Wetting-induced formation of void-free metal halide perovskite films by green ultrasonic spray coating for large-area mesoscopic perovskite solar cells

A void-free metal halide perovskite (MHP) layer on a mesoscopic TiO2(m-TiO2) film was formedviathe wetting-induced infiltration of MHP solution in the m-TiO(2)filmviaa green ultrasonic spray coating process using a non-hazardous solvent. The systematic investigation of the behavior of ultrasonic-sprayed MHP micro-drops on the m-TiO(2)film disclosed that the void-free MHP layer on the m-TiO(2)film can be formed if the following conditions are satisfied: (1) the sprayed micro-drops are merged and wetted in the mesoscopic scaffold of the m-TiO(2)film, (2) the MHP solution infiltrated into the m-TiO(2)film by wetting is leveled to make a smooth wet MHP film, and (3) the smooth wet MHP film is promptly heat treated to eliminate dewetting and the coffee ring effect by convective flow in order to form a uniform void-free MHP layer. A void-free MHP layer on the m-TiO(2)film was formed under optimal ultrasonic spray coating conditions of substrate temperature of similar to 30 degrees C, spray flow rate of similar to 11 mL h(-1), nozzle to substrate distance of similar to 8 cm, and MHP solution-concentration of similar to 0.6 M under a fixed scan speed of 30 mm s(-1)and purged N(2)carrier gas pressure of 0.02 MPa. The mesoscopic MHP solar cells with an aperture area of 0.096, 1, 25, and 100 cm(2)exhibited 17.14%, 16.03%, 12.93%, and 10.67% power conversion efficiency at 1 sun condition, respectively.