Enhancing Stability and Thermoelectric Performances of Doped Oriented Regioregular Poly(3-Hexythiophene) Using Anion Exchange Doping

Doping polymer semiconductors is an elegant method to produce conducting polymer films for various electronic applications with effective structural control. In this contribution, we present an innovative method for preparing stable and efficient conducting polymer films by combining high-temperature rubbing with anion exchange doping using bis(trifluoromethylsulfonyl)imide (TFSI) as the dopant. We combine two doping methods i.e. anion exchange and incremental concentration doping to investigate the impact of progressive dopant introducting in the semi-crystalline structure of P3HT on the structural and thermoelectric properties of aligned thin films using a combination of Transmission electron microscopy, Polarized optical absorption spectroscopy and transport measurements. Our results demonstrate that anion exchange strongly depends on both the ratio of the dopants (F₄TCNQ and LiTFSI in n-butylacetate) but also on their concentration. Two regimes of doping are evidenced with different distributions of dopants in the semi-crystalline structure of P3HT: first crystalline domains are doped by exchange of F₄TCNQ^- with TFSI^- followed by a progressive doping of amorphous regions of P3HT. The best thermoelectric performances are comparable to the ones observed for P3HT doped with tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic blue) with power factors of 150-160 μW/mK². In addition, we demonstrate a long-term stability of the doped films under inert atmosphere over > 50 days. The enhanced stability of TFSI-doped P3HT films is directly related to the specific distribution of dopants inside the semi-crystalline polymer structure. The enhanced stability is analysed in the frame of a modulation-doping mechanism considering the dopant distribution in the polymer structure and its impact on the accumulation layer that is created at the crystal/amorphous interface.