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

Abstract

π-Conjugated donor (D)-acceptor (A) copolymers have been extensively studied as organic photovoltaic (OPV) donors yet remain largely unexplored in organic thermoelectrics (OTEs) despite their outstanding mechanical bendability, solution processability and flexible molecular design. Importantly, they feature high Seebeck coefficient (S) that are desirable in room-temperature wearable application scenarios under small temperature gradients. In this work, the authors have systematically investigated a series of D-A semiconducting copolymers possessing various electron-deficient A-units (e.g., BDD, TT, DPP) towards efficient OTEs. Upon p-type ferric chloride (FeCl₃ ) doping, the relationship between the thermoelectric characteristics and the electron-withdrawing ability of A-unit is largely elucidated. It is revealed that a strong D-A nature tends to induce an energetic disorder along the π-backbone, leading to an enlarged separation of the transport and Fermi levels, and consequently an increase of S. Meanwhile, the highly electron-deficient A-unit would impair electron transfer from D-unit to p-type dopants, thus decreasing the doping efficiency and electrical conductivity (σ). Ultimately, the peak power factor (PF) at room-temperature is obtained as high as 105.5 µW m^-1 K^-2 with an outstanding S of 247 µV K^-1 in a paradigm OPV donor PBDB-T, which holds great potential in wearable electronics driven by a small temperature gradient.

Keywords: D−A copolymers; doping; molecular packing; p-type thermoelectrics; semiconducting polymers.

Figure 1

p‐Type D−A copolymers of PBDP‐T, PTB7‐Th, and PBDB‐T: a) molecular structures, b) ultraviolet–visible absorption spectra, and c) energy level diagrams. Note D = donor unit: BDT; A = acceptor unit: DPP > TT > BDD in electron‐withdrawing capability.

Figure 2

UV–vis–NIR absorption spectra of pristine and FeCl₃ (10 mM) doped a) PBDP‐T, b) PTB7‐Th, and c) PBDB‐T thin films. The insets are the energy bands and corresponding absorption peaks at pristine and doped state, respectively. Cyclic voltammograms of pristine and FeCl₃ (10 mM) doped d) PBDP‐T, e) PTB7‐Th, and f) PBDB‐T thin films deposited on the working electrode immersed in 0.1 M n‐Bu₄PF₆ acetonitrile solution scanned at 50 mV s⁻¹.

Figure 3

GIWAXS profiles of pristine (solid line) and doped (dash‐dot line) polymers. a) Out‐of‐plane (⊥) and b) in‐plane (//) patterns. c) Lamellar stacking and π–π stacking distances as extracted from both (a) and (b).

Figure 4

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

Figure 5

Charge carrier mobilities of pristine and FeCl₃ (10 mm) doped D–A copolymers extracted from the saturated region of the transfer curves of OFET devices. The charge carrier density of FeCl₃ (10 mm) doped D–A copolymers is calculated by an equation of n = σ/eµ.