Scalable Processing of Low-Temperature TiO₂ Nanoparticles for High-Efficiency Perovskite Solar Cells

Most high-efficiency perovskite solar cells (PSCs) rely on titanium dioxide (TiO2) electron transport layers (ETLs) that are usually processed at high temperature (>450 °C). Consequently, a fully solution-based process of PSCs with TiO2 ETL on inexpensive flexible polymer substrates is not feasible. Therefore, a scalable low-temperature TiO2 ETL is developed based on presynthesized crystalline nanoparticle TiO2 (np-TiO2). The presented synthesis process offers control over the doping, the hydrodynamic diameter, and the solvent of the np-TiO2. High initial power conversion efficiency (PCE) of 19.5% (stabilized at 18.2%) of PSC processed on the np-TiO2 ETL is demonstrated with spin-coated methylammonium lead iodide (CH3NH3PbI3). Furthermore, it is shown that other perovskite absorber layers that include evaporated CH3NH3PbI3 and triple cation perovskite, Cs0.1(MA0.17FA0.83)0.9Pb(I0.83Br0.17)3, also show very high initial PCE, 14.5% and 19%, respectively. The possibility of doping the np-TiO2 with niobium (Nb5+) also reduces the resistance of the np-TiO2 ETL, allowing for thick ETLs that are beneficial for rough substrates. Facile upscaling of the deposition of the np-TiO2 ETL by inkjet printing, blade coating, and slot-die coating is also demonstrated, having high uniformity, resulting in prototype PSCs with a stable PCE > 15%.