Probing local photocurrent in inorganic CsPbBr3 perovskite films by scanning probe microscopy

Hybrid organic-inorganic halide perovskites are among the most promising materials for solar cells. Rapid growth of their power conversion efficiency (PCE) from 3.8% in 2009 [1] to more than 22% in 2017 [2] made them attractive for commercial applications. However, rapid degradation of organic-inorganic perovskites under operating conditions – photon flux, electric field, heating – remains unsolved in spite of tremendous research effort [3,4]. Their fully inorganic counterparts possess similar optoelectronic properties [5] and better stability [6]. Still, their PCE is below 10%, which requires further investigation and optimization

XPS SPECTRA AS A TOOL FOR STUDYING PHOTOCHEMICAL AND THERMAL DEGRADATION IN APbX3 HYBRID HALIDE PEROVSKITES

Our results show that the resistance of hybrid lead halide perovskites to stress factors (light or heat) is gradually increased while substituting the A-site cation going from CH3NH3+ to CH(NH2)2+ and then to Cs+.This study was supported by Russian Foundation for Basic Research (grants 20-24-660003 and 21-52-52002)

Influence of Oxygen Ion Migration from Substrates on Photochemical Degradation of CH3NH3PbI3 Hybrid Perovskite

Measurements of XPS survey, core levels (N 1s, O 1s, Pb 4f, I 3d), and valence band (VB) spectra of CH3NH3PbI3 (MAPbI3) hybrid perovskite prepared on different substrates (glass, indium tin oxide (ITO), and TiO2) aged under different light-soaking conditions at room temperature are presented. The results reveal that the photochemical stability of MAPbI3 depends on the type of substrate and gradually decreases when glass is replaced by ITO and TiO2. Also, the degradation upon exposure to visible light is accompanied by the formation of MAI, PbI2, and Pb0 products as shown by XPS core levels spectra. According to XPS O 1s and VB spectra measurements, this degradation process is superimposed on the partial oxidation of lead atoms in ITO/MAPbI3 and TiO2/MAPbI3, for which Pb–O bonds are formed due to the diffusion of the oxygen ions from the substrates. This unexpected interaction leads to additional photochemical degradation. © 2021 by the author. Licensee MDPI, Basel, Switzerland.Funding: The sample preparation, aging experiments, UV‐vis, XRD, and SEM characterization were supported by Russian Science Foundation (project No. 19‐73‐30020). The XPS measurements were supported by the Ministry of Education and Science of the Russian Federation (project FEUZ‐2020‐0060), Theme ‘Electron’, no. AAAA‐A18‐118020190098‐5 and Russian Foundation for Basic Research (projects No. 21‐52‐52002/21 and 20‐42‐660003). C.‐C.C. acknowledges the financial support from the Ministry of Science and Technology in Taiwan (MOST 110‐2923‐E‐002‐007‐MY3) and the Top University Project from National Taiwan University (110L7836 and 110L7726)

Temperature Dependence of Photochemical Degradation of MAPbBr3 Perovskite

The experimental results of X-ray diffraction (XRD), optical absorbance, scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS) of the core levels and valence bands of MAPbBr3 (MA-CH3NH3+) perovskite before and after exposure to visible light for 700 h at temperatures of 10 and 60 °C are presented. It reveals that the light soaking at 60 °C induces the decomposition of MAPbBr3 perovskite accompanied with the decay of organic cation and the release of a PbBr2 phase as a degradation product whereas the photochemical degradation completely disappears while the aging temperature is decreased to 10 °C. © 2022 by the authors.Russian Foundation for Basic Research, РФФИ: 21-52-52002; Ministry of Education and Science of the Russian Federation, Minobrnauka: AAAA-A18–118020190098-5, FEUZ 2020-0060; Ministry of Science and Technology, Taiwan, MOST: 110-2923-E-002-007-MY3; Russian Science Foundation, RSF: 19-73-30020I.S.Z. and A.I.K. thank the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program) for support.This work was supported by the Russian Science Foundation (Project 19-73-30020) at IPCP RAS. The XPS measurements were supported by the Ministry of Science and Higher Education of the Russian Federation under the theme “Electron” No. AAAA-A18–118020190098-5 and Project FEUZ 2020-0060 as well as the Russian Foundation for Basic Research (Project No. 21-52-52002). The research fundings from the Ministry of Science and Technology in Taiwan (MOST 110-2923-E-002-007-MY3) are gratefully acknowledged

