PHOTORESPONSE OF POLY(PARA-PHENYLENEVINYLENE) LIGHT-EMITTING-DIODES

We have studied the photoresponses of poly(para-phenylene vinylene) (PPV) light-emitting diodes (LED's) with PPV derivatives sandwiched between indium tin oxide (ITO) and metals including calcium, aluminum, and copper. Under illumination all diodes exhibit relatively large photoconductive I(V) responses which cross the dark I(V) curve at a forward-bias voltage V0 that scales with the difference in work functions between the ITO and metal electrodes. This causes the open-circuit voltage to saturate easily at V0 and consequently to be temperature independent, in contrast to the properties of the photovoltaic effect exhibited by conventional Schottky-barrier-type photodiodes. We have also found a dramatic, large enhancement in the electroluminescence intensity of the illuminated LED's which correlates with the photocurrent.