No abstract

Thermoelectric (TE) energy conversion in conjugated polymers is considered a promising approach for low‐energy harvesting and self‐powered temperature sensing. To enhance the TE performance, it is necessary to understand the relationship between the Seebeck coefficient (α) and electrical conductivity (σ). Typical doped polymers exhibit α–σ relationship that is distinct from that of inorganic materials due to their large structural and energetic disorder, which prevents them from achieving the maximum TE power factor (PF = α2σ). Here, an ideal α–σ relationship in the Kang–Snyder model following a transport parameter s = 1 is demonstrated with two degenerately doped semi‐crystalline polymers, poly[(4,4′‐(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1‐b:3,4‐b′]dithiophene)‐alt‐(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT) and poly[(2,5‐bis(2‐hexyldecyloxy)phenylene)‐alt‐(5,6‐difluoro‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) using a sequential doping method. The results allow the realization of the PFs reaching theoretic maxima (i.e., 112.01 µW m−1 K−2 for PPDT2FBT and 49.80 µW m−1 K−2 for PCPDTSBT) and close to metallic behavior in heavily doped films. Additionally, it is shown that the PF maxima appear when the doping state switches from non‐degenerate to degenerate. Strategies towards an optimal α–σ relationship enable optimization of the PF and provide an understanding of the charge transport of doped polymers.

Two types of p-type thermoelectric (TE) polymers with alkyl (PCPDTSBT) and oligoethylene glycol (OEG) side chains (PCPDTSBT-A) on an sp 2 -hybridized olefinic bis-(alkylsulfanyl)methylene-substituted cyclopentadithiophene backbone are synthesized. Interestingly, the OEG-substituted polymer, PCPDTSBT-A, exhibits significant self-doping compared to PCPDTSBT, where the polaron density of the former is 2.3 × 10 16 mm −3 (vs 7.9 × 10 14 mm −3 for PCPDTSBT) without external doping. Changing the side chains also induces a completely different polymer chain orientation in the PCPDTSBT-A (face-on) and PCPDTSBT (edge-on) films. The effect of doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) on the morphological and TE properties of the polymers with different side chains is studied. Sequential solution doping (SQD) is performed by overcoating the preannealed polymer films with F 4 TCNQ solution, which affords highly effective doping without disrupting the morphology of the crystalline films, especially for PCPDTSBT-A with OEG side chains. Resulting from the synergistic effect of the OEG side chains and SQD, PCPDTSBT-A exhibits remarkably improved electrical conductivity (53.8 S cm −1 ) with a higher power factor (40.4 μW m −1 K −2 ), compared to PCPDTSBT, for which the maximum electrical conductivity is 1.4 S cm −1 and the power factor is 1.8 μW m −1 K −2 . In addition, the transport coefficient of PCPDTSBT-A, determined by applying the Kang−Snyder model (2.40 × 10 −2 S cm −1 ), is superior to that of PCPDTSBT (3.59 × 10 −3 S cm −1 ), thereby showing the excellence of the developed strategy for improving the performance of TE polymers.

A series of ionically interconnected polypyrrole (PPy) films are fabricated through two-monomer-connected-precursor polymerization by varying diacid linkers, thereby significantly influencing the crystalline morphology and electrical properties. The structure obtained using 1,5-napthalenedisulfonic acid (PPy-Nap) as a fused aromatic linker exhibits a higher electrical conductivity (∼78 S cm–1) than that (6.7 S cm–1) without a linker (PPy-ref). Cryogenic conductivity measurements reveal that the percolation carrier transport barrier of PPy-Nap is significantly smaller than that of PPy-ref, and the calculated carrier mobility of PPy-Nap is ∼5 times higher compared to PPy-ref. The carrier transport characteristics show a good agreement with morphological data by 2D grazing-incidence X-ray scattering. All PPys have similar doped charge carrier concentrations and, thus, similar Seebeck coefficients (5–8 μV K–1) but very different electrical conductivities. Consequently, PPy-Nap exhibits a higher power factor than that of PPy-ref (0.21 vs 0.043 μW m–1 K–2). The results show that the trade-off relationship between the Seebeck coefficient and electrical conductivity can be overcome by improving crystalline morphology and carrier transport. Thus, both the electrical conductivities and thermoelectric power factors can be improved with maintaining the Seebeck coefficients by enhancing the ordered conductive domains and carrier mobility while maintaining the doping level.

An increase in the demand for completely foldable electronics has motivated efforts for the development of conducting polymer electrodes having extraordinary mechanical stability. However, weak physical adhesion at intrinsic heterojunctions has been a challenge in foldable electronics. This paper reports the completely foldable polymer thin-film transistors (PTFTs) and logic gate arrays. Homojunction-based PTFTs were fabricated by selectively doping p-type diketopyrrolopyrrole-based semiconducting polymer films with FeCl3 to form source/drain electrodes. The doping process caused a gradual work function change with depth, which promoted charge injection to semiconducting regions and provided a low contact resistance. In addition, the interfacial adhesion in the PTFTs was improved by interfacial cross-linking between adjacent component layers. The electrical performance of the resulting PTFTs was maintained without noticeable degradation even after extreme folding, suggesting that the proposed fabrication strategy can further be applied to various semiconducting polymers for the realization of foldable electronics.