Measurement of built-in voltage of organic semiconductor devices

The built-in voltage in organic semiconductors is usually measured by electroabsorption measurements. In this paper, we show that for tris-(8-hydroxy quinolato)aluminium (Alq) devices, modulated photocurrent spectroscopy provides a simple viable alternative to measuring the built-in voltage. This is possible because the quantum efficiency for photoconductivity in Alq is independent of the electric field. We also examine the limitations of modulated photocurrent spectroscopy in obtaining the built-in electric field.