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Artificial light harvesting, a process that involves converting sunlight into chemical potential energy, is considered to be a promising part of the overall solution to address urgent global energy challenges. Conjugated polyelectrolyte complexes (CPECs) are particularly attractive for this purpose due to their extended electronic states, tunable assembly thermodynamics, and sensitivity to their local environment. Importantly, ionically assembled complexes of conjugated polyelectrolytes can act as efficient donor-acceptor pairs for electronic energy transfer (EET). However, to be of use in material applications, we must understand how modifying the chemical structure of the CPE backbone alters the EET rate beyond spectral overlap considerations. In this report we investigate the dependence of the EET efficiency and rate on the electronic structure and excitonic wave function of the CPE backbone. To do so, we synthesized a series of alternating copolymers where the electronic states are systematically altered by introducing comonomers with electron withdrawing and electron-rich character while keeping the linear ionic charge density nearly fixed. We find evidence that the excitonic coupling may be significantly affected by the exciton delocalization radius, in accordance with analytical models based on the line-dipole approximation and quantum chemistry calculations. Our results imply that care should be taken when selecting CPE components for optimal CPEC EET. These results have implications for using CPECs as key components in water-based light-harvesting materials, either as standalone assemblies or as adsorbates on nanoparticles and thin films.

The ability to form robust, optoelectronically responsive, and mechanically tunable hydrogels using facile processing is desirable for sensing, biomedical, and light-harvesting applications. We demonstrate that such a hydrogel can be formed using aqueous complexation between one conjugated and one nonconjugated polyelectrolyte. We show that the rheological properties of the hydrogel can be tuned using the regioregularity of the conjugated polyelectrolyte (CPE) backbone, leading to significantly different mesoscale gel morphologies. We also find that the exciton dynamics in the long-time limit reflect differences in the underlying electronic connectivity of the hydrogels as a function CPE regioregularity. The influence of excess small ions on the hydrogel structure and the exciton dynamics similarly depends on the regioregularity in a significant way. Finally, electrical impedance measurements lead us to infer that these hydrogels can act as mixed ionic/electronic conductors. We believe that such gels possess an attractive combination of physical-chemical properties that can be leveraged in multiple applications.

Conjugated polyelectrolytes (CPEs) have the potential to serve as building blocks of artificial light-harvesting systems. This is primarily due to their delocalized electronic states and potential for hierarchical self-assembly. We showed previously that inter-CPE complexes composed of oppositely charged exciton-donor and exciton-acceptor CPEs displayed efficient electronic energy transfer. However, near ionic charge equivalence, complexed CPE chains become net-neutral and thus experience a precipitous drop in aqueous solubility. To increase the stability and to rationally manipulate the phase behavior of inter-CPE complexes, we synthesized a series of highly water-soluble exciton-donor CPEs composed of alternating ionic and polar nonionic fluorene monomers. The nonionic monomer contained oligo(ethyleneglycol) sidechains of variable length. We then formed exciton donor-acceptor complexes and investigated their relative energy transfer efficiencies in the presence of a fixed exciton-acceptor CPE. We find that, even when the polar nonionic sidechains become quite long (nine ethyleneglycol units), the energy transfer efficiency is hardly affected so long as the inter-CPE network retains a net polyelectrolyte charge. However, near the onset of spontaneous phase separation, we observe a clear influence of the length of the oligo(ethyleneglycol) sidechains on the photophysics of the complex. Our results have implications for the use of polyelectrolyte phase separation to produce aqueous light-harvesting soft materials., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Viscoelastic liquid coacervate phases that are highly enriched in nonconjugated polyelectrolytes are currently the subject of highly active research from biological and soft-materials perspectives. However, formation of a liquid, electronically active coacervate has proved highly elusive, since extended π-electron interactions strongly favor the solid state. Herein we show that a conjugated polyelectrolyte can be rationally designed to undergo aqueous liquid/liquid phase separation to form a liquid coacervate phase. This result is significant both because it adds to the fundamental understanding of liquid/liquid phase separation but also because it opens intriguing applications in light harvesting and beyond. We find that the semiconducting coacervate is intrinsically excitonically coupled, allowing for long-range exciton diffusion in a strongly correlated, fluctuating environment. The emergent excitonic states are comprised of both excimers and H-aggregates.

