Layer-by-Layer Growth of CH_3NH_3PbI_(3-x)Cl_xfor Highly Efficient Planar Heterojunction Perovskite Solar Cells

摘要

Owing to their broad spectral absorption, high charge-carrier mobility, small exciton binding energy (≈50 meV), and long exciton diffusion length, the organic-inorganic hybrid methylammonium lead halide perovskites (e.g., CH_3NH_3PbX_3, X = Cl, Br, I), have recently attracted enormous attention for thin-film photovoltaics. High-performance perovskite solar cells in both mesoporous scaffold and planar heterojunction architectures have been reported to show power conversion efficiencies (PCEs) over 19% with some dedicated energy-level engineering. Since conventional spin-casting has been currently used to form the perovskite active layer from a mixture of PbX_2 and CH_3NH_3X in a common solvent (e.g., dimethylformamide (DMF)), the poor solubility of PbX_2 usually leads to large morphological variations (even with pinholes) for the solution-cast perovskite layer, and hence unstable device performance. Despite the recent rapid increase in PCEs of various perovskite solar cells, therefore, morphological control is a bottleneck in further development of the state-of-the-art perovskite solar cells. In this context, a sequential deposition method has been recently reported for the formation of the perovskite by first introducing lead iodide (PbI_2) into a nanoporous titanium dioxide film, followed by exposing it to a solution of CH_3NH_3I to produce the perovskite layer without the uncontrollable precipitation of the perovskite in the casting solution. Although this two-step method permitted a much better control over the perovskite morphology (and hence a high PCE of approximately 15%) with respect to the one-step solution casting, it is still difficult to control the morphology of PbI_2 film made from solution casting due to its intrinsically poor solubility, and the porous metal oxide film is needed for the surface-assisted complete transformation of PbI_2 and CH_3NH_3I into the perovskite layer.