Photovoltaic measurement of the built-in potential in organic light emitting diodes and photodiodes

We measure the voltage at which the current under illumination in poly2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene based light emitting diodes is equal to the dark current. At low temperatures, this voltage, which we term the “compensation” voltage, is found to be equal to the built-in potential, as measured with electroabsorption on the same diode. Diffusion of thermally injected charges at room temperature, however, shifts the compensation voltage to lower values. A model explaining this behavior is developed and its implications for the operation of organic light emitting diodes and photovoltaic cells are briefly discussed.