Researchers at Soochow University, Sichuan University and Empa (Swiss Federal Laboratories for Materials Science and Technology), recently devised a new strategy to create high-quality perovskite absorbers with grains in the micrometer scale and prolonged carrier lifetimes. This new strategy is based on a close-space annealing (CSA) process, a heat-based technique that can be used to change a material's chemical properties.
According to the team, controllable crystallization plays a crucial role in the formation of high-quality perovskites. The researchers reported a universal CSA strategy that increases grain size, enhances crystallinity and prolongs carrier lifetimes in low-bandgap (low-Eg) and wide-bandgap (wide-Eg) perovskite films. The CSA strategy devised by the team is universal, as it can be applied to perovskites with various bandgaps to produce high-quality absorbers with enlarged grains and longer carrier lifetimes. As part of their recent study, the team demonstrated its generalizability by successfully using it to prepare absorbers based on perovskites with different chemical compositions.
The new strategy involves growing perovskites through the CSA process, while managing the presence of residual solvents inside the intermediate-phase perovskites (i.e., the perovskite's form before the final annealing steps take place). The researchers found that this promoted the growth of grains, merging neighboring crystals as solvents were slowly released from the perovskites.
"By placing the intermediate-phase perovskite films with their faces towards solvent-permeable covers during the annealing process, high-quality perovskite absorber layers are obtained with a slowed solvent releasing process, enabling fabrication of efficient single-junction perovskite solar cells (PVSCs) and all-perovskite tandem solar cells," Soochow University's Changlei Wang and his colleagues explained in their paper.
In initial evaluations, the CSA strategy enabled the creation of high performance perovskite absorbers with both low and wide bandgaps. These absorbers were then used to fabricate 4-T and 2-T all-perovskite TSCs that exhibited remarkable power conversion efficiencies.
"The best PCEs of 21.51% and 18.58% for single-junction low-Eg and wide-Eg PVSCs are achieved and thus ensure the fabrication of 25.15% efficiency 4-terminal and 25.05% efficiency 2-terminal all-perovskite tandem solar cells," Wang and his colleagues wrote in their paper.
In the future, the CSA strategy introduced by this team of researchers could be used to create better absorbers for low-cost and efficient TSCs that are solely based on perovskites. This could facilitate the large-scale implementation of these highly promising energy solutions.
