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The III–V semiconductor GaN is a promising material for photoelectrochemical (PEC) cells, however the large bandgap of 3.45 eV is a considerable hindrance for the absorption of visible light. Therefore, the substitution of small amounts of N anions by isovalent Sb is a promising route to lower the bandgap and thus increase the PEC activity under visible light. Herein we report a new chemical vapor deposition (CVD) process utilizing the precursors bis(N,N′-diisopropyl-2-methyl-amidinato)-methyl gallium (III) and triphenyl antimony (TPSb) for the growth of GaSbxN1−x alloys. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements show crystalline and homogeneous thin films at deposition temperatures in the range of 500–800 °C. Rutherford backscattering spectrometry (RBS) combined with nuclear reaction analysis (NRA) shows an incorporation of 0.2–0.7 at% antimony into the alloy, which results in a slight bandgap decrease (up to 0.2 eV) accompanied by enhanced sub-bandgap optical response. While the resulting photoanodes are active under visible light, the external quantum efficiencies remained low. Intriguingly, the best performing films exhibits the lowest charge carrier mobility according to time resolved THz spectroscopy (TRTS) and microwave conductivity (TRMC) measurements, which showed mobilities of up to 1.75 cm2 V−1 s−1 and 1.2 × 10–2 cm2 V−1 s−1, for each timescale, respectively.

Recently, α-SnWO4 attracted attention as a material to be used as a top absorber in a tandem device for photoelectrochemical water splitting due to its nearly optimum band gap of ∼1.9 eV and an ear...

The temporal evolution of photogenerated carriers in CuWO4, CuO and WO3 thin films deposited via a direct chemical vapor deposition approach was studied using time-resolved microwave conductivity and terahertz spectroscopy to obtain the photocarrier lifetime, mobility and diffusion length. The carrier transport properties of the films prepared by varying the copper-to-tungsten stoichiometry were compared and the results related to the performance of the compositions built into respective photoelectrochemical cells. Superior carrier mobility was observed for CuWO4 under frontside illumination.

The III-V semiconductor GaN is a promising material for photoelectrochemical (PEC) cells, however the large bandgap of 3.45 eV is a considerable hindrance for the absorption of visible light. Therefore, the substitution of small amounts of N anions by isovalent Sb is a promising route to lower the bandgap and thus increase the PEC activity under visible light. Herein we report a new chemical vapor deposition (CVD) process utilizing the precursors bis(

The rational combination of tetracene Tc with crystalline silicon c Si could greatly enhance c Si solar cell efficiencies via singlet fission. The Tc c Si energy level alignment ELA is thought to be central to controlling the required interface transfer processes. We modified hydrogen terminated c Si H Si with 2,2 amp; 8242; perfluoronaphthalene 2,6 diylidene dimalononitrile F6TCNNQ , C60, or NF3 and probed the effect on the c Si surface chemistry, the Tc c Si ELA, the Tc morphology, and solar cell characteristics using ultraviolet and X ray photoelectron spectroscopy, atomic force microscopy, X ray diffraction, photoluminescence transients, device measurements, and transfer matrix optical modeling. Submonolayer interlayers of F6TCNNQ shifted the Tc H Si 111 ELA by up to 0.55 eV. C60 showed no notable effect on the ELA and proved detrimental for the Tc film morphology and solar cell performance. Neither F6TCNNQ nor C60 improved the Tc related photocurrent significantly. NF3 CVD substituted the H termination of H Si 100 with more electronegative species and resulted in work functions as high as 6 eV. This changed the Tc H Si 100 ELA by up to 0.45 eV. NF3 plasma from a remote source caused pronounced c Si oxidation and a diminished c Si photoluminescence lifetime, which was not observed for NF3 plasma created in close proximity to the c Si surface or neutral NF3. We discuss possible reasons for why the improved ELA does not lead to an improved singlet fission harvest

