Perovskite solar cells have shown a rapid increase of performance and overcome the threshold of 20% power conversion efficiency (PCE). The main issues hampering commercialization are the lack of deposition methods for large areas, missing long-term device stability and the toxicity of the commonly used Pb-based compounds. In this work, we present a novel chemical vapor deposition (CVD) process for Pb-free air-stable methylammonium bismuth iodide (MBI) layers, which enables large-area production employing close-coupled showerhead technology. We demonstrate the influence of precursor rates on the layer morphology as well as on the optical and crystallographic properties. The impact of substrate temperature and layer thickness on the morphology of MBI crystallites is discussed. We obtain smooth layers with lateral crystallite sizes up to 500 nm. Moreover, the application of CVD-processed MBI layers in non-inverted perovskite solar cells is presented.

Recently, Pb-based organometal halide perovskite solar cells have passed 20% power conversion efficiency (PCE). However, the main issue hampering commercialization is toxic Pb contained in these cells. Therefore, great attention is devoted to replace Pb by less harmful metals such as Bi. Yet, the most efficient methylammonium bismuth iodide (MBI) perovskite solar cells reported in literature reach PCE up to 0.2%. In this work, MBI perovskite solar cells, which are processed by spin-coating under inert nitrogen atmosphere, employing a standard non-inverted stack are presented. The control of perovskite morphology by modifying the process has been highlighted and the impact on photovoltaic (PV) characteristics has been shown. It is observed that the concentration of the perovskite solution (0.15-0.30 M) has a huge impact on the crystallite size, and the rotation speed during the spin-coating process determines the layer coverage. Exposure of MBI solar cells to ambient air is found to be essential to obtain the highest short-circuit current and open-circuit voltage. The PCE increases over time, from 0.004% directly after processing up to 0.17% after 48 h. The fabricated cells exhibit an open-circuit voltage of 0.72 V, which is the highest value published for this type of solar cell. Experimental SectionUnless otherwise noted, materials were purchased from Sigma-Aldrich and used as received without further purification.The device layout of the processed perovskite solar cells is shown in Figure 1. First, F-doped SnO 2 (FTO)-on-glass substrates (6-9 Ω/sq, VisionTek Systems Ltd.) were cleaned thoroughly with dimethyl sulfoxide (DMSO), acetone, isopropyl alcohol, and deionized water (DI) and finally dried with nitrogen.

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