유기태양전지의 분자구조-모폴로지-성능 관계 연구

Abstract

Organic solar cells (OSCs) have attracted considerable interest in the photovoltaic community due to their superior properties such as light-weight, solution processability, tunable energy level, transparent properties, and mechanical flexibility. With tremendous efforts such as material development and device manufacturing process optimization, OSCs have now achieved efficiencies of over 19%.

However, identifying the nano-morphology of bulk-heterojunction (BHJ) photoactive blends is one of the most important tasks in the field of OSCs. The complex morphology, which ranges from binary blends to ternary systems, displays a wide range of structural characteristics that must be well defined in order to relate them to device performance. Structure optimization is challenging due to the non-equilibrium nature of the BHJ thin-film morphology. In addition, fundamental principles relating to molecular structure, fabrication techniques, and device performance are still not well understood. In the meanwhile, a variety of high-power methods, from internal equipment to synchrotron technology, have been used to tackle this extraordinarily challenging research topic. Quantitative techniques may be used to get more structural information, such as the length scale of chain/molecule orientation, structural order, phase separation, surface topology, etc., which will ultimately help identify the molecular structure-device performance correlations.

This is the inspiration and subject of my Ph.D. research. I performed material synthesis, characterization, and morphological analysis except for the device fabrication. The novel materials to correlate the molecular structure-morphology-performance were synthesized by Dr. Z. Abbas and Z. Rehman for BDTID-X, Y6-analogs, and PTF terpolymers. In addition, device fabrication was conducted by M. Haris, M. Zahankhan, and D. H. Ryu.

In Chapter 1, the brief introduction of the development of OSCs including donor polymers and NFAs is described. After then, I have provided the importance of morphology control to achieve high-performance OSCs and the morphology control strategies for organic semiconductors. Finally, the morphological characterization method utilizing the X-ray synchrotron is briefly introduced.

In Chapter 3, I have aimed to elucidate the molecular structure-morphology-device performance relationship. First, systematic halogenation is performed on the end-groups of the small molecule donor material. The chlorinated compound has high backbone coplanarity and strong dipole moment. These molecular characteristics facilitate intermolecular interactions, leading to efficient charge transport. The well-mixed blend morphology without large-scale phase segregation forms interconnected networks comprising excellent charge transport pathways. This morphological characteristic can assist efficient exciton dissociation and charge transport, improving the photovoltaic parameters. Second, systematic alkyl chain engineering is conducted on Y6 NFA to increase the solubility in the non-halogen solvent. The subtle change in alkyl chain improves solubility in halogen-free solvent and increases the device performance. In particular, FF values change dramatically according to the chain length. In-depth morphological analyses reveal that the systematic Y6 inner side-chain modulation not only provides non-halogen solvent processability, but also enables the formation of the donor polymer-rich top surface with favorable vertical phase separation. Third, a random copolymerization strategy is employed to synthesize high-performance terpolymer donors for non-halogen solvent processing, the terpolymer strategy effectively reduces aggregation behavior, resulting in better device performance. Finally, narrow bandgap donor polymers for semitransparent OSC (ST-OSC) applications are designed and synthesized. The morphological analyses revealed that the sulfur containing polymer has favorable BHJ morphology with acceptor materials, resulting in higher photovoltaic performance. Furthermore, when the material introduced on the ST-OSC, high AVT with high LUE are achieved.

In this study, an in-depth morphological analysis was conducted to identify the molecular structure-morphology-device performance relationship. I believe that the results of this study will provide valuable insights and guidance to the OSC field, opening new avenues for the development of novel materials for morphology optimization and performance improvement.