In this paper, a direct chemical vapor deposition (CVD) approach is applied for the first time to synthesize high quality copper oxide (CuO), copper tungstate (CuWO4) and tungsten oxide (WO3) on F:SnO2 (FTO) substrates for photocatalytic water splitting. Variation of process parameters enables us to tune the stoichiometry of the deposits to obtain stoichiometric, W-rich, and Cu-rich deposits. It is found that the presence of Cu in WO3 thin films reduces the bandgap and enhances the absorption properties of the material in the visible range. The photoelectrocatalytic performance of stoichiometric CuWO4 was found to be superior to that of WO3 oxide under frontside illumination when thin films were used. However, detailed photoelectrochemical investigations of both thin and thicker CuWO4 films reveal that the incorporation of copper also decreases the mobility of both electrons and holes, the latter being the performance-limiting factor. These results are in line with our first-principles calculations of the electronic structure of CuWO4. A charge carrier mobility and diffusion length of ∼6× 10−3 cm2 V−1 s−1 and 30 nm were determined by time-resolved microwave conductivity measurements, values comparable to those of undoped bismuth vanadate (BiVO4). Our findings establish new insights into the advantages and limits of CuWO4-based photoanodes, and suggest a possibility of using very thin CuWO4 films on top of highly absorbing semiconductors with optimal electronic properties.

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination to structure and processing of photovoltaic absorber materials is critical to designing efficient solar cells. Using three complementary techniques, the authors elucidate photoexcited carrier dynamics in a high-quality kesterite crystal. These dynamics help to explain limitations in charge-carrier collection observed in a device's $J\ensuremath{-}V$ relation and external quantum efficiency. Combining ultrafast spectroscopy and device measurements in this way can lead to a more detailed understanding of the performance-limiting photophysical processes, accelerating the improvement of solar cells.

Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development., While cuprous oxide is a promising solar-to-fuel conversion material, photoelectrochemical devices substantially underperform. Here, the authors use femtosecond time-resolved two-photon photoemission spectroscopy to correlate photoexcited electron energetics and dynamics with performance losses.

BiVO4 is a promising photoanode candidate for water splitting applications, but its microscopic charge carrier transport properties are not yet fully understood. We investigated the photoinduced carrier mobility for undoped and 1% tungsten-doped BiVO4 thin films in an early time window from 1 ps to 1 ns using THz spectroscopy. The combined electron–hole effective mobility gradually decreases with time by 1 order of magnitude starting at an upper limit of ∼0.4 cm2 V–1 s–1. The loss is attributed to carrier localization. We provide for the first time direct time-resolved evidence of hole polaron formation accompanied by the temporal buildup of a polaron population in parallel to initial carrier trapping. A mobility of 0.02 cm2 V–1 s–1 is found for the self-trapped carriers, which leads to a thermal hopping activation energy of ∼90 meV.

CuSbS2 is a promising nontoxic and earth-abundant photovoltaic absorber that is chemically simpler than the widely studied Cu2ZnSnS4. However, CuSbS2 photovoltaic (PV) devices currently have relatively low efficiency and poor reproducibility, often due to suboptimal material quality and insufficient optoelectronic properties. To address these issues, here we develop a thermochemical treatment (TT) for CuSbS2 thin films, which consists of annealing in Sb2S3 vapor followed by a selective KOH surface chemical etch. The annealed CuSbS2 films show improved structural quality and optoelectronic properties, such as stronger band-edge photoluminescence and longer photoexcited carrier lifetime. These improvements also lead to more reproducible CuSbS2 PV devices, with performance currently limited by a large cliff-type interface band offset with CdS contact. Overall, these results point to the potential avenues to further increase the performance of CuSbS2 thin film solar cell, and the findings can be transferred t...

The effect of compositional variation on charge carrier lifetimes of Cr_1Fe_(0.84)Al_(0.16)O_3, a promising material for solar water splitting recently identified using combinatorial materials science, is explored using ultrafast time-resolved optical reflectance. The transient signal can be described by a biexponential decay, where the shorter time constant varies over 1 order of magnitude with changing Cr content while the longer one stays constant. Intrinsic performance limitations such as a low charge carrier mobility on the order of 10^(–3) cm^2/(Vs) are identified. Charge carrier lifetime and mobility are discussed as screening criteria for solar water splitting materials.

