Impact of NiCo2O4/SrTiO3 p–n Heterojunctions on the Interface of Photoelectrochemical Water Oxidation
Hongxia WangYan WangYu‐Mei LinXiao‐Chun HuangMiguel García‐TecedorVíctor A. de la Peña O’SheaConnor MurrillVlado K. LazarovF. PalacioKelvin H. L. Zhang
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Forming semiconductor heterojunctions is a promising strategy to boost the efficiency of solar-driven photoelectrochemical (PEC) water splitting by accelerating the separation and transport of photogenerated charge carriers via an interfacial electric field. However, there is limited research considering the influence of electrolytes on the band alignment of the heterojunction under PEC conditions. In this work, we use a single crystal NiCo2O4/SrTiO3 (NCO/STO) heterojunction with atomic-precision controlled thickness as a model photoelectrode to study the band structure modulations upon getting in contact with the electrolyte and the correlation with the PEC activity. It is found that the band alignment can be tuned by the control of p-n heterojunction film thickness and regulated by the water redox potential (Eredox). When the Fermi level (EF) of the heterojunction is higher/lower than the Eredox, the band bending at the NCO/STO-electrolyte interface will increase/decrease after contacting with the electrolyte. However, when the band bending width of the NCO layer is thinner than its thickness, the electrolyte will not influence the band alignment at the NCO/STO interface. In addition, PEC characterization results show that the 1 nm NCO/STO heterojunction photoanode exhibits superior water-splitting performance, owing to the optimum band structure of the p-n heterojunction and the shorter charge transfer distance.Keywords:
Band bending
Depletion region
Changes in the surface Fermi-level position in n-type epitaxial GaAs samples are determined by Hall-effect measurements of the corresponding changes in the sheet concentrations and theoretical calculations of the surface depletion thickness. The changes are induced and reversed repeatedly by alternating wet chemical treatments in hydrogen peroxide and ammonium hydroxide. This is the first known use of hydrogen peroxide to restore the surface Fermi level to near its starting value and demonstrate the repeated variation of the surface Fermi level by ammonium hydroxide. The results agree with the predictions of the advanced unified defect model and with published reports of increased band bending on n-type material, rather than with conflicting reports of decreased band bending. The results also indicate that problems may exist with other techniques used to measure surface- potential changes.
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We use capacitance techniques to directly measure the Fermi level at the crystalline/amorphous interface in n-type silicon heterojunction solar cells. The hole density calculated from the Fermi level position and the inferred band-bending picture show strong inversion of (n)crystalline silicon at the interface at equilibrium. Bias dependent experiments show that the Fermi level is not pinned at the interface. Instead, it moves farther from and closer to the crystalline silicon valence band under a reverse and forward bias, respectively. Under a forward bias or illumination, the Fermi level at the interface moves closer to the crystalline silicon valence band thus increases the excess hole density and band bending at the interface. This band bending further removes majority electrons away from the interface leading to lower interface recombination and higher open-circuit voltage.
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Band bending at GaAs interfaces with either metals or insulators is of considerable importance. Recently a literature has developed, which shows for both metals and insulators, that this band bending can be modified, i.e., the Fermi level position Efi at the interface changes. Of the five metals studied in detail, Ag, Au, Al, Ti, and Cr, three (Au, Al, and Ti) have been shown to produce Fermi level changes on the order of 0.1 eV (i.e., change in the Schottky barrier height φ) on thermal annealing. For n-type GaAs φ decreases for Au, and increases for Al and Ti. These changes of Efi can be explained in terms of the generation of excess As or Ga at the interface due to chemical reactions between the metal and the GaAs. The Fermi levels move toward the conduction-band minimum (CBM) when the As/Ga ratio is increased and toward the valence-band maximum (VBM) when it decreases. The changes in As/Ga ratio have been confirmed independently. No movement is seen for Ag, which is unique in having little reaction with GaAs, or for Cr, which bonds to both As and Ga. The Fermi level position can also be changed at the GaAs insulator interface. Na2S⋅H2O and (NH4)2S treatments of the GaAs surface reduce surface recombination. Recent studies show that such treatments increase rather than decrease the band bending on n-GaAs. All of these results agree with a model in which AsGa and GaAs antisites dominate the interface and Fermi level changes are explained in terms of changes in the relative numbers of these defects.
