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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.

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.

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.

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

The activation of C-H bonds requires the generation of extremely reactive species, which hinders the study of this reaction and its key intermediates. To overcome this challenge, we synthesized an iron(III) chloride-pyridinediimine complex that generates a chlorine radical proximate to reactive C-H bonds upon irradiation with light. Transient spectroscopy confirms the formation of a Cl·|arene complex, which then activates C-H bonds on the PDI ligand to yield HCl and a carbon-centered radical as determined by photocrystallography. First-principles molecular dynamics-density functional theory calculations reveal the trajectory for the formation of a Cl·|arene intermediate. Together, these experimental and computational results show the complete reaction profile for the preferential activation of a C-H bond in the solid state.

Primary defects introduced in boron-doped silicon by an alpha-particle source with a fluence rate of 7 × 106 cm−2 s−1 at cryogenic temperatures were investigated using deep-level transient spectroscopy (DLTS). The data showed that the defects observed between 35 K and 120 K were not detectable when irradiation was carried out at room temperature. The defect levels were observed at 0.10 eV, 0.14 eV and 0.18 eV above the valence band maximum. These levels were attributed to the boron-substitutional vacancy complex, the mono vacancy and a vacancy-related defect respectively.

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

Cesium-based perovskite nanocrystals (NCs) have outstanding photophysical properties improving the performances of lighting devices. Fundamental studies on excitonic properties and hot-carrier dynamics in perovskite NCs further suggest that these materials show higher efficiencies compared to the bulk form of perovskites. However, the relaxation rates and pathways of hot-carriers are still being elucidated. By using ultrafast transient spectroscopy and calculating electronic band structures, we investigated the dependence of halide in Cs-based perovskite (CsPbX3 with X=Br, I, or their mixtures) NCs on the hot-carrier relaxation processes. All samples exhibit ultrafast ( CsPbBr1.5I1.5 (380 fs)>CsPbI3 NC (580 fs). These result accounts for a reduced light emission efficiency of CsPbI3 NC compared to CsPbBr3 NC.

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.

We have studied changes of sub-gap defect states in amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) by means of photoinduced transient spectroscopy (PITS). It is found that the sub-gap states in the channel region of a-IGZO TFTs were drastically modified as back-channel etch process proceeded. The change was due to in-and out-diffusion of hydrogen atoms in the channel region of a-IGZO TFTs, which affects the performance of the a-IGZO TFTs.

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.

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.

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.

An overview is given on how to tackle the question of the electrical activity of extended defects which are inevitably present in hetero-epitaxial III-V layers on silicon. Analysis methods are described which rely on simple device structures containing a specific type of extended defect (here, threading dislocations). Applying the same methods to real scaled FinFETs is rather challenging. Instead Generation-Recombination noise spectroscopy provides data that can be compared with other more standard techniques, like Deep-Level Transient Spectroscopy (DLTS) in order to identify the presence of an electrically active extended defect in the channel material.

The spectrum of defects produced by 5 MeV electron irradiation in oxygen-lean p-type silicon strongly contaminated with interstitial copper (Cui) is studied using the deep-level transient spectroscopy. It is observed that the room-temperature irradiation creates a large amount of CuPL centers (complexes including one substitutional and three interstitial Cu atoms). The analysis shows that this process is govern by formation of the substitutional copper atoms due to the direct reaction between irradiation-induced vacancies and mobile Cui species. This reaction consumes nearly all irradiation-induced vacancies and affects strongly the standard spectrum of radiation defects.

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]

In this work we present a detailed study of the influence of the GaAs substrate orientation on the electrical properties of heterojunctions based on GaAs and sulfonated polyaniline (SPAN) using Current-Voltage (I-V), Capacitance-Voltage (C-V), Deep-Level Transient Spectroscopy (DLTS) and Laplace DLTS techniques. Three different GaAs substrate orientations have been investigated, namely (1 0 0), (3 1 1)A and (3 1 1)B. The I-V results revealed that the turn-on voltage (Von) of SPAN/(3 1 1)B GaAs heterojunction is higher than that for SPAN/(1 0 0) GaAs and SPAN/(3 1 1)A GaAs heterojunctions. The DLTS results showed that the number of electrically active defects present in devices based on the lower index (1 0 0) plane of GaAs substrate is higher than those of higher index (3 1 1)A and (3 1 1)B GaAs substrates, corroborating with I-V results. In order to investigate the role of interface states, capacitance-frequency measurements were performed in forward bias on all three devices.

