Effects of Pressure, Bending and Annealing Temperature on the Mechanical and Optoelectronic Properties of Perovskite Solar Cells
The increasing needs for clean and sustainable energy stimulate the growing interest in photovoltaic (PV) technology using organic-inorganic hybrid perovskite materials. However, perovskite solar cells (PSCs) are faced with stability problem due to the presence of cracks or defects within the perovskite absorber and along the interfaces of the multilayered PSC structures. It is therefore important to improve on our understanding of their degradation pathways and mechanical stabilities. In this thesis, the effects of pressure, bending and processing annealing temperature on the mechanical and optoelectronic properties of perovskite solar cells are studied. First, the effects of pressure on photoconversion efficiencies of perovskite solar cells (PSCs) are studied using a combined experimental and analytical/computational technique. The results show that crystallization, absorbance, and the power conversion efficiencies of PSCs can be significantly improved by the application of pressure. This leads to the closing-up of voids and the corresponding increase in the interfacial surface contact lengths, which increase with increasing pressure. The observed improvement in the power conversion efficiencies (9.84 to 13.67%) was observed with increased pressure between 0 and 7 MPa, attributed largely to the effects of increased surface contact and the compaction and infiltration of the TiO2 layers with perovskite during the application of pressure. At higher pressure values (> 7 Mpa), the damage due to sink of the perovskite layers into the mesoporous layers results in reductions in the photoconversion efficiencies of PSCs. The understanding of the variations in the mechanical properties of organic-inorganic hybrid perovskites structures that are processed at different annealing conditions is then studied for ultimate device performance and robustness. We show that the temperature at which perovskite film is annealed affects the mechanical properties of the devices fabricated. The size dependence ii of hardness is due to the increase in the density of geometrically necessary dislocations (GNDs) with decreasing indentation size. The indentation size effects are characterized between the micron- and nanoscales by a bi-linear strain gradient plasticity (SGP) framework with source limited and established dislocation substructures. The measured microstructural length scales decrease with increasing annealing temperature to 130oC, after which it began to increase, causing films annealed beyond 130oC to have reduced strengths because the larger microstructural length scales correspond to larger dislocation spacings and weaker dislocation interactions. Perovskite solar devices annealed at temperatures above 130oC have poor performance. The results show that perovskite solar cell devices annealed at 130oC exhibit optimal performance and attractive combinations of mechanical properties. Finally, the underlying failure mechanisms associated with flexible perovskite solar cells (FPSCs) are elucidated for deformation and cracking under monotonic and cyclic bending. The mechanical robustness of the inverted flexible PSCs is increased with increasing fraction of polyethylene oxide (PEO) in the double-cation perovskite precursor, which promotes the grain size and passivates the defects of the film. The associated changes in the optical transmittance of the perovskite-PEO absorber and the PCEs of the multilayered FPSCs structures are elucidated under monotonic and cyclic bending. The failure mechanisms of the perovskite films for different radii of bending were observed using a scanning electron microscope before computing the interfacial fracture energies in the multilayer devices using finite element simulations. The failure mechanisms are then used to explain the degradation of the optoelectronic properties of flexible perovskite solar cells.