Surface modification of indium tin oxide by plasma treatment: An effective method to improve the efficiency, brightness, and reliability of organic light emitting devices

Abstract

We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from Ͼ20 to Ͻ10 V, the external electroluminescence quantum efficiency ͑backside emission only͒ increases by a factor of 4 ͑from 0.28% to 1%͒, a much higher drive current can be applied to achieve a much higher brightness ͑maximum brightness ϳ10,000 cd/m 2 at 1000 mA/cm 2 ), and the forward-to-reverse bias rectification ratio increases by orders of magnitude ͑from 10 2 to 10 6 -10 7 ). The lifetime of the device is also enhanced by two orders of magnitude.Because of its transparency, high conductivity, and efficiency as a hole injector into organic materials, indium tin oxide ͑ITO͒ has been widely used as the anode contact for organic light emitting devices ͑OLEDs͒. These devices usually consist of a sandwich structure with the organic thin film deposited onto the ITO-coated glass substrate and covered by patterned top metal cathode contacts. 1,2 Since the organic thin film is in direct contact with the ITO, the surface properties of the ITO are expected to directly affect the characteristics of the device. Abnormal device behaviors such as shorting, unstable I -V characteristics, and damage on the surface of the top cathode contact after continuous operation of the device have been observed in OLEDs built on bare cleaned ITO surfaces. 3-5 Furthermore, as-grown ITO contacts have been found to be less efficient for hole injection than low work function metal cathodes for electron injection, resulting in hole-limited devices. 6,7 The mitigation of these problems has so far involved changing the properties of the organic materials or introducing an intermediate stabilization layer with proper carrier injection/transport characteristics between the ITO and the active luminescent layers. 7,8 The alternative of modifying the ITO itself, however, has not been extensively investigated. In this paper, we report that an oxygen plasma treatment is an effective way to modify the surface of ITO. We perform a comprehensive investigation of the correlation between ITO surface properties such as morphology and chemical composition, and the device characteristics. We demonstrate that ITO surface modification leads to good performance even in single layer polymer based OLEDs, where the ITO surface properties are more critical than in multilayer devices.The ITO coated glass substrates used in this study were purchased from Donnelly Applied Films Co. The 1.1-mmthick polished soda lime float glass was coated with a 200 Å SiO 2 barrier layer and a 1400 Å ITO film. ITO was sputtered from an In 2 O 3 -SnO 2 ͑90 wt %-10 wt %͒ oxide target in an Ar/O 2 ambient at an elevated temperature using a planar dc magnetron sputtering system. The ITO was annealed in situ during the deposition and no postdeposition annealing was performed. The sheet resistance and transmittance of the ...

Correlation between the Indium Tin Oxide morphology and the performances of polymer light-emitting diodes

“…The samples were polarized at 200 V to trap the secondary electrons. The diameter of the 4 He + incident beam is typically about 2 mm. A silicon surface barrier detector was used.…”

“…2(a), three RBS spectra corresponding to 0, 30 and 60 min of acidic treatment. For In and Sn, the energies of the backscattered 4 He + particles are theoretically 1.751 and 1.758 MeV respectively. As the resolution is 13.5 keV, In and Sn cannot be distinguished using this technique.…”

“…But, without surface modification of the ITO, PLED devices usually exhibit poor performance. Many surface treatments of the ITO have been employed to improve the device performance, including aqua regia treatment [3], oxygen plasma treatment [4], [5], [6] and [7], atmospheric plasma treatment [8], ultra-violet ozone treatment [9], carbon tetrafluoride/oxygen treatment [10] and various coating treatments such as for example self-assembled monolayers [11] and [12]. Alternatively, by inserting an additional thin layer, such as copper-phtalocyanine [13], polyaniline [14], poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonate (PEDOT-PSS) [15] and [16], inorganic or organic insulators [17] and [18], emission intensity and device stability can be improved by lowering the driving voltage.…”

High-efficiency transparent organic light-emitting diode with one thin layer of nickel oxide on a transparent anode for see-through-display application

“…Using oxygen inductively coupled plasma, the decrease in Sn:In ratio and the increase in oxygen concentration of ITO are assumed to be closely related to the enhanced work function of the ITO [13]. As regards other metal species, the formation of oxidation of metallic film may lead to increasing pinning-free state, which is contributed to the increased work function of metal species.…”

Performance enhancement of metal nanowire transparent conducting electrodes by mesoscale metal wires

“…T ransparent conducting electrodes (TCEs) are essential components for numerous optoelectronic devices that require transport of both electrons and photons, including solar cells, displays, touch screens and smart windows [1][2][3][4][5] . Recently, there have been increasing efforts in developing nanomaterial TCEs including carbon nanotubes (CNTs) 1,[6][7][8][9] , graphene 4,[10][11][12][13] , metal nanowires [14][15][16][17][18][19][20][21] and nanoparticles 22,23 to replace conventional indium tin oxide (ITO) films, in order to enable low-cost and mechanical flexibility while maintaining low sheet resistance (R s ) and high transmittance (T).…”