Photoresponse and electroresponse of polymers light emitting diodes

We have studied the photoresponse and electroresponse of light emitting diodes (LEDs ) made from a variety of soluble conducting polymers sandwiched between indium tin oxide (ITO) and metals including calcium, aluminum and copper. Under illumination all freshly prepared LEDs exhibit relatively large photoconductive current-voltage (I-V) responses which cross the dark I-V curve at a forward-bias voltage V 0 that scales with the difference in the work functions between the ITO and metal electrodes. This causes the open-circuit voltage to easily saturate at V 0 and consequently to be temperature independent, in contrast to the properties of the photovoltaic effect exhibited by conventional Schottky-barrier type photodiodes. Some LEDs, prepared under less ideal conditions exhibit I-V curve, electroluminescence (EL) intensity-voltage ( I E L-V) curve, and EL spectra identical in forward and reverse bias. The I-V curves were also symmetric under illumination, with I ~ 0 at V = 0, suggesting a negligibly small internal electric field. These diodes are discussed in relation to Fermi-level pinning at defect states in the interfaces between the polymer and the electrodes.