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

Abstract

In this work, we report an in-depth investigation on the π-stacking and interdigitating structures of poly(p-anthracene-ethynylene)-alt-poly(p-phenylene-vinylene) copolymer with octyl and ethyl-hexyl side chains and the resulting electronic band structures using density functional theory calculations. We found that in the π-stacking direction, the preferred stacking structure, determined by the steric effect of the branched ethyl-hexyl side chains, is featured by the anthracene-ethynylene units stacking on the phenylene-vinylene units of the neighboring chains and vice versa. This stacking structure, combined with the interdigitating structure where the branched side chains of the laterally neighboring chains are isolated, defines the energetically favorable structure of the ordered copolymer phase, which provides a good compromise between light absorption and charge-carrier transport.

Density Functional Study on the Intercalation of Fullerenes into AnE-PV Copolymer Layers

“…For each fixed value of the shear, we varied the interlayer distance d from 12.4 to 13.4 Å with a step width of 0.2 Å, and performed geometry optimization at each step. The band structure calculations were performed for the optimized structures of the intercalated AnE-PV:C 60 systems with the side chains replaced by H atoms, since the side chains have nearly no influence on the electronic bands relevant for the photovoltaic performance, as we verified in our previous work …”

“…The remaining lattice constant c and the angle β can be replaced by the interlayer distance d and the shear s through s = c cos β and d = c sin β (see Figure ). From a previous study we knew that the band structure of neat AnE-PV ab stacks depends crucially on the shear in the backbone direction . Since the copolymer layer distances of the intercalated structures are evidently larger than those of the neat AnE-PV stacks, the effects of shearing along the backbone on interchain interaction are assumably much weaker.…”

“…From a previous study we knew that the band structure of neat AnE-PV ab stacks depends crucially on the shear in the backbone direction. 22 Since the copolymer layer distances of the intercalated structures are evidently larger than those of the neat AnE-PV stacks, the effects of shearing along the backbone on interchain interaction are assumably much weaker. Therefore, we chose only three fixed values of s: 0.0 Å representing the face-to-face configuration, 3.5 Å corresponding to a shear of about one PV unit, and 13.0 Å corresponding to a shear of about one AnE unit along the backbone.…”

“…The band structure calculations were performed for the optimized structures of the intercalated AnE-PV:C 60 systems with the side chains replaced by H atoms, since the side chains have nearly no influence on the electronic bands relevant for the photovoltaic performance, as we verified in our previous work. 22 All the calculations were performed by using density functional theory (DFT) applying the PBE functional 25 in the framework of the PAW potential method as implemented in the VASP package. 26,27 The energy cut off was 490 eV, and for geometric optimization the Brillouin zone was sampled over a 4 × 2 × 2 k-point grid.…”

“…In the present study, we investigate the intercalated structures of C 60 and poly­(p-anthracene-ethynylene)- alt -poly­(p-phenylene-vinylene) (AnE-PV). As a new photovoltaic material, the AnE-PV copolymer is becoming increasingly interesting since the structural order of its thin films can be controlled by specific side chain substitution. , The copolymer chains with linear octyl side chains substituted to the phenylenes next to the anthracene-ethynylene units and branched 2-ethyl-hexyl to the remaining phenylenes (named in what follows as AnE-PV ab and shown in Figure ) form ordered stacks of copolymer layers with approximately 3.8 Å stacking distance, − whereas the reverse substitution, i.e., branched 2-ethyl-hexyl side chains substituted to the phenylenes next to the anthracene units and linear octyl side chains to the remaining phenylenes (AnE-PV ba; shown in Figure ), results in almost amorphous films. This unique property makes AnE-PV an ideal material for studying the structural order related issues in BHJ.…”