Understanding the effect of solvent additive in polymeric thin film: turning a bilayer into a bulk heterojunction-like photovoltaic device

Here we report the effect of an additive solvent, 1,8-diiodooctane (DIO), on the performance of a bilayer organic photovoltaic device in which the active layer comprises poly[2,7-(9,9-bis(2 ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) as the electron donor material and C60 as the electron acceptor material. We observed that when the donor layer was treated with 1% of DIO the power conversion efficiency (PCE) of the device increased by 138.4% in relation to the device with an untreated donor layer, and 21.3% in relation to the device containing a donor layer submitted to thermal annealing. The main effects that lead to this increase in PCE are the large interfacial area between donor and acceptor materials and the improved conductivity at low voltages. The increase in polymer surface roughness leads to a more effective PSiF-DBT/C60 interface for exciton dissociation. This effect, as well as the increase in the conductivity, raised the short circuit current density (JSC) to 13.89 mA cm−2 and the PCE to 4.84%. Our conclusions are supported by morphological analysis, chemical cross-sectional evaluations with advanced microscopy techniques, charge mobility measurements, as well as by theoretical simulations of the devices in which the changes on the donor/acceptor interfacial area were considered. The outcomes suggest that solvent additives could be an alternative treatment to replace the thermal annealing, which imposes further difficulties in performing lab-to-manufacturing upscaling.