ABSTRACT
Following the discovery of organic light-emitting diodes (OLEDs)
with vacuum-evaporated small molecules in 1987 [1], conjugated
polymers were found to exhibit electroluminescence in 1990 [2].
Conjugated polymers are organic semiconductors with bandgap
between 1.5 and 3 eV. In earlier works luminescent polymers
were shown to be insoluble in common organic solvents and were
converted from soluble precursors by thermal curing. Later, orange
emitting soluble poly(p-phenylene vinylene) (PPV) was developed and shown, in 1993, to have a promising efficiency [3]. Since then,
extensive researches have been devoted to polymer light-emitting
diodes (PLEDs) by the solution process. The device structure is
quite simple. On the transparent conductive ITO glass a conducting
polymer, typically poly(3,4-ethylenedioxythiophene) doped with
Figure 2.1 (a) In a single-layer PLED, electron and hole current balance is controlled by the transport properties of the emissive material. Work
functions of ITO, PEDOT:PSS, and cathode are labeled. Electrons and holes
recombine across the bandgap, as indicated by the arrow. (b) In amulti-layer
PLED, electron and hole balance is achieved by blocking layers. The electron
blocking layer also acts as hole transport layer. The hole blocking layer is
also the electron transport layer.
