Recently, Pb-based organometal halide perovskite solar cells have passed 20 % power conversion efficiency (PCE). However, the main issue hampering commercialization is the amount of toxic lead used in this type of solar cells. For that reason, research is focused on Pb-free materials, especially Sn- or Bi-based perovskites. In literature, Sn-based perovskites are the most efficient alternative achieving maximum PCE of 6 %, but suffer from instantaneous degradation in ambient air. The more stable but less investigated counterpart is Bi-based perovskites.
Most efficient hybrid organic-inorganic Bi-based perovskite solar cells reported in literature reach PCE up to 0.2 % and consist of methylammonium bismuth iodide (MBI). Due to the air stability of Bi-based perovskites, it becomes possible to analyze effects of ambient conditions on the solar cell characteristics. In this work, we present efficient MBI perovskite solar cells which are stable in ambient air. The cells are processed by spin-coating under inert atmosphere of nitrogen, employing a standard non-inverted stack composed of compact and mesoporous TiO2, the perovskite layer and Spiro-MeOTAD sandwiched between FTO and evaporated gold contacts. Structural and morphological characterization of processed layers and devices are carried out by scanning electron microscopy. Photovoltaic measurements are performed under simulated AM1.5 sunlight of 100 mW/cm2 illumination. We highlight the influence of several concentrations of the perovskite solution (0.15 M - 0.3 M) on the photovoltaic characteristics. The highest PCE was achieved with 0.2 M yielding a maximum photocurrent of 0.56 mA/cm2. We observed that exposing the solar cell to ambient air is essential to obtain the largest short-circuit current and open-circuit voltage. The cells exhibit reproducible open-circuit voltages of 0.73 V, which is one of the highest value published for this type of solar cell. The PCE increases over time from 0.004 % directly after processing up to 0.17 % after 48 h. This is most likely caused by the oxidation process of Spiro-MeOTAD which strongly increases its conductivity, overcompensating a potential degradation of the MBI perovskite. Extending the exposure to ambient air leads to a slow decrease of photovoltaic performance. Our experiments show that after 300 h exposure to air, the cells still deliver 56 % of their maximum PCE and 84 % of their maximum open-circuit voltage.
In order to improve Bi-based perovskite solar cells, the use of mixed-cation compositions is beneficial to accomplish higher photocurrents and consequently cell efficiency. It is obvious that Bi-based perovskites, as high-bandgap materials, provide great potential in tandem devices and hybrid organic-inorganic solar cells to be a low-cost and environmentally friendly alternative for expensive inorganic solar panels.