Light is ubiquitous in the urban environment – from the sun that shines down upon us to the artificial sources that light-up our devices and homes. While some of this light is used very effectively, for example by plants in the process of photosynthesis, much is wasted, either due to inefficient capture or poor recycling of the broad spectrum of photon energies available. Photon conversion materials can help bridge the energy mismatch between a light source and the collector (e.g. a solar photovoltaic (PV) cell or fibre optic) through use of a photoluminescence process to convert the incident photon energy.[1] For ease of processing and eventual integration with (opto)electronic devices, the photon converter must usually be integrated within a solid-state host. Judicious consideration of the host material is essential to ensure that host-emitter interactions enhance rather than diminish the optical performance.

In this talk, recent highlights from our research into the bottom-up design of photon conversion materials utilising organic-inorganic hybrid hosts will be presented. It will be shown that materials chemistry strategies can be used to control the packing, orientation and placement of emitters, which provides a means of modulating the optical properties – from enhanced photoluminescence quantum yields[2], to tunable photon energies via Förster resonance energy transfer[3,4] or triplet-triplet annihilation upconversion (TTA-UC). These characteristics can be exploited to improve light-harvesting and trapping, which can be used to develop highly efficient luminescent solar concentrators,[4] optical amplifiers for visible light communications[5], and sensor platforms for bioimaging.

Organic semiconductors are an emerging class of materials with various optoelectronic applications. To enable faster commercialisation of this technology, there are two requirements: (i) improve their performance and (ii) follow eco-friendly manufacturing strategies.

Improving their performance is a key requirement to enable faster commercialisation. Charge carrier mobility is one crucial parameter for electronic applications, particularly organic thin film transistors (OTFTs). A generic approach to achieve this is through the addition of molecular dopants in the organic semiconductor layer. Here, we report the use of novel materials as molecular dopants for both p^1,2 and n-type^3,4 organic semiconductors in OTFT devices. We show that key device parameters such as charge carrier mobility, contact resistance and threshold voltage improve dramatically upon adding the dopant. The effect of the dopant was analysed with Electron Paramagnetic Resonance (EPR) and by extracting the activation energy (EA) from low-temperature electrical characterisation. The impact of the dopant on the morphology of the OSCs will also be discussed as studied from Atomic Force Microscopy (AFM) and X-Ray diffraction (XRD). Overall, this work highlights that controlled doping of organic semiconductor materials is the key to enhanced electronic devices.

In the second part of the talk, I will introduce sustainable routes to manufacture solution-processed organic electronics. Particular focus will be given to organic solar cells, which recently attracted immense attention due to the development of a new family of semiconductors that allows highly efficient light harvesting in indoor and outdoor conditions. One current limitation, though, is the use of not eco-friendly solvents and materials during the device development stages. Most organic electronic devices require halogenated and non-halogenated aromatic solvents during their fabrication. For large-scale production and further commercialisation, this is a key limitation. This arises from the fact that organic semiconductors are highly soluble in this category of solvents, which are often carcinogenic or toxic to the human reproductive systems and negatively impact the environment. Here I will show high-performing organic solar cells fabricated from biomass-based solvents.⁵. Overall, this work highlights the importance of replacing harmful chemicals and materials in the organic electronics fabrication stages, resulting in faster and wider commercialisation and new market opportunities.

