Effects of Counter‐Ion Size on Delocalization of Carriers and Stability of Doped Semiconducting Polymers

Abstract

Molecular charge transfer dopants either oxidize or reduce the polymeric backbone through accepting or donating an electron. In such cases, the neutralizing counter-ion to the charge carrier on the polymer is the dopant molecule. [1] Protonating the polymer backbone with a Brønsted acid provides a similar effect with the proton donor acting as the counter-ion. [2,3] Electrochemical methods can be used to supply, or remove, electrons if the polymer is supported by a conductive substrate with infiltration of a counter-ion from an electrolyte. [4] These methods effectively tune the electrical conductivity in polymeric semiconductors for emerging applications including bioelectronics [5] and thermoelectrics. [6] For all doping mechanisms, the interactions that exist between a conjugated polymer, a charge carrier, and its corresponding counter-ion are difficult to ascertain. Our lack of understanding stems from a multitude of complications that arise from doping polymers. The morphology of thin films can evolve upon infiltration of dopants, which convolutes the effects of morphology and carrier concentration on the resulting electrical properties. [7] The use of dopants of varying molecular sizes simultaneously changes steric interactions and the energetics of charge transfer, along with the additional possibility of the formation of charge transfer complexes. [8-10] These confounding factors make simple relationships, like how the degree of interaction between the dopant counter-ion and charge carrier impacts the electronic mobility, challenging to determine. A recent formalism to elucidate the importance of interactions between charge carriers and their associated counterions in semiconducting polymers was developed through examining their spectroscopic signatures in the infrared region. The optical properties of polaronic charge carriers in semiconducting polymers are affected by factors such as electronic/vibrational coupling between chains, coulombic interactions, and disorder. [11,12] One model, developed by Spano and coworkers, rationalizes the optical transitions of polaronic carriers in poly(3-hexylthiophene) (P3HT) using a Holstein Hamiltonian modified to incorporate disorder that is present in crystallites of polymers. [13] Both the predicted energies of the optical transitions and their intensities were in good agreement with experimental observations of field-induced charge carriers 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]-C 14 to 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...

Unraveling the effect of defects, domain size, and chemical doping on photophysics and charge transport in covalent organic frameworks

“…The two dimensional Holstein model has been successful in rationalizing various experiments carried out on semiconducting polymers. 33,36,41,[63][64][65]67 Using our model we can not only obtain quantitative agreement with experiments but also extract detailed information about the polaron coherence lengths, i.e., how far the hole delocalizes along the x and y directions in the 2D COF plane and through the columns (z direction). The basis set expressing the Hamiltonian is for single polarons and we omit the inuence of bipolarons in this work.…”

“…4a). 41,65 Therefore, investigating the mid to near IR spectral signatures (0.05-1 eV) of doped COF lms as a function of pore sizes, linker lengths, building blocks, and dopant type will provide a critical understanding of the extent of intra-and inter-framework polaron transport, structure property relationships, location of the dopant counter-anion, and electrical properties of COF lms.…”

“…This trend may not hold if inter-domain charge transport is achieved in COFs since the electrical properties of organic materials depend on the long-range order and mesoscale connectivity between the crystalline domains. 34,65 In all our comparisons with the available experimental data, we have considered 3D tetragonal lattices. COF topologies vary across different structures.…”

“…In particular, defects arising from non-crystalline regions can trap the polarons, resulting in localization and subsequently yielding lower values of hole mobility and conductivity. The effects of domain size, 34,57 electronic defects, [58][59][60] crystallinity, 61,62 molecular weight, 39 side chain engineering, 63 and chemical dopants 36,41,64,65 have been previously discussed in the context of conjugated polymers. In all these cases, mid-IR polaron signatures have provided fundamental insights into the local structure-property relationships and have successfully established the key correlations between polaron coherence and hole mobility.…”

Unraveling the Effect of Defects, Domain Size, and Chemical Doping on Photophysics and Charge Transport in Covalent Organic Frameworks

“…This trend may not hold if inter-domain charge transport is achieved in COFs since the electrical properties entirely depend on long-range order and mesoscale connectivity between domains. 28,59 It should be mentioned that in all our comparisons with the available experimental data, we have considered 3D tetragonal lattices. COF topologies vary across different structures.…”