Theoretical Studies On Structures And Properties Of Thiophene Derivative-based Conjugated Polymers In Organic Solar Cells

Organic solar cells (OSCs) have attracted considerable attention owing to their special advantages, including simple, low cost fabrication, flexibility and lightweight. In recent years, OSCs have gained a rapid progress in the power conversion efficiency (PCE). For the conventional and inverted single-junction solar cells, PCE has been brought up to ～10% through developing novel D-A conjugated polymers with lower band gap and optimization of device fabrication. But to realize the commercialization application of OSCs, its PCE needs further enhance. As the development of computer technology and quantum theory, computational simulations are applied in material science field. In this dissertation, we have designed some D-A conjugated polymers and A-D-A small molecules, and their structures, electronic and optical properties have been theoretically studied using TDDFT and MBGFT methods.In addition, halogen bonds are specific noncovalent interactions between a covalently bonded halogen atom in compounds of R-X (X= F, Cl, Br and I) and a Lewis base. Because the chargedensity on the covalent halogen atom is anisotropic distribution, halogen bond possesses a greater tendency to linear directionality than hydrogen bond. And interactional energies fall into the range of 5～180 kJ/mol. In this dissertation, we have been studied the influences of metal atoms and polyvalent halogen atoms on the strength and bonding characteristics of halogen bonds. The main contents are summarized as follows:1) Benzotrithiophene (BTT) is symmetric, perfect planar and electron-rich, which facilitate the intermolecular Ï€-Ï€ interactions and charge transport. BTT family should have seven possible isomers, including benzo[2,1-b:-3,4-b’:5,6-c"]trithiophene (BTT1), benzo[2,1-b:-3,4-b’:5,6-b"]trithiophene (BTT2), benzo[l,2-b:-4,3-b’:5,6-c"]-trithiophene (BTT3), benzo[1,2-b:4,3-b’:5,6-c"]trithiophene (BTT4), benzo[1,2-b:-3,4-c’:5,6-c"]trithiophene (BTT5), benzo[1,2-b:3,4-b’:5,6-b"]trithiophene (BTT6), benzo[1,2-c:3,4-c’:5,6-c"]trithiophene (BTT7). These BBTs have high coplanarity and extended Ï€-conjugation, and are expected to serve as donor units for designing low-band gap D-A conjugated polymers. A new BTT2-based polymer possesses a broad bandgap and deeper HOMO energy level, and the assembled device based on BTT2-based polymer achieved the highest PCE of 5.06%. In this chapter, we have investigated the structures, electronic and optical properties of five conjugated copolymers (BTT1-BTz, BTT2-BTz, BTT3-BTz, BTT4-BTz and BTT5-BTz) featuring BTT isomers as donor units and benzothiadiazole (BTz) as acceptor units, linked through thiophene spacers, employing MBGFT. The calculated low and high energy absorption bands (660 and 413 nm) for BTT1-BTz agree well with the experimental ones (645 and 430 nm) with a small offset of ～15 nm. Importantly, both BTT1-BTz and BTT2-BTz exhibit intense adsorption around 660 and 623 nm, indicating their promising application in solar cells, whereas BTT3-BTz and BTT4-BTz display the intense adsorption at 569 and 551 nm, which are also usable in the tandem solar cells.2) To acquire high efficient PSCs, relative broad absorption, suitable energy levels and good solubility in organic solvents are fundamental prerequisites for conjugated polymers. By selecting D and A units of the D-A polymers, HOMO and LUMO energy levels as well as the bandgaps of polymers can be tuned at some extent. Up to now, many high performance polymers have been designed and successfully synthesized utilizing this strategy. Because NDT have large coplanar area and longe conjugation length, NDT may be used as a potential candidate in the development of D-A type conjugated polymers. In this chapter, a series of NDT-based polymers, coded as NDT-A0 to NDT-A12, have been designed and theoretically investigated using the CAM-B3LYP/6-311G(d,p) method for geometrical optimization and TD-PBE0/6-31G(d) method for TDDFT calculation. The absorption spectra of NDT-A7, NDT-A10 and NDT-A11 can extend much farther into the near-infrared region, whereasthe absorption spectra of the other polymers fall into the visible region within 900 nm. These polymers may meet the particular requirements of the tandem and ternary solar cells on the donor components.3) Compared with D-A conjugated polymers, acceptor-donor-acceptor (A-D-A) small molecules have some advantages, e.g. exact structure, high open circuit voltage (Voc) as well as good solubility and chemical and thermalstability. Chen et al. have designed and synthesized a new BDT-based A-D-A small molecule and, a PCM of OSCs based on this organic molecule is over 9%. In this chapter, the electronic structures and optical properties of BDT-Tm-ED (m=0-7) were performed by MBGFT. Our calculation results reveal that Ï€-conjugated bridge can be beneficial to the charge transfer between A and D units. When five thiophenes are used as Ï€-conjugated bridge, A-D-A small molecule has the best optical properties.4) Halogen bonds are a directional and attractive interaction between a positives-hole of a covalent halogen atom and a Lewis base. Traditional halogen bonds can be depicted as R-X…Y-R’, in which X such as F, Cl, Br, and I are usually monovalent. However, halogen elements have tervalent, pentavalent and septivalent oxidation state. In order to understand the nature and properties of halogen bonds involving these polyvalent halogen, we have studied halogen-bonded complexes of FXOn (X= Cl, Br; n= 0-3)-CH3CN using M05-2X and MP2 methods. Our calculations reveal that, in thelinear halogen bonding complexes, the most important orbital interactions are nâ†'σ*, while in the cyclic complexes, the mostimportant orbital interactions are Ï€â†'σ*. And halogen bonds are mainly electrostatic interactions and the nature of the intermolecular interactions involving polyvalent halogen is similar to traditional halogen bonds.5) In recent years, due to widely applicationin the supramolecular networks, the metal-influenced halogen bonds C-X…X’-M (Mrepresents metal) attracted much attention. Perutz et al. have studied metal-bound halogen bonds between group 10 metal monohalide compounds and iodoperfluoroarenes. But we do not know the nature of halogen bonds invoving the group 10 metal chlorides and bromides, and the reason why the dates of halogen bonding energies follow a trend of Ni> Pd> Pt. Thus, we have studied halogen bonding interactions between C6F5I and a series of transition metal monohalides trans-[M(X)(2-C5NF4)-(PR3)2] (M= Ni, Pd, Pt; X= F, Cl, Br; R= Me, Cy) by quantum chemical calculations. Our calculations reveal that, for trans-[M(F)(2-C5NF4)-(PR3)2] (M= Ni, Pd, Pt) monomers, the formed halogen bonding complexes are stabilized by local concentration of the charge of intermediate character, while for the metal monomers containing chlorine and bromine, a typical closed-shell interaction exist.In the other part, ab initio MP2 calculations have been performed to study halogen bonds formed between MA13" (M= Si, Ge, Sn, Pb) and YX (Y= HCC-, F3C-, HO-; X = Cl, Br, I). Our calculations have found that four halogen-bonded interaction modes have been recognized for complexes of MA13"-XY, and are coded as MAl3--XY-1, MAl3--XY-2, MAl3--XY-3 and MAl3--XY-4. The halogen bonds in the HCCCl/Br- and F3CCl/Br-containing complexes belong to traditional halogen bonds, while those in SiAl3--BrCF3-1, GeAl3--BrCF3-1, HCCI-and F3CI-containing complexes are halogen-shared halogen bonds. The halogen bonds in the HOX-containing complexes belong to covalent bonds or halogen-shared halogen bonds.