Investigation of vacuum and solution-processed halide perovskites and their applications in heterojunction photovoltaics

Perovskite solar cells (PSCs) have emerged as a promising renewable energy technology in recent years. However, their path towards industrial production and commercialization presents challenges that demand innovative solutions. This doctoral thesis is dedicated to addressing two pivotal issues in the development of PSCs: (1) the development of a fabrication method compatible with traditional semiconductor industry processes and (2) the exploration of approaches to improve device performance and stability. The present thesis separates these two issues into three parts. First, the fabrication method of MA-free perovskites via vacuum vapor deposition process is proposed. CsBr is added to FAPbI${}_3$ giving FA${}_x$Cs${}_{1-x}$PbI${}_{3-x}$Br${}_x$. to maintain the stable black perovskite phase. Furthermore, the effect of the thermal annealing process on perovskite films with different stoichiometric ratios was explored. It was found that thermal annealing enhances the crystallinity of both FAI-poor and stoichiometric films. For FAI-poor perovskite films, an increase in absorption as well as reduction in defect concentrations was achieved through annealing process. However, the opposite effect was observed for FAI-rich films. By optimizing the fabrication processes, a solar cell device with an efficiency of 16.6% was acquired. However, these vapor-deposited devices still exhibit lower performances compared to those prepared using solution processes, indicating the need for further improvements in perovskite layer composition and interfacial properties to enhance their efficiency. The second part of this dissertation demonstrates the concept of phase heterojunction (PHJ) solar cells by combining two polymorphs of the same material from the evaporation process (γ-CsPbI${}_3$) and solution process (β-CsPbI${}_3$). It was discovered that the photovoltaic parameters of these PHJ devices significantly surpass those of either single-phase device, resulting in a maximum power conversion efficiency of 20.1%. The enhancement comes from the following three factors: efficient passivation of the β-CsPbI${}_3$ by the larger bandgap γ-CsPbI${}_3$, an increase in the built-in potential of the PHJ devices enabled by the energetic alignment between the two phases, and enhanced absorption of light resulting from narrower band-gap β-CsPbI${}_3$ in the PHJ structure. The approach demonstrated here offers new possibilities for developing photovoltaic devices based on polymorphic materials. In the final part, a 1D-3D dimensional junction formed spontaneously by a two-step process is presented. It was found that the morphology, energy alignment, and defects of the buried interface were improved by creating 1D perovskites. In addition, strip-shaped PbI${}_2$ domains and voids are eliminated which ultimately enhances photovoltaic performance and stability. These improvements can be attributed to the passivation effect of the 1D perovskite and better energetic alignment. Furthermore, this dimensional junction strategy also shows potential for use in large-area devices.