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Abstract

In order to study the relationship between the bandgaps and linking bonds, three kinds of model polymers (DA1, DA2 and DA3) consisting of electron-rich (D) and electron-deficient units (A) were synthesized. They had the same electron-rich and electron-deficient groups, but were linked together by aryl-aryl (DA1), aryl-vinyl-aryl (DA2), and aryl-ethynyl-aryl (DA3), respectively. This result indicated that changing the linkers between the electron-rich and electron-deficient units can tune the HOMO energy levels of the copolymers.

a. The optical and electrochemical properties results show that, compared to the aryl-aryl and aryl-vinyl-aryl based polymers, the aryl-vinyl-aryl structure can keep almost the same LUMO energy level and increase the HOMO energy level of copolymer.  Therefore, it led to the lowest bandgap of copolymer.

b. The quantum-chemical calculations results show that DA2 and DA3 have a higher degree of co-planarity (no larger than 5° dihedral angle), which may improve carrier mobility of solid film.

1.     Experimental section

According to the synthesis route shown in Figure 1, three kinds of model polymers (DA1, DA2 and DA3) consisting of electron-rich (D) and electron-deficient units (A), were synthesized via Suzuki, Stille or Sonogashira polycondensation, respectively.  They had the same electron-rich and electron-deficient units (disubstituted 3,4-dialkyl thiophene monomers and benzothiadiazole monomers), but were linked together by aryl-aryl, aryl-vinyl-aryl, and aryl-ethynyl-aryl respectively. 

Figure 1. Chemical structures and synthesis route of copolymers with different bonds.

2.     Optical and Electrochemical Properties.

The UV-vis absorption spectra of the three copolymers in solution and on solid films are shown in Figure 2.  The polymers' bandgaps are in the following order: DA1 (2.22 eV) > DA3 (1.91eV) > DA2 (1.65 eV).  Comparing the solution and solid film absorption, the DA2 was red-shifted by 62nm, whereas the DA1 was 10nm and DA3 was 50nm, indicating the DA2 unit promotes better interchain or intermolecular interaction in solid films. DA2 has the strongest red-shifted and the lowest bandgap, which is likely due to a better p-p stacking of the polymer backbone in the solid state.

Figure 2. UV-vis absorption spectra of DA1, DA2 and DA3 in dilute chloroform solutions and on solid film (spin-coating from chloroform solution).

The HOMO energy levels were determined from cyclic voltammetry, and LUMO energy levels were roughly estimated from the difference between HOMO and the optical bandgap (E_g) of the polymer films. Interestingly they exhibited almost the same LUMO energy levels, but their HOMO energy levels were very different (Table 1). Thus, they show different bandgaps. DA2 was found to have the highest HOMO energy level. This result indicated that changing the linkers between the electron-rich and electron-deficient units can tune the HOMO energy levels of the copolymers.

Figure 3. Cyclic voltammetry of the copolymers DA1, DA2 and DA3 in 0.05M TBAPF₆ acetronitrile.

3. Quantum-chemical Calculations.

All the calculations were performed with Gaussian03 program, using the density functional theory (DFT) at B2LYP/6-31G (d, p) levels.  Figure 4 shows the side-view and front-view of a single chain oligomer backbone structure.  The optimized structure and geometrical parameters of oligomers show that DA1 had a dihedral angle (about 125^o) between the thiophene and thiadiazole units, which would reduce the delocalization in the backbone. DA2 and DA3 exhibited a small dihedral angle (no larger than 5^o), which might has little effect on the delocalization on the coplanar backbone structure.

DA1

DA2

DA3

Figure 4. Side-view (left) and front-view (right) configuration of the copolymer single chain calculated from density functional theory (DFT) at B2LYP/6-31G (d, p).

As shown in Table 1, the bandgaps of DA2 and DA3 are in good agreement between the theoretically predicted values and experimentally optical bandgap (E_g) values. However, the bandgap of calculated results for DA1 (2.74 eV) is somehow different from the experimental value (2.22 eV). Regarding to the order of bandgaps, the DFT calculation results were found to be in good agreement with the experimental results: DA1>DA3>DA2. This indicates that the linkers between the electron-rich and electron-deficient units can tune the bandgaps of copolymers, of which the aryl-vinyl-aryl structure achieved the lowest bandgap.

Table 1.  HOMOs, LUMOs, and bandgaps of DA1, DA2 and DA3 from the experiments and calculation by DFT B3LYP/6-31G (d,p).

Figure 5. The bandgaps of DA1, DA2, and DA3 calculated by DFT B3LYP/6-31G (d,p).

Conclusion

Three kinds of model copolymers consisting of the same electron-rich and electron-deficient units, but linked with different chemical bonds, were synthesized via Suzuki, Stille or Sonogashira polycondensation, respectively. Both the DFT calculation and experimentally obtained results show that the linking groups between the electron-rich and electron-deficient units affect the overall copolymer energy levels (LUMO and HOMO) and bandgaps.  The aryl-vinyl-aryl (D-vinyl-A) structure showed the greatest potential for light harvesting in organic solar cells. 

Acknowledgement

Acknowledgment is made to Donors of the American Chemical Society Petroleum Research Fund for support of this research.   ADDIN EN.REFLIST