Microstructure and heteroatom dictate the doping mechanism and thermoelectric properties of poly(alkyl-chalcogenophenes)

Abstract

Heteroatom substitution is one promising way to favorably alter electronic transport in conductive polymers to improve their performance in thermoelectric devices. This study reports the spectroscopic, structural, and thermoelectric properties of poly (3-(3',7'-dimethyloctyl) chalcogenophenes) (P3RX) doped with 2, 3,5,7,8,, where the doping methodology, the heteroatom (X = Thiophene (T), Selenophene (Se), Tellurophene (Te)) and the extent of doping are systematically varied. Spectroscopic measurements reveal that while all P3RX polymers are appreciably doped, the doping mechanism is inherently different between the polymers. Poly(3-hexylthiophene) (P3HT, used in this study as a control) and P3RTe doped primarily via integer charge transfer (ICT), whereas P3RSe and P3RT appear to be doped via charge-transfer complex (CTC) mechanisms. Despite these differences, all polymers saturate with roughly the same number of F4TCNQ counterions (1 dopant per 4 to 6 heterocycles), reinforcing the idea that the extent of charge transfer from polymer to dopant varies significantly on the preferred doping mechanism. Grazing incidence wide-angle X-ray scattering measurements provide insight into the structural driving forces behind these different doping mechanisms -P3RT and P3RSe have similar microstructures in which F4TCNQ intercalates between the π-stacked backbones resulting in CTC doping (localized charge carriers), while P3HT and P3RTe have microstructures in which F4TCNQ intercalates in the alkyl-side chain region, giving rise to ICT doping (delocalized charge carriers). These structural and spectroscopic observations shed light on why P3HT and P3RTe

Significant Enhancement of the Electrical Conductivity of Conjugated Polymers by Post-Processing Side Chain Removal

“…W H is the localization energy, and this localization can arise from many physical phenomena, including spatial and electrostatic effects. , The SLoT model shows that, as the carrier ratio ( c ) and/or charge carrier density ( n ) increases, W H decreases because the carriers become spatially closer to one another (note that c and n are directly related). W H systematically reduces σ (such as the Mott polaron and Marcus models); however, as n (and/or c ) increases, W H → 0 and (in general − ). Finally, σ 0 is a scalar constant for a polymer-dopant-processing system, and it can be related to characteristic mobilities in an ideal system. , …”

Group 16 conjugated polymers based on furan, thiophene, selenophene, and tellurophene

“…Notably, heavier polychalcogenophenes require smaller dopant concentrations for similar increases in conductivity. 184–186 P3AS and P3ATe with low dopant concentrations can also achieve comparable power factors to highly doped P3AT. 185 Since the polymer structures are saturated at similar dopant levels for different chalcogens, this implies different doping mechanisms across the series.…”

“…Most importantly, the degree of oxidation from doping increased from S to Se to Te. 186 Differences in lamellar spacings also influence the doping mechanism. When doped, conjugated polymers will structurally change in order to accommodate dopants, and polychalcogenophene lamellar spacing will shift to fairly uniform values for the same dopant.…”

Poly(3-hexylthiophene)-stat-poly(3-dodecylselenophenes): Conjugated Statistical Copolymers and Their Gels

“…49 When a doped conjugated polymer primarily undergoes the ICT mechanism, dopants mostly intercalate into the lamellar sidechain region, while when the CTC mechanism dominates, dopants mostly intercalate between the π-stacks. 50 Such changes can be probed through GIWAXS studies by observing the difference of the lamellar lattice spacing before and after doping the conjugated polymer. 51 Strong (h00) peaks along with the (010) peak were found in all conjugated polymer gels films (Figures 5a and S5), indicating an edge-on orientation for the gel films.…”