Molecularly Engineered Polymer LEDs
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ABSTRACT Polymeric light emitting devices may be fabricated from a simple structure consisting of a low work function cathode (typically calcium or magnesium), a conjugated semiconducting polymer and a transparent anode (typically indium-tin oxide). Optimum device efficiencies require the balanced injection of electrons and holes. This paper describes the application of molecular engineering in the design of a family of poly(cyanoterephthalylidenene)s which show increased electron affinity over the unsubstituted analogue [poly(p-phenylenevinylene) PPV]. In particular these polymers as the emissive layer in a bilayer device with indium tin oxide (ITO, positive transparent contact) and aluminum (stable negative contact) and PPV as a hole transporting layer exhibit internal efficiencies up to 4%.
INTRODUCTION About three years ago we reported that the conjugated organic semiconductor, poly(pphenylenevinylene) (PPV) could be used as the emissive layer in a light emitting device when the polymer was sandwiched between charged electrodes (metal/polymer/metal) [1]. Electroluminescence in devices constructed from sublimed molecular films had been known for some time, and although rather high internal efficiencies (measured as photons emitted per electron injected) had been reported with these materials, it has been assumed that they are likely to suffer from changes in morphology which could lead to breakdown of the device. The advantage of organicsemiconductors is that the nature of the conjugated unit (dye) can be varied very easily to span an energy gap between excited and ground state which covers the whole range of the visible spectrum. The attraction of using a polymer as an emissive layer is that it may be processed (either as a precursor polymer or as a solvent-soluble material) and then deposited as a thin film over relatively large areas with few defects. For example PPV can be prepared as a tough intractable light emitting material via a sulfonium salt precursor route which involves processing the precursor in methanol, followed by conversion in the form of a thin film (typically 100 nm thick) into the fully conjugated material with expulsion of the sulfonium leaving group [2]. In an electroluminescent device the efficiency of light emission depends on the number of positive and negative charges which combine to form singlet excitons, and whether these decay radiatively to the ground state. Double charge injection produces three times as many triplet states as singlets. Thus maximum attainable internal efficiencies are limited in theory to one quarter of that observed for photoluminescence. DISCUSSION In this paper we show how efficiencies may be improved by molecular engineering. The earliest reported efficiency with PPV was 0.01% [1]. Using copolymers we engineered 371 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
interruptions in conjugation to limit exciton migration to posssible quenching sites, and raised efficiencies to 0.3% [3]. We knew that the barrier to injection of negative
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