Transient Surface Photovoltage Research Articles

Charge reaction kinetics on TiO2 nanotubes under photoelectrochemical water oxidation condition

Artificial photosynthesis relies on the photoreactions between the chemical molecules and photogenerated charges. The oxygen evolution reactions are regarded as the most difficult procedure, because they are multiple-step reactions with multiple charges transfer. Notwithstanding the extensive study on photoelectrochemical and photochemical water oxidation, understanding how the surface charge reacts with the chemical molecules is limited, especially the reaction kinetics of the surface charges. We prepared anatase TiO2 nanotubes array and dense polycrystalline TiO2 films on Ti substrates, where the photocurrent increased about 4–4.5 times on nanotubes compared with the dense films. Moreover, the surface charge reaction rate and the accumulated charge densities were analyzed by transient surface photovoltage and transient photocurrent techniques. It was found a 2nd-order charge reaction behavior could be found when the charge density was high, differently, a 1st-order charge reaction could be observed when the charge density was low. The nanotubes structures would in situ grow from the Ti substrates, which simultaneously enhanced charge separation and transfer kinetics to improve the PEC performance.

Transient surface photovoltage spectroscopy of diamond

Contactless and highly sensitive probing of electronic transitions in diamond over a wide spectral range from near infrared to deep ultraviolet is still challenging. Surface photovoltage (SPV) signals depend on electronic transitions and transport phenomena leading to charge separation in space and allow for a contactless study of electronic transitions. Here, transient SPV spectroscopy in an arrangement with a charge amplifier and a laser tunable over a wide range was applied to study an undoped diamond single crystal between 0.8 and 5.9 eV at room temperature in ambient air. SPV transients were measured without and with weak visible bias light, which allowed for suppression of possible parasitic contributions in SPV signals not related to diamond and distinction of processes of charge separation that were independent of band bending. Transitions at 1.0 and 3.1 eV led to preferential separation of photogenerated holes toward the surface. In contrast, a transition at 1.8 eV caused preferential separation of photogenerated electrons toward the surface. Transitions near the indirect bandgap of diamond were observed at 5.27, 5.32, 5.48, and 5.53 eV and could be assigned to absorption assisted (i) by an indirect exciton and absorption of longitudinal optical and acoustic phonons, (ii) by absorption of transverse acoustic phonons, (iii) by emission of transverse acoustic phonons, and (iv) by emission of longitudinal optical and acoustic phonons, respectively. Charge separation under excitation at 5.27 eV was caused by directed charge transfer at/near the diamond surface after exciton diffusion followed by exciton dissociation.

Transient Surface Photovoltage Spectroscopy of (NH4)2Mo3S13/WSe2 Thin-Film Photocathodes for Photoelectrochemical Hydrogen Evolution.

Hydrogen produced from solar energy has the potential to replace petroleum in the future. To this respect, there is a need in the abandoned and efficient materials that can continuously split water molecules using solar energy. In this report, an ammonium thiomolybdate (ATM: (NH4)2Mo3S13) is evaluated as a p-type semiconductor film photocathode for hydrogen evolution reaction. The ATM thin films are prepared by spin-coating on fluorine-doped tin oxide substrates, and their structural, morphological, optical, photoelectrical, and photoelectrochemical (PEC) properties are studied. Transient surface photovoltage (TSPV) spectroscopy and spectroscopic ellipsometry indicate the band gap Eg = 1.9 eV for the ATM thin films. Furthermore, the photovoltage of the ATM thin films measured by TSPV is correlated to the photocurrents measured by the PEC characterization that can be used to evaluate the material potential for hydrogen generation. The films exhibit a low photocurrent density of 46 μA cm-2 at 0 VRHE. However, its combination with WSe2 thin-film photocathodes results in a significant increase in photocurrent density up to 4.6 mA cm-2 at 0 VRHE (100 times). The reason for such a strong charge carrier transfer effect for ATM/WSe2 heterojunction photocathodes is studied by TSPV spectroscopy that allows a comprehensive evaluation of potential photovoltaic materials toward PEC hydrogen production. Furthermore, the photovoltage generated by a WSe2 thin film is 30 times lower than that of its single crystal, which indicates that the quality of WSe2 thin films should be improved for faster PEC hydrogen evolution.

