Graphene: Help Solar Cells "Blank Blooming"

Imagine such scenarios: In the future, solar cells are everywhere, be it windows and walls, or mobile phones and laptops. Professor Jing Jing from the Department of Electronic Engineering and Computer Science at the Massachusetts Institute of Technology (MIT) recently used graphene-based flexible transparent solar cells to make this dream scenario one step closer to reality. This type of solar cell does not require a separate installation and can be integrated into mobile phones and computer screens, which is expected to significantly reduce the manufacturing costs of these electronic products.

Graphene "stepped in"

In the past 10 years, researchers have been developing a variety of transparent organic solar cells and made significant progress. Compared with silicon-based solar cells, these batteries have many advantages: simple manufacturing process, low cost, light and easy bending, and easy transportation to remote areas without power grid. However, these studies are faced with a long-term difficult problem: they cannot find suitable electrode materials that combine conductivity and optical transparency.

At present, the most widely used material is Indium Tin Oxide (ITO). This material has both conductivity and transparency, but it is too hard. When it is bent, it easily breaks and breaks. In addition, indium is a rare metal. The cost of producing solar cells is too high.

The graphene layer is the best alternative to ITO. This kind of material made of carbon, which is made of everywhere, is not only highly conductive, bendable and transparent, but also has an electrode made of only one nanometer thick, which is more in line with the demand for ultra-thin organic solar cells.

New process to overcome bottlenecks

However, the two major bottlenecks have always restricted the popularity of graphene electrodes in solar cells. The first bottleneck is that the two electrodes of graphene are difficult to deposit on solar cells. Most solar panels are glass or plastic. When one of the graphene electrodes (bottom electrode) is deposited directly, an aqueous solution and heating are required, resulting in a particularly complex top electrode deposition process. Kong Jing said: “The hole transport layer (HTL) between two layers of graphene electrodes is easily dissolved and therefore is particularly sensitive to water and heat. As a result, other research teams often replace the top electrode with ITO and use it only at the bottom. Graphene electrode."

Another bottleneck of the graphene electrode is that the top electrode and the bottom electrode must perform different work performances, which is not easy to achieve.

Prof. Kong Jing led the team's laboratory team to develop a specific process that could solve these two bottlenecks at once. They use copper foil, polymer layers, silica gel and a layer of ethylene vinyl acetate (EVA). They not only successfully deposited two layers of graphene electrodes on solar panels, but also changed the working properties of top graphene electrodes to make them The performance of the underlying graphene is completely different, ensuring smooth current flow.

Highest transparency so far

In order to test whether the graphene electrode is practical, Kong Jing team used a solar panel in another laboratory of the school to integrate the graphene electrode, the ITO electrode and the aluminum electrode on the glass plate, and compared the solar energy conversion efficiency of the three electrodes. The test results show that the conversion efficiency of the graphene electrode and the ITO electrode is comparable; the conversion efficiency of the aluminum electrode is the highest. Kong Jing explained that this is because the aluminum electrode can reflect part of the sunlight back to the panel and absorb more solar energy, so it is the most efficient.

They conducted transparency tests on solar cells made with two layers of graphene electrodes and found that their optical transparency reached 61% with a maximum of 69%, which is the highest among current transparent solar cells.

Kong Jing said that their graphene solar cells can spread to any surface, regardless of the hardness and transparency of the surface. They also used transparent plastic, opaque paper and semi-transparent tape as the bottom plate respectively, and made double-layered graphene electrodes deposited on them to make solar cells. It was found that the conversion efficiency of the three is quite similar, and slightly lower than the solar cell conversion efficiency of the glass bottom plate. This means that graphene solar cells are very versatile for the future. Graphite batteries can be laid on top of the wall and glass, as well as mobile phones and computers, to provide the necessary power.

Although the conversion efficiency of graphene batteries is only 4% at present, according to the theoretical calculations of the team of Kong Jing, the conversion efficiency of graphene solar cells can be increased to 10% without increasing the transparency. This is a great improvement. Next research focus. (Reporter Nie Cuirong)

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