Thin film solar cell

Cheaper solar material explained.

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In the near future, solar panels will not only be more efficient but also a lot cheaper and affordable for everyone, thanks to research by Nanyang Technological University (NTU) scientists.

This next generation solar cell, made from organic-inorganic hybrid perovskite materials, is about five times cheaper than current thin-film solar cells, due to a simpler solution-based manufacturing process.

Perovskite is known to be a remarkable solar cell material as it can convert up to 15 per cent of sunlight to electricity, close to the efficiency of the current solar cells, but scientists did not know why or how, until now.

In a paper published in the world’s most prestigious academic journal, Science, NTU’s interdisciplinary research team was the first in the world to explain this phenomenon.

The team of eight researchers led by Assistant Professor Sum Tze Chien and Dr Nripan Mathews had worked closely with NTU Visiting Professor Michael Grätzel, who currently holds the record for perovskite solar cell efficiency of 15 per cent, and is a co-author of the paper. Prof Grätzel, who is based at the Swiss Federal Institute of Technology in Lausanne (EPFL), has won multiple awards for his invention of dye-sensitised solar cells.

The high sunlight-to-electricity efficiency of perovskite solar cells places it in direct competition with thin film solar cells which are already in the market and have efficiencies close to 20 per cent.

The new knowledge on how these solar cells work is now being applied by the Energy Research Institute @ NTU (ERI@N), which is developing a commercial prototype of the perovskite solar cell in collaboration with Australian clean-tech firm Dyesol Limited (ASX: DYE).

Asst Prof Sum said the discovery of why perovskite worked so well as a solar cell material was made possible only through the use of cutting-edge equipment and in close collaboration with NTU engineers.

“In our work, we utilise ultrafast lasers to study the perovskite materials. We tracked how fast these materials react to light in quadrillionths of a second (roughly 100 billion times faster than a camera flash),” said the Singaporean photophysics expert from NTU’s School of Physical and Mathematical Sciences.

“We discovered that in these perovskite materials, the electrons generated in the material by sunlight can travel quite far. This will allow us to make thicker solar cells which absorb more light and in turn generate more electricity.”

The NTU physicist added that this unique characteristic of perovskite is quite remarkable since it is made from a simple solution method that normally produces low quality materials.

His collaborator, Dr Nripan Mathews, a senior scientist at ERI@N, said that their discovery is a great example of how investment in fundamental research and an interdisciplinary effort, can lead to advances in knowledge and breakthroughs in applied science.

“Now that we know exactly how perovskite materials behave and work, we will be able to tweak the performance of the new solar cells and improve its efficiency, hopefully reaching or even exceeding the performance of today’s thin-film solar cells,” said Dr Mathews, who is also the Singapore R&D Director of the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) NRF CREATE programme.

“The excellent properties of these materials, allow us to make light weight, flexible solar cells on plastic using cheap processes without sacrificing the good sunlight conversion efficiency.”

Professor Subodh Mhaisalkar, the Executive Director of ERI@N said they are now looking into building prototype solar cell modules based on this exciting class of materials.

“Perovskite-based solar cells have the potential to reach 20 per cent solar cell efficiencies and another great benefit of these materials is their amenability to yield different translucent colours, such as red, yellow or brown. Having such colourful solar glass will create new opportunities for architectural design,” he added.

The NTU team, consisting of six scientists, one postgraduate and one undergraduate, took six months to complete this fundamental research project, which was funded by NTU and the National Research Foundation, Prime Minister’s Office, Singapore.

Solar Hydrogen Production Breakthrough.

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Using a simple solar cell and a photo anode made of a metal oxide, HZB and TU Delft scientists have successfully stored nearly five percent of solar energy chemically in the form of hydrogen. This is a major feat as the design of the solar cell is much simpler than that of the high-efficiency triple-junction cells based on amorphous silicon or expensive III-V semiconductors that are traditionally used for this purpose. The photo anode, which is made from the metal oxide bismuthvanadate (BiVO4) to which a small amount of tungsten atoms was added, was sprayed onto a piece of conducting glass and coated with an inexpensive cobalt phosphate catalyst.

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Basically, we combined the best of both worlds,” explains Prof. Dr. Roel van de Krol, head of the HZB Institute for Solar Fuels: “We start with a chemically stable, low cost metal oxide, add a really good but simple silicon-based thin film solar cell, and — voilà — we’ve just created a cost-effective, highly stable, and highly efficient solar fuel device.”

Thus the experts were able to develop a rather elegant and simple system for using sunlight to split water into hydrogen and oxygen. This process, called artificial photosynthesis, allows solar energy to be stored in the form of hydrogen. The hydrogen can then be used as a fuel either directly or in the form of methane, or it can generate electricity in a fuel cell. One rough estimate shows the potential inherent in this technology: At a solar performance in Germany of roughly 600 Watts per square meter, 100 square meters of this type of system is theoretically capable of storing 3 kilowatt hours of energy in the form of hydrogen in just one single hour of sunshine. This energy could then be available at night or on cloudy days.

Metal oxide as photo anode prevents corrosion of the solar cell

Van de Krol and his team essentially started with a relatively simple silicon-based thin film cell to which a metal oxide layer was added. This layer is the only part of the cell that is in contact with the water, and acts as a photo anode for oxygen formation. At the same time, it helps to prevent corrosion of the sensitive silicon cell. The researchers systematically examined and optimized processes such as light absorption, separation of charges, and splitting of water molecules. Theoretically, a solar-to-chemical efficiency of up to nine percent is possible when you use a photo anode made from bismuth vanadate, says van de Krol. Already, they were able to solve one problem: Using an inexpensive cobalt phosphate catalyst, they managed to substantially accelerate the process of oxygen formation at the photo anode.

A new record: More than 80 percent of the incident photons contribute to the current!

The biggest challenge, however, was the efficient separation of electrical charges within the bismuth vanadate film. Metal oxides may be stable and cheap, but the charge carriers have a tendency to quickly recombine. This means they are no longer available for the water splitting reaction. Now, Van de Krol and his team have figured out that it helps to add wolfram atoms to the bismuth vanadate film. “What’s important is that we distribute these wolfram atoms in a very specific way so that they can set up an internal electric field, which helps to prevent recombination,” explains van de Krol. For this to work, the scientists took a bismuth vanadium wolfram solution and sprayed it onto a heated glass substrate. This caused the solution to evaporate. By repeatedly spraying different wolfram concentrations onto the glass, a highly efficient photo-active metal oxide film some 300 nanometers thick was created. “We don’t really understand quite yet why bismuth vanadate works so much better than other metal oxides. We found that more than 80 percent of the incident photons contribute to the current, an unexpectedly high value that sets a new record for metal oxides” says van de Krol. The next challenge is scaling these kinds of systems to several square meters so they can yield relevant amounts of hydrogen.

Source: http://www.sciencedaily.com