Perovskite Single Crystals: Fundamentals and Applications

Self-Healing Cs3Bi2Br3I6 Perovskite Wafers for X-Ray Detection

Surface versus Bulk Currents and Ionic Space-Charge Effects in CsPbBr3 Single Crystals

Characterization of Aerosol Deposited Cesium Lead Tribromide Perovskite Films on Interdigited ITO Electrodes

Overcoming Temperature‐Induced Degradation of Silver Nanowire Electrodes by an Ag@SnOx Core‐Shell Approach