Process-, Solvent- and Chemical Engineering for Solution Processed Organic-Inorganic Lead Halide Perovskite Solar Cells

In this thesis, thin films of organic-inorganic lead halide perovskites (ABX3) have been prepared via solution processing approaches and integrated into thin film solar cell devices. For this purpose, a low temperature single-step fast crystallization method - the instantaneous perovskite crystallization process (IPC) - was developed for the deposition of CH3NH3PbX3 (X = halide) thin films. The influence of short-chain lead (II) carboxylate precursors, processing parameters and the impact of chemical modification on the X-site of the hybrid perovskite thin films with methylammonium cation have been studied and evaluated by a variety of spectroscopic and analytical techniques. Efficiently operating planar organic-inorganic lead halide perovskite solar cells in both device stacks (n-i-p and p-i-n) could be obtained by this approach. The highest power conversion efficiency of 15.6% was obtained via IPC process at a processing temperature of 75 °C for CH3NH3PbI3 thin films in n-i-p device stack configuration prepared using Pb(CH3COO)2 and CH3NH3I precursors in N, N-dimethylformamide solvent.

Furthermore, protic ionic liquids with methylammonium cation and carboxylate anion have been synthesized and successfully employed as alternative solvents for hybrid lead halide perovskite processing in their pristine state and in binary blends with co-solvents such as water, alcohols and acetonitrile. By using a ternary solvent system, based on the new protic ionic liquid methylammonium propionate, acetonitrile and dimethyl sulfoxide, (MA0.15FA0.85)Pb(I0.85Br0.15)3 perovskite thin films could be prepared by a sequential deposition method and successfully integrated into solar cell devices. In addition, the effects of surface modification of mesoporous TiO2 thin films by alkali bis-(trifluoromethanesulfonyl) imide (TFSI) treatment were studied by X-ray photoelectron spectroscopy (XPS). Here, a trade-off between type of alkali-metal and precursor concentration was observed. Through potassium-TFSI surface treatment with optimized precursor concentration of 0.2 mol/l, air processed perovskite solar cells with stabilized power conversion efficiency of 20.6% could be obtained.