The orientational correlation length of domains in a semiconducting polymer controls its thermoelectric performance.

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled--yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, especially for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resolution imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resolution of approximately 30 nanometres and 15 milliseconds, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resolution (about 390 nanometres), the photocurrent associated with water oxidation, and find that the most active sites for water oxidation are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod--even though more improvement in photocurrent efficiency correlates with less reduction in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential reduction the optimal sites are the sites with more positive onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.

The heterogeneous microstructure of semicrystalline polymers complicates the relationship between their electrical conductivity and carrier concentration. Charge transport models typically describe conductivity with an assumption of uniform doping throughout the material. Here, the evolution in morphology and optoelectronic properties of poly(3-hexylthiophene) (P3HT) is reported as a function of carrier concentration in an organic electrochemical transistor using a polymeric ionic liquid (PIL) as the gate insulator. Operando grazing incidence X-ray scattering reveals that negatively charged ions from the dielectric first infiltrate the amorphous regions of the semiconductor, and then penetrate the crystalline regions at a critical carrier density of 4 × 10 20 cm −3 . Upon infiltration, the crystallites expand by 12% in the alkyl stacking direction and compress by 4% in the π-π stacking direction. The change in crystal structure of P3HT correlates with a sharply increasing effective carrier mobility. UV-visible spectroscopy reveals that holes induced in P3HT first reside in the crystalline regions of the polymer, which verifies that a charge carrier need not be in the same physical domain as its associated counterion. The dopant-induced morphological changes of P3HT rationalize the dependence of mobility on carrier concentration, suggesting a phase transition of crystalline regions at high carrier concentration.

Predicting the interactions between a semiconducting polymer and dopant is not straightforward due to the intrinsic structural and energetic disorder in polymeric systems. Although the driving force for efficient charge transfer depends on a favorable offset between the electron donor and acceptor, we demonstrate that the efficacy of doping also relies on structural constraints of incorporating a dopant molecule into the semiconducting polymer film. Here, we report the evolution in spectroscopic and electrical properties of a model conjugated polymer upon exposure to two dopant types: one that directly oxidizes the polymeric backbone and one that protonates the polymer backbone. Through vapor phase infiltration, the common charge transfer dopant, F 4 -TCNQ, forms a charge transfer complex (CTC) with the polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT), a conjugated polymer with the same backbone as the well-characterized polymer P3HT, resulting in a maximum electrical conductivity of 3 × 10 −5 S cm −1 . We postulate that the branched side chains of P3EHT force F 4 -TCNQ to reside between the π-faces of the crystallites, resulting in partial charge transfer between the donor and the acceptor. Conversely, protonation of the polymeric backbone using the strong acid, HTFSI, increases the electrical conductivity of P3EHT to a maximum of 4 × 10 −3 S cm −1 , 2 orders of magnitude higher than when a charge transfer dopant is used. The ability for the backbone of P3EHT to be protonated by an acid dopant, but not oxidized directly by F 4 -TCNQ, suggests that steric hindrance plays a significant role in the degree of charge transfer between dopant and polymer, even when the driving force for charge transfer is sufficient.

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...

A fundamental understanding of charge transport in polymeric semiconductors requires knowledge of how the electrical conductivity varies with carrier density. The thermopower of semiconducting polymers is also a complex function of carrier density making it difficult to assess structure–property relationships for the thermoelectric power factor. We examined the thermoelectric properties of poly­[2,5-bis­(3-tetradecylthiophen-2-yl)­thieno­[3,2-b]­thiophene] (pBTTT-C14) by measurements of an electrochemical transistor using a polymeric ionic liquid (PIL) gate dielectric that can modulate the carrier concentration from 4 × 1018 to 3 × 1020 cm–3. As carrier density increases, so does the concentration of associated counterions, leading to a greater degree of energetic disorder within the semiconductor. Using thermopower measurements, we show experimentally that the electronic density-of-states broadens with increasing carrier density in the semiconducting polymer. The origin of a commonly observed power law relationship between thermopower and electrical conductivity is discussed and related to the changes in the electronic density-of-states upon doping.

Two disparate modes of operation can occur in an electrolyte-gated transistors (EGTs) depending on the permeability of the semiconductor to ions that are opposite in charge to the induced carriers. High permeability to ions promotes volumetric doping whereas a low permeability promotes electric double layer (EDL) formation and field-effect doping at the semiconductor/dielectric interface. Here, we present a generalized method to control the mode of charge accumulation in an EGT with a constant semiconducting layer by gating with anhydrous polymeric ionic liquids (PILs) of opposite polarity. The polarity of the acrylate-based PILs was controlled by tethering ionic units of positive or negative charge to the backbone. In situ optical spectroscopy of EGTs with poly­(3-hexylthiophene), P3HT, reveals that selectively tethering either the anion or the cation dictates whether ions infiltrate P3HT. Gating with both dielectric materials results in similar trends in the dependence of the charge carrier mobility on carrier concentration, despite the difference in doping mechanisms. The EDL (interfacial) doping of the anion-tethered PIL leads to higher mobilities at low carrier concentrations in P3HT with lower onset voltages. Optical measurements during gating show that the anion-tethered PIL gate results in a more even distribution of carriers between ordered and less ordered domains, promoting the formation of a percolated network in the film.

Semiconducting polymers exhibiting lyotropic liquid-crystalline (LC) mesophases allow for precise control over crystallinity and crystallite orientation using scalable equilibrium processing techniques. The ability to control morphologies from molecular to macroscopic scales is desirable for optimizing polymer electronic devices such as field-effect transistors. LC mesophases can form in semiconducting polymers with branched side chains, but steric repulsion from the side chain structure increases π-stacking distances. This change in unit cell negatively impacts electronic transport, negating any improvements gained by achieving more favorable mesostructures. Herein, we show that the π-stacking distance in cyclopenta­dithiophene-benzo­thiadiazole donor–acceptor copolymers is successively shortened by moving the branch point further away from the conjugated backbone. These materials retain their favorable LC properties, and the change in unit cell is accompanied by a monotonic increase in charge carrier mobility in field effect transistors. Mobility values span from ∼10–4 to 0.41 cm2 V–1 s–1, reaching their highest value when the branch point is furthest from the polymer backbone. In addition, these LC materials can be aligned across large length scales on rubbed polyimide, resulting in an improvement of charge carrier mobility by a factor of 56 when comparing transistors fabricated parallel and perpendicular to the alignment direction.