Here, we delve into symmetry-breaking charge transfer (SB-CT) and triplet formation dynamics, particularly their interplay with conformational diversity in 9,9’,10,10’-tetraphenyl-2,2’-bianthracene (TPBA). TPBA distinguishes itself from the traditional 9,9’-bianthracene through its notably planar structure, attributed to the diminished steric hindrance between its anthracene chromophores. This structural attribute engenders pronounced short-range CT coupling, underpinned by significant overlap integrals of electron and hole, in stark contrast to the 9,9’-bianthracene, which is characterized by dominant long-range Coulombic coupling. As a result, TPBA shows an adiabatic mixture of locally-excited and CT diabats, with the degree of their contribution being influenced by the dielectric properties of the surrounding medium. Our research further uncovers that the SB-CT reaction in TPBA is mainly affected by solvation dynamics. Remarkably, we observe that triplet formation in TPBA follows an unprecedented multiexponential behavior, and its efficiency significantly varies with solvent polarity. This can be rationalized by considering the multiple conformers and their energetic landscapes.

Singlet fission is a unique process wherein a photoexcited molecule shares half its energy with a neighbor, generating a pair of spin-entangled triplet excitons. If properly harnessed, singlet fission can enhance photocatalysts and solar energy technologies and be utilized to produce new optically-addressable qubits. However, achieving these goals requires singlet fission materials wherein the rate of triplet pair production and spatial diffusion of triplet excitons can be controlled. Using femtosecond transient absorption microscopy, we have characterized how grain boundaries impact both singlet fission and energy transport within two prototypical fission materials, N,N’-bis(2-phenylethyl)-3,4,9,10-perylenedicarboximide (EP-PDI) and rubrene. We find that while grain boundaries act to enhance triplet exciton production in rubrene, in EP-PDI they play an opposite role, suppressing both energy transport and accelerating exciton decay.

The conformation of non-covalent assemblies is exploited to tune the semiconducting properties of light-harvesting nanoscale materials and semiconducting interfaces. We will show new tools to covalently tether non-covalent assemblies to engineer nanoscale materials and functionalized Silicon electrodes, demonstrating emergent electronic properties.

High-order harmonic generation spectroscopy and imaging and were used to study energy transfer in 2D polar metal-graphene heterostructures. Using Fourier transform SHG microscopy, sub-cycle energy transfer between electronic states of crystalline 2D Ag monolayers was resolved. This rapid carrier transfer created a population inversion, which was reflected in depletion of perturbative time-domain SHG signals. In a second example, non-perturbative high harmonic generation was used to measure interfacial carrier transfer from a series of 2D metals (Ag, Ga, In, Pb) to graphene.

We present evidence that electron-transfer in model organic photovoltaic blends can be modeled as a competition between short and long-range electron transfer events, each described by a Marcus parabola having different reorganization energies for the most localized charge-transfer (CT) state and the mobile free charge (CT) state. Time-resolved Microwave Conductivity (TRMC) combined with photoluminescence excitation (PLE), photoinduced-absorption detected magnetic resonance (PADMR), and femtosecond transient absorption (fsTA) spectroscopy show that when electron transfer is confined to the immediate interfacial region between the donor and the acceptor very little free charge is produced. Instead, excitons split into a highly localized charge transfer state that does not produce photoconductivity. These results provide an alternative way of thinking about charge separation in organic photovoltaic materials, unify solid-state and solution phase models of charge separation, and provide unique design rules for functional donor/acceptor interfaces.

Helical conjugated polymers are of great interest for their potential as sources of circularly polarized luminescence for numerous electro-optical device applications including display technologies. Due to their relatively strong absorption cross sections and high emissivity in the visible wavelength range, these materials permit a detailed investigation of how the transition between helical and random coil forms are driven by polymer structural features such as chain length and chemical defects as well as environmental properties such as solvent and temperature. Bulk methods such as circular dichroism, absorption, and fluorescence as well as single-particle microscopy is used to probe the helix-to-coil phase transition in a model chiral polyfuran and to determine whether the conformations favored in solution are retained in the solid state. In addition, the transient dynamics and the effects of chemical doping on the electronic properties of the helix and coiled forms are explored.

