Letter pubs.acs.org/NanoLett
High Efficiency Solar-to-Hydrogen Conversion on a Monolithically Integrated InGaN/GaN/Si Adaptive Tunnel Junction Photocathode Shizhao Fan,† Bandar AlOtaibi,† Steffi Y. Woo,‡ Yongjie Wang,† Gianluigi A. Botton,‡ and Zetian Mi*,† †
Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 0E9, Canada ‡ Department of Materials Science and Engineering, Canadian Centre for Electron Microscopy, McMaster University, 1280 Main Street West, Hamilton, Ontairo L8S 4M1, Canada S Supporting Information *
ABSTRACT: H2 generation under sunlight offers great potential for a sustainable fuel production system. To achieve high efficiency solar-to-hydrogen conversion, multijunction photoelectrodes have been commonly employed to absorb a large portion of the solar spectrum and to provide energetic charge carriers for water splitting. However, the design and performance of such tandem devices has been fundamentally limited by the current matching between various absorbing layers. Here, by exploiting the lateral carrier extraction scheme of one-dimensional nanowire structures, we have demonstrated that a dual absorber photocathode, consisting of p-InGaN/tunnel junction/n-GaN nanowire arrays and a Si solar cell wafer, can operate efficiently without the strict current matching requirement. The monolithically integrated photocathode exhibits an applied bias photon-to-current efficiency of 8.7% at a potential of 0.33 V versus normal hydrogen electrode and nearly unity Faradaic efficiency for H2 generation. Such an adaptive multijunction architecture can surpass the design and performance restrictions of conventional tandem photoelectrodes. KEYWORDS: Nanowire, InGaN, Si solar cell, photoelectrochemical water splitting, hydrogen
A
that provides the smaller maximum photocurrent density.16−18 Recently, the use of 1D nanostructures, such as nanowires has been intensively studied, which can enable highly efficient carrier extraction and proton reduction on the large area lateral surfaces.7,8,19−21 To date, however, there have been no reports on InGaN nanowire-based monolithically integrated multijunction photoelectrodes. Compared to other semiconductor photocatalysts, it has been recently discovered that the band edges of InGaN can straddle water oxidation and hydrogen reduction potentials under deep visible light irradiation.22−24 In this work, we have developed an adaptive double-junction photocathode by integrating such nanowire arrays onto a planar Si solar cell wafer, wherein the maximum achievable current is not limited by the current matching related issues. Compared to the conventional buried multijunction light absorbers,4,25,26 such adaptive junction can reduce chemical loss by allowing charge carriers with different overpotentials to participate hydrogen/ oxygen evolution reaction simultaneously.27 Schematically shown in Figure 1a, the device heterostructure consists of a planar n+-p Si solar cell wafer, 150 nm n-GaN and 600 nm pInGaN nanowire segments along the axial direction. The top InGaN nanowire arrays with an indium composition of ∼25% is designed to absorb the ultraviolet and a large portion of the
n essential component of a solar-hydrogen production system is a high efficiency and highly stable photocathode, wherein photoexcited electrons lead to H2 generation. Over the past decades, extensive studies have been performed to develop photocathodes that can absorb a large part of the solar spectrum and can lead to efficient charge carrier separation and proton reduction.1,2 The semiconductor light absorber should have a conduction band minimum (CBM) more negative than that required for hydrogen evolution reaction (HER) (4.44 eV below the vacuum level in solutions of pH = 0).3 This requirement limits the choice of high efficiency semiconductor photocathodes mainly to Si and a few III−V materials, including GaP, InP, and their alloys.4−8 Various HER catalysts or protection layers integrated with Si9−12 exhibited improved performance compared to platinized p-Si photocathodes.13 Some other materials have also been studied but are often limited by either rapid degradation14 or very poor absorption of visible light.15 To effectively utilize photons within a wide range of the solar spectrum, a dual light absorber with a narrow bandgap material like Si at the bottom and direct wide-bandgap materials on top can provide energetic electrons for H2 production. However, the design and performance of such multijunction devices is limited by the current matching related issues between the two absorbers, because the carrier collection and extraction is only available on the front surfaces. For such photoelectrodes consisting of dual or multiple light absorbers, although the required external bias can be reduced, the photocurrent density is ultimately limited by the light absorber © XXXX American Chemical Society
Received: February 7, 2015 Revised: March 18, 2015
A
DOI: 10.1021/acs.nanolett.5b00535 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Design of integrated InGaN/Si photocathode. (a) The schematic of the photocathode formed by InGaN tunnel junction nanowires on n+p Si solar cell substrate. (b) The energy band diagram of InGaN tunnel junction nanowires on n+-p Si substrate under illumination. Proton reduction on the lateral surfaces of GaN and InGaN nanowire segments is also illustrated. Schematic of the device structure is shown in the lower panel. (c) SEM image of nanowires. (d) Photoluminescence emission spectrum of nanowires measured at room temperature.
InGaN nanowires were subsequently grown on the n+-Si surface by radio frequency plasma-assisted molecular beam epitaxy (MBE) (Supporting Information S4). Growth of similar nanowire structures on n+-Si substrates was also performed as a control experiment. A detailed schematic of the studied samples is shown in the Supporting Information (Figure S1). Due to the large bandgap, conventional GaN p++/n++ junction generally has very low tunneling efficiency. In this study, we have designed p++-GaN(20 nm)/In0.4Ga0.6N/n++-GaN(20 nm) polarization-enhanced tunnel junction structures28 (Supporting Information S4). On top of the tunnel junction, ∼600 nm pInGaN nanowire was grown. The scanning electron microscope (SEM) image of nanowires is shown in Figure 1c. It is seen that such nanowires are vertically aligned on the Si substrate with relatively uniform lengths of ∼800 nm and diameters varying from 50−150 nm. Previous studies revealed that such nanowires were N-polar30 with their sidewalls being the nonpolar m-plane.31 Shown in Figure 1d is the photoluminescence emission spectrum measured at room temperature (Supporting Information S5). The peak wavelength is at ∼520 nm, corresponding to an average indium composition of ∼25% and an energy bandgap of ∼2.39 eV. Therefore, photoexcited electrons in p-InGaN can in principle reduce protons without external bias.32 The downward surface band bending of p-InGaN facilitates the flow of electrons toward the electrolyte. In addition, the accumulation of photoexcited holes in p-InGaN and their transport across the tunnel junction can enhance the injection of photoexcited electrons from the n+-p
visible solar spectrum. The rest of the photons with wavelengths
