Molecular Design and Device Engineering for High-efficiency Organic Solar Cells

Abstract

Organic solar cells (OSCs) have emerged as a highly efficient and viable pathway for harnessing solar energy. This technology utilizes carbon-based semiconductors instead of inorganic materials, which facilitates low-cost, lightweight, and flexible roll-to-roll manufacturing of large-area panels with a minimal energy payback period. Recent advancements have propelled power conversion efficiencies beyond 20%, positioning OSCs as increasingly competitive with established solar technologies. The development of high-performance OSC systems rests on three critical pillars: stability, efficiency and reproducibility. Research guided by these principles largely falls into two interconnected domains: material development and device engineering. This thesis will elaborate on my research contributions from both of these pivotal perspectives.

In Chapter II, a novel fluorine-and methoxy-co-substituted terminal group is synthesized and successfully developed the corresponding asymmetric small molecular acceptors (SMAs), namely BTP-BO-3FO. A remarkably power conversion efficiency (PCE) over 19% for non-toxic solvent was achieved when BTP-BO-3FO was used as the third component in PM6: BTP-eC9 based OSCs, emphasizing the importance of halogen-and methoxy-co-substituted ending groups and the asymmetric substitution strategy in developing high performance small molecule acceptors. In addition, a PCE exceeding 19% can be attained by substituting blade coating in the deposition of the blend films. This study centers on clarifying issues that are commonly encountered yet often overlooked in the construction of ternary blends aimed at achieving cutting-edge device performance; thus, it is expected to offer guidance for other research on ternary OSCs in the future.

In Chapter III, we are trying to solve the problem of inadequate charge separation caused by BTP-BO-3FO's large bandgap and suboptimal active layer morphology, which results in relatively low PCE in the binary system. To minimize the offset in energy levels while ensuring adequate charge separation, we later introduced a fluorine atom into the fluorine-and methoxy-co-substituted terminal group to synthesize an IC-2FOMe terminal group and synthesized the corresponding asymmetric SMA, namely BTP-BO-4FO. By pairing with the polymer donor PM6, it realizes a remarkable efficiency as high as 18.62%, which is one of the highest PCEs for binary device based on NFAs with asymmetric terminal groups. This study presents a novel end group that enables state-of-the-art PCE in asymmetric acceptor-based organic photovoltaics, curating an expanded material selection and delivering strategic design insights.

In Chapter IV, we report a study of case of combining solid additive (newly proposed 2-monobromo-1,3-dichloro-bezene (BDCB)) and layer-by-layer (LBL) processing. By varying the BDCB’s concentration in PM6 solution and L8-BO solution, the best device fabrication condition is located to be PM6 -20 mg mL^-1 and L8-BO -10mg mL^-1, achieving an excellent PCE of 19.03% higher than 18.03% of bulk heterojunction (BHJ) structure. Revealed by plenty of morphology characterizations, the LBL method with proper BDCB in L8-BO solution can lead to acceptor’s fibrillization, and varying the BDCB’s concentration in PM6 can change the small molecule’s locating and crystallizing condition. Overall, not only a cutting-edge efficiency for binary organic solar cells (OSCs) are reported in this work, but a smart strategy and deep understanding is produced by this work focusing on layer-by-layer optimization and solid additive’s working mechanism.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	He YAN (Supervisor) & Sai Ho PUN (Supervisor)