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Jinhong Du1 Dingdong Zhang1 Wencai Ren1 Hui-Ming Cheng1

1, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, , China

Graphene has a great potential to replace indium tin oxide (ITO) as transparent conductive films (TCFs) for flexible opto-electronic devices, such as organic photovoltaic (OPV) cells and organic light emitting diodes (OLEDs), due to high electrical conductivity, optical transmittance, mechanical flexibility, chemical stability and the successful synthesis of large-area graphene film. However, the low work function and poor compatibility with hole injection layer (HIL) of graphene make the performance of graphene-based opto-electronic devices to be far from satisfactory and usually worse than those using ITO TCFs,[1] and large surface roughness causes the active area of devices to be very small. For example, the available lighting area of OLEDs and the active area of OPV cells using graphene TCFs are usually less than 1 and 0.6 cm2, respectively.
Here we propose a graphene oxide/graphene (GO/G) vertical heterostructure TCFs, which are fabricated by directly oxidizing the top layer of three-layer graphene films through ozone treatment and show greatly improved optical transmittance, a large work function, and high stability.[2, 3] Moreover, the GO/G heterostructure TCFs have much better compatibility with HIL materials than pristine graphene and ITO, allowing a uniform MoO3 HIL deposition on its surface with good smoothness. Besides high flexibility, OLEDs with different colors based on the GO/G heterostructure TCFs show much better performance than those based on pristine graphene and ITO TCFs. Furthermore, a natural organic small molecular-assisted wet etching transfer method is developed to fabricate a clean and damage-free graphene TCF with a very low surface roughness with a maximum height of about 15 nm over a large area. Such clean and damage-free graphene greatly improves the current efficiency and power efficiency of OLEDs, with maxima as high as 89.7 cd A-1 and 102.6 lm W-1, respectively. More importantly, a 4-inch flexible green OLED with uniform light emitting and high luminance (ca. 10000 cd m-2 at 16 V) has been successfully fabricated for the first time, showing a strong potential of graphene TCFs for next generation flexible opto-electronics. [4]

References

[1] Du J. H. et al., Carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices, Adv. Mater. 2014, 26 (13): 1958.
[2] Jia S and Du J. H. et al., Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes, Nanoscale 2016, 8(10): 10714.
[3] Yuan J. T. and Du J. H. et al., Tuning the electrical and optical properties of graphene by ozone treatment for patterning monolithic transparent electrodes, ACS Nano 2013, 7 (5): 4233.
[4] Zhang Z. K. and Du J. H. et al., Rosin-enabled ultraclean and damage-free transfer of graphene for large-area flexible organic light-emitting diodes, Nat. Commun. 2017, 8:14560.

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