Flexible InGaN nanowire membranes for enhanced solar water splitting

Abstract

III-Nitride nanowires (NWs) have recently emerged as potential photoelectrodes for efficient solar hydrogen generation. While InGaN NWs epitaxy over silicon is required for high crystalline quality and economic production, it leads to the formation of the notorious silicon nitride insulating interface as well as low electrical conductivity which both impede excess charge carrier dynamics and overall device performance. We tackle this issue by developing, for the first time, a substrate-free InGaN NWs membrane photoanodes, through liftoff and transfer techniques, where excess charge carriers are efficiently extracted from the InGaN NWs through a proper ohmic contact formed with a high electrical conductivity metal stack membrane. As a result, compared to conventional InGaN NWs on silicon, the fabricated free-standing flexible membranes showed a 10-fold increase in the generated photocurrent as well as a 0.8 V cathodic shift in the onset potential. Through electrochemical impedance spectroscopy, accompanied with TEM-based analysis, we further demonstrated the detailed enhancement within excess charge carrier dynamics of the photoanode membranes. This novel configuration in photoelectrodes demonstrates a novel pathway for enhancing the performance of III-nitrides photoelectrodes to accelerate their commercialization for solar water splitting.

A Single-Junction Cathodic Approach for Stable Unassisted Solar Water Splitting

“…Open circuit potential (OCP) measurement (data not shown) further suggests the downward surface band bending of p-type InGaN nanowires in electrolyte solution, which is well suited for electron extraction to drive proton reduction reaction. [52][53][54][55] Furthermore, the effect of using heavily doped silicon substrate is also studied (see Figure S5), which does not contribute to solar water splitting under light illumination but may lead to leakage current under negative biasing conditions. Additional LSV measurements were performed under light using 360 nm and 700 nm bandpass filters.…”

InGaN/Si Double-Junction Photocathode for Unassisted Solar Water Splitting

“…In this context, III-nitride semiconductors, i.e., Ga­(In)­N, have emerged as one of the most promising materials for next-generation semiconductor photoelectrodes. − They exhibit tunable direct band gap from 0.65 eV (InN) to 3.4 eV (GaN) and have large carrier mobility and high light absorption coefficient . As such, III-nitrides have been intensively studied for PEC water splitting − as well as for other artificial photosynthesis devices. ,− Recently, it has been discovered that the surfaces of III-nitride nanostructures can be engineered to be N-rich, not only for their top c -plane but also for their nonpolar sidewalls, which can protect against photocorrosion and oxidation , and are stable in various electrolytes. ,, Moreover, such N-terminated III-nitride nanostructures can be grown directly on Si wafer without the formation of extensive dislocations, providing distinct opportunities to realize Si-based double-junction PEC water-splitting devices with a nearly ideal energy band gap configuration, i.e., ∼1.1 and 1.8 eV for the bottom Si and top In 0.46 Ga 0.54 N junction, respectively.…”

Improved solar hydrogen production by engineered doping of InGaN/GaN axial heterojunctions

“…The u-InGaN/p-GaN sample was supposed to have the lowest carrier concentration among the three samples as only the GaN segment in it was intentionally doped with Mg. The incorporation the band-edg bandgap of bandgaps are while overcom ) would introd hotons to be lo sion peaks [17 /GaN NWs to d ments [11,22]. The ionized carrier concentrations of the different InGaN/GaN NWs were quantified using Mott-Schottky plots, which were depicted by extracting the fitted capacitance of InGaN NWs from the Nyquist plots, as shown in Fig.…”

“…Magnesium is a typical p-type dopant for III-nitride materials [16][17][18][19]. It has advantages including low formation energy and efficient dopant incorporation into NWs [19][20][21], and Mg-doped InGaN-based NWs have already been applied to PEC devices [2,17,22]. The previous study has been done on investigating the influence of doping concentration on tuning the surface Fermi level.…”