Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency

Sequential doping and dedoping increased the conductivity and optimized the oxidation level of transparent and flexible poly(3,4-ethylenedioxythiophene): poly(4-styrene sulfonic acid) (PEDOT: PSS) films, resulting in an improvement in the thermoelectric figure of merit ZT. The electrical conductivity (sigma) increased from 970 to 1260 S cm(-1) and the power factor from 66.5 to 70.7 mu W mK(-2) at the optimum concentration of the chemical dopant p-toluenesulfonic acid monohydrate (TSA). Then, the doped PEDOT: PSS films were treated with hydrazine/DMSO solutions with different hydrazine concentrations to precisely control the oxidation level. During the hydrazine/DMSO treatment (dedoping), s of the films continuously decreased from 1647 to 783 S cm(-1) due to a decrease in the carrier concentration, whereas the Seebeck coefficient (S) steeply increased from 28 to 49.3 mu V K-1 at the optimum oxidation level. A power factor of 318.4 mu W mK(-2) (sigma = 1310 S cm(-1), S = 49.3 mu V K-1), the highest among all existing thermoelectric nanofilms, was achieved while maintaining polymer film flexibility and transparency (88.3% of optical transmittance). In addition, the thermal conductivity (kappa) of the PEDOT: PSS films decreased from 0.38 to 0.30 W mK(-1) upon removal of PSS. At the lowest kappa value, a high ZT value of 0.31 was achieved at room temperature.