Wavelength dependent photoinduced charge carrier dynamics of heterojunction materials: The case of CuO/ZnO

Heterojunction materials composed of narrow and wide band-gap semiconductors possess different photoelectric responses under different wavelength incident light, and show distinct photogenerated charge carrier dynamics. In this work, taking the hydrothermal method fabricated CuO nanosheets / ZnO nanorods heterojunction materials as example, the wavelength dependent photoinduced charge carrier dynamics in this heterojunction material have been carefully studied. Transient surface photovoltage (TSPV) measurements reveal that the photogenerated holes gather at the surface of the fabricated CuO / ZnO with 355 nm light illumination, but photogenerated electrons gather at the surface under 532 nm light. Photocatalytic degradation experiments suggest that CuO / ZnO heterojunction material has better performance under ultraviolet light than that under visible light.

SnO2 as thin conformal layer over BiVO4 surface for enhanced charge carrier separation towards O2 evolution from water oxidation

Recently, bismuth vanadate (BiVO4) has attracted substantial attention for photocatalytic applications owing to its many advantages as semiconductor to drive water oxidation. Despite possessing promising characteristics to be applied in photocatalysis, the fast electron-hole pairs recombination is still one of the drawbacks involving this semiconductor. The use of conformal layers to minimize such limitation and to improve charge separation is the aim of this work. Herein, we report on the use of SnO2 as a conformal coating layer over the BiVO4 surface for O2 evolution reaction. The incorporation of SnO2 over the BiVO4 surface showed the beneficial effect in O2 evolution compared to the pristine material, resulting in 490 μmol after 6 h of irradiation. Transient absorption and surface photovoltage spectroscopies indicate enhanced charge carrier separation and transport by passivating BiVO4 with SnO2 thin layer. Even though the O2 evolution tests in this work were carried out without the presence of a co-catalyst, the use of a thin conformal coating of SnO2 over BiVO4 showed promising results for further studies.

Charge transfer rates and electron trapping at buried interfaces of perovskite solar cells

<h2>Summary</h2> Identification of electronic processes at buried interfaces of charge-selective contacts is crucial for photovoltaic and photocatalysis research. Here, transient surface photovoltage (SPV) is used to study the passivation of different hole-selective carbazole-based SAMs. It is shown that transient SPV and transient photoluminescence provide complementary information on charge transfer kinetics and trapping/de-trapping mechanisms, and that trap-assisted non-radiative recombination losses originate from electron trapping at the SAM-modified ITO/perovskite interface. The hole transfer rates and the density of interface electron traps, obtained by fitting SPV transients with a minimalistic kinetic model, depended strongly on the SAM's chemical structure, and densities of interface traps as low as 10⁹ cm⁻², on par with highly passivated c-Si surfaces, were reached for Me-4PACz, previously used in record perovskite/silicon tandem solar cells. The extracted hole transfer rate constants and interface trap densities correlated well with the corresponding fill factors and open-circuit voltages of high-efficiency solar cells.

Surface Photovoltage Spectroscopy over Wide Time Domains for Semiconductors with Ultrawide Bandgap: Example of Gallium Oxide

A nonconventional approach is proposed for the measurement of surface photovoltage (SPV) signals over very wide ranges in photon energy and time. Regimes for AC, DC, and combined AC–DC measurements are defined and applied for the characterization of a β‐Ga2O3 crystal by transient and modulated SPV spectroscopy, spectroscopy in the mode of a Kelvin probe and single‐pulse SPV transients from 10 ns to 1000 s with the same electrode. Numerous electronic transitions are distinguished in β‐Ga2O3 depending on the measurement regime, the time response and the history of measurement. An accumulation of negative charge at the surface of β‐Ga2O3 is observed at long times independent whether the SPV signals are positive or negative before. The nonconventional approach of measuring SPV signals opens new opportunities for investigating electronic properties of semiconductors with ultrawide bandgaps and any other photoactive materials.

