Study on Molecular Engineering for Improving Stability of Perovskite Solar Cells

Abstract

Perovskite solar cells (PSCs) have been highlighted as a promising candidate for next generation solar cells, showing a dramatic efficiency increase (3.8% to 25.8%) after a first application in 2009. However, despite the efficiency increase, the long-term stability of PSCs is still in a low-level when compared to that of the conventional Si-based solar cells. The low long-term stability issue of perovskite is originated from intrinsic issues (vacancy formation, and ion migration), extrinsic stimuli (moisture, heat, light, and oxygen), and component layers (electron/hole transport layers). In a n-i-p structure, the stability-determining materials are pointed out to be perovskite and hole transport material (HTM). Therefore, to deal with issues, molecular engineering strategies have been implemented to PSCs in a form of introducing and replacing molecules to perovskites, HTM, and the interface between perovskite and HTM. It is very clear that introducing proper materials to alleviate aforementioned issues could largely improve the stability of PSCs. However, to further overcome the stability bottleneck and move toward to the commercialization of PSCs, the perovskite surface, HTM, and interface between perovskite and HTM should all be considered, not indiviually. This is the inspiration and subject of my Ph.D. research. I have participated and led each of researches described in Chapter 3. I performed everything from the perovskite material synthesis to device fabrication, characterization, and stability assessment. The novel molecule to conduct researches is synthesized by my co-workers Dr. J. Lee, Dr. X. Liu, and H. Lim for F-PBTBDT, IDTT-ThCz, and OAN3, respectively. In Chapter 1, the brief introduction of the development in PSCs including deposition method, device architecture is provided and origin of instability in PSCs is described. After then, I have provided a summary and the on-going molecular engineering strategies in improving the stability of PSCs with the criteria of molecular engineering in (1) HTM (2) crystallization (3) interface. In Chapter 3, I have aimed to perform molecular enginnering in HTM part to to replace the conventionally used HTM, spiro-OMeTAD and introduce dopant-free polymeric HTM. The novel dopant-free HTM possesses F-containing side chain, which could attribute to the moisture stability of PSCs by preventing moisture penetration. Additionally, this dopant-free polymer exhibits a suitable energy level matching with perovskite and facilitates the hole extraction. Because of these advantages, PSCs with novel HTM achieved not only a high efficiency, but also high stability under extremely humid conditions for 1000 h. After that, I have tried to perform molecular engineering on perovskite filmperovskite film by depositing additives during the perovskite crystallization process. Through the previous research, I have found out that changing the conventional HTM to dopant-free and hydrophobic HTM could increase the long-term stability. However, the side penetration of external stimulus could not be protected. Therefore, modification on the perovskite film is required and I have tried to introduce a novel molecule during perovskite crystallization, which could interact with perovskite crystals and passivate the defects on the perovskite surface. Furthermore, I have also introduced polymeric HTM to further improve the stability. The resulting PSCs exhibits a higher power conversion efficiency and long-term stability compared to the non-treated PSCs. Lastly, based on the previous researches, I employed a dual functional additive, octylammonium azide, which could passivate the defects in perovskite and provide interaction with dopant-free HTM, improving both efficiency and long-term stability. What is noteworthy is that the HTM used in this part do not contain hygroscopic dopants. In this study, I have implemented in-depth study to improve the development of stability in PSCs by the step-by-step molecular engineering. I believe that this result suggests a promising new way in the field of perovskite based optoelectonic devices not only solar cells but also different electronic devices.