Formation of a Secondary Phase in Thermally Evaporated MAPbI₃ and Its Effects on Solar Cell Performance

Abstract

Thermal evaporation is a promising deposition technique to scale up perovskite solar cells (PSCs) to large areas, but the lack of understanding of the mechanisms that lead to highquality evaporated methylammonium lead triiodide (MAPbI 3 ) films gives rise to devices with efficiencies lower than those obtained by spin coating. This work investigates the crystalline properties of MAPbI 3 deposited by the thermal coevaporation of PbI 2 and MAI, where the MAI evaporation rate is controlled by setting different temperatures for the MAI source and the PbI 2 deposition rate is controlled with a quartz crystal microbalance (QCM). Using grazing incident wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD), we identify the formation of a secondary orthorhombic phase (with a Pnma space group) that appears at MAI source temperatures below 155 °C. With synchrotron-based X-ray fluorescence (XRF) microscopy, we show that the changes in crystalline phases are not necessarily due to changes in stoichiometry. The films show a stochiometric composition when the MAI source is heated between 140 to 155 °C, and the samples become slightly MAI rich at 165 °C. Increasing the MAI temperature beyond 165 °C introduces an excess of MAI in the film, which promotes the formation of films with low crystallinity that contain low-dimensional perovskites. When they are incorporated in solar cells, the films deposited at 165 °C result in the champion power conversion efficiency, although the presence of a small amount of low-dimensional perovskite may lead to a lower open-circuit voltage. We hypothesize that the formation of secondary phases in evaporated films limits the performance of PSCs and that their formation can be suppressed by controlling the MAI source temperature, bringing the film toward a phase-pure tetragonal structure. Control of the phases during perovskite evaporation is therefore crucial to obtain high-performance solar cells.

Vapor transport deposition of metal-halide perovskites solar cells

“…They are inherently solvent-free and offer exquisite control over film composition and grain morphology, permitting additional material tunability. [9][10][11][12][13] While prior work has shown high efficiency using vapor deposition with high vacuum thermal evaporation (10 -6 ~10 -7 Torr), [14][15][16] the vapor transport deposition (VTD) approach of interest for this work uses an inert carrier gas to transport precursor vapor to a cooled substrate in low vacuum (0.1~10 Torr). VTD has been shown to realize deposition rates that are >10 times larger than high-vacuum based techniques.…”

Evolution of Structure and Optoelectronic Properties During Halide Perovskite Vapor Deposition

“…Research on the vapor-based deposition of perovskites is not predominantly focused on the crystallographic variations of the structure during growth, presumably due to the need for a custom-designed system. In most studies, the diffraction patterns are used to determine the crystal phases, i.e., tetragonal or cubic, while changing external parameters, such as the working pressure, precursor temperature, or substrate temperature. ,,− Here, we investigated the changes in the crystallographic structure during MAPbI 3 growth in a vacuum.…”

“…Numerous fabrication methods have been developed to prepare perovskite layers, with the solution-processed spin coating being the most commonly used. Although the spin coating is advantageous for basic laboratory research due to its rapid manufacturing processes and low equipment costs, it is not suitable for large-scale industrial/commercial production. − One of the reasons for this is a high material waste rate of over 90% . Furthermore, poor control of crystallinity and stoichiometry occurs in the case of the incomplete reaction of precursors during annealing, leading to poor batch-to-batch reproducibility …”

“…Since vapor deposition, requiring dedicated/specialized high-vacuum deposition chambers, is not an extensively utilized technique for perovskite fabrication, the majority of published work is focused on the specific deposition parameters and their influence on the perovskite layer quality. ,,− The main cause is the nontrivial evaporation of the organic precursor, typically MAI, that has a low sticking coefficient influencing its adsorption onto the substrate. , However, to the best of our knowledge, a comprehensive understanding of the perovskite evolution during vapor deposition in terms of morphology and electronic and structural properties is missing. To obtain an unbiased picture during perovskite formation, noninvasive in-situ experimental techniques need to be employed.…”

“…In conventional vacuum deposition, the layer thickness is typically deduced by monitoring the material deposited on the quartz crystal microbalance (QCM). However, this technique is not directly applicable to organic-based perovskites due to the volatile nature of the MAI precursor. ,,, However, the deposited layer thickness can be inferred from the intensity oscillations along the diffuse scattering rod in the q ⊥ direction. Figure e shows the oscillations along q ⊥ in time.…”