ABSTRACT
A titanium dioxide (TiO2) compact film is a widely used electron transport layer (ETL) for n-i-p planar perovskite solar cells (PSCs). However, TiO2 sufferers from poor electrical conductivity, leading to high energy loss at the perovskite/ETL/transparent conductive oxide interface. Doping the TiO2 film with alkali- and transition-metal elements is an effective way to improve its electrical conductivity. The conventional method to prepare these metal-doped TiO2 films commonly requires time-consuming furnace treatments at 450-600 °C for 30 min to 3 h. Herein, a rapid one-step laser treatment is developed to enable doping of tantalum (Ta) in TiO2 (Ta-TiO2) and to simultaneously induce the crystallization of TiO2 films from its amorphous precursor to an anatase phase. The PSCs based on the Ta-TiO2 films treated with the optimized fiber laser (1070 nm) processing parameters (21 s with a peak processing temperature of 800-850 °C) show enhanced photovoltaic performance in comparison to that of the device fabricated using furnace-treated films at 500 °C for 30 min. The ambient-processed planar PSCs fabricated under high relative humidity (RH) of 50-70% display power conversion efficiencies (PCEs) of 18.34% and 16.04% for devices based on Cs0.1FA0.9PbI3 and CH3NH3PbI3 absorbers, respectively. These results are due to the improved physical and chemical properties of the Ta-TiO2 films treated by the optimal laser process in comparison to those for the furnace process. The laser process is rapid, simple, and potentially scalable to produce metal-doped TiO2 films for efficient PSCs.
ABSTRACT
Perovskite solar cells (PSCs) based on organic-inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA1-xCsxPbI3 and (FA1-xCsx)1-yRbyPbI3 (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs+ and Rb+ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA1-xCsxPbI3 by providing a source of I- that fills iodine vacancy sites in the perovskite lattice, while Rb+ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.
