Electroluminescence efficiency in bilayer organic light-emitting devices with LiF/Al cathode

An analytical model to calculate electroluminescence (EL) efficiency of bilayer organic light-emitting devices, considering the influence of introducing LiF insulating buffer layer at metal/organic interface on the barrier height for electron injection, was presented. The relations of EL efficiency versus the applied voltage and injection barrier or internal interfacial barrier or the thickness of organic layer were discussed. The results indicate that: (1) when δ e/δ h < 2, metal/organic (M/O) interface is ohmic contact; when δ e/δ h > 2, M/O becomes contact limited; and when δ e/δ h = 2 (Φ h ～ 0.2 eV, Φ e ～ 0.3 eV), there is a transition from ohmic contact to contact limited; (2) η EL decreases with the increase of δ′e / δ′h; however, when δ′e / δ′h > 2.5 (H ′h～ 0.2 eV, H ′e～ 0.4 eV), the changes of η EL are very small, which shows that η EL is dominated by the carrier’s injection; (3) when increasing Lh/L, η R has a descending trend at low voltage and a rising one at higher voltage. For a given Lh/L, η EL first increases and then decreases with the increasing applied voltage, and as Lh/L further increases, the variation tendency of η EL is more obvious. These conclusions are in agreement with the reported theoretic and experimental results.

Electroluminescence efficiency in bilayer organic light-emitting devices with LiF/Al cathode

An analytical model to calculate electroluminescence (EL) efficiency of bilayer organic light-emitting devices, considering the influence of introducing LiF insulating buffer layer at metal/organic interface on the barrier height for electron injection, was presented. The relations of EL efficiency versus the applied voltage and injection barrier or internal interfacial barrier or the thickness of organic layer were discussed. The results indicate that: (1) when δ _e/δ _h < 2, metal/organic (M/O) interface is ohmic contact; when δ _e/δ _h > 2, M/O becomes contact limited; and when δ _e/δ _h = 2 (Θ _h ∼ 0.2 eV, Θ ∼ 0.3 eV), there is a transition from ohmic contact to contact limited; (2) η _EL decreases with the increase of δ′ _e/δ′ _h; however, when δ′ _e/δ′ _h > 2.5 (H′ _h ∼ 0.2 eV, H′ _e∼ 0.4 eV), the changes of η _EL are very small, which shows that η _EL is dominated by the carrier’s injection; (3) when increasing L _h/L, η _R has a descending trend at low voltage and a rising one at higher voltage. For a given L _h/L, η _EL first increases and then decreases with the increasing applied voltage, and as L _h/L further increases, the variation tendency of η _EL is more obvious. These conclusions are in agreement with the reported theoretic and experimental results. Supported by the Excellent Youth Foundation of Hunan Province (Grant No. 03JJY1008), the China Postdoctoral Science Foundation (Grant No. 2004035083), and the Science Foundation of Central South University (Grant No. 0601059)