Organometal halide perovskite-based solar cells have recently been reported to be highly efficien. However, much of the fundamental physical and chemical properties underlying this performance have remained unknown.
In this work, we report on our recent studies on the effects of film processing conditions (e.g., solvent and temperature), film thickness and alternative top contacts on charge transport, recombination, and device characteristics of perovskite-based solar cells.
Particularly, we will present our newly developed one-step solution approach to prepare perovskite CH3NH3PbI3 on a mesoporous TiO2 film or on a planar, compact TiO2 layer on FTO. In this new synthetic approach, CH3NH3Cl is added to the standard CH3NH3PbI3 precursor (equimolar mixture of CH3NH3I and PbI2) solution to adjust the crystallization process for CH3NH3PbI3. Depending on the amount of MACl used in the precursor solution and the annealing temperature, the optimum crystallization time for forming pure CH3NH3PbI3 with the strongest absorption varies from a few minutes to several tens of minutes. The use of MACl not only leads to enhanced absorption of CH3NH3PbI3 but also improves significantly coverage of CH3NH3PbI3 on a planar substrate.
Moreover, pushing our device optimization and better understanding of its operation, we examined charge transport, recombination, and device characteristics of solid-state mesostructured perovskite CH3NH3PbI3 solar cells based on 0.24 to 1.65 μm thick TiO2 films and spiro-MeOTAD hole conductor. Charge transport and recombination in the solid-state mesostructured perovskite cells are similar to those in the solid-state DSSC and exhibit little dependence on the TiO2 film thickness. The performance of perovskite cells increases with TiO2 film thickness up to 650−850 nm, resulting primarily from the enhanced light harvesting. Further increasing film thickness results in lower cell efficiencies, mainly caused by the reduced FF or Jsc. The electron diffusion length in mesostructured perovskite cells is found to be longer than 1 μm under normal cell operation conditions.
Finally, we will demonstrate the effectiveness of using a combination of a thin layer of molybdenum oxide and aluminum as the top-contact structure for extracting photogenerated holes from perovskite solar cells, instead of nobel metals such as Au or Ag. The device performance of perovskite solar cells using a MoOx/Al top contact is comparable to that of cells using the standard Ag top contact. Impedance measurements suggest that the extraction of photogenerated holes is not affected by the MoOx/metal interface when proper MoOx thickness is used. Using a thicker (20-nm) MoOx layer leads to decreased cell performance resulting primarily from a reduced fill factor.
We will discuss our result in terms of device performance in order to serve as guide toward highly efficiency cells. These results have direct implications for device manufacturing and upscaling issues.