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This dissertation focuses on the interrogation of the photoacid sensitization (Förster) cycle to understand mechanisms for light-to-protonic energy conversion. I define protonic species as those capable of undergoing proton-transfer reactions, including H2O(l), H+(aq), OH-(aq), and Brønstead–Lowry (conjugate) acids/bases. The central theme of this work is relating operating principles between traditional electronic photovoltaics (electron/holes) and protonic photovoltaics (OH-/H+) including charge carrier generation and conduction with the goal of developing design strategies for efficient light-to-ionic power conversion. This is achieved by investigating ground-state proton transfer processes in the Förster cycle using electrochemical and spectroscopic techniques and leveraging this knowledge to understand photo-initiated ion transport in ion-exchange membranes.Transient absorption spectroscopy studies of aqueous photoacids provided insight into the protonic species generated during the Förster cycle and the inefficiencies of current state-of-the-art photoacids for generating large photoresponses in photoacid-modified ion-exchange membranes. An abundance of ultrafast spectroscopy data exist that revealed mechanistic details for excited-state proton transfer, but there was limited information regarding the mechanisms for the subsequent ground-state proton transfer. Proton acceptance in the excited-state and proton donation in the ground-state from H2O(l) in the Förster cycle results in the transient generation of H+ and OH-, analogous to photogeneration of electrons and holes in electronic photovoltaics. An obstacle for realizing this ideal mechanism is kinetics. These were analyzed via a systematic study using various photoacids by varying the concentration of deprotonated photoacids and protons, photoacid ground-state acidity, solution acidity, and total photoacid concentration.Photoacids were covalently bonded to ion-exchange membranes to assess the impact that their photochemical mechanisms had on their ability to exhibit photovoltaic action. Existing methodology for four-probe electrochemical measurements was revamped to produce reproducible signals and temporally stable baselines. The key factor was eliminating the introduction of external electrolyte from reference electrodes filled with saturated internal electrolyte by replacing those electrodes with reference electrodes immersed directly into the electrolyte in contact with the photoacid-modified ion-exchange membranes. This introduced experimental constraints that required physical modification of the experimental apparatus as well as deconvolution of the Nernst electrode potential from the membrane potential. Implementation of this new setup resulted in the observation of a “reverse” photovoltage, meaning that it was opposite in sign to photovoltages measured in traditional electronic photovoltaics and that observed in our group in all prior studies of these systems. A series of control experiments was performed, and a mechanism coined “electrolyte crossover induced bulk membrane polarization” was hypothesized to explain this reverse photovoltage.

Singlet fission (SF) has attracted much attention on account of its great potential for applications in high efficiency solar energy conversion. The major roadblock to realize this potential is roo...

Transient spectroscopies are sensitive to charge carriers released from trapping centres in semiconducting devices. Even though these spectroscopies are mostly applied to reveal defects causing states that are localised in the energy gap, these methods also sense-charge from quantum wells in heterostructures. However, proper evaluation of material response to external stimuli requires knowledge of material properties such as electron effective mass in complex structures. Here we propose a method for precise evaluation of effective mass in quantum well heterostructures. The infinite well model is successfully applied to the InGaAsN/GaAs quantum well structure and used to evaluate electron effective mass in the conduction and valence bands. The effective mass m/m0 of charges from the conduction band was 0.093 ± 0.006, while the charges from the valence band exhibited an effective mass of 0.122 ± 0.018.

We theoretically study the dynamics of charge transfer induced by femtosecond laser-pulse excitation. Models involving coupled electronic states of symmetrically bridged organic mixed-valence molecules are investigated, where the motion proceeds along two reaction coordinates. Linear absorption spectra of two species that differ in the energetical position of the bridge, relative to acceptor and donor states, are determined and compared to experimental results. From the wave packet dynamics it emerges that relaxation dominates the charge transfer. This behavior is reflected in transient absorption spectra, which are obtained from a directional decomposition of the time-dependent polarization. Due to the nature of the coupled dynamics the extraction of the relevant contributions needs an extension of well-known techniques for the decomposition.