The mobilities of electrons and holes determine the applicability of any semiconductor, but their individual measurement remains a major challenge. Here, we show that time-resolved terahertz spectroscopy (TRTS) can distinguish the mobilities of minority and majority charge carriers independently of the doping-type and without electrical contacts. To this end, we combine the well-established determination of the sum of electron and hole mobilities from photo-induced THz absorption spectra with mobility-dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu

The mobilities of electrons and holes determine the applicability of any semiconductor, but their individual measurement remains a major challenge. Here, we show that time-resolved terahertz spectroscopy (TRTS) can distinguish the mobilities of minority and majority charge carriers independently of the doping-type and without electrical contacts. To this end, we combine the well-established determination of the sum of electron and hole mobilities from photo-induced THz absorption spectra with mobility-dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu2ZnSnSe4 thin film and reveals a minority (electron) mobility of 128 cm2/V-s and a majority (hole) carrier mobility of 7 cm2/V-s in the vertical transport direction relevant for light emitting, photovoltaic and solar water splitting devices. Additionally, the TRTS analysis yields an effective bulk carrier lifetime of 4.4 ns, a surface recombination velocity of 6 * 104 cm/s and a doping concentration of ca. 1016 cm−3, thus offering the potential for contactless screen novel optoelectronic materials.

We have realized a tandem solar cell design that combines a pin-junction with a photovoltaic intersubband absorber. This concept allows harvesting light in the visible range and the near- and mid-infrared at the same time, and theoretically, energy conversion efficiencies beyond the Shockley–Queisser-limit could be achieved. A test structure was grown, and the operation of this concept could be confirmed, in principal with an optical two-beam experiment. The basic characteristics of the device can be explained with an equivalent circuit design that consists of three individual cells, and we find an obvious analogy to the concept of the intermediate band solar cell with noteworthy advantages at some points. Our results show, that for a working device it is crucial to adjust the properties of the photovoltaic intersubband absorber for optimal charge separating performance at the working point of the solar cell. Copyright © 2015 John Wiley & Sons, Ltd.

We show that charge carrier mobilities can be measured by reflection time resolved THz spectroscopy (R-TRTS) even for thin films on metal contacts, such as polycrystalline Cu2SnZnSe4 grown on molybdenum. In the measurement a reduced THz reflection upon photo-excitation is observed in contrast to increased THz reflection commonly observed on insulating substrates, and which excludes standard analytic R-TRTS analyses. Instead, a numerical transfer matrix method is used to model the THz reflection from which we derive carrier mobilities of 100 cm2/Vs consistent with literature. We show that R-TRTS on metal substrates is ~100x less sensitive compared to measurements on insulating substrates. These sensitivity of these R-TRTS measurements can be increased by using lower substrate refractive indices, lower substrate conductivities, thicker sample layers or higher THz probe frequencies.

Widespread application of solar water splitting for energy conversion is largely dependent on the progress in developing not only efficient but also cheap and scalable photoelectrodes. Metal oxides, which can be deposited with scalable techniques and are relatively cheap, are particularly interesting, but high efficiency is still hindered by the poor carrier transport properties (i.e., carrier mobility and lifetime). In this paper, a mild hydrogen treatment is introduced to bismuth vanadate (BiVO4), which is one of the most promising metal oxide photoelectrodes, as a method to overcome the carrier transport limitations. Time-resolved microwave and terahertz conductivity measurements reveal more than twofold enhancement of the carrier lifetime for the hydrogen-treated BiVO4, without significantly affecting the carrier mobility. This is in contrast to the case of tungsten-doped BiVO4, although hydrogen is also shown to be a donor type dopant in BiVO4. The enhancement in carrier lifetime is found to be caused by significant reduction of trap-assisted recombination, either via passivation of deep trap states or reduction of trap state density, which can be related to vanadium antisite on bismuth or vanadium interstitials according to density functional theory calculations. Overall, these findings provide further insights on the interplay between defect modulation and carrier transport in metal oxide photoelectrodes, which will benefit the development of low-cost, highly efficient solar energy conversion devices.

