This PDF file contains the front matter associated with SPIE Proceedings Volume 13127, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

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.

Photochemical processes in complex molecules and materials are central to controlling the movement of charge and energy on the microscopic level. The ability to tailor the motion of charge through synthetic modification enables new material properties to be realized. Dynamic couplings and correlations occurring on ultrafast timescales between the electronic motions of the system and the microscopic structure of the system are key aspects to understand in order to discover new photoinduced and optoelectronic behaviors in molecules and materials. In nanoparticle systems and colloidal suspensions, the capping ligands can be used both to stabilize the material and to tune optoelectronic properties of the materials. This paper discusses studies using femtosecond transient-IR absorption spectroscopy and two-dimensional electronic-vibrational (2D EV) spectroscopy on Co-CN-Fe (i.e., Prussian blue analogue) nanoparticles with polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) stabilizing ligands. These mixed-valence, mixed-metal nanoparticle systems contain broad and complex electronic absorption features with different charge transfers accessible throughout the UV and visible spectral region. The cyano bridging ligand stretching vibration is a sensitive reporter of charge transfer dynamics that is used in this study to directly probe how photoinduced charge motion in 11 nm Co-CN-Fe nanoparticles occurs. The initial data discussed in this paper show different CN stretching dynamics and spectral features are observed that are capping ligand dependent. The results of this study suggest that ultrafast electronic-vibrational spectroscopies will be crucial methods to understand vibronic coupling dynamics in complex systems, such as nanoparticle-ligand materials.

Quantum dots absorbing mid-infrared light have been synthesized and electron dynamics induced by infrared field is examined with IR pump-probe (IRPP) spectroscopy. We observed the ultrafast electron relaxation dynamics of dodecanethiol-doped HgS CQDs. IR pump-probe experiments reported pump power-independent ultrafast 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-sublevel transition between the split states due to spin-orbit coupling was suggested. To further study these intermediate dynamics and the size-dependence of intra-band Auger process, we prepared HgSe CQDs with three different size and investigated intraband exciton dynamics with IR pump-probe spectroscopy. Surprisingly, we found that the oscillation signal, appearing around 10 ps, becomes more pronounced with the increase in CQD size. Furthermore, the observed oscillation signal shows the contribution of photoinduced absorption and excludes the previous assignment of oscillation with the increased bleaching signal from the refilling electrons after Auger process. To reveal the origin of oscillation signal, we employed two-color IRPP (2CIRPP) to track the electron dynamics within the whole intraband.

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 (V_loss). 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 V_loss to improve the efficiency of solar cell devices.

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.

Herein, we have studied interfacial charge generation and recombination in polymer solar cells and perovskite solar cells. First, we focused on 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 anions upon the photoexcitation of dye molecules. This finding shows that dye molecules should be located at the donor/acceptor interface in the ternary blend. We also discussed why dye molecules are spontaneously located at the interface. Next, we focused on interfacial charge recombination in perovskite solar cells. More specifically, we discussed how ambient-storage results in energy matching at the interface between perovskite and hole-transporting layers and hence can suppress interfacial charge recombination effectively. In addition, passivation to the interface also can suppress interfacial charge recombination even before the ambient-storage. In either case, open-circuit voltage is effectively improved because of suppressed interfacial charge recombination.

Supercapacitors are gaining significance owing to their remarkable ability to develop advanced energy storage devices. This paper uses a new approach to optimize the energy density of supercapacitors using molybdenum disulfide properties by simulating it using the convex optimization method in Python programming by optimizing its crucial parameters. This study provides valuable insights into the potential of this method as a tool to enhance the production 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.