The bandgaps of 3D lead-halide perovskites are mostly varied by changing the halide. As the electronegativity of the halide decreases, so does the bandgap, leading to gaps that are suitable for absorbing sunlight in a solar cell. Unfortunately, the stability of halide perovskites also decreases as we move to the larger and less electronegative halides, like iodides. We recently found a way to circumvent this dichotomy by mixing organochalcogenides (RS-; R = organic group) with halides in perovskites. We can now access the higher stability of bromide or chloride perovskites, while the chalcogen (S, Se) orbitals form the highest-energy filled electronic bands, affording lower bandgaps than pure-bromide or -chloride perovskites. I will present our latest findings on how mixing the halide and mixing the chalcogenide can tune the bandgap of the perovskites, in the ideal range for various photovoltaic applications. I will discuss the potential, as well as the remaining challenges, for extracting photocurrent from this new family of perovskites that may combine the properties of lead-halide and lead-chalcogenide solar absorbers.

Hybrid organic metal halides are solution-processable semiconductors that have unusually good electronic properties for materials deposited at low temperatures. Organic metal halides can be used to form solar cells and have potential as light emitting diodes. Because these materials combine organic and inorganic bonding, there is significant coupling between electronic excitations and the lattice. Towards understanding this relationship, we will present our work investigating the optoelectronic properties of layered organic metal halide systems and the relationship to structure and growth conditions. We will discuss the nature of optical excitations in layered organic metal halide compounds. These systems show formation of self-trapped excitons that can be interpreted as occurring through optical frequency magnetic dipole transitions. We will then discuss how mechanical strain during growth influences photoluminescence. We find evidence that broad emission can be strongly impacted by strain in model systems. Our results suggest that broad emission of layered organic metal halides can be tuned in thin films providing a route towards controlling LEDs.

The need for self-powered electronics is progressively growing in parallel to the flourishing of the Internet of Things (IoT). Although batteries are dominating as powering devices, other small systems are attracting attention, such as piezoelectrics, thermoelectrics and photovoltaics. These last ones can be adapted from their classical outdoor configuration to work preferentially under indoor illumination, i.e. through the harvesting of the spectrum emitted by LEDs and/or fluorescent lamps. While lead- based halide perovskites cannot represent a valuable solution for this scope, due to the strong environmental and health concerns associated to the presence of Pb, analogous compounds based on the heaviest pnictogen, i.e. bismuth, could work as sustainable light-harvesters for indoor photovoltaic devices. In this contribution, we will show our most recent results obtained from the integration of the double perovskite Cs2AgBiBr6 in carbon-based perovskite solar cells, devices characterized by a high degree of sustanaibility, also due to the use of recycled materials within the carbon electrodes.

Inorganic lead halide perovskite nanocrystals (IHP NCs) are known for their exceptional photoluminescence efficiency. This presentation will explore their capacity for converting thermal energy into light via one-photon optical upconversion, or anti-Stokes photoluminescence (ASPL). ASPL is pivotal for advancing "thermophotonic" technologies where highly luminescent semiconductors transform heat into light, driving various thermodynamic engines.

Triplet-triplet annihilation (TTA) enables efficient and versatile photon-upconversion in organics. The same process also can enhance OLED efficiencies by utilising dark triplets. Here, we present progresses on efficient TTA-UC in novel composite systems: First, we use a hybrid nanocrystal as triplet-sensitizer, and two different organic emitters, we observe for the first time, a multi-fold enhancement in the upconversion efficiency in such a composite hybrid system. Next, solid-state photon-upconversion always suffers from low efficiencies and high threshold excitation power. We showcase a novel composite-sensitizer, inspired by organic photovoltaics, for TTA-UC devices, leading to highly-efficient solid-state upconversion devices with low excitation power, through a one-step solution method. We also scaled-up this strategy on highly-flexible large-area substrates. Lastly, we explore overcoming the efficiency bottleneck in simple fluorescent OLEDs. First, in a dual-dopant system, lifting rubrene-derivative-based OLEDs to >20% EQE. Secondly, in single-dopant devices, by triplet-fusion mechanism, drawing parallels to TTA-UC, to enhance device efficiency ceiling.

