Dielectric spectroscopy of [P(NID2OD-T2)]n thin films: Effects of UV radiation on charge transport

• Ultra-violet (UV) radiation dopes the polymer.
• The doping is n-type.
• UV radiation enhances charge mobility without post polymer processing.
• Dielectric spectroscopy and field effect transistor results are self-consistent.

Poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyll-alt-5,5′-(2,2′-bithiophene)]-[P(ND12OD-T2)]n is a n-doped polymer that is stable in air. Low frequency (40 Hz–30 kHz) dielectric spectroscopy shows that the polymer impedance strength is reduced under ultra-violet (UV) radiation as a result of charge increase in the bulk polymer. Photo-excitation and the creation of electron-hole pairs and subsequent hole recombination with electron trapping species adsorbed by the polymer are suggested as possible doping mechanisms. The relaxation times were also faster in the presence of UV indicating multiple pathways for oscillating dipoles to relax. These results imply increased polymer conductance with corresponding enhancement of charge mobility due to reduced scattering in the presence of UV radiation. A thin film field effect transistor was fabricated using this polymer as the active material and characterized in the presence of UV radiation. As expected, the device exhibited n-type behavior with a charge mobility of 3.0 × 10− 3 cm2/V-s. Exposure to UV radiation increased the channel current, shifted the threshold voltage to more negative values and doubled the value of the mobility. These results are consistent with dielectric measurements and suggest an easy method of increasing device currents and charge mobility in this polymer via UV irradiation.