Photoelectrochemical Behavior of Planar and Microwire‐Array Si|GaP Electrodes

Abstract

Gallium phosphide exhibits a short diffusion length relative to its optical absorption length, and is thus a candidate for use in wire array geometries that allow light absorption to be decoupled from minority carrier collection. Herein is reported the photoanodic performance of heteroepitaxially grown gallium phosphide on planar and microwire‐array Si substrates. The n‐GaP|n‐Si heterojunction results in a favorable conduction band alignment for electron collection in the silicon. A conformal electrochemical contact to the outer GaP layer is produced using the ferrocenium/ferrocene (Fc+/Fc) redox couple in acetonitrile. Photovoltages of ∼750 mV under 1 sun illumination are observed and are attributed to the barrier formed at the (Fc+/Fc)|n‐GaP junction. The short‐circuit current densities of the composite microwire‐arrays are similar to those observed using single‐crystal n‐GaP photoelectrodes. Spectral response measurements along with a finite‐difference‐time‐domain optical model indicate that the minority carrier diffusion length in the GaP is ∼80 nm. Solid‐state current–voltage measurements show that shunting occurs through thin GaP layers that are present near the base of the microwire‐arrays. The results provide guidance for further studies of 3D multi‐junction photoelectrochemical cells.

Modeling, Simulation, and Implementation of Solar‐Driven Water‐Splitting Devices

“…[11g,i] Attempts to design and fabricate tandem-junction microstructured devices have been challenging because of the complex nature of the exposed crystal facets on which a material must be grown. Two routes have been investigated: epitaxial growth of high-performance compound semiconductors (GaP, GaInP), [57] and growth of defective, nano-/ microcrystalline materials that may provide intrinsic advantages in terms of stability over the known higher performance materials, [11j, 55, 58] where a maximum STH conversion efficiency of 0.12 % has been reported. [11j] For microstructured devices to achieve more than just scientific interest, clear, quantitative performance and/or economic advantages over planar equivalents must be demonstrated including perhaps impacts on balance-of-systems requirements and costs.…”

Photoelectrochemical oxidation of anions by WO3 in aqueous and nonaqueous electrolytes

“…In contrast, valuable information can in principle be obtained by photoelectrochemical investigations of the same series of anions in nonaqueous electrolytes. Compared with main-group semiconductors such as Si 15 and GaP, 16 the photoelectrochemistry of metal oxides in nonaqueous solvents is not well elaborated. Early work regarding n-type TiO 2 17 and ZnO 18 electrodes in acetonitrile solutions placed emphasis on the behavior of the conduction-band electrons in these semiconductors.…”

Mesoscale modeling of photoelectrochemical devices: light absorption and carrier collection in monolithic, tandem, Si|WO_3 microwires

“…Deposition of a series-connected photoanode material on Si microwires enables a tandem device to be realized, with an open circuit voltage suitable for water splitting, and enables greater use of the solar spectrum in a tandem photoabsorber [9,10]. This tandem, microwire design affords many advantages including (i) the series addition of the photovoltages in both components of the tandem to exceed the minimum voltage required to split water, while simultaneously absorbing visible light [8], (ii) the orthogonalization of light absorption (in wire axial direction) and carrier transport (in wire radial direction), which is especially advantageous for indirect bandgap semiconductors, which have long absorption lengths [11], (iii) increased surface area for catalysis, (iv) an approximately 90% decrease in photoelectrode material usage relative to planar monolithic device designs, depending on wire pitch and diameter, (v) a source for radial strain relief, enabling the integration of lattice mismatched materials [12,13], (vi) a monolithic structure enabling straightforward incorporation of an ion-conducting membrane, and (vii) short electrolyte diffusion pathways for low solution resistance [6].…”