Fast Near-Infrared Photodetectors Based on Nontoxic and Solution-Processable AgBiS2.

Solution-processable near-infrared (NIR) photodetectors are urgently needed for a wide range of next-generation electronics, including sensors, optical communications and bioimaging. However, it is rare to find photodetectors with >300 kHz cut-off frequencies, especially in the NIR region, and many of the emerging inorganic materials explored are comprised of toxic elements, such as lead. Herein, solution-processed AgBiS2 photodetectors with high cut-off frequencies under both white light (>1 MHz) and NIR (approaching 500 kHz) illumination are developed. These high cut-off frequencies are due to the short transit distances of charge-carriers in the ultrathin photoactive layer of AgBiS2 photodetectors, which arise from the strong light absorption of this material, such that film thicknesses well below 120 nm are sufficient to absorb >65% of NIR to visible light. It is also revealed that ion migration plays a critical role in the photo-response speed of these devices, and its detrimental effects can be mitigated by finely tuning the thickness of the photoactive layer, which is important for achieving low dark current densities as well. These outstanding characteristics enable the realization of air-stable, real-time heartbeat sensors based on NIR AgBiS2 photodetectors, which strongly motivates their future integration in high-throughput systems.

Surface-Mediated Charge Transfer of Photogenerated Carriers in Diamond.

Solvated electrons are highly reductive chemical species whose chemical properties remain largely unknown. Diamond materials are proposed as a promising emitter of solvated electrons and visible light excitation would enable solar-driven CO2 or N2 reductions reactions in aqueous medium. But sub-bandgap excitation remains challenging. In this work, the role of surface states on diamond materials for charge separation and emission in both gaseous and aqueous environments from deep UV to visible light excitation is elucidated. Four different X-ray and UV-vis spectroscopy methods are applied to diamond materials with different surface termination, doping and crystallinity. Surface states are found to dominate sub-bandgap charge transfer. However, the surface charge separation is drastically reduced for boron-doped diamond due to a very high density of bulk defects. In a gaseous atmosphere, the oxidized diamond surface maintains a negative electron affinity, allowing charge emission, due to remaining hydrogenated and hydroxylated groups. In an aqueous electrolyte, a photocurrent for illumination down to 3.5 eV is observed for boron-doped nanostructured diamond, independent of the surface termination. This study opens new perspectives on photo-induced interfacial charge transfer processes from metal-free semiconductors such as diamonds.

Enhancing Hydrogen Evolution Reaction via Synergistic Interaction between the [Mo3S13]2- Cluster Co-Catalyst and WSe2 Photocathode.

A thiomolybdate [Mo3S13]2- nanocluster is a promising catalyst for hydrogen evolution reaction (HER) due to the high number of active edge sites. In this work, thiomolybdate cluster films are prepared by spin-coating of a (NH4)2Mo3S13 solution both on FTO glass substrates as hydrogen evolving electrodes and on highly 00.1-textured WSe2 for photoelectrochemical water splitting. As an electrocatalyst, [Mo3S13]2- clusters demonstrate a low overpotential of 220 mV at 10 mA cm-2 in 0.5 M H2SO4 electrolyte (pH 0.3) and remain structurally stable during the electrochemical cycling as revealed by in situ Raman spectroscopy. Moreover, as a co-catalyst on WSe2, [Mo3S13]2- clusters enhance the photocurrent substantially by more than two orders of magnitude (from 0.02 to 2.8 mA cm-2 at 0 V vs RHE). The synergistic interactions between the photoelectrode and catalyst, i.e., surface passivation and band bending modification by the [Mo3S13]2- cluster film, promoted HER catalytic activity of [Mo3S13]2- clusters influenced by the WSe2 support, are revealed by intensity-modulated photocurrent spectroscopy and density functional theory calculations, respectively. The band alignment of the WSe2/[Mo3S13]2- heterojunction, which facilitates the electron injection, is determined by correlating UV-vis with photoelectron yield spectroscopy results.

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination.

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Chemical Interaction at the MoO3/CH3NH3PbI3-xClx Interface.

The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO3. However, the use of MoO3 as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO3/CH3NH3PbI3-xClx (MAPbI3-xClx) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO3 deposition, significant chemical interaction is induced at the MoO3/MAPbI3-xClx interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI2 on the MoO3 cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO4, PbN2O2, and PbO as a result of the MAPbI3-xClx decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO3/MAPbI3-xClx interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO3 as a hole-transport material and in which there is a direct contact between MoO3 and perovskite.

BaZrS3 Chalcogenide Perovskite Thin Films by H2S Sulfurization of Oxide Precursors.

