Since doped polymers require a charge‐neutralizing counter‐ion to maintain charge neutrality, tailored and high degrees of doping in organic semiconductors requires an understanding of the coupling between ionic and electronic carrier motion. A method of counter‐ion exchange is utilized using the polymeric semiconductor poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] ‐C14to deconvolute the effects of ionic/polaronic interactions with the electrical properties of doped semiconducting polymers. In particular, exchanging the counter‐ions of the dopant nitrosonium hexafluorophosphate enables investigation into the role of counter‐ion size from 5.2 to 8.2 Å in diameter. The orientational order of the polymeric crystallites is not affected with this exchange process while effectively modifying the counter‐ion distance to the charge carrier. Doped films have electrical conductivities of 320 S cm−1and are not sensitive to an increased ion‐polaron distance. It is posited that other factors dominate the electrical properties at a device scale, such as the morphology and presence of domain boundaries. Interestingly, the temperature stability of the doped film can be drastically improved with the use of counter‐ions containing less labile bonds. This platform serves as a unique way to retain the morphology of polymeric thin films while studying charge interactions at the local scale.
2D polymers (2DPs) are promising as structurally well‐defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)‐containing 2DP semiconductors is enhanced by controllably n‐doping the NDI units using cobaltocene (CoCp2). Optical and transient microwave spectroscopy reveal that both as‐prepared NDI‐containing 2DPs are semiconducting with sub‐2 eV optical bandgaps and photoexcited charge‐carrier lifetimes of tens of nanoseconds. Following reduction with CoCp2, both 2DPs largely retain their periodic structures and exhibit optical and electron‐spin resonance spectroscopic features consistent with the presence of NDI‐radical anions. While the native NDI‐based 2DPs are electronically insulating, maximum bulk conductivities of >10−4 S cm−1are achieved by substoichiometric levels of n‐doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out‐of‐plane (π‐stacking) crystallographic directions, which indicates that cross‐plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well‐defined, paramagnetic, 2DP n‐type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Award ID(s):
- Publication Date:
- NSF-PAR ID:
- Journal Name:
- Advanced Materials
- Wiley Blackwell (John Wiley & Sons)
- Sponsoring Org:
- National Science Foundation
More Like this
Effects of Counter‐Ion Size on Delocalization of Carriers and Stability of Doped Semiconducting Polymers
Efficient doping of polymer semiconductors is required for high conductivity and efficient thermoelectric performance. Lewis acids, e.g., B(C6F5)3, have been widely employed as dopants, but the mechanism is not fully understood. 1:1 “Wheland type” or zwitterionic complexes of B(C6F5)3are created with small conjugated molecules 3,6‐bis(5‐(7‐(5‐methylthiophen‐2‐yl)‐2,3‐dihydrothieno[3,4‐b][1,4]dioxin‐5‐yl)thiophen‐2‐yl)‐2,5‐dioctyl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione [oligo_DPP(EDOT)2] and 3,6‐bis(5''‐methyl‐[2,2':5',2''‐terthiophen]‐5‐yl)‐2,5‐dioctyl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione [oligo_DPP(Th)2]. Using a wide variety of experimental and computational approaches, the doping ability of these Wheland Complexes with B(C6F5)3are characterized for five novel diketopyrrolopyrrole‐ethylenedioxythiophene (DPP‐EDOT)‐based conjugated polymers. The electrical properties are a strong function of the specific conjugated molecule constituting the adduct, rather than acidic protons generated via hydrolysis of B(C6F5)3, serving as the oxidant. It is highly probable that certain repeat units/segments form adduct structures in
p‐type conjugated polymers which act as intermediates for conjugated polymer doping. Electronic and optical properties are consistent with the increase in hole‐donating ability of polymers with their cumulative donor strengths. The doped film of polymer (DPP(EDOT)2‐(EDOT)2) exhibits exceptionally good thermal and air‐storage stability. The highest conductivities, ≈300 and ≈200 S cm−1, are achieved for DPP(EDOT)2‐(EDOT)2doped with B(C6F5)3and its Wheland complexes.
Persistent Conjugated Backbone and Disordered Lamellar Packing Impart Polymers with Efficient n‐Doping and High Conductivities
Solution‐processable highly conductive polymers are of great interest in emerging electronic applications. For p‐doped polymers, conductivities as high a nearly 105S cm−1have been reported. In the case of n‐doped polymers, they often fall well short of the high values noted above, which might be achievable, if much higher charge‐carrier mobilities determined could be realized in combination with high charge‐carrier densities. This is in part due to inefficient doping and dopant ions disturbing the ordering of polymers, limiting efficient charge transport and ultimately the achievable conductivities. Here, n‐doped polymers that achieve a high conductivity of more than 90 S cm−1by a simple solution‐based co‐deposition method are reported. Two conjugated polymers with rigid planar backbones, but with disordered crystalline structures, exhibit surprising structural tolerance to, and excellent miscibility with, commonly used n‐dopants. These properties allow both high concentrations and high mobility of the charge carriers to be realized simultaneously in n‐doped polymers, resulting in excellent electrical conductivity and thermoelectric performance.
Molecular doping—the use of redox‐active small molecules as dopants for organic semiconductors—has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox‐active character of these materials. A recent breakthrough was a doping technique based on ion‐exchange, which separates the redox and charge compensation steps of the doping process. Here, the equilibrium and kinetics of ion exchange doping in a model system, poly(2,5‐bis(3‐alkylthiophen‐2‐yl)thieno(3,2‐b)thiophene) (PBTTT) doped with FeCl3and an ionic liquid, is studied, reaching conductivities in excess of 1000 S cm−1and ion exchange efficiencies above 99%. Several factors that enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl3, are demonstrated. In this high ion exchange efficiency regime, a simple connection between electrochemical doping and ion exchange is illustrated, and it is shown that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential.
Ionic Dopant‐Induced Ordering Enhances the Thermoelectric Properties of a Polythiophene‐Based Block Copolymer
Conjugated polymer‐based block copolymers (CP‐BCPs) are an unexplored class of materials for organic thermoelectrics. Herein, the authors report on the electronic conductivity (σ) and Seebeck coefficient (α) of a newly synthesized CP‐BCP, poly(3‐hexylthiophene)‐
block‐poly (oligo‐oxyethylene methacrylate) (P3HT‐ b‐POEM), upon solution co‐processing with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and subsequently vapor‐doping with a molecular dopant, 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ). It is found that the addition of the hydrophilic block POEM greatly enhances the processability of P3HT, enabling homogeneous solution‐mixing with LiTFSI. Notably, interactions between P3HT‐ b‐POEM with ionic species significantly improve molecular order and unexpectedly cause electrical oxidizing doping of P3HT block both in solution and solid‐states, a phenomenon that has not been previously observed in Li‐salt containing P3HT. Vapor doping of P3HT‐ b‐POEM‐LiTFSI thin films with F4TCNQ further enhances σ and yields a thermoelectric power factor PF =α2σ of 13.0 µW m−1 K−2, which is more than 20 times higher than salt‐free P3HT‐ b‐POEM sample. Through modeling thermoelectric behaviors of P3HT‐ b‐POEM with the Kang‐Snyder transport model, the improvement in PFis attributed to higher electronic charge mobility originating from the enhanced molecular ordering of P3HT. The results demonstrate that solution co‐processing CP‐BCPs with a salt is a powerful method to control structure and performance of organic thermoelectric materials.