Impact of Ar Ion Beam Milling on Metal-Halide Perovskite Solar Cells

Abstract

Abstract Body: Metal-halide perovskite solar cells (PSCs) are the most promising photovoltaic (PV) technology owing to their cost-effective processing, tunable energy band-gaps, long carrier diffusion lengths, unique defect tolerance, and “self-healing” capabilities. Despite considerable efforts, the physical mechanisms for the inferior stability of PSCs are not well understood. Some researchers propose that structural defects are the main source of deterioration because they serve as a channel for moisture/oxygen stressors. Others claim that grain boundaries are benign, and the degradation is attributed to trapped charges in the perovskite absorber itself. A robust measurement approach that can characterize individual microstructures (e.g., grain, grain boundary, interface) is required. Focused ion beam (FIB) milling is a well-established sample preparation method that can remove the surface roughness of polycrystalline thin-films. An atomically-smooth surface allows local optoelectronic measurements of a microstructured PV with minimum artifacts from its innate rough surface. While extremely useful, high-energy ion beams (< 30 keV) irradiated on a hybrid organic-inorganic PSC may induce structural damage and/or chemical degradation. In this work, we investigate possible beam damage on PSCs prepared by focused argon (Ar) ion-beam at various doses. In addition, we use Monte-Carlo simulations to estimate the thickness of the damage layer. Our PSC devices were fabricated in a multilayer configuration: Au (60 nm) / Spiro-OMeTAD (220 nm) / MAPbI3 (550 nm) / MoOx (5 nm) / ITO (200 nm). Here, MA is methylammonium, and Spiro-OMeTAD stands for 2,2',7,7'-Tetrakis [N, N-di(4-methoxyphenyl) amino]-9,9'- spirobifluorene. We performed a series of Ar ion milling processes at room temperature (Fischione Model 1060), with the incident beam irradiated at shallow angles of 1° and 3°. The beam voltage was fixed at 4 kV, and the PSCs were milled for 5 min, 10 min, 15 min, and 20 min. At a first glance, we did not observe any notable changes with the samples milled at an incident beam angle of 1°. In contrast, the 3° milled PSCs showed color changes with an increase of milling time. We are currently working on quantitative EDS (Energy Dispersive X-Ray Spectroscopy) analysis to obtain the evolution of I/Pb ratio at different ion beam doses. It has been reported in the literature that the decrease of the I/Pb from 3 to a lower value indicates compositional deterioration (i.e., MAPbI3 is partially converted to PbI2). To gain a deep understanding into the interaction of the Ar ion beam with PSCs, we performed Monte-Carlo Simulations. In our model, 20,000 Ar ions at 4 keV were irradiated into MAPbI3 layer at a shallow angle of 3° and 30° from the surface. The estimated damage layer was calculated based on the substrate displacement density. Using a constant contour representing 5 % of the peak damage density, we obtained the damage depth of 8 nm and 13 nm for the incident Ar+ beam angle of and, respectively. The integrated analysis based on the simulations and the EDS measurements are in progress. Our results provide qualitative information of possible beam damage of PSCs that can occur during sample preparation. We will discuss the mitigation strategies to minimize the beam damage while characterizing the microstructural properties of PSCs. This work was supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the DE-FOA-0002064 program award number DE-EE0008985. The assistance of Utah Nanofab is also acknowledged.