Physical Chemistry of Semiconductor Materials and Interfaces XXIII

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.

Colloidal quantum dots (CQDs) are excellent semiconductor materials with unique properties, such as tunable electronic transitions achieved by exploiting the confinement effect and transition selectivity. Due to the wide spectral window realized by varying the size of the materials, CQDs have been intensively used for various applications, including displays, photovoltaics, bioimaging, etc. When an exciton is created in the CQD under photoexcitation, the electron and hole lie in the conduction and valence bands, respectively. Studying the separate dynamics of each charge carrier is critical but challenging. Here, we present the steady-state intraband transition, the unique optical property of self-doped quantum dots, and the methods to control the self-doping density of CQDs with direct and indirect methods.

There is increasing need to go beyond simple models of strong coupling and develop an understanding of the complex dynamics of exciton-polariton systems, including multiply excited exciton-polariton states and long-lived dark excitons. Experimental studies of an InGaAs quantum well embedded in a wedged microcavity are described in this talk, using both non-collinear and collinear polarization-dependent multidimensional coherent spectroscopy (MDCS). Using a noncollinear MDCS system, which uses three beams impinging at three different angles, we have observed a peak with polarization dependence and energy consistent with a biexciton that is nearly independent of cavity detuning. Its magnitude depends on the detuning, indicating that strong coupling plays an important role in the interaction. Using a collinear MDCS system, we can excite with three beams along the same wavevector, potentially simplifying the interaction enough to compare with simple theoretical models. We observe the biexciton peak and additionally, at certain negative values of the detuning, new peaks whose magnitude depends on excitation fluence that may arise from interaction with an optical weak (dark) state.

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.

Understanding the connection between photo-induced electronic motion on nanoparticle surfaces and the molecular-level vibrational motions of surface-bound ligands is important for developing tailored optical and photochemical properties in new materials. This talk will discuss current efforts for implementing multidimensional electronic-vibrational spectroscopies that may be capable of directly studying vibronic couplings between photoexcited material properties and molecular vibrational motions of ligands with femtosecond time resolution.

Quantum dots absorbing mid-infrared light have been synthesized and electron dynamics induced by infrared field examined with IR pump-probe (IRPP) spectroscopy. We observed the interesting electron relaxation dynamics of Dodecanethiol-doped HgS CQDs. IR pump-probe experiments reported pump power-independent fast decaying dynamics (1.2 ± 0.1 ps) accelerated by Auger process in CQDs with biexciton generation and slow decay process (>300 ps) caused by Phonon bottleneck for CQDs with single photon absorption. However, the origin of the observed intermediate component (20~60 ps) remains unclear although the inter-state transition between the split state due to spin-orbit coupling was suggested. For a further study of this intermediate dynamics and the size-dependence of the intra-band Auger process, we prepared HgSe CQDs with three different sizes and investigated intraband exciton dynamics with IR pump-probe spectroscopy.

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 stability of perovskite solar cells made of inorganic CsPbI3 is impressive, but the power conversion efficiency still needs improvement due to high open circuit loss (Vloss). This issue is often attributed to a mismatch in energy levels between the inorganic perovskite layer and the charge-selective contacts at the interface. Therefore, it is essential to understand the interface dynamics in these solar cells. Despite significant efforts since 2015 to address CsPbI3 crystallisation and phase stability at operational temperatures, a more comprehensive understanding of the interface is necessary to advance CsPbI3-based devices. We focus on presenting a detailed surface chemistry and interfacial dynamics analysis using spectroscopic and optical techniques. In this talk, we will have a comprehensive discussion on the effects of the annealing environment on the intrinsic properties of the interface. Additionally, we will discuss the implementation of interface engineering using dipole molecules to mitigate Vloss to improve the efficiency of solar cell devices.

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.

This talk will consider the future of metal halide perovskite (MHP) photovoltaic (PV) technologies as photovoltaic deployment reaches the terawatt scale. The requirements for significantly increasing PV deployment beyond current rates and what the implications are for technologies attempting to meet this challenge will be addressed. In particular how issues of CO2 impacts and sustainability inform near and longer-term research development and deployment goals for MHP enabled PV will be discussed. To facilitate this, an overview of current state of the art results for MHP based single junction, and multi-junctions in all-perovskite or hybrid configurations with other PV technologies will be presented. This will also include examination of performance of MHP-PVs along both efficiency and reliability axes for not only cells but also modules placed in context of the success of technologies that are currently widely deployed.

