Symmetric light emitting devices from poly(p-di ethynylene phenylene) (p-di phenylene vinylene) derivatives

Light emitting devices were fabricated from 2,5-dialkoxy derivatives of poly(p-di ethynylene phenylene-p-di phenylene vinylene) (PDEPDPV) sandwiched between indium tin oxide (ITO) and Al. The current–voltage (I–V) curve, electroluminescence (EL) intensity-voltage (IEL–V) curve, and the EL spectra were identical in forward and reverse bias. The I–V curves were also symmetric under illumination, with I≊0 and V=0, suggesting a negligibly small internal electric field. At high bias voltage, carrier injection was found to be dominated by tunneling at the interfaces. At low bias voltage, tunneling among localized states at the Fermi level prevailed. The behavior is discussed in relation to Fermi-level pinning at defect states in the interfaces with the ITO and Al.