Effect of Molecular Weight on Thermoelectric Performance of P3HT Analogues with 2-Propoxyethyl Side Chains

Abstract

By replacing hexyl chains in poly(3-hexylthiophene) (P3HT) with 2-propoxyethyls, four poly(3-(2-propoxyethyl)thiophene) (P3POET) homopolymers with comparable polydispersity indexes (PDI) and regioregularities were prepared herein in addition with step increment of about 7 kDa on number-average molecular weight (M _n) from around 11 to 32 kDa (accordingly denoted as P11k, P18k, P25k, and P32k). When doped in film by FeCl₃ at the optimized conditions, the maximum power factor (PF _max) increases greatly from 4.3 µW·m⁻¹·K⁻² for P11k to 8.8 µW·m⁻¹·K⁻² for P18k, and further to 9.7 µW·m⁻¹·K⁻² for P25k, followed by a slight decrease to 9.2 µW·m⁻¹·K⁻² for P32k. The close Seebeck coefficients (S) at PF _max are observed in these doped polymer films due to their consistent frontier orbital energy levels and Fermi levels. The main contribution to this PF _max evolution thus comes from the corresponding conductivities (σ). The σ variation of the doped films can be rationally correlated with their microstructure evolution.

Keywords

conjugated polymer / molecular weight / microstructure / thermoelectric performance

References

[1] Glaudell AM, Cochran JE, Patel SN, et al. Impact of the Doping Method on Conductivity and Thermopower in Semiconducting Polythiophenes[J]. Adv. Energy Mater., 2015, 5(4): 1 401 072

[2] Mcculloch I, Heeney M, Chabinyc ML, et al. Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin-Film Organic Transistors[J]. Adv. Mater., 2009, 21(10–11): 1 091-1 109.

[3] Zhao W, Ding J, Zou Y, et al. Chemical Doping of Organic Semiconductors for Thermoelectric Applications[J]. Chem. Soc. Rev., 2020, 49(20): 7 210-7 228.

[4] Kim GH, Shao L, Zhang K, et al. Engineered Doping of Organic Semiconductors for Enhanced Thermoelectric Efficiency[J]. Nat. Mater., 2013, 12(8): 719-723.

[5] Deng L, Liu Y, Zhang Y, et al. Organic Thermoelectric Materials: Niche Harvester of Thermal Energy[J]. Adv. Funct. Mater., 2022, 33(3): 2 210 770

[6] Fan Z, Du D, Yao H, et al. Higher Pedot Molecular Weight Giving Rise to Higher Thermoelectric Property of PEDOT:PSS: A Comparative Study of Clevios P and Clevios PH1000[J]. ACS Appl. Mater. Interfaces, 2017, 9(13): 11 732-11 738.

[7] Massetti M, Jiao F, Ferguson AJ, et al. Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications[J]. Chem. Rev., 2021, 121(20): 12 465-12 547.

[8] Bahk JH, Fang H, Yazawa K, et al. Flexible Thermoelectric Materials and Device Optimization for Wearable Energy Harvesting[J]. J. Mater. Chem. C, 2015, 3(40): 10 362-10 374.

[9] Li QY, Yao ZF, Wang JY, et al. Multi-Level Aggregation of Conjugated Small Molecules and Polymers: From Morphology Control to Physical Insights[J]. Rep. Prog. Phys., 2021, 84(7): 076 601

[10] Park KS, Kwok JJ, Kafle P, et al. When Assembly Meets Processing: Tuning Multiscale Morphology of Printed Conjugated Polymers for Controlled Charge Transport[J]. Chem. Mater., 2021, 33(2): 469-498.

[11] Jacobs IE, Aasen EW, Oliveira JL, et al. Comparison of Solution-Mixed and Sequentially Processed P3HT:F4TCNQ Films: Effect of Doping-Induced Aggregation on Film Morphology[J]. J. Mater. Chem. C, 2016, 4(16): 3 454-3 466.

