Bringing Conducting Polymers to High Order: Toward Conductivities beyond 10⁵ S cm⁻¹ and Thermoelectric Power Factors of 2 mW m⁻¹ K⁻²

Abstract

Here, an effective design strategy of polymer thermoelectric materials based on structural control in doped polymer semiconductors is presented. The strategy is illustrated for two archetypical polythiophenes, e.g., poly(2,5‐bis(3‐dodecyl‐2‐thienyl)thieno[3,2‐b]thiophene) (C12‐PBTTT) and regioregular poly(3‐hexylthiophene) (P3HT). FeCl3 doping of aligned films results in charge conductivities up to 2 × 105 S cm−1 and metallic‐like thermopowers similar to iodine‐doped polyacetylene. The films are almost optically transparent and show strongly polarized near‐infrared polaronic bands (dichroic ratio >10). The comparative study of structure–property correlations in P3HT and C12‐PBTTT identifies three conditions to obtain conductivities beyond 105 S cm−1: i) achieve high in‐plane orientation of conjugated polymers with high persistence length; ii) ensure uniform chain oxidation of the polymer backbones by regular intercalation of dopant molecules in the polymer structure without disrupting alignment of π‐stacked layers; and iii) maintain a percolating nanomorphology along the chain direction. The highly anisotropic conducting polymer films are ideal model systems to investigate the correlations between thermopower S and charge conductivity σ. A scaling law S ∝ σ−1/4 prevails along the chain direction, but a different S ∝ −ln(σ) relation is observed perpendicular to the chains, suggesting different charge transport mechanisms. The simultaneous increase of charge conductivity and thermopower along the chain direction results in a substantial improvement of thermoelectric power factors up to 2 mW m−1 K−2 in C12‐PBTTT.

Electron transport in a sequentially doped naphthalene diimide polymer

“…3,4 Sequential doping avoids possible complications that arise if the doped material is poorly soluble in the casting solvent and in some cases may allow for the preservation of some of the structural order present in the pristine semiconductor film. Although widely applied to the p-doping of P3HT [5][6][7][8] and PBTTT, 9,10 there are only few reports of the sequential n-doping of solution-processed polymers. 11 The air-stability of oxidants, hole-transport materials, and their doped combinations often allow for easy handling in air.…”

“…Highly conductive hole-transport materials are achievable in many polymer-dopant systems. 7,[12][13][14][15][16] In contrast, electrontransport materials and their n-doped derivatives lag behind p-type polymers in terms of both mobility and conductivity. [17][18][19] One reason is that the dopant-induced polaronic charge carriers tend to be localized on acceptor moieties, and the electronic coupling between adjacent acceptor sites is often poor.…”

Insights into the structural complexity of semi-crystalline polymer semiconductors: electron diffraction contributions

“…72 Recent studies have demonstrated that the dopant molecules intercalate inside the crystal lattice of PSCs. [73][74][75] The key role of dopant intercalation and ordering on the structure and thermoelectric properties of PSCs has been highlighted. Important and fundamental aspects deal with the precise position and distribution of dopant molecules in the crystalline structure of the PSCs and how it controls the electronic properties of the doped systems.…”

“…On a more applied basis, crystallization and orientation of TE materials has emerged as a valuable and effective means to reach high TE power factors. [74][75][76] The know-how developed in monitoring the crystallization of PSCs is essential to further optimize the structure of the doped conducting systems. New strategies for the preparation of highly ordered conducting polymers must be developed.…”

Impact of Morphology on Charge Carrier Transport and Thermoelectric Properties of N‐Type FBDOPV‐Based Polymers

“…[ 14 ] The development of both n‐type and p‐type efficient materials is then required to benefit from voltage summation. While high control over the organization of conjugated polymer chains allowed to achieve a metallic‐like electrical conductivity in p‐doped polymer films, [ 15,16 ] the development of n‐type polymers with efficient transport properties in their intrinsic and doped states remains a challenge. [ 17,18 ] To date, the highest electron mobility and thermoelectric performance for n‐type polymers were achieved for those composed of fluorinated derivatives of the electron‐deficient benzodifurandione‐based oligo( p ‐phenylene vinylene) (BDOPV) unit.…”