The ability to assemble artificial systems that mimic aspects of natural light-harvesting functions is fascinating and attractive for materials design. Given the complexity of such a system, a simple design pathway is desirable. Here, we argue that associative phase separation of oppositely charged conjugated polyelectrolytes (CPEs) can provide such a path in an environmentally benign medium: water. We find that complexation between an exciton-donor and acceptor CPE leads to formation of a complex fluid. We interrogate exciton transfer from the donor to the acceptor CPE within the complex fluid and find that transfer is highly efficient. We also find that excess molecular ions can tune the modulus of the inter-CPE complex fluid. Even at high ion concentrations, CPEs remain complexed with significantly delocalized electronic wavefunctions. Our work lays the rational foundation for complex, tunable aqueous light-harvesting systems via the intrinsic thermodynamics of associative phase separation., (© 2022 Wiley-VCH GmbH.)

The mechanical properties of π-conjugated (semiconducting) polymers are a key determinant of the stability and manufacturability of devices envisioned for applications in energy and healthcare. These properties─including modulus, extensibility, toughness, and strength─are influenced by the morphology of the solid film, which depends on the method of processing. To date, the majority of work done on the mechanical properties of semiconducting polymers has been performed on films deposited by spin coating, a process not amenable to the manufacturing of large-area films. Here, we compare the mechanical properties of thin films of regioregular poly(3-heptylthiophene) (P3HpT) produced by three scalable deposition processes─interfacial spreading, solution shearing, and spray coating─and spin coating (as a reference). Our results lead to four principal conclusions. (1) Spray-coated films have poor mechanical robustness due to defects and inhomogeneous thickness. (2) Sheared films show the highest modulus, strength, and toughness, likely resulting from a decrease in free volume. (3) Interfacially spread films show a lower modulus but greater fracture strain than spin-coated films. (4) The trends observed in the tensile behavior of films cast using different deposition processes held true for both P3HpT and poly(3-butylthiophene) (P3BT), an analogue with a higher glass transition temperature. Grazing incidence X-ray diffraction and ultraviolet-visible spectroscopy reveal many notable differences in the solid structures of P3HpT films generated by all four processes. While these morphological differences provide possible explanations for differences in the electronic properties (hole mobility), we find that the mechanical properties of the film are dominated by the free volume and surface topography. In field-effect transistors, spread films had mobilities more than 1 magnitude greater than any other films, likely due to a relatively high proportion of edge-on texturing and long coherence length in the crystalline domains. Overall, spread films offer the best combination of deformability and charge-transport properties.

Electron transfer is key to the operation of devices based on molecular (organic) semiconductors. Others have shown that electron transfer in the solid state often proceeds on sub-50 fs timescales, the details of which can be difficult to temporally resolve using pump-probe spectroscopy. A popular technique to measure average time scales for such rapid electron-transfer events is the core-hole clock implementation of resonant Auger electron spectroscopy at a single X-ray absorption edge. This is often done on relatively small molecules with core-excited states that are highly localized. We have used resonant Auger spectroscopy to probe sub-50 fs electron dynamics of two relatively large model organic semiconductors: Cu phthalocyanine (CuPc) along with its fluorinated analog, F 16 CuPc. We have interrogated electron dynamics simultaneously at N and C K-edges, along with calculations of initial and final states participating in the core-excited states. Our measurements show that the electron dynamics differ substantially across the two absorption edges for a given molecule, and that there are significant differences at a given edge between the two derivatives. X-ray spectroscopy calculations suggest that the extension of π-electron density onto peripheral F atoms in F 16 CuPc is implicated in the large change in ultrafast electron dynamics upon fluorination. We believe our results have important implications for analysis of core-hole clock measurements on relatively large organic semiconductors.

We report the first demonstration of using trivalent metal hydrated nitrate coordination complexes (TMHNCCs) as novel passivation ligands to control the synthesis of magic sized clusters (MSCs) and quantum dots (QDs) of CsPbBr 3 perovskite at room temperature. We can easily tune from QDs to MSCs or produce a mixture of the two by changing the amount of TMHNCC ligands used, with more ligands favoring MSCs. The original TMHNCC introduced, aluminum nitrate nonahydrate [ANN, Al(NO 3 ) 3 ·9H 2 O], led to the production of aluminum dihydroxide nitrate tetrahydrate {ADNT, [Al(OH) 2 (NO 3 )]·4H 2 O}, with the assistance of oleic acid (OA) and oleylamine (OAm). Through several control experiments, we determined that ADNT is the primary ligand for effectively passivating the MSCs and QDs, with OAm being essential for deprotonating ANN and OA for adjusting the pH of the reaction system. We suggest that ADNT is planar on the surface of the MSCs or QDs with its NO 3 - and OH - groups binding to the Cs + defect sites and Al 2+ defect sites and Al 3+ binding to the Br - defect sites of the MSCs or QDs.