A low molecular weight purely organic Forster resonance energy transfer (FRET) sensitizer system combining multiple chromophores into a single molecule via covalent attachment is designed to adjust the ratio and relative position of the fluorol donor and coumarin acceptor units. A phenyl based scaffold accommodates both FRET partners and is connected to a carboxylic anchor group for adsorption on a semiconductor surface. The functionality of the complete ensemble is demonstrated by UV–vis and luminescence lifetime measurements. The energy and charge transfer dynamics of the FRET assemblies in solution and adsorbed on ZnO nanorods are measured by femtosecond transient absorption spectroscopy. Optical pump THz probe (OPTP) spectroscopy is applied to detect the injected electrons in the ZnO electrode, confirming the full electronic pathway from FRET light harvesting to subsequent charge carrier injection.

Semisquarylium dyes use a novel acyloin anchor group to strongly bind to TiO2 semiconductors. Efficient acyloin anchor mediated electron injection into nanocrystalline TiO2 is demonstrated, allowing highly efficient dye-sensitized solar cells with IPCEs > 80%. The acyloin anchor can thus be viewed as a true alternative to the standard carboxylic acid anchor group. The opto-electronic and electron injection properties of the most basic semisquarylium dye SY404 are compared to the modified semisquarylium dye DD1 and the carboxylic acid anchored indoline dye D131 using a combination of ultrafast and photoemission spectroscopy. For SY404, ultrafast injection times of ∼50 fs are found despite a small energetic driving force between dye excited states and TiO2 conduction band minimum. This is possible due to the strong electronic coupling of the semisquarylium dyes to the TiO2 surface mediated by the acyloin anchor. For a better overlap with the solar spectrum, the semisquarylium dyes are modified by substituti...

Electron injection from photoexcited chemisorbed dyes into zinc oxide is known to proceed in a stepwise manner, yet the origin of the injection retardation remains controversial. Here we present a complementary time-resolved spectroscopy study on the electron injection dynamics from organic dyes into ZnO using model perylene derivatives with systematically lengthened bridge units to clarify the influence of the positively charged cation on the escape of the injected electron. The combination of transient absorption, optical-pump terahertz-probe, and time-resolved two-photon photoemission spectroscopy reveals that the delayed release of charges into ZnO is independent of Coulomb attraction between the dye cation and the injected electron. Rather, following dye photoexcitation the primary acceptor states of electron transfer into ZnO are interface states formed between the dye and the ZnO surface, which retard the formation of free charges.

We track charge carriers dynamics from femtoseconds to nanoseconds in Cu 2 ZnSn(S/Se) 4 thin films by time resolved terahertz spectroscopy (TRTS) and time resolved micro wave conductivity measurements (TRMC). This includes trapping of photo carriers into band tail states followed by recombination with a time constant of 9.1 ns. The relatively long life time is likely caused by the localization into long living trap states rather than by a low concentration of recombination centers.

Intra grain charge carrier mobilities measured by time resolved terahertz spectroscopy TRTS in state of the art Cu2ZnSn S,Se 4 kesterite thin films are found to increase from 32 to 140 cm2V 1s 1 with increasing Se content. The mobilities are limited by carrier localization on the nanometer scale which takes place within the first 2 ps after carrier excitation. The localization strength obtained from the Drude Smith model is found to be independent of the excited photocarrier density. This is in accordance with bandgap fluctuations as cause of the localized transport. Charge carrier localization is a general issue in the probed kesterite thin films which were deposited by coevaporation, colloidal inks and sputtering followed by annealing with varying Se S contents and yield 4.9 to 10.0 efficiency in the completed device

Solar cells based on ZnO nanorods with thin In2S3 shells have recently shown promising solar conversion efficiencies. Using optical-pump terahertz-probe (OPTP) spectroscopy, the charge separation across ZnO/In2S3 interfaces is analyzed for ZnO nanorods with systematically varied In2S3 absorber thicknesses, measuring transient photoconductivities with subpicosecond time resolution. Whereas for neat In2S3 films the photoconductivity is dominated by fast multiple trapping and second order recombination, the ZnO/In2S3 heterostructures exhibit slow electron injection dynamics occurring within hundreds of picoseconds, and long-lived charge-separated states. The transient photoconductivity of the ZnO/In2S3 core/shell system is analyzed with a correlated three component effective medium approach, yielding a significant decrease of the charge separation efficiency with increasing shell thickness.

