Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN2, and find that above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 to above 30 THz. Our results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN2. Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. Our study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields.

The interaction of water molecules with semiconductor surfaces is relevant to various optoelectronic phenomena and physicochemical processes. Despite advances in fundamental understanding of water‐exposed surfaces, the detailed time‐ and energy‐resolved behavior of excited electrons remains largely unexplored. Here, the effects of water exposure on the near‐surface electron dynamics of phosphorus‐terminated p(2×2)/c(4×2)‐reconstructed indium phosphide (100) (P‐rich InP) are studied experimentally and matched to theoretical calculations. The P‐rich InP surface, consisting of H‐passivated P‐dimers, serves as a model for other P‐containing III‐V semiconductors such as gallium phosphide (GaP) or aluminum indium phosphide (AlInP). Electron dynamics near the surface are probed with femtosecond resolution using time‐resolved two‐photon photoemission (tr‐2PPE), a pump‐probe spectroscopic technique. Pulsed water exposure preserves electronic states and significantly increases lifetimes at the conduction band minimum (CBM). Density‐functional theory (DFT) calculations attribute these findings to suppression of surface vibrational modes in the top P‐layer by water exposure, reducing electronic transition probabilities of near‐band‐gap surface states. The results suggest that many near‐surface state lifetimes reported in ultra‐high vacuum may change significantly upon electrolyte exposure. These states may thus contribute more strongly to surface reactions than traditionally assumed. Demonstrating this effect for the technologically relevant P‐rich InP surface opens new opportunities in this underexplored area of surface electrochemistry.

Renewable ("green") hydrogen production through direct photoelectrochemical (PEC) water splitting is a potential key contributor to the sustainable energy mix of the future. We investigate the potential of indium phosphide (InP) as a reference material among III-V semiconductors for PEC and photovoltaic (PV) applications. The p(2 x 2)/c(4 x 2)-reconstructed phosphorus-terminated p-doped InP(100) (P-rich p-InP) surface is the focus of our investigation. We employ time-resolved two-photon photoemission (tr-2PPE) spectroscopy to study electronic states near the band gap with an emphasis on normally unoccupied conduction band states that are inaccessible through conventional single-photon emission methods. The study shows the complexity of the p-InP electronic band structure and reveals the presence of at least nine distinct states between the valence band edge and vacuum energy, including a valence band state, a surface defect state pinning the Fermi level, six unoccupied surface resonances within the conduction band, as well as a cluster of states about 1.6 eV above the CBM, identified as a bulk-to-surface transition. Furthermore, we determined the decay constants of five of the conduction band states, enabling us to track electron relaxation through the bulk and surface conduction bands. This comprehensive understanding of the electron dynamics in p-InP(100) lays the foundation for further exploration and surface engineering to enhance the properties and applications of p-InP-based III-V-compounds for, e.g., efficient and cost-effective PEC hydrogen production and highly efficient PV cells.

The current efficiency records for generating green hydrogen via solar water splitting are held by indium phosphide (InP)-based photo-absorbers, protected by TiO2 layers grown through atomic layer deposition (ALD). InP is also a leading material for photonic integrated circuits and computing, where ultrafast near-surface behavior is key. A previous study described electronic pathways at the phosphorus-rich (P-rich) surface of p-doped InP(100) using time-resolved two-photon photoemission (tr-2PPE) spectroscopy. Here, the intricate electron pathways of the P-rich InP surface modified with ALD-deposited TiO2 are explored. Photoexcited bulk InP electrons migrate through a bulk-to-surface transition cluster of states and surface states and inject into the TiO2 conduction band (CB). Energy levels and occupation dynamics of CB states in P-rich InP and TiO2 adlayers are observed, with discrete states preserved up to 10 nm TiO2 deposition. Thermalization lifetimes of excited electrons > 0.8 eV above the InP conduction band minimum (CBM) are preserved for layer thicknesses up to 2.5 nm. Annealing at 300 degrees C to achieve crystalline TiO2 reconstructions destroys interfacial states, affecting charge transfer. These observations enable innovative engineering of the P-rich InP/TiO2 heterointerface, opening new possibilities for studying hot-carrier extraction, adsorbate effects, surface plasmons, and improving photovoltaic and PEC water-splitting devices.

