| Organic light-emitting device (OLED) is considered as one of the most important candidates for the next generation display technology due to its self-emitting property, wide viewing angle, high efficiency, low power consumption and flexiblity. In OLEDs, low work-function metals such as Ca and Ba are commonly ultilized to promote electron injection. However, these low work-function metals are very sensitive to moisture and oxygen in the ambient and the degradation of the cathode would significantly affect the device operation and stability. Inverted devices using air-stable metal oxides such as ZnO or TiO2 as electron injection layer are proposed to solve this issue. Nevertheless, there is significant energy barrier for electron injection from such metal oxides into organic materials. This thesis aims to improve electron injection efficiency through incorporation of a polyethylenimine ethoxylated (PEIE) interlayer. The main work includes the following aspects:(1) Implementation of a PEIE layer between ITO or ITO/ZnO and MEH-PPV layer significantly enhances the luminance efficiency by a factor of 7-10. In addtion, ca.50 times increase in device efficiency has been also observed upon modification of Al with a PEIE interlayer. The current density of single electron devices with a PEIE layer is higher than that of the analogous devices without a PEIE layer by a factor of 5-10, suggesting that PEIE layer significantly improves electron injection and as a result enhances device efficiency. In order to explore the underlying mechanism, we use ultraviolet and X-ray photoelectron spectroscopy methods to measure the work function of the PEIE/Al and ZnO/PEIE samples, and find that the work function values of ZnO and Al are decreased by ca.1.2 and 1.0 eV upon additon of a PEIE layer. The reduction of work function is mainly due to the formation of interfacial dipoles. X-ray photoelectron spectroscopy measurements also reveal that neutral amines rather than protonated amines are mainly responsible for the reduction of ZnO work function.(2) To explore electron injection efficiency from PEIE into light emitting polymers with various LUMO levels, we study organic-inorganic light-emitting devices using PF-TBT, SY or PFA as the emissive layer with the respective LUMO level of ca.-3.6,-2.7 or-2.1 eV. Luminous efficiencies of these hybrid devices are similar to those of the analoguous conventional devices using PEDOT:PSS and CsF as hole and electron injection layer, suggesting that electron injection barrier from PEIE modified ZnO into light emitting polymers is vanishingly small. Thus, PEIE serves an effective electron injector for manifold light emitting polymers with wide distribution of LUMO levels.(3) Compared to the commonly adopted Cs2CO3 electron injection layer, PEIE, which possesses similar surface energy to the organic materials and can form homogeneous film, is expected to collaborate with the adjoining organic layers, without significantly interrupting their morphology. To exploit this advantage, we study organic-inorganic light emitting devices using solution-processed TCTA:OXD-7:TEG emissive layer and PEIE electron injection layer. The peak luminance efficiency of the devices is 87.6 cd A-1 and external quantum efficiency at 1000 cd m-2 is 20.9%. Compared to the previously reported devices based on ZnO:Cs2CO3 electron injection layer, the luminous efficiency of the present devices is more than four times higher, which can be attributed to excellent electron injection and hole/exciton blocking ability of PEIE, and in particular to the benigh compatiblity between PEIE and emissive layer. |
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