The recent advent of robust, refractory (having a high melting point and chemical stability at temperatures above 2000°C) photonic materials such as plasmonic ceramics, specifically, transition metal nitrides (TMNs), MXenes and transparent conducting oxides (TCOs) is currently driving the development of durable, compact, chip-compatible devices for sustainable energy, harsh-environment sensing, defense and intelligence, information technology, aerospace, chemical and oil & gas industries. These materials offer high-temperature and chemical stability, great tailorability of their optical properties, strong plasmonic behavior, optical nonlinearities, and high photothermal conversion efficiencies. This lecture will discuss advanced machine-learning-assisted photonic designs, materials optimization, and fabrication approaches for the development of efficient thermophotovoltaic (TPV) systems, lightsail spacecrafts, and high-T sensors utilizing TMN metasurfaces. We also explore the potential of TMNs (titanium nitride, zirconium nitride) and TCOs for switchable photonics, high-harmonic-based XUV generation, refractory metasurfaces for energy conversion, high-power applications, photodynamic therapy and photochemistry/photocatalysis. The development of environmentally-friendly, large-scale fabrication techniques will be discussed, and the emphasis will be put on novel machine-learning-driven design frameworks that leverage the emerging quantum solvers for meta-device optimization and bridge the areas of materials engineering, photonic design, and quantum technologies.

Artificial intelligence (AI) techniques have boosted the capability of optical imaging, sensing, and communication. Concurrently, photonics facilitate the tangible realization of deep neural networks, offering potential benefits in terms of latency, throughput, and energy efficiency. In this talk, I will discuss our efforts in AI photonics with two examples. The first involves employing a convolutional neural network for achieving single-shot full-field measurement of optical signals. The second example pertains to implementing a deep neural network with a multiple-scattering system featuring structural nonlinearity, thereby enabling nonlinear computations using linear optics.

With the proliferation of networked sensors and artificial intelligence, there is an increasing need for edge computing where data is processed at the sensor level to reduce bandwidth and latency while still preserving energy efficiency. In this talk, I will discuss how meta-optics can be used to implement computation for optical edge sensors, serving to off-load computationally expensive convolutional operations from the digital platform, reducing both latency and power consumption. I will discuss how meta-optics can augment, or replace, conventional imaging optics in achieving parallel optical processing across multiple independent channels for identifying, and classifying, both spatial and spectral features of objects.

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.

We report the design of an Extreme Ultraviolet microscope relying on Ultrafast Ptychographic Coherent Diffractive Imaging. This compact tool is capable of imaging the functional response of interfaces and heterogeneous nanomaterials activated by light pulses, across length scales, with high spatio-temporal resolutions, and with exquisite contrast to their chemical composition and to their morphology. Keywords: High-Harmonic Generation, EUV, microscopy, nanoparticles

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.

Supercapacitors are gaining significance owing to their remarkable ability to develop advanced energy storage devices. This paper uses a new approach of using convex optimization to optimize the energy density of a supercapacitor using molybdenum disulfide as a dielectric. The convex optimization is performed using Python programming. This study provides valuable insights into the potential of using this method as a tool to enhance the performance of supercapacitors, paving the path for the manufacturing of energy storage devices with smaller sizes and higher performance. The findings with the proposed model highlight the effectiveness of convex optimization in enhancing the performance of supercapacitors offering a robust approach to achieve higher energy density.

Zn0.5Cd0.5S quantum dots (QDs) were synthesized in low-temperature and non-coordinating solvent. QDs synthesized at 150°C exhibited band-edge and surface-state emission wavelengths of 411 nm and 525 nm, respectively, with a relative quantum yield (QY) of up to 175%. The encapsulated warm white LED achieved CIE coordinates of (0.37, 0.40), a CCT of 4298 K, a CRI of 79, and the luminous efficacy of 14.7 lm/W.

In this study, fatty acids with varying chain lengths were utilized to modify nucleation and growth rates through steric effects, aiming to prepare high-quality green InP quantum dots (QDs). Uniform particle size distribution and small size of InP QDs can be obtain by using palmitic acid and stearic acid. The sample with optimal performances exhibits an emission wavelength of 492 nm, a full half-maximum width of 39 nm, and a maximum quantum efficiency of 87%. The quantum efficiency increases with the lengthening of the fatty acid chain. This trend is beneficial for the application of InP QDs in the display industry, as higher quantum efficiency enhances the performance and potential applications of these QDs in display technologies.

