Comparative Study of Effects of Terminal Non‐Alkyl Aromatic and Alkyl Groups on Small‐Molecule Solar Cell Performance

Abstract

Small‐molecule donors for solar cells are usually end‐capped with π‐systems or aliphatic chains extending the π‐conjugation of the molecules's backbone. Compared with alkyl terminals, π‐systems can form π−π arrangements, for example, with an aligning spherical fullerene π‐system. To study the effects of two kinds of terminals on the solar cell performance, the non‐alkyl, branched aromatic and electron‐donating diphenylamine (DPA) and the aliphatic n‐butyl (n‐Bu) unit are selected as end‐capping groups on a diketopyrrolo­pyrrole‐based linear backbone, affording two new solution‐processable small‐molecule donors. Photovoltaic data indicate that by changing the end‐function from n‐Bu to DPA, the photocurrent significantly increases from 8.35 to 15.64 mA cm−2 and the efficiency from 3.2 to 5.8%. Characterization of absorption, morphology, recombination, and carrier transportation clearly demonstrates that the higher photocurrent can be attributed to a higher density of the mobile carriers (i.e., free holes, in this case). The DPA end‐functions enhance the light‐harvesting capacity, improve the charge dissociation, and reduce the recombination loss, all of which lead to more carriers being collected by the electrode. This work demonstrates that the choice of end‐function along the molecular backbone is as important to improve the cell performance as the light‐harvesting backbone and the side‐chains.