Improved quantum efficiency for electroluminescence in semiconducting polymers

Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attaina ble far electroluminescence(1,2). If the lowest-energy excited states are s trongly bound excitons (electron-hole pairs in singlet or triplet spin stat es), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the pi*-band and a hole (o r missing electron) in the pi-band can form a triplet,vith spin multiplicit y of three, or a singlet with spin multiplicity of one, but only the single t will decay radiatively(3). But if the electron-hole binding energy is suf ficiently weak, the ratio of the maximum quantum efficiencies for electrolu minescence and photoluminescence can theoretically approach unity. Here we report a value of similar to 50% for the ratio of these efficiencies (elect roluminescence:photoluminescence) in polymer light-emitting diodes, attaine d by blending electron transport materials with the conjugated polymer to i mprove the injection of electrons. This value significantly exceeds the the oretical limit for strongly bound singlet and triplet excitons, assuming th ey comprise the lowest-energy excited states. Our results imply that the ex citon binding energy is weak, or that singlet bound states are formed with higher probability than triplets.