Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems.

Energy transfer across the donor-acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system.

Beyond n-dopants for organic semiconductors: use of bibenzo[d]imidazoles in UV-promoted dehalogenation reactions of organic halides.

2,2'-Bis(4-dimethylaminophenyl)- and 2,2'-dicyclohexyl-1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazole ((N-DMBI)2 and (Cyc-DMBI)2) are quite strong reductants with effective potentials of ca. -2 V vs ferrocenium/ferrocene, yet are relatively stable to air due to the coupling of redox and bond-breaking processes. Here, we examine their use in accomplishing electron transfer-induced bond-cleavage reactions, specifically dehalogenations. The dimers reduce halides that have reduction potentials less cathodic than ca. -2 V vs ferrocenium/ferrocene, especially under UV photoexcitation (using a 365 nm LED). In the case of benzyl halides, the products are bibenzyl derivatives, whereas aryl halides are reduced to the corresponding arenes. The potentials of the halides that can be reduced in this way, quantum-chemical calculations, and steady-state and transient absorption spectroscopy suggest that UV irradiation accelerates the reactions via cleavage of the dimers to the corresponding radical monomers.

Metal-Carbodithioate-Based 3D Semiconducting Metal-Organic Framework: Porous Optoelectronic Material for Energy Conversion.

Solar energy conversion requires the working compositions to generate photoinduced charges with high potential and the ability to deliver charges to the catalytic sites and/or external electrode. These two properties are typically at odds with each other and call for new molecular materials with sufficient conjugation to improve charge conductivity but not as much conjugation as to overly compromise the optical band gap. In this work, we developed a semiconducting metal-organic framework (MOF) prepared explicitly through metal-carbodithioate "(-CS2)nM" linkage chemistry, entailing augmented metal-linker electronic communication. The stronger ligand field and higher covalent character of metal-carbodithioate linkages─when combined with spirofluorene-derived organic struts and nickel(II) ion-based nodes─provided a stable, semiconducting 3D-porous MOF, Spiro-CS2Ni. This MOF lacks long-range ordering and is defined by a flexible structure with non-aggregated building units, as suggested by reverse Monte Carlo simulations of the pair distribution function obtained from total scattering experiments. The solvent-removed "closed pore" material recorded a Brunauer-Emmett-Teller area of ∼400 m2/g, where the "open pore" form possesses 90 wt % solvent-accessible porosity. Electrochemical measurements suggest that Spiro-CS2Ni possesses a band gap of 1.57 eV (σ = 10-7 S/cm at -1.3 V bias potential), which can be further improved by manipulating the d-electron configuration through an axial coordination (ligand/substrate), the latter of which indicates usefulness as an electrocatalyst and/or a photoelectrocatalyst (upon substrate binding). Transient-absorption spectroscopy reveals a long-lived photo-generated charge-transfer state (τCR = 6.5 µs) capable of chemical transformation under a biased voltage. Spiro-CS2Ni can endure a compelling range of pH (1-12 for weeks) and hours of electrochemical and photoelectrochemical conditions in the presence of water and organic acids. We believe this work provides crucial design principles for low-density, porous, light-energy-conversion materials.

Charge Concentration Limits the Hydrogen Evolution Rate in Organic Nanoparticle Photocatalysts.

Time-resolved microwave conductivity is used to compare aqueous-soluble organic nanoparticle photocatalysts and bulk thin films composed of the same mixture of semiconducting polymer and non-fullerene acceptor molecule and the relationship between composition, interfacial surface area, charge-carrier dynamics, and photocatalytic activity is examined. The rate of hydrogen evolution reaction by nanoparticles composed of various donor:acceptor blend ratio compositions is quantitatively measured, and it is found that the most active blend ratio displays a hydrogen quantum yield of 0.83% per photon. Moreover, it is found that nanoparticle photocatalytic activity corresponds directly to charge generation, and that nanoparticles have 3× more long-lived accumulated charges relative to bulk samples of the same material composition. These results suggest that, under the current reaction conditions, with ≈3× solar flux, catalytic activity by the nanoparticles is limited by the concentration of electrons and holes in operando and not a finite number of active surface sites or the catalytic rate at the interface. This provides a clear design goal for the next generation of efficient photocatalytic nanoparticles.

Controlled n-Doping of Naphthalene-Diimide-Based 2D Polymers.

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-1 are 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.

