Solar-to-Hydrogen Production on Multi-Band Photoelectrodes: Surpassing the Current Matching Requirements of Conventional Tandem Devices
Authors: Mi Zetian , AlOtaibi Bandar , Fan Shizhao 
Published: February 2020

In a photoelectrochemical (PEC) system, to harvest a wider wavelength range of sunlight, a relatively narrow bandgap photoelectrode is often used, which, however, requires an external bias to produce hydrogen, due to its insufficient photovoltage. One of the solutions to overcome the low photovoltage of single-photoelectrodes is to implement a buried junction below the photocatalyst (known as photovoltaic-PEC or PV-PEC). However, the design and performance of PV-PEC is severely limited by the stringent requirement of current matching between the buried PV and the top complementary light absorption photocatalyst. In this context, by exploiting the lateral carrier extraction scheme of 1-dimensional nanowire structures, we have developed an adaptive PV-PEC photocathode, consisting of monolithically integrated GaN/p-InGaN nanowire arrays on a planar Si solar cell wafer, that can surpass the current matching requirements of conventional tandem electrode. An applied bias photon-to-current efficiency of 8.7% is measured.

The Si/GaN/p-InGaN PV-PEC device heterostructure consists of a planar n+-p Si solar cell wafer, ~ 150 nm n-GaN and ~ 600 nm p-InGaN nanowire segments along the axial direction. The top p-InGaN nanowire arrays are designed to absorb the ultraviolet and a large portion of the visible solar spectrum. The remaining photons with wavelengths up to 1.1 µm are absorbed by the underlying planar Si p-n junction. In contrary to conventional semiconductor photocatalysts, InGaN can uniquely straddle water oxidation and hydrogen reduction potentials under deep visible light irradiation. The n-GaN and p-InGaN are connected via an n++-GaN/InGaN/p++-GaN polarization-enhanced tunnel junction, which enables the transport of photo-excited holes from the p-InGaN to the n-GaN within each single nanowire. The presented device differs from conventional PV-PEC electrodes in that both the top p-InGaN and the bottom GaN/Si light absorbers can simultaneously drive proton reduction due to the lateral carrier extraction scheme of nanowires. That is, due to the relatively small offset between the n+
-Si and n-GaN conduction band edges, photo-excited electrons of the bottom Si solar cell can readily inject into the n-GaN nanowire segment. A large fraction of the injected electrons can drive proton reduction on GaN surfaces, with the rest recombining with holes from the p-InGaN in the tunnel junction. It is seen that such a novel design, with the use of Si/GaN-nanowire as the bottom light absorber, can surpass the restriction of current matching in conventional dual absorber devices and simultaneously provide energetic photo-excited electrons to the hydrogen-evolution-reaction (HER) catalyst. Compared to the conventional PEC-PV, such adaptive junction can reduce chemical loss by allowing charge carriers with different overpotentials to be utilized for HER. Such a monolithically integrated photocathode shows an applied bias photon-to-current efficiency of 8.7% at a potential of 0.33 V vs. normal hydrogen electrode. The Faradaic efficiency for hydrogen generation is also measured to be nearly unity.