Correlating photocurrent spectra and electrical transport parameters in organic field effect transistors

We used photocurrent spectroscopy to study the electronic density of states distribution in thin films of sexithiophene and an alkylated analog, both at the band edge and inside the band-gap. The presented photocurrent spectroscopy analyses provide direct experimental evidence for a modification of the distribution of the electronic density of states in an organic field effect transistor (OFET) channel induced by the modulation of the effective carrier density by the gate voltage. We observe a correlation between a gate voltage induced increase of the carrier density in the channel and a broadening of the distribution of the electronic states around the band edge. We also detected the formation of deep electrically active states within the band-gap of sexithiophene OFETs, induced by the exposure to air and light. These results are in good agreement with the evolution of the macroscopic transport parameters of the devices, i.e., carrier mobility and threshold voltage, as a function of the exposure time to light and atmosphere.