Short and long-range electron transfer compete to determine free-charge yield in organic semiconductors.

Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies.

PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst.

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF3)ppy)2(bpy)][PF6] (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF3)ppy)2(BIP-bpy)][PF6] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV2+), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.

Active-Site Environmental Factors Customize the Photophysics of Photoenzymatic Old Yellow Enzymes.

The development of non-natural photoenzymatic systems has reinvigorated the study of photoinduced electron transfer (ET) within protein active sites, providing new and unique platforms for understanding how biological environments affect photochemical processes. In this work, we use ultrafast spectroscopy to compare the photoinduced electron transfer in known photoenzymes. 12-Oxophytodienoate reductase 1 (OPR1) is compared to Old Yellow Enzyme 1 (OYE1) and morphinone reductase (MR). The latter enzymes are structurally homologous to OPR1. We find that slight differences in the amino acid composition of the active sites of these proteins determine their distinct electron-transfer dynamics. Our work suggests that the inside of a protein active site is a complex/heterogeneous dielectric network where genetically programmed heterogeneity near the site of biological ET can significantly affect the presence and lifetime of various intermediate states. Our work motivates additional tunability of Old Yellow Enzyme active-site reorganization energy and electron-transfer energetics that could be leveraged for photoenzymatic redox approaches.

Reversible multicolor chromism in layered formamidinium metal halide perovskites.

Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties. The crystal-liquid duality enables optoelectronic devices with unprecedented performance and a unique opportunity to chemically manipulate the structure with low energy input. In this work, we leverage the low formation energy of metal halide perovskites to demonstrate multicolor reversible chromism. We synthesized layered Ruddlesden-Popper FAn+1PbnX3n+1 (FA = formamidinium, X = I, Br; n = number of layers = 1, 2, 3 … ∞) and reversibly tune the dimensionality (n) by modulating the strength and number of H-bonds in the system. H-bonding was controlled by exposure to solvent vapor (solvatochromism) or temperature change (thermochromism), which shuttles FAX salt pairs between the FAn+1PbnX3n+1 domains and adjacent FAX "reservoir" domains. Unlike traditional chromic materials that only offer a single-color transition, FAn+1PbnX3n+1 films reversibly switch between multiple colors including yellow, orange, red, brown, and white/colorless. Each colored phase exhibits distinct optoelectronic properties characteristic of 2D superlattice materials with tunable quantum well thickness.

Slow charge transfer from pentacene triplet states at the Marcus optimum.

Singlet fission promises to surpass the Shockley-Queisser limit for single-junction solar cell efficiency through the production of two electron-hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~107 s-1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.

Measuring Photoexcited Free Charge Carriers in Mono- to Few-Layer Transition-Metal Dichalcogenides with Steady-State Microwave Conductivity.

Photoinduced generation of mobile charge carriers is the fundamental process underlying many applications, such as solar energy harvesting, solar fuel production, and efficient photodetectors. Monolayer transition-metal dichalcogenides (TMDCs) are an attractive model system for studying photoinduced carrier generation mechanisms in low-dimensional materials because they possess strong direct band gap absorption, large exciton binding energies, and are only a few atoms thick. While a number of studies have observed charge generation in neat TMDCs for photoexcitation at, above, or even below the optical band gap, the role of nonlinear processes (resulting from high photon fluences), defect states, excess charges, and layer interactions remains unclear. In this study, we introduce steady-state microwave conductivity (SSMC) spectroscopy for measuring charge generation action spectra in a model WS2 mono- to few-layer TMDC system at fluences that coincide with the terrestrial solar flux. Despite utilizing photon fluences well below those used in previous pump-probe measurements, the SSMC technique is sensitive enough to easily resolve the photoconductivity spectrum arising in mono- to few-layer WS2. By correlating SSMC with other spectroscopy and microscopy experiments, we find that photoconductivity is observed predominantly for excitation wavelengths resonant with the excitonic transition of the multilayer portions of the sample, the density of which can be controlled by the synthesis conditions. These results highlight the potential of layer engineering as a route toward achieving high yields of photoinduced charge carriers in neat TMDCs, with implications for a broad range of optoelectronic applications.

The Effect of Ring Expansion in Thienobenzo[b]indacenodithiophene Polymers for Organic Field-Effect Transistors.