Metal oxides are considered as stable and low‐cost photoelectrode candidates for hydrogen production by photoelectrochemical solar water splitting. However, their power conversion efficiencies usually suffer from poor transport of photogenerated charge carriers, which has been attributed previously to a variety of effects occurring on different time and length scales. In search for common understanding and for a better photo‐conducting metal oxide photoabsorber, CuFeO 2 , α‐SnWO 4 , BaSnO 3 , FeVO 4 , CuBi 2 O 4 , α‐Fe 2 O 3 , and BiVO 4 are compared. Their kinetics of thermalization, trapping, localization, and recombination are monitored continuously 100 fs–100 µs and mobilities are determined for different probing lengths by combined time‐resolved terahertz and microwave spectroscopy. As common issue, we find small mobilities < 3 cm 2 V ‐1 s ‐1 . Partial carrier localization further slows carrier diffusion beyond localization lengths of 1–6 nm and explains the extraordinarily long conductivity tails, which should not be taken as a sign of long diffusion lengths. For CuFeO 2 , the localization is attributed to electrostatic barriers that enclose the crystallographic domains. The most promising novel material is BaSnO 3 , which exhibits the highest mobility after reducing carrier localization by annealing in H 2 . Such overcoming of carrier localization should be an objective of future efforts to enhance charge transport in metal oxides.

Long diffusion lengths of photoexcited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Time-resolved photoconductance measurements are often used to determine diffusion lengths in conventional semiconductors. However, effects such as polaron formation or multiple trapping can lead to time-varying mobilities and lifetimes that are not accounted for in the conventional calculation of the diffusion length. Here, a generalized analysis is presented that is valid for time-dependent mobilities and time-dependent lifetimes. The diffusion length is determined directly from the integral of a photoconductivity transient and can be applied regardless of the nature of carrier relaxation. To demonstrate our approach, photoconductivity transients are measured from 100 fs to 1 µs by the combination of time-resolved terahertz and microwave spectroscopy for BiVO_{4}, one of the most studied metal oxide photoanodes for photoelectrochemical water splitting. The temporal evolution of charge carrier displacement is monitored and converges after about 100 ns to a diffusion length of about 15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a-Si:H, c-Si, and halide perovskite, which underlines its potential to determine the diffusion length in a wide range of semiconductors, including disordered materials.

A major problem toward using renewable energy is the lack of appropriate storage possibilities. Photoelectrochemical water splitting to produce hydrogen as solar fuel represents a possible solution. As potential semiconductor photoelectrodes, metal oxides have come into the focus of research activities. BiVO4 is one of the best performing materials. However, localization and slow transport of photoexcited charge carriers lead to early recombination before they reach the semiconductor surface where they can perform a chemical reaction. Carrier trapping at intrinsic and extrinsic defects in the crystal lattice as well as polaron formation are main efficiency limiting factors in most metal oxides, including BiVO4. Ultrafast optical laser spectroscopy and time-resolved X-ray techniques along with theoretical calculations provide a vast complementary view of the related photodynamics and underlying physics of carrier localization processes. This chapter reviews some of the main findings regarding transport and loss dynamics of photogenerated charge carriers in BiVO4.

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.

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 though...