Unabated, worldwide trends in CO2 production project growth to > 43-BMT per year over the next two decades. Efficient power electronics are crucial to fully realizing the CO2 mitigating benefits of a worldwide smart grid (~ 18% reduction for the United States alone). Even state-of-the-art SiC high voltage junction devices are inefficient because of slow transition times (~ 0.5-μs) and limited switching rates at high voltage (~ 20-kHz at ≥ 15-kV) resulting from the intrinsically limited charge carrier drift speed (7-cm-s−1). Slow transition times and limited switch rates waste energy through transition loss and hysteresis loss in external magnetic components. Bulk conduction devices, where carriers are generated and controlled nearly simultaneously throughout the device volume, minimize this loss. Such devices are possible using below bandgap excitation of semi-insulating (SI) SiC single crystals. We explored carrier dynamics with a 75-fs single wavelength pump/supercontinuum probe and a modified transient spectroscopy technique and also demonstrated a new class of efficient, high-speed, high-gain, bi-directional, optically-controlled transistor-like power device. At a performance level six times that of existing devices, for the first time we demonstrated prototype operation at multi-10s of kW and 20-kV, 125-kHz in a bulk conduction transistor-like device using direct photon-carrier excitation with below bandgap light.

Transient Absorption Spectroscopy (TAS) is a pump probe technique with the ability of directly probe the photogenerated species and their evolution with time. This technique consists in two light sources, one of them generates the photoexcited species while the other one probe those as-generated species. TAS allows to study energy transfer dynamics in a wide range of materials (organic molecules, metal complexes, inorganic materials, etc) in a wide range of media (solution, solid state, etc). Following the School spirit of this ‘Escuela de Materiales Moleculares’ this talk is going to explain the capabilities and functioning of TAS. This will include both, picosecond and microsecond TAS, with their differences in setup and applicability. This talk will focus on the application of this technique in the study of organic materials used to generate energy. Among the variety of the studied examples, there will be small molecules donors, diradicals materials. And the information that TAS is able to obtain from those. RSEQ. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.

