Cyano-Derivatives of Poly(P-Phenylene Vinylene) For use in Thin-Film Light-Emitting Diodes
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ABSTRACT Conjugated polymers can be use to provide charge transport and emissive layers in a range of thin-film electroluminescent devices. Electroluminescence results from radiative decay of excitons which are formed by electron-hole capture. Device efficiency is particularly sensitive to the balancing of electron and hole currents injected from opposite electrodes, and this is best achieved at the heterojunction between two polymer layers with different electronegativities. We report here the properties of a conjugated polymer with high electron affinity, a cyano derivative of poly(p-phenylene vinylene), PPV. This polymer shows an electron affinity considerably higher than than that of PPV, and is therefore suitable for use as the electron transporting layer in heterostructure LEDs. It shows efficient photoluminescence due to radiative decay of singlet excitons, and photoinduced absorption in the IR, similar to that in PPV, which we attribute to excitations of photogenerated triplet excitons. EL devices formed with a heterojunction between PPV and this cyano derivative of PPV, with indium/tin oxide as positive electrode, PPV, cyanoPPV layers and an aluminium negative electrode, show internal quantum efficiencies as high as 4%. INTRODUCTION Interest in conjugated polymers has largely been directed to their properties as electrical conductors when formed as charge-transfer complexes with oxidising and reducing agents [1]. The realisation that these polymers could show high luminescence efficiency if suitably free of defects that would act as quenching centers was not made until the mid 1980's with improvements in methods of synthesis and processing of some of these polymers in the undoped state, notably poly(p-phenylene vinylene), PPV [2, 3], and the poly(3-alkyl thienylene)s [4]. Improvements in methods of processing of these polymers from solution had also allowed fabrication of thin polymer films of high quality which proved suitable for use as the active layers in thin film transistors [5, 6]. Since our first report of electroluminescence (EL) in PPV [7], a range of conjugated polymers have been shown to exhibit EL [8-13]. A typical polymer light-emitting diode (LED) comprises a thin film of polymer sandwiched between metal electrodes, one of which is semitransparent. Under an applied bias, oppositely-charged carriers are injected from the opposing contacts and are swept through the device by the electric field. These carriers may then capture one another within the device to form excitons, and the singlets amongst them may then decay radiatively, giving out light at a wavelength characteristic of the energy gap of the polymer. There are a number of processes which control the performance of the LED. We require: (i) injection and (ii) transport of holes; (iii) injection and (iv) transport of electrons from the negative 351 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
electrode; (v) electron-hole capture to form excitons, and (vi) radiative decay of the excitons to give light emission.
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