White organic light emitting devices (WOLEDs) are being considered as substitutes for conventional white light sources. They are efficient solid-state lighting sources and their power efficiencies have surpassed those of the incandescent light sources, especially due to recent improvement in device architectures, molecular engineering in synthesis of new materials and the incorporation of electrophosphorescent emitters. This paper reviews the various approaches to achieve white light emission from organic light emitting diodes (OLEDs), their advantages, disadvantages and recent progress. The device architecture and problems related to various device designs have been discussed.

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When the transport and Poisson equations are integrated a constant of integration C should be used. If the Schottky barrier (ϕB) between metal electrode and organic semiconductor is zero, the injected carrier density at the contact is equal to that at the Fermi surface of the metal and can be taken as infinity to a good approximation. In this case the constant of integration C becomes zero. But for most cathodes and anodes used in organic devices the Schottky barriers are not zero. In these cases the constant C comes out to be A[J∕P(0)m], where the constant A depends on material parameters, m is a constant more than 1, J is the current density, and P(0)∝exp(−ϕB∕kT) is the injected carrier density. Even for a small value of the Schottky barrier ϕB=0.12eV, P(0) is reduced by two orders of magnitude. For this or smaller value of P(0), C plays an important role in determining the J-V characteristics. We report a theory of J-V characteristics for finite (nonzero) Schottky barriers. Even for small values of the Schottky barrier the results are strikingly different from the conventional theory based on zero barrier. As the barrier increases the deviation from the conventional theory becomes large. For very large values of C, J-V characteristics become Ohmic. We have fabricated ITO/PEDOT:PSS/MEH-PPV/Au diode and measured their J-V characteristics. The theory given in this paper agrees with the experimental data at different temperatures and different sample thicknesses.

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