Article

Highly Efficient Organic Devices Based on Electrically Doped Transport Layers

Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany.
Chemical Reviews (Impact Factor: 46.57). 05/2007; 107(4):1233-71. DOI: 10.1021/cr050156n
Source: PubMed

ABSTRACT

In this review, we have discussed the controlled doping of organic semiconductors by coevaporation with suitable dopant molecules and its application for highly efficient devices, such as organic LED and organic solar cells. The experimental data show that the conductivities can be raised many orders of magnitude above the conductivity of nominally undoped materials. Due to low mobilities, the conductivity of the materials is still much lower than those of inorganic semiconductors but sufficient for many devices that do not need too high current densities, such as organic light-emitting diodes and solar cells. Although some basic doping effects like Fermi level shifts can be well compared to the standard behavior of inorganic semiconductors, there are deviations that cannot be explained by the simple models used for crystalline inorganic semiconductors. A detailed understanding of the dependence of conductivity on doping concentration requires models that take effects like localization and percolation into account. While molecular p-type doping has been available for some time, impressive progress has recently also been made for n-type doping, which is more difficult since electrons have to be transferred into rather high-lying orbitals. We have further discussed that doped organic semiconductors are well suited for device applications. For OLEDs, the conductivities achieved are high enough to avoid significant voltage drops even in thicker layers. A key effect of doping is the generation of Ohmic contacts by tunneling through a thin barrier formed by space charge layers, an effect which works in organic semiconductors very well. This is particularly important for OLED devices where the nominally undoped transport layers have required extensive measures to achieve low barriers at the interfaces and have made the devices very sensitive to the contact properties. It has been demonstrated that doped transport layers allow realization of very efficient inverted top-emitting and transparent OLED devices. The application of doped transport layers to organic solar cells has progressed much less than that for OLEDs. Again, one key advantage is the decoupling of the electrical and optical optimization, which allows the placement of the active region of the solar cells at the regions where the optical field is the largest. Other points are that the use of doped window layers allows the extension of the quasi-Fermi level splitting from the active layers in the most efficient way toward the contacts, thus allowing maximum open-circuit voltage independent of the detailed nature of the contact materials. Many of the aspects of doped organic layers that we have discussed here are directly taken from inorganic semiconductors. It is thus easily predictable that the multitude of device principles that have developed over decades in the field of inorganic semiconductors can be explored as well for organic semiconductors, with some modifications. We thus believe that in the future, there will be ample space for further investigations of organic devices with doped layers. From a materials perspective, the progress on new organic semiconductors is rather rapid since the commercial application in devices like OLEDs has spurred large interest from industry, and a systematic search for new materials with improved properties has begun. Part of these investigations should also address new dopants, since the experiments and materials being reported here are still a very small part of what is possible.

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    • "The improvement of hole injection can contribute to the reduction in the power loss of OLEDs. During the past decade, p-doped hole transport layers (HTLs) have been widely used to enhance hole injection of OLEDs111213, since (1) the p-doped HTL can offer ohmic contacts with metal even with medium work function, e.g., Al, as a result of the thin depletion zone formed between p-doped HTL and metal, and (2) the p-doped HTL can lead to the significant decrease in ohmic loss during hole conduction relative to the undoped HTL. The MoO 3 doped N,N 0 -bis-(1-naphthyl)-diphe nyl-1,1 0 -biphenyl-4,4 0 -diamine (NPB:MoO 3 ) and 4,4 0 -N,N 0 -dicarba zole-biphenyl (CBP:MoO 3 ) are among the most frequently employed p-doped HTLs. "
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    ABSTRACT: The MoO3 doped N,N′-bis-(1-naphthyl)-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB:MoO3 in 2:1 mass ratio) and 4,4′-N,N′-dicarbazole-biphenyl (CBP:MoO3 in 2:1 mass ratio) as p-doped hole transport layers have been used in inverted organic light emitting diodes (IOLEDs). Compared to the NPB/20 nm NPB:MoO3 structure, the NPB/10 nm CBP:MoO3/10 nm NPB:MoO3 structure showed increased device performance, mostly because the hole transport barrier from CBP:MoO3 to NPB was smaller than that from NPB:MoO3 to NPB; it also presented improved device performance than the NPB/20 nm CBP:MoO3 structure, ascribed to the higher conductivity of NPB:MoO3 than that of CBP:MoO3. We provide a manageable way to unlock the merits of p-doped hole transport layers for markedly increasing the performance of IOLEDs.
    Full-text · Article · Dec 2015 · Displays
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    • "The degeneration is attributed to the electric field induced migration of cesium ions towards ETL/MoO 3 interface, which will further lead to the reaction of cesium cations with MoO 3 to form complexes. When Cs or Li are doped into the transporting layers, they would be ionized to provide free electrons in the matrix [35], leaving the Cs or Li cations which are relatively easy to migrate in the organic layers under electrical field, due to their small size compare to the organic molecules [36], [37]. Considering Li and Cs are the most frequently used n-dopant in OLEDs, the degenerating problem of TMOs based interconnectors might be broad. "
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    ABSTRACT: In this paper, we present a comprehensive review on recent progress of interconnectors in tandem organic light emitting diodes, from structure design to performance and their working principles. We will introduce the most commonly used interconnectors, discuss in detail the main features of them, as well as the advantages and disadvantages of each type. Especially, organic heterojunciton type interconnectors are highlighted.
    Full-text · Article · Jan 2015 · IEEE Journal of Selected Topics in Quantum Electronics
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    • "In all devices the organic transport layers are doped with electron atomic donors (Cs atoms for n-type layer) or electron molecular acceptors (F 4 TCNQ for p-type layer) compounds [21]. Doping reduces the resistivity of transport layers, improving LE, and allows for a tunnel injection of the carriers from the metal layers into the organic stack, making the interfaces injection properties almost independent of the metal electrodes layers [21]. This makes possible to use silver layers for both cathode and anode, and obtaining a microcavity architecture [16]. "
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    ABSTRACT: The technology of white organic light-emitting diodes (WOLEDs) is attracting growing interest due to their potential application in indoor lighting. Nevertheless the simultaneous achievement of high luminous efficacy (LE), high color rendering index (CRI), very low manufacturing costs and compatibility with flexible thin substrates is still a great challenge. Indeed, very high efficiency devices show usually low values of CRI, not suitable for lighting applications, and use expensive indium tin oxide (ITO) electrodes which are not compatible with low cost and/or flexible products. Here we show a novel low cost ITO-free WOLED structure based on a multi-cavity architecture with increased photonic mode density and still broad white emission spectrum, which allows for simultaneous optimization of all device characteristics. Without using out-coupling optics or high refractive index substrates, CRI of 85 and LE as high as 33 lm W−1 and 14 lm W−1 have been demonstrated on ITO-free glass and flexible substrates, respectively.
    Full-text · Article · Nov 2013 · Organic Electronics
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