This dissertation presents a series of research projects designed and carried out to elucidate the physical chemistry and assess the atmospheric relevance of (1) carbonyl oxide radicals (i.e., Criegee intermediates) produced in alkene ozonolysis and (2) nitrous oxide (N2O) produced in lightning-induced corona discharges. The results provide UV absorption spectra and reaction rate coefficients for Criegee intermediates that will help constrain the formation and loss pathways of aerosol nucleation precursors such as H2SO4 and oxidized volatile organic compounds, and the isotopic signature of N2O formed in lightning that can help distinguish various N2O sources in atmospheric measurements.Criegee intermediates are byproducts of the reaction of alkenes with ozone. Bimolecular reactions of Criegee intermediates can lead to the production of low-volatility organic compounds and acids in the atmosphere, which in turn play a role in determining the concentration, size, and optical properties of aerosols. Recently, a novel method for producing measurable quantities of stabilized Criegee intermediates in the laboratory paved the way for the development of new experimental techniques to study their chemical properties and predict their importance in the atmosphere. For this dissertation, a unique apparatus combining time-resolved UV absorption in a flow cell with laser depletion in a molecular beam was adapted to obtain the absolute absorption spectrum of CH3CHOO with high resolution and accuracy relative to previous spectral measurements by other groups. The resulting absorption cross sections imply a photolysis lifetime of about seven seconds in the atmosphere, long enough for CH3CHOO to participate in unimolecular and bimolecular reactions. The broad absorption band with weak structure in the long-wavelength region of the spectrum represents a “spectral fingerprint” for identifying CH3CHOO in future studies, and the cross sections provide valuable benchmarks for theory to characterize electronically excited states of CH3CHOO.The fast reaction of CH2OO with water dimer is thought to dominate CH2OO removal in the atmosphere. However, reaction rates can vary considerably under different conditions of temperature, humidity, and pressure. A temperature-controlled flow cell was designed to measure the transient absorption of CH2OO and obtain rate coefficients for its reaction with water dimer from 283 to 324 K. The rate of the reaction of CH2OO with water dimer was found to exhibit a strong negative temperature dependence, pointing to the participation of a hydrogen-bonded pre-reactive complex between CH2OO and two water molecules. Due to the strong temperature dependence, and shifting competition between water dimer and water monomer (which has a positive temperature dependence), the effective loss rate of CH2OO by reaction with water vapor is highly sensitive to atmospheric conditions. The role played by the stable pre-reactive complex suggests that similar complexes could form between water dimer and other larger Criegee intermediates, and that the stability and relative energy of these complexes control the reaction rate with water and its temperature dependence.Effective loss rates of Criegee intermediates due to bimolecular reactions in the atmosphere are limited by their rates of unimolecular decomposition. The rates of decomposition depend strongly on the molecular geometry, which affects the accessible isomerization pathways and dissociation products. (CH3)2COO is the main product of tetramethylethylene ozonolysis, and has been found to react slowly with water dimer and rapidly with SO2. While CH2OO decomposes slowly via isomerization to dioxirane, (CH3)2COO may decompose faster via intramolecular hydrogen transfer to form vinyl hydroperoxide. Fast (CH3)2COO decomposition could affect the significance of the Criegee intermediate H2SO4 source, as well as the non-photolytic production of OH radicals. In this dissertation, measurements of the transient absorption of (CH3)2COO to obtain thermal decomposition rate coefficients from 283 to 323 K by extrapolating the observed loss rate to zero concentration are reported. The rate of unimolecular decomposition is ~400 s-1 at 298 K and varies by nearly an order of magnitude within the studied temperature range. The effective loss rate of (CH3)2COO in the atmosphere due to thermal decomposition is thus competitive with its loss due to reaction with water vapor and with SO2, suggesting that the unimolecular decomposition pathway is a significant sink for (CH3)2COO and possibly other di-substituted Criegee intermediates, and should be included in models of Criegee chemistry in the atmosphere as well as in kinetic models of tetramethylethylene ozonolysis.N2O is the third most important greenhouse gas after CO2 and methane, and is mainly emitted to the atmosphere as a byproduct of microbial activity in soils. The expanding use of nitrogen-containing fertilizers in agriculture has led to an increase in N2O atmospheric concentrations since preindustrial times. Isotopic measurements are a valuable tool to distinguish the influence of different sources of N2O, but the isotopic composition of N2O formed from corona discharge in lightning has not previously been measured. Here, a corona discharge cell apparatus was used to generate a corona discharge in flowing or static zero air, and the N2O formed at discharge cell pressures from ~0.1 to 10 Torr and discharge voltages from 0.25 to 5 kV was collected and measured with isotope ratio mass spectrometry to determine its isotopic composition. The results show enrichments in 15N of N2O up to 32‰ relative to the reactant N2, and even larger enrichments in 15N of up to 77‰ at the central nitrogen atom. Large depletions in 18O as large as -71‰ relative to reactant O2 were also measured. The isotopic composition measured here may help to elucidate the chemical mechanisms leading to N2O formation and destruction in a corona discharge. Furthermore, the isotope-isotope relationships of the N2O produced in the corona discharge experiments are distinct from those of N2O from other sources, implying that isotopic measurements can be used to determine whether local variations in the atmospheric concentration of N2O – e.g., the enhanced N2O levels recently measured in the upper tropical and subtropical troposphere – are due to lightning activity, soil emissions, or biomass burning.

