Abstract

Organic light emitting diodes (OLEDs) have attracted much attention in recent years as next generation lighting and displays, due to their many advantages, including superb performance, mechanical flexibility, ease of fabrication, chemical versatility, etc. In order to fully realize the highly flexible features, reduce the cost and further improve the performance of OLED devices, replacing the conventional indium tin oxide with better alternative transparent conducting electrodes (TCEs) is a crucial step. In this review, we focus on the emerging alternative TCE materials for OLED applications, including carbon nanotubes (CNTs), metallic nanowires, conductive polymers and graphene. These materials are selected, because they have been applied as transparent electrodes for OLED devices and achieved reasonably good performance or even higher device performance than that of indium tin oxide (ITO) glass. Various electrode modification techniques and their effects on the device performance are presented. The effects of new TCEs on light extraction, device performance and reliability are discussed. Highly flexible, stretchable and efficient OLED devices are achieved based on these alternative TCEs. These results are summarized for each material. The advantages and current challenges of these TCE materials are also identified.

Highlights

  • Organic light emitting diode (OLED) has emerged as a potential candidate for generation flexible, large-area, high resolution display and solid state lighting panels, because of its high color quality, attractive appearance, ease of fabrication, low manufacturing and materials cost, etc [1]

  • We focus our attention on the following materials: carbon nanotubes (CNTs), metallic nanowires, conducting polymers and graphene

  • Thicknesses [34]; (b) the wavelength dependence of the refractive index and the extinction coefficient of indium tin oxide (ITO) and PANI films [35]; (c) the device structure of ITO-free transparent OLEDs based on a PEDOT:PSS transparent conducting electrodes (TCEs); (d) the comparison of external quantum efficiencies (EQEs) and corresponding photon fluxes for bottom and top emission and the sum of both (experiment and optical simulation results); (e) aging characteristics of PEDOT:PSS-based OLEDs with a different PEDOT:PSS

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Summary

Introduction

Organic light emitting diode (OLED) has emerged as a potential candidate for generation flexible, large-area, high resolution display and solid state lighting panels, because of its high color quality, attractive appearance, ease of fabrication, low manufacturing and materials cost, etc [1]. The work function of ITO is around 4.7 eV, which forms a low barrier for hole injection into the emission layer made of commonly used organic materials (Figure 1b) [9] Despite these advantages, ITO is far from being a perfect candidate for OLED applications for the following reasons. We focus our attention on the following materials: carbon nanotubes (CNTs), metallic nanowires, conducting polymers and graphene These materials have shown the potential to fulfill standard requirements on the sheet resistance and transmission values of TCE and can be formed by low-cost processes, such as spin coating, spray coating and even roll-to-roll processes [12]. (b) engery level diagram of a simple OLED device consisting of N,N′-Bis(3methylphenyl)-N,N′-diphenylbenzidine (TPD), N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1'biphenyl)-4,4'-diamine (NPB), 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl (CBP), Bathocuproine (BCP), and Tris-(8-hydroxyquinoline)aluminum (Alq3); (c) sheet resistance and transmission chart for various types of transparent conducting electrode (TCE) materials including carbon nanotube (CNT), silver nanowire (AgNW), conductive polymers, and graphene

Carbon Nanotubes
Metallic Nanowires
Conductive Polymers
Graphene
Findings
Conclusions

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