Studies on Non-Fullerene Electron Acceptors Based on Benzothiadiazole and its Derivatives for Organic Solar Cells

Abstract

Solar cells that convert sunlight to electricity can be divided into inorganic solar cells (ISCs) and organic solar cells (OSCs). OSCs have been extensively studied due to their advantages of low production cost, light weight, flexibility and semitransparency compared to ISCs. As one of the important key factors for high-efficiency OSCs, an active layer in conventional OSCs generally consists of the bulk heterojunction mixture of the polymers as an electron donor and fullerene and its derivatives, including PC61BM, PC71BM and ICBA, as electron acceptor. In the early stage of OSCs development, fullerene derivatives were widely used as an electron acceptor for OSCs due to their strong electron affinity, high electron mobility and isotropic charge-transport behaviors. However, the development of high-efficiency OSCs is still limited due to inherent weak light absorption, inferior ambient photochemical stability, limited energy level variability and high production cost. For this reason, the development of new electron acceptors to replace fullerene derivatives has been required. As alternatives to fullerene derivatives, non-fullerene small molecule electron acceptors (NFAs) have been vigorously explored due to the advantages of high purity, easy purification, batch-to-batch reproducibility, and well-defined chemical structure. Recently, most of the high-efficiency NFAs are A-DD′D-A type (or A-DA′D-A-type) fused-ring electron acceptors (FREAs), which have a large ladder-type fused-ring central core. FREAs consist of the conjugated large planar fused-ring as the central core and two strongly electron-withdrawing groups as the terminal units. The rigid and planar fused-ring core is beneficial for electron delocalization and intermolecular stacking, and two terminal units are favorable for enhancing the push-pull electronic effect. However, the synthesis of FREAs is very complex, low yield, and expensive. Their drawbacks limit their commercial application of OSCs. Therefore, the development of simple and efficient NFAs is demanded to overcome the drawbacks of FREAs. A-D-A′-D-A-type nonfused-ring electron acceptors (NFREAs) have recently been considered as an alternative to FREAs due to their simple synthesis, high yields, and low cost. NFREAs generally consist of one non-fused core (A′), two bridge units (D), and two terminal groups (A). The structure of NFREAs can increase the rotation of the C–C single bond between A′ and D units and decrease their conformational stability. In this way, the crystallinity of NFREAs can be decreased, resulting in poor charge transport. To overcome these problems, careful design and synthesis of NFREAs are required. In chapter 2, I investigated how the symmetric conformation of NFREAs the microstructural crystallinity and efficiency of donor polymer:NFREA OSCs. Because of the rotatable C–C single bond between A′ and D units in NFREA, NFREAs suffer from weak intermolecular π–π interactions and inferior crystallinity. To overcome these problems, I successfully designed and synthesized two NFREAs, maned NTz-4F and BT-4F. They have different A′ cores: NTz-4F has a centrosymmetric NTz core, whereas BT-4F has an axisymmetric BT core. A comparison of NTz-4F, which has a centrosymmetric core, and BT-4F, which has an axisymmetric core, enables an investigation on the effects of the A′ core symmetry on the microstructural crystallinity. In pristine films, NTz-4F shows substantially enhanced intermolecular interaction and microstructural crystalline ordering compared with BT-4F. When blended with PBDB-T as a donor polymer, the power conversion efficiency (PCE) for NTz-4F-based devices (9.14%) is higher than that of BT-4F-based devices (7.18%). The higher PCE of NTz-4F-based devices is attributable to the strong crystalline nature of NTz-4F, which leads to improved charge transport properties and suppressed nonradiative voltage loss. These results demonstrate that introducing a centrosymmetric A′ core is also an effective method to improve the PCEs of OSC devices. In chapter 3, I investigated how the extended conjugation length of A’ core in NFREAs affects their energy level and the miscibility of donor polymer:NFREA OSCs. I systematically designed and synthesized three NFREAs: BT-4F, which has a BT core: DTBT-4F, which has two fused thiophene rings on the BT core: and BTST-4F, which has two fused thienothiophene rings on the BT core. In these NFREAs, the LUMO level is upshifted in proportion to the length of the extended π-conjugated core: BT-4F < DTBT-4F < BTST-4F. Open-circuit voltage (VOC) is related to the gap between the LUMO of NFREAs and HOMO of donor polymer, so VOC in OSCs that used them increased from 0.66 V for BT-4F to 0.80 V for DTBT-4F and to 0.85 V for BTST-4F. PBTB-T:BTST-4F blends have a narrower light-absorption range than PBTB-T:BT-4F blends, but because of the miscible morphologies of PBTB-T:BTST-4F blends, OSCs that use BTST-4F (BTST-F4 OSCs) show comparable short-circuit current density JSC to BT-4F OSCs.