[12] Tang J, Pai YH, Liang Z. Strategic Insights into Semiconducting Polymer Thermoelectrics by Leveraging Molecular Structures and Chemical Doping[J]. ACS Energy Lett., 2022, 7(12): 4 299-4 324.

[13] Hynynen J, Kiefer D, Muller C. Influence of Crystallinity on the Thermoelectric Power Factor of P3HT Vapour-Doped with F4TCNQ[J]. RSC Adv., 2018, 8(3): 1 593-1 599.

[14] Nam GH, Sun C, Chung DS, et al. Enhancing Doping Efficiency of Diketopyrrolopyrrole-Copolymers by Introducing Sparse Intramolecular Alkyl Chain Spacing[J]. Macromolecules, 2021, 54(17): 7 870-7 879.

[15] Wang D, Ding J, Dai X, et al. Triggering ZT to 0.40 by Engineering Orientation in One Polymeric Semiconductor[J]. Adv. Mater., 2023, 35(2): 2 208 215

[16] Untilova V, Biskup T, Biniek L, et al. Control of Chain Alignment and Crystallization Helps Enhance Charge Conductivities and Thermoelectric Power Factors in Sequentially Doped P3HT:F4TCNQ Films[J]. Macromolecules, 2020, 53(7): 2 441-2 453.

[17] Russ B, Glaudell A, Urban JJ, et al. Organic Thermoelectric Materials for Energy Harvesting and Temperature Control[J]. Nat. Rev. Mater., 2016, 1(10): 16 050

[18] Xu S, Shi XL, Dargusch M, et al. Conducting Polymer-Based Flexible Thermoelectric Materials and Devices: From Mechanisms to Applications[J]. Prog. Mater. Sci., 2021, 121(2021): 100 840

[19] Li S, Han Y, Ye L. Effect of Molecular Weight on Microstructure and Electrical Properties of Conjugated Polymer Films[J]. Sci. Sin. Chim., 2021, 52(4): 523-535.

[20] Koch FPV, Rivnay J, Foster S, et al. The Impact of Molecular Weight on Microstructure and Charge Transport in Semicrystalline Polymer Semiconductors-Poly(3-Hexylthiophene), a Model Study[J]. Prog. Polym. Sci., 2013, 38(12): 1 978-1 989.

[21] Shi M, Wang T, Wu Y, et al. The Intrinsic Role of Molecular Mass and Polydispersity Index in High-Performance Non-Fullerene Polymer Solar Cells[J]. Adv. Energy Mater., 2020, 11(1): 2 002 709

[22] Wadsworth A, Hamid Z, Bidwell M, et al. Progress in Poly (3-Hexylthiophene) Organic Solar Cells and the Influence of Its Molecular Weight on Device Performance[J]. Adv. Energy Mater., 2018, 8(28): 1 801 001

[23] Wang Z, Gao M, He C, et al. Unraveling the Molar Mass Dependence of Shearing-Induced Aggregation Structure of a High-Mobility Polymer Semiconductor[J]. Adv. Mater., 2022, 34(7): 2 108 255

[24] Guo X, Zhang Y, Hu Y, et al. Molecular Weight Engineering in High-Performance Ambipolar Emissive Mesopolymers[J]. Angew. Chem. Int. Ed. Engl., 2021, 60(27): 14 902-14 908.

[25] Qu S, Yao Q, Yu B, et al. Optimizing the Thermoelectric Performance of Poly(3-Hexylthiophene) through Molecular-Weight Engineering[J]. Chem. Asian J., 2018, 13(21): 3 246-3 253.

