Theoretical Investigation On The D-A Donor And D-A Acceptor Of Organic Solar Cells

Organic solar cells (PSCs), whose the active layers typically consist of donor materials and acceptor materials, are becoming very attractive because of their advantages of low cost, light weight, and easy processing. According to the materials of active layer, organic solar cells can be divided into organic small molecules solar cells and polymer solar cells. In particular, organic polymer solar cells have become a hot research area. Significant progress has achieved in power conversion efficiencies (PCE), however, the devices have not yet entered the market for commercial applications. Hereon, we attempted to design the new efficient donor and acceptor materials for improving the performances of organic solar cells. We hope that the present results could provide a theoretical guidance for experimental workers in the research on organic solar cells. This paper generally includes the following three aspects:1. Theoretical Studies on the Effect of New Ladder-Type Structure for the Performance of D-A Polymer DonorConsiderable papers have reported the ladder-type polymer donors in previous work, and almost all of them focus on electron-rich units of the polymer donors. Actually, it is the first time that the electron-deficient unit of D-A polymer donor was designed as the ladder-type multifused structure. DFT and TD-DFT were applied to calculate the HOMO/LUMO levels, optical absorption spectrum and charge transfer rate of D-A donors. Compared to the D-A donors based on the ladder-type electron-rich unit, the D-A donors composed of the ladder-type electron-deficient unit also possess the following advantages:(1) extending intramolecular π conjugation, (2) reducing the energy gap and reorganization energy, (3) facilitating the interchain π stacking and so on. Then the substituent groups (electron-donating or electron-withdrawing groups) were introduced at the ladder-type electron-deficient unit of D-A polymer donor to further improve the electronic, optical and charge transfer properties of those D-A donors. According to the calculated results, we conclude that the introduction of a suitable ladder-type electron-deficient unit at D-A donor can evidently improve the performance of the donors in the PSCs.2. Explore the Effect of the Double-Substituted Bithiophene Bridge with Different Substituent Groups (R=H, CH3,OCH3 and CN) in the Performance of D-π-A DonorUp to now, there has been already considerable research on the design and synthesis of new D or A building blocks of the donors for the sake of enhancing the photoelectric conversion efficiency, nevertheless, the studies focusing on the bridges of D-π-A donor are insufficient. Although some papers have reported that the introduction of different π-bridges can improve the photovoltaic performances of D-π-A donors, however, few studies concentrated on the modification of D-π-A donors by introducing different substituent groups at π-bridges. Hereon, we investigate the effect of a bithiophene bridge with different substituent groups on the HOMO/LUMO levels, light-absorbing efficiency, reorganization energy and carrier mobility of D-π-A polymer donors. The calculated results demonstrate that the substituents in the bithiophene-bridge have a significant effect on the ground state structure, electronic, optical properties and hole mobility of D-π-A copolymers. In addition, we found that the introduction of the electron-donating groups and electron-withdrawing groups into the same bithiophene-bridge can not only easily tune the energy levels and energy gaps of the copolymers, but can also improve the charge transport properties and the photovoltaic performances.3. A Theoretical Strategy to Design Novel n-Type Copolymers Based on Anthracene Diimide and Pyrido[2,3-g]quinoline Diimide for Organic Solar CellsGreat attention has been devoted to the donors for polymer solar cells (PSCs) in previous works, however, much less attention has been paid to the acceptors. PC61BM/PC71BM ([6,6]-Phenyl-C61/C71-butyric acid methyl ester) is the most used acceptor in PSCs, nevertheless, the weak light absorption in the visible region and low LUMO energy level are two disadvantages for PC61BM/PC71BM as an excellent acceptor. Thus, the design and synthesis of efficient acceptors instead of PC61BM/PC71BM are a promising research field. In recent years, the aromatic diimide derivatives are the most promising candidates for acceptor materials in PSCs due to their excellent electronic and optical properties, such as naphthalene and perylene diimide derivatives. Here, we first theoretically explore the problem that what is the effect of the molecular backbone planarity and the electron-drawing ability of electron-deficient unit on the HOMO/LUMO levels, light absorption range and light-absorbing efficiency, the exciton separation efficiency at donor/acceptor interface as well as intermolecular electron mobility of the D-A non-fullerene copolymer acceptors. From the calculated results, the performance of D-A copolymer acceptors can significantly improve by enhancing the coplanarity of polymer backbone and increasing the electron-withdrawing ability of electron-deficient unit. This study presents us with a designed guideline for designing the non-fullerene copolymer acceptors.