Most energy and electron transfer processes in solution occur in the weak-coupling limit. Here, oxidation states are long-lived; the words "acceptor" and "donor" have real physical meanings, and the theory describing these phenomena is now decades old. When photochemically relevant organic molecules with sparse densities of states covalently bond with solids and nanoparticles with large densities of states, the molecule and solid can form hybrid electronic states with some unusual energy and electron transfer characteristics. Photophysics in the strong-coupling limit is much less explored and holds some surprises and new opportunities for design.

Chirality is a fundamental molecular property that plays a crucial role in biophysics and drug design. Our simulations demonstrate that X-ray Circular Dichroism (CD) can exploit the localized and element-specific nature of X-ray electronic transitions. X-ray CD therefore is more sensitive to local structures and the chirality probed with it can be referred to as local which in contrast to a conventional Optical CD probing the global molecular chirality. Inducing chiroptical activity into semiconductors is challenging due to difficulties of creating asymmetric crystal structures. We further explore chirality transfer in hybrid perovskite quantum dots capped with chiral ligands. Our atomistic modeling suggests the observed chirality transfer is best rationalized by a dipole – dipole coupling. To maximize the bulk effect, both strategic functionalization and limited conformationally degrees of freedom of the ligands are important for obtaining high-intensity nanomaterial chiroptical signatures through chirality transfer. These computational insights provide synthetic mechanistic guidelines towards improving chiroptical functionality in semiconductor nanomaterials.

A hybrid nanostructure formed from colloidal semiconductor quantum dots (QDs) and a monolayer of a transition metal dichalcogenide (TMD) has superior performance over the pristine monolayer TMD in solar harvesting and photodetector applications. This is because the QD component provides most of the light harvesting through its large absorption cross section which sometimes spans a spectral range from ultraviolet to visible and up to near infrared, depending on the QD’s material composition and size. In this presentation we discuss results of time resolved photoluminescence and pump-probe spectroscopic measurements addressing the charge carrier dynamics at the interface of a hybrid nanostructures composed of core/shell PbS/CdS QDs and a monolayer MoS2 where the size of the core QD is varied. We observe long exciton diffusion in photoexcited QDs followed by electron transfer with a core size dependent rate which is maximal for QDs of smallest core size. And a core-size dependent hole transfer from photoexcited MoS2 onto QD with a rate also dependent of the size of the QD.

Molecular chromophores provide a broad range of possibilities for developing next-generation light-energy conversion applications. However, it remains difficult to control photoactive due to complex potential energy landscapes and a mixture of electronic and vibrational couplings. These in turn mediate photoactive processes such as light-harvesting, energy transport, and charge separation. It is essential to understand the interplay between molecular structure, inter-chromophore coupling, and the influence of the surrounding environment in order to design and optimize new robust materials for emerging applications. This contribution will discuss our recent work elucidating the photophysics of structurally dynamic molecular chromophore systems. Our results demonstrate how the balance of competitive processes can be controlled by molecular design and interactions with the surrounding environment.

Studying the bulk and surface/interface properties of advanced solid-state material often requires femto- and picosecond laser pulses at wavelength ranging from the DUV to the Mid-IR region. We describe recent developments in laser technology that result in the reliable production of femtosecond pulses at 100s of kHz repetition rates. To address the variety of novel functional materials these pulses need to be tuned to sample-specific wavelength that may range from the DUV to the MIR region. In this presentation, we describe recent advances in Iaser sources and their tunable parametric amplifiers.

In classic electrodynamics, circular polarization of light is characterized by the electromagnetic field of the wave rotating in a plane perpendicular to its propagation direction. State-of-the-art CPL emission and detection materials rely on the inorganic metamaterials, helicenes, chiral polymeric materials, chiral nanostructures and most recently, organic-inorganic chiral perovskites. Low-dimensional organic-inorganic hybrid halide crystals are an emerging class of semiconductors with a general formula of (L)2(A)n-1MnX3n+1 (L, A=organic cation, M=metal, X=halides), that recently received much attention for their excellent optoelectronic performance. In this presentation, I will showcase our latest research on low-dimensional chiral hybrid perovskites, highlighting our approach to enhancing their circularly polarized luminescence and nonlinear optical properties. Additionally, I will present our recent observations regarding the structural variations and their impact on spin relaxation processes in these materials, which we've studied using ultrafast pump-probe spectroscopy.

