The use of light-emitting diodes (LEDs) continues to increase dramatically, for example in color computer screens based on flat LED boards which are intended to replace the large and expensive cathode-ray tubes. In the past, it has proved difficult to economically extend the emission spectra of the LEDs into the blue spectral region.The story of the LED goes back to 1907,L'I and in the last 35 years much effort has been invested in the study of inorganic semiconductors e.g. GaN, ZnS, ZnSe and S i c in order to produce an effective blue LED. However, large scale applications have not been practicable due to the problems in fabrication and the rather low luminescence efficiency. Therefore, the use of alternative materials was proposed[21 in order to extend the spectral region into the blue.Organic materials have always been considered for use in electroluminescent devices,[31 but up to now they have not been widely used except for applications in scintillation detectors. Only recently has there been a noticable effort in electroluminescent device fabrication with organic laye r~. [~. ' 1 The observed transfer function between the current input and luminescence output of organic materials is linear, which allows optocouplers without any distortion to be devised. This feature cannot be provided by existing optocoupling devices. A blue light-emitting electroluminescent device, based on low-molecular-weight organic materials with a substantial efficiency was presented recently.I6] A serious disadvantage of devices consisting of organic materials of rather low molecular weight is the tendency of the materials to recrystallize (initiated by the heat produced in the device), which leads to a drastic decrease in quantum efficiency.16] Intensive research activities on the physical properties and synthesis of conjugated polymers[71 have shown these materials to be promising candidates for applications in stable optoelectronic devices. The first encouraging results on LEDs based on semiconducting polymers were reported by Burroughes et al.[*l and confirmed by Braun et al.'91 These devices emitted in the yellowish ( x 600 nm) sprectral range.In the search for new materials for blue LEDs and linear optocouplers, we have investigated the polymer polybphenylene) (PPP, see Fig.

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