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 for electroluminescence, ,. 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 π-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively. 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 ∼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.

In this paper it is demonstrated that the characteristics of light-emitting diodes based upon MEH-PPV [more fully known as poly(2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene- vinylene)] are determined by tunneling of both the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating voltage of the device as well as its efficiency. A model is developed that clearly explains the device characteristics of a wide range of diodes based upon MEH-PPV. The turn-on voltage for an ideal device is shown to be equal to the band gap, i.e., 2.1 eV for MEH-PPV, and is slightly lower at 1.8 eV for an indium-tin oxide/MEH-PPV/Ca device. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I–V characteristics, while the larger barrier determines the device efficiency. In indium-tin-oxide/MEH-PPV/Ca devices, the barrier to hole injection is 0.2 eV and the barrier to electron injection is only 0.1 eV. This combination of electrodes is close to optimal for MEH-PPV, but lowering the hole barrier can still lead to a doubling of the device efficiency.

Photoelectron spectroscopy has been used to map out energy level alignment of conjugated polymers at various organic-organic and hybrid interfaces. Specifically, we have investigated the hole-injection interface between the substrate and light-emitting polymer. Two different alignment regimes have been observed: (i) Vacuum-level alignment, which corresponds to the lack of vacuum-level offsets (Schottky–Mott limit) and (ii) Fermi-level pinning, where the substrate Fermi level and the positive polaronic level of the polymer align. The observation is rationalized in terms of spontaneous charge transfer whenever the substrate Fermi level exceeds the positive polaron/bipolaron formation energy per particle. The charge transfer leads to the formation of an interfacial dipole, as large as 2.1 eV.

45Communications Scheme 3. Monomers and polymers under discussion. Synthetic route to the new monomer M5 and to the polymers P1 to P9: a) Pd(PPh 3 ) 4 , toluene, water, Na 2 CO 3 , reflux, 95 %. b) LiAlH 4 , THF, reflux, 99 %. c) SOCl 2 , room temperature, distillation, 46 %. d) KO t Bu, dioxane, reflux, reprecipitation. C 10 = 3,7-dimethyloctyl.

The operational lifetime of polymer light-emitting diodes was studied at several temperatures in the range from 25 to 85 °C. When operated in constant current mode, at luminances greater than 100 cd/m 2, lifetimes of around 20 000 h were noted. Two significant changes in performance were found during continuous operation: the luminance of the devices varied in a nonmonotonic fashion, and the operating voltage increased in a linear fashion. Both of these changes were thermally activated, with the changes accelerated at higher temperatures. These changes are also accelerated at higher current densities. We discuss possible mechanisms for these degradation processes.