A ternary calabash model photocatalyst (Pd/MoP)/CdS for enhancing H2 evolution under visible light irradiation

Noble metal and non-noble metal co-catalysts have their own advantages for improving photocatalytic hydrogen evolution. Here, we report a new composite co-catalyst (Pd/MoP) and a ternary photocatalyst (Pd/MoP)/CdS for hydrogen evolution. The optimum photocatalytic hydrogen evolution of (Pd/MoP)/CdS reaches 943 μmol/h, which is 2.1 times of MoP/CdS and 4.4 times of Pd/CdS. Transient photocurrent responses, photoluminescence spectroscopies, and surface photovoltage spectroscopies demonstrated the combination of (Pd/MoP) makes high-efficiency of charge separation on CdS. Galvanostatic charge/discharge and cyclic voltammetry tests revealed MoP mainly serves as a reservoir to store photo-excited electrons from CdS. Electrochemical impedance spectra and linear sweep voltammetry measures revealed Pd mainly acts as active sites for hydrogen evolution. Most importantly, the functions of each part of ternary photocatalyst for hydrogen evolution have been clarified, and a ternary calabash model photocatalyst for hydrogen evolution is put forward.

Frequency-Dependent Sonochemical Processing of Silicon Surfaces in Tetrahydrofuran Studied by Surface Photovoltage Transients.

The field of chemical and physical transformations induced by ultrasonic waves has shown steady progress during the past decades. There is a solid core of established results and some topics that are not thoroughly developed. The effect of varying ultrasonic frequency is among the most beneficial issues that require advances. In this work, the effect of sonication of Si wafers in tetrahydrofuran on the photovoltage performance was studied, with the specific goal of studying the influence of the varying frequency. The applied ultrasonic transducer design approach enables the construction of the transducer operating at about 400 kHz with a sufficient sonochemical efficiency. The measurements of the surface photovoltage (SPV) transients were performed on p-type Cz-Si(111) wafers. Sonication was done in tetrahydrofuran, methanol, and in their 3:1 mixture. When using tetrahydrofuran, the enhanced SPV signal (up to ≈80%) was observed due to increasing sonication frequency to 400 kHz. In turn, the signal was decreased down to ≈75% of the initial value when the frequency is lowered to 28 kHz. The addition of methanol suppressed this significant difference. It was implied that different decay processes with hydrogen decomposed from tetrahydrofuran could be attempted to explain the mechanism behind the observed frequency-dependent behavior.

Using a CdS under-layer to suppress charge carrier recombination at the Ag2S/FTO interface

The recombination of photoinduced charge carriers is detrimental to the photovoltaic performance of solar cells. In this work, we used CdS as an under-layer to reduce the recombination of photoinduced charge carriers at the Ag2S/FTO (F-doped SnO2) interface. The CdS under-layer was deposited onto an FTO substrate using a facile chemical bath deposition (CBD) method. By controlling the thickness of the CdS film between Ag2S and FTO, the recombination of photoinduced charge carriers was successfully suppressed. The effect of the CdS under-layer on the charge carrier recombination was carefully evaluated by transient surface photovoltage (TSPV) measurements, electrochemical impedance spectroscopy (EIS) and a photoelectrochemical method. The power conversion efficiency (PCE) of solar cell devices with a 40 nm CdS under-layer is 1.16%, which is ~52% enhancement compared to those of FTO/Ag2S-based solar cell devices. Our work may provide a useful strategy to suppress photoinduced charge carrier recombination at the electrode and absorber semiconductor interface.

