Highly Efficient Organic Devices Based on Electrically Doped Transport Layers
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.
- SourceAvailable from: Fabrizio Mariano
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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 . 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 . This makes possible to use silver layers for both cathode and anode, and obtaining a microcavity architecture . "
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.Organic Electronics 11/2013; 14(11):2840-2846. DOI:10.1016/j.orgel.2013.07.034 · 3.83 Impact Factor
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- "For example, at a typical application luminance level of 1000 cd/m 2 , the devices with the Au tri-cathode-layer demonstrate an efficiency of 23 cd/A and 10.5 lm/W, higher than 21 cd/A and 9.1 lm/W for the devices with the Ag based tri-cathodelayer . Further optimization, for example, electrical doping the charge transporting layer , employing new emitters/host with high carrier mobility and efficiency, applying out-coupling techniques  , can be employed to boost the device performance. "
ABSTRACT: It is challenging to obtain broadband emission spectra in top-emitting white organic light-emitting diodes (WOLEDs) due to the well known microcavity effects. In this work, we demonstrate that the microcavity effects can be greatly alleviated by employing a low reflection tri-cathode-layer Yb (5nm)/Au (15nm)/MoO3 (35nm). Top-emitting WOLEDs with evenly separated red, green and blue emission peaks have been achieved. The white emission spectra show weak angle dependence as well. At a luminance of 1000cd/m2, the top-emitting WOLEDs show an efficiency of 23cd/A, 10.5lm/W, a 1931 Commission International de L’Eclairage coordinate of (0.36, 0.42) and a high color rendering index of 85. The top-emitting WOLEDs have been successfully applied in the Si based microdisplay.Organic Electronics 12/2011; 12(12):2065-2070. DOI:10.1016/j.orgel.2011.08.014 · 3.83 Impact Factor
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- "In recent years, organic molecular materials have gained considerable importance from the viewpoint of their applications in optoelectronics: these materials can be used to fabricate optoelectronic devices such as organic light-emitting diodes, organic thin-film transistors, and organic solar cells         . Devices fabricated using organic molecular materials are expected to be lightweight and flexible; moreover, the fabrication process is environmentally friendly   . "
ABSTRACT: Bisazomethine dyes, which are synthesized using diaminomaleonitrile and aminobenzaldehydes, exhibit red fluorescence in solution and in the solid state. Several bisazomethine dyes are known to form J-aggregates in vapour-deposited films. In this work, novel bisazomethine dyes were synthesized and the effect of phenyl ring substitution on the J-aggregate formation in vapour-deposited films was examined. The optical properties of the dyes were examined in solution and in the solid state through molecular orbital calculations. Four derivatives were found to form J-aggregates in vapour-deposited films as determined from the shape of the spectrum and the absorption edge.Dyes and Pigments 07/2011; 90(1):56-64. DOI:10.1016/j.dyepig.2010.11.009 · 3.97 Impact Factor