Photon-assisted capacitance–voltage study of organic metal–insulator–semiconductor capacitors

The results are reported of a detailed investigation into the photoinduced changes that occur in the capacitance–voltage ( C – V ) response of an organic metal–insulator–semiconductor (MIS) capacitor based on the organic semiconductor poly(3-hexylthiophene), P3HT. During the forward voltage sweep, the device is driven into deep depletion but stabilizes at a voltage-independent minimum capacitance, C min , whose value depends on photon energy, light intensity and voltage ramp rate. On reversing the voltage sweep, strong hysteresis is observed owing to a positive shift in the flatband voltage, V FB , of the device. A theoretical quasi-static model is developed in which it is assumed that electrons photogenerated in the semiconductor depletion region escape geminate recombination following the Onsager model. These electrons then drift to the P3HT/insulator interface where they become deeply trapped thus effecting a positive shift in V FB . By choosing appropriate values for the only disposable parameter in the model, an excellent fit is obtained to the experimental C min , from which we extract values for the zero-field quantum yield of photoelectrons in P3HT that are of similar magnitude, 10 −5 to 10 −3 , to those previously deduced for π -conjugated polymers from photoconduction measurements. From the observed hysteresis we deduce that the interfacial electron trap density probably exceeds 10 16 m −2 . Evidence is presented suggesting that the ratio of free to trapped electrons at the interface depends on the insulator used for fabricating the device.