Solution-processed nanocomposite films comprising small molecule organic semiconductors (OSCs) and inorganic colloidal quantum dots (QDs) are promising systems for low-cost, high efficiency, solar energy harvesting technologies [1]. In these systems, OSCs capable of singlet fission (SF) offer a mechanism to surpass the radiative efficiency limits of single-junction photovoltaics (PV). The SF-generated triplet excitons can be harvested by the inorganic QDs, where they radiatively recombine to achieve photon multiplication, converting a single high-energy photon into two low-energy photons. Such a SF photon multiplication film (SF-PMF) has the potential to enhance the efficiency of the best Si-PV from 26.7% to 32.5%, a substantial gain [2]. For efficient SF-PMF, the ideal film nanomorphology consists of QDs that are highly dispersed throughout the OSC phase at a length scale comparable to the triplet exciton diffusion length. However, controlling QD dispersibility in OSC:QD blends is challenging given the strong tendency of QDs to aggregate and phase-separate due to the mismatch of their size, shape and surface energies. Understanding the self-assembly mechanisms of the organic and inorganic components during large-scale, high throughput film coating methods is therefore crucial for precise control of film nanomorphology [3]. This talk will demonstrate how in-situ grazing incidence X-ray scattering (GIXS) offers direct insights into the self-assembly of OSC:QD blends during blade coating. It will outline some of our latest strategies to control structure formation in nanocomposite films via subtle changes in composition and processing conditions. The results provide routes for the structural design and optimization of solution-processed nanocomposites that are compatible with large-scale coating techniques, essential for driving the commercialisation of SF-PMF architectures for solar energy harvesting applications.

Optoelectronics devices based on closed-shell organic small molecules and polymers have proven themselves as disruptive technology in display applications. Their ability to interchange light absorption or emission with electrical generation or excitation has allowed for the development of organic light emitting diodes (OLED), organic photovoltaics (OPV) and other devices with commercially relevant performance and costs. However, the fundamental electronic structure of these closed-shell materials imposes limitations on their use. The excitonic nature of these materials means that for every bright, optically accessible (singlet) state there is a corresponding dark (triplet) state at a lower energy. The presence of dark triplet states is the cause for both the lower efficiency and lower stability in current generation organic optoelectronic devices relative to their inorganic counterparts. We propose to generate an entirely new set of optoelectronic materials based on open-shell (radical) organic materials where the lowest energetically excited state is an optically bright state, thus eliminating the major loss mechanism in current organic optoelectronic devices. In addition, the proposed materials can have high-spin which can be optically (or otherwise) manipulated opening up the possibility of using organic materials in next generation quantum technologies.

Masers, the microwave version of lasers, are a promising emerging class of ultralow-noise microwave amplifiers with the potential as quantum sensors in a range of commercial applications.^[1] These devices rely on the photogeneration of electron spin-polarized triplet states in organic crystals of acene-doped-p-terphenyl or nitrogen-vacancy (NV) diamond. The stimulated collapse of these states by injected microwaves results in microwave emission and amplification which is highly sensitive to magnetic fields. However, due to inefficient electronic processes and triplet spin dynamics, these materials require large crystals with strong light pump sources to overcome the maser threshold, increasing their bulk and expense and damaging the gain media. Therefore, before masers can be widely applied, we must develop new materials with enhanced spin dynamics to ease their operation while increasing their power.

To tackle these issues, we have designed novel approaches to tune the properties of maser candidate molecular systems.^[2,3] We have synthesized several new triplet and radical-based materials capable of producing extremely strong and long-lived electron spin polarisation through intersystem crossing, singlet fission, and triplet-radical interactions. We employed transient photoluminescence and absorption spectroscopy alongside electron paramagnetic resonance to understand the link between their spin dynamics and their merit as maser devices. Our new materials demonstrate the capacity to operate at various resonant frequencies and can be optically pumped at more easily generated wavelengths. Furthermore, their triplet spin dynamics suggest that similar systems could exhibit enhanced maser cooperativity. These results pave the way for the synthesis of more efficient and applicable maser technologies.

References:

[1]      D. M. Arroo, N. M. Alford, J. D. Breeze, Appl. Phys. Lett. 2021, 119, 140502.

[2]      W. Ng, X. Xu, M. Attwood, H. Wu, Z. Meng, X. Chen, M. Oxborrow, Adv. Mater. 2023, 35, 2300441.

[3]      M. Attwood, X. Xu, M. Newns, Z. Meng, R. A. Ingle, H. Wu, X. Chen, W. Xu, W. Ng, T. T. Abiola, V. G. Stavros, M. Oxborrow, Chem. Mater. 2023, 35, 4498–4509.