A fused donor, thienobenzo[b]indacenodithiophene (TBIDT), was designed and synthesized using a novel acid-promoted cascade ring closure strategy, and then copolymerized with a benzothiadiazole (BT) monomer. The backbone of TBIDT is an expansion of the well-known indacenodithiophene (IDT) unit and was expected to enhance the charge carrier mobility by improving backbone planarity and facilitating short contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm2 V-1 s-1, lower than the performance of IDT-BT (∼1.5 cm2 V-1 s-1). Mobilities extracted from time-resolved microwave conductivity measurements were consistent with the trend in hole mobilities in organic field-effect transistor devices. Scanning tunneling microscopy measurements and computational modeling illustrated that TBIDT-BT exhibits a less ordered microstructure in comparison to IDT-BT. This reveals that a regular side-chain packing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that TBIDT-BT, despite containing a larger, planar unit, showed less stabilization of planar backbone geometries in comparison to IDT-BT. This is due to the reduced electrostatic stabilizing interactions between the peripheral thiophene of the fused core and the BT unit, resulting in a reduction of the barrier to rotation around the single bond. These insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT-type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors.

Carotenoid Nuclear Reorganization and Interplay of Bright and Dark Excited States.

We report quantum chemical calculations using multireference perturbation theory (MRPT) with the density matrix renormalization group (DMRG) plus photothermal deflection spectroscopy measurements to investigate the manifold of carotenoid excited states and establish their energies relative to the bright state (S2) as a function of nuclear reorganization. We conclude that the primary photophysics and function of carotenoids are determined by interplay of only the bright (S2) and lowest-energy dark (S1) states. The lowest-lying dark state, far from being energetically distinguishable from the lowest-lying bright state along the entire excited-state nuclear reorganization pathway, is instead computed to be either the second or first excited state depending on what equilibrium geometry is considered. This result suggests that, rather than there being a dark intermediate excited state bridging a non-negligible energy gap from the lowest-lying dark state to the lowest-lying bright state, there is in fact no appreciable energy gap to bridge following photoexcitation. Instead, excited-state nuclear reorganization constitutes the bridge from S2 to S1, in the sense that these two states attain energetic degeneracy along this pathway.

Excited-State Electronic Properties in Zr-Based Metal-Organic Frameworks as a Function of a Topological Network.

Molecular assemblies in metal-organic frameworks (MOFs) are reminiscent of natural light-harvesting (LH) systems and considered as emerging materials for energy conversion. Such applications require understanding the correlation between their excited-state properties and underlying topological net. Two chemically identical but topologically different tetraphenylpyrene (1,3,6,8-tetrakis( p-benzoicacid)pyrene; H4TBAPy)-based ZrIV MOFs, NU-901 ( scu) and NU-1000 ( csq), are chosen to computationally and spectroscopically interrogate the impact of topological difference on their excited-state electronic structures. Time-dependent density functional theory-computed transition density matrices for selected model compounds reveal that the optically relevant S1, S2, and S n states are delocalized over more than four TBAPy linkers with a maximum exciton size of ∼1.7 nm (i.e., two neighboring TBAPy linkers). Computational data further suggests the evolution of polar excitons (hole and electron residing in two different linkers); their oscillator strengths vary with the extent of interchromophoric interaction depending on their topological network. Femtosecond transient absorption (fs-TA) spectroscopic data of NU-901 highlight instantaneous spectral evolution of an intense S1 → S n transition at 750 nm, which diminishes with the emergence of a broad (580-1100 nm) induced absorption originating from a fast excimer formation. Although these ultrafast spectroscopic data reveal the first direct spectral observation of fast excimer formation (τ = 2 ps) in MOFs, the fs-TA features seen in NU-901 are clearly absent in NU-1000 and the free H4TBAPy linker. Furthermore, transient and steady-state fluorescence data collected as a function of solvent dielectrics reveal that the emissive states in both MOF samples are electronically nonpolar; however, low-lying polar excited states may get involved in the excited-state decay processes in polar solvents. The present work shows that the topological arrangement of the linkers critically controls the excited-state electronic structures.

The Role of the Side Chain on the Performance of N-type Conjugated Polymers in Aqueous Electrolytes.

We report a design strategy that allows the preparation of solution processable n-type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol-based side chain. A series of random copolymers was prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90, and 100 with respect to the alkyl side chains. These were characterized to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions, and performance in OECTs when operated in aqueous electrolytes. We observed that glycol side chain percentages of >50% are required to achieve volumetric charging, while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.

