Structure Dependence of Kinetic and Thermodynamic Parameters in Singlet Fission Processes

Singlet fission—whereby one absorbed photon generates two coupled triplet excitons—is a key process for increasing the efficiency of optoelectronic devices by overcoming the Shockley–Queisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in coevaporated films of pentacene in p-terphenyl. The rate of dissociation is higher for parallel dimers than for their herringbone counterparts, as is the rate of recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that weaker triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and a stronger triplet–triplet interaction result in slower dissociation and recombination. The identification and understanding of how the intermolecular geometry promotes efficient triplet dissociation provide the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance

A response surface model to predict and experimentally tune the chemical, magnetic and optoelectronic properties of oxygen-doped boron nitride

Porous boron nitride (BN), a combination of hexagonal, turbostratic and amorphous BN, has emerged as a new platform photocatalyst. Yet, this material lacks photoactivity under visible light. Theoretical studies predict that tuning the oxygen content in oxygen-doped BN (BNO) could lower the band gap. This is yet to be verified experimentally. We present herein a systematic experimental route to simultaneously tune BNO's chemical, magnetic and optoelectronic properties using a multivariate synthesis parameter space. We report deep visible range band gaps (1.50–2.90 eV) and tuning of the oxygen (2–14 at.%) and specific paramagnetic OB3 contents (7–294 a.u. g−1). Through designing a response surface via a design of experiments (DOE) process, we have identified synthesis parameters influencing BNO's chemical, magnetic and optoelectronic properties. We also present model prediction equations relating these properties to the synthesis parameter space that we have validated experimentally. This methodology can help tailor and optimise BN materials for heterogeneous photocatalysis

Paramagnetic states in oxygen-doped boron nitride extend light harvesting and photochemistry to the deep visible region

A family of boron nitride (BN)-based photocatalysts for solar fuel syntheses have recently emerged. Studies have shown that oxygen doping, leading to boron oxynitride (BNO), can extend light absorption to the visible range. However, the fundamental question surrounding the origin of enhanced light harvesting and the role of specific chemical states of oxygen in BNO photochemistry remains unanswered. Here, using an integrated experimental and first-principles-based computational approach, we demonstrate that paramagnetic isolated OB3 states are paramount to inducing prominent red-shifted light absorption. Conversely, we highlight the diamagnetic nature of O–B–O states, which are shown to cause undesired larger band gaps and impaired photochemistry. This study elucidates the importance of paramagnetism in BNO semiconductors and provides fundamental insight into its photophysics. The work herein paves the way for tailoring of its optoelectronic and photochemical properties for solar fuel synthesis

Comparison of organic and inorganic layers for structural templating of pentacene thin films

Pentacene is a key organic semiconductor, which has achieved prominence in transistor applications and as an archetypal material for singlet fission, the process whereby the absorption of one photon leads to the formation of two triplet states. Functional properties of molecules are highly anisotropic, and control over the molecular orientation in thin films with structural templating is commonly implemented as a route for governing the morphology and structure of organic films. Among the structural templating layers, 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA) and copper (I) iodide (CuI) have been shown to effectively template aromatic systems such as phthalocyanines. Here, we extend their use to pentacene thin films and find that a successful transition to a flat-lying arrangement is achieved with CuI films grown at high temperatures, but not with PTCDA. As a result, we postulate a model based on quadrupole interactions as the driving force behind the molecular orientation of pentacene. A 0.25 eV increase in work function and a two-fold increase in absorption are recorded for the induced flat-lying orientation. Therefore, our templating methodology provides design opportunities for optoelectronic devices that require a predominantly flat-lying orientation

Correction: Comparison of organic and inorganic layers for structural templating of pentacene thin films

Correction for ‘Comparison of organic and inorganic layers for structural templating of pentacene thin films’ by Dong Kuk Kim et al., Mater. Horiz., 2019, DOI: 10.1039/c9mh00355j. The authors wish to amend the Acknowledgements section of the originally published manuscript. The correct Acknowledgements section is shown below