Singlet fission (SF) is a charge carrier multiplication process that can occur in organic semiconductors, which has potential to enhance (opto)electronic device performance. We examine how SF depends on molecular packing with functionalized tetracene (R-Tc) crystals which have similar monomer properties but different crystal packing with ‘slip-stack’ (R=TES, TMS, tBu) and ‘gamma’ (R=TBDMS) motifs. Using temperature and magnetic field dependent photoluminescence spectroscopy, we find that the 1(TT) state in R-Tc systems under study is non-emissive, and the emission is dominated by that from low-energy emissive trap states ‘Sx’ in ‘slip-stack’ derivatives and from J-aggregate S1 states in ‘gamma’ TBDMS-Tc, with the emissive channels competing with SF. We also report on magnetic field-dependent photodegradation in R-Tc and the relationship between photodegradation and SF and find that the 'gamma’-packed TBDMS-Tc is more stable than the ‘slip-stacked’ derivatives.

Chemical reactions can scale from diatoms to multi-atomic biomolecules. Large molecules have 3N-5/3N-6 degrees of freedom. However, as we go from their configurational space to their reactive space, the motions that drive a reaction are only a few. Thus, understanding a chemical reaction largely boils down to understanding the vibrations that participate in a reaction. We use time-resolved optical spectroscopy and ultrafast electron diffraction to capture the motions that couple to the reaction trajectory of a photo-induced formation of chemical bond in a carefully designed dimetallic Platinum complex.

The efficiency of organic donor-acceptor heterojunctions in solar cell devices relies on factors such as charge separation, transport mobility, exciton diffusion lengths, and energy losses from exciton recombination. Despite significant improvements in organic photovoltaic materials, a disorder often impedes exciton transport, hindering device efficiency. Recently, long-range exciton propagation of nearly 100 μm was observed in strongly cavity-coupled organic materials forming quasiparticles known as exciton-polaritons. The delocalized nature of polaritons facilitates the efficient transport of excitons; nonetheless, the rapid decay kinetics impose temporal constraints on subsequent charge transfer events. A promising candidate is the non-dissipative Bloch Surface Wave (BSW) in distributed Bragg reflectors (DBRs), offering high exciton-polariton velocities and extended propagation lengths. Here, we investigate a DBP-C70 donor-acceptor bilayer/blend system on a DBR cavity, exploring the effects of strong coupling on the charge transfer dynamics using transient absorption spectroscopy.

Understanding the molecular doping in organic semiconductors is crucial for the operation of organic electronic devices, including field-effect transistors, solar cells, thermoelectrics and bioelectronics. In this study, we investigated the dip-doping solution method to an efficient polymer, bithiophene-thiophene (Pg32T-T) copolymer with an oligo ethylene glycol side chain. At controlled doping levels, we demonstrate pure integer charge transfer (ICT) in 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ)-doped Pg32T-T. AC Hall measurement was used to characterize charge transport properties, revealing high carrier concentration and mobility in F4TCNQ-doped Pg32T-T. To further explore the understanding of the doping mechanism, we performed the UV-visible/NIR absorption spectroscopy, temperature-dependent measurement, Electron Paramagnetic Resonance (EPR), and Secondary Ion Mass Spectrometry (SIMS). Results obtained elucidate changes in charge transport properties induced by doping. The implications of these observed mechanisms are also discussed for the future development of efficient doping methods for organic semiconductors.

Understanding how exciton polariton formation can be used to manipulate singlet fission and competing processes is of considerable interest for enhancing optoelectronic device performances. We present a systematic study of strong coupling in a benchmark singlet fission material, functionalized tetracene (R-Tc), and how it affects its photophysics and photochemistry depending on film morphology, placement in the cavity (to achieve various degrees of overlap with the cavity electric field), and cavity design. We observe magnetic field-enhanced emission from exciton and polariton states and cavity-suppressed emission from low-energy trap states and establish how these processes depend on the film properties and cavity characteristics. We also report on effects of polariton formation on photodimerization of R-Tc and discuss how concurrent studies of photochemistry and photophysics promote understanding of singlet fission and polariton formation through the evolution of excited states during photodegradation.

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.