[26] Yoon SE, Kim B, Chun SY, et al. Impact of Molecular Weight on Molecular Doping Efficiency of Conjugated Polymers and Resulting Thermoelectric Performances[J]. Adv. Funct. Mater., 2022, 32(32): 2 202 929

[27] Pingel P, Schwarzl R, Neher D. Effect of Molecular P-Doping on Hole Density and Mobility in Poly(3-Hexylthiophene)[J]. Appl. Phys. Lett., 2012, 100(14): 143 303

[28] Kang YH, Ko SJ, Lee MH, et al. Highly Efficient and Air Stable Thermoelectric Devices of Poly(3-Hexylthiophene) by Dual Doping of Au Metal Precursors[J]. Nano Energy, 2021, 82(2021): 105 681

[29] Gregory SA, Menon AK, Ye S, et al. Effect of Heteroatom and Doping on the Thermoelectric Properties of Poly(3-Alkylchalcogenophenes) [J]. Adv. Energy Mater., 2018, 8(34): 1 802 419

[30] Chen L, Liu W, Yan Y, et al. Fine-Tuning the Solid-State Ordering and Thermoelectric Performance of Regioregular P3HT Analogues by Sequential Oxygen-Substitution of Carbon Atoms Along the Alkyl Side Chains[J]. J. Mater. Chem. C, 2019, 7(8): 2 333-2 344.

[31] Osaka I, Mccullough RD. Advances in Molecular Design and Synthesis of Regioregular Polythiophenes[J]. Acc. Chem. Res., 2008, 41(9): 1 202-1 214.

[32] Song CK, Eckstein BJ, Tam TL, et al. Conjugated Polymer Energy Level Shifts in Lithium-Ion Battery Electrolytes J]. ACS Appl. Mater. Interfaces, 2014, 6(21): 19 347-19 354.

[33] Lee Y, Park J, Son J, et al. Degenerately Doped Semi-Crystalline Polymers for High Performance Thermoelectrics[J]. Adv. Funct. Mater., 2020, 31(9): 2 006 900

[34] Liu J, Van Der Zee B, Alessandri R, et al. N-Type Organic Thermoelectrics: Demonstration of ZT > 0.3[J]. Nat. Commun., 2020, 11(1): 5 694

[35] Johnston DE, Yager KG, Hlaing H, et al. Nanostructured Surfaces Frustrate Polymer Semiconductor Molecular Orientation[J]. ACS Nano, 2014, 8(1): 243-249.

[36] Peng Z, Xian K, Liu J, et al. Unraveling the Stretch-Induced Microstructural Evolution and Morphology-Stretchability Relationships of High-Performance Ternary Organic Photovoltaic Blends[J]. Adv. Mater., 2023, 35(3): 2 207 884

[37] Pouliot JR, Wakioka M, Ozawa F, et al. Structural Analysis of Poly(3-Hexylthiophene) Prepared Via Direct Heteroarylation Polymerization[J]. Macromol. Chem. Phys., 2016, 217(13): 1 493-1 500.

[38] Pokrop R, Verilhac JM, Gasior A, et al. Effect of Molecular Weight on Electronic, Electrochemical and Spectroelectrochemical Properties of Poly(3,3″-Dioctyl-2,2′:5′,2″-Terthiophene)[J]. J. Mater. Chem., 2006, 16(30): 3 099-3 106.

[39] Ma J, Li SH, Jiang YS. A Time-Dependent Dft Study on Band Gaps and Effective Conjugation Lengths of Polyacetylene, Polyphenylene, Polypentafulvene, Polycyclopentadiene, Polypyrrole, Polyfuran, Polysilole, Polyphosphole, and Polythiophene[J]. Macromolecules, 2002, 35(3): 1 109-1 115.

[40] Liu F, Chen D, Wang C, et al. Molecular Weight Dependence of the Morphology in P3HT:PCBM Solar Cells[J]. ACS Appl. Mater. Interfaces, 2014, 6(22): 19 876-19 887.

[41] Li H, Plunkett E, Cai Z, et al. Dopant-Dependent Increase in Seebeck Coefficient and Electrical Conductivity in Blended Polymers with Offset Carrier Energies[J]. Adv. Electron. Mater., 2019, 5(11): 1 800 618

[42] Ko DK, Murray CB. Probing the Fermi Energy Level and the Density of States Distribution in PbTe Nanocrystal (Quantum Dot) Solids by Temperature-Dependent Thermopower Measurements[J]. ACS Nano, 2011, 5(6): 4 810-4 817.

