Morphology Optimization of Perovskite Layer for Efficient Solar Cell Applications
Student thesis: Doctoral Thesis
With diminishing supply of fossil fuels, organometal halide perovskites, as a new class of solar energy conversion material, have received extensive attention due to their high efficiency as well as cost-effective solution processability. Remarkable progress has been achieved in recent years with power conversion efficiency (PCE) increased from 3.8% to over 22.1%. One strategy for performance enhancement is to improve the quality of the perovskite films.
Perovskite solar cells were fabricated with a simple one-step, low-temperature solution-process using lead acetate as the lead source. Solvent annealing was applied to enhance grain growth for obtaining better film morphology. Uniform perovskite films without pinholes can be obtained by solvent annealing for 5 min at 100 oC. Planar solar cells based on the high quality perovskite films deliver power conversion efficiency up to 12.7% with negligible hysteresis and good reproducibility. In addition, the substrate surfaces have little effect on the crystallization of the perovskite when lead acetate was used, leading to uniform films on different substrates, which can provide a wide choice of substrates and interfacial materials.
The quality of perovskite films was further improved by chlorine incorporation. The effect of chlorine incorporation on the crystallization process of perovskite film was studied based on a lead acetate precursor, which demonstrated a fabrication process for fast grain growth with highly preferred (110) orientation upon only 5 min of annealing at 100 oC. By studying the correlation between precursor composition and morphology, the growth dynamic of perovskite film is discussed. In particular, it is found that both lead acetate precursor and Cl incorporation are beneficial to perovskite growth. While lead acetate allows a fast crystallization process, Cl improves crystallinity. Planar perovskitesolar cells with optimized parameters deliver a best power conversion efficiency of 15.0% and an average efficiency of 14.0% with remarkable reproducibility and good stability.
Recently formamidinium (FA) based perovskite solar cell was demonstrated to show high performance andbetter stability upon partial substitution of FA with Cs cation. However, the fabrication of device required high-temperature processing on TiO2 electrode and thus limits the use of flexible polymeric substrates. Here, a low temperature approach for the fabrication of p-i-n perovskite solar cells based on Cs0.15FA0.85PbI3 was presented. Furthermore, we investigated the effects of chlorine on the morphology and crystallinity of the perovskite films and the corresponding photovoltaicperformance. Chlorine incorporation can significantly enlarge the size ofgrains and improve the crystallinity of perovskite films with full surface coverage. A best power conversion efficiency of 14.5% was realized for planar perovskite solar cells with negligible hysteresis and remarkable reproducibility.
Improved long-term stability can be also obtained by using layered two-dimensional (2D) perovskitefilms, which, however shows lower efficiency. The main reasons for their relatively poor efficiency are low crystallinity, high defect densities and poor charge transport caused by large amount of insulating bulk organic cations in 2D perovskite films. Here, a simple and universal method was demonstrated to control the crystallization of 2D perovskite film by incorporating dimethylsulfoxide (DMSO) and CH3NH3Cl (MACl) additives in the precursor solution. The resulting film shows much improved crystallinity and reduced defect densities. In addition, the additives can induce growth of highly oriented perovskite frameworks, which are vertically aligned to the substrate facilitating efficient charge transport in the device thickness direction. More importantly, the 2D perovskite films with the additives show graded distribution of multiple perovskite phases with various n values, forming type-II band alignment that favors self-driven charge separation. Planar 2D perovskite solar cells based on (PEA)2(MA)3Pb4I13(PEA = C6H5(CH2)2NH3) showsa best power conversion efficiency over 12% with no hysteresis and good reproducibility. Furthermore, the devices without encapsulation can retain their initial PCE for10 days and maintain 50% after one month in ambient air, exhibiting greatly improved stability compared to their three-dimensional counterparts.