Influence of Oxygen Ion Migration from Substrates on Photochemical Degradation of CH3NH3PbI3 Hybrid Perovskite

Measurements of XPS survey, core levels (N 1s, O 1s, Pb 4f, I 3d), and valence band (VB) spectra of CH3NH3PbI3 (MAPbI3) hybrid perovskite prepared on different substrates (glass, indium tin oxide (ITO), and TiO2) aged under different light-soaking conditions at room temperature are presented. The results reveal that the photochemical stability of MAPbI3 depends on the type of substrate and gradually decreases when glass is replaced by ITO and TiO2. Also, the degradation upon exposure to visible light is accompanied by the formation of MAI, PbI2, and Pb0 products as shown by XPS core levels spectra. According to XPS O 1s and VB spectra measurements, this degradation process is superimposed on the partial oxidation of lead atoms in ITO/MAPbI3 and TiO2/MAPbI3, for which Pb–O bonds are formed due to the diffusion of the oxygen ions from the substrates. This unexpected interaction leads to additional photochemical degradation

XPS evidence of degradation mechanism in CH3NH3PbI3 hybrid perovskite

In this study, we investigate the photo-/thermal degradation mechanism of hybrid perovskites by using x-ray photoelectron (XPS) valence band (VB) spectra coupling with density functional theory (DFT) calculations. Herein, CH3NH3PbI3 is respectively subjected to irradiation with visible light and annealing at an exposure of 0-1000 h. It is found from XPS survey spectra that, in both cases (irradiation and annealing), a decrease in the I:Pb ratio is observed with aging time, which unambiguously indicates the formation of PbI2 as the product of photo/thermal degradation. The comparison of the XPS VB spectra of irradiated and annealed perovskites with the DFT calculations of CH3NH3PbI3 and PbI2 compounds have showed a systematic decrease in the contribution of I-5p states, which allows us to determine the respective threshold for degradation, which is 500 h for light irradiation and 200 h for annealing. This discrepancy might be due to the fact that the relaxation of thermal excitations of the system is carried out only by the phonons (which are non-radiative physical processes) while the radiative processes occurred during the photoexcitation will elastically or inelastically divert part of the external energy from the system to reduce its impact on perovskite degradation. © 2019 IOP Publishing Ltd

Reversible and Irreversible Electric Field Induced Morphological and Interfacial Transformations of Hybrid Lead Iodide Perovskites

We report reversible and irreversible strain effects and interfacial atomic mixing in MAPbI₃/ITO under influence of external electric bias and photoillumination. Using conductive-probe atomic force microscopy, we locally applied a bias voltage between the MAPbI₃/ITO and the conductive tip and observed local dynamic strain effects and current under conditions of forward bias. We found that the reversible part of the strain is associated with a current spike at the current onset stage and can therefore be related to an electrochemical process accompanied by local molar volume change. Similar partly reversible surface deformation was observed when the tip–sample contact was illuminated by light. Time-of-flight secondary ion mass spectrometry of electrically biased regions revealed massive atomic mixing at the buried MAPbI₃/ITO interface, while the top MAPbI₃ surface, subjected to strong morphological damage at the tip–surface contact, revealed less significant chemical decomposition

Probing the Intrinsic Thermal and Photochemical Stability of Hybrid and Inorganic Lead Halide Perovskites

We report a careful and systematic study of thermal and photochemical degradation of a series of complex haloplumbates APbX₃ (X = I, Br) with hybrid organic (A⁺ = CH₃NH₃) and inorganic (A⁺ = Cs⁺) cations under anoxic conditions (i.e., without exposure to oxygen and moisture by testing in an inert glovebox environment). We show that the most common hybrid materials (e.g., MAPbI₃) are intrinsically unstable with respect to the heat- and light-induced stress and, therefore, can hardly sustain the real solar cell operation conditions. On the contrary, the cesium-based all-inorganic complex lead halides revealed far superior stability and, therefore, provide an impetus for creation of highly efficient and stable perovskite solar cells that can potentially achieve pragmatic operational benchmarks