Intercalation and Ordering of F₆TCNNQ and F₄TCNQ Dopants in Regioregular Poly(3-hexylthiophene) Crystals: Impact on Anisotropic Thermoelectric Properties of Oriented Thin Films

Abstract

Redox doping is widely used to transform polymer semiconductors such as regioregular poly(3-hexylthiophene) into conducting polymers. Strong acceptor molecules such as F 6 TCNNQ and F 4 TCNQ have been used for sequential doping of aligned P3HT films to produce films with enhanced thermoelectric properties along the chain direction. This study uses Polarized UV-vis-NIR spectroscopy and electron diffraction to clarify the way dopant molecules are introduced in the crystals of P3HT. A structural model is obtained for F 6 TCNNQ-doped P3HT (P-1 symmetry, a=18.8 Å, b=8.95 Å, c=7.75Å, =107.6°, =101.5° and =89.3°). The proposed structure is consistent with the electron diffraction pattern and polarized UV-vis-NIR results. The model predicts intercalation of one dopant per four thiophene monomers, reduced -stacking distance and expanded unit cell along alkyl side chains. Both TEM and polarized absorption spectroscopy demonstrate that F 6 TCNNQ molecules are better ordered than F 4 TCNQ in the P3HT crystals, leading to improved charge conductivities reaching 500 S/cm with thermoelectric power factors close to 80 W m -1 K -2 .

Preferential Location of Dopants in the Amorphous Phase of Oriented Regioregular Poly(3‐hexylthiophene‐2,5‐diyl) Films Helps Reach Charge Conductivities of 3000 S cm−1

“…The recent structural refinement for F 6 TCNNQ-doped P3HT has shown that doping P3HT with F 6 TCNNQ induces a shift of P3HT backbones so as to group alkyl side chains and create cavities to host the dopant molecules in the crystal. [19] The present results demonstrate that doping with MB does not induce such a structural reorganization within π -stacks. There is thus evidence that MB is only marginally intercalated inside the P3HT lattice.…”

“…The incremental concentration doping (ICD) of P3HT films leads to an improvement of TE properties based on a progressive intercalation of dopants in the pristine polymer structure as compared to direct doping using dopant solutions of high concentration. [18,19,30] The same oriented film was thus dipped in solutions of MB/ACN of increasing concentration [MB] and the anisotropic conductivity and Seebeck coefficient were measured along (//) and normal (⊥) to the rubbing direction (chain direction) (see Figure 1b-d).…”

“…Previous studies have demonstrated that the evolution of the (h 0 2) reflections (h = 0, 1, 2) with doping is a fingerprint of the dopant intercalation in the P3HT crystal lattice. [19,30] Intercalation of dopants inside P3HT crystals results in the disappearing of the 1 0 2 and 2 0 2 reflections and a streaking of the 0 0 2 reflection whose intensity increases substantially. These changes are the fingerprints of a re-organization of P3HT backbones within individual π-stacks.…”

“…For instance, ICD or vapor phase doping that preserve better the pristine structure of the polymers than direct doping should help determine more accurately the S-σ correlations. [18][19][20]…”

“…[10][11][12][13][14][15] Understanding of the correlations between the microstructure of the doped polymers and their TE properties is central to the optimization path of TE organic materials. So far, detailed studies on structure-TE property correlations are investigated for polythiophenes doped with F 4 TCNQ, F 6 TCNNQ [16][17][18][19] FeCl 3 [20] and Mo(tdf-COCF 3 ) 3 . [21][22][23] Generally speaking, the studies of doping of polymer semiconductors such as P3HT and PBTTT have demonstrated the key role of the following parameters: i) dopant's electron affinity, [21] ii) the method of doping (sequential vs. direct doping from solution, vapor phase vs. solution doping), [15,24,14] iii) the structure of the pristine polymer prior to doping, [25,26] and iv) the structure of alkyl side chain layers that host the dopant molecules.…”