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Light-emitting devices based on polymeric/inorganic heterojunction

Since the initial discovery of electroluminescence in PPV [poly (p-phenylenevinylene)], polymer light emitting devices have been widely investigated. Aiming at overcoming the lack of electron type polymer can be used in polymer electroluminescent devices, using n-type semiconducting inorganic material ZnO:Zn as electron transport layer and p-type semiconducting polymer PDDOPV as hole transport layer, device based on heterojunction with structure ITO/PDDOPV/ZnO:Zn/Al is successfully fabricated. The highest ratio luminescent efficiency of heterojunction device to that of single layer device of PDDOPV is 38.6 times at 8V, the brightness and current of heterojunction device are 19.4 times and 0.5 time of that of single layer device respectively. The results demonstrate that the insertion of ZnO:Zn layer can transport electrons and block holes that lead to the decrease of current and increase of luminescent brightness and efficiency. The electroluminescent spectra of single layer device were bias-independent while that of heterojunction device were bias-dependent. The emitting color of single layer device is orange, while that of heterojunction device shifts from orange to green-yellow and finally becomes green as the applied voltage increases. The full width at half maximum (FWHM) of single layer device is 50 nm, while that of heterojunction device is from 70 to 84 nm. Different photoluminescent spectra of heterojunction device are obtained at different excitation wavelength. Exciplex theory is often used to explain the origin of new emitting color, but it is not suitable here because exciplex emission should leads to red-shift emission. We attribute this phenomenon to the conformation of new light-emitting unit.