efficiency but no corrections were made for reflectivity at the device interface quantum yields for devices made with mg as the electronegative electrode were found to vary systematically with the thickness of the ppv layer with typical values of 1 for ppv layers of thickness 50 nm and 0 5 at 90 nm for power levels of 0 1 mw cm 2 these quantum efficiencies were measured under short circuit conditions i e at a bias voltage of zero volts the spectral response of the current for an ito ppv mg device with a relatively thick layer of ppv 220 nm is shown in fig7 at biases between 1 0 v and 1 5 v no correction has been made for the response of the light source and monochromator or for reflectance from the device the spectra show a sharp peak at the onset of the optical gap about 2 5 ev and a more shallow tail at higher photon energies between 1 0 and 1 5 v the polarity of the photocurrent reverses which indicates that the polarity of the electric field inside the device has reversed between these voltages this is consistent with the open circuit voltage for magnesium diodes of.