Abstract
Superior photoluminescence characters and thermodynamic phase stability endow halide perovskite optoelectronics with exceptional performance and operational durability. In line with widely discovered performance limiting factors, such as defects in perovskite devices, the strain, that describes the lattice deformation of an element under stress, has been found to deliver a profound influence on the physic characteristics in halide perovskites. This thesis aims to develop approaches to harness, especially, the detrimental tensile strain in thin film perovskites and elaborate on the strain effect on thin film perovskites and perovskite solar cells.
Based on optimised one-step thin film perovskite where perovskites have a medium bandgap, it is found that these perovskites deposited on a rigid substrate can be readily tensile strained at the in-plane direction. For perovskite of different tensile strain levels, compliant interlayers were experimentally introduced at the bottom interface between perovskite and rigid substrate. It was discovered that these long alkyl chain amino acids interlayers can help to partially relax the residual tensile strain in perovskites and perovskites of reduced tensile strain exhibited enhanced optoelectronic properties and phase stability. The strain-modulated thin film perovskites further enable solar cells to reach an encouraging power conversion efficiency (PCE) of 24%, without using surface passivation. The approach here developed can offer insights into interfacial engineering that realises, concurrently, strain relaxation and fast carrier transfer by using long alkyl chain ligands albeit these ligands are considered to be poorly conductive.
The defective perovskite top surface is another termination that necessitates a careful investigation from the perspective of mechanical strain. With the use of 2D perovskite capping layer, it is found that 2D perovskites modified 3D perovskites can have the enclosed tensile strain being relaxed. The strain relaxation effect was attributed to the lattice partition effect brought by 2D perovskite. The stronger chemical affinity of long chain-alkylamine salt and 3D perovskite leads to easier generation of 2D perovskites and strain relaxation. Besides, the benign optoelectronic modification of 3D perovskites can only be realised when there is limited lattice partition and strain relaxation. The theory developed here answers a long-lasting puzzle about how tensile strain is relaxed for 2D/3D perovskites with respect to 3D perovskite systems, offering clues to modulate the physical properties of 3D perovskite more effectively by using 2D perovskite capping layer.