Electron-conducting/hole-blocking layers

The design of PLEDs relies on a variety of different materials, including a transparent substrate, the (semi)transparent anode, a hole-transporting layer in case of the usual two-layer structure, the EL polymer itself, an additional electron-conducting/hole-blocking layer in case of a three-layer device, the cathode, and finally some additional packaging material(s). 

In single layer devices the low efficiency frequently is due to the imbalance of electrons and holes. Inserting a hole-transport (electron-blocking) or electron transport (hole-blocking) layer provides a means to enhance minority carriers and block the majority carriers and confine them to the emitter layer, which increases the probability of recombination. Poly(N-vinylcarbazole) (PVK) has been used as a hole transport layer and occasionally in blends with the emitter polymer. PVK is a well-studied photo-conductive polymer which often forms exciplexes with organic molecules, e.g., dimethyl terephthalate. An exciplex is a transient donor-acceptor complex between the excited state of the donor and the ground state of the acceptor. ... 

A high resistance photo-conductive detector structure is disclosed in US-A-4731640. An n-type photosensitive layer connected via two n+-type regions is disposed between two blocking regions which confine electrons and holes within the photosensitive layer. The increase of the resistance of the photosensitve layer is accomplished by electrically depleting majority carriers, electrons, out of the photosensitive layer. 

More complex phenomena occur when current crosses interfaces between semiconductors. The most typical example is the rectification produced at interfaces between p- and n-type semiconductors. Electric current freely flows from the former into the latter semiconductor, but an electric field repelling the free carriers from the junction arises when the attempt is made to pass current in the opposite direction Holes are sent back into the p-phase, and electrons are sent back into the n-phase. As a result, the layers adjoining the interface are depleted of free charges, their conductivities drop drastically, and current flow ceases ( blocking the interface). 

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