Spray-Coated MoO₃ Hole Transport Layer for Inverted Organic Photovoltaics

This study focuses on the hole transport layer of molybdenum trioxide (MoO₃) for inverted bulk heterojunction (BHJ) organic photovoltaics (OPVs), which were fabricated using a combination of a spray coating and low-temperature annealing process as an alternative to the thermal evaporation process. To achieve a good coating quality of the sprayed film, the solvent used for solution-processed MoO₃ (S-MoO₃) should be well prepared. Isopropanol (IPA) is added to the as-prepared S-MoO₃ solution to control its concentration. MoO₃ solutions at concentrations of 5 mg/mL and 1 mg/mL were used for the spray coating process. The power conversion efficiency (PCE) depends on the concentration of the MoO₃ solution and the spray coating process parameters of the MoO₃ film, such as flow flux, spray cycles, and film thickness. The results of devices fabricated from solution-processed MoO₃ with various spray fluxes show a lower PCE than that based on thermally evaporated MoO₃ (T-MoO₃) due to a limiting FF, which gradually increases with decreasing spray cycles. The highest PCE of 2.8% can be achieved with a 1 mg/mL concentration of MoO₃ solution at the sprayed flux of 0.2 mL/min sprayed for one cycle. Additionally, S-MoO₃ demonstrates excellent stability. Even without any encapsulation, OPVs can retain 90% of their initial PCE after 1300 h in a nitrogen-filled glove box and under ambient air conditions. The stability of OPVs without any encapsulation still has 90% of its initial PCE after 1300 h in a nitrogen-filled glove box and under air conditions. The results represent an evaluation of the feasibility of solution-processed HTL, which could be employed for a large-area mass production method.