Rapid Performance Optimization Method for Photoelectrodes

We report the development
of a rapid method for the performance
optimization of photoelectrodes. The method requires as an input incident
photon-to-current efficiency (IPCE) measurements of a material at
two or more wavelengths, an estimation of the complex refractive index,
the permittivity, the doping concentration, the flatband potential,
and the photoelectrode thickness, and it estimates in return the diffusion
length, the optical loss, the bulk and surface recombination losses,
and the space charge region
loss. The diffusion optical number, defined as the product of the
absorption coefficient and the diffusion length at 500 nm, was used
to quantify the performance of photoelectrodes. The method was validated
using planar Cu2O water-splitting photoelectrodes. Subsequently,
it was applied to planar water-splitting photoelectrodes made of Cu2O, Si, Fe2O3, BiVO4, Cu2V8O3, and CuFeO2 and to nanostructured
photoelectrodes made of Fe2O3 and LaTiO2N. The projected diffusion optical number of Fe2O3 was improved by 1 order of magnitude when nanostructuring
compared to the diffusion optical number of flat Fe2O3 photoelectrodes. Thus, a modification of the synthesis or
deposition method should be prioritized instead of developing nanostructuring
techniques for any photoelectrode with a diffusion optical number
2 orders of magnitude below the one required to obtain an internal
quantum efficiency of 95%. Using this benchmark, the investigated
Si photoelectrode performed well without the need of nanostructuring.
Cu2O and LaTiO2N photoelectrodes were found
to benefit from nanostructuring. In contrast, nanostructuring is not
advised for Fe2O3, BiVO4, Cu2V8O3, and CuFeO2 photoelectrodes;
instead, their synthesis method should be modified. Approaches for
performance improvements by nanostructuring, doping concentration
optimization, or surface passivation or by changing the photoelectrode
thickness were presented for all investigated photoelectrode materials
and nanostructures. We predictedconsistent with previous reportingthat
nanostructuring improves the projected diffusion length but also increases
the surface recombination losses, which partially counteract the performance
improvement, exemplified by the investigated nanostructured Fe2O3. The validated tool is well suited to evaluate
if nanostructuring can bring a photoelectrode to high performance
and/or if the deposition or synthesis methods should be optimized.
This tool can also be used for developing new approaches to passivate
surfaces and bulk defects, modifying doping concentration, and investigating
their impact on the photoelectrode performance and, more specifically,
on the surface and bulk losses. This tool is not restricted to water-splitting
reaction but can be applied to any photoelectrochemical reaction.