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In semiconductor electrochemistry there is considerable confusion concerning the potential distribution at the semiconductor/solution interface under weak depletion and accumulation conditions. The applied potential is partitioned between the space charge layer in the semiconductor and the Helmholtz layer on the solution side of the interface. Under deep depletion conditions, a change in the applied potential usually appears across the space charge layer and the band bending can be determined using the Mott−Schottky relation. Under conditions of weak depletion or accumulation, however, the applied potential is partitioned between the two double layers and determination of band bending is not straightforward. In this paper, expressions for the dependence of the band bending on the applied potential are derived and the consequences for charge-transfer processes are discussed.
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The effects of photochemical oxidation in water, Na2 S deposition and exposure to NH3 and HCl gas ambients on the Fermi level pinning, and the surface recombination process at GaAs surfaces were studied by measuring the band edge photoluminescence (PL) intensity, x-ray photoelectron spectroscopy spectra, and surface current transport(SCT). Computer simulation of the surface recombination process was also made on the basis of the disorder-induced gap state model. Marked increase of PL intensity was observed after both photochemical oxidation and deposition of Na2 S as previously reported. However, SCT measurements on n-type materials detected increase of surface band bending in the dark. Exposure to NH3 resulted in a slight reduction in the band bending with little change in the PL intensity. Exposure to HCl, on the other hand, resulted in marked reduction of the band bending with marked increase of the PL intensity. The computer simulation shows that the contradictory behavior of the PL intensity and band bending, as observed after photochemical oxidation and Na2 S deposition, is explicable by a shift of the Fermi level pinning position towards the valence band edge due to a fixed negative interface charge which reduces the effective recombination velocity. On the other hand, exposure to HCl reduces the surface-state density and weakens the pinning remarkably.
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For nickel on the chemically clean surface of undoped semi-insulating GaAs at room temperature, an upward surface band bending of 0.062 eV and a barrier height of 0.690 eV have been observed by the photovoltage and the internal photoemission techniques, respectively. The observed surface band bending is in excellent agreement with its predicted value, and the observed barrier height also agrees very well with its value from the very careful analysis of reversed I-V data. It has been determined that the interfacial Fermi level lies at 0.690 eV below the GaAs conduction band minimum at the interface. The interfacial Fermi level is found to coincide with the energy level of the EL2 native defect, indicating the importance of the EL2 in the Fermi level pinning at the interface.
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Photoemission studies demonstrate that temperature and dopant concentration dependent movement of the surface Fermi level is controlled by coupling between adatom-induced and bulk states. At a low temperature for lightly doped n- or p-GaAs, initial band bending inhibits tunneling and EF remains near the band edges until the onset of metallicity. For heavy doping, greater band bending reflects a thinner depletion region. Thermal cycling for 20≤T≤300 K for low coverages demonstrates that band bending is reversible.
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Band bending is a fundamental issue for discussing organic devices. Band bending with Fermi level alignment between semiconductors and metals are often assumed, although the validity of this scheme in the case of organic semiconductors has been not yet established. In this paper, our recent efforts to examine band bending in organic semiconductors using Kelvin probe method (KPM) are reported. After discussing the applicability of KPM to organic thick film – metal substrate system, the results for C60, TPD, and Alq3 are shown to discuss band bending of the films without intentional doping in ultrahigh vacuum condition. Gradual band bending was observed for C60/metal interfaces although the width of the space charge layer is in the order of 100 nm. In contrast, flat band feature was observed for TPD/metal interfaces probably because of its high purity. These results demonstrate that the frame work of band bending used in inorganic semiconductor interfaces is still valid for organic semiconductors although much thicker films are often necessary to achieve bulk Fermi level alignment. For Alq3/metal interfaces formed in dark condition, we found a new type of band bending where the energy levels change as a linear function of the distance from the interface. The observed location of the vacuum level was far below the Fermi level of the metal substrates, clearly indicating that Fermi level varies place by place in the system. Such electronically non-equilibrium state was quite stable for the order of years. The concept of Fermi level alignment is also discussed in relation to the observed energy diagrams. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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