Studies on Efficient Nonfullerene Acceptors Based on Dicyanodistyrylbenzene and Naphthalimide Units for Organic Solar Cells

Abstract

Developing advanced organic materials for solution-processed bulk-heterojunction (BHJ) organic solar cells has been attracting much attention in the past few decades, exhibiting power conversion efficiencies (PCEs) of the resulting devices exceeding 10%. Until now, such material developments have focused mostly on the high-performance donor materials because fullerene-based PC61BM/PC71BM have been exploited almost exclusively as the acceptor in BHJ devices. Although the conventional PC61BM and PC71BM acceptors have outstanding electron accepting/transporting abilities and favorable compatibility with a large number of the donor materials, fullerene derivatives have some drawbacks of weak visible light absorption and difficulty of energy level control, limiting the further improvement of the device efficiency. Accordingly, nonfullerene electron acceptors are emerging as promising acceptor alternatives to overcome the difficulties of fullerene derivatives in tuning optical and electronic properties. Through the innovative molecular designs, nonfullerene acceptors could have enhanced light harvesting abilities and finely tuned frontier molecular orbital energy levels, providing possibilities of enhanced device efficiencies, especially with increased open circuit voltage (Voc). To date, a variety of nonfullerene small molecule/polymeric acceptors have been reported in combination with several high-performance donor materials to have PCEs over 8%. This research focuses on the development and application of novel nonfullerene acceptor materials for BHJ organic solar cells. The main structure of the new acceptors used in this work is a dicyanodistyrylbenzene moiety because its derivatives have exhibited outstanding electron transporting properties in various optoelectronic applications. However, their too strong self-assembly power to form meso or micro scale structures have limited their use for BHJ solar cells. Therefore, as a specific method for modulating self-assembly tendency, a bulky naphthalimide moiety, another typical electron-withdrawing unit, was incorporated at the terminal position of the molecule (DCS-NI acceptor). The resulting compounds, NIDCS, NIDCS-MO, and NIDCS-HO, showed uniform film formation behavior attributed to twisted molecular conformation by steric hindrance between the two moieties. Furthermore, by synergistic effect of both moieties on electron accepting and transporting abilities, optical, electrochemical, and electrical characteristics of the DCS-NI acceptors were demonstrated to be suitable as the acceptor for organic solar cells. The solar cells using a prototype p-type polymer P3HT as the donor exhibited reasonable PCEs of a maximum 2.7% that come close to that of PC61BM-based devices. They also showed different device characteristics with different alkoxy substituents on a core phenyl unit of the DCS-NI acceptor. (Chapter 2) Next, to realize high-performance devices, further nonfullerene organic solar cells using other high performance donor materials with the DCS-NI acceptors were fabricated. By comprehensive investigations of diverse donor–acceptor combinations, a small molecule donor p-DTS(FBTTh2)2 with the NIDCS-MO acceptor and a polymer donor PPDT2FBT with the NIDCS-HO acceptor combinations were selected, showing remarkable solar cell PCEs of 5.4% and 7.6%, respectively. In addition, exceptionally high Voc of 1.03 V was also attained in the latter system. In both combinations, by thermal annealing, nanoscale structures and morphologies were successfully modulated with forming favorable nanoscale phase separation toward efficient charge generation and transport. Furthermore, complementary absorption, a high and balanced charge transport property, and minimized charge recombination processes all helped to improve the device performances. (Chapter 3) Lastly, a new structure of the DCS-NI acceptor, NIDCSN, having better processability in various organic solvents was designed and synthesized. Compared to the previous acceptors which have been optimized only in CF solvent processing, NIDCSN showed uniform film formation characteristics even in other solvents like CB, THF, toluene, and o-xylene. The resulting nonfullerene all-small-molecule solar cells comprising p-DTS(FBTTh2)2 as the donor exhibited a maximum power conversion efficiency of 3.5% with a remarkable Voc of 1.04 V in the chloroform solvent condition. Besides, the solar cells also showed similar device performances when fabricated in five different solvents including environmentally benign ones. (Chapter 4)