The impact of the chemical environment on heterogeneous electron transfer processes of two perylene derivatives chemically bound to ZnO or TiO2 colloids in a film was investigated by means of absor...

It is well-known that the efficiency of dye sensitized solar cells can be improved by controlling the interface energetics using molecular interface modifiers. Whereas this leads to a beneficial band level shifting, it also affects the interfacial electron injection and recombination dynamics. Here we demonstrate a significant retardation of the injection process and a loss of ultrafast recombination components by coadsorbing inert gases and solvents with increasing dipole moments on a TiO2/perylene interface. Model perylene dyes with different electronic couplings to the colloidal TiO2 films were subject to precisely defined chemical environments, and the electron transfer dynamics was investigated with femtosecond transient absorption spectroscopy. The coadsorption of N2 and Ar doubled the injection times compared to the ultrafast sub-60 fs electron injection in vacuum. The introduction of solvents led to injection retardations by up to 2 orders of magnitude. This slow-down correlates well with the degr...

The ultrafast injection dynamics, early recombination dynamics, and spectral signatures of four systematically varied dye-metal oxide hybrid systems were investigated using transient absorption spectroscopy techniques. First, photoinduced electron transfer from two different perylene derivatives into zinc oxide (ZnO) colloidal films is reported. Here, the electronic coupling of the perylene chromophore 2,5-Di-tert-butyl-perylene-9-yl-propionic acid (1) to the ZnO colloids was weaker than the electronic coupling of the chromophore 2,5-Di-tert-butyl-perylene-9-yl-acrylic acid (2). Second, the photoinduced electron transfer of the same two molecules attached to TiO2 colloids was measured and compared to the results for the ZnO colloids using the same techniques. The temporal traces at both the excited-state and the cationic state of the chromophores attached to the semiconductor surfaces were measured simultaneously and showed very good agreement, which indicated a direct injection into the semiconductor. Th...

High-throughput combinatorial methods have been useful in identifying new oxide semiconductors with the potential to be applied to solar water splitting. Most of these techniques have been limited to producing and screening oxide phases formed at temperatures below approximately 550 °C. We report the development of a combinatorial approach to discover and optimize high temperature phases for photoelectrochemical water splitting. As a demonstration material, we chose to produce thin films of high temperature CuNb oxide phases by inkjet printing on two different substrates: fluorine-doped tin oxide and crystalline Si, which required different sample pyrolysis procedures. The selection of pyrolysis parameters, such as temperature/time programs, and the use of oxidizing, nonreactive or reducing atmospheres determines the composition of the thin film materials and their photoelectrochemical performance. XPS, XRD, and SEM analyses were used to determine the composition and oxidation states within the copper niobium oxide phases and to then guide the production of target Cu(1+)Nb(5+)-oxide phases. The charge carrier dynamics of the thin films produced by the inkjet printing are compared with pure CuNbO3 microcrystalline material obtained from inorganic bulk synthesis.

Cu${}_{2}$SnS${}_{3}$ is a promising, earth-abundant photovoltaic material, but disorder in the positions of Cu and Sn cations in its crystal lattice complicate its electronic transport. The authors find that even after substantially increasing its cation ordering via annealing, minority-carrier transport is still dominated by disorder effects, as revealed by spectroscopy plus Monte Carlo simulations. These results identify important challenges to the implementation of Cu${}_{2}$SnS${}_{3}$ in solar cells, and carry implications for other semiconductors of this type, such as Cu${}_{2}$ZnSn(S,Se)${}_{4}$ (CZTS).

We investigated the scattering dynamics of hot electrons at the atomically well-defined In-rich $(2\ifmmode\times\else\texttimes\fi{}4)$-reconstructed InP(100) surface in the presence of different surface states. Using energy- and time-resolved femtosecond two-photon photoemission spectroscopy, we determined the electron transfer between three-dimensional bulk states and the two-dimensional surface state C2, located high above the conduction band minimum (CBM) avoiding energetic overlap with relaxed bulk electrons. At excitation energies, where C2 was populated through hot bulk electrons, relaxing towards the CBM, we found evidence that the energy loss rate of the photoexcited electron distribution is mostly determined by inelastic electron-phonon scattering between bulk states. This was confirmed by measurements repeated after quenching the surface states with oxygen. For resonant photoexcitation, on the other hand, we measured a time constant of ${\ensuremath{\tau}}_{\mathrm{fast}}=20\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ for the depopulation of C2, showing that electron-phonon scattering between bulk and surface states might have direct consequence on ultrafast relaxation dynamics for materials with a high surface-to-bulk ratio.

