Formation of a secondary phase in thermally evaporated MAPbI3 and its effects on solar cell performance
Andres Felipe Castro Mendez a, Juan-Pablo Correa-Baena a, Carlo Andrea Perini a, Junita Hidalgo a, Jacob Vagott a
a School of Materials Science and Engineering, Georgia Institute of Technology, North Avenue, Atlanta, United States
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Andres Felipe Castro Mendez, presentation 147
Publication date: 20th April 2022

Thermal evaporation is a promising deposition technique to scale up perovskite solar cells (PSCs) to large areas, but the lack of understanding of mechanisms that lead to high quality, thermally evaporated methylammonium lead triiodide (MAPbI3) films give rise to devices with lower efficiencies than those obtained by spin coating. This work investigates the crystalline properties of MAPbI3 deposited by thermal coevaporation of PbI2 and MAI, where the MAI deposition rate is controlled by setting different temperatures for the MAI source with a constant PbI2 deposition rate. Using grazing incident wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD) we identify the formation of a secondary phase – identified as orthorhombic MAPbI3 at MAI source temperatures below 155 °C. We find that the formation of this phase is limiting for the performance of PSCs and that the secondary phase formation can be suppressed using MAI source temperatures above 155 °C, which brings the film towards a pure tetragonal phase. Conversely, evaporating MAI at high temperatures results in films with low crystallinity. We show that forming the orthorhombic phase is possible by controlling the temperature we use during the evaporation process and that this phase is detrimental to solar cell performance in MAPbI3 perovskites. Control of the phases during perovskite evaporation is therefore crucial to obtain high-performance solar cells.

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