Boosting visible-light-driven photocatalytic performance of waxberry-like CeO2 by samarium doping and silver QDs anchoring

In this work, waxberry-like CeO2 photocatalyst (denoted ASC) with prominent visible-light-driven photocatalytic performances for multi-model reactions was achieved by Sm doping and Ag quantum dots (QDs) anchoring. For instance, the as-fabricated ASC acquired 7.08-times and 6.83-times higher activities for CH3CHO removal and H2 production than those of pure CeO2 counterpart, respectively. The concentration of oxygen vacancies (Ov) in CeO2 is distinctly increased by Sm doping, resulting in a narrower bandgap of the Sm-doped CeO2 (SC). Under visible light irradiation, the Ov caused by doping can capture the photo-excited electrons and construct a doping-related transition state between the conduction band (CB) and the valence band (VB), which can effectively limit the recombination of photo-excited electrons and holes. These captured electrons further fleetly transfer to the co-catalytic sites of anchored Ag QDs, strengthening the absorption utilization for visible-light synchronously. The migration of charge carriers and proposed mechanisms were well elaborated by transient photovoltage (TPV), surface photovoltage (SPV) and density functional theory (DFT) calculation. It is hoped our work in this paper could provide potential and meaningful strategies for the design of noble metal quantum dots modified metal oxide semiconductors and facilitate their applications in other photocatalytic fields effectively.

The study of photogenerated charge behavior and photocatalytic hydrogen evolution on g-C3N4 decorated with PtCox bimetal

Exploration of unique and useful photocatalysts is of great importance to address the energy and environmental concerns by utilizing solar energy. Graphite carbon nitride (g-C3N4) is regarded as an effective photocatalyst for the photocatalytic hydrogen evolution. However, the hydrogen production rate of bare g-C3N4 is almost negligible due to a quick recombination of photogenerated electrons and holes. Herein, we modified g-C3N4 with PtCox cocatalyst to accelerate the transfer of photogenerated charges and to further restrain the recombination of photogenerated electrons and holes in bulk. As a result, the superior electrical conductivity of PtCox promotes the migration of photogenerated charges and further improves the separation efficiency of photogenerated charges for PtCox/g-C3N4 photocatalyst, meanwhile, the low overpotential of PtCox accelerates the reaction kinetics. The enhanced photocatalytic activity of 2.5PtCox results in hydrogen evolution rate of 11.9 mmol h−1 g−1. PtCox/g-C3N4 photocatalyst was characterized by systematically examining the photogenerated charges behavior through studying surface photovoltage, transient surface photovoltage, surface photocurrent, and photoacoustic technology. Based on the obtained results, a photocatayltic mechanism is proposed and discussed in detail. This work improves the photocatalytic performance of g-C3N4 and contributes to a theoretical knowledge for the fabrication of future cocatalyst/photocatalyst systems.

Hydrogen evolution with fluorescein-sensitized Pt/SrTiO3 nanocrystal photocatalysts is limited by dye adsorption and regeneration

Photocatalysts are one of several technologies for the conversion of sunlight and water into hydrogen fuel. Here, we show that the photocatalytic activity of fluorescein-sensitized platinum modified SrTiO3 nanocubes for visible light hydrogen evolution from aqueous solutions of methanol, potassium iodide, potassium hexacyanoferrate (II), and triethanolamine is limited by light absorption from the fluorescein and by its self-oxidation, even in the presence of the sacrificial donors. The highest hydrogen evolution rate was observed with Pt/SrTiO3 suspended in 0.5 mM fluorescein at pH 13 with triethanolamine as a sacrificial donor (60 μmol h−1, 0.67 % AQE at 2.64 eV). The other donors (methanol and KI) gave lower H2 rates or no H2 (hexacyanoferrate (II)). This trend can be rationalized with surface photovoltage spectroscopy (SPS) measurements on FL-sensitized SrTiO3 films on indium tin oxide substrates. Electron transfer from the dye to SrTiO3 under 2.75 eV illumination and hole transfer to the sacrificial donors are modulated by the redox potentials of the donors and acceptors. The low activity of hexacyanoferrate appears to be due to e/h recombination, while triethanolamine appears to replace the dye from the SrTiO3 nanocrystals under the conditions of the SPS experiment. Photovoltage values suggest charge transfer is accompanied by irreversible self-oxidation of FL and this is confirmed by transient surface photovoltage measurements. Overall these results suggest that system optimization requires dyes with greater redox stability and stronger binding affinity to the SrTiO3 nanocrystal surface.

