Chad Risko

Understanding the mechanisms of nucleation and growth in organic molecular crystals is critical for controlling solution processing and the resulting properties of organic materials.

The spin-entangled triplet-pair state produced by singlet fission is promising for quantum information applications, but its optical darkness severely limits spectroscopic access and optical readout. Here we show that monolayer WS₂ can act as a long-range excitonic sensitizer for triplet-pair emission in the singlet-fission molecule 6,13-bis(triisopropylsilylethynyl)-tetramethyl pentacene (TCPS). Strong excitonic resonances and spin–orbit–split valleys in WS₂ mediate long-range excitonic energy transfer into the triplet-pair manifold, enhancing triplet-pair emission by more than two orders of magnitude. This enhancement arises from the exceptionally large exciton transition dipole moment of WS₂, which enables efficient dipole–dipole coupling across the hybrid interface. Temperature-dependent and time-resolved spectroscopy reveals ultrafast picosecond energy transfer. Distance-dependent measurements demonstrate efficient transfer beyond 100 nm, inconsistent with classical Förster scaling and instead indicative of radiative, near-field–assisted coupling with a shallow ~ 1/d dependence. These results establish a mechanism for long-range triplet-pair sensitization in hybrid molecular/transition-metal dichalcogenide heterostructures and provide a route for coupling correlated multiexciton states to optically active excitonic materials.

Charge carriers (i.e., polarons) in electrochemically doped organic (semi)conductors are proposed to be regulated by several physicochemical features, including the chemical compositions and structures of the π-conjugated frameworks of the semiconductor building blocks, the chemistry of the electrolyte (considering both the salt and solvent), multiscale and time-dependent morphology variations across the material as a function of electrochemical processes, and assorted permutations of these and other factors. To address these hypotheses, we investigate the energetic, optoelectronic, chemical, and local structural properties of negative polarons (radical anions) as a function of electrochemical doping in the donor–acceptor, π-conjugated redox copolymer P(NDI2OD-T2), also referred to as N2200. A critical finding is that there is not just “one type” of polaron in electrochemically doped P(NDI2OD-T2). Rather, an ensemble of polarons exists, with the polarons having optoelectronic signatures that are defined by their nanoscale environments. Importantly, the polaron optical signatures serve as local probes for how the operando electrochemical environments are dynamically working in concert to facilitate charge transport. Collectively, the distinctive and extensive integration of theory and measurement science presented here establishes a baseline for the roles that semiconductor and electrolyte chemistries and dynamic structural features have on polarons in electrochemically doped organic semiconductors and how these factors influence the energetics and rates of polaron transport.

Strong p-dopants are required to dope high-ionization energy organic semiconductors for a variety of potential applications, but strong, simple one-electron oxidants are typically sensitive to reduction by atmospheric moisture and thus challenging to store or handle. Here we show that bis(3,5-bis(trifluoromethyl)benzenesulfonyl) peroxide─a dimer formed by two highly oxidizing radicals─can function as a water-stable yet powerful oxidant, cleanly reacting with some organic semiconductors to form two radical cations and two 3,5-bis-(trifluoromethyl)benzenesulfonate anions, although in other cases sulfonylation reactions can also occur. Notably, this peroxide is capable of p-doping the high-ionization energy polymer poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)] (F8BT) to afford electrical conductivities of up to 0.03 S cm–1, while its use with electron-rich poly(3,4-dialkoxythiophene-2,5-diyl) derivatives can afford values up to 100 S cm–1. Quantum-chemical calculations reveal the peroxide oxidant behaves in a fashion mirroring that of relatively oxygen-stable, but highly reducing, n-dopants that have been developed based on dimers of organic radicals or organometallic sandwich compounds.

Organic molecules exhibiting second-scale room-temperature phosphorescence (RTP) in the red/near-infrared are particularly rare because low-energy excited states that are characteristic of these chromophores are susceptible to nonradiative deactivation. Here, we observe second-scale red RTP from contorted hexabenzocoronene (cHBC) embedded in a rigid polymer. This RTP is uniquely efficient as approximately 23% of the steady-state photoluminescence originates from triplets. We propose that this efficient triplet generation stems from intersystem crossing (ISC) that outcompetes symmetry-forbidden fluorescence. Density functional theory and time-dependent density functional theory calculations suggest that ISC occurs from the lowest energy singlet state of cHBC into a nearly resonant triplet state. Perdeuterating cHBC substitutes C–H stretches with lower-energy C–D stretches, which further suppresses nonradiative recombination and prolongs red RTP. The phosphorescence lifetime of perdeuterated cHBC-polymer composites exceeds 5 s, and has a steady-state phosphorescence fraction of 44%.

The design and discovery of functional molecular materials can be greatly accelerated through in silico approaches. Machine learning (ML) models, in particular, demonstrate significant promise for the rapid analysis and manipulation (and reanalysis) of both molecular and crystal structures, essential steps to hasten materials design and discovery. A critical element of these approaches, however, is that robust ML potentials be developed that allow for accurate predictions of energies and forces acting on atoms within a given crystal. Here, we present an ML potential trained on the OCELOT Crystal Relaxation v1 data set, which is introduced in this work and consists of 15,000 molecular crystal structures and over 2.2 million crystal geometries. The ML potential can predict the energies and forces on atoms in crystal structures comprised of π-conjugated organic molecules with a mean absolute error of 0.008 eV/atom for energy and 0.034 eV/Å for forces when compared with density functional theory calculations. To enable the use of the ML potential for in silico discovery of functional molecular crystals, we develop workflows for analyzing crystal surfaces and morphologies and manipulating and relaxing crystal structures. Furthermore, we deploy these workflows on an open-access, web-based tool, OCELOT XtalTransform, that bridges the gap between sophisticated simulations and user-friendly interfaces. By offering these capabilities, OCELOT XtalTransform aims to democratize access to advanced ML potentials for functional organic molecular crystals, thereby providing new mechanisms for materials design and discovery to the broader scientific community.

Figures of merit are used to translate science to technology. For organic mixed ionic–electronic conductors (OMIECs), the product of mobility and volumetric capacitance, i.e., the µC* product, is the figure of merit used to guide the development of technologies ranging from bioelectronics to neuromorphics. While organic electrochemical transistors (OECTs) are used extensively to measure the µC* product, if there is a kink in the transistor current, then the µC* product is overestimated. Here, we show that the origin of the kink is a change in the rate with which the ions diffuse into the channel. We also discover that the OECT channel geometry has an unprecedented role on how the ions are distributed within the OMIEC. Finally, we observe that electronic charge carriers are first be injected and drift in the OMIEC, before ions can drift from the electrolyte into the OMIEC. Overall, the diverse charge transport phenomena identified here are essential for understanding the complexities of integrating novel OMIECs into traditional device structures, providing the key information needed to realize their promising application potential.

Noncovalent interactions between aromatic rings can dictate the crystal structures of organic optoelectronic materials. While cofacial interactions of fluorinated and non-fluorinated arenes are useful supramolecular synthons, the impacts of the...