Chemical and Electronic Structure Characterization of Lead Halide Perovskites and Stability Behavior under Different Exposures—A Photoelectron Spectroscopy Investigation

Abstract

The past few years, two perovskite materials have attracted much attention in the solar cell community: CH3NH3PbI3 and CH3NH3PbI3–x Cl x . While these materials are usually characterized using their structure (via X-ray diffraction (XRD)) and performance within solar cell communities, not so much attention has been devoted to their surface chemical composition and, specifically, the surface composition. Photoelectron spectroscopy (PES) can easily fulfill this task, and, in addition to chemical information, PES provides an overall picture of the electronic structure of the perovskite and its relation to mesoporous TiO2 when studied with hard X-rays. In this work, CH3NH3PbI3 and CH3NH3PbI3–x Cl x have been compared with each other and also to CH3NH3PbCl3, and it appears that, despite very different morphologies and kinetics of formation, the two former materials present a very similar electronic structure and chemical composition (i.e., no chlorine is observed in the final CH3NH3PbI3–x Cl x materials). Nevertheless, chlorine is very important during the preparation, because it affects the formation of crystalline CH3NH3PbI3. We have also exposed the classical CH3NH3PbI3 to various environments, such as water, temperature, and long-time storage in air and argon, and followed changes of the surface composition with PES. The main result of the different exposures is that the perovskite is decomposed into PbI2, but an important point is that this degradation seems to occur already at 100 °C and is not only related to large humidity. Indeed, even in an inert atmosphere such as argon, a slow degradation to PbI2 is observed. The results obtained are crucial for a better understanding of this material and will help to improve not only the post-conditioning of the cells but also their synthesis.

Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of opto-electronic properties and photovoltaic performance

“…11 By comparison, MAPbCl 3 possesses a large bandgap of 3.1 eV. 16 In mixed halide preparations where chloride-based precursors are used, however, no signicant inclusion of chloride is observed in the perovskite crystal structure. Instead it appears either as separate MAPbCl 3 crystals, 17,18 or it disappears upon annealing.…”

Long term stability of air processed inkjet infiltrated carbon-based printed perovskite solar cells under intense ultra-violet light soaking

“…1,2 Despite exhibiting higher efficiencies, the biggest challenge associated with the traditional geometry of PSCs 3-5 is the lack of long term device stability. [6][7][8][9][10][11] The main cause reported for their degradation is the solubility of the perovskite light absorbing layer in many solvents including water except a few unipolar ones rendering the cell operation in the traditional geometry highly sensitive to moisture intrusion and humidity. 6,7 Therefore several types of new materials, strategies and device geometries have been introduced to address the aforementioned issues.…”

Pathway to oxide photovoltaics via band-structure engineering of SnO

“…13,18 In fact, it is now recognized as an important factor 21,22 in the stunning efficiencies achieved within a very short time scale of development in perovskite photovoltaics based on methyl-ammonium lead iodide (MAPbI 3 ). 23 Due to stability issues in these perovskites, 24 it would be highly desirable to transfer this design principle to the realm of oxide photovoltaics. Since, in SnO, the Sn 5p like conduction band is also highly dispersive 16 with a small electron effective mass of about 0.4 m 0 , p-type SnO could support long minority lifetimes, which is one of the most important metrics for PV application.…”