Influence of Surface States and Mobility on Charge Transport in BiVO4 Investigated By Surface Photovoltage Spectroscopy

In order to enable mass production of photocatalytically active electrodes for direct conversion of solar energy into hydrogen, abundant materials and inexpensive preparation methods are needed. BiVO4 photoelectrodes prepared by aerosol deposition satisfy these requirements, in addition offering reasonable photocurrent densities, and can potentially contribute to the establishment of a prospective economy based on hydrogen as energy source.Nevertheless, in order to allow the economically feasible implementation of BiVO4 electrodes for direct water splitting, there is need for further improvement of the electrodes efficiency.In particular, an increased understanding of factors limiting the photocurrent is needed. In this work, BiVO4 photoelectrodes, prepared by aerosol deposition of BiVO4 particles on FTO-substrates, were investigated by surface photovoltage (SPV) as well as photocurrent and Mott-Schottky measurements.Surface photovoltage spectroscopy under front and back illumination gives evidence of limitation of charge transport by a significant lower mobility of electrons compared to holes, leading to a nearly tenfold higher photocurrent, when illuminated from the back, compared to illumination from the front. Furthermore, the presence of traps for holes as well as for electrons on the BiVO4 surface has been confirmed by transient surface photovoltage spectroscopy and their influence on the photocurrent is discussed. Figure 1

