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 attainable far electroluminescence(1,2). If the lowest-energy excited states are strongly bound excitons (electron-hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the pi*-band and a hole (or missing electron) in the pi-band can form a triplet,vith spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively(3). But if the electron-hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of similar to 50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.