Minerals and metals serve important roles in the organic geochemistry of natural environments. Mobility of organics, catalysis of degradation, and redox catalysis are among the processes affected by minerals. With the addition of ultraviolet light a new suite of photo-induced redox reactions is possible including reductive and oxidative ligand-to-metal/mineral charge transfer. Such reactions allow for novel chemistry that has relevance to the modern Earth as well as the pre-biotic origin of life.This thesis describes processes by which electrons transfer between minerals/metals and organic ligands relevant to natural systems as well as the origins of life. I present evidence of ultrafast electron transfer and the production of radical intermediates essential to deducing redox reaction mechanisms. I also present methods for communicating understanding of interfacial chemistry to the public that promote engagement in science. This thesis is broadly applicable to those interested in mineral organic photochemistry, electron transfer, the origin of life, and science teaching methods.I probed the chemistry between organic molecules and minerals/metals, using pump/probe transient absorption (TA) spectroscopy to observe the dynamics of electrons and vibrational modes at timescales ranging from picoseconds to nanoseconds. This technique can be conducted in solution and can be highly sensitive to intermediate reaction products.I examined the photolysis of the metal carboloto, ferric oxalate, under UV irradiation using mid-infrared TA spectroscopy in both D2O and H2O. Ferric oxalate is a model molecule for natural systems and is used to measure photo flux due to its well-characterized quantum efficiency. However, the mechanism of its photolysis is debated. This was the first time the intermediates of ferric oxalate photolysis were observed using techniques sensitive to the vibrational states of organic molecules. I observed the rapid intramolecular charge transfer and the production of CO2 and tentatively CO2•–. Additionally, we observed intermediate states that we interpret to be CO2 disassociating from ferrous iron, a signature never before reported. Investigations of photo-induced electron transfer were expanded to ZnS nanoparticles and fumarate. Fumarate is an intermediate metabolite in the tricarboxilic acid (TCA) cycle, which is a part of core metabolism in modern organisms. It undergoes a two-electron reduction to form succinate. Reductive versions of the TCA cycle may have been important for the origin of prebiotic metabolism. I measured the effect of adsorbed fumarate on the electronic states of photo-excited ZnS and observed electron transfer both at short (1 ns) timescales. Additionally, I observed an electronic signature tentatively attributed to fumarate radical, which persisted for at least 8 nanoseconds. The appearance of a long-lived radical intermediate product and the rapid initial electron transfer from the mineral to the organic suggests that ZnS could be a viable catalyst for prebiotic metabolism on the early Earth.To better educate students on the importance of mineral surface chemistry I designed and implemented a classroom experiment wherein students performed electrolysis of water using mineral electrodes. The experiment emphasized both the mineral catalysis and mineral redox chemistry, which occur at the solid/liquid interface. Concepts in interfacial chemistry are often difficult to exhibit, making this teaching tool unique and useful. Students were guided through a set of investigations and constructed their understanding through observations and sharing of ideas. The experiment was successfully implemented in a college level mineralogy course.

Strong light–matter coupling to form exciton– and vibropolaritons is increasingly touted as a powerful tool to alter the fundamental properties of organic materials. It is proposed that these states and their facile tunability can be used to rewrite molecular potential energy landscapes and redirect photophysical pathways, with applications from catalysis to electronic devices. Crucial to their photophysical properties is the exchange of energy between coherent, bright polaritons and incoherent dark states. One of the most potent tools to explore this interplay is transient absorption/reflectance spectroscopy. Previous studies have revealed unexpectedly long lifetimes of the coherent polariton states, for which there is no theoretical explanation. Applying these transient methods to a series of strong-coupled organic microcavities, we recover similar long-lived spectral effects. Based on transfer-matrix modeling of the transient experiment, we find that virtually the entire photoresponse results from photoexcitation effects other than the generation of polariton states. Our results suggest that the complex optical properties of polaritonic systems make them especially prone to misleading optical signatures and that more challenging high-time-resolution measurements on high-quality microcavities are necessary to uniquely distinguish the coherent polariton dynamics.

The formidable rise of lead‐halide perovskite photovoltaics has energized the search for lead‐free perovskite‐inspired materials (PIMs) with related optoelectronic properties but free from toxicity limitations. The photovoltaic performance of PIMs closely depends on their defect tolerance. However, a comprehensive experimental characterization of their defect‐level parameters—concentration, energy depth, and capture cross‐section—has not been pursued to date, hindering the rational development of defect‐tolerant PIMs. While mainstream, capacitance‐based techniques for defect‐level characterization have sparked controversy in lead‐halide perovskite research, their use on PIMs is also problematic due to their typical near‐intrinsic character. This study demonstrates on four representative PIMs (Cs3Sb2I9, Rb3Sb2I9, BiOI, and AgBiI4) for which Photoinduced Current Transient Spectroscopy (PICTS) offers a facile, widely applicable route to the defect‐level characterization of PIMs embedded within solar cells. Going beyond the ambiguities of the current discussion of defect tolerance, a methodology is also presented to quantitatively assess the defect tolerance of PIMs in photovoltaics based on their experimental defect‐level parameters. Finally, PICTS applied to PIM photovoltaics is revealed to be ultimately sensitive to defect‐level concentrations

I present a fully abinitio scheme to model transient spectroscopy signals in presence of strongly bound excitons. Using LiF as a prototype material, I show that the scheme is able to capture the exciton signature both in time-resolved ARPES and transient absorption experiments. The approach is completely general and can become the reference scheme for modelling pump and probe experiment in a wide range of materials., Comment: 6 pages, 5 figures plus supplemental material

Electronic traps are the primary factor stifling the performance of quantum-dot (QD) solar cells to nearly half their theoretical potential. Yet, the exact origin of these traps remains largely unknown, making it difficult to address the problem. In the inaugural issue of Matter, Gilmore et al. employ advanced transient spectroscopy to reveal that QD dimerization can be as detrimental as unpassivated surface states in QD films.

