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Mesoscale modeling of photoelectrochemical devices: light absorption and carrier collection in monolithic, tandem, Si|WO3 microwires

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Abstract

We analyze mesoscale light absorption and carrier collection in a tandem junction photoelectrochemical device using electromagnetic simulations. The tandem device consists of silicon (Eg,Si = 1.1 eV) and tungsten oxide (Eg,WO3 = 2.6 eV) as photocathode and photoanode materials, respectively. Specifically, we investigated Si microwires with lengths of 100 µm, and diameters of 2 µm, with a 7 µm pitch, covered vertically with 50 µm of WO3 with a thickness of 1 µm. Many geometrical variants of this prototypical tandem device were explored. For conditions of illumination with the AM 1.5G spectra, the nominal design resulted in a short circuit current density, JSC, of 1 mA/cm2, which is limited by the WO3 absorption. Geometrical optimization of photoanode and photocathode shape and contact material selection, enabled a three-fold increase in short circuit current density relative to the initial design via enhanced WO3 light absorption. These findings validate the usefulness of a mesoscale analysis for ascertaining optimum optoelectronic performance in photoelectrochemical devices.

© 2014 Optical Society of America

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Figures (6)

Fig. 1
Fig. 1 One-dimensional representation of a tandem photoelectrochemical device with a proton-conducting membrane, illustrating the integrated nature of light absorption, carrier transport, catalysis, and reactant and product transport through solution and membrane.
Fig. 2
Fig. 2 Diagram of a monolithic, tandem, microwire based PEC device, including photoanode, photocathode, Ohmic contact material between the two photoelectrodes, oxygen (OER) and hydrogen (HER) evolution catalysts, and ion-conducting membrane. Reactions are written for acidic conditions, and therefore, the membrane is labeled as a proton exchange membrane (PEM).
Fig. 3
Fig. 3 Schematics for two proposed optoelectronic tandem PEC designs; (a) an opaque contact with the p|n junction in the bottom half of the Si microwire (b) a transparent contact design with the p|n junction in the top half of the Si microwire.
Fig. 4
Fig. 4 (a) Schematic of simulation unit cell, simulated as a 2D infinite array using TE-polarized illumination at normal incidence; (b) absorption vs. wavelength for the photoanode (WO3), photocathode (Si), and contact (Al, opaque) for the structure in (a); (c) absorption vs. wavelength for the photoanode (WO3), photocathode (Si), and contact (ITO, transparent) for the structure in (a).
Fig. 5
Fig. 5 Snapshots of the propagation of the electric field along an Al|air interface within an infinite 2D array of wires indicating the coupling into a surface plasmon-polariton mode (a) at λ = 800 nm, illustrating the evanescent decay of the electric field away from the interface; (b) at λ = 200 nm, illustrating the slower propagation of light at the interface.
Fig. 6
Fig. 6 Schematic of (a) the original, (b) the partially optimized, and (c) the optimized microwire array designs with the transparent, indium tin oxide contact; (d) plot of WO3 absorption vs. wavelength of each structure and their planar equivalences, demonstrating the absorption enhancement.

Tables (2)

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Table 1 Short Circuit Current Densities (mA/cm2), Day-integrated Hydrogen Production (mmol day−1cm−2), and Internal Quantum Efficiencies for the Opaque and Transparent Contact Designs and Their Planar Equivalence

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Table 2 Short Circuit Current Densities (mA cm−2), Day-integrated Hydrogen Production (mmol day−1cm−2), and Internal Quantum Efficiencies for the Original, Partially-optimized, and Optimized Transparent Contact Designs and Their Planar Equivalences

Equations (2)

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P abs = 1 2 ω | E | 2 imag(ε)
C gen = π | E | 2 imag(ε) h
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