Electroluminescent Mechanism Analysis Of Hybrid Light Emitting Diode Based On ZnO Nanorods/Polymers

To be a direct wide band gap semiconductor, the group II-VI Zinc oxide (ZnO) showsexcellent optical properties and then indicates the promising application in light emittingdiodes (LED), ultraviolet laser operating devices and sensors. In recent years, nanostructureZnO, which possesses a large number of extrinsic and intrinsic defects, has attracted particularattention for visible or white light emission LED. In this paper, a research was carried outbased on ZnO nanorods organic/inorganic LEDs. The main results are as follows:(1) A two step chemical bath deposition (CBD) method was adopted for the ZnO NRsfabrication on different substrate such as ITO glass, silicon and flexible PET surface under alow temperature of75oC with no annealing. The growth mechanism of ZnO NRs wasdiscussed based on various temperatures and attributed to the growth rate differences betweennucleation and axial growth. At higher temperatures, the competition between twogrowth mechanisms leads to the short of the length. Two parameters including the seriesresistance (Rs) and the total trap concentration (Nt) were introduced to interpret the ELbehavior of ZnO NRs devices with different polymer functional layer. The optimal ELintensity was achieved in the device with p-type polymer of MEH-PPV because of its suitableenergy level and high carrier mobility.(2) Due to the intrinsic n-type behavior, the imbalance between electrons and holes inZnO leads to an excess of negative effect and often limits the ZnO-based LEDs efficiency. Ap-n-p dual heterojunction white light emitting device was fabricated including two polymerlayers. The as-prepared device showed stable white light electroluminescence and low turn-onvoltage. The carrier transport mechanisms under different forward bias region for the dualheterojunction device were discussed on the basis of current-voltage (I-V) curve. The resultrevealed different carrier transport mechanisms in low voltage region (SCLC) and highvoltage region (TCLC), respectively. The influence of two functional layers on EL spectrawas verified. With its hole conductivity, the p-PVK layer cannot only improve the holestunnel into ZnO nanorods layer, but also lower the barrier between ITO and the valance bandof ZnO nanorods. On the other hand, p-MEH-PPV could be regarded as block layer for theinjection of electrons from the Al electrode. In the way of the value of Nt, the EL behavior ofthe dual heterojunction device was related to trap-filling process.(3) A flexible WLED was fabricated by using PET as substrate. The two functionallayers including PEDOT: PSS and PVK layer were introduced and displayed obviousimprovement in the balance of injected carriers. To be as hole-injection layer, the PEDOT:PSS layer could enhance the injection efficiency of holes as well as lower the barrier betweenAu and the HOMO level of PVK. On the other hand, p-PVK was severed as hole conductivitylayer which increase the holes tunnel into the valance band of ZnO nanorods.