Investigation of trap states and mobility in organic semiconductor devices by dielectric spectroscopy: Oxygen-doped P3HT:PCBM solar cells

We investigate the dielectric response of solar cell devices based on oxygen-doped poly(3-hexylthiophene):[6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) blends as a function of temperature between 133 K and 303 K. The spectra are analyzed using a recently introduced model [O. Armbruster, C. Lungenschmied, and S. Bauer, Phys. Rev. B 84, 085208 (2011)] which is based on a trapping and reemission mechanism of charge carriers. A dominating trap depth of 130 meV is determined and the broadening of this trap level identified as purely thermal. In addition we estimate the density of charge carriers after doping as well as their mobility. We show that the concentration of mobile holes approximately doubles by heating the device from the lowest to the highest measured temperature. This is indicative of a second, shallow trap level of approximately 14 meV. Dielectric spectroscopy hence proves to be a valuable tool to assess device parameters such as dopant concentration, charge carrier transport characteristics, and mobility which are of crucial interest for understanding degradation in organic semiconductor devices.