Solution-Processed Organic Light-Emitting Diodes

Abstract

Following the discovery of organic light-emitting diodes (OLEDs)with vacuum-evaporated small molecules in 1987 [1], conjugatedpolymers were found to exhibit electroluminescence in 1990 [2].Conjugated polymers are organic semiconductors with bandgapbetween 1.5 and 3 eV. In earlier works luminescent polymerswere shown to be insoluble in common organic solvents and wereconverted from soluble precursors by thermal curing. Later, orangeemitting 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-emittingdiodes (PLEDs) by the solution process. The device structure isquite simple. On the transparent conductive ITO glass a conductingpolymer, typically poly(3,4-ethylenedioxythiophene) doped withFigure 2.1 (a) In a single-layer PLED, electron and hole current balance is controlled by the transport properties of the emissive material. Workfunctions of ITO, PEDOT:PSS, and cathode are labeled. Electrons and holesrecombine across the bandgap, as indicated by the arrow. (b) In amulti-layerPLED, electron and hole balance is achieved by blocking layers. The electronblocking layer also acts as hole transport layer. The hole blocking layer isalso the electron transport layer.