Quantum sensors can harness the sensitivity of the entangled electron spin states to external stimuli to probe physical properties such as temperature, electric and magnetic fields. However, improvements are still required for efficient and reliable read-out of the stored quantum information to expand the applicability of devices. Molecular systems are the most recent candidates for quantum device applications as these systems can be tuned to have specific magnetic properties and much work is being done to achieve optical readout, analogous to NV centers in diamond. Here, we investigate the possibility of a digital state readout by combining a molecular spin system to a valleytronic material.

Coordination compounds use structural tuning to provide precise tailoring of magnetic properties, with phthalocyanines established as a versatile system. Specifically, vanadyl phthalocyanine (VOPc) is considered, a spin ½ system with long (μs) coherence times that can be easily and predictably deposited on a range of substrates. Films consisting of a few layers of transition metal dichalcogenides (TMDC) have demonstrated circular polarized photoluminescence from the transition to a direct band gap semiconductor. Chiral emission is relevant to spin-selective optoelectronics and combining the molecular spin qubit with a TMDC substrate could lead to interesting opportunities in quantum sensing. However, at this stage little is known of the electronic and spin interactions at such an interface and the extent of spin-valley cooperativity. We therefore present an optical study of the VOPc thermally evaporated on WSe2. Transient absorption (TA) spectroscopy and time resolved circular dichroism (TRCD) are performed to extract spin-valley relaxation parameters and correlate these to the molecular orientation on the 2D layer. This provides critical information to design new architectures for quantum sensors.

In recent years, the investigation of strong light-matter coupling has emerged as a powerful avenue to manipulate and control the excitonic properties of organic materials. Strong light-matter coupling leads to the formation of hybrid light-matter states called exciton-polaritons, which have the potential to revolutionise the future of molecular electronics owing to their promise to induce long-range energy transfer [1]. One intriguing aspect of strong coupling is its potential impact on processes involving excited triplet states, such as triplet-triplet annihilation (TTA), which plays a significant role in organic photophysics and optoelectronic devices through upconversion.

This talk discusses the intricate interplay between strong light-matter coupling and triplet-triplet annihilation in organic materials. We explore how the formation of exciton-polaritons modifies the TTA process, potentially affecting the rate of triplet-state quenching and singlet exciton generation which influences the delayed-emission dynamics [2]. Through a combination of advanced spectroscopic techniques, we explore the underlying mechanisms governing the interactions between exciton-polaritons and triplet excitons.

Our findings provide valuable insights into the fundamental photophysical processes in the strong-coupling regime and pave the way for novel strategies to enhance the efficiency of organic optoelectronic devices. By elucidating the intricate dynamics of triplet-triplet annihilation under strong light-matter coupling, we contribute to the broader understanding of excitonic interactions and offer a new perspective on designing advanced organic materials for next-generation optoelectronics.

Ortho-fused aromatic rings form helically shaped chiral molecules such as carbo[6]helicenes, that wind in a left-handed (M) or a right-handed (P) sense.^[1-3] The helical topology combined with extended pi-conjugation provides helicenes with peculiar properties such as strong photophysical and chiroptical properties (high optical rotation values, intense electronic circular dichroism - CD, and circularly polarized emission - CPL). The molecular engineering of helicenes using organometallic and heteroaromatic chemistries offers a convenient way to tune the properties of these helically shaped ligands. Indeed, their combination with metallic or organic assembling units leads to chiral materials with appealing properties (circularly polarized phosphorescence, magnetochirality, spin selectivity). Applications in materials science (Circularly Polarized OLEDs, Chiroptical Switches, Spintronics) are targetd. A set of representative examples will be presented while emphasizing the specific features of each system. When possible, theoretical calculations enable to shed light on the phenomena (exciton coupling chirality, singlet or triplet excited state features, molecular orbitals involvement, dissymmetry factors improvement, ...).^[4-6]