Conformational Change on a Bithiophene-Based Copolymer Induced by Additive Treatment: Application in Organic Photovoltaics

Abstract

In recent years the use of solvent additives for fabrication of polymer-based solar cells has become an important procedure to induce morphological changes at the system nanoscale, a critical step to improve device performance. Yet the actual effects of those additives on the polymer’s backbone conformations (with coupled variations on the electronic structure) remain very elusive. By combining different experimental and theoretical techniques, we show that the use of the solvent additive 1,8-diiodooctane (DIO) might influence the conformation of poly­[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) chains resulting in improved properties of the film. We correlate this conformational variation with the photovoltaic response of F8T2:fullerene­(C60) devices prepared using different DIO concentrations. We find that the efficiency of the devices increases more than 100%, and the hole mobility in the F8T2 films increases almost 1 order of magnitude with the use of DIO. A comparison between experimental data and the properties of the calculated structures suggests that the additive induces a higher density of syn conformers in the bithiophene unit of the copolymer backbone, planarizing the polymer’s geometry. The simulations indicate that this transition is very likely mediated by the electrostatic interaction between the iodine atom of the DIO and the heteroatoms of the bithiophene moiety. The higher degree of electronic delocalization and the enhancement of the interchain interactions improve the transport and the photovoltaic features of F8T2 layers. The use of solvent additive treatments to control conformational variations of the backbone might be a promising strategy to improve the optoelectronic properties of polymer-based devices.

Charge Transfer Dynamics and Device Performance of Environmentally Friendly Processed Nonfullerene Organic Solar Cells

“…The use of o -Ma systematically reduces the absorption of the blend. Because the measurements of Figure d were taken in films with approximately the same thickness (120 nm, see Discussion below), the reduction of the absorption indicates that the layer processed by o -Ma is less dense than the layer processed by o -DCB. , …”

“…Different film density will directly affect their optical properties, as also observed in the references. 38,39 This difference in the film density will lead to an absorption intensity reduction, because lower light will interact with the film as there are lower polymer chains. Observing the optical properties of PTB7-Th film, there is a reduction in the absorption intensity when the film is processed by the green solvent compared to the halogenated one.…”

“…Because the measurements of Figure 1d were taken in films with approximately the same thickness (120 nm, see Discussion below), the reduction of the absorption indicates that the layer processed by o-Ma is less dense than the layer processed by o-DCB. 38,39 Conventional solar cells based on a bottom anode composed by PEDOT:PSS-modified ITO and a top LiF/Al bilayer cathode were fabricated. A schematic description of the device used in this work is represented in Figure 2a.…”

Comparing C60 and C70 as acceptor in organic solar cells: Influence of the electronic structure and aggregation size on the photovoltaic characteristics

“…Usually, a p-type semiconductor is blended with a n-type material that acts as the electron acceptor, such as the functionalized fullerenes in the bulk heterojunction (BHJ) devices configuration [5] . However, to gain insights on the key processes that underlie the energy conversion, one interesting approach is to process these materials in separated layers configuring the bilayer geometry [6][7][8][9][10][11] . Furthermore, fullerenes (C 60 and C 70 ) [12] are significantly cheaper than its functionalized analogues.…”

Diketopyrrolopyrrole‐based polymer fibrils formation by changing molecular conformation during film formation

“…The planarization conformation could be tuned by (1) increasing electronic delocalization of the molecular backbone, (2) decreasing the steric hindrance of the side chain, (3) restricting the torsion of the molecular backbone, and (4) improving the ability of adjusting conformation using annealing techniques . Alternatively, the planarization conformation can be tuned during the film formation process using solvent additives, which is the primary focus of the current study.…”