Although high-temperature operation (i.e., beyond 150°C) is of great interest for many electronics applications, achieving stable carrier mobilities for organic semiconductors at elevated temperatures is fundamentally challenging. We report a general strategy to make thermally stable high-temperature semiconducting polymer blends, composed of interpenetrating semicrystalline conjugated polymers and high glass-transition temperature insulating matrices. When properly engineered, such polymer blends display a temperature-insensitive charge transport behavior with hole mobility exceeding 2.0 cm 2 /V·s across a wide temperature range from room temperature up to 220°C in thin-film transistors., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Electron delocalization in conjugated organic molecules is a rate-limiting step in maximizing the charge generation efficiency of next generation photovoltaics and molecular electronics. In particular, ultrafast (<50 fs) delocalization is an important aspect that has been beyond the scope of traditional optical experiments. In this work, we use resonant photoemission spectroscopy to probe electron delocalization timescales as a function of conjugation length by examining an oligothiophene chemical series containing 4-, 5- and 6-mers. We find that above a certain photon energy threshold, the 5-mer, quinquenthiophene, displays the largest ultrafast tunneling rates, roughly three times faster than the 6-mer, sexithiophene. We argue that differences in thin-film molecular packing cannot satisfactorily explain our results, and we speculate that the differences in ultrafast electron dynamics may be a manifestation of the odd/even effect.

Melt-processing of complementary semiconducting polymer blends provides an average charge carrier mobility of 0.4 cm 2 V -1 s -1 and current on/off ratios higher than 10 5 , a record performance for melt-processed organic field-effect transistors., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Photosynthetic organisms have mastered the use of "soft" macromolecular assemblies for light absorption and concentration of electronic excitation energy. Nature's design centers on an optically inactive protein-based backbone that acts as a host matrix for an array of light-harvesting pigment molecules. The pigments are organized in space such that excited states can migrate between molecules, ultimately delivering the energy to the reaction center. Here we report our investigation of an artificial light-harvesting energy transfer antenna based on complexes of oppositely charged conjugated polyelectrolytes (CPEs). The conjugated backbone and the charged side chains of the CPE lead to an architecture that simultaneously functions as a structural scaffold and an electronic energy "highway". We find that the process of ionic complex formation leads to a remarkable change in the excitonic wavefunction of the energy acceptor, which manifests in a dramatic increase in the fluorescence quantum yield. We argue that the extended backbone of the donor CPE effectively templates a planarized acceptor polymer, leading to excited states that are highly delocalized along the polymer backbone.

Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

In this work, we synthesize large-area thin films of a conjugated, imine-based, two-dimensional covalent organic framework at the solution/air interface. Thicknesses between ∼2-200 nm are achieved. Films can be transferred to any desired substrate by lifting from underneath, enabling their use as the semiconducting active layer in field-effect transistors.

A highly sensitive single-walled carbon nanotube/C60 -based infrared photo-transistor is fabricated with a responsivity of 97.5 A W(-1) and detectivity of 1.17 × 10(9) Jones at 1 kHz under a source/drain bias of -0.5 V. The much improved performance is enabled by this unique device architecture that enables a high photoconductive gain of ≈10(4) with a response time of several milliseconds., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C60 acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation.

Organic semiconductors with higher carrier mobility and better transparency have been actively pursued for numerous applications, such as flat-panel display backplane and sensor arrays. The carrier mobility is an important figure of merit and is sensitively influenced by the crystallinity and the molecular arrangement in a crystal lattice. Here we describe the growth of a highly aligned meta-stable structure of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) from a blended solution of C8-BTBT and polystyrene by using a novel off-centre spin-coating method. Combined with a vertical phase separation of the blend, the highly aligned, meta-stable C8-BTBT films provide a significantly increased thin film transistor hole mobility up to 43 cm(2) Vs(-1) (25 cm(2) Vs(-1) on average), which is the highest value reported to date for all organic molecules. The resulting transistors show high transparency of >90% over the visible spectrum, indicating their potential for transparent, high-performance organic electronics.