Growth, morphology and structure of mixed pentacene films

Thin films of pentacene and p-terphenyl were grown via organic molecular beam deposition to enable solid-state dilution of functional molecules (pentacene) in an inert matrix (p-terphenyl) at higher concentrations than permitted by traditional crystal growth methods, such as melts. Growth rates were first optimised for single component films to ensure a precise control over the dopant/host concentrations when the mixed films were deposited. Both thin film and bulk phases can be identified in pentacene growths, with the precise lattice parameters dependent on the deposition rates. The effect on the microstructure, resulting from progressive dilution of pentacene in a p-terphenyl host, was then investigated. Although disorder increases and the crystallite size decreases in the mixture, with a minimum at a 1 : 1 ratio, phase segregation is not observed on the length scale (limit) that can be probed in our measurements. This indicates that the mixed films form homogeneous solid-solutions that may be employed for the investigation of solid-state phenomena. Our methodology can be extended to other compatible host-dopant systems used in optoelectronic and spintronic devices

Porous boron nitride for combined CO2 capture and photoreduction

Porous and amorphous materials are typically not employed for photocatalytic purposes, like CO2 photoreduction, as their high number of defects can lead to low charge mobility and favour bulk electron–hole recombination. Yet, with a disordered nature can come porosity, which in turn promotes catalyst/reactant interactions and fast charge transfer to reactants. Here, we demonstrate that moving from h-BN, a well-known crystalline insulator, to amorphous BN, we create a semiconductor, which is able to photoreduce CO2 in the gas/solid phase, under both UV-vis and pure visible light and ambient conditions, without the need for cocatalysts. The material selectively produces CO and maintains its photocatalytic stability over several catalytic cycles. The performance of this un-optimized material is on par with that of TiO2, the benchmark in the field. For the first time, we map out experimentally the band edges of porous BN on the absolute energy scale vs. vacuum to provide fundamental insight into the reaction mechanism. Owing to the chemical and structural tunability of porous BN, these findings highlight the potential of porous BN-based structures for photocatalysis particularly solar fuel production

Structure dependence of kinetic and thermodynamic parameters in singlet fission processes.

Singlet fission-whereby one absorbed photon generates two coupled triplet excitons-is a key process for increasing the efficiency of optoelectronic devices by overcoming the Shockley-Queisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in coevaporated films of pentacene in p-terphenyl. The rate of dissociation is higher for parallel dimers than for their herringbone counterparts, as is the rate of recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that weaker triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and a stronger triplet-triplet interaction result in slower dissociation and recombination. The identification and understanding of how the intermolecular geometry promotes efficient triplet dissociation provide the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance

Identifying triplet pathways in dilute pentacene films

Building efficient triplet-harvesting layers for photovoltaic applications requires a deep understanding of the microscopic properties of the components involved and their dynamics. Singlet fission is a particularly appealing mechanism as it generates two excitons from a single photon. However, the pathways of the coupled triplets into free species, and their dependence on the intermolecular geometry, has not been fully explored. In this work, we produce highly ordered dilute pentacene films with distinct parallel and herringbone dimers and aggregates. Using electron paramagnetic resonance spectroscopy, we provide compelling evidence for the formation of distinct quintet excitons in ambient conditions, with intrinsically distinctive electronic and kinetic properties. We find that the ability of quintets to separate into free triplets is promoted in the parallel dimers and this provides molecular design rules to control the triplets, favouring either enhanced photovoltaic efficiency (parallel) or strongly bound pairs that could be exploited for logic applications (herringbone)

Comprehensive Studies of Magnetic Transitions and Spin–Phonon Couplings in the Tetrahedral Cobalt Complex Co(AsPh₃)₂I₂

A combination of inelastic neutron scattering (INS), far-IR magneto-spectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh3)2I2 (1), a reported single-molecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D′) shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm–1, as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin–phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin–phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P21/c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1–2 cm–1. The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized