(Invited) High Efficiency Solar-to-Hydrogen Conversion on InGaN Nanowire Arrays
In this design, the photocathode and the photoanode are connected side by side, wherein the solar light can be split spectrally and spatially. Different split light spectra are spread out to strike the corresponding photoelectrodes. n-InGaN/GaN and p-InGaN/GaN decorated with Pt nanoparticles are used as the photoanode and photocathode, respectively. The design of paired InGaN nanowire photoelectrodes can provide high photovoltage without trading off on the harvesting of longer wavelength of sunlight. Catalyst-free, vertically aligned InGaN/GaN nanowire arrays were grown on n-type Si(111) substrate by radio frequency plasma-assisted molecular beam epitaxy under nitrogen rich conditions. After an in situ oxide desorption at ~ 770 °C, A GaN nanowire template with a height of ~ 200 nm was first grown on Si substrate. To enhance the carrier transport between the n-type Si substrate and p-InGaN nanowire, a tunnel junction consisting of n+-GaN/p+-InGaN/p+-GaN was implemented beneath the p-InGaN nanowire. The nanowires are doped n-type or p-type using silicon or magnesium, respectively.
The photoelectrochemical (PEC) properties of the photocathode and the photoanodes were investigated separately using a cell with a three-electrode configuration in 1 mol/L HBr. A linear sweep voltammogram for n-InGaN/GaN photoanode was performed under both dark and illuminated conditions. The maximum incident photon-to-current conversion efficiency (IPCE) measured at 1 V vs. Ag/AgCl for the n-InGaN/GaN photoanode is ~ 50% at 350 nm, while the maximum IPCE for p-InGaN/GaN photocathode (at -0.6 V vs. Ag/AgCl) is ~ 65% at 350 nm. The IPCE shows a decreasing trend with increasing wavelength, limited by the effective light absorption of the nanowire structures.
In the paired configuration, the incident light was split by using long- and short-pass optical filters. The parallel-connected photoanode was illuminated with light with wavelengths in the range of 300-480nm. The photocathode was illuminated with light with wavelengths of 485-600 nm. Using a two-electrode configuration, the photoanode was considered to be the working electrode, and the photocathode p-InGaN/GaN was connected as the counter electrode in 1 mol/L HCl. With this novel design, the power conversion efficiency in the range of 300-600 nm has been increased by nearly one order of magnitude, compared to efficiency of a single electrode. The maximum power conversion efficiency of this device can reach >5%. Work is currently in progress on the integration of InGaN nanowires with solar cells to achieve further increased efficiency for solar-to-hydrogen conversion.