The dynamics of heterogeneous electron transfer (ET) from the polycyclic aromatic chromophore perylene to nanostructured TiO2 anatase was investigated for two different anchor groups with transient absorption spectroscopy in an ultrahigh vacuum. Data from ultraviolet photoelectron spectroscopy and from linear absorption spectroscopy showed that the donor state of the chromophore was located around 900 meV above the lower edge of the conduction band. With the wide band limit fulfilled the rate of the heterogeneous ET reaction was only controlled by the strength of the electronic coupling and not reduced by Franck-Condon factors. Two different time constants for the electron transfer, i.e., 13 and 28 fs, were measured with carboxylic acid and phosphonic acid as the respective anchor groups. The difference in the ET time constants was explained with the different extension of the donor orbital onto the respective anchor group to reach the empty electronic states of the semiconductor. The time constants were extracted by means of a simple rate equation model. The validity of applying this model on this ultrafast time scale was verified by comparing the rate equation model with an optical Bloch equation model.

An improved setup for femtosecond two-photon photoemission spectroscopy (TR-2PPE) is presented. Two noncollinear optical parametric amplifiers (NOPA) were operated simultaneously at a repetition rate of 150 kHz. The frequencies of the NOPA outputs were tuned such that subsequent second harmonic generation (SHG) provided the two different ultraviolet wavelengths required for the pump and probe pulse. The width of the crosscorrelation function (CC) for pump and probe pulse was sub-30 fs (FWHM).

The influence of structure geometry on THz emission from Black Silicon (BS) surfaces fabricated by reactive ion etching (RIE) has been investigated by a comprehensive study including optical simulations, optical-pump THz probe and THz emission studies. A strong enhancement of THz emission is observed with increasing structure depth, which is mainly related to the increased number of carriers created within the silicon needles and not due to the overall absorption enhancement as previously claimed for silicon nanowires.

Discovery of novel semiconducting materials is needed for solar energy conversion and other optoelectronic applications. However, emerging low-dimensional solar absorbers often have unconventional crystal structures and unusual combinations of optical absorption and electrical transport properties, which considerably slows down the research and development progress. Here, the effect of stronger absorption and weaker carrier collection of 2D-like absorber materials are studied using a high-throughput combinatorial experimental approach, complemented by advanced characterization and computations. It is found that the photoexcited charge carrier collection in CuSbSe2 solar cells is enhanced by drift in an electric field, addressing a different absorption/collection balance. The resulting drift solar cells efficiency is

Multiple quantum well (InAlAs/InGaAs) based photovoltaic cells and subsystems were grown by molecular beam epitaxy on (001) InP substrates to study the dynamics of the intersubband transitions and the photocurrent behavior with ultrafast and continuous wave (cw) laser spectroscopy, respectively. The surface recombination velocity was determined with photoluminescence and time domain THz spectroscopy by varying the well thickness. Two-photon absorption was observed in a photovoltaic device as an increase in photocurrent from room temperature down to 5 K by illumination of the structures at two different wavelengths.

We use optical pump Thz probe spectroscopy to access the microscopic mobilities and fast charge carrier dynamics processes in polycrystalline chalcopyrite and kesterite thin films grown by coevaporation. In order to avoid complicating effects from the presence of Ga-gradients, ternary CuInSe 2 samples were used as a model system. Significantly different DC mobilities were found for stoichiometric and Cu-poor samples, respectively. While the stoichiometric samples exhibit Drude-like free carrier mobilities with DC mobilities up to 1200cm2/Vs at room temperature, the Cu-poor samples show non-Drude behavior, with much lower DC mobilities, indicative of carrier localization. Kesterite materials are found to show even stronger signatures of carrier localization than the chalcopyrites.