(Invited) Surface Photovoltage Analysis of Photocatalytic Materials

A brief review of surface photovoltage (SPV) techniques and their application to photocatalytic materials is given. SPV is defined by the light-induced change of the contact potential difference (∆CPD) between two electrodes, i.e. SPV signals can be obtained whenever photogenerated charge carriers are separated in space [1]. SPV signals depend on processes of light absorption, photogeneration, directed charge transfer, charge transport and recombination. Therefore, SPV techniques can be well applied to the analysis of electronic, transport and recombination properties of photoactive materials [2].Basics of SPV are introduced including the relaxation of a disturbed space charge, the center of charge approach and mechanisms of charge separation. Principles of SPV measurements will be given and illustrated with applications. Depending whether fixed capacitors, Kelvin probes, electron beams or photoelectrons are applied (see also figure 1), SPV signals can be measured at crystals, layers or powders of photocatalysts with high sensitivity over wide ranges in time or high resolution in space and in different ambient (gases, vacuum, electrolytes). In-operando SPV measurements are realized.For measurements with fixed capacitors, SPV signals are coupled out directly with a high-impedance buffer. Due to the discharge of the measurement capacitor via a very large measurement resistance, the application of fixed capacitors is limited to transient [3] and modulated [4] SPV measurements. With fixed capacitors, the widest range in time ((sub)ns...ms...h, transient, example: nano-composite of TiO2/In2S3 [5]) and the highest sensitivity (10...100 nV, modulated, see [6] for defect analysis in (FA0.85MA0.1Cs0.05)PbBr3) are reached whereas no additional interactions are involved. Attention will be paid to the application of transient SPV spectroscopy [7] at constant photon flux for the investigation of defect states and charge transfer across interfaces [8] (c-Si(n++)/TiO2). Furthermore, principles of random walk simulations for the modeling of transient [9] and modulated [10] SPV signals in small systems will be discussed [2].With Kelvin probes, the ac current between a sample and a vibrating electrode (vibrating electrode [11], most applied technique, see, for example [12]) or the electrostatic force between the sample surface and a vibrating tip electrode (Kelvin probe force microscopy [13] [14], KPFM) are nulled by applying an external bias potential. The highest resolution in space is reached for SPV measurements with KPFM (tens of nm). Examples will be shown for SPV measurements with vibrating electrodes for TiO2 [15] and KPFM for Cu2O [16] and BiVO4 with co-catalysts [17].In SPV measurements with electron beams [18, 19], a low electron current is kept constant with a compensating bias potential. In photoelectron spectroscopy, core levels shift with regard to ∆CPD. Additional interactions of the sample with electrons and/or x-rays are possible. The application of synchrotron radiation allows for SPV measurements at extremely short times [20, 21]. Further opportunities opened for SPV measurements by ambient pressure photoelectron emission spectroscopy [22].[1] review of L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1.[2] Th. Dittrich, S. Fengler, Surface photovoltage analysis of photoactive materials, World Scientific (2019), ISBN: 978-1-78634-765-7.[3] E. O. Johnson, J. Appl. Phys. 28 (1957) 1349.[4] V. Duzhko, et al., Phys. Rev. B 64 (2001) 075204.[5] Th. Dittrich, et al., Rev. Sci. Instrum. 88 (2017) 053904 and 90 (2019) 026102.[6] I. Levine, et al., ACS Energy Lett. 4 (2019) 1150.[7] Th. Dittrich, et al., Appl. Phys. Lett. 110 (2017) 023901.[8] S. Fengler, et al., submitted.[9] S. Fengler, et al., J. Phys. Chem. C 117 (2013) 6462.[10] S. Fengler, Th. Dittrich, J. Phys. Chem. C 120 (2016) 17777.[11] K. Besocke, S. Berger, Rev. Sci. Instrum. 47 (1976) 840.[12] M. A. Melo et al., Nano Lett. 18 (2018) 805.[13] M. Nonnenmacher, et al., Appl. Phys. Lett. 58 (1991) 2921.[14] T. Glatzel, S. Sadewasser (edts.) Kelvin probe force microscopy – from single charge detection to device characterization, Springer, Cham. (2018).[15] M. K. Nowotny, et al., Mater. Lett. 64 (2010) 928.[16] R. Chen, et al., Nano Lett. 19 (2019) 426.[17] J. Zhu, et al., Nano Lett. 17 (2017) 6735; R. Chen, et al., Chem. Soc. Rev. 47 (2018) 8238.[18] F. Steinrisser, R. E. Hatrick, Rev. Sci. Instrum. 42 (1971) 3014.[19] J. Clabes, M. Henzler, Phys. Rev. B 21 (1980) 625.[20] W. Widdra, et al., Surf. Sci. 543 (2003) 87.[21] S. Yamamoto, I. Matsuda, J. Phys. Soc. Jpn. 82 (2013) 021003.[22] J. R. Harwell et al., Phys. Chem. Chem. Phys. 18 (2016) 19738. Figure 1

Defects Near c-Si(n++)/TiO2 Interfaces Revealed By Persistent Charging Analysis in Modulated Surface Photovoltage Spectroscopy

