Solar panels are great and all, but they can be a bit of an eyesore. While companies like Tesla are disguising them as roof tiles, others are finding ways to make them transparent, allowing them to hide on windows, walls or other surfaces. Now an MIT team has developed solar cells that are not only invisible, but flexible as well, thanks to electrodes made of – what else? – graphene.

Organic solar cells may not be as efficient as their silicon-based brethren, but they do have plenty of other benefits: they're cheaper and easier to make, as well as being lighter, more flexible, and potentially transparent. The problem is that in order for the devices to be at all practical, the electrode layer often tints or colors the material, and making it thinner comes at the cost of the already relatively-low efficiency.

Enter everybody's favorite 2D wonder material. Graphene is an excellent conductor of electricity and in sheets only one atom thick it's virtually invisible and allows for thinner devices. But making transparent solar cells with graphene poses its own challenges: namely, getting the two electrodes to stick together and to the substrate, as well as ensuring that electrons only flow out of one of the graphene layers.

Using heat or glue to stick the bottom graphene layer to the substrate can damage the material and reduce its conductivity, so the MIT team developed a new technique. Rather than applying an adhesive between the graphene and the substrate, they sprayed a thin layer of ethylene-vinyl acetate (EVA) over the top, sticking them together like tape instead of glue. That process not only kept the layers together, but it unexpectedly solved the second problem as well.

"We got lucky," says Yi Song, co-author of the study. "Our top and bottom electrodes just happen to have the correct work functions (electron flow) as a result of the processes we use to make them."

To test their creation, the team made samples of solar cells using electrodes of different materials, and compared their current density (CD) and power conversion efficiency (PCE) (Credit: Stuart Darsch)

The team then compared their graphene electrode solar cells against others made from standard materials like aluminum and indium tin oxide (ITO), built on rigid glass and flexible substrates. The power conversion efficiency (PCE) of the graphene solar cells peaked at 4.1 percent: far lower than regular solar panels, but much better than previous transparent solar cells.

But as expected, that efficiency is a trade-off from the graphene solar cells being flexible and transparent. In that regard they performed well, transmitting almost 70 percent of the light in the middle of the human range of vision.

"Those values [for transmittance] are among the highest for transparent solar cells with comparable power conversion efficiencies in the literature," says Jing Kong, lead author of the study.

According to the researchers' calculations though, the efficiency of these graphene solar cells could be pushed as high as 10 percent without losing any transparency, and doing just that is the next step in the project. The researchers are also working on ways to scale up the system to cover windows and walls.

The research was published in the journal Advanced Energy Materials.

Source: MIT

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