| Author: | Zhang, Hengkai |
| Title: | Device engineering approaches towards high performance perovskite solar cells |
| Advisors: | Li, Gang (EIE) |
| Degree: | Ph.D. |
| Year: | 2021 |
| Subject: | Perovskite solar cells Solar cells -- Materials Photovoltaic cells -- Materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electronic and Information Engineering |
| Pages: | xxvii, 185 pages : color illustrations |
| Language: | English |
| Abstract: | The energy problem is the most urgent problem we need to solve. Without cheap, sustainable, and safe energy alternatives for fossil fuels, everyone on the earth cannot avoid facing the dilemma of the climate crisis and non-renewable resources exhausting. Therefore, the biggest challenge of the whole society is to find large-scale energy, which is sustainable, affordable, and safe, to replace fossil fuels. Solar energy is the most precious and luckily abundant gift to human being which can fulfill all the energy requirement of the whole world. This dissertation is focused on the development of high performance and stable perovskite solar cell which is regarded as the game-changer of the energy field. The perovskite photovoltaic technology is much advantageous over a conventional solar cell, including much lower material cost, tunable bandgap, high efficiency, solution-processed manufacture, and so on. However, compared with the commercialized silicon solar cell, the stability issue of perovskite solar cell is still a big challenge toward its practical application. Thus, my Ph.D. research work is around the strategies to enhance perovskite stability. In this dissertation, three major strategies are introduced to enhance the device performance and stability, including crystallization manipulation, strain engineering, and lead iodide management, with deepened scientific understanding of the physical chemical processes within these systems. The first work is on exploring novel perovskite film formation process inspired by classical semiconductor physics. Here a crystallization manipulation approach of solution-processed quasi-epitaxial growth is demonstrated. The ideal of this novel crystallization manipulation came from the conventional thin-film semiconductor epitaxial growth technique, where all the highest-quality semiconductors are obtained, which is associated with expensive and slow process. We systematically investigate and elucidate the crystallization kinetics of the perovskite via a modified two-step method, and find out the perovskite material has similar features to the conventional semiconductor epitaxial growth. More specifically, we introduce a template-guided quasi-epitaxial process to fabricate the high-quality α-FAPbI3 based perovskite film, which is manipulated by a SYNERGETIC effect of (a) methylammonium chloride (MACl) additive and (b) large-organic cation (BABr et. al). The in situ and ex situ GIWAXS measurements have been conducted to visualize the crystallization dynamics of solution-processed quasi-epitaxial growth of the α-FAPbI3 film. From the in situ GIWAXS measurements, we prove the n-butylammonium (BA)-based intermediate phase is grown preferably at the bottom of the intermediate layer. The depth-resolved ex-situ GIWAXS results provide clear evidence of bottom-up perovskite growth during the annealing process, which is the prerequisite of template-guided epitaxial growth (detail shown in Chapter 3). The solution-processed bottom-up quasi-epitaxial growth of high-quality perovskite film is proven to be relatively general, confirmed by several widely used large cation organic salts (BAI, PEAI, and PEABr). The perovskite PSCs by the introduced crystallization manipulation approach exhibited much-enhanced performance and stability over the control device. The second piece of work is on exploring two key issues we believe to e promising on PSC technology – how to apply the crosslinking concept to enhance PSC stability, and strain engineering in a new way. The intrinsic tensile strain is recently found to be one of the key sources of PSCs' instability. Strain engineering is one important topic in the study of PSCs. Based on this topic, we introduce a novel and effective strain engineering strategy – Cross-linking enabled Strain Regulating Crystallization (CSRC) – to precisely modulate the largest lattice distortion region through synchronous cooperation of in situ chemical cross-linking and perovskite crystallization. In chapter 4, trimethylolpropane triacrylate (TMTA), with multiple functional groups of -OH and C=O (could provide effective passivation for the perovskite device), was selected as a chemical cross-linking agent to prove the newly introduced strain regulation approach, i.e., CSRC. TMTA is introduced into the anti-solvent chlorobenzene (CB), aiming at regulating the top region of the perovskite film during the crystallization process. The in situ chemical cross-linking strategy during the perovskite crystallization process is proven to be much more effective in strain regulation over the conventional strain compensation method, in which TMTA in isopropanol is applied as post-treatment. Furthermore, the strain engineering strategy CSRC is proven to be one general method that can be applied to other cross-linking material systems. The reported in situ CSRC approach exhibits multiple functions including strain regulation, humidity repulsion, effective trap passivation, and therefore highly efficient and stable PSCs are obtained simultaneously. The last important strategy introduced in my Ph.D. is the novel composition/stable complex engineering for excess PbI2 management in mainstream high-efficiency PSCs, aiming at excelling in stability and efficiency in the PSCs. Excess lead iodide passivation is one well-recognized strategy in PSCs to reduce non-radiative recombination and enhance device performance. However, the excess lead iodide in perovskite film has been well-recognized to be detrimental to PSCs' stability due to the photolysis of lead iodide. Currently, the introduced lead iodide management approaches are very limited. In chapter 5, we demonstrate an Ionic Liquid (imidazolium-based, [BMIM]X) assisted supramolecule self-assemble engineering (ILaSS) for excess PbI2 management. The as-formed [BMIM]Pb2X5 supramolecule is shown to be extremely stable under continuous light illumination in the ambient environment, thus efficiently eliminates the detrimental stability effect of unreacted PbI2. Moreover, the [BMIM]+ was found to track the distribution of PbI2, reduce the defect density, and suppress the recombination inside the films. Additionally, we find the perovskite film with excess PbI2 induces a larger tensile strain than perovskite film with stoichiometric PbI2. We also demonstrate that the ILaSS is an effective method to release the film tensile strain, likely by eliminating the thermal expansion coefficient mismatch between perovskite and lead iodide. Through the synergetic regulation on both unreacted PbI2 and tensile strain, the ILaSS approach enabled PSCs to exhibit excellent device stability and better performance. Apart from the introduced three main works, the dissertation also includes my publication efforts in the application side - solution-processed electrode deposition technique to replace precious metal electrode in perovskite solar cells, which eliminates the vacuum process and may greatly reduce the manufacturing cost. The work is compatible with the flexible substrate and large area coating and holds promise in versatile applications. The fifth section of the dissertation is on the exploration of quantum dots - CsPbI3- xBrx QDs – in PSC, in which we demonstrate these QDs effectively functioned as "surface patches" to "cure" the defects of perovskite grains and enhance PSCs stability and performance. The working mechanism of the surface patch effect is again studied in detail. In summary, the objectives of my Ph.D. study include three aspects: a. Deepened understanding of the physical-chemical phenomena of perovskite solar cells; b. Novel scientific and engineering approaches discovery and mechanism elucidation aiming at providing guidance for the PSC fabrication, and c. Achieving high efficiency and high stability perovskite solar cells. We hope the methodology and the mechanism studies introduced in the dissertation can provide a useful reference or guidance on the field of PSCs research and further promotes the development of PSCs towards a real-world technology, contributing to our global energy, environment, and quality of life. |
| Rights: | All rights reserved |
| Access: | open access |
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