Optimization of Thermoelectric Properties of Polymers by Incorporating Oligoethylene Glycol Side Chains and Sequential Solution Doping with Preannealing Treatment

Abstract

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.

Achieving Efficient p‐Type Organic Thermoelectrics by Modulation of Acceptor Unit in Photovoltaic π‐Conjugated Copolymers

“…Meanwhile, the strong electron-withdrawing capability of A moiety could strengthen the energetic disorder along the polymer chains and tuning the shape of density-ofstates (DOS), thus enlarging the value of E F − E T , which is proportional to S. [27] Accordingly, the peak room-temperature PFs of PBDP-T, PTB7-Th, and PBDB-T are acquired to be as high as 20.1, 46.0, and 105.5 μW m −1 K −2 at [FeCl 3 ] = 10, 5, and 10 mM, respectively, as shown in Figure 4c. When comparing with the literature results of D-𝜋, [14][15][16][17][18][19][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] A-𝜋, [20,[49][50][51] and D-A [23][24][25][26][27][52][53][54] type copolymers as summarized in Figure 4d, it has been rarely reported with S values over 100 μV K −1 , most of which were obtained by those D-A copolymers. This work achieves an impressively high PF over 100 μW m −1 K −2 while preserving a superior S beyond 200 μV K −1 as shown in Table S6, Supporting Information, holding great promise in wearable skin electronics, taken as one example.…”

“…The calculated lamellar packing and 𝜋-𝜋 stacking distances along the out-of-plane direction are as small as 19.19 (q z = 0.33 Å −1 ) and 3.69 Å (q z = 1.70 Å −1 ), respectively, suggesting that PBDB-T has remarkably compact molecular packing compared to both PBDP-T and PTB7-Th. Interestingly, the absence of 𝜋-bridges (thiophenes) in PTB7-Th [14][15][16][17][18][19][20][23][24][25][26][27] between the alternating rigid D and A units may hinder the sidechains from packing densely to the 𝜋-backbone and hence the largest lamellar and 𝜋-stacking distances are obtained.…”

“…[ 25 ] An alternate substitution by polar oligoethylene glycol as side‐group led to PCPDTSBT‐A, which presented significant self‐doping and greatly improved σ of 53.8 S cm −1 after sequential doping with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F 4 TCNQ). [ 26 ] On the other side, Li and colleagues reported such a random copolymerization of D−D and D−A type moieties into the backbone, the former of which contributes to effective charge transfer while the latter ensures efficient carrier transport. The resulting optimal copolymer PDPP‐g 3 2T 0.3 with finely tuned D/A unit ratios delivered an excellent PF as high as 110 µW m −1 K −2 with a moderate S of 56 µV K −1 .…”

“…Thermoelectric characteristics of doped polymer thin films as a function of dopant concentration. a) Seebeck coefficient (S), b) electrical conductivity (𝜎), c) power factor (PF), and d) their comparison with literature results among different types of p-type semiconducting polymers [14][15][16][17][18][19][20][23][24][25][26][27].…”

Revealing the effect of oligo(ethylene glycol) side chains on n‐doping process in FBDPPV‐based polymers

“…10 Introducing oligo(ethylene glycol) (OEG) side chains into conjugated polymers is a promising strategy to enhance doping efficiency. [11][12][13] Possessing higher local dipole moment and polarity than the alkyl side chains, 14 OEG side chains could stabilize doped products like dopant ions and polymer ions through the polar environment, thus enhancing the doping efficiency and miscibility of dopant with polymers. 15,16 However, OEG chains also bring unexpected low mobility and poor film morphology.…”