To achieve organic solar cells with a broadened spectral absorption, we aim to promote the growth of the near-infrared (NIR)-active polymorph of lead phthalocyanine (PbPc) on a relevant electrode for solar cell applications. We studied the effect of different substrate modification layers on PbPc thin film structure as a function of thickness and deposition rate (rdep). We characterized crystallinity and orientation by grazing incidence X-ray diffraction (GIXD) and in situ X-ray reflectivity (XRR) and correlated these data to the performance of bilayer solar cells. When deposited onto a self-assembled monolayer (SAM) or a molybdenum oxide (MoO3) buffer layer, the crystallinity of the PbPc films improves with thickness. The transition from a partially crystalline layer close to the substrate to a more crystalline film with a higher content of the NIR-active phase is enhanced at low rdep, thereby leading to solar cells that exhibit a higher maximum in short circuit current density (JSC) for thinner donor layers. The insertion of a CuI layer induces the formation of strongly textured, crystalline PbPc layers with a vertically homogeneous structure. Solar cells based on these templated donor layers show a variation of JSC with thickness that is independent of rdep. Consequently, without decreasing rdep we could achieve JSC=10 mA/cm2, yielding a bilayer solar cell with a peak external quantum efficiency (EQE) of 35% at 900 nm, and an overall power conversion efficiency (PCE) of 2.9%.

In an effort to favor the formation of straight polymer chains without crystalline grain boundaries, we have synthesized an amphiphilic conjugated polyelectrolyte, poly(fluorene-alt-thiophene) (PFT), which self-assembles in aqueous solutions to form cylindrical micelles. In contrast to many diblock copolymer assemblies, the semiconducting backbone runs parallel, not perpendicular, to the long axis of the cylindrical micelle. Solution-phase micelle formation is observed by X-ray and visible light scattering. The micelles can be cast as thin films, and the cylindrical morphology is preserved in the solid state. The effects of self-assembly are also observed through spectral shifts in optical absorption and photoluminescence. Solutions of higher-molecular-weight PFT micelles form gel networks at sufficiently high aqueous concentrations. Rheological characterization of the PFT gels reveals solid-like behavior and strain hardening below the yield point, properties similar to those found in entangled gels formed from surfactant-based micelles. Finally, electrical measurements on diode test structures indicate that, despite a complete lack of crystallinity in these self-assembled polymers, they effectively conduct electricity.

We examine the ultrafast dynamics of exciton migration and polaron production in sequentially processed 'quasi-bilayer' and preblended 'bulk heterojunction' (BHJ) solar cells based on conjugated polymer films that contain the same total amount of fullerene. We find that even though the polaron yields are similar, the dynamics of polaron production are significantly slower in quasi-bilayers than BHJs. We argue that the different polaron production dynamics result from the fact that (1) there is significantly less fullerene inside the polymer in quasi-bilayers than in BHJs and (2) sequential processing yields polymer layers that are significantly more ordered than BHJs. We also argue that thermal annealing improves the performance of quasi-bilayer solar cells not because annealing drives additional fullerene into the polymer but because annealing improves the fullerene crystallinity. All of the results suggest that sequential processing remains a viable alternative for producing polymer/fullerene solar cells with a nanometer-scale architecture that differs from BHJs.

Anthradithiophene was incorporated in a polymer structure by extending its conjugation from the 5,11-positions, through in situ desilylation followed by acetylenic coupling with a dibromo-monomer. The resulting polymer showed largely redshifted order in a thin film as well as order in thin film, forming lamellar structures out of the substrate plane. As a result, it exhibits field-effect hole mobilities, on the order of 0.1 cm(2) V(-1) s(-1), a ten to hundred-fold improvement as compared to previous acene-containing polymers.

We introduce a novel siloxane-terminated solubilizing group and demonstrate its effectiveness as a side chain in an isoindigo-based conjugated polymer. An average hole mobility of 2.00 cm(2) V(-1) s(-1) (with a maximum mobility of 2.48 cm(2) V(-1) s(-1)), was obtained from solution-processed thin-film transistors, one of the highest mobilities reported to date. In contrast, the reference polymer with a branched alkyl side chain gave an average hole mobility of 0.30 cm(2) V(-1) s(-1) and a maximum mobility of 0.57 cm(2) V(-1) s(-1). This is largely explained by the polymer packing: our new polymer exhibited a π-π stacking distance of 3.58 Å, while the reference polymer showed a distance of 3.76 Å., (© 2011 American Chemical Society)

The fullerene adducts 1a and 1b, whose molecular shapes either promote or hinder the formation of 1-D stacks, have been examined for their potential to form 1-D wire-like domains in bulk-heterojunction organic solar cells. The photovoltaic efficiency of solar cells based on blends of the stacking fullerene 1a with regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) is greatly enhanced compared to nonstacking model fullerene 1b.