Si-based Metal Insulator Semiconductor Research Articles

Enhancing Photovoltage of Silicon Photoanodes by a High Work-Function Coordination Polymer

A metal–insulator–semiconductor (MIS) structure based on an inhomogeneous junction has been recently proven to be highly efficient at photoelectrochemical (PEC) water oxidation. Engineering the surrounding layer of the MIS nanojunction is crucial to maximizing the photovoltage. Specifically, for an n-type photoanode, a high work-function material is required to create a large Schottky barrier that assists the hole transfer to oxidize water while blocking the electron transfer. Normally, the surrounding layer is a native oxidized phase of the metal layer that forms unintentionally. Herein, we demonstrate the use of copper(I) thiocyanate (CuSCN), a transparent p-type coordination polymer semiconductor with a high work function, to specifically surround the n-Si/SiOx/Cu nanojunctions, resulting in an increased effective barrier height from 0.71 to 1.03 eV. This phenomenon, known as the pinch-off effect, is also applied to improve the performance of the planar n-Si/SiOx/Cu electrode via a PEC dissolution method that creates an inhomogeneous surface covered with CuOx. The Cu/CuSCN nanojunction still shows superior characteristics due to the favorable energetics of CuSCN. This work shows a rational route for engineering the surrounding layer to improve the performance of Si-based MIS photoelectrodes, enabled by the facile chemistry of coordination polymers.

Scalable, highly stable Si-based metal-insulator-semiconductor photoanodes for water oxidation fabricated using thin-film reactions and electrodeposition

Metal-insulator-semiconductor (MIS) structures are widely used in Si-based solar water-splitting photoelectrodes to protect the Si layer from corrosion. Typically, there is a tradeoff between efficiency and stability when optimizing insulator thickness. Moreover, lithographic patterning is often required for fabricating MIS photoelectrodes. In this study, we demonstrate improved Si-based MIS photoanodes with thick insulating layers fabricated using thin-film reactions to create localized conduction paths through the insulator and electrodeposition to form metal catalyst islands. These fabrication approaches are low-cost and highly scalable, and yield MIS photoanodes with low onset potential, high saturation current density, and excellent stability. By combining this approach with a p+n-Si buried junction, further improved oxygen evolution reaction (OER) performance is achieved with an onset potential of 0.7 V versus reversible hydrogen electrode (RHE) and saturation current density of 32 mA/cm2 under simulated AM1.5G illumination. Moreover, in stability testing in 1 M KOH aqueous solution, a constant photocurrent density of ~22 mA/cm2 is maintained at 1.3 V versus RHE for 7 days.

Hybrid porous silicon/silver nanostructures for the development of enhanced photovoltaic devices

Si-based metal–insulator–semiconductor (MIS) Schottky junction solar cells with the basic structure Al/Si/TiO2/Au were fabricated. This structure was modified by the addition of nanostructured porous silicon (nanoPS) layers and silver nanoparticles (AgNPs), resulting in devices with the following structures: Al/Si/nanoPS/TiO2/Au and Al/Si/nanoPS+AgNPs/TiO2/Au. The key performance parameters of the three MIS Schottky junction solar cells were determined, including spectral photocurrent response, short-circuit current density, open-circuit voltage, fill factor, and efficiency. The experimental results show a remarkable enhancement in the overall performance of the solar cells upon the addition of nanoPS and AgNPs layers to the basic structure. An energy band model is proposed for the Si-based MIS Schottky junction solar cells to understand the different photogeneration and conduction mechanisms.

Bifacial passivation of n-silicon metal–insulator–semiconductor photoelectrodes for efficient oxygen and hydrogen evolution reactions

This paper describes a bifacial passivation strategy for the metal/Si interface of metal–insulator–semiconductor (MIS) photoelectrodes, achieving record high activities for water oxidation and reduction for Si-based MIS electrodes.

Electrical and photoelectric properties of Si-based metal–insulator–semiconductor structures with Au nanoparticles at the insulator–semiconductor interface

It is shown that the formation of Au nanoparticles at the insulator–silicon interface in structures with a high density of surface states results in a shift of the Fermi-level pinning energy at this interface towards the valence-band ceiling in silicon and in increasing the surface-state density at energies close to the Fermi level. In this case, a band with a peak at 0.85 eV arises on the photosensivity curves of the capacitor photovoltage, which is explained by the photoemission of electrons from the formed Au-nanoparticle electron states near the valence-band ceiling in silicon.

Study of Randomly Distributed Charge Traps by Measuring Frequency- and Time-Dependence of a DNTT-Based MIS Capacitor

The frequency and time dependence of capacitance of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT)-based metal-insulator-semiconductor (MIS) capacitors were studied. The frequency spectra show a long and gradual slope over many orders of magnitude in frequency, and the time evolution in the subthreshold region exhibits a logarithmic dependence. These two phenomena are analyzed from a unified point of view on the basis of charge trapping models that were originally developed for Si-based MIS capacitors. The charge trapping model with a Gaussian-distributed surface potential in DNTT successfully reproduces both the frequency and time dependences with the same parameter set.

H2 evolution at Si-based metal–insulator–semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover

Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal-insulator-semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2 cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO2/Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting.

Improving the Accuracy in Determining the Insulator Capacitance in Metal–Insulator–Semiconductor Structures

The results of the voltage–capacitance spectroscopy of interface states in metal–insulator–semiconductor (MIS) structures are critical functions of the accuracy in determining the insulator capacitance C i, which is typically no higher than a few fractions of a percent. This substantially limits the energy range of the observed spectrum of the interface states (ΔE ≅ 0.5 eV for Si-based MIS structures) and the sensitivity to the density of the interface states at the spectrum edges (ΔN ss ≅ 1 × 1010 cm–2 eV–1). We propose a method for minimizing these errors that is based on a sequential variation of the initial estimate C i ≅ C i 0 → C ij, j = 0, 1, 2, ... and the identification of singular points in the dependences $$\overline {\delta V_g^{ac,in} } $$ and $$\overline \delta _{ac,in} $$ on C ij, where $$\overline {\delta V_g^{ac,in} } $$ are the mean arithmetic values of the voltage difference between the experimental and ideal voltage–capacitance characteristic and $$\overline \delta _{ac,in} $$ are the rms deviations of the voltage $$\overline {\delta V_g^{ac,in} } $$ values taken in the high-accumulation (ac) and inversion (in) regions from $$\delta V_g^{ac,in} $$ values. The highest (≅10–4%) accuracy in determining C i is achieved in the regions of the equidistant experimental and ideal voltage–capacitance characteristic. This method, combined with the technique of Ψ s ′/Ψ s diagrams, ensures an extension of ΔE to ≅0.9 eV at ΔN ss ≅ 1 × 1010 cm–2 eV–1 and the possibility of determining the sign and density of the fixed charge in the gate insulator.