[43] Yazdani S, Pettes MT. Nanoscale Self-Assembly of Thermoelectric Materials: A Review of Chemistry-Based Approaches[J]. Nanotechnology, 2018, 29(43): 432 001

[44] Fritzsche H. A General Expression for the Thermoelectric Power[J]. Solid State Commun., 1971, 9(21): 1 813-1 815.

[45] Kim NY, Lee TS, Lee DY, et al. Enhanced Doping Efficiency and Thermoelectric Performance of Diketopyrrolopyrrole-Based Conjugated Polymers with Extended Thiophene Donors[J]. J. Mater. Chem. C, 2021, 9(1): 340-347.

[46] Böhm W, Fritz T, Leo K. Charge Transport in Thin Organic Semiconducting Films: Seebeck and Field Effect Studies[J]. Phys. Stat. Sol. A, 1997, 160(1): 81-87.

[47] Zou Y, Huang D, Meng Q, et al. Correlation between Seebeck Coefficient and Transport Energy Level in Poly(3-Hexylthiophene)[J]. Org. Electron., 2018, 56(2018): 125-128.

[48] Enengl C, Enengl S, Pluczyk S, et al. Doping-Induced Absorption Bands in P3HT: Polarons and Bipolarons[J]. Chem. Phys. Chem., 2016, 17(23): 3 836-3 844.

[49] Tripathi A, Lee Y, Jung C, et al. A Heavily Doped D–D′-Type Polymer with Metal-Like Carrier Transport Via Hybrid Doping[J]. J. Mater. Chem. C, 2023, 11(17): 5 646-5 656.

[50] Patel SN, Glaudell AM, Peterson KA, et al. Morphology Controls the Thermoelectric Power Factor of a Doped Semiconducting Polymer[J]. Sci. Adv., 2017, 3(6): e1700434

[51] Tang J, Ji J, Chen R, et al. Achieving Efficient P-Type Organic Thermoelectrics by Modulation of Acceptor Unit in Photovoltaic Pi-Conjugated Copolymers[J]. Adv. Sci., 2022, 9(4): 2 103 646

[52] Wu L, Li H, Chai H, et al. Anion-Dependent Molecular Doping and Charge Transport in Ferric Salt-Doped P3HT for Thermoelectric Application[J]. ACS Appl. Electron. Mater., 2021, 3(3): 1 252-1 259.

[53] Lee J, Kim J, Nguyen TL, et al. A Planar Cyclopentadithiophene-Benzothiadiazole-Based Copolymer with sp²-Hybridized Bis(Alkylsulfanyl)Methylene Substituents for Organic Thermoelectric Devices[J]. Macromolecules, 2018, 51(9): 3 360-3 368.

[54] Lee H, Li H, Kim YS, et al. Novel Dithienopyrrole-Based Conjugated Copolymers: Importance of Backbone Planarity in Achieving High Electrical Conductivity and Thermoelectric Performance[J]. Macromol. Rapid Commun., 2022, 43(19): 2 200 277

[55] Gu K, Snyder CR, Onorato J, et al. Assessing the Huang-Brown Description of Tie Chains for Charge Transport in Conjugated Polymers [J]. ACS Macro Lett., 2018, 7(11): 1 333-1 338.

[56] Sirringhaus H, Brown PJ, Friend R H, et al. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers[J]. Nature, 1999, 401(6754): 685-688.

[57] Jiang Y, Chen J, Sun Y, et al. Fast Deposition of Aligning Edge-on Polymers for High-Mobility Ambipolar Transistors[J]. Adv. Mater., 2019, 31(2): 1 805 761

[58] Li B, Li X, Yang F, et al. Enhanced Thermoelectric Performance of a Donor-Acceptor-Based Two-Dimensional Conjugated Polymer with High Crystallinity[J]. ACS Applied Energy Materials, 2021, 4(5): 4 662-4 671.