Deposition of 2D Cs₃Bi₂Br₃I₆ Perovskites—Thermal Evaporation vs Spin Coating

Halide perovskites have gained significant attention in the scientific community due to their remarkable potential in photovoltaic applications. These materials are known for their exceptional properties, including high absorption coefficients, defect tolerance, long diffusion lengths, and long carrier lifetimes, which contribute to the high efficiencies observed in solar cells. However, conventional halide perovskites face significant challenges such as instability when exposed to air, humidity, or heat, and the toxicity associated with their lead content.<br/><br/>Efforts to develop non-toxic, lead-free alternatives have primarily focused on tin-based systems, with relatively less research dedicated to bismuth and antimony-based systems. In this work, we present for the first time the co-evaporation of cesium bromide (CsBr) and bismuth iodide (BiI₃) to form the mixed halide perovskite system Cs₃Bi₂Br₃I₆ (CBBI). CBBI is of great interest, since it possesses the 2D phase and similar band gap energies as its pure-iodide derivative Cs₃Bi₂I₉ (CBI), which mainly crystallizes in the 0D phase. The higher dimensionality is beneficial for charge carrier properties. Our deposition method is compared to the more conventional spin-coating technique.<br/><br/>For the thermally evaporated films, an ex-situ method using atomic force microscopy (AFM) analysis of the precursor films was employed to determine and adjust the evaporation rate of the precursors, ensuring an accurate stoichiometric ratio satisfying the Cs:Br ratio. Thermally evaporated films were characterized and compared to spin-coated films using a variety of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy. XRD and Raman spectroscopy confirmed the successful deposition of the thermally evaporated films. XRD data indicated that these films do not possess a large degree of preferential orientation, whereas Raman data suggested that the 2D phase is the main structure present, with some 0D phase also detected. SEM and UV-vis data revealed morphological and optical differences between the two types of films.<br/><br/>Furthermore, we constructed solar cells based on a planar architecture. Incorporating compact TiO₂ (c-TiO₂) as the electron transport layer and spiro-OMeTAD as the hole transport layer, we observed highest efficiencies when solution-processed CBBI was deposited on mesoporus TiO₂. For the thermally evaporated CBBI films, we find that phenyl-C61-butyric acid methyl ester (PCBM) between c-TiO₂ and the active layer is more suitable. Moreover, for the thermally evaporated CBBI films, the inclusion of a thin 7.5 nm layer of MoO_x between the hole transport layer and the Au electrode led to increased device efficiency. The thermally evaporated films deposited on different surfaces exhibited significantly larger grain sizes, indicating thermal evaporation to be a more attractive method for photovoltaic applications.<br/><br/>The field of thermal evaporation of lead-free perovskite-inspired systems has been notably underexplored. Our work addresses this gap by presenting comprehensive results from the thermal evaporation of these systems.