Synthesis of Perovskite Nanocrystals and Their Applications in Perovskite Solar Cells

Abstract

Perovskite solar cells are a very promising photovoltaic technology which was first reported in 2009 and developed very rapidly. The crystallisation within perovskite films is highly dependent on processing environments, such as temperature, humidity, atmosphere, even light, which makes the fabrication of perovskite solar cells rather lab-dependent and poorly reproducible. One strategy to overcome this problem is to develop a controlled synthesis of perovskite nanocrystals which can then be ordered into films in a separatestep.In this thesis, optimisation of planar perovskite solar cells is carried out by the engineering of perovskite film fabrication methods. Different deposition methods along with different process factors such as solvents, temperature and precursor recipes are compared. One step spin-coating method with the recipe of MAI:PbCl2=3:1 gives the best PCE of 12.1 ± 0.7 % in air with controlled humidity of < 35%, showing high reliability and reproducibility.Doping of TiO2 layers with Zn2+, Sn4+ and Nb5+ ions are carried out to investigate the impacts of doping ions in different valence states on the electron-transporting properties of TiO2 ETLs. The different doping ions shift the flat band potential differently. Zn2+ largely negatively shifts the flat band potential, whereas Nb5+ positively shifts and Sn4+ barely changes the flat band potential of TiO2. the Zn-doping of the TiO2 ETL decreases the performance of the cells. However, when a thin layer of Zn-doped TiO2 is deposited on top of the pristine TiO2 layer as interlayer, the cell efficiency is slightly improved.Following the cell optimisation, to achieve better control over the crystallisation process, a facile flow reactor is developed for the synthesis of MAPbX3 perovskite nanocrystals at low temperature, which are further used for perovskite solar cells. The nanocrystals show narrow size distribution, good emissive properties and high stability. The bandgap of the nanocrystals was easily tuned between 485-745 nm by changing the halide composition. The photoluminescence of the MAPbI3 NCs in the first supernatant can also be tuned by changing the process parameters such as temperature, residence time and ligand concentration. However the impacts are more complex in the second supernatant in toluene with the appearance of multiple peaks in the PL spectra. It could be resultedfrom the formation of smaller NCs due to the reprecipitation of the incompletely removed reactants when added into toluene, or the fragmentation of the NCs upon dispersion into toluene, but better understanding is still needed.In the last part of the thesis, the synthesised MAPbI3 nanocrystals are investigated in perovskite solar cell applications. They have been applied as interlayers at the perovskite HTM interface, where they improved the stability of the devices towards moisture. The nanocrystals and their bulk by-products are also used as active light-absorbing layers for perovskite solar cells, delivering the best PCEs of 0.51% and 1.2% respectively, and notably showing outstanding water resistance. Further improvements in the cell performance could potentially be achieved by the removal of the insulating long chain ligands using effective ligand exchange treatments.

Date of Award	31 Oct 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Petra Cameron (Supervisor) & Frank Marken (Supervisor)