Device structures with silicon or germanium nanodots embedded in an oxide matrix may find application in the field of Non-Volatile Memories (NVMs) [1,2] or photonics [3,4]. In the former case, the nanodots can become charged by tunneling of electrons or holes from the silicon substrate through a tunnel oxide. Successful operation requires that the charge does not leak away by the assistance of traps or defects in the material stack. The latter can be probed by techniques like Admittance Spectroscopy [5] or Deep-Level Transient Spectroscopy (DLTS) [2,5]. In the present work we report on a systematic investigation of Metal-Oxide-Semiconductor (MOS) capacitors fabricated on n-type Czochralski silicon substrates and containing Ge nanodots with different size. Ge nanodots with average diameter of 1, 2 or 3 nm have been deposited by Molecular Beam Epitaxy (MBE) on a 5 nm tunnel oxide, thermally grown on n-type Czochralski silicon wafers. An Atomic Force Microscopy (AFM) picture is shown in Fig. 1. Next, 50 nm of SiO2 is deposited. MOS capacitors are prepared by thermal evaporation of 2 mm diameter Al gate contacts, while InGa+In foil ohmic contacts are prepared on the silicon substrate side. The device structure is schematically shown in Fig. 2. Capacitance-Voltage (C-V) characterization is performed at a fixed frequency f=1 MHz, using both a forward and reverse gate voltage sweep. As can be seen in Fig. 3, the presence of the Ge nanodots has a pronounced impact on the C-V characteristics: there is a shift towards positive flat-band voltage, compared with the zero dot reference, a strong increase of the accumulation capacitance and, for the nanodot samples, the hysteresis increases with the increase in nanodot size. For temperature (T-) scan DLTS, the capacitors are mounted in a helium contact gas cryostat cooled with liquid nitrogen. Measurements at different bias pulses from reverse bias (VR) to the pulse bias (VP) are performed in parallel in order to probe the electron traps in different parts of the structure. An example is given in Fig. 4, for the 1 nm nanodots sample. It shows that in deep depletion, there is a broad peak between 150-250 K which could stem from the response of filled interface traps. The increase found at room temperature may originate from minority carrier generation in the depletion region [2,5,6]. When probing closer to the interface, for more positive VR, a pronounced but broad peak around 200 K dominates the spectra. From the Arrhenius plot shown in Fig. 5, it is concluded that it corresponds to the dangling bond (Pb) acceptor level at about 0.31 eV from the conduction band [2,5,6]. Maximum densities are estimated in the range of 5x1011 cm-2eV-1. A comparison of the spectra near the interface for the different samples studied is provided in Fig. 6 and will be discussed in more detail in the conference paper. References [1] S. Tiwari, H. Hanafi, A. Hartstein, E.F. Crabbe and K. Chan, Appl. Phys. Lett., 68, 1377 (1996). [2] R. Beyer, J. von Borany and H. Burghardt, Microelectron. Eng., 86, 1859 (2009). [3] J. von Borany, R. Grötzschel, K.H. Heinig, A. Markwitz, W. Matz, B. Schmidt and W. Skorupa, Appl. Phys. Lett., 71, 3215 (1997). [4] D.A. Grachev, A.V. Ershov, A.V. Belolipetsky, L.V. Krasilnikova, A.N. Yablonskiy, B.A. Andreev and O.B. Gusev, Phys. Stat. Sol. A, 213, 2867 (2016). [5] E. Simoen, J. Lauwaert and H. Vrielinck, Semiconductors and Semimetals, Eds. L. Romano, V. Privitera and C. Jagadish, 91, pp. 205-250, Elsevier 2015. [6] S.N. Volkos, E.S. Efthymiou, S. Bernardini, I.D. Hawkins, A.R. Peaker and G. Petkos, J. Appl. Phys., 100, 124103 (2006). Figure 1

