Rapid progress has been achieved in perovskite solar cells, improving the efficiency from 3.8 % to 25.7 % in less than a decade. However, the stability of perovskites still need to be improved before commercialization. This study reports the thermal stability of perovskites exposed to an ion beam irradiation. Such combined stressors are seen in atomic/nanoscale microscopy, where a perovskite lamella is characterized using a controlled heating/cooling stage. Focused ion beams (FIBs) are frequently used to section perovskites of interest. Previous studies proposed that high-energy electron beams could cause unexpectedly fast thermal degradation. Alternatively, the perovskite surface may be already altered during FIB processes, accelerating the deterioration. We use a grazing angle argon ion (Ar+) beam directly irradiated on methyl-ammonium lead iodide (MAPbI3) to test the impact of ion beams to degradation mechanisms. This perovskite film was spin-coated on a stack of glass / FTO (fluorine-doped tin oxide; 200 nm) / SnO2 (35 nm). A 4 kV Ar+ ion beam was injected into a part of the MAPbI3 sample at an angle of 3° for 10 minutes. A strong photoluminescence (PL) peak was observed near 765 nm (1.62 eV) for both samples, attributed to the bandgap emission. However, the PL intensity decreased by 40 % after the Ar+ polishing. A finite difference time domain (FDTD) simulation estimates an approximately 40 % of the PL excitation (244 nm laser) can be absorbed within ≈15 nm of the MAPbI3 subsurface. This result agrees with the ion-beam penetration depth, calculated by Monte Carlo simulations. The PL analysis and the numerical simulations indicate that the 4 kV ion beam irradiation at an incident angle of 3° introduces an optically inactive “dead layer (≈15 nm)” on MAPbI3. We performed x-ray photoemission spectroscopy (XPS) to investigate possible surface compositional changes of MAPbI3 under Ar+ polishing. Quantitative XPS analysis shows an apparent reduction of the organic components (e.g., C, O, and N) after the ion beam irradiation. In contrast, XPS signals of the inorganic species (e.g., Pb, I) significantly increase with introducing the doublet metallic Pb (0) near the bonding energy of 137 eV and 142 eV. Overall, the I/Pb ratio decreases from 2.6 (pristine) to 2.1 (polished), suggesting the ion beam irradiation modifies the subsurface of MAPbI3 to be a lead-rich, iodine-deficient layer (< 10 nm). The impact of the modified subsurface of MAPbI3 on thermal deterioration was qualitatively investigated using a heat lamp at 150 °C. Owing to the partial surface decomposition, we observed the ion-beam exposed MAPbI3 shows a faster decay than the control under the heating. Apparent deterioration of MAPbI3 (yellow color) on the Ar+ polished section after applying the heating stressor for 20 minutes. At this stage, the color of the pristine area remains unchanged (dark-brown color). The deterioration of the pristine area began at around 40 minutes. Both sample sections fully deteriorated at 70 minutes. Our experiment confirms that subsurface modification (≈15 nm) can significantly influence the deterioration mechanisms of MAPbI3. This talk will discuss a new method of monitoring the deterioration of perovskites in real-time. A prototype design consists of a patch antenna (≈ 3 cm × 4 cm) and an array of four MAPbI3 solar cells (1 cm × 1 cm). The distance of the adjacent solar cells is set to 1.4 cm in width and 0.6 cm in length. Computer Simulation Technology (CST) simulations display the resonance frequency of the patch antenna changes from 7.13 GHz to 7.07 GHz as the MAPbI3 film decomposes to volatile species and PbI2. While the shift is small, our pilot design demonstrates the feasibility of wireless monitoring the deterioration of perovskite solar cells. Tailoring antenna design and experimental validations are in progress.

Monitoring Stability of Metal-Halide Perovskites Under Combined Stressors of Ion-Beam and Heat

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.

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

L: Organic and Hybrid Materials for Optoelectronic Devices

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

EMC 2023

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Z: Solar Cell Materials and Devices

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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Yu-Lin Hsu

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Monitoring Stability of Metal-Halide Perovskites Under Combined Stressors of Ion-Beam and Heat

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

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