| Author: | Yang, Chen |
| Title: | Unraveling the phase transition of formamidinium lead iodide thin film mediated by defects using in-situ and ex-situ microscopy |
| Advisors: | Zhu, Ye (AP) Li, Gang (EEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Subject: | Perovskite materials Photovoltaic power generation Solar energy Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xv, 115 pages : color illustrations |
| Language: | English |
| Abstract: | Over a prolonged period, the energy problem has been a focal point of global concern. The direct application of solar energy is regarded as one of the most promising solutions to alleviate or resolve this issue. The development of FAPbI3 has substantially advanced the field of photovoltaics, largely due to its exceptional electronic properties and an optimal bandgap of approximately 1.48 eV. However, this material has perpetually faced numerous challenges, among which the issue of stability remains a significant concern for scientists. This problem has attracted considerable attention, leading to extensive efforts in both mechanistic studies and engineering improvements. In the fabrication of FAPbI3 devices, annealing induces the transformation of FAPbI3 from the non-photoactive δ phase to the photoactive α phase. The advancement of FAPbI3 devices is intrinsically linked to a profound understanding of this phase transition process, therefore, intuitive observation of this phase transition is essential. Particularly, 4D STEM (scanning transmission electron microscopy) is invaluable for capturing the orientation relationship at the phase boundary and the formation of defects during the phase transition. In this thesis, in-situ optical microscopy and ex-situ 4D STEM techniques are employed to explore the dynamic phase transition process, the preferred orientation during the phase transition, and the formation of defects resulting from the phase transition in FAPbI3 from the δ to α phase. Building upon the understanding gained from the characterizations, the preferred orientation of the α phase resulting from the phase transition was ultimately determined and verified by X-ray diffraction. Initially, millimeter-sized highly oriented single crystal δ phase thin films with precisely defined [101̅0]δ surface orientations were synthesized using an innovative space confined anti-solvent method. In situ annealing polarized optical microscopy was conducted at various temperatures to determine the activation energy of the δtoαphase transition in two dimensions. Furthermore, the preferred phase transition orientation <0002>δ over <2 1̅1̅0 >δ directions is examined. Subsequent ex-situ 4D-STEM characterization of the transferred thin films elucidated a specific phase transition relationship: <10 1̅0>δ//<001>α, {12̅ 10}δ//{210}α, and {0002}δ//{120}α. Boundary analysis revealed a preferred {210}α interphase boundary, the non-coherent interphase boundary reveals the reconstructive behavior of this phase transition, further evidenced by the observed ledge growth mode. Furthermore, analysis of the regions near the transition front, the degree of amorphousness and the defect density progressively increases. Hence, this study proposed a systematic approach to investigate the underlying mechanisms of the δ to α phase transition in FAPbI3, examining the anisotropy inherent in this transition. This work advances the understanding of the phase transition, providing insights into the anisotropic nature of the transformation. Additionally, the study elucidates the prevalent [001]α orientation commonly observed in FAPbI3. This thesis also scrutinizes the degradation of FAPbI3 under moist conditions, focusing not only on the δ to α phase transition in perovskite but also utilizing X-ray diffraction (XRD) and transmission electron microscopy (TEM) with selected area electron diffraction (SAED) to investigate the degradation phenomenon. In this experimental section, the active layer, consistent with device fabrication, was prepared as a freestanding sample. The XRD analysis revealed a distinct peak at 2𝜃=11.78°, while SAED confirmed this peak to be confined 101̅0δ peak. Subsequent degradation experiments under ultraviolet and high humidity conditions demonstrated an initial strengthening followed by decay of the confined 101̅0δ peak. This observation suggests that the degradation of FAPbI3 samples maybe initiated by such defects. These findings shed light on the phase transition from δ to α and both degradation mechanisms of FAPbI3, highlighting the utility of XRD and TEM techniques in comprehending the underlying phase transition mechanism. This understanding fills a critical gap which significantly aids in further modifications of this material. |
| Rights: | All rights reserved |
| Access: | open access |
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