Molecular Design Of DA Alternative Donor Materials And Theoretical Investigation Of The Optoelectronic Performances For Organic Solar Cells

In this paper, the photoelectric properties of three kinds of DA alternative donor materials for organic solar cells were investigated by Density Functional Theory(DFT) and Time-dependent Density Functional Theory(TD-DFT). We simulated polymer and small molecule donor materials to explore the relationship between molecular structure and properties. Our main purpose is to provide some effective strategies of donor molecular design for experimental work.In Chapter 1, we presented a brief introduction about the divice structures and the working principles of organic solar cells. Besides, the donor/acceptor materials and the research status of organic solar cells were also introduced. Furthermore, we also adressed the importance of our work.In Chapter 2, a detailed introduction of the theoretical methods(DFT and TD-DFT) mainly applied in this paper were presented. On this basis, we introduced some other important theretical methods and measures, including atoms in molecular(AIM) theory, nucleus-independent chemical shifts(NICS) and nature bond orbital(NBO). In addition, the Marcus theory for electron transfer in photoelectric material applications was also briefly presented.In Chapter 3, A series of novel copolymers containing benzo[1,2-b:4,5-b’]dithiophene(BDT) and thieno[3,4-c]pyrrole-4,6-dione(TPD) derivatives were simulated by DFT and TD-DFT. We performed a systematic study on the influences on molecular geometry parameters, electronic properties, optical properties, photovoltaic performances, and intermolecular stacking as well as hole mobility when different chalcogenophenes in TPD derivatives were used and functional groups with different electron-withdrawing abilities such as alkyl, fluorine, sufonyl, and cyano were introduced to the nitrogen positions in electron-deficient units. The substitution position of electron-withdrawing groups may cause little steric hindrance to the neighboring donor units,especially fluorine and cyano group. It was found that the incorporation of these newelectron-deficient substituents and sulfur-selenium exchange can be applicable to further modify and optimize existing molecular structures. Our findings will provide valuable guidance and chemical methodologies for a judicious material design of conjugated polymers for solar cell applications with desirable photovoltaic characteristics.In Chapter 4, Based on the already synthesized donor molecule(1s) used in organic heterojunction solar cells, six new A-A-D-A-A type small molecules composed of different donor(D,electron rich) and same acceptor(A, electron poor) moieties were designed and characterized by using DFT, TD-DFT and Marcus theory. Through calculating frontier molecular orbital energy levels and spectra properties, we found that the value of HOMO level goes up while the value of Eg goes down along with the cycle extension of the center part flanked by bithiophene. Next, character of excited state, ionization potentials(IPs) and charge transport properties were also investigated to provide an in-depth insight into the charge transfer/transport characteristics and the JSC of organic solar cells. The results reveal that compounds 3c, 3n and 3o exhibit favorable JSCand comparable performance to original molecule(1s) and show promising potential in small-molecule organic solar cells.In Chapter 5, Two novel strategies to tune diketopyrrolopyrrole and its derivatives by lactam-lactim and alkoxy-thioalkoxy exchange were theoretically presented for improving the efficiency of organic small-molecule solar cells. The structural tunings could synergistically reduce the HOMO level and the energy gap of donors due to the disrupted aromaticity of the linked pyrrole derivatives. Compared with the parent molecule, the new designed donors not only create a more red-shift of absorption spectrum but also show better hole transport rates, larger fill factor, higher open circuit voltage and more favorable solar cell efficiency. Moreover, the arrangements at D/A interface and the optical properties of donor-PC61 BM complexes were computationally investigated to get insight into the absorption of charge transfer states. Consequently, the strategies are judicious approaches to enhance their intrinsic properties of donors and can be used for further improving the performance of other DPP-based molecules in bulk heterojunction solar cells.