Workfunction-Tunable, N-Doped Reduced Graphene Transparent Electrodes for High-Performance Polymer Light-Emitting Diodes

Abstract

Graphene is a promising candidate to complement brittle and expensive transparent conducting oxides. Nevertheless, previous research efforts have paid little attention to reduced graphene, which can be of great benefit due to low-cost solution processing without substrate transfer. Here we demonstrate workfunction-tunable, highly conductive, N-doped reduced graphene film, which is obtainable from the spin-casting of graphene oxide dispersion and can be successfully employed as a transparent cathode for high-performance polymer light-emitting diodes (PLEDs) as an alternative to fluorine-doped tin oxide (FTO). The sheet resistance of N-doped reduced graphene attained 300 Ω/□ at 80% transmittance, one of the lowest values ever reported from the reduction of graphene oxide films. The optimal doping of quaternary nitrogen and the effective removal of oxygen functionalities via sequential hydrazine treatment and thermal reduction accomplished the low resistance. The PLEDs employing N-doped reduced graphene cathodes exhibited a maximum electroluminescence efficiency higher than those of FTO-based devices (4.0 cd/A for FTO and 7.0 cd/A for N-doped graphene at 17,000 cd/m(2)). The reduced barrier for electron injection from a workfunction-tunable, N-doped reduced graphene cathode offered this remarkable device performance.

Solution-Processed rGO/AgNPs/rGO Sandwich Structure as a Hole Extraction Layer for Polymer Solar Cells

“…It is likely that the incorporation of electron withdrawing groups such as -OH to the surface of graphene increases the work function, whereas N-doping of rGO by hydrazine increases the electron density and thus decreases the work function as compared to GO. Our value is slightly higher than the reported experimental values within the range of 4.2-4.4 eV [11]. In case of HEL, the work function is found to be 4.9 eV, which is similar to the work function of graphene doped with AgNPs.…”

Chemical structures of hydrazine-treated graphene oxide and generation of aromatic nitrogen doping

“…For the last half decade, among several chemical reductants 4,5 , hydrazine has been the most commonly used reductant due to its ease of use (for example, via a one-pot synthesis in either liquid or gas phase) and its ability to achieve a high degree of reduction of graphene oxide without the need for further treatment 3,[6][7][8] . Hydrazine-treated graphene oxide (chemically reduced graphene oxide, 'CReGO') 3 is one of the most promising graphene-based materials for several applications, such as polymer composites, ultracapacitors, rechargeable batteries, chemical/biosensors and thin films [9][10][11][12][13][14] . A large fraction of the oxygen-based functional groups of graphene oxide are removed by exposure to hydrazine; however, the resulting graphene materials contain a small amount of O and N atoms (with approximate C/O and C/N ratios of 10 and 22, respectively) 3,6 .…”

Graphene–Ferroelectric Hybrid Structure for Flexible Transparent Electrodes

“…26 For instance, the realization of n-type doped graphene is attractive for low work function cathode in light emitting diodes and solar cell devices. 50 …”