Graphene-Perovskite Schottky Barrier Solar Cells

Perovskite solar cells have attained incredible power conversion efficiencies but it is still unclear whether photogenerated carriers are free or excitonic in nature. Originally, it is believed that they are exciton-based devices, similar to organic or dye-sensitized solar cells. However, the emergence of efficient planar devices as well as measurements of exciton binding energy in the range of 10-100 meV suggest that they may be free carrier-based. In this work, the free carrier model is confirmed by building graphene/perovskite Schottky barrier solar cells, analogous to conventional metal/semiconductor Schottky barrier solar cells. To address the challenges of building such devices, solution-processing techniques are extensively investigated for depositing perovskite films directly onto graphene in order to obtain an intimate contact between the graphene and perovskite. Interestingly, these graphene/perovskite Schottky barrier devices have reasonably good efficiencyup to 10.6%and short circuit current densities only slightly lower than control devices. Furthermore, devices with neither a hole transport layer nor an electron transport layers have power conversion efficiencies of up to 6%. These results provide convincing evidence supporting the free carrier model for methylammonium lead iodide perovskites and offer insights on potential alternative designs for perovskite solar cells.