If monolayer materials can be further confined in an additional dimension, such as being compressed into one-dimensional structures, novel physical and chemical properties will emerge that go beyond what is observed in their bulk and monolayer counterparts. To achieve this objective, it is evident that a versatile technique is required to precisely manipulate the size, shape, and dimensionality of a wide range of two-dimensional materials. We present deterministic top-down fabrication methods that enable the efficient organization of monolayer transition metal dichalcogenide and graphene into one-dimensional periodic arrays, including regularly arranged nanobubbles and nanoribbons, with lateral dimensions in the nanometer scale and longitudinal dimension approaching millimeter. Compared with monolayers, the nanoribbons demonstrate increased sensitivity to strain, with distinct doping and conductivities on the edges compared with the center. The aligned one-dimensional bubbles exhibit unique directional transport properties, including charge and thermal transport, which are distinct from flat monolayers.

Since the discovery of monolayer ferromagnets, magneto-optics plays a compelling role in revealing new physics of magnetism in the extreme nanoscale limit. Here, I will first discuss our recent discovery of the ultra-sharp exciton emission in the van der Waals antiferromagnetic NiPS3 from bulk to atomically thin flakes. Magneto-optical measurements under in-plane field is used to reveal the strong coupling between the spin the electrical dipole oscillator, leading to the linear polarization of the exciton emission. We will further discuss the splitting of the spin-correlated emission in NiPS3 under in-plane magnetic field along various directions of the crystal, supporting the Zhang-Rice exciton origin of the emission. Benefiting from the spin-correlated emission in NiPS3, the Néel vector orientation can be optically detected as perpendicular to the exciton polarization, providing an easy, fast, nondestructive strategy to determine the Néel vector orientation. We further utilize the magneto-optic effect to reveal the three-state nematicity and domain evolution in NiPS3.

Organic semiconductors are an attractive class of semiconductor combining simple fabrication with the scope to tune properties by changing the structure. Their high oscillator strength and exciton binding energy make them attractive candidates for strong light-matter coupling, and polariton lasers operating at room temperature have been demonstrated. Many of these lasers operate at high thresholds (>100 uJ/cm2). We have explored fluorene oligomers and polymers as a route to much lower thresholds. We demonstrate low-threshold (<20 µJ/cm2) polariton lasing in a range of fluorene-based materials. Building on these results we have explored 1D and 2D lattices of polariton condensates. In the 2D case we observe a polariton condensate 2-3 µm from the pump spots, showing that polaritons can travel much further than excitons in organic semiconductors.

In this presentation, I will primarily discuss our recent results on the influence of strong coupling in polariton organic light-emitting diodes using time-resolved electroluminescence measurements [1]. We strategically decided to fabricate and examine metal-clad microcavity OLEDs consisting of the well-established polariton TDAF organic emitter because of its stable electroluminescent and high fluorescence PLQY with marginal ISC/RISC contribution. The latter is essential for allowing us to observe marginal polariton-induced TADF. Fitting our experimental data on a model of coupled rate equations that considered all major mechanisms contributing to delayed electroluminescence, we found that emission dynamics remained unmodified in the presence of strong coupling in our devices. In addition, I will discuss our recent efforts to transition to solution-based methods for fabricating polariton microcavities [2]. We fabricated DBR microcavities with Q near 100 by developing an automated deep-coating procedure. [1] Abdelmagid et. al., Nanophotonics, 8986, 1–9 (2024). [2] Palo et. al., The Journal of Physical Chemistry C 127, 14255 (2023)

The degree of strong exciton-photon coupling in organic microcavities is continuously tuned by engineering molecular transition dipole moment (TDM) orientation through the choice of thin film processing conditions. Microcavities based on amorphous organic thin films of 4, 4’-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) achieve ultrastrong coupling in a metal reflector microcavity with a Rabi splitting greater than 1.0 eV. By fabricating BSB-Cz optical microcavities as a function of substrate temperature during deposition, a ~20% variation in Rabi splitting is realized. This study adds a new axis for control over the strength of the exciton-photon interaction and polariton formation.