Thin layers of semiconductors with wide band gaps (wide gap SCs), such as TiO2, are of great interest for the passivation and/or specific activation of semiconductor surfaces in photoelectrochemical (PEC) systems [1]. Defect states in the bulk of wide gap SCs and at related semiconductor hetero-junctions play an important role for the electronic properties of PEC systems. Due to high activation energies in wide gap SCs, electronic states can be charged for relatively long times. This persistent charging of wide gap SCs can have large impact on photogeneration and modulated charge separation in modulated surface photovoltage (SPV) spectroscopy and thus opens a new perspective for the investigation of electronic defect states at/near interfaces with wide gap SCs. In this work, persistent charging is applied to the investigation of defect states in c-Si(n++)/TiO2(ALD) systems before and after conversion from amorphous TiO2 to anatase by post annealing.Thin layers of amorphous TiO2 with thicknesses of up to 100nm were deposited onto highly doped silicon wafers (c-Si(n++)) and converted into anatase by annealing for 30 min at 450°C. Modulated SPV measurements were performed at room temperature in the fixed capacitor arrangement [2] with a quartz prism monochromator and a xenon arc lamp for excitation over a range from 0.8 to 4.5 eV. Incidentally, the in-phase (X) or phase-shifted by 90° (Y) signals are much faster or slower, respectively, than the modulation period (see for details also [3]).The X- and Y- signals of two modulated SPV spectra were measured subsequently within about 30 min. This kind of measurement regime opened the opportunity for the analysis of the influence of charging on electronic transitions of defect states in c-Si(n++)/TiO2(ALD) systems and it will be straight forward to compare those results with the optical properties of holes and electrons trapped in TiO2 [4].As an example, the figure shows modulated SPV spectra of an as prepared (a) and annealed (b) sample. The X-signals of the as-prepared sample where quenched with ongoing absorption in amorphous TiO2 whereas the onset energy for quenching shifted by more than 0.1 eV towards lower energies for the second scan. In contrast, the Y-signals of the as-prepared sample became more negative between about 3 and 3.5 eV and changed to more positive and changed sign at higher photon energies. Electrons photogenerated in amorphous TiO2 near the interface with c-Si were transferred into c-Si and partially trapped near the interface resulting in an increase of the SPV signals related to photogeneration in c-Si(n++) and in quenching of charge transfer from amorphous TiO2 into c-Si(n++) in the second scan.For the annealed sample, the signals were much larger and modulated charge separation with opposite direction at the c-Si and TiO2 surfaces became dominant. The Y-signals for the virgin scan were larger than for the second scan at photon energies up to about 2.5 eV, but smaller for higher photon energies, i.e. photogeneration by defects in anatase caused an increase of positive charge at the interface with c-Si(n++) and of negative charge at the external surface whereas the signature of the hole traps nearly disappeared in the second scan shown in the figure.Furthermore, the influence of the specific spectrum of the photon flux could be widely eliminated by analyzing the spectra of the ratio of the amplitudes before and after charging (not shown) which allowed for the extraction of defect peaks induced by charging.Acknowledgement: The authors are grateful to A. Schwartzberg, D. Olynick and S. Cabrini for their insight and support for the preparation and coating of the samples by atomic layer deposition. The authors furthermore gratefully acknowledge financial support by the Federal Ministry of Education and Research of Germany, in the framework of the project "FocusH2" (No. 03SF0479A). The transient SPV spectroscopy instrumentation was developed at HZG with support from the Helmholtz Association of German Research Centres, within the "HEMF" platform (Helmholtz Energy Materials Foundry). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.[1] S. Hu et al., Science 344 (2014) 1005.[2] V. Duzhko et al., Phys. Rev. B 64 (2001) 075204.[3] Th. Dittrich, S. Fengler, Surface photovoltage analysis of photoactive materials, World Scientific. ISBN No. 978-1786347657 (2019).[4] D. Bahnemann et al., J. Phys. Chem. 88 (1984) 709; Bahnemann et al., J. Phys. Chem. B 101 (1997) 4265; R. Memming, Semiconductor Electrochemistry, Wiley-VCH (2001 2nd ed. 2015); J. Schneider and D. Bahnemann, J. Phys. Chem. C 122 (2018) 13979. Figure 1

Ultraviolet-ozone modification on TiO2 surface to promote both efficiency and stability of low-temperature planar perovskite solar cells

Abstract As a classical electron transport layer, the high crystallinity TiO2 has been widely used in perovskite solar cells (PSCs), however, its high-temperature preparation process elevates the fabrication cost and limits its application. Here, we report an ultraviolet-ozone assisted strategy to modify low-temperature TiO2 interface for PSCs. In addition to the more appropriate work function and reinforced built-in potential, the lattice strain of perovskite films crystallized on modified TiO2 has also been released in some degree. Ultrafast transient absorption technique is employed to provide a deep insight into the carrier dynamics, revealing that less non-radiative recombination exists in the modified device. Interestingly, transient surface photovoltage results demonstrate that ultraviolet-ozone modification can efficiently suppress the decomposition of perovskite films under light illumination. Taking advantage of these facts, this device exhibits better efficiency and remarkable stability. This demonstrated low-temperature strategy is a promising way for fabricating low-cost, efficient and stable perovskite device.

