Nickel as an alternative semitransparent anode to indium tin oxide for polymer LED applications

Abstract

We report on the possibility of using a thin Ni layer, instead of ITO, as a semitransparent hole-injecting electrode for bottom polymer LEDs. Thin metal layers of Ni were deposited by a sputtering technique and their electrical and optical properties with different deposition times have been investigated. Both square resistance and transmittance were seen to decrease with deposition time (thickness). The films showed a transmittance of around 30-40%, which is quite low compared to the 86% of ITO, while their square resistance was higher than that of ITO. Nevertheless, diodes based on a blue emitting polymer, polyfluorene (PFO), showed the same efficiency for either ITO or thin Ni electrodes, although the Ni transmittance is around 2.5 times lower than the ITO transmittance. Such preliminary results definitively suggest that indium-free organic devices can be achieved.

High figure-of-merit ultrathin metal transparent electrodes incorporating a conductive grid

“…Although ITO has excellent transmission and low sheet resistance, it possesses several drawbacks, for example, high cost due to indium scarcity, need of postdeposition treatments, dependence of performance on doping, deposition conditions, and their multicomponent structure which can lead to incompatibilities with some active materials in devices. [2][3][4][5] These facts have led to search for alternative materials, such as single walled carbon nanotubes, graphene films, and ultrathin metal films ͑UTMFs͒. [3][4][5][6][7] Among them, UTMFs can overcome the high cost of raw materials and be grown using a simple process technique.…”

“…[2][3][4][5] These facts have led to search for alternative materials, such as single walled carbon nanotubes, graphene films, and ultrathin metal films ͑UTMFs͒. [3][4][5][6][7] Among them, UTMFs can overcome the high cost of raw materials and be grown using a simple process technique. Contrary to TCOs, they also possess high compatibility with nearly all organic and semiconductor materials and related device fabrication steps.…”

“…Contrary to TCOs, they also possess high compatibility with nearly all organic and semiconductor materials and related device fabrication steps. 4,5 However, if one wants to achieve high optical transmission, the thickness of UTMFs should be limited to several nanometers, at the expense of sheet resistance. For example, 40 nm Ni films can achieve sheet resistance values of less than 5 ⍀ / ᮀ but are opaque.…”

Highly stable Al-doped ZnO transparent conductors using an oxidized ultrathin metal capping layer at its percolation thickness

“…Additional details on the fabrication steps and characterization of the OLEDs can be found elsewhere. 14 Too thick metal capping layer would result in serious optical loss in the visible-light region, while too thin films would tend to form discrete island structures, 15 thus resulting in an incomplete protection of the underlying AZO against the environment or detrimental interaction with the other layers forming the device. Therefore, it is intuitive that the optimum thickness of the metal capping layer is around its percolation threshold, i.e., the thickness corresponding to which the layer morphology changes from an island distribution to a continuous layer.…”

ITO-free large-area organic light-emitting diodes with an integrated metal grid

“…Transparent conductive electrodes used in prior studies to replace ITO in small-area OLEDs devices include the highly conductive polymer poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) [11], semitransparent metal electrodes [12] as well as graphene composite electrodes [13]. While all these electrodes can achieve a performance comparable to that of ITO in terms of transmittance and conductance, they generally do not provide solutions to the fabrication of large-area OLEDs where a large potential gradient is expected as the area of the device increases, hence degrading the uniformity of the light emitted.…”