A polymer including a region having a conjugated backbone capable of transporting negative charge carriers, which region includes a plurality of repeat units each including a core group which includes an [Ar] group substituted with at least one pendant group X and the [Ar] groups form at least a part of the backbone, wherein each Ar includes an aromatic or heteroaromatic group and each X includes NAr₂ in which each Ar is the same or different and independently includes a substituted or unsubstituted aromatic or heteroaromatic group and N is conjugated with the backbone.