Quantum dots (QDs) are nanomaterials with high luminous efficiency and tunable wavelengths, suitable for applications in lighting, displays, and solar cells. White light QDs feature both band-edge and surface-state emissions, with a broad full width at half maximum (FWHM), making them suitable for lighting applications. However, challenges persist in commercialization due to the low quantum yield (QY, <86%) and low device efficacy (approximately 11 lm/W). To address these issues, we utilized unsaturated fatty acid oleic acid (OA) and a non-coordinating solvent, 1-octadecene (ODE), to synthesize Zn_0.5Cd_0.5S QD at low temperatures, effectively preventing surface defect passivation. A significant finding in this study is the successful preparation of Zn_0.5Cd_0.5S white light QDs at low temperatures (120°C to 180°C). At 150°C, the band-edge and surface-state emission wavelengths were 411 nm and 525 nm, respectively, achieving a remarkable relative quantum yield (QY) of up to 175%, marking the highest relative QY among the white QDs. These QDs exhibit a broad emission, enabling single-material encapsulation. This simplifies the process and prevents color shifts in light-emitting diode (LED) caused by varying lifespans of QDs. The encapsulated QDs resulted in a warm white LED with CIE coordinates of (0.37, 0.40), a correlated color temperature (CCT) of 4298 K, a color rendering index (CRI) of 79, and the luminous efficacy of 14.7 lm/W. This research confirms the successful synthesis of QDs in low-temperature oil phases and their application in solid-state lighting.

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 and has potential to enhance (opto)electronic device performance. We examine how SF depends on molecular packing with functionalized tetracene (R-Tc) crystals which have the same monomer properties but different crystal packings with ‘slip-stack’ (R=TES) and ‘gamma’ (R=TBDMS) packing structures. Using temperature-dependent photoluminescence spectroscopy, we find that the triplet pair state (TT) in R-Tc systems under study is non-emissive, and the PL is dominated by that from lowenergy emissive trap states in TES-Tc and from aggregate states in TBDMS-Tc, with the emissive channels competing with SF. We also study the effects of photodegradation from endoperoxide formation on R-Tc and the relationship between photodegradation and SF and find that the ‘gamma’-packed TBDMS-Tc is more photostable than the ‘slip-stacked’ TESTc derivative. To analyze SF and competitive pathways, we constructed a 4-state kinetic model to reproduce the observed PL data, which predicts maximum SF free triplet yields of 190% for TES-Tc and 185% for TBDMS-Tc at room temperature.

The performance of organic donor-acceptor heterojunctions in solar cell devices is fundamentally determined by factors such as charge separation efficiency, charge carrier mobility, exciton diffusion lengths, and energy losses due to exciton recombination. Despite advancements in organic photovoltaic materials, exciton transport is frequently hindered by structural disorder, which limits overall device efficiency. Exciton-polaritons, formed through the strong coupling of cavity-coupled organic materials, exhibit delocalized states that enhance exciton transport and reduce the effects of disorder. Thus, it is of interest to elucidate the influence of strong coupling on charge transfer dynamics in organic photovoltaic materials. Using transient absorption spectroscopy, we explore the effect of cavity-strong-coupling in DBPC70 donor-acceptor blends integrated with a distributed Bragg reflector cavity. We find that the presence of the cavity slows the charge transfer process in such heterojunction systems.

Understanding singlet fission, a charge carrier multiplication process, and how the singlet fission and the competing processes can be manipulated with external parameters within the same material system is of considerable interest for enhancing optoelectronic device performance. Exciton polaritons, formed by strong exciton-photon coupling in organic films in microcavities, have been shown to manipulate the energy landscape that may be used to control the photophysics and photochemistry in existing singlet fission materials. To efficiently utilize exciton polariton formation to enhance singlet fission and suppress competing processes such as relaxation into low-energy trap states, it is necessary to establish how the properties of polaritons in singlet fission materials depend on molecular photophysics and microcavity configurations. We present a systematic study of strong coupling in 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 probe cavity-coupled and uncoupled molecular populations and examine the effects of intermolecular interactions on the excited state dynamics and polariton formation and properties. By varying magnetic field, we create different excited states relaxation scenarios and determine how the polariton states participate in the competition between the singlet fission and relaxation into trap states. We observe magnetic field-enhanced emission from exciton and polariton states and cavity-suppressed emission from low-energy trap states. 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.