The earth-abundant ternary compound BaZrS3, which crystallizes in the perovskite-type structure, has come into view as a promising candidate for photovoltaic applications. We present the synthesis and characterization of polycrystalline perovskite-type BaZrS3 thin films. BaZrO3 precursor layers were deposited by pulsed laser deposition and sulfurized at various temperatures in an argon-diluted H2S atmosphere. We observe increasing incorporation of sulfur for higher annealing temperatures, accompanied by a red shift of the absorption edge, with a bandgap of Eg = 1.99 eV and a large absorption strength >105 cm-1 obtained for sulfurization temperatures of 1000 °C. X-ray diffraction analysis and SEM indicate enhanced crystallization at the higher annealing temperatures, but no evidence for a crystalline solid solution between the BaZrO3 and BaZrS3 phases is found. The charge carrier sum mobility estimated from optical-pump-terahertz-probe spectroscopy indicates increasing mobilities with increasing sulfurization temperature, reaching maximum values of up to ∼2 cm2 V-1 s-1.

Electronic Structure of the CdS/Cu(In,Ga)Se2 Interface of KF- and RbF-Treated Samples by Kelvin Probe and Photoelectron Yield Spectroscopy.

Ambient-pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments (KF-PDT, RbF-PDT) on the electronic features of Cu(In,Ga)Se2 (CIGSe) thin films and the CdS/CIGSe interface in a CdS thickness series that has been sequentially prepared during the chemical bath deposition (CBD) process depending on the deposition time. We observe distinct features correlated to the CBD-CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum (VBM) of the CIGSe surface is significantly shifted (by 180-620 mV) toward the Fermi level. However, VBM positions at the surface of the CIGSe are still much below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of postdeposition treatment, showing values between 1.46 and 1.58 eV, characteristic for a copper-poor CIGSe surface composition. At the CdS/CIGSe interface, the lowest VBM discontinuity is observed for the RbF-PDT sample. At this interface, a thin layer with a graded band gap is found. We also find that K and Rb act as compensating acceptors in the CdS layer. Detailed energy band diagrams of the CdS/CIGSe heterostructures are proposed.

Water Adsorption Enhances Electrical Conductivity in Transparent P-Type CuI.

CuI has been recently rediscovered as a p-type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a 2-fold increase in the conductivity of CuI after exposure to ambient humidity for 5 h, followed by slight long-term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is maximized at intermediate humidity levels. On the basis of the large intragrain mobility measured by THz spectroscopy, we suggest that hydration of grain boundaries may be beneficial for the overall hole mobility.

Tunability of MoO3 Thin-Film Properties Due to Annealing in Situ Monitored by Hard X-ray Photoemission.

The chemical and electronic structure of MoO3 thin films is monitored by synchrotron-based hard X-ray photoelectron spectroscopy while annealing from room temperature to 310 °C. Color-coded 2D intensity maps of the Mo 3d and O 1s and valence band maximum (VBM) spectra show the evolution of the annealing-induced changes. Broadening of the Mo 3d and O 1s spectra indicate the reduction of MoO3. At moderate temperatures (120-200 °C), we find spectral evidence for the formation of Mo5+ and at higher temperatures (>165 °C) also of Mo4+ states. These states can be related to the spectral intensity above the VBM attributed to O vacancy induced gap states caused by partial filling of initially unoccupied Mo 4d-derived states. A clear relation between annealing temperature and the induced changes in the chemical and electronic structure suggests this approach as a route for deliberate tuning of MoO3 thin-film properties.

Waveguide effect enhancement of local photocurrent in hybrid organic/inorganic solar cells.

The local efficiency of hybrid ZnO-nanorods/C60/ZnPc:C60/MoO3/Ag solar cells, with different nanorods length has been investigated by scanning near-field optical microscopy. Simultaneous spatially resolved measurements of topography and photocurrent suggest a waveguide effect enhancement of the local photocurrent. This interpretation is supported by finite element method simulations of the light propagation in the solar cell.

Metal-free photocatalytic graphitic carbon nitride on p-type chalcopyrite as a composite photocathode for light-induced hydrogen evolution.

Recently, it has been shown that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. We present herein the preparation and characterization of graphitic carbon nitride (g-C(3)N(4)) films on p-type semiconducting CuGaSe(2) chalcopyrite thin-film substrates by thermal condensation of a dicyandiamide precursor under inert-gas conditions. Structural and surface morphological studies of the carbon nitride films suggest a high porosity of g-C(3)N(4) thin films consisting of a network of nanocrystallites. Photoelectrochemical investigations show light-induced hydrogen evolution upon cathodic polarization for a wide range of proton concentrations in the aqueous electrolyte. Additionally, synchrotron radiation-based photoelectron spectroscopy has been applied to study the surface/near-surface chemical composition of the utilized g-C(3)N(4) film photocathodes. For the first time, it has been shown that g-C(3)N(4) films coated on p-type CuGaSe(2) thin films can be successfully applied as new photoelectrochemical composite photocathodes for light-induced hydrogen evolution.