Fullerene-free and processing additive-free 8.5% efficient polymer solar cells are achieved by using a new 3,4-ethylene-dioxythiophene- linked arylene diimide dimer with a 76 degrees twist angle. The devices combine high (78-83%) external quantum efficiency with high (0.91-0.95 V) photovoltages and thus have relatively low optical bandgap energy loss.