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Conjugated polymers with oligoether side chains make up a promising class of thermoelectric materials. In this work, the impact of the side-chain length on the thermoelectric and mechanical properties of polythiophenes is investigated. Polymers with tri-, tetra-, or hexaethylene glycol side chains are compared, and the shortest length is found to result in thin films with the highest degree of order upon doping with the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ). As a result, a stiff material with an electrical conductivity of up to 830 +/- 15 S cm(-1) is obtained, resulting in a thermoelectric power factor of about 21 mu W m(-1) K-2 in the case of as-cast films. Aging at ambient conditions results in an initial decrease in thermoelectric properties but then yields a highly stable performance for at least 3 months, with values of about 200 S cm(-1) and 5 mu W m(-1) K-2. Evidently, identification of the optimal side-chain length is an important criterion for the design of conjugated polymers for organic thermoelectrics., Funding Agencies|European Union [955837]; Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship Prolongation grant; Spanish Ministerio de Ciencia e Innovacion [CEX2019-000917-S, PID2020-119777GBI00, PDC2021-121814-I00]; China Scholarship Council [CSC201806950006]; Universitat Autonoma de Barcelona

Three lactone-based rigid semiconducting polymers were designed to overcome major limitations in the development of n-type organic thermoelectrics, namely electrical conductivity and air stability. Experimental and theoretical investigations demonstrated that increasing the lactone group density by increasing the benzene content from 0 % benzene (P-0), to 50 % (P-50), and 75 % (P-75) resulted in progressively larger electron affinities (up to 4.37 eV), suggesting a more favorable doping process, when employing (N-DMBI) as the dopant. Larger polaron delocalization was also evident, due to the more planarized conformation, which is proposed to lead to a lower hopping energy barrier. As a consequence, the electrical conductivity increased by three orders of magnitude, to achieve values of up to 12 S cm and Power factors of 13.2 mu Wm(-1) K-2 were thereby enabled. These findings present new insights into material design guidelines for the future development of air stable n-type organic thermoelectrics., Funding Agencies|KAUSTKing Abdullah University of Science & Technology; Office of Sponsored Research (OSR) [OSR-2018-CRG/CCF-3079, OSR-2019-CRG8-4086, OSR-2018-CRG7-3749]; ERC Synergy Grant SC2 [610115]; European Unions Horizon 2020 research and innovation program [952911, 862474]; EPSRC ProjectUK Research & Innovation (UKRI)Engineering & Physical Sciences Research Council (EPSRC) [EP/T026219/1B]; National Science FoundationNational Science Foundation (NSF) [NSF DMR-1751308]; DOE Office of ScienceUnited States Department of Energy (DOE) [DE-AC02-06CH11357]; Knut and Alice Wallenberg foundationKnut & Alice Wallenberg Foundation; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2020-03243]; Olle Engkvists Stiftelse [204-0256]; ECEuropean CommissionEuropean Commission Joint Research Centre [GA-955837]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]