Ligand-engineered perovskites for achieving high-performance and stable solar cells

Abstract

Due to their outstanding optoelectronic properties, metal halide perovskites have been intensively studied in recent years. A certificated power conversion efficiency (PCE) of 22.7% has recently been achieved in perovskite solar cells (PVSCs), which enables them as a very promising candidate to be used for next-generation photovoltaics. However, most of the state-of-the-art PVCSs reported so far require tedious layer-dependent high-temperature sintering or annealing processes. This will not only increase the fabrication complexity, cost, and energy consumption, but also hinder the use of flexible substrates for roll-to-roll printing. Moreover, thermal annealing for extended periods is known to cause perovskite decomposition, which degrades device performance and stability. It is still challenging to fabricate low-temperature processed PVSCs, owing to a trade-off between processing temperature and device efficiency. In this dissertation, we focus on the ligand-engineered perovskites for room-temperature solution processed high-performance and stable PVSCs. Firstly, we demonstrated a ligand-engineered PbI2 nanostructure for forming high-quality (large crystal size and smooth) perovskite without PbI2 residue via two-step deposition method. On one hand, ligand-engineered PbI2 nanostructure was formed by incorporating small amount of rationally chosen ligand (4-tert-butylpyridine) into the PbI2 precursor solutions, which significantly facilitate the conversion of perovskite. On the other hand, through employing a relatively high CH3NH3I concentration in the second step, a firmly crystallized and smooth CH3NH3PbI3 film was formed. As a result, a promising PCE of 16.21% was achieved in planar-heterojunction PVSCs. Furthermore, we experimentally demonstrated that the PbI2 residue in perovskite film has a negative effect on the long-term stability of devices. Secondly, we developed a ligand vapor treatment for forming the porous PbI2(ligand)x intermediates, which promote the perovskite formation at room temperature. The ligand selection rules for forming high-quality perovskite films were established through a combined experimental and theoretical study. Using suitable ligand (i.e., pyridine), high-quality perovskite films featuring large grain-sizes, and high crystallinity while being free of PbI2 residues can be prepared. As a result, all-room-temperature solution-processed PVSCs showed high PCE of 17.10% with negligible hysteresis on rigid substrate, which is the best PCE reported to date for PVSCs fabricated by all room-temperature solution-processed techniques. The PVSC was very stable which can maintain 95% of its original PCE after being stored 1000 hours in ambient environment. Furthermore, our all room-temperature solution-processed technique enables the fabrication of flexible PVSCs with high PCE of 14.19 % and remarkable power-per-weight of 23.26 W/g. Finally, we invented the first kind of simple post-device ligand (PDL) treatment to further improve the photovoltaic performance and operational stability of complete PVSCs. The PCE of completely fabricated PVSCs can be improved from 18.7% to 20.13%. Meanwhile, the stability of the treated devices without any encapsulation remarkably improves that 70% PCE is maintained in ambient conditions after 500-hour maximum-power-point tracking test while the control unencapsulated device will completely break down within 100 hours. Equally important is that this PDL treatment shows a special ‘stitching effect’, namely repairing the as-fabricated ‘poor devices’ by healing the defects of perovskite active region and can significantly improve their PCEs.