• I-Ju Chen, Yang Chen and Xulu Zeng are all on the track to find better ways of harvesting sunlight using solar cells made of nanowires.
    I-Ju Chen, Yang Chen and Xulu Zeng are all on the track to find better ways of harvesting sunlight using solar cells made of nanowires.

The sunhunters – with knowledge to collect the light

Increased use of solar cells in the future requires higher efficiency and lower production costs. Innovative research from the interdisciplinary centre NanoLund wants to optimize the nanowires so that sunlight can be harvested more efficiently. Meet three young international researchers in the PhD4Energy project, working on hunting the sun.

Solar energy, which reaches our earth every day, corresponds to 15,000 times more than all the electricity we need in our daily lives. We could solve our great challenge of finding alternative sources of energy for oil, coal and gas, if we were able to capture the sun’s rays in smarter ways. At NanoLund at Lund University, extremely thin and incredible small nanowires that are about 1% of the thickness of a hair, are manufactured. Nanotechnology, which has many uses that can change our everyday life in the future, is about controlling small materials down at the atomic level. Nano Lund are one of the best in the world when it comes to taking advantage of the nanostructure’s properties and produce nanowires in a cheap and efficient manner.

Xulu Zeng, I-Ju Chen and Yang Chen, all devoted researchers from Asia, have now spent the last four years at NanoLund within EU financed PhD4Energy project that involves 12 international researchers who focus on nanowire and nanotechnology. These three sunhunters are all on the track to find better ways of harvesting sunlight using solar cells made of nanowires.

Xulu Zeng is involved in the manufacturing of semiconductor nanowires. , and is a type of material that can extract electricity from light.

The researchers in Lund previously held the record of the efficiency of nanowire solar cells with 13,8 % of the sun’s conversion into electricity. That was a big leap from the prevoius record that attracted the world attention from both academia and industry.

Recently, we have updated the efficiency of our devices to 15 %. So far, our solar cells consist of one single p-n junction with very high quality. However, the single junction can only harvest a limited range of the solar energy, therefore a single-junction solar cell has limited the efficiency roof, explains Xulu Zeng and continues:

We need to optimize the wire for light collection

– Our next goal is to introduce the tandem junction design into nanowires, which is the strategy used in the most efficient solar cells of all kinds. In a tandem junction solar cell several junctions are connected in series so that different parts of the solar spectrum can be more effectively absorbed by each junction. This tandem geometry is predicted to increase the solar cell efficiency to above 35%.

It is the combination of different materials in each junction that makes the nanostructured solar cell be able to better respond to a larger proportion of the solar energy.

-Our current goal is to take the step from research to manufacturing a full solar cell. I think we will reach our goal in three years and hopefully Sol Voltaics will take it on. Once the technique is there, it will be a good transition from lab to production

All forecasts are clear: improved solar technology generates lower production costs and will enable us to rely more on the sun’s rays, which are both free, endless and environmentally friendly, for our energy usage. And the price of solar cell has been reduced a lot in the past 20 years and is now competitive with other energy resources.

Other research that excites Xulu Zeng and his colleagues aims to transfer nanowires from the substrate where they are grown to another foreign substrate, which can be cheaper materials like silicon, glass or plastic, by embedding the nanowires into a polymer film. A polymer film of nanowire devices is much lighter, flexible and bendable. You are no longer locked to only silicon plates, which are the standard base for building a solar cell, so the applications increase. The nanowire film can be conveniently applied on different surfaces, such as building sidewalls, mobile electronics, and wearable devices. In this ‘‘cut and paste’’ technique of nanowires, the cost of devices will be further lowered by reusing the substrate.

But will nanowire-based solar cells outnumber the ones made of silicon?

– I don´t see a clear winner. They have different features and their own suitable applications. Despite competition, an idea is to rather combine them side by side so that the advantages of both can be utilized.

Researcher I-Ju Chen is also involved in developing the future energy landscape in understanding how to optimize harvesting the energy that comes from the sun.

Today when solar cells absorb the sunlight they get really hot. This shows that a lot of the absorbed solar energy don´t become electricity, but instead just become heat. It also creates another problem, that the solar cell efficiency actually drops when they get hot.

I-Ju Chen and her colleagues are looking into how to more efficiently turn the absorbed light into electricity. The effect that they are studying is often called hot-electron solar energy conversion because it collect electrons that are very hot due to absorption of light.

I-Ju Chen works in the cleanroom at NanoLund, a laboratory where nanostructures are manufactured. The question is: can the researchers implement new concepts so that a bigger portion of the energy of light can be turned into electricity and a smaller portion into waste heat.

-We use equipment in the cleanroom to make nanoscale solar cells for our measurements. The hot-electron device is based on a nanowire, but it’s composed of a stack of multiple materials. Our goal is to minimize the loss of solar energy into heat.

In the long term, researchers predicts that by collecting these very hot electrons we can convert the energy from the sun with an up to 60 – 85 % efficience.

But how do nanowires grow? The original method is called epitaxi. A slow and expensive way of producing the extremely thin threads by placing gold particles on the crystalline substrate, a kind of semiconductor crystal on a baking plate.

Another technology in Lund today is aerotaxy where gold particles and semiconductor materials are blown into an oven of about 400 degrees Celsius. There they can float freely. What comes out of the oven is crystalline nanowires, perfect in shape and grown at a much faster speed than in the original method.

Aerotaxy is the key to building an industry with nanowire solar cells and it was at NanoLund the birth of this method took place. Today NanoLund is world-leading in using this technique.

The thin nanowires are built atomic layer by atomic layer to control when a material begins and one ends in the nanowire. You can then see what different characters they have, and sections can be made out of completely different materials. To make it as efficient as possible NanoLund uses computer simulation to check what are the best combinations. In order to really achieve the maximum effect of taking care of the solar rays in a solar cell, all properties in the material need to be accurate.  For this to work, very careful examinations of the material and advanced electrical and optical modeling are required. Such work employs Yang Chen.

– We need to optimize the wire for light collection, so we try to change the distance between nanowire and the diameter in the computer for that to happen, and we also try to improve the use of material. The second aspect is that we can optimize the combination of material for harvesting the energy.

The cleanroom at NanoLund is a laboratory where nanostructures are manufactured.

The cleanroom at NanoLund is a laboratory where nanostructures are manufactured.

It’s expensive to carry out experimental tests, that’s why the researchers do careful modeling to see which is the best model to use.

– We need to know where to go, so that we don’t waste money. Clear instructions for the tests make the experiments more efficient.

How will we use solar energy in the future? Taking a realistic view, we can use it for many of our everyday energy needs, but we can´t rely on it fully; there are sometimes clouds in the sky and there will always be nighttime. In sunny countries all around the world the solar energy has become the cheapest way to get energy, even cheaper than electricity that is produced by wind power and fossil fuels. And Yang Chen has a vision of how we could help each other in a network around the world.

– Imagine, if the sun shines in the US it could be distributed to countries that have night at the same time. We could store and reuse. The most stable thing is not the sun cell but the sun itself.

Text: Bodil Malmström

Photo: Kennet Ruona

Facts

PhD4Energy project

http://www.phd4energy.lu.se/about-the-project/

 

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