Abstract: A material comprising an electron-accepting unit of formula (I): Ar is an aromatic ring; Ar1 is a substituted or unsubstituted 5- or 6-membered heteroaromatic ring containing N and C ring atoms; when Ar1 is a substituted or unsubstituted 6-membered heteroaromatic ring, Ar2 is a substituted or unsubstituted 6-membered heteroaromatic ring wherein the ring atoms are selected from N and C; when Ar1 is a 5-membered heteroaromatic ring, Ar2 is a substituted or unsubstituted 5- or 6-membered heteroaromatic ring; Ar3 is a 5-membered ring or a substituted or unsubstituted 6-membered ring; Ar4 is a 5-membered ring or a substituted or unsubstituted 6-membered ring or is absent; Ar5 is a substituted or unsubstituted monocyclic or polycyclic group containing at least one aromatic or heteroaromatic ring; Ar6 is a substituted or unsubstituted monocyclic or polycyclic group containing at least one aromatic or heteroaromatic ring or is absent; and each X is independently a substituent bound to a C atom of Ar3, and where prese