Deep-level transient spectroscopy measurements on β-Ga 2 O 3 crystals reveal the presence of three defect signatures labeled E 2 a, E 2 b, and E 3 with activation energies at around 0.66 eV, 0.73 eV, and 0.95 eV below the conduction band edge. Using secondary ion mass spectrometry, a correlation between the defect concentration associated with E 3 and the Ti concentration present in the samples was found. Particularly, it is found that E 3 is the dominant Ti-related defect in β-Ga 2 O 3 and is associated with a single Ti atom. This finding is further corroborated by hybrid functional calculations that predict Ti substituting on an octahedral Ga site, denoted as Ti GaII, to be a good candidate for E 3. Moreover, the deep level transient spectroscopy results show that the level previously labeled E 2 and attributed to Fe substituting on a gallium site (Fe Ga) consists of two overlapping signatures labeled E 2 a and E 2 b. We tentatively assign E 2 a and E 2 b to Fe substituting for Ga on a tetrahedral or an octahedral site, respectively.

Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime., Comment: 21 pages plus supplement

In this study, we demonstrate an approach to identify defects in wide band gap semiconductors by comparing accumulatively-recorded derivative steady-state photo-capacitance (SSPC) spectra to simulations using results from first-principles calculations. Specifically, we present a method to simulate SSPC spectra which adopts inputs both from first-principles calculations and the experimental conditions. The applicability of the developed method is demonstrated using the cases of subsitutional Fe (FeGa) and Ti (TiGa) defects in β-Ga2O3. Using deep-level transient spectroscopy, we identify defect levels associated with Fe GaI 0 / − (E A = 0.66 eV), Fe GaII 0 / − (E A = 0.79 eV) and Ti GaII + / 0 (E A = 1.03 eV) in the β-Ga2O3 samples studied here. Accumulatively-recorded SSPC spectra reveal several defect levels labeled T 1 E F G – T 6 E F G with onsets for optical absorption between 1.5 eV and 4.3 eV. The signature T 1 E F G consists of several overlapping defect signatures, and is identified as being related to Fe GaI 0 / − , Fe GaII 0 / − and Ti GaII + / 0 by comparing measured and simulated accumulatively-recorded derivative SSPC spectra.

Evidence of subtle effects in the dynamic reorganization of a protic solvent in its first- and farther-neighbor shells, in response to the sudden change in the solute's electronic distribution upon excitation, is unveiled by a multilevel computational approach. Through the combination of nonequilibrium molecular dynamics and quantum mechanical calculations, the experimental time evolution of the transient T1 absorption spectra of a heteroleptic Ru(II)-polypyridine complex in ethanol or dimethyl sulfoxide solution is reproduced and rationalized in terms of both fast and slow solvent re-equilibration processes, which are found responsible for the red shift and broadening experimentally observed only in the protic medium. Solvent orientational correlation functions and a time-dependent analysis of the solvation structure confirm that the initial, fast observed red shift can be traced back to the destruction-formation of hydrogen bond networks in the first-neighbor shell, whereas the subsequent shift, evident in the [20-500] ps range and accompanied by a large broadening of the signal, is connected to a collective reorientation of the second and farther solvation shells, which significantly changes the electrostatic embedding felt by the excited solute.

This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No: OSR-2018-CARF/CCF-3079. M.W. and F.L. thank Denis Andrienko and Julien Gorenflot for fruitful discussions and comments on the work.

International audience; It is shown that deep level transient spectroscopy can be carried out on Schottky diodes to investigate, in addition to majority carrier traps, minority carrier traps. This is possible thanks to the application of a large reverse bias to the device which allows minority carrier injection by lowering their corresponding effective Schottky barrier height. Indeed, when increasing the reverse bias voltage, the deep level transient spectroscopy signal, initially negative and thus showing only majority carrier traps signature, becomes positive, revealing minority carrier traps involvement. A careful analysis of the recorded spectra leads to the identification of four minority carrier traps which have been so far only evidenced using dedicated technique such as minority carrier transient spectroscopy.

Interface traps (ITs) and border traps (BTs) in Al2O3/GeOx/p-Ge gate stacks were characterized using deep-level transient spectroscopy. Through evaluating the gate stacks with different GeOx thicknesses, the respective BTs in Al2O3, the Al2O3/GeOx interface region, and GeOx were detected. The density of ITs (Dit) near the midgap is lower in the metal-oxide-semiconductor (MOS) capacitors with thicker GeOx, while Dit near the valence band is lower in the MOS capacitor with thinner GeOx. The density of BTs (Nbt) in Al2O3 (6–9 × 1017 cm−3) is lower than those in GeOx (∼2 × 1018 cm−3), and the highest Nbt (∼1 × 1019 cm−3) was found in the Al2O3/GeOx interface region. Ge p-channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) with Al2O3/GeOx/p-Ge gate stacks were fabricated and analyzed. We confirmed that the ITs and the BTs near the valence band edge of Ge affect the effective mobility of Ge p-MOSFETs in the high-field region.

