Félix Urbain

Multijunction Si photocathodes with tunable photovoltages from 2.0 V to 2.8 V for light induced water splitting

We report on the development of high performance triple and quadruple junction solar cells made of amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) for the application as photocathodes in integrated photovoltaic–electrosynthetic devices for solar water splitting. We show that the electronic properties of the individual sub cells can be adjusted such that the photovoltages of multijunction devices cover a wide range of photovoltages from 2.0 V up to 2.8 V with photovoltaic efficiencies of 13.6% for triple and 13.2% for quadruple cells. The ability to provide self-contained solar water splitting is demonstrated in a PV-biased electrosynthetic (PV-EC) cell. With the developed triple junction photocathode in the a-Si:H/a-Si:H/μc-Si:H configuration we achieved an operation photocurrent density of 7.7 mA cm−2 at 0 V applied bias using a Ag/Pt layer stack as photocathode/electrolyte contact and ruthenium oxide as counter electrode. Assuming a faradaic efficiency of 100%, this corresponds to a solar-to-hydrogen efficiency of 9.5%. The quadruple junction device provides enough excess voltage to substitute precious metal catalyst, such as Pt by more earth-abundant materials, such as Ni without impairing the solar-to-hydrogen efficiency.

Development of Thin Film Amorphous Silicon Tandem Junction Based Photocathodes Providing High Open-Circuit Voltages for Hydrogen Production

Hydrogenated amorphous silicon thin film tandem solar cells (a-Si:H/a-Si:H) have been developed with focus on high open-circuit voltages for the direct application as photocathodes in photoelectrochemical water splitting devices. By temperature variation during deposition of the intrinsic a-Si:H absorber layers the band gap energy of a-Si:H absorber layers, correlating with the hydrogen content of the material, can be adjusted and combined in a way that a-Si:H/a-Si:H tandem solar cells provide open-circuit voltages up to 1.87 V. The applicability of the tandem solar cells as photocathodes was investigated in a photoelectrochemical cell (PEC) measurement set-up. With platinum as a catalyst, the a-Si:H/a-Si:H based photocathodes exhibit a high photocurrent onset potential of 1.76 V versus the reversible hydrogen electrode (RHE) and a photocurrent of 5.3 mA/cm2 at 0 V versus RHE (under halogen lamp illumination). Our results provide evidence that a direct application of thin film silicon based photocathodes fulfills the main thermodynamic requirements to generate hydrogen. Furthermore, the presented approach may provide an efficient and low-cost route to solar hydrogen production.

Upscaling of integrated photoelectrochemical water-splitting devices to large areas

Photoelectrochemical water splitting promises both sustainable energy generation and energy storage in the form of hydrogen. However, the realization of this vision requires laboratory experiments to be engineered into a large-scale technology. Up to now only few concepts for scalable devices have been proposed or realized. Here we introduce and realize a concept which, by design, is scalable to large areas and is compatible with multiple thin-film photovoltaic technologies. The scalability is achieved by continuous repetition of a base unit created by laser processing. The concept allows for independent optimization of photovoltaic and electrochemical part. We demonstrate a fully integrated, wireless device with stable and bias-free operation for 40 h. Furthermore, the concept is scaled to a device area of 64 cm2 comprising 13 base units exhibiting a solar-to-hydrogen efficiency of 3.9%. The concept and its successful realization may be an important contribution towards the large-scale application of artificial photosynthesis.

Photoelectrochemical and photovoltaic characteristics of amorphous-silicon-based tandem cells as photocathodes for water splitting.

In this study amorphous silicon tandem solar cells are successfully utilized as photoelectrodes in a photoelectrochemical cell for water electrolysis. The tandem cells are modified with various amounts of platinum and are combined with a ruthenium oxide counter electrode. In a two-electrode arrangement this system is capable of splitting water without external bias with a short-circuit current of 4.50 mA cm(-2). On the assumption that no faradaic losses occur, a solar-to-hydrogen efficiency of 5.54% is achieved. In order to identify the relevant loss processes, additional three-electrode measurements were performed for each involved half-cell.

A modular device for large area integrated photoelectrochemical water-splitting as a versatile tool to evaluate photoabsorbers and catalysts

We present a stand-alone integrated solar water-splitting module with an active area of 64 cm2 and a long-term stable operation. As a photocathode we employ multijunction thin film silicon solar cells that were optimized to deliver a suitable output voltage for spontaneous water-splitting. Two approaches for the design of a suitable front contact are presented to reduce series resistance losses related to the upscale of the photoelectrodes. The photoelectrode is protected from the electrolyte by a sheet metal which connects the rear contact of the solar cell with the hydrogen evolving catalyst. Thereby, the sheet metal ensures long-term stability while the electrical and thermal coupling of the solar cell and the electrolysis cell is maintained. Due to the modular setup, which allows us to vary and optimize the device components (i.e. the solar cell, catalysts, membrane, and electrolyte) individually, the presented water-splitting device provides a convenient toolbox for the optimization of such systems.

Spectral matching and outdoor solar to electrical conversion efficiency in thin-film silicon multi-junction solar cells

Semi-empirical computer modelling is used to investigate spectral matching in tandem and triple-junction thin film silicon solar cells. In amorphous/microcrystalline silicon (a-Si:H/µc-Si:H) tandem cells, current mis-match is offset by an increase in fill-factor, resulting in a broad peak in efficiency versus average photon energy. For a-Si:H/a-Si:H tandem cells, photo-generated currents in both sub-cells increase with increasing average photon energy, and efficiency is predicted to increase monotonically over a wide spectral range. a-Si:H/a-Si:H/µc-Si:H triple cells exhibit spectral behaviour similar to a-Si:H/µc-Si:H tandem cells, but with a smaller fill-factor dependence. Variations in spectral quality are predicted to account for only a small reduction in annual electrical energy yield, of some 2 to 4%.