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Xinjing Huang, a PhD student in applied physics, demonstrates the semi-transparent view through the solar cell. The new manufacturing process could make it possible to produce electricity-generating windows on a metre scale. Photo: Silvia Cardarelli, Electrical and Computer Engineering

When it comes to turning the tide of the energy and climate crisis, at the top of the wish list are buildings that can produce the energy they consume themselves, without emissions. One way to achieve this is through photovoltaics. Conventional silicon-based solar cells are opaque and therefore work well on roofs and in large-scale solar farms. They are not suitable for window surfaces that can be seen through. Organic solar cells, on the other hand, where the light absorber is a type of plastic, can be transparent.

At the University of Michigan, an important step has now been taken towards transparent solar cells for house windows. Researchers have developed a method for producing transparent solar cells: “In principle, we can now scale semi-transparent organic solar cells to two metres by two metres, which brings our windows much closer to realisation,” explained Stephen Forrest, professor of electrical engineering. Although development is still in its early stages, cells from Forrest’s lab have recently achieved a record-breaking efficiency of 10 % and an estimated lifetime of up to 30 years.

One of the challenges in the production of transparent solar panels is to connect the individual cells to each other in the micrometre range to form modules. To avoid damaging the cells with lasers, which are usually used to structure them, the team developed a multi-stage peeling process to divide the thin plastic layers into narrow strips. They then applied the organic and metallic layers and peeled off the strips, creating very fine electrical connections between the cells. Eight cells were connected in this way to form a 13 square centimetre module and the flexible solar cell panel was clamped between two window panes. With a transparency of almost 50 % and a greenish colour, the cells are suitable for use in commercial windows. In order to achieve even better transparency for the residential sector, the industry is now to be involved in further development.

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