Impact of White Light Illumination on the Electronic and Chemical Structures of Mixed Halide and Single Crystal Perovskites

Abstract

devices. Due to substantial research efforts, the power conversion efficiency of perovskite-based PV cells has been pushed beyond 20%. [3] The most widely studied materials are the triiodide CH 3 NH 3 PbI 3 and mixed halide CH 3 NH 3 PbI 3−x Cl x perovskites, which can be either solution processed [4] or thermally evaporated. [5] In particular, the mixed halide perovskites have been widely investigated due to their long carrier high diffusion length (over 1 µm [6] ) and low density of trap states, [7] making them especially efficient for charge generation and collection. Though substantial achievements for solar cells using solution processed CH 3 NH 3 PbI 3−x Cl x thin films have been reported, large variations in efficiency still persist among the devices even for nominally identical device configurations, [8,9] indicating significant differences in perovskite film properties. This is certainly to be related to the different preparation methods, e.g., one-step or twostep, [10] solvent treatment, [4] as well as the use of chlorine containing precursors, [11] which can greatly influence the film morphology and charge transport properties.Despite the rapid improvement in device performance, most of the fundamental questions regarding the physico-chemical properties of these materials remain to be unequivocally answered. In this context, the determination of the electronic structure and interface energetics with electron and hole transport layers, as well as contacts are crucial prerequisites to further device optimization.Photoelectron spectroscopy (PES) is the experimental technique of choice to investigate the core level and valence electronic structure of semiconductors. Particularly relevant parameters are the work function (φ), the ionization energy (IE), and the energy difference between the valence band maximum (VBM) and the Fermi level (E VBM − E f , where E f = 0 eV). To date, ultraviolet photoelectron spectroscopy (UPS) studies reporting the valence electronic properties of CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3−x Cl x thin films have shown a wide range of VBM binding energies. [12][13][14][15][16] In most UPS studies, E f was reported very close to (or even within) the conduction band edge, thus suggesting n-type character. The transport gap of these two perovskites was determined by Schulz et al. via combining UPS This study investigates the effect of white light illumination on the electronic and chemical properties of mixed halide perovskite (CH 3 NH 3 PbI 3−x Cl x ) thin films and CH 3 NH 3 PbI 3 single crystals using photoelectron and absorption spectroscopy. The pristine materials' surfaces are found to be n-type because of surface band bending due to the presence of donor levels, likely consisting of reduced lead (Pb 0 ) that acts as surface traps. When illuminating the sample with white light (up to 1 sun), the valence features shifted to lower binding energy due to surface photovoltage, i.e., the bulk of the materials is much less n-type. However, the surface photovoltage is only partially revers...

Exploring Red, Green, and Blue Light‐Activated Degradation of Perovskite Films and Solar Cells for Near Space Applications

“…The intensities of the characteristic peak at 14.25° that features (110) crystal panes of the perovskite structures gradually decrease, synchronously, the new peak at 12.65° appears. It suggests that the degradation of the perovskite films under RGB light illumination in vacuum leads to the presence of the cubic PbI 2 , which is in agreement with the previous reports . For blue light illumination, we note that there are additional peaks at 31.25° and 36.26° emerging that represent the (111) and (002) peaks of metallic Pb, pointing to a decomposition from PbI 2 to Pb, consistent with the XPS results.…”

Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells

“…34 Our results for (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 are therefore very different from those obtained in photoinduced PES studies of MAPbI 3 and MAPbI 3– x Cl x , where a degradation of the perovskite and formation of PbI 2 and much larger amounts of Pb 0 were observed. 37,38 In our measurements, we therefore observe a better stability of (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 than what others have observed for MAPbI 3 . At the same time, our results indicate a process linked to a phase separation.…”

Understanding Hydrogen Bonding Interactions in Crosslinked Methylammonium Lead Iodide Crystals: Towards Reducing Moisture and Light Degradation Pathways

“…[4] In the case of CH 3 NH 3 PbI 3 ,t he degradation reaction proceeds first by reversibly hydrating the perovskite crystal via Reaction (1). [4,42,43] ðCH 3 NH 3 Þ 4 PbI 6 Á 2H 2 OðsÞ!4CH 3 NH 3 IðaqÞþPbI 2 ðsÞþ2H 2 OðlÞ ð2Þ 4CH 3 NH 3 IðaqÞ!4CH 3 NH 2 ðaqÞþ4HIðaqÞð 3Þ When CH 3 NH 3 PbI 3 is exposed to UV light in the presence of oxygen and moisture,d egradation has been shown to be accelerated compared to degradation in dark conditions as per Reactions (4)- (7). [4,42,43] ðCH 3 NH 3 Þ 4 PbI 6 Á 2H 2 OðsÞ!4CH 3 NH 3 IðaqÞþPbI 2 ðsÞþ2H 2 OðlÞ ð2Þ 4CH 3 NH 3 IðaqÞ!4CH 3 NH 2 ðaqÞþ4HIðaqÞð 3Þ When CH 3 NH 3 PbI 3 is exposed to UV light in the presence of oxygen and moisture,d egradation has been shown to be accelerated compared to degradation in dark conditions as per Reactions (4)- (7).…”

“…[4][5][6][7][8] These approaches include modification of the perovskite structure via halide engineering, [9][10][11][12] cation substitution and the addition of dopants, [13][14][15][16][17] interface engineering, [18][19][20][21][22] and surface passivation [23] or encapsulation via small molecules or waterproof layers. [4][5][6][7][8] These approaches include modification of the perovskite structure via halide engineering, [9][10][11][12] cation substitution and the addition of dopants, [13][14][15][16][17] interface engineering, [18][19][20][21][22] and surface passivation [23] or encapsulation via small molecules or waterproof layers.…”