Recently, alpha-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 similar to 1.9 eV and an early photocurrent onset potential of similar to 0 V versus RHE. However, the mismatch between the charge carrier diffusion length and light penetration depth. which is typical for metal oxide semiconductors currently hinders the realization of high photoconversion efficiencies. In this work, the pulsed laser deposition process and annealing treatment of alpha-SnWO4 thin films are elucidated to optimize their charge carrier transport properties. A high-temperature treatment is found to enhance the photoconductivity of alpha-SnWO4 by more than 1 order of magnitude, as measured with time-resolved microwave conductivity (TRMC). A complimentary analysis by time-resolved terahertz spectroscopy (TRTS) shows that this improvement can be assigned to an increase of the grain size in the heat-treated films. In addition, TRTS reveals electron-hole charge carrier mobilities of up to 0.13 cm(2) V-1 s(-1) in alpha-SnWO4. This is comparable to values found for BiVO4, which is one of the best performing metal oxide photoanode materials to date. These findings show that there is a significant potential for further improving the properties of alpha-SnWO4 photoanodes.

The charge carrier dynamics of epitaxial hematite films is studied by time-resolved microwave (TRMC) and time-resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of approximate to 20-50 ns and 1-2 mu s. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.

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.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL), but TRPL may not be suitable or straightforward for all absorbers. Alternative non-contact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here we demonstrate the complementary use of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIRTR) spectroscopy along with TRPL to elucidate photoexcited carrier dynamics in a high-quality copper-poor, zinc-rich kesterite Cu2ZnSnSe4 (CZTSe) single crystal. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin film growth. A single-crystal-based PV device exhibited an efficiency of 6.2% and Voc of 400 mV, consistent with the quasi-Fermi level splitting determined using absolute photoluminescence. NIR-TR showed picosecond-scale cooling and relaxation of carriers into a distribution of band tail states while TRTS revealed a characteristic time scale of 200 – 260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of 50 – 100 cm/Vs. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed

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.

Time‐resolved two‐photon photoemission spectroscopy (tr‐2PPE) directly probes the kinetic energy and dynamics of photoemitted electrons. At the same time, the electronic structure and temporal occupation of surface‐near states can be accessed, which allows to unravel the fundamental processes governing electron dynamics and energetics in semiconductor surfaces. Here, recent studies on epitaxial III–V semiconductors and II–VI nanostructures are reviewed and the feasibility to study electron dynamics in III–V surface quantum wells (SQW) with tr‐2PPE is demonstrated. On InP(100), for example, surface states are filled by electrons relaxing from higher energetic bulk states. In the case of nanostructured materials, these effects play an even larger role due to the high surface to bulk ratio. For CdSe quantum dots, Auger recombination strongly competes with the exploitation of the quantum size dependent phonon bottleneck. The electron cooling dynamics in CdSe platelets are extremely fast and exhibit complete independence of Auger‐like processes. Finally, an InGaAs SQW/InP structure is shown to exhibit much longer lifetime of the quantum confined states. The SQW may act as a carrier accumulation layer for bulk electrons diffusing to the surface. Implications for future use in energy material systems for photovoltaic and photocatalytic applications are discussed.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL), but TRPL may not be suitable or straightforward for all absorbers. Alternative noncontact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here, we demonstrate the complementary use of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIRTR) spectroscopy along with TRPL to elucidate photoexcited carrier dynamics in a high-quality copper-poor, zinc-rich kesterite Cu2ZnSnSe4 (CZTSe) single crystal. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin-film growth. A single-crystal-based PV device exhibited an efficiency of 5.7% and an open circuit voltage (V-oc) of 400 mV, consistent with the quasi-Fermi-level splitting determined using absolute photoluminescence. NIRTR showed picosecond-scale cooling and relaxation of carriers into a distribution of band-tail states while TRTS revealed a characteristic time scale of 200-260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of 50-100 cm(2)/V s. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed understanding of performance-limiting photophysical processes and can accelerate the development of more efficient PVs.

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.

The effect of compositional variation on charge carrier lifetimes of Cr1Fe0.84Al0.16O3, 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 cm2/(Vs) are identified. Charge carrier lifetime and mobility are discussed as screening criteria for solar water splitting materials.