Light-emitting diodes (LEDs) that are based on conjugated polymers have attracted much attention because of their potential applicability to flat, large area displays that can be operated at a relatively low driving voltage. [1,2] Electroluminescence in the semiconducting polymer is achieved by injecting electrons into the conduction band and holes into the valence band; these capture each other and recombine with an emission of visible radiation. In polymer lightemitting diodes, the majority carriers of the emissive polymers are often holes, and the electron±hole (E±H) collision area is located at the interface between the polymer film and the cathode or nearby. [3,4] One of the major reasons for the low quantum yield of single-layer LEDs is that the electron injection current is too low. To make a more efficient electroluminescent (EL) device, it is necessary to enhance the injection efficiency of the negative carriers. To improve the electron injection properties, several methods have been reported: i) Use of a cathode with a low work function, such as Li, Ca, or Mg can reduce the minority carrier injecting barrier. [3] ii) Introducing an electron injecting or transporting layer can improve the quantum efficiency by increasing the electron±hole recombination rate. [5±8] iii) Introducing an insulating layer such as Al 2 O 3 can reduce the effective energy barrier and exciton quenching at the interface between the emissive polymer and the metal cathode. [9] In this work, heat treatment was performed to enhance the efficiency of the EL devices and its effect was studied. The temperature dependence of the performance of organic EL devices is reported elsewhere, [10] and the effect of thermal elimination conditions of poly(p-phenylenevinylene) on device performance is also reported. [11] However, the effect of the thermal treatment conditions during fabrication of the EL device on quantum efficiency has not been reported. Our experiment revealed that the performance of polymeric EL devices greatly depends on the fabrication conditions of the polymer thin film, mostly the heat treatment temperature and its procedure. Therefore, we tried to systematically investigate the optimum heat treatment conditions (temperature, time, procedure, and thickness) for the most efficient EL device. Poly[2-methoxy-5-(2¢-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) was used as an emissive amorphous polymer with only glass transi-tion temperature (T g ). The T g of the MEH-PPV (M n = 400 000, M w = 626 000) we synthesized was 65 C, which is the same as the T g reported by Liu et al. [12] Baking (or drying)Ðheat treatment below the T g of the thin filmÐwas carried out to remove the residual solvent after film casting. Thermal annealingÐheat treatment above the T g Ð was tried in an attempt to change the morphological properties of the EL polymer and interfacial properties of the EL device. Thermal annealing was performed via two different schemes, before and after Al deposition, to discriminate between the effects of the change in ...

Light-emitting diodes (LEDs) that are based on conjugated polymers have attracted much attention because of their potential applicability to flat, large area displays that can be operated at a relatively low driving voltage. [1,2] Electroluminescence in the semiconducting polymer is achieved by injecting electrons into the conduction band and holes into the valence band; these capture each other and recombine with an emission of visible radiation. In polymer lightemitting diodes, the majority carriers of the emissive polymers are often holes, and the electron±hole (E±H) collision area is located at the interface between the polymer film and the cathode or nearby. [3,4] One of the major reasons for the low quantum yield of single-layer LEDs is that the electron injection current is too low. To make a more efficient electroluminescent (EL) device, it is necessary to enhance the injection efficiency of the negative carriers. To improve the electron injection properties, several methods have been reported: i) Use of a cathode with a low work function, such as Li, Ca, or Mg can reduce the minority carrier injecting barrier. [3] ii) Introducing an electron injecting or transporting layer can improve the quantum efficiency by increasing the electron±hole recombination rate.[5±8] iii) Introducing an insulating layer such as Al 2 O 3 can reduce the effective energy barrier and exciton quenching at the interface between the emissive polymer and the metal cathode. [9] In this work, heat treatment was performed to enhance the efficiency of the EL devices and its effect was studied. The temperature dependence of the performance of organic EL devices is reported elsewhere, [10] and the effect of thermal elimination conditions of poly(p-phenylenevinylene) on device performance is also reported.[11] However, the effect of the thermal treatment conditions during fabrication of the EL device on quantum efficiency has not been reported. Our experiment revealed that the performance of polymeric EL devices greatly depends on the fabrication conditions of the polymer thin film, mostly the heat treatment temperature and its procedure. Therefore, we tried to systematically investigate the optimum heat treatment conditions (temperature, time, procedure, and thickness) for the most efficient EL device. Poly[2-methoxy-5-(2¢-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) was used as an emissive amorphous polymer with only glass transition temperature (T g ). The T g of the MEH-PPV (M n = 400 000, M w = 626 000) we synthesized was 65 C, which is the same as the T g reported by Liu et al. [12] Baking (or drying)Ðheat treatment below the T g of the thin filmÐwas carried out to remove the residual solvent after film casting. Thermal annealingÐheat treatment above the T g Ð was tried in an attempt to change the morphological properties of the EL polymer and interfacial properties of the EL device. Thermal annealing was performed via two different schemes, before and after Al deposition, to discriminate between the effects of the change in poly...

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