We present preliminary results on minority carrier traps in as-grown n-type 4H&ndash, SiC Schottky barrier diodes. The minority carrier traps are crucial for charge trapping and recombination processes. In this study, minority carrier traps were investigated by means of minority carrier transient spectroscopy (MCTS) and high-resolution Laplace-MCTS measurements. A single minority carrier trap with its energy level position at Ev + 0.28 eV was detected and assigned to boron-related defects.

Charge transport in π-conjugated polymers is characterized by a strong degree of disorder in both the energy of conjugated segments and the electronic coupling between adjacent sites. This disorder arises from variations in the structure and conformation of molecular units, as well as the weak intermolecular binding interactions. Although disorder in molecular conformation can be expected to influence the density of states (DOS) distribution—and hence, optoelectronic properties of the material—until now, there has been no direct study of the relationship between a distinct conformational defect and the charge transport properties of a conjugated polymer. Here, we investigate the impact of introducing an extended, planarized chain geometry, known as the “β-phase,” on hole transport through otherwise amorphous films of poly(9,9-dioctylfluorene) (PFO). We show that while β-phase introduces a striking drop of about a hundredfold in time-of-flight (TOF) hole mobility (μh) at room temperature, it reduces the steady-state μh measured from hole-only devices by a factor of less than about 5. In order to reconcile these observations, we combine high-dynamic-range TOF photocurrent spectroscopy and energy-resolved electrochemical impedance spectroscopy to extract the hole DOS of the conjugated polymer. Both methods show that the effect of the β-phase content is to introduce a sharp sub-bandgap feature into the DOS of glassy PFO lying about 0.3 eV above the highest occupied molecular orbital. The observed energy of the conformational trap is consistent with electronic structure calculations using a tight-binding approach. Using the obtained DOS with a drift-diffusion model capable of resolving charge carriers in both time and energy, we show how the seemingly contradictory transport phenomena obtained via the time-resolved, frequency-resolved, and steady-state methods are reconciled. The results highlight the significance of energetic redistribution of charge carriers in affecting transport behavior. This work demonstrates how charge-carrier mobility in organic semiconductors can be controlled via molecular conformation, and it resolves a long-standing debate over how different (equilibrium versus nonequilibrium) transport techniques reveal electronic properties of disordered solids in a unified manner., Physical Review X, 9 (2), ISSN:2160-3308

In this work, deep levels present in n-type In0.53Ga0.47As hetero-epitaxial layers grown lattice-matched on n-type InP substrates by molecular beam epitaxy have been studied by deep-level transient spectroscopy (DLTS). Metal–oxide–semiconductor capacitors are employed, based on an Al2O3 gate oxide. It is shown that a single, near mid-gap electron trap dominates the DLT-spectra, whatever the surface pre-or post-gate oxide deposition treatment. At the same time, it is shown that the deep level parameters vary significantly from capacitor to capacitor and from wafer to wafer. Only after Forming Gas Annealing, a stable value for the activation energy of 0.39 ± 0.01 eV is obtained. These results are tentatively interpreted in terms of antisite defects in the epitaxial layer, which form a family of related complexes with close deep-level parameters.

Hybrid functional calculations reveal the Zn-O divacancy in ZnO, consisting of adjacent Zn and O vacancies, as an electrically active defect exhibiting charge states ranging from $2+$ to $2-$ within the band gap. Notably, the divacancy retains key features of the monovacancies, namely the negative-\textit{U} behavior of the O vacancy, and the polaronic nature of the Zn vacancy. The thermodynamic charge-state transition levels associated with the negative-\textit{U} behavior $\varepsilon$($0$/$2-$), $\varepsilon$($-$/$2-$) and $\varepsilon$($0$/$-$) are predicted to occur at 0.22, 0.42 and 0.02 eV below the conduction band minimum, respectively, resulting in \textit{U} = $-$0.40 eV. These transition levels are moved closer to the conduction band and the magnitude of \textit{U} is lowered compared to the values for the O vacancy. Further, the interaction with hydrogen has been explored, where it is shown that the divacancy can accommodate up to three H atoms. The first two H atoms prefer to terminate O dangling bonds at the Zn vacancy, while the geometrical location of the third depends on the Fermi-level position. The calculated electrical properties of the divacancy are in excellent agreement with those reported for the E4 center observed by deep-level transient spectroscopy, challenging the O vacancy as a candidate for this level., Comment: 8 pages, 5 figures

