Optical, electrical, and solar energy-conversion properties of gallium arsenide nanowire-array photoanodes

Abstract

Periodic arrays of n-GaAs nanowires have been grown by selective-area metal-organic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced near-unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with nonaqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of $8.1% under 100 mW cm À2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 AE 15 mV and short-circuit current densities of 24.6 AE 2.0 mA cm À2 . The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes. Broader contextDue to the voltage requirements to produce fuels from sunlight, water, and CO 2 as the inputs, two light-absorbing materials, with band gaps of 1.7 eV and 1.1 eV, respectively, are attractive as the foundation for high-efficiency articial photosynthesis. The integration of materials with 1.7 and 1.1 eV band gaps is, however, very challenging. Accordingly, a nanowire-growth strategy has been developed to integrate single crystal III-V nanowires (e.g. GaAs) with highly mismatched Si substrates. In this work, GaAs nanowire arrays grown on Si were studied using a non-destructive contact method involving non-aqueous photoelectrochemistry. The approach has allowed us to understand the interplay of nanowire growth with the optical absorption and electrical properties of such systems, and will aid in the design and optimization of nanowire-based systems for solar energy-conversion applications.Photoelectrolysis of water for the production of renewable H 2 from sunlight faces a constraint in that a potential difference of 1.23 V is required thermodynamically to sustain the watersplitting reaction under standard conditions. In an integrated photoelectrochemical system for water splitting, the operating voltage produced by the light absorber should exceed the sum of the thermodynamic minimum required potential difference, the resistance losses of electrolytes, and any overpotentials that are required to drive water splitting at a given current density in the device structure of interest. When this requirement is met, the solar-to-hydrogen (STH) efficiency is limited solely by the photocurrent density generated from the light absorber.By sequentially stacking light-absorbing materials with different band gaps to utilize various wavelengths of light in the solar spectrum, a dual-junction tandem structure can provide a sufficient photovoltage as well as high current densities to...

Near-unity broadband absorption designs for semiconducting nanowire arrays via localized radial mode excitation

“…Previously, we reported the very strong light absorption properties of sparse (<5% fill fraction) GaAs nanowire arrays grown on both GaAs and Si substrates via selective area growth MOCVD, supported by experiments, simulations and analytical theory [27]. We focus our design in the range of wire geometries employed in this previous study and investigate the simulated wavelength-dependent absorption for a 5% fill fraction, uniform array of GaAs nanowires with heights of 3 µm and radii of 65 nm, which is shown in (red) in Fig.…”

“…Note that partial spectral averaging has been used for the planar layer to smooth out the Fabry-Perot resonances (see Methods for details). All nanostructures are positioned on top of an infinite Si substrate and embedded in a 30 nm layer of SiO x to emulate SAG-MOCVD as-grown structures [27]. (45,55,65, 75 nm) with inset of aerial layout, and (iii) truncated nanocones with tip radii of 40 nm and base radii of 100 nm; (b) Cross sections of normalized power absorbed at the TM 11 resonance at 675nm for (i) a 65nm radius nanowire in a uniform array, (ii) a truncated nanocone at r = 65 nm and (iii) a 65 nm radius nanowire in the multiradii nanowire array (black circles outline the edges of the wire); (c) Simulated absorption vs. wavelength for the geometrically-optimized GaAs arrays of truncated nanocones shown in (a) and the planar equivalent thickness (t = 150 nm).…”

“…The array pitch and wire diameter are easily tunable due to the use of patterned masks, created by electron-beam lithography [20] or nanoimprint lithography [22]. Reported light trapping mechanisms include coupling into photonic crystal modes [4,[23][24][25][26], coupling into leaky radial waveguide modes [8,13,15,[27][28][29], and absorption in media considered using effective index models [30,31]. In this paper, we discuss mechanisms for broadband absorption in sparse arrays, where light coupling into localized radial modes dominates over coupling into Bloch-like photonic crystal modes that depend on array periodicity.…”

Resonant absorption in semiconductor nanowires and nanowire arrays: Relating leaky waveguide modes to Bloch photonic crystal modes

“…These modes are well-defined and well understood. For nanowire arrays, mode identification is less straight forward, due to the introduction of periodicity; absorption enhancements in nanowire arrays have been attributed to both leaky waveguide modes of individual nanowires [19][20][21][22] and photonic crystal Bloch modes. 12,[23][24][25][26] Previous reports have examined a wide variety of array dimensions, nanowire geometries, and illumination angles, which also exhibit varying dominant mode resonances.…”

Characterization and analysis of InAs/p–Si heterojunction nanowire-based solar cell

“…Among the applications of III-V arrays on silicon are tunnel diodes [23], photoelectrochemical water splitting [24,25] and solar cells [26,27]. By combining a GaAs nanowire array on a silicon cell, a dual junction with a theoretical efficiency higher than 30% could, in theory, be achieved [28].…”