Direct water electrolysis was achieved with a novel, integrated, monolithic photoelectrochemical-photovoltaic design. This photoelectrochemical cell, which is voltage biased with an integrated photovoltaic device, splits water directly upon illumination; light is the only energy input. The hydrogen production efficiency of this system, based on the short-circuit current and the lower heating value of hydrogen, is 12.4 percent.

https://science.sciencemag.org/content/sci/280/5362/425.full.pdf

A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting

We describe the development of solar water-splitting cells comprising earth-abundant elements that operate in near-neutral pH conditions, both with and without connecting wires. The cells consist of a triple junction, amorphous silicon photovoltaic interfaced to hydrogen- and oxygen-evolving catalysts made from an alloy of earth-abundant metals and a cobalt|borate catalyst, respectively. The devices described here carry out the solar-driven water-splitting reaction at efficiencies of 4.7% for a wired configuration and 2.5% for a wireless configuration when illuminated with 1 sun (100 milliwatts per square centimeter) of air mass 1.5 simulated sunlight. Fuel-forming catalysts interfaced with light-harvesting semiconductors afford a pathway to direct solar-to-fuels conversion that captures many of the basic functional elements of a leaf.

https://science.sciencemag.org/content/sci/early/2011/09/28/science.1209816.full.pdf

Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts

To convert the energy of sunlight into chemical energy, the leaf splits water via the photosynthetic process to produce molecular oxygen and hydrogen, which is in a form of separated protons and electrons. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into spatially separated electron–hole pairs. The holes of this wireless current are captured by the oxygen evolving complex (OEC) of photosystem II (PSII) to oxidize water to oxygen. The electrons and protons produced as a byproduct of the OEC reaction are captured by ferrodoxin of photosystem I. With the aid of ferrodoxin–NADP+ reductase, they are used to produce hydrogen in the form of NADPH. For a synthetic material to realize the solar energy conversion function of the leaf, the light-absorbing material must capture a solar photon to generate a wireless current that is harnessed by catalysts, which drive the four electron/hole fuel-forming water-splitting reaction under benign conditions and under 1 su...

The artificial leaf

The photoactivity of metal oxide electrodes for water splitting is often limited by poor charge separation. Abdi et al. improve the solar-to-hydrogen efficiency in a hybrid device that comprises a gradient-doped bismuth vanadate photoanode and a double-junction amorphous silicon tandem solar cell.

/pdf/efficient-solar-water-splitting-by-enhanced-charge-r7t2sa6al0.pdf

Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode

Laboratory demonstrations of spontaneous photoelectrochemical (PEC) solar water splitting cells are reviewed. Reported solar-to-hydrogen (STH) conversion efficiencies range from 10% STH efficiency using potentially less costly materials have been reported. Device stability is a major challenge for the field, as evidenced by lifetimes of less than 24 hours in all but a few reports. No globally accepted protocol for evaluating and certifying STH efficiencies and lifetimes exists. It is our recommendation that a protocol similar to that used by the photovoltaic community be adopted so that future demonstrations of solar PEC water splitting can be compared on equal grounds.

/pdf/experimental-demonstrations-of-spontaneous-solar-driven-s6paarltnk.pdf

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

A new direct one step method to split the water into hydrogen and oxygen by using a triple stack amorphous silicon solar cell with an n‐i‐p‐n‐i‐p‐n‐i‐p structure is reported. The ruthenium oxide layer, which is directly painted on the back surface of the solar cell, catalytically aids the evolution of the oxygen. The isolated islands of platinum deposited on the top side of the cell materially help the production of hydrogen. The conversion efficiency of 5% of solar to chemical energy in steady state is achieved under simulated AM1, 100 mW/cm2 solar radiation.

One step method to produce hydrogen by a triple stack amorphous silicon solar cell

A one-unit photovoltaic electrolysis system based on a triple stack of amorphous silicon (pin) cells

An amorphous silicon-based one-unit photovoltaic electrolyzer

Amorphous silicon-selenium alloys were prepared by plasma-enhanced chemical vapor deposition. It was found that the SiSe alloy is a wide band-gap semiconductor. The optical band-gap of the alloy changed from 1.7 to 2.0 eV, as a function of the selenium concentration in the film. The light to dark conductivity ratio was of the order of 1.5 × 10 3 . The dangling bond density was estimated from electron spin resonance measurements, and the elemental content and distribution were analyzed by the secondary ion mass spectrometry, energy dispersive X-ray spectroscopy and electron microp analysis techniques. The results show that the amorphous silicon selenium alloy is a promising materials for multi-band-gap photovoltaic devices.

Plasma-enhanced chemical vapor deposition of amorphous silicon-selenium alloy films

High and low bandgap amorphous silicon thin film alloys (a-Si:Al, a-Si:Se, a-Si:S, and a-Si:Ga) were prepared by plasma-enhanced chemical vapor deposition. It was found that Al and Ga amorphous silicon alloys are low bandgap materials whereas a:SiS and a:SiSe are high bandgap semiconductors. The optical band gap of these systems could be changed from 1.0 eV to 2.0 eV, depending on the alloying element and its concentration in the film. The dark to light conductivity ratio was measured. The elemental content and distribution was analyzed by the SIMS and EPMA techniques. The results show that some of the amorphous silicon alloys studied are promising materials for multi-bandgap photovoltaic devices.

M. Kapur

Papers

One step method to produce hydrogen by a triple stack amorphous silicon solar cell

A one-unit photovoltaic electrolysis system based on a triple stack of amorphous silicon (pin) cells

An amorphous silicon-based one-unit photovoltaic electrolyzer

Plasma-enhanced chemical vapor deposition of amorphous silicon-selenium alloy films

The Electrical and Optical Properties of Amorphous Silicon Alloys by Plasma-Enhanced CVD Method