The photophysics of a donor-acceptor system in which efficient electronic energy transfer occurs is analyzed and discussed by the combined use of steady-state and time-resolved spectroscopy and DFT/TD-DFT computations. The donor and acceptor units belong to the class of BODIPY chromophores, and are conveniently linked through a calixarene scaffold, which allows the control of the mutual orientation and distance between chromophores. Our results highlight that the energy transfer process occurs with multiexponential dynamics strongly influenced by the solvent. Although the conformation adopted by the system is very similar in all the analyzed solvents, highly polar media favour fast and efficient energy transfer. On the contrary, in non-polar media, the concomitant occurrence of backward energy transfer causes a significant slowdown of the process. The inverse of the energy transfer rates calculated at the TDDFT level are in very good agreement with the experimental kinetics measured with transient absorption spectroscopy.

Time-resolved spectroscopy is a powerful tool for studying fundamental photophysics in optoelectronic materials on a molecular temporal scale. In this review, we describe transient spectroscopic studies on fundamental photovoltaic conversion processes in polymer solar cells, which consist of a series of conversion processes such as photon absorption, exciton diffusion into a donor/acceptor interface, charge transfer at the interface, charge dissociation into free charge carriers, and charge collection to each electrode. These conversion processes are ultrafast phenomena and are ranging over the wide temporal scale from femtoseconds to microseconds, which can be directly observed by transient spectroscopy.

Photochemical reactivity of 2-hydroxymethylphenol (1) has been investigated experimentally by photochemistry at cryogenic conditions and the detection of reactive intermediates by IR, and the nanosecond and femtosecond transient absorption spectroscopic methods in solutions at room temperature. In addition, theoretical studies were performed to facilitate the interpretation of the experimental results and also to simulate the reaction pathway for a better understanding of the reaction mechanism. The main finding of this work is that photodehydration of 1 takes place in an ultrafast adiabatic photochemical reaction without any obvious intermediate, delivering quinone methide (QM) in the excited state. Upon photoexcitation to a higher vibrational level of the singlet excited state, 1 undergoes vibrational relaxation leading to two photochemical pathways with one being the synchronous elimination of H2O to give a QM (2) in its S1 state and the other being a homolytic cleavage of the phenolic O-H bond to produce a phenoxyl radical (S0). Both are ultrafast processes that occur within a ps. The excited state of QM 2 (S1) probably deactivates to S0 via a conical intersection giving QM 2 (S0) and subsequently delivers benzoxete 4. Elucidation of the reaction mechanisms for the photodehydration of phenols where QMs are formed is important for the tuning the reactivity of QMs with DNA and proteins for potential applications of QMs in medicine as therapeutic agents.

Point defects in n- and p-type 4H-SiC grown by fluorinated chemical vapor deposition (CVD) have been characterized optically by photoluminescence (PL) and electrically by deep-level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS). The results are considered in comparison with defects observed in non-fluorinated CVD growth (e.g., using SiH4 instead of SiF4 as silicon precursor), in order to investigate whether specific fluorine-related defects form during the fluorinated CVD growth, which might prohibit the use of fluorinated chemistry for device-manufacturing purposes. Several new peaks identifying new defects appear in the PL of fluorinated-grown samples, which are not commonly observed neither in other halogenated chemistries, nor in the standard CVD chemistry using silane (SiH4). However, further investigation is needed in order to determine their origin and whether they are related to incorporation of F in the SiC lattice, or not. The electric characterization does not find any new electrically-active defects that can be related to F incorporation. Thus, we find no point defects prohibiting the use of fluorinated chemistry for device-making purposes. Funding Agencies|Knut & Alice Wallenberg Foundation (KAW) project; SSF project "SiC - the Material for Energy-Saving Power Electronics [2016-05362]; VR Grant [2016-05362]