Thickness dependence of hole mobility in regioregular poly(3-hexylthiophene) thin films

Methods of time of flight (TOF) and charge extraction by linearly increasing voltage (CELIV) are used to investigate the hole mobility in regioregular poly(3-hexylthiophene) (RR-P3HT) thin films. The film thickness was varied by changing the RR-P3HT solution concentration and spin-coating speed. The hole mobility is found to monotonically increase from 10$^{-4}$ cm$^{2}$/Vs to 10$^{-3}$ cm$^{2}$/Vs along with film thickness from 100 nm to 700 nm and saturate at 10$^{-3 }$cm$^{2}$/Vs beyond 700 nm. X-ray diffraction and ellipsometry data showed that the film morphology changes with the thickness. The structural change supports the weak dependence of energetic and positional disorder on thickness analyzed by the Gaussian Disorder Model (GDM).