Fast photoresponse of functionalized pentacene and anthradithiophene thin films

We present photoconductivity of high-performance functionalized pentacene and anthradithiophene thin films on time scales from picoseconds to many seconds after photoexcitation. The polycrystalline thin films were deposited from solution on glass substrates with patterned interdigitated aluminum electrodes. In studies of fast transient photoconductivity, the samples were excited with laser pulses of ̃100 fs duration at a wavelength of 400 nm, and the photocurrent due to transport of photoexcited charge carriers was monitored using 50 GHz digital sampling oscilloscope. The photoconductivity at longer (milliseconds through seconds) time scales was investigated using continuous wave (cw) illumination and a source-delay- measure unit. Both experiments were performed under conditions of varied electric field strength, fluence and temperature. In all samples, we observed fast charge carrier photogeneration (<30 ps, limited by time resolution of our setup) followed by decay of the photocurrent over the period of 5-50 ns, depending on the material, due to charge trapping and recombination, linear dependence of the peak photoconductivity on the fluence and super-linear dependence of the peak photocurrent on the applied electric field.