FAPbI₃ Perovskite Solar Cells—Thermal Evaporation vs Solution Processing

Thermal evaporation is an upscalable vacuum deposition technique for fabricating perovskite solar cells (PSCs). The advantages of thermal evaporation include better control of film thickness and reproducibility as well as conformal coating over large substrate areas which could be better suited for multi-junction solar cell fabrication. Furthermore, various thermal sources from inorganic to organic, for example PbI₂, PbBr₂, SnI₂, CsBr, FAI (formamidinium iodide), and MAI (methylammonium iodide) can be used allowing maximum compositional control and flexibility during the solvent-less deposition. To date, evaporation has been far less explored versus more conventional solution processing techniques¹.<br/><br/>The FAPbI₃ perovskite composition has attracted much attention given its optimal bandgap for single-junction PSCs. In this study, we compare the properties of thermally co-evaporated FAPbI₃ (PbI₂ and FAI sources) versus solution processed spin-coated FAPbI₃, by characterizing both pristine FAPbI₃ films and solar cells. We examine chemical properties such as PbI₂ to FAI ratio, structural properties such as the perovskite crystallization conditions, and device architectures comprising different electron/hole transport layers and contacts. In literature, cation (Cs⁺ and MA⁺) or other additives are typically used to improve the properties of the FAPbI₃ perovskite allowing higher efficiency devices and long-term stability as a result of octahedral tilting in their crystal structure².<br/><br/>Interestingly, scanning electron diffraction (SED) results show that thermally co-evaporated FAPbI₃ films exhibit some inherent octahedral tilt in their crystal structure, which results in the slower conversion to the non-perovskite yellow hexagonal δ-phase in air over time. This intrinsic thermodynamic conversion occurs faster in our solution processed pristine FAPbI₃ films as shown through various characterization techniques, which could be due to the different crystallization process or effects of residual solvents. This unwanted conversion of FAPbI₃ is exacerbated in the presence of moisture, heat, and light as shown widely in literature³.<br/><br/>Currently, solution processed FAPbI₃ solar cells have achieved much higher efficiency (record 25.7%⁴) compared to thermally evaporated FAPbI₃ devices (&lt;21%⁵^,⁶). This is largely due to the fact that vastly more time and effort has been invested in optimizing solution processed PSCs, compared to thermal evaporation. Our work suggests that thermal evaporation could have a marked advantage over solution processing of FAPbI₃ based PSCs and will attract other researchers to further optimize thermally evaporated PSCs. If performance similar to solution processed methods can be achieved, the better scalability will make thermal evaporation a promising route to commercializing PSCs.<br/><br/><u>References</u><br/>1. Vaynzof, Y. The Future of Perovskite Photovoltaics — Thermal Evaporation or Solution Processing ? <b>2003073</b>, (2020).<br/>2. Doherty, T. A. S. <i>et al.</i> Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases. <i>Science (80-. ).</i> <b>374</b>, 1598–1605 (2021).<br/>3. Liu, Z. <i>et al.</i> Efficient and Stable FA-Rich Perovskite Photovoltaics: From Material Properties to Device Optimization. <i>Adv. Energy Mater.</i> <b>12</b>, 1–35 (2022).<br/>4. NREL. NREL Efficiency Chart. <i>Chart, Best Research-Cell Efficiency</i> https://www.nrel.gov/pv/cell-efficiency.html (2019).<br/>5. Roß, M. <i>et al.</i> Co-Evaporated Formamidinium Lead Iodide Based Perovskites with 1000 h Constant Stability for Fully Textured Monolithic Perovskite/Silicon Tandem Solar Cells. <i>Adv. Energy Mater.</i> <b>11</b>, (2021).<br/>6. Borchert, J. <i>et al.</i> Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells. <i>ACS Energy Lett.</i> <b>2</b>, 2799–2804 (2017).