Mechanism Study of High Efficiency Perovskite Solar Cells Based on Planar or Mesoporous Metal-Doped TiO₂ Electron Extraction Layer

Organic-inorganic halide perovskite solar cells (PSCs) is considered as one of the renewable energy sources. Over the last decade, the power conversion efficiency (PCE) of PSCs has rapidly promoted from 3.8% to 25.7%. The charge collection and defect states in a device are the most significant issues hindering the progress of PCE. On the other hand, hysteresis phenomenon in n-i-p PSCs, namely mismatch current density at different scanning directions, is one main issue that needs to deal with. Note that several factors that result in the hysteresis phenomenon. First, the imbalance between electron and hole flux in a device, attributed to lower electron mobility of TiO₂ can lead to severe hysteresis. Secondly, shallow trap states close to the conduction band increasing the charge recombination. As a result, carrier pathway between the perovskite active layer and the electron extraction layer (EEL) and smooth contact interface with the perovskite active layer play critical roles in charge transportation. Previous studies have investigated metal doped TiO₂ as EEL, such as Mg, W, Co, Zn, Ag, and Sn, in depth. With the suitable dopant, it could tune the band alignment between the EEL and perovskite active layer. That contributes to improve charge transportation in a device. Typically, the inferior contact between planar EEL and perovskite layer and low conductivity of pristine planar TiO₂ EEL deteriorate the hysteresis phenomenon in a perovskite solar cell. Therefore, mesoporous structure TiO₂being intimate contact between perovskite layer allow perovskite to infiltrate into mesoporous microstructure and to provide large contact area for electron transportation. Also, the intimate contact can reduce the defect density between an interface. As mentioned above, we believed that developing metal doped meso-TiO₂ EEL is promising for improving electron properties and passivating defects. In this study, we adopted three metals (Ag, Zn, Sn) as dopants into the planar or mesoporous TiO₂ to compare the difference between planar and mesoporous EEL. In the planar PSCs series, a promising Sn-doped TiO₂PSC showed an average PCE of 14.4%. In the mesoporous PSCs series, the device with optimized 1.0 mol% Sn-doped meso-TiO₂ achieved an average PCE of 19.51%. Further characterization revealed that the absorption edge of Sn-doped TiO₂ red shifted along with the increased doping concentration. Also, the defect density in EEL can be passivated by Sn doping. The upper shift of the conduction band and valance band edge made the energy level alignment in devices much more appropriate. That facilitates electron injection into the EEL with minor recombination. Finally, the champion device with 1.0 mol% Sn-doped meso-TiO₂ EEL delivered a PCE of 20.55%, a high fill factor (<i>FF</i>) of 81.72%, and a minor hysteresis index of 0.03.