The ability to harness and conserve solar energy is of utmost importance to guarantee a continuous (energy) output and satisfy our energy demands despite its intermittent availability. Energy storage using artificial photosynthesis is one promising technology to store the captured solar energy into energy-dense chemicals, e.g. hydrogen, methanol, or methane.^[1] Such a solar-energy-to-chemical-energy conversion could be achieved at high efficiency through the use of photo-electrochemical devices. Herein, via the absorption of solar photons, semiconducting photocathodes or photoanodes (or a combination thereof) generate electrons and holes, that are capable of reduction and oxidation of target compounds; e.g. water to H₂ and O₂, or CO₂ to formic acid, methanol, etc.

Typical fabrication of such photoelectrodes occurs through the use of CVD and sputtering to obtain well-controlled and high-purity semiconductors. Instead, we will explore the use CSD routes towards such semiconductors, offering increased scalability as well as synthetic flexibility. Deposition from a liquid phase enables nearly-free control of the precursor composition, and ultimately, the obtained end-product. In this work we demonstrate the strength of this precise chemical control towards fabrication and finetuning of the opto-electronic properties of two Bi-based semiconductors.

First, a bismuth-based oxide semiconductor, i.e. copper bismuth oxide CuBi₂O₄, was prepared by exploiting the flexibility of the aqueous-solution gel route to introduce an elemental gradient across the semiconductor film and obtain an internal electric field.^[2] The latter should enhance the charge separation of the excitons, and increase the efficiency of the photoelectrodes.^[3] The phase-purity of CuBi₂O₄ was confirmed using X-ray diffraction and Raman spectroscopy, and TOF-SIMS measurements reveal the introduced elemental gradient is maintained throughout the photoelectrode fabrication. Only a limited diffusion of copper and bismuth during thermal annealing is observed, which would counter the elemental gradient. The tight synthetic control enables a photocurrent increase of up to 20% compared to homogeneous stoichiometric CuBi₂O₄, similar to values found for CVD routes.^[2]

Secondly, a bismuth-based oxyhalide semiconductor, i.e. bismuth oxyiodide BiOI, was prepared by an aqueous chemical conversion of pre-deposited BiI₃. The low-dimensionality of the bismuth oxyhalide, similar to antimony chalcogenides, renders its performance very sensitive to its morphological features.^[4] Controlling the aqueous environment enables kinetic control over the conversion (typically seconds-long) while retaining the crystallographic properties of the oxyhalide. This enables us a time-window to impact the ultimate morphology.

The results shown will highlight the ability of CSD routes towards finetuning semiconductors for use to photoelectrodes but can also be applied to photovoltaics, sensors, etc.