An oligomer or polymer comprising a first repeat unit and a second repeat unit that may be the same or different, the first repeat unit having formula (I): wherein each E independently represents optionally substituted nitrogen or optionally substituted phosphorus, with the proviso that at least one E is optionally substituted phosphorus; each Ar1, Ar2 and Ar3 is the same or different and independently represents an optionally substituted aryl or heteroaryl; n is 0-3; and in the case of unsubstituted nitrogen and phosphorus, the second repeat unit is directly conjugated to the first repeat unit.