Novel optical materials based on oxide hydrate/poly(vinyl alcohol) hybrids are presented that have a readily tunable refractive index and can be solution-processed into photonic structures, including dielectric mirrors, gratings, and beyond. In planar microcavities, strong light–matter coupling is achieved at the target excitation. The agreement between classical electrodynamic simulations of the microcavity response and the experimental data demonstrates that the entire microcavity stack can be controllably produced as designed. Because of the versatility of the hybrid material used in these microcavities as high refractive index material, structures with a wide spectral range of optical modes might be designed and produced with straightforward coating methodologies, enabling fine-tuning of the energy and lifetime of the microcavities‘ optical modes to harness strong light–matter coupling in a wide variety of solution processable active materials.

Advances in time-resolved pump-probe spectroscopies have enabled us to follow the microscopic dynamics of quantum materials on femtosecond time scales. This gives us a glimpse into the inner workings of how complex, emergent functionalities of quantum many-body systems develop on ultrafast time scales or react to external forces. The ultimate dream of the community is to use light as a tuning parameter to create new states of matter on demand with designed properties and new functionalities, perhaps not achievable by other means. In this talk I will discuss recent progress in controlling and engineering properties of quantum materials through light-matter interaction. I will highlight work on Floquet engineering — the creation of effective Hamiltonians by time-periodic drives — on sub-cycle time scales combining theory and pump-probe experiments at the limits of energy and time resolution. I will then showcase recent theories on inducing superconductivity with light by employing enhanced light-matter interaction in the near-field involving polaritonic excitations.

We explore the effects of crystal structure and counterion position on the formation of polarons, strongly coupled polarons, and bipolarons using both spectroscopic and X-ray diffraction experiments and time-dependent density functional theory (TD-DFT) calculations. The counterion positions control whether two polarons spin-pair to form a bipolaron or whether they strongly couple without spin-pairing. When two counterions lie close to the same polymer segment, bipolarons can form, with an absorption spectrum that is blueshifted from that of a single polaron. Otherwise, polarons at high concentrations do not spin-pair, but instead J-couple, leading to a redshifted absorption spectrum. The counterion location needed for bipolaron formation is accompanied by a loss of polymer crystallinity, so that bipolarons can form only in disordered regions of conjugated polymer films. Our experiments and calculations also suggest that the ease with which charge carriers can be produced depends on the barrier to transforming the neutral polymer crystal structure into the doped structure that is able to incorporate the counterions.

Semiconducting polymers have broad device application potential, particularly when doped, either chemically or electrochemically, to improve conductivity. Carrier mobility in doped polymers is highly variable, however, and much of that variability stems from strong electrostatic attraction between dopants and their counter-ions. Here, we first explore how polymer and dopant structure can be used to mitigate that electrostatic attraction, considering the interplay between dopant size, polymer chain packing, polymer crystallinity, and doping mechanism. We next consider applications for doped conjugated polymers, focusing on their use as binders in lithium ion batteries. Battery binders are usually chosen only for chemical inertness, but adding electronic conductivity can improve battery cycling. If polymer doping energies are matched to the electrode material, highly conductive binders can be produced. By tuning the side chains, ionic conductivity can further be mixed with electronic conductivity, both of which are needed for fast battery operation.

In this work, charge pairs of varying separation distance are interpreted from photoinduced absorption detected magnetic resonance (PADMR) spectroscopy of organic, small molecule dilute donor/acceptor thin films. We report that a donor/acceptor film that generates few free charges at room temperature yet has a relatively large, calculated driving force for electron transfer generates a large concentration of tightly bound CT states when measured with PADMR. These states are markedly absent in films with smaller driving forces yet higher free charge yields which instead only show charge-separated state signals with weaker spin coupling. We interpret this result to be in support of a hypothesis where a larger reorganization energy associated with charge transfer to tightly bound CT states means that they are primarily generated in systems far from the Marcus optimum for free charge yield. And the highest free charge yielding systems instead predominantly undergo long-range charge separation into the acceptor host.