Fabrication of Oriented n‐Type Thermoelectric Polymers by Polarity Switching in a DPP‐Based Donor–Acceptor Copolymer Doped with FeCl₃

Abstract

n‐Type‐doped polymers are key elements to fabricate all‐polymer thermoelectric generators but they are challenging to produce. Herein, a new strategy is proposed, which is based on polarity switching upon doping of a donor–acceptor (D–A) copolymer based on diketopyrrolopyrrol (DPP) and quintethiophene (5T) with FeCl3. Polarity switching from p‐type to n‐type is observed upon increasing the doping concentration of FeCl3. An analysis based on nonmonotonic density of states is proposed, which accounts for the main experimental trends and demonstrates that the polarity switch is governed by the electronic band filling that is determined by the dopant concentration. The influence of the curvature of the density of states is in addition discussed and a complete description of the doping induced transport regimes is proposed. This polarity switching depends on the molecular weight Mn of the polymer and shifts to higher FeCl3 concentrations with increasing Mn. This behavior is attributed to the change of the width of the density of states with Mn. The combination of polarity switching and alignment is a means to produce n‐type‐like oriented and conducting polymers with enhanced power factors up to 10 µW K−2 m−1 along the chain direction.

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.…”

“…[ 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 m m , 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.…”

“…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 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 ]…”

“…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. [ 14–20,23–27,32–54 ]…”

Thermopower, figure of merit and Fermi integrals

“… Here, the characteristic energies , and , and the constants , and have been introduced in order to focus on energy dependence. This formalism is general in the sense it describes both metallic and insulating regimes, and appears especially relevant as long as the density of states is a monotonic function of the energy 19 .…”