The electronic structures of two significant atomically well-defined (100)-surface reconstructions of gallium phosphide were investigated with two-photon-photoemission spectroscopy (2PPE). We allocated a series of occupied and unoccupied surface states and deduced the influence of each particular reconstruction on the electronic structure of the surface. Photoemission signals arising from bulk optical transitions were distinguished from surface-state related signals by studying the influence of oxygen exposure to the surfaces and by comparing our results to reflectance anisotropy spectroscopy (RAS) measurements. This revealed that the features in the RAS signal correlate with the transition energies between unoccupied and occupied surface-related states that were observed in the 2PPE measurements. An anisotropy around the ${E}_{0}^{\ensuremath{'}}$ critical point, previously known from RA spectra, was also investigated with 2PPE and is ascribed to modifications of the bulk electronic structure in the vicinity of the surface.

The relaxation and transport dynamics of photoexcited carriers in $p$-type stoichiometric polycrystalline CuInSe${}_{2}$ is investigated by optical-pump terahertz-probe spectroscopy. For all time delays and temperatures studied, the optically measured photoconductivity exhibits a characteristic free carrier Drude response, which allows analyzing the carrier scattering and relaxation phenomena in detail. The hot carrier distribution initially present after photoexcitation is found to relax within the first 200 ps by electron-phonon interaction with polar longitudinal optical (LO) phonons. The relaxed carrier distribution found after 200 ps indicates room temperature minority carrier mobilities close to 1000 cm${}^{2}$/Vs, in excellent agreement with Hall effect carrier mobilities previously determined for $n$-type single crystals. Analysis of the temperature dependence shows that the mobility at low temperatures is limited by ionized impurity scattering, while at room temperature the scattering of electrons with polar LO phonons dominates.

By application of 20 fs laser pulses, vibrational wave packets of low-energy modes (mainly 357 and 421 cm-1) were generated in the perylene chromophore that gave rise to periodic beats that lasted ...

The influence of near-surface crystal damage on carrier dynamics in silicon has been investigated with optical-pump THz-probe and THz emission studies. The surface damage is caused by a plasma process used for the fabrication of the ultrathin insulator within semiconductor–insulator–semiconductor (SIS) solar cells. The ion bombardment during the plasma process introduces a highly damaged subsurface region. Furthermore, the integration of positive interface charges leads to the formation of a depletion region that can be detected via the emitted THz radiation. The results are compared with classic I–U-characterization demonstrating that THz spectroscopy can be used as a supplementary tool for the investigation of process-induced surface damages.

Efficient photoelectrochemical devices for water splitting benefit from the highest material quality and dedicated surface preparation achieved by epitaxial growth. InP(100)-based half-cells show significant solar-to-hydrogen efficiencies, but require a bias due to insufficient voltage. Tandem absorber structures may provide both adequate potential and efficient utilization of the solar spectrum. We propose epitaxial dilute nitride GaPNAs photocathodes on Si(100) substrates to combine close-to-optimum limiting efficiency, lattice-matched growth, and established surface preparation. Prior to a discussion of the challenging III–V/Si(100) heterojunction, we describe the closely related epitaxial preparation of InP(100) surfaces and its beneficial impact on photoelectrochemical water-splitting performance. Analogies and specific differences to GaP(100) surfaces are discussed based on in situ reflectance anisotropy and on two-photon photoemission results. Preliminary experiments regarding GaP/Si(100) photoelectrochemistry and dilute nitride GaPN heteroepitaxy on Si(100) confirm the potential of the GaPNAs/Si tandem absorber structure for future water-splitting devices.

Some interesting phenomena have been described recently and attempts were made to understand them mechanistically. The keywords of these studies are, for example, excitonic, trapped, and delayed fluorescence.10-14 Beyond that, attention has been drawn to nonlinear optical effects,'s20 transitions to higher excited electron transfer from the conduction band of CdS to quenching molecule^^^-^^ or from one colloidal particle to others such as AgI," ZnO,= AB~S,~~ or solid, and colloidal Ti02.31J2,3436 CdS is a very attractive model substance due to its stability, easy preparation and handling, and last, but not least, its distinct band gap that enables one to easily detect a number of optical properties. All the performed experiments underline the importance of the surface condition of the respective CdS particles. This is due to their very small size