The results of investigations of the pyroelectric current in a layered TlInS2 crystal doped with lanthanum are presented. A comparative analysis of the obtained results and data of photo-induced current transient spectroscopy has been performed. The revealed features of the contribution from electrically active defects to pyroelectric properties of the crystal have been discussed.

Pulsed-laser deposited ZnO thin films were implanted with Ar-ions to generate intrinsic defects within the material. The defects created were analyzed by deep-level and optical deep-level transient spectroscopy. Deep-level defects with thermal activation energies of 980 and appear after implantation. In order to study the annealing behavior of these defects, the implanted samples were isochronously annealed under an oxygen ambient of at temperatures ranging from 120 to . Both defects are annealed out after annealing for 1 h at . (O)DLTS signal of an 40Ar-implanted ZnO thin film under (a) dark condition and (b) under illumination with photons having a wavelength of .

Local electronic states of regular dislocation networks produced by n- and p-type silicon wafer bonding with different screw dislocation density were studied with deep-level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS). A drastic sadden changes of the electric level spectrum with increasing of dislocation density from two shallow bands located near the edges of valence and conduction bands towards two deep bands with energy positions about Ec - (0.22-0.26) eV and Ev + (0.4-0.53) eV were found. The origin of the electric level spectrum changes is ascribed to the changes of dislocation core structure from dissociated to perfect ones that occur when interdislocation distances became comparable with the dislocation equilibrium dissociation width. The obtained results correlate well with the results of recent studies of recombination activity of grain boundaries in mc-Si.

Defect levels in high purity semi-insulating 4H silicon carbide substrates are studied by charge transient spectroscopy using 5.5 MeV alpha particles. A shallow defect level with the activation energy around 0.3 eV is found in all samples annealed at temperatures from 1400 to 1600 °C. Some other defect levels lying at deeper in the bandgap are found in samples annealed at 1400 and 1500 °C. As these deep levels are annealed out by 1600 °C, the series resistance of samples is decreased.

The ultrafast mechanisms underlying the initial photoisomerization (Pr → Lumi-R) in the forward reaction of the cyanobacterial photoreceptor Cph1 were explored with multipulse pump-dump-probe transient spectroscopy. A recently postulated multi-population model was used to fit the transient pump-dump-probe and dump-induced depletion signals. We observed dump-induced depletion of the Lumi-R photoproduct, demonstrating that photoisomerization occurs via evolution on both the excited- and ground-state electronic surfaces. Excited-state equilibrium was not observed, as shown via the absence of a dump-induced excited-state “Le Châtelier redistribution” of excited-state populations. The importance of incorporating the inhomogeneous dynamics of Cph1 in interpreting measured transient data is discussed.

We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (&gt, 1 ns). This “two-step” charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM.

Despite the astonishing values of the power conversion efficiency reached, in just less than a decade, by the carbon nanotube/silicon (CNT/Si) solar cells, many doubts remain on the underlying transport mechanisms across the CNT/Si heterojunction. Here, by combining transient optical spectroscopy in the femtosecond timescale, X-ray photoemission, and a systematic tracking of I–V curves across all phases of the interlayer SiOx growth at the interface, we grasp the mechanism that adequately preserves charge separation at the junction, hindering the photoexcited carrier recombination. Moreover, supported by ab initio calculations aimed to model the complex CNT–Si heterointerface, we show that oxygen-related states at the interface act as entrapping centers for the photoexcited electrons, thus preventing recombination with holes that can flow from Si to CNT across the SiOx layer.

The novel dendritic-shaped, heptanuclear compound 1 – the first mixed Os–Ru dendrimer with tris-chelating bridging ligands – has been synthesized and its photophysics studied by fs pump–probe transient spectroscopy. In 1, all light energy absorbed by six identical Ru(II) chromophores is funnelled to the luminescent central Os(II) core, with a time constant of 11 ps, by Dexter energy transfer.

