The sunlight’s intermittent nature is one of the issues limiting widespread harvesting of solar energy for power infrastructures. A leading approach is to store as chemical fuels the energy produced by this discontinuous renewable energy source. The development of photoelectrochemical cells (PECs) used to produce H₂ and O₂ from water photo-splitting has, therefore, attracted a great deal of interest.

Nowadays, the efficiency of the photoelectrochemical cells (PEC) ranges from 12 to 18% depending on the materials and the type of cells (single- or multiple-junction) while the theoretical limit is 24.4 and 30% for tandem and multi-junction cells, respectively. High efficiency at a high cost has been shown from multi-junction cells but no sufficient improvements leading to a market compliant PEC have been reported yet. The photoelectrochemical technology remains at a low technology readiness level (TRL 1 to 2) and the main issue is the actual production cost of H₂. To solve this question, it is mandatory to reduce significantly the costs and to increase the photoconversion efficiency. Many research groups are currently investigating different routes to fabricate PECs that could respond to the market demand. To improve efficiency, stability and price, one has, of course, to select a cost-effective photosensitive materials and the appropriate cell design. It has been recently shown that micro- or nanostructuring and/or surface functionalization of the photoelectrodes can lead to higher performances. Among the numerous approaches and techniques that have been used since nanosciences and nanotechnologies have emerged, atomic layer deposition (ALD) has recently demonstrated its high effectiveness to fabricate both two- and three-dimensional (2D, 3D) nano-objects. Energy storage and production are part of the fields of applications in which ALD has shown highly promising perspectives. It will be shown in this presentation that, during the last five years, ALD has been effectively integrated in various fabrication strategies of photoelectrodes.

After a brief introduction on the various methods of surface micro- and nanostructuring methods, a short description of the ALD process will be presented. The goal of this presentation is to report the different types of uses of ALD in the field of solar fuel production: active materials, surface state passivation and corrosion protection.

A special attention will be drawn on the recent results obtained in our laboratory where a rapid, inexpensive two-step method for structuring n-type (100) Si surfaces with micron-sized cavities, the process is based on the photoelectrochemical etching (PEE) of the Si surface and its subsequent alkaline etching. This method produces a layer of random macropores over a large area, which renders the Si surfaces antireflective over the visible spectrum. We demonstrate that such surfaces can be conformably coated by anatase TiO₂ layers by atomic layer deposition (ALD) and that they can be used as stable photoanodes producing enhanced photocurrents under simulated sunlight with respect to their planar counterparts. These TiO₂-protected Si microstructured surfaces were highly stable in strongly alkaline solutions and were used as photoanode for several hours under simulated sunlight. Such photoanodes surfaces showed 50 % photocurrent enhancements and ~400 mV negative shift of onset potential without any co-catalysts, demonstrating their high potential for solar energy conversion applications [3].

[1] N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. 103, 15729 (2006).

[2] A. Fujishima and K. Honda, Nature 238, 37 (1972).

[3] L. Santinacci, M. W. Diouf, M. K. S. Barr, B. Fabre, L. Joanny, F. Gouttefangeas, and G. Loget, ACS Appl. Mater. Interfaces, 8, 24810 (2016)