Thermoelectric Performance of an Open-Shell Donor–Acceptor Conjugated Polymer Doped with a Radical-Containing Small Molecule

Abstract

Thermoelectric devices based on conducting polymers are promising energy conversion systems; however, the intrinsic semiconducting nature inherent to the macromolecular architecture of common conjugated polymers (CPs) in their neutral form requires doping to impart electrical conductivity and requires optimization of the complex dopant–polymer interactions in order to enhance thermoelectric performance. Therefore, designing and synthesizing CPs that have readily tunable properties and that can be doped in a facile manner using stable and noncorrosive dopants is a significant opportunity in the field. Here, we report the expedient synthesis of a donor–acceptor CP based on an alternating cyclopenta­dithiophene and thiadiazolo­quinoxaline framework that exhibits a narrow band gap, an open-shell electronic ground state, intrinsic electrical conductivity (σ ∼ 10–3 S cm–1), and a large Seebeck coefficient (S > 1000 μV K–1) in the absence of dopants. The addition of a tailored open-shell dopant significantly increases σ and allows for the systematic manipulation of the thermoelectric properties, resulting in an optimized power factor of >10 μW m–1 K–2, one of the largest values reported for nontraditional CP thermoelectric systems. This combination of a next-generation, radical-containing macromolecule with an open-shell small molecule dopant opens a new pathway by which to control charge transport and enable improved behavior in next-generation polymer thermoelectric systems.

Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

“…[ 30,31 ] Strong, proquinoidal TQ acceptors lower the lowest unoccupied molecular orbital (LUMO) and facilitate strong intramolecular interactions, promoting very narrow bandgaps, a quinoidal bonding pattern, backbone rigidity, and unpaired spin densities. [ 26,27 ] Substitution of the TQ acceptor with methyl ( P1 ), phenyl ( P2 ), and thiophene ( P3 ) moieties affords the capability to fine‐tune structural and electronic features. The polymers were synthesized using a modified microwave‐assisted Stille cross‐coupling copolymerization between (4,4‐dihexadecyl‐4 H ‐cyclopenta[2,1‐ b :3,4‐ b ′]dithiophene‐2,6‐diyl)bis(trimethylsta‐nnane) [ 32 ] and 4,9‐dibromo‐6,7‐dimethyl‐[1,2,5]thiadiazolo[3,4‐ g ]quinoxaline [ 33 ] for P1 , 4,9‐dibromo‐6,7‐diphenyl‐[1,2,5]thiadiazolo[3,4‐ g ]quinoxaline [ 34 ] for P2 , and 4,9‐dibromo‐6,7‐di(thiophen‐2‐yl)‐[1,2,5]thiadiazolo[3,4‐ g ]quinoxaline [ 35 ] for P3 .…”

“…Thin films of the polymers show absorption profiles with maxima (λ max ) of 1.60 μm ( P1 ), 1.45 μm ( P2 ), and 1.66 µm ( P3 ) and relatively sharp band‐tail characteristic of undoped polymers ( Figure a). [ 26 ] The optical bandgap ( E g opt ) of P1 is 0.57 eV, as estimated from the absorption onset of the thin film. Cyclic voltammetry shows that the HOMO is located at −4.93 eV and the LUMO at −3.90 eV, which gives an electrochemical bandgap ( E g elec ) of 1.03 eV (Figure 1b and Table S1 (Supporting Information)).…”

“…While most conventional conjugated polymers are closed‐shell species accommodating their π‐electrons in bonding orbitals, donor–acceptor (DA) copolymers with quinoidal character, extended π‐systems, and narrow bandgaps demonstrate open‐shell electronic structures. [ 4,26–29 ] We previously demonstrated that DA copolymers based on cyclopentadithiophene–thiadiazoloquinoxaline (CPDT–TQ) frameworks exhibit very narrow bandgaps (0.5 eV < E g < 1 eV), room‐temperature conductivities (σ RT ) of ≈10 −2 –10 −3 S cm −1 , and controlled spin multiplicities, emanating from a high degree of electronic coherence along the π‐conjugated backbone. [ 26,27 ] In these materials, narrow bandgaps increase configuration mixing, while extension of the π‐system promotes topological localization of α and β singly occupied molecular orbitals to opposite sides of the macromolecule, diminishing the covalency of the ground state and increasing the diradical character ( y ).…”

“…[ 4,26–29 ] We previously demonstrated that DA copolymers based on cyclopentadithiophene–thiadiazoloquinoxaline (CPDT–TQ) frameworks exhibit very narrow bandgaps (0.5 eV < E g < 1 eV), room‐temperature conductivities (σ RT ) of ≈10 −2 –10 −3 S cm −1 , and controlled spin multiplicities, emanating from a high degree of electronic coherence along the π‐conjugated backbone. [ 26,27 ] In these materials, narrow bandgaps increase configuration mixing, while extension of the π‐system promotes topological localization of α and β singly occupied molecular orbitals to opposite sides of the macromolecule, diminishing the covalency of the ground state and increasing the diradical character ( y ). Both the singlet ( S = 0) and triplet ( S = 1) states are nearly degenerate and there is extensive delocalization, promoting thermodynamic stabilization, unpaired spin densities, and electrical conductivity.…”

Wide Potential Window Supercapacitors Using Open‐Shell Donor–Acceptor Conjugated Polymers with Stable N‐Doped States

“…It was recently demonstrated that electronic states in this class of polymers can be tuned from closed-shell singlets to biradicaloids with varying amounts of open-shell character, to biradicals in singlet and triplet spin states. [28,30] Electron spin resonance spectra, collected at room temperature, shows a welldefined signal at g e = 2.006, indicating unpaired or weakly paired electrons ( Figure S2, Supporting Information). These results are consistent with density functional theory (DFT) calculations at the unrestricted (U)B3LYP/6-31+G** level of theory and basis set on an oligomer with four repeat units (n = 4).…”