Electrically active defects in silicon-based epitaxial layers on silicon substrates have been studied by Deep-Level Transient Spectroscopy (DLTS). Several aspects have been investigated, like, the impact of the pre-epi cleaning conditions and the effect of a post-deposition anneal on the deep-level properties. It is shown that the pre-cleaning thermal budget has a strong influence on the defects at the substrate/epi layer interface. At the same time, a post-deposition Forming Gas Anneal can passivate to a large extent the active defect states. Finally, it is shown that application of a post-deposition anneal increases the out-diffusion of carbon from a Si:C stressor layer into the p-type CZ substrate.

In this paper a systematic and detailed study on the photophysical behavior of silicon nanocrystals (alkyl-capped, surface oxidized, oxide-encapsulated) is presented in order to elucidate the influence of local environment and surface species on the luminescence properties. Experiments at different temperatures – ranging from room temperature (RT) to 7 K – confirmed the important influence of the silicon nanocrystal (ncSi) surface on the photoluminescence (PL). Our comparative study of ncSi embedded in different matrices shows a strong increase of the exciton lifetimes at low temperature and the dominant widening of the bandgap for small ncSi after deactivation of traps below 100–150 K. The present findings have significant implications for the use of ncSi in optoelectronic and biomedical applications, since they show that surface oxidation can occur in ligand-capped ncSi which limits the accessible emission wavelength for these applications to the orange/red spectral region when current passivation strategies are used.

Formation and annealing behavior of the 1.014-eV copper center and its dissociation product (center) in silicon are characterized by photoluminescence (PL) and deep-level transient spectroscopy (DLTS) measurements. On the basis of the findings reported in this study, the structures of the centers are discussed.

Two donor-π-acceptor (D-π-A) dyes are synthesized for application in dye- sensitized solar cells (DSSC). These D-π-A sensitizers use triphenylamine as donor, oligothiophene as both donor and π-bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO2 sur- face. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid-electrolyte-based dye-sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells’ performance leading to an over 6.5 times higher efficiency (η = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (η = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO2-photoanode.

Deep-level transient spectroscopy (DLTS) has been used to study p-n junctions fabricated by implantation of boron into epitaxial 4H-SiC films with n-type conductivity and the donor concentration (8–9) × 1014 cm−3. A DLTS signal anomalous in sign is observed; this signal is related to recharging of deep compensating boron-involved centers in the n-type region near the metallurgical boundary of the p-n junction.

Interaction between hydrogen (H) and irradiation induced defects in p-type silicon (Si) have been studied in H implanted pn-junctions, using deep level transient spectroscopy (DLTS), as well as minority carrier transient spectroscopy (MCTS). Two H related levels at Ev+0.27 eV and Ec-0.32 eV have been observed (Ev and Ec denote the valence and conduction band edge, respectively). Both levels form after a 10 min anneal at 125C, concurrent with the release of H from the boron-hydrogen (B-H) complex. The correlated formation rates and absolute concentrations of the two levels support the notion that they are due to the same defect. In addition, a level at Ec-0.45 eV is observed and discussed in terms of vacancy-hydrogen related defects.

The features of the DLTS detection of defects with retrappability of thermally excited carriers due to a large value of effective capture cross section are considered. Indications allowing identification of the implementation of the given model are found.

In the present paper, 4H-SiC JBS diodes with "boron" p–n junctions have been investigated by means of deep-level transient spectroscopy (DLTS). The sign of the DLTS signal for all the 4H-SiC diodes under study, was positive. The "anomaly" of the DLTS spectra measured is apparently connected with the properties of "boron" p–n junctions. In particular, is presented the role of deep D-centers in recompensation of donors in the JBS diodes.

Discharge current transient spectroscopy (DCTS) is a promising technique for detecting the trap level and density in dielectrics because it is based on a simple emission process. In order to confirm the validity of DCTS, we compare the results from the conventional deep-level transient spectroscopy (DLTS) technique for samples of CVD-grown silicon nitride (SiNx) films. Results indicated that a trap level, about 0.6 eV below the energy level of the conduction band edge in the SiNx thin films estimated by DCTS is in good agreement with that obtained from DLTS analysis, and it is found that the trap density increases with the decreasing N/Si ratio in the SiNx film. As a proposed estimation for energy level in defects, it will be originated from hydrogen-incorporated defects in the SiNx matrix. This study demonstrates that the DCTS method will be a useful electrical method for the evaluation of defects.

