External quantum efficiency versus charge carriers mobility in polythiophene/methanofullerene based planar photodetectors

The paper studies the role of electrons and holes mobility in determining the external quantum efficiency (EQE) in photodetectors based on a 1:1 in weight blend of poly(3-hexylthiophene) (P3HT) and C61-butyric acid methyl ester (PCBM). In order to fairly correlate the optoelectronic properties (efficiency and transient photocurrent) measured on photodetectors with the transport properties measured on field effect transistors, the photodetectors and the transistors have been produced with an identical planar geometry, where in both cases the charges flow occurs in the same direction with respect to the substrate. The measurements have been performed on devices based on pristine P3HT, on pristine PCBM, and on P3HT:PCBM 1:1 blend, before and after annealing, and have revealed the importance of a balanced carrier transport, showing that quantum efficiency increases as long as the mobility of electrons, which are the slowest carrier, increases. On the other end, the response time is mainly affected by the extension of the effective area.