Solution-processing of Ruddlesden-Popper lower-dimensional (LD) perovskites have inherent limitations that lead to heterogeneous crystallization of several LD phases, inhibiting charge transport and introducing electronic defects. Thermal co-evaporation of LD perovskites is an alternative, scalable, solvent-free deposition route for the fabrication of such layers, allowing a high degree of control on layer thickness and conformality, and limiting competing crystallization pathways. In this work, the co-evaporation deposition of such materials is demonstrated for phenethylammonium-based 2D and quasi-2D perovskite layers. Co-evaporation yields crystalline, phase-pure and homogeneous layers that are coated conformally on the underlying surface. We demonstrate their use in perovskite solar cells as interfacial passivation layers, which boost performance efficiencies by 1.5% absolute compared to their solution-processed counterparts. We furthermore study the crystallization and charge-carrier dynamics of quasi-2D layers and find that a templated growth using a phosphonic acid surface modifier increases the charge-carrier lifetime in the quasi-2D phase.

The impact of energy disorder to the delocalization character of vibrational polaritons in the strong-coupling regime is explored by numerical simulations of the Tavis–Cummings model. We concluded the Rabi splitting must be substantially larger than the inhomogeneous linewidth to achieve the delocalization character of polaritons. We identified a critical ratio of 4 between the collective Rabi splitting and the inhomogeneous linewidth that distinguishes delocalized from localized polaritons. This result provides a much tighter criterion for strong coupling than conventionally accepted. Our findings revealed the sensitivity of delocalization to energy disorder. Additionally, dynamic simulations predicted faster decoherence with increasing inhomogeneous linewidths, aligning with the decreasing degree of polariton delocalization.

Doping conjugated polymers (CPs) with molecular dopants is essential for advancing organic electronics, yet achieving stable high doping efficiency and rational co-design of dopant and CP remain challenging. Here, we investigate the effects of sidechain chemistry (alkyl vs. oligo(ethylene glycol) - OEG) and energy level alignment on the miscibility of dopants and polymers and its relation to doping efficiency. Using UV-vis absorption spectroscopy, cyclic voltammetry, and ToF-SIMS analyses, we reveal the crucial role of charge transfer and electrostatic interactions as driving forces for higher effective miscibility and interdiffusion of dopant molecules into CP films. This study offers insights into co-designing CP-dopant systems for enhanced performance and high doping efficiency

Perhaps the single most important problem confronting the development of OLED displays and lighting today is how to achieve sufficiently long triplet-controlled emission device lifetime to prevent rapid color change during operation, while achieving 100% internal emission efficiency. It has been shown1 that bimolecular (e.g. triplet-polaron, triplet-triplet) annihilation provides a source of energy sufficient to destroy the blue triplet chromophore (whether a phosphor or a TADF molecule) or its host. Since that time, many materials, structures and strategies to extend blue emission lifetime based on this understanding have been demonstrated. Furthermore, various molecular fragments have been identified whose presence leads to the observed luminance loss. Unfortunately, a fully satisfactory solution has not been shown where blue triplet emitter lifetime is sufficient to meet the standards of high performance displays, although white OLED illumination sources may now have adequate lifetime to meet industry standards. In this talk I will discuss progress in extending blue phosphorescent OLED (PHOLED) lifetime, and in understanding of the limitations to extending the lifetime of blue triplet emitters. In particular, I will focus on the relationship between radiative state lifetime, exciton density, and the longevity of the PHOLED. I will review efforts that have resulted in increasing the deep blue phosphorescent longevity by at least 14 X via emitter design, polaritons, and optical cavity engineering. Prospects for future advances will be discussed. 1. “Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions”. N.C. Giebink, B.W. D’Andrade, M.S. Weaver, P.B. Mackenzie, J.J. Brown, M.E. Thompson, and S.R. Forrest, J. Appl. Phys., 103, 044509 (2008).

Commercial carbazole (Cz) has been widely used to synthesize organic functional materials, which are entwined with recent breakthroughs in ultralong organic phosphorescence, thermally activated delayed fluorescence, organic luminescent radicals, and organic semiconductor lasers. Recently, we discovered that different from commercial Cz, the fluorescence of lab-synthesized-Cz (Lab-Cz) is blue-shifted by 54 nm and the well-known room-temperature ultralong phosphorescence almost disappears. Detailed studies reveal the presence of a Cz isomer as the impurity, which is widespread in commercial Cz sources with <0.5 mol%. Ten representative Cz derivatives were resynthesized from the Lab-Cz and all failed to show the reported ultralong phosphorescence in the same crystal states. However, even 0.1 mol% isomer doping can recover the reported ultralong phosphorescence. The presence of the isomer in commercial carbazole triggers us to re-examine the structure-property of many optically active materials with important discoveries.

