The Basic Polymer LED Device Architecture

The simplest polymer-based EL device consists of a single layer of semiconducting fluorescent polymer, e.g., PPV, sandwiched between two electrodes, one of which has to be transparent (Fig. 1-1). When a voltage or bias is applied to the material, charged carriers (electrons and holes) are injected into the emissive layer and these carriers are mobile under the influence of the high ( 10 V cm ) electric field. 

Some of these carriers may recombine within the emissive layer yielding excited electron-hole pairs, termed excitons. These excitons may be produced in either the singlet or triplet states and may radiatively decay to the ground state by phosphorescence (PL) or fluorescence (FL) pathways (Fig. 1-2). An important figure of merit for electroluminescent materials is the number of photons emitted per electron injected and this is termed the internal quantum efficiency. It is clear, therefore, that the statistical maximum internal efficiency for an EL device is 25% as only one quarter of the excitons are produced in the singlet state. In practice, this maximum value is diminished further because not all of the light generated is visi- 

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Poly(l,4-phenylene vinylcne) and its Derivatives 2 The Basic Polymer LED Device Architecture 4 Substituted Poly(phcnylene vinylcne)s 6 Poly(anthrylenevinylcne)s 10 Step-Growth Routes to PPV Derivatives 10 PPV Copolymers 11... 

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