Cathodic and Anodic Material Diffusion in Polymer/Semiconductor‐Nanocrystal Composite Devices

Abstract

In the present day, the information technologies and telecommunications sector continually increase their demand for low cost, low power consumption, high performance electroluminescent devices for display applications. Furthermore, general lighting applications, such as white light and large array colour displays, would also benefit from an increase in the overall efficiency. Several technologies are being investigated to fulfill these needs, such as organic light emitting diodes (OLED), polymeric light emitting diodes (PLED) and field effect emission devices. A new and promising technology is light emitting devices (LEDs) based on nanostructured materials. With organic LEDs (OLEDs) already making an impact on the market in an increasingly large number of applications, hybrid technologies based on organic/inorganic nano-composites are a potential the next step. The incorporation of highefficiency fluorescent semiconductor nanoparticles has been shown to have a beneficial effect on device performance, [1] modify the colour output from the device [2] and provide a simplified route to generation of LED type devices.[3]Fluorescent semiconductor crystallites offer a high spectral purity and excellent luminescence quantum efficiency. These properties stem from the confinement of the carriers within the nanocrystals.[4] However, this confinement also reduces the carrier transport capabilities of the nanocrystals. [5,6] Carrier transport is reduced further when the nanocrystals are embedded in a polymer matrix.[3] Therefore, most devices incorporating semi-conductor nanocrystals work at fields in the order of 10 7 V m -1 or higher. The implications of high applied fields in device degradation have been investigated by various groups in other structures where similar values of field are applied, particularly in organic LEDs (OLEDs). Gao et al. [7] have described damage in hybrid polymer/nanocrystal devices caused by the current-induced oxidation of the aluminum cathode. In the case of OLEDs, the formation of dark spots has been related to the localized degradation of metallic cathodes, caused by electromigration at high operational fields. [8,9] Furthermore, Gautier et al. [10] found that field-induced damage on ITO electrodes can lead to device failure. Here, electrode diffusion-related degradation mechanisms in nanoparticles/polymer composites LEDs have been studied. The structure used for this purpose has been described in detail elsewhere. [3,11] Briefly, a composite multilayer made of a dielectric polymer (poly-diallyl-dimethyl-ammonium, PDDA) and CdTe nanoparticles was sandwiched between an aluminum cathode and an ITO anode. The multilayer was deposited through a layer-by-layer process carried out in air.The thicknesses of the different materials are 35 nm, 220 nm and 130 nm for aluminum, multilayer and ITO, respectively. Each substrate had up to 6 identical independently addressable devices with an active area of 4 mm 2 (Fig. 1). Four such devices were biased at 4.0 V for 15 s, two under moderate vacuum (10 -5 mba...

Light‐Emitting Diodes with Semiconductor Nanocrystals

“…explained the field‐induced diffusion as a consequence of the decomposition of the ITO. We studied electrode‐diffusion‐related degradation mechanisms in nanocrystal/polymer composite LEDs of the sort described above 66. The film thicknesses of the different materials are 35 nm, 220 nm, and 130 nm for aluminum, multilayer, and ITO, respectively.…”

Three‐Dimensional Self‐Assembly of Thiol‐Capped CdTe Nanocrystals: Gels and Aerogels as Building Blocks for Nanotechnology

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Novel assemblies and self-assembly architectures are currently of great interest for nanochemistry and chemical nanotechnology. The assemblies are generally considered as bottom-up alternative to top-down methods (i.e., nanolithography, e-beam epitaxy, and others).Stable, water-soluble, strongly light-emitting thiol-capped CdTe nanocrystals (NCs) [1,2] are promising materials for nanotechnological applications such as bioimaging, [3,4] labeling [5][6][7][8] and coding, [9] light-emitting diodes, [10][11][12][13] nanophotonic devices, [14,15] and energy scavengers. [16] In particular, lightemitting diodes (LEDs) based on thiol-capped NCs are the only known devices that are assembled by utilizing environmentally friendly layer-by-layer (LbL) technology under ambient conditions and show electroluminescence that is visible to the naked eye.

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Effect of degradation on electronic properties of polymer solar cells

“…Studies show that from ITO, indium (In) diffuses into active layer and even up to Al electrode. [46] with time and stabilized to~0.27 after~140 h. Similar to J sc , the efficiency decayed first rapidly and then slowly with time and the decay could be expressed by eqn (2) with different values of the fitting parameters. Symbols in Fig.…”

“…[17,[21][22][23][24] However, there is a rough classification between physical and chemical degradation. Physical degradation is because of delamination of top electrode, change in the active layer morphology, diffusion of electrode materials into organic layers and degradation of different interfaces, [25][26][27] whereas chemical degradation includes the formation of organic molecules with inferior electrical and optical properties due to reaction of original organic molecules with oxygen, moisture and electrode materials. [28][29][30][31] Photo-electrochemical reactions, photo-oxidation and oxidation of top low work-function electrodes are other important chemical degradation processes.…”