Research On Graphene Based Organic Light-emitting Diodes

Organic light-emitting diode(OLED) has shown wide application prospect in next-generation display technology including flexible, pollution- free and wearable portable displays due to its many advantages of rich colours, high contrast ratio, fast response volecity, light weight, high luminous efficiency and fine mechanical flexibility.Graphene is a very promising electrode material in flexible, stretchable displays and lighting applications. Employing graphene as electrodes can eliminate those drawbacks in ITO-based OLEDs and is compatible with current low-cost, mass- manufacturable and large-scale printing technology. Unfortunately, luminous efficiencies in most graphene OLEDs are still very limited due to low work function and large sheet resistance of graphene. In the past several years, the luminous efficiency in graphene-based OLEDs has been gradually improved via manufacturing a high-quality graphene with a low sheet resistance or modifying its surface with a high work function material.In this work, negative differential resistance(NDR)/hysteretic behaviors as important factors were observed to seriously affect emission stability and performances in graphene OLEDs. O ur results indicate that the NDR and the hysteresis are respectively originated from the hole tunneling induced by residual PMMA and traps in bulk organic materials, which can be removed via a current annealing approach, accompanied with a significant increase in luminous efficiency. Under a low voltage region residual PMMA covered on the surface of graphe ne results in part of voltage drop onto the ultrathin PMMA, thus generating a hole tunneling accompanied by N DR phenomenon. The large hole tunneling current passing through the conducting device, just like a current annealing process, generates Joule heating and significantly increases the sample temperature around residual PMMA to several hundred centigrades. This large Joule heating leads to the decomposition of PMMA. O ur calculation results indicate that the hysteresis phenomenon under a high voltage region is related to the traps in bulk organic materials and it can be removed after these traps are fully filled after several J-V scans.The work presented here offers a new approach for further device improvements, and the combination of this approach with previously reported methods of reducing sheet resistance and increasing work function will be very useful for fabricating high-performance graphene optoelectronic devices. Notably, the luminance and current efficiency of graphene O LED based on the Iridium(III)bis(4-phenylthieno[3,2-c]pyridinato-N,C2’)acetylacetonate(PO-01) yellow phosphor are as high as >60000 cd/m2 and >90 cd/A in this thesis.