2, AIXTRON SE, Herzogenrath, , Germany
Meanwhile, the efficiency of Pb-based perovskite solar cells is comparable to that of silicon photovoltaics. However, upscaling devices for production is still a challenge. In recent studies, chemical vapor deposition (CVD) of perovskite solar cells has been recognized as an alternative to the widely used solution processing techniques leading to homogeneous perovskite layers on large areas. Moreover, increasing concern is related to the toxicity of Pb. Therefore, less toxic alternatives like Sn- or Bi-based perovskites are being explored. Compared to Sn perovskite solar cells, Bi-based devices showed a great stability over days of storage in ambient conditions, whereas power conversion efficiency (PCE) is still much lower. Additionally, halide compounds of Bi feature significantly higher vapor pressures and thus are more suitable for CVD compared to their Pb halide counterparts. Without the need for orthogonal solvents, CVD Bi-based perovskites also offer a great potential as high-bandgap material in tandem devices.
In this work, CVD of methylammonium bismuth iodide employing N2 as carrier gas is studied. In the pressure regime of 10 hPa, methylammonium iodide (MAI) is heated up to 150 °C whereas BiI3 is heated to 250 °C. The evaporation of both precursors is controlled via the carrier gas flux through each source. To avoid parasitic gas phase prereactions of both precursors, perovskite films are formed by alternating CVD processes. Here BiI3 and MAI are consecutively deposited over several cycles with time lengths varied between 600 and 3600 s. Single-layer depositions on titanium oxide coated FTO substrates are carried out to investigate film formation and thicknesses. Finally, complete solar cells employing an additional solution-processed Spiro-MeOTAD film with evaporated gold as top electrode were fabricated. SEM measurements are used to investigate the effect of the process parameters. The crystal structure and porosity of the perovskite layer depend strongly on the deposition rate of the perovskite and the substrate temperature. It was found that substrate temperatures in the range of 60°C to 80°C are needed to achieve an in-situ perovskite formation. IV measurements show that CVD results in significantly denser layers with reduced parasitic series resistance and improved fill factors as compared to solution-processed reference layers. Overall, a maximum PCE of 0.03% and an open-circuit voltage of 0.7 V could be achieved with vapor-deposited methylammonium bismuth iodide which is comparable to those of solution-processed solar cells. Cross-section SEM images reveal that the vapor-deposited perovskite poorly penetrates the mesoporous titanium dioxide structure and show adhesion defects at the surface resulting in relatively low currents. Possibilities for future improvements are choosing planar cell geometries, introducing a vapor-deposited hole transport layer and further process optimizations.