Fluorescence-based sensing has the potential for sensitive (trace), rapid and selective detection of chemical threats and is compatible with low power portable detectors that can be used in the field by military personnel, first responders, healthcare workers and those tasked with environmental monitoring. Chemical threats can include illicit drugs, toxic industrial chemicals, pesticides improvised explosive devices, and chemical warfare agents. This presentation will use practical examples to introduce different modes of fluorescence sensing, illustrate the key issues relating to solid-state detection of chemical vapours, and multivariate strategies to achieve selective chemical threat detection.

Solar radiation will be the largest single source of electricity in a low-carbon future. To maximise the potential of solar power, new materials will be needed to harvest and convert solar energy alongside existing photovoltaic technologies. Molecular electronic materials, such as conjugated polymers and molecules, can achieve photovoltaic conversion through a process of photon absorption, charge separation and charge collection. The materials are appealing because of the potential to tune their properties through chemical design and their compatibility with high-throughput manufacture. They are also interesting model systems for photochemical energy conversion because of their parallels with natural photosynthesis. Through a remarkable series of advances in materials design, the efficiency of photovoltaic energy conversion in molecular materials has risen from 1% to around 20% within two decades, surpassing most predictions. We will discuss the factors that control the function of molecular solar cells including the nature of the charge separating heterojunction, and the impact of chemical and physical structure on phase behaviour, energy and charge transport, light harvesting, and loss pathways. Finally, we will address the limits to conversion efficiency in such systems.

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.

First, I will discuss interfacial charge generation in ternary blend polymer solar cells based on a conjugated donor polymer, a fullerene derivative acceptor, and a near-IR dye molecule. On the basis of transient absorption analysis, we found rapid charge generation of polymer polarons and fullerene anion upon the photoexcitation of dye molecules, suggesting that dye molecules should be located at the donor/acceptor interface in the ternary blend. Next, I will discuss how energy matching and passivation at the interface between perovskite and hole-transporting layers can suppress interfacial charge recombination effectively. In either case, open-circuit voltage is effectively improved because of suppressed interfacial charge recombination.

A next target in photovoltaic energy conversion can possibly be met by developing perovskite triple or even quadruple junction solar cells. These require developing stable perovskite sub-cells with bandgaps in the range of 1.8 to 2.3 eV, i.e., a range that has not received much attention so far. Guided by photocurrent spectroscopy and absolute photoluminescence spectroscopy, in combination with bulk and interface passivation strategies, tandem and triple junction solar cells with a power-conversion efficiency of 26% have been reached. Photoluminescence of individual sub-cells provides information on the internal voltage in each absorber layer and offers a detailed understanding of the performance-limiting components in the tandem solar cell following prolonged continuous operation.

Solution-processed light-emitting diodes (LEDs) are attractive for applications in low-cost, large-area lighting sources and displays. Organometal halide perovskites can be processed from solutions at low temperatures to form crystalline direct-bandgap semiconductors with intriguing optoelectronic properties, such as high photoluminescence yield, good charge mobility and excellent color purity. In this talk, I will present our effort to boost the efficiency of perovskite LEDs to a high level which is comparable to organic LEDs. More importantly, organic LEDs are difficult to maintain high efficiency at high current densities due to their excitonic nature and low charge mobilities. Low temperature solution-processed perovskite LEDs demonstrate remarkably high efficiency at high current densities, suggesting unique potential to achieve large size planar LEDs with high efficiency at high brightness.

Microcavity exciton-polaritons are bosonic quasiparticles that result from the hybridization of excitons and modes of a confined electromagnetic field in a regime known as strong light-matter coupling. Having a low effective mass, polaritons can undergo condensation, the macroscopic occupation of the lowest energy and momentum state. Two-dimensional (2D) perovskites are promising candidates for polariton condensation due to their high exciton binding energies, low non-radiative recombination rates and strong oscillator strengths. However, despite their optimal optoelectronic properties, there are no reports of room temperature polariton condensation in 2D perovskites and only one unreproduced report at low temperature. In this study, we systematically examine the interplay between the emission from the exciton reservoir and the population of the lower polariton. We gain insights on how the spectral features of the emission of 2D perovskites affect polariton relaxation and onto one of the mechanisms making polariton condensation challenging in 2D perovskites.