Non-fullerene Small Molecule Acceptors For Organic Photovoltaic Application

Fullerenes and their derivatives have been dominant electron acceptors in the field of organic solar cells (OSCs) for their merits of large electron affinity, high electron mobility, and isotropy of charge transport. Nevertheless, their drawbacks such as limited absorption in the visible region and high production cost, strongly restrict their practical applications. Thereby, novel non-fullerene acceptors with narrow band gap and readily accessible synthetic processes attract more and more interests. Taking these considerations into account, in this dissertation, a series of D-A non-fullerene small molecule acceptors with isoindigo (IID) and diketopyrrolopyrrole (DPP) as electron-accepting units are contructed, and their photovoltaic properties are investigated. The details are described as follows:1. A linear small molecule acceptor based on a central cyclopentadithiophene (CPDT) moiety flanked with two isoindigo (IID) units, CPDT(IID)2, has been desigened and synthesized. Owing to its narrow band gap as well as the suitable energy levers, the OSCs with CPDT(IID)2 as the acceptor and poly(3-hexyl thiophene) (P3HT) as the donor give a high open-circuit voltage (VOC) of 1.02 V and a power conversion efficiency (PCE) of 0.97%. It is firstly reported that the IDD unit-based non-fullerene small molecule acceptor is applied in OSCs.2. A linear small molecule acceptor with a fluorene (F) moiety as the core and two IID units as the end-groups, F(IID)2, has been designed and synthesized. The OSCs based on P3HT:F(IID)2 blend provide a PCE of 0.79%. Through the further chemical modifications of F(IID)2, three small molecule acceptors are prepared:one is the fluorinated derivative of F(IID)2 (F(FIID)2); Due to the electronegativity of F atom, the HOMO and LUMO energy levers of F(IIDF)2 are both 0.05 eV lower than those of F(IID)2, in favor of the dissociation of the photogenerated excitons; The other two acceptors are obtained through the insertion of one or two thiophene rings between fluorine and IID units ((F(TIID)2 and F(TTIID)2), leading to higher light-havesting capabilities than F(IID)2. However, the photovoltaic properties of three acceptors are all worse than that of F(IID)2, which can be ascribed to their bad miscibility with P3HT.3. Three small molecule acceptors with a phenyl ring (Ph) as the core and DPP units as the arms, Ph(DPP)2, PhDMe(DPP)2, and Ph(DPP)3, have been designed and synthesized. For PhDMe(DPP)2, because two methyl groups are attached to 2,5-positions of phenyl ring, the dihedral angles between the core and the DPP arms increase, leading to the worst molecular planarity. Therefore, P3HT:PhDMe(DPP)2 blend gives fine phase-seperation domains, and the resulting OSCs show the best PCE of 0.65% among three types of OSCs. Our results proves that the geometry structure of non-fullerene acceptors play a key role in the performances of OSCs.