Electronic and optical excitations of the PTB7 crystal: First-principles GW-BSE calculations

Abstract

Given the recent success in achieving efficient organic photovoltaic solar cells based on thieno [3,4-b]thiophene/benzodithiophene polymers (PTB7) and growing efforts to further improve the power conversion efficiency of the PTB7-based devices, a detailed atomic-scale picture of the electronic structure and the excitonic properties of PTB7 crystal is highly desirable.We report electronic and optical properties of PTB7 on the basis of first-principles density functional theory and GW many-body plus Bethe-Salpeter equation (GW-BSE) calculations. It is established that the first two highest valence bands (HVB) and the first two lowest conduction bands (LCB) originate from the benzodithiophene (BDT) and thieno [3,4-b]thiophene (TT) functional units, respectively, thus confirming the donor-acceptor nature of PTB7. A significant difference of band-splitting between HVBs and LVBs is found and its origins are explained. Our results strongly suggest that the strength of the inter-chain π-π interaction is not only a function of inter-chain distance, but is also highly dependent on the nature of the fused rings. The experimental optical absorption spectrum of PTB7 is well reproduced and explained by our 2 GW-BSE calculations. Further analysis shows that the nature of the lowest singlet (triplet) excitons in polymeric crystals such as PTB7 differs from that of organic molecular crystals. A possible reason is explored by combining BSE calculations with a simple Hamiltonian model.

Donor–Acceptor Sequence Engineering in π-Conjugated Polymers for Enhanced Hole Transport and Photocurrent

“…Here, we take the optical absorption of D 4 A 4 polymer as an example to illustrate the change of absorption spectra with the D−A pattern. It should be noticed that our GGA obtained optical gap is about 1.1 eV (Figure 2) for the normal [(DA) n ] pattern, which is much smaller than the G 0 W 0 43 and experimental 44 value of 1.65 eV. To stay consistent with the previous studies, we assume that the same error is introduced by GGA functional in all patterns and a rigid shift of 0.55 eV is applied in Figure 2.…”

Density Functional Study of Stacking Structures and Electronic Behaviors of AnE-PV Copolymer

“…Therefore, it is interesting to study these aspects by density functional theory (DFT) calculations. The fundamental importance of shearing and interdigitation for the electronic band structure of conjugated polymer crystallites has been already shown for other polymers, for example, P3HT 14−17 and PTB7, 18 as well as for conjugated systems in general. 19,20 In our case, besides the known distance between the layers, it is in particular the relative positions of the anthracene units on neighboring chains, that is, stacking just above each other or on ethynylene, phenylene, or vinylene units, that determine the overlap between the π-electron systems and, consequently, the band structures, the optical transitions, and the interchain charge transport properties along the π-stacking direction.…”

Absorption-induced scattering and surface plasmon out-coupling from absorber-coated plasmonic metasurfaces

“…9 ). PCDTBT and PTB7 had a dip in the short wavelength scattering due to higher-energy excitonic absorption transitions of the polymers 34 35 ( Supplementary Fig. 5a ).…”