Controlling Long-Lived Triplet Generation from Intramolecular Singlet Fission in the Solid State.

The conjugated polymer poly(benzothiophene dioxide) (PBTDO1) has recently been shown to exhibit efficient intramolecular singlet fission in solution. We investigate the role of intermolecular interactions in triplet separation dynamics after singlet fission. We use transient absorption spectroscopy to determine the singlet fission rate and triplet yield in two polymers differing only by side-chain motif in both solution and the solid state. Whereas solid-state films show singlet fission rates identical to those measured in solution, the average lifetime of the triplet population increases dramatically and is strongly dependent on side-chain identity. These results show that it may be necessary to carefully engineer the solid-state microstructure of these "singlet fission polymers" to produce the long-lived triplets needed to realize efficient photovoltaic devices.

300% Enhancement of Carrier Mobility in Uniaxial-Oriented Perovskite Films Formed by Topotactic-Oriented Attachment.

Organic-inorganic perovskites with intriguing optical and electrical properties have attracted significant research interests due to their excellent performance in optoelectronic devices. Recent efforts on preparing uniform and large-grain polycrystalline perovskite films have led to enhanced carrier lifetime up to several microseconds. However, the mobility and trap densities of polycrystalline perovskite films are still significantly behind their single-crystal counterparts. Here, a facile topotactic-oriented attachment (TOA) process to grow highly oriented perovskite films, featuring strong uniaxial-crystallographic texture, micrometer-grain morphology, high crystallinity, low trap density (≈4 × 1014 cm-3 ), and unprecedented 9 GHz charge-carrier mobility (71 cm2 V-1 s-1 ), is demonstrated. TOA-perovskite-based n-i-p planar solar cells show minimal discrepancies between stabilized efficiency (19.0%) and reverse-scan efficiency (19.7%). The TOA process is also applicable for growing other state-of-the-art perovskite alloys, including triple-cation and mixed-halide perovskites.

Experimental demonstration of photon upconversion via cooperative energy pooling.

Photon upconversion is a fundamental interaction of light and matter that has applications in fields ranging from bioimaging to microfabrication. However, all photon upconversion methods demonstrated thus far involve challenging aspects, including requirements of high excitation intensities, degradation in ambient air, requirements of exotic materials or phases, or involvement of inherent energy loss processes. Here we experimentally demonstrate a mechanism of photon upconversion in a thin film, binary mixture of organic chromophores that provides a pathway to overcoming the aforementioned disadvantages. This singlet-based process, called Cooperative Energy Pooling (CEP), utilizes a sensitizer-acceptor design in which multiple photoexcited sensitizers resonantly and simultaneously transfer their energies to a higher-energy state on a single acceptor. Data from this proof-of-concept implementation is fit by a proposed model of the CEP process. Design guidelines are presented to facilitate further research and development of more optimized CEP systems.

Through-Space Ultrafast Photoinduced Electron Transfer Dynamics of a C70-Encapsulated Bisporphyrin Covalent Organic Polyhedron in a Low-Dielectric Medium.

Ultrafast photoinduced electron transfer (PIET) dynamics of a C70-encapsulated bisporphyrin covalent organic polyhedron hybrid (C70@COP-5) is studied in a nonpolar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)-acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined cofacial D-A orientation with a fixed edge-to-edge separation, REE (2.8 Å), without the aid of covalent bonds. Sub-picosecond PIET (τET ≤ 0.4 ps) and very slow charge recombination (τCR ≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D-A coupling (V = 675 cm-1) and extremely small reorganization energy (λ ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C70@COP-5 complex.

Mechanism for rapid growth of organic-inorganic halide perovskite crystals.

Optoelectronic devices based on hybrid halide perovskites have shown remarkable progress to high performance. However, despite their apparent success, there remain many open questions about their intrinsic properties. Single crystals are often seen as the ideal platform for understanding the limits of crystalline materials, and recent reports of rapid, high-temperature crystallization of single crystals should enable a variety of studies. Here we explore the mechanism of this crystallization and find that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization. We use this knowledge to demonstrate a broader range of processing parameters and show that these can lead to improved crystal quality. Our findings are therefore of central importance to enable the continued advancement of perovskite optoelectronics and to the improved reproducibility through a better understanding of factors influencing and controlling crystallization.