Electric field-induced transition from heterojunction to bulk charge recombination in bilayer polymer light-emitting diodes

We investigate the photo- and electroluminescence from bilayers of electron- and hole-transporting polyfluorene derivatives at different device temperatures. We show that barrier-free charge capture at the heterojunction is the sole capture mechanism at low driving voltages (below 2.4×105V∕cm2 at room temperature). In this mechanism, which we suggested recently [Morteani et al., Adv. Mater. 15, 1708 (2003)], charge capture produces an interfacial excited state (exciplex) directly and bulk exciton electroluminescence is only achieved through endothermic transfer (activation energy 200meV) from the exciplex. For high driving voltages (above 8.3×105V∕cm2 at 43K), however, we find that charges are injected over the heterojunction barriers and subsequent charge recombination occurs in the polymer bulk.