Photocatalytic reaction in the shadow

January 25, 2021

Solar-driven photoelectrochemical (PEC) water splitting is an attractive approach to convert solar energy into chemical energy. Among many photoelectrode materials, crystalline silicon (c-Si) has drawn considerable attention because of its earth abundance, narrow bandgap, and suitable band edge position for hydrogen evolution reaction (HER). However, c-Si suffers from low photovoltage generated from the solid-liquid junction.

Various strategies, such as the construction of p-n homojunctions, metal-insulator-semiconductor (MIS) junctions and p-n heterojunctions, have been adopted to obtain high photovoltage. The MIS junctions have been the focus of attention in PEC water splitting due to their simple fabrication and the potential to achieve higher efficiencies than p-n junctions. However, there are very limited Si-based MIS photocathodes reported with efficiency exceeding 5%, much lower than that of p-n junction photocathode (10%).

One of the major challenges of p-Si MIS photocathodes for higher efficiency is the parasitic light absorption from HER catalysts such as Pt, Ni-Mo, etc. Traditional MIS photocathodes are fabricated from p-Si, where the photogenerated minority carriers (electrons) drive the reduction reaction at the front surface. This could be translated into the fact that the catalyst must be placed at the same side of MIS junction. Thus, the parasitic light absorption from catalysts will severely limit the photocurrent density. The metal layers in MIS junction also cause optical loss. Another limiting factor is the lack of low work function metals to form a large band offset with p-Si in MIS junction, resulting in a low photovoltage.

In a research article published in National Science Review, scientists at Tianjin University present a unique illumination-reaction decoupled MIS photocathode fabricated from n-Si, which surmounts the challenges that seriously impede the development of p-Si MIS photocathode.

Different from previous works that employ minority carriers to drive the surface reduction reaction, the majority carriers (electrons) of n-Si MIS photocathode are used in this work. Upon this simple, unconventional yet effective modification, the MIS junction and catalyst can be placed on the opposite sides of n-Si, which avoids the light-shielding problem of catalyst.

Moreover, this MIS photocathode constructed from n-Si addresses the drawback of lacking metallic materials with suitable work function to generate a large band offset for p-Si MIS photocathode. By using indium tin oxide (ITO) with a high transmittance as the high work function metallic material for n-Si MIS photocathode, the trade-off between metal coverage and light absorption confronted by high work function metals is further eliminated.

As a result, this illumination-reaction decoupled n-Si MIS photocathode exhibits a light absorption higher than 90%, a photovoltage up to 570 mV, and a recorded efficiency of 10.3%, exceeding traditional p-Si MIS photocathodes.

This facile strategy exhibits a potential to inspire the rational design of solar powered photoelectrochemical systems that use catalysts with poor light transmittance, a step forward towards future large-scale commercialization of solar water splitting.
-end-
This research is supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Natural Science Foundation of Tianjin City, and the Program of Introducing Talents of Discipline to Universities.

See the article:
Shujie Wang, Tuo Wang, Bin Liu, Huimin Li, Shijia Feng and Jinlong Gong
Spatial decoupling of light absorption and reaction sites in n-Si photocathodes for solar water splitting
Natl Sci Rev 2020; doi: 10.1093/nsr/nwaa293
https://doi.org/10.1093/nsr/nwaa293

Science China Press

Related Efficiency Articles from Brightsurf:

Theoretically, two layers are better than one for solar-cell efficiency
Solar cells have come a long way, but inexpensive, thin film solar cells are still far behind more expensive, crystalline solar cells in efficiency.

Using physics to improve root canal efficiency
In Physics of Fluids, scientists report calculations with a model of a conical-shaped root canal inside a tooth.

Record efficiency for printed solar cells
A new study reports the highest efficiency ever recorded for full roll-to-roll printed perovskite solar cells.

New 5G switch provides 50 times more energy efficiency than currently exists
As 5G hits the market, new US Army-funded research has developed a radio-frequency switch that is more than 50 times more energy efficient than what is used today.

On the trail of organic solar cells' efficiency
Scientists at TU Dresden and Hasselt University in Belgium investigated the physical causes that limit the efficiency of novel solar cells based on organic molecular materials.

Wearable health tech gets efficiency upgrade
North Carolina State University engineers have demonstrated a flexible device that harvests the heat energy from the human body to monitor health.

Photoelectrochemical water-splitting efficiency hits 4.5%
Solar-to-fuel conversion offers a promising technology to solve energy problems, yet device performance could be limited by undesired sunlight absorption.

Green hydrogen: Research to enhance efficiency
Laboratory experiments and a parabolic flight campaign have enabled an international team of researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) to gain new insights into water electrolysis, in which hydrogen is obtained from water by applying electric energy.

New efficiency world record for organic solar modules
Researchers from Nuremberg and Erlangen has set a new record for the power conversion efficiency of organic photovoltaic modules.

A new method for quantifying crystal semiconductor efficiency
Japanese scientists have found a new way to successfully detect the efficiency of crystal semiconductors.

Read More: Efficiency News and Efficiency Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.