The method of transient spectroscopy is one of the basic methods widely used for determining the parameters of defects, which are the origin of deep levels in the semiconductor-material band gap. With the purpose of increasing the accuracy of determining the concentration and deep-level parameters describing the character of the isothermal capacitance relaxation, a statistical method of processing the results of its measurements based on the statistical solution-search algorithm is proposed.

The energy level of electron traps in silicon nitride (SiN x ) thin films was investigated by discharging current transient spectroscopy (DCTS). Results indicate that the trap level of the SiN x thin films becomes deeper with decreasing composition (N/Si) and shallower after hydrogen annealing. The dependence of the trap level on the SiN x composition and the modulation of the trap level by hydrogen annealing are possibly related to the change in the number of Si–H bonds in the SiN x thin films.

The properties of GaAs/AlGaAs solar cells irradiated with 40, 70, 100 and 170 keV protons have been studied. Current–voltage (IV) measurement showed that the worst degradation was found in the cells irradiated by 100 keV protons. The degradation was found defect dependent. Defect profile was obtained by the stopping and range of ions in matter (SRIM) simulation and deep-level transient spectroscopy (DLTS) measurement. In order to obtain the deep-level defect profile in depletion layer by DLTS measurement, the traditional calculation formula of defect concentration has been modified. DLTS measurement showed good agreement with that of the SRIM simulation.

Stationary and transient spectroscopy of excitonic induction is applied for measuring the rates of the processes controlling the efficiency of the coherence transfer under the photon-exciton transformations in GaAs/AlGaAs structures with single quantum wells. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using deep-level transient spectroscopy and capacitance-voltage measurements, the effect of electric field on ionization of the sulfur-related DX center is investigated for n-In1 − xGaxP layers (x = 0.65) grown by vapor-phase epitaxy. It is shown that, upon increasing the electric field strength from 1.3 × 104 to 1.9 × 105 V/cm, the activation energy of the center decreases from ∼0.38 to ∼0.26 eV, and the ionization process is governed by multiphonon tunneling. By the estimated tunneling time of the defect, it is concluded that the DX center corresponds to a model of large lattice relaxation.

Electrical characteristics and an origin of traps in silicon nitride film were investigated using discharge current transient spectroscopy (DCTS), electron spin resonance (ESR), and photoluminescence (PL). It was found from DCTS that the density of traps of 1.1 eV from the conduction band edge increased with the increase of the gas-mixture ratio of SiH2Cl2 over NH3 for the fabrication of silicon nitride film. The energy levels of traps in silicon nitride film of 0.25, 0.68, 0.83–0.91, and 1.25–1.3 eV from the conduction band edge were found from photoluminescence observation. The ESR signals of the K-center that indicated a silicon dangling-bond increased with the increase of the gas-mixture ratio of SiH2Cl2 over NH3. It is suggested that the origin of traps of 1.1 eV from the conduction band in silicon nitride film is the silicon dangling-bond.

Defect levels in the n-type bulk 6H-SiC have been investigated by deep level transient spectroscopy (DLTS) and photoinduced transient spectroscopy (PITS). From the DLTS spectra, four electron traps at Ec - 0.53 eV (T1), Ec - 0.64 eV (T2), Ec - 0.67 eV (T3) and Ec - 0.69 eV (T4) with concentrations of 6.4x1015, 1.1x1016, 3.0x1016 and 1.6x1016 cm–3, respectively, were revealed. The centers T1, tentatively identified with carbon vacancies, were found to be located in the vicinity of dislocations. The centers T2 and T3 were assigned to the known deep-level defect Z1/Z2 occurring both in 4H and 6H SiC polytypes. The PITS measurements revealed two shallow traps with activation energies 20 and 60 meV, as well as a deep trap at 0.66 eV. The origin of the first trap still remains unclear. The latter two traps are tentatively identified with a nitrogen atom in hexagonal site and a complex involving a boron atom and silicon antisite, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A method to extract the carrier capture kinetics of traps in the neutral region by isothermal deep-level transient spectroscopy using two sets of bias pulses was applied to the traps with similar energy levels around Ec–0.6 eV in GaN grown by metalorganic chemical vapor deposition on GaN and sapphire substrates. The Ec–0.6 eV trap in GaN on GaN is found to be the isolated point defects. The Ec–0.6 eV trap in GaN on sapphire shows the logarithmic capture kinetics, expected for dislocation-related defects. This is ascribed to the difference in dislocation density between GaN on GaN and sapphire substrates. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)