Abstract
Over a decade, perovskite solar cells (PVSCs) have attracted tremendous attention in the photovoltaic community, and the power conversion efficiency (PCE) of single-junction PVSCs has rapidly increased from 3.8% to 26.7%. Meanwhile, tandem structured solar cells with large and small bandgaps perovskite/perovskite or perovskite/silicon have demonstrated promising performance. In particular, a 34.6% PCE was recently reported for a perovskite/silicon tandem device, exceeding the intrinsic theoretical efficiency limit of single-junction PVSCs (33.7%). It marks perovskites have great potential in future photovoltaic markets.Optimizing perovskite film quality is always the primary strategy for achieving high-performance PVSCs. However, the deteriorated morphology generally observed in wide-bandgap (WBG) PVSCs has been a major challenge. Consequently, understanding the crystallization process is crucial for determining the limiting factors during the formation of perovskite films. Organometallic halide perovskites undergo multiple ion coordination reactions during nucleation and crystallization. It is well known that the kinetics of crystal growth are very sensitive to the ionic composition, solvent concentration, and ambient conditions. Therefore, a noninvasive and sensitive in-situ characterization technique is crucial for revealing the ion coordination process. Another major concern with WBG perovskites is their long-term stability, where phase segregation due to the dissociation of ions in the crystal lattice has been commonly observed. However, it is still unclear whether such dissociation is solely triggered by external stimuli such as light and electrical field stresses or it is caused by intrinsic lattice instability. Getting into this degradation issue requires an in-situ technique to monitor the ion dissociation process during PVSC operation.
This thesis describes in detail the research works of the in-situ techniques developed during my PhD study to monitor the crystallization and degradation processes in WBG perovskites. It aims to uncover the technical limitations of existing in-situ optical methods and explore strategies to modulate the crystallization of WBG perovskites and enhance crystal stability. Accordingly, this thesis is divided into three major areas: (1) to develop a nondestructive in-situ optical method to monitor the crystallization mechanism of perovskites; (2) based on the in-situ measurement results, to investigate the crystallization modulation strategies for WBG mixed-halide perovskites (MHP); (3) to investigate the passivation mechanisms of different strategies.
Chapter 1 will discuss the current understanding of the crystallization mechanism in metal halide perovskites and the existing development of the in-situ measurement approach in detail. Chapter 2 will highlight the experimental techniques employes in this study. Chapter 3 will present an investigation of the in-situ optical characterization techniques, including photoluminescence (PL) and optical absorption, to probe the crystallization process in perovskite. A modified optical probing method was developed to eliminate the impact of continuous laser excitation during the in-situ measurement. First, to verify the impact of continuous excitation on the study of the crystallization mechanism, we conducted in-situ PL and UV-vis absorption on MAPbI3 perovskite during the spin-coating and annealing processes. This is because MAPbI3 has a simple composition, and its crystallization process is relatively well-studied. Our results confirm that continuous laser excitation not only induces fast crystallization but also damages the perovskite film. To solve this issue, we modified the setup by introducing a dynamic probe that can scan across the film instead of illuminating at the same spot. Using the dynamic mode of in-situ PL, we observed reliable and distinctive nucleation features that have not been observed using the static method in the literature, bringing insight into the crystallization process in various perovskite systems during the fabrication process.
In Chapter 4, we introduced Br- into MA-based perovskites to simultaneously increase the bandgap and investigate the impact of mixed-halide on the crystallization process. By comparing the in-situ results of MAPbI3 and MAPbI2Br during spin-coating, it is found that the nucleation is less efficient in the mixed-halide systems. It results in a large amount of aggregated PbI2 formed along with perovskite crystals during thermal annealing. The excessive PbI2 deteriorated the device’s performance. To overcome this issue, two strategies were developed to promote the nucleation in MHPs and retard the PbI2 formation. The first approach is to implement a two-step annealing method. A pre-heating process at 60 °C is found to effectively facilitate nucleation and form a compact film without PbI2 formation. After that, complete crystallization can be achieved through coarsening by annealing the film at 100 °C, with only a small amount of PbI2 forming around the grain boundaries as a passivator. The second approach is to engineer the organic cation composition to promote nucleation in the mixed-halide system. It is found that partially replacing MA+ with FA+ can facilitate stronger nucleation during the spin-coating. As a result, even using only the one-step annealing without the pre-heating process, high-quality perovskite films can be obtained. This work demonstrates that in-situ optical characterization provides valuable information for optimizing fabrication to obtain high-quality perovskite films and high-performance PVSCs.
Chapter 5 presents an in-situ optical investigation of the phase segregation issue along with different passivation strategies to enhance the perovskite film stability. Combined with the experimental results and theoretical calculations of ion coordination in the MHP crystal lattice, it is found that MHPs have a higher surface energy than single-halide perovskites. Such high surface energy induces ion migration from the crystal bulk and triggers a reaction with adjacent grains. The conventional physical passivation approach, such as distributing PbI2 around grain boundaries, can only provide a barrier to block the dissociated ions from reacting with adjacent grains, but cannot suppress ion dissociation. In contrast, it is found that chemical passivation using polar molecules attached to the lattice surface can reduce the surface energy and ultimately prevent ion dissociation from the crystal lattice. This work provides insight into the origin of phase segregation in MHPs and the mechanism of physical and chemical passivation.
| Date of Award | 5 Sept 2024 |
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| Original language | English |
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| Supervisor | Sai Wing TSANG (Supervisor) |