Fabrication of novel CuFe2O4/MXene hierarchical heterostructures for enhanced photocatalytic degradation of sulfonamides under visible light

The overuse of sulfonamides, causing serious pollution of water bodies, has drawn great attention from society. To address these problems, a novel CuFe2O4/MXene (CFO/Ti3C2) heterojunction photocatalyst was used to photodegrade the antibiotic sulfamethazine (SMZ, a typical pollutant) under visible light, and the synergy and coupling function of the two components in the heterojunction system were analyzed. With the aid of time-resolved photoluminescence (TRPL) and transient surface photovoltage (TPV) spectra, the carrier lifetimes and kinetic behaviors were studied, revealing that the lifetime of photoinduced carriers was prolonged by loading Ti3C2, inhibiting the reorganization of photogenerated electron holes. More importantly, the organic intermediates and mineralization degree were identified by high-performance liquid chromatography (HPLC)-mass spectrometry (MS) and total organic carbon (TOC) techniques. The results show that the breaking of SN bonds, the oxidation of aniline and deamination were dominated by the attack of •OH. This work shows a new model for the degradation mechanism of SMZ over CFO/MXene heterostructures.

Charge Transfer in c-Si(n++)/TiO2(ALD) at the Amorphous/Anatase Transition: A Transient Surface Photovoltage Spectroscopy Study.

Electronic properties and charge transfer processes were studied in an n-Si(n++)/TiO2(ALD) system at an amorphous TiO2/anatase transition by transient surface photovoltage spectroscopy at constant photon flux. The TiO2 layers were deposited by atomic layer deposition (ALD) onto highly doped silicon (c-Si(n++)), and the phase composition of the TiO2 layers changed with increasing thickness from amorphous to the anatase polymorph as anatase crystallites started to grow at the surface. Depending on phase composition, the band gap of TiO2 correlated with the characteristic energy of exponential tails. In most cases, photogenerated electrons were separated toward the back contact. For photogeneration in c-Si(n++), electron back transfer was limited by Auger recombination with holes in the surface space charge region of c-Si(n++), and by electron transfer across the interface, either via exponentially distributed states near the conduction band edge of amorphous TiO2 or via distance-dependent recombination with holes trapped in anatase. For photogeneration in TiO2, electron back transfer was limited by trapping in TiO2. Under strong light absorption in amorphous TiO2 with anatase crystallites on top, electrons were preferentially separated toward the TiO2 surface.

Reducing the Schottky Barrier by SnS2 Underlayer Modification to Enhance Photoelectric Performance: The Case of Ag2S/FTO.

The transfer and recombination of photoinduced charge carriers play the crucial roles in a photoelectric conversion system. In this work, the Ag2S/fluorine-doped tinoxide (FTO) was used as the platform to understand the photoinduced charge carrier transfer and recombination at the light absorber and electrode interface. SnS2 was evaporated onto the FTO surface to cooperate the Fermi level with Ag2S, which reduced the Schottky barrier at the Ag2S/FTO interface. Kelvin probe force microscopy measurements reveal that the Fermi level of FTO can be tuned from -4.93 to -4.75 eV by various SnS2 with different evaporation amounts. Transient surface photovoltage tests confirm that the recombination of the photogenerated charge carrier can be drastically suppressed. The photoelectric conversion efficiency of the resulting solar cell devices has been significantly improved.