Tri-segmented side-chain conjugated polymers for fine-tuning the performance of doped organic electronics – TriPODE

The project TriPODE aims at providing a breakthrough in the field of doped polymeric systems through a strategy of side-chain engineering of conjugated semiconducting polymers to fine-tune the performance of doped organic electronics. Two applications are targeted in particular: thermoelectricity device (TE) to convert thermal loss into useful electricity, and electrochemical transistors (OECTs) which is the core of next-generation bioelectronic devices.

The consortium driving TriPODE brings together members of the IPCMS, ICPEES and ICS laboratories, all located on the same Strasbourg campus. In this project, they uniquely combine their expertise in organic synthesis, controlled polymer alignment control, morphological characterization of thin films, fundamental mechanistic study of doping of these organic systems not only in films but also in operando TE device and OECTs.

The developed molecular approaches that are being developed aim to simultaneously address most factors limiting device performance, namely doping efficiency, doping/dedoping kinetics, induced charge carrier mobility, structural and energetic disorders, mechanical stresses, film morphological rearrangement and delamination, stability over cycling.

To do so, TriPODE proposes the original use of tri-segmented side chains based on the three following segments: alkyl / ether (polar) / siloxane. By tuning the position and length of each segment of the side chain, we will build stable multi-layered mesomorphic polymer thin films with well-defined location and thickness of each functional sublayer. The systematic variation of molecular parameters will allow to rationalize and exploit the effect of each segment on the polymer properties. In particular, the siloxane segments will enhance the structural film cohesion to preserve the order favourable to electronic transport despite dopant insertion. Concomitantly, the polar segments are designed to increase the amount of dopant inserted and confine them at defined spatial positions in order to study the impact of Coulomb interactions in carrier mobility and doping stability. The results will essentially i) provide an in-depth understanding of the doping mechanisms and of their impact on conducting properties, and ii) define a toolbox for the development of high-performing doped systems for TE and OECTs that will be very likely generalizable to other applications.

Although the basis of this project is fundamental, the organic polymers that will be synthesized in the framework of TriPODE aim at achieving the following performances:

For TE, a figure of merit ZT with valuess of about 0.3 are targeted for both p-type and n-type polymers. These values are to be compared with the best systems obtained for the particular cases of PEDOT:PSS (p-type, ZT = 0.3-0.5) and fullerene (n-type, ZT=0.3). Note that one of the objectives of this project is to equip the consortium with the technical facilities to measure the thermal conductivity of thin films.

For OECT, a figure of merit ??C* with values over 8 000 F cm-1 V-1 s-1 and 80 F cm-1 V-1 s-1 are targeted for p-type and n-type polymers, respectively. In addition to these performances, our side chain engineering approach is expected to lead to a significant improvement in stability over ON/OFF transistor cycling (measured over about 10 000 cycles). Current values reported in the state of the art are about 2 000 and 55 F cm-1 V-1 s-1 for p-type and n-type, respectively.