Abstract
Today, organic photovoltaics is a promising and rapidly developing field of renewable energy. The key materials responsible for light absorption, charge generation, and charge transport to the electrodes within the device are organic semiconductors. Organic semiconductor materials possess a number of unique properties, including light weight, flexibility, environmental friendliness, biodegradability, and relatively low production costs [1]. One of the main challenges in organic photovoltaics currently is the instability and poor controllability of the donor–acceptor composite. The morphology of the active layer can be improved by introducing substances with specific properties (volatile and non-volatile additives). Volatile additives are compounds introduced into the solution during film formation and completely removed from the active layer during spin-coating or thermal annealing steps. In this work, the volatile additives 2-(2-methylphenyl)benzo[b]thiophene (MPBT) and 2-(2-(trifluoromethyl)phenyl)benzo[b]thiophene (TFBT) were used to improve the morphology of the PM6:Y6 composite, owing to their high solubility in organic solvents and low boiling points. Organic photovoltaic cells based on the PM6:Y6 composite with MPBT additive at various concentrations relative to the solvent mass, as well as based on the pristine composite, were fabricated and tested. For the active layer deposited from chlorobenzene, the optimal additive concentration was found to be about 5 wt.% relative to the solvent. This led to a decrease in short-circuit current and fill factor, while the overall power conversion efficiency (PCE) increased compared to the additive‑free active layer [2]. Analysis of the additive properties showed that MPBT preferentially interacts with the acceptor component Y6. X‑ray diffraction and AFM revealed significant changes in the molecular ordering of Y6 and in the film morphology upon MPBT evaporation. The selective action of the volatile additive on one of the composite components can be used for fine‑tuning the active layer morphology and deliberately modifying its properties. This approach may be useful not only for the PM6:Y6 active layer but also for other donor–acceptor blends with non‑fullerene acceptors. The TFBT additive was also tested for the PM6:Y6 system deposited from ortho‑xylene. O‑xylene, being a “green” solvent, is halogen‑free, has low toxicity, and is relatively inexpensive, making it suitable for large‑scale application. The introduction of the volatile TFBT additive into the active layer increased the PCE from 1% (devices based on the pristine composite) to 5% (devices with 5 wt.% TFBT relative to the solvent). The results obtained so far demonstrate that the use of volatile additives is an effective tool for controlling the active layer morphology and improving the performance of organic solar cells.