Solution-processable semiconducting materials in organic solar cells (OSCs) enable the roll-to-roll printing of functional devices. However, the dissolution of these materials commonly requires toxic solvents, thus hindering the translation to industry. To enable large scale printing in industry, they should be formulated as inks using a less toxic medium such as alcohol or even water.
Researchers have fabricated OSCs by dispersing semiconducting materials in water/alcohol with the help of surfactants. However, significant surfactants remained in the active layer even after extensive dialysis, reducing the device performance and solar cell life-time. Recently, devices were fabricated using a dispersion of poly-3-hexylthiophene (P3HT) with indene-C60 bisadduct (ICBA) in methanol and showed a 3.8% device efficiency without the need of a surfactant. However, in this case, the performance was highly dependent on the batch of polymer used for dispersion. Hence, there is a requirement of an appropriate surfactant which not only controls the size of nanoparticles but also increases nanoparticle stability whilst maintaining device efficiency.
To address this problem, we synthesized P3HT end-capped with pyridine. We expected this pyridine-P3HT to be protonated on addition of acid and act as a surfactant by making an electrical double layered around the nanoparticles, inducing a repulsive interaction and preventing aggregation, however under thermal annealing the pyridinium salt would decompose removing all trace of the acid. By using a combination of 1.5 wt% pyridine-P3HT and trifluoroacetic acid as additives, stable P3HT:ICBA nanoparticle dispersions of up to 30 mg/mL in methanol were achieved. The nanoparticles in these dispersions were 120 ± 5 nm, and the dispersions were stable up to 60 days. Inverted architecture OSC built using these nanoparticle dispersions exhibited 3.4% efficiency, a result that is comparable to the state-of-the-art P3HT:ICBA solar cells fabricated under optimized conditions using chlorinated solvents.