A newly developed lithium cobalt oxide super hydrophilic film for large area, thermally stable and highly efficient inverted perovskite solar cells

A new inorganic hole transporting layer, a sputtering made LiCoO₂ film, was developed and used in an inverted perovskite solar cell (PSC) and sub-module (PSM). The LiCoO₂ film prepared by RF magnetron sputtering is composed of nano-sized particles and a superhydrophilic surface after treating it with UV-ozone, and therefore can be wetted evenly with a (MAI + PbI₂)/(DMF + DMSO) precursor solution. By applying chlorobenzene as an anti-solvent, a very dense film with big perovskite grains was formed. After depositing the C₆₀ electron transporting layer, BCP hole blocking layer and Ag electrode, the best perovskite solar cell achieves a power conversion efficiency (PCE) of 19% with negligible current hysteresis. The high-efficiency cell is stable up to 90 °C in the inert atmosphere without encapsulation, and the PCE only decreases by 2% when the cell was heated at 100 °C for 30 minutes. When the cell was heated at 100 °C for 5 days, the PCE decreases by only 40%; nevertheless, under the same heating conditions, the efficiency of the PSC based on the PEDOT:PSS HTL is lost totally. The superhydrophilic surface of LiCoO₂ made the even wetting of the large surface area with the perovskite precursor solution possible. Therefore the perovskite solar sub-module with an active area of 25.2 cm² (on a 10 cm × 10 cm substrate) can be fabricated to achieve a power conversion efficiency of 16% which was further verified to be 15%. The high-efficiency sub-module based on the LiCoO₂ HTL also shows good thermal stability, and ca. 10% of the efficiency was lost by heating at 100 °C for 30 minutes. The development of new inorganic hole transporting layers for large area, thermally stable and highly efficient perovskite solar sub-modules closes the gap for their near-future market exploitation.