Light, flexible, efficient: Perovskite-based tandem solar cells

Roof tiles have gotten a factor of the previous: Right this moment, increasingly Swiss roofs boast giant black and blue rectangles that convert daylight into electrical energy. The blueish coloration comes from silicon crystals, as the vast majority of photo voltaic cells accessible right this moment are primarily based on this semiconductor materials. However silicon isn’t the one approach to make a photo voltaic cell—and probably not even the very best.

Silicon-based photovoltaic cells have been perfected to date that they’re reaching the bounds of their effectivity. Though a number of proportion factors of enchancment might nonetheless be gained, the theoretical higher restrict for the effectivity of a single silicon cell is 33%. In follow, it’s considerably decrease, as small vitality losses inevitably happen in the course of the development and operation of the cells.

The explanation for this tough effectivity restrict lies within the materials properties of silicon. The so-called band hole of the fabric signifies that solely photons with a sure vitality might be transformed into electrical energy. If the vitality of the photon is simply too excessive, it can’t be totally “utilized” by the photo voltaic cell.

Two layers are higher than one

Photo voltaic cells constituted of different supplies provide a approach to overcome this limitation, says Empa researcher Fan Fu. The group chief within the Laboratory for Skinny Movies and Photovoltaics is researching extremely environment friendly photo voltaic cells made from perovskite.

A single perovskite cell alone doesn’t obtain a better effectivity, as a result of perovskite as a semiconductor additionally has a restricted band hole. The actual energy of this progressive materials lies in the truth that, not like silicon, this band hole might be simply adjusted by various the precise composition of the perovskite materials.

If two perovskites with completely different band gaps are processed into thin films photo voltaic cells and “stacked” on high of one another, the result’s a so-called tandem photo voltaic cell. One perovskite layer “catches” the photons with excessive vitality, the opposite one these with low vitality.

In idea, this permits efficiencies of as much as 45%—a lot greater than the 33% of single-junction cells. Alternatively, a perovskite layer can be added to a silicon cell to create a extremely environment friendly tandem cell.

Nonetheless, Fu and his group are at the moment specializing in all-perovskite tandem cells, particularly as a part of the EU analysis mission SuPerTandem, wherein a complete of 15 European main analysis establishments and firms are concerned. The intention of the mission is to develop versatile perovskite tandem modules with an effectivity of over 30%, which can be produced utilizing scalable and cost-effective processes.

That is one other energy of perovskite photo voltaic cells: “Silicon solar cells usually require high-purity silicon monocrystals that are produced at high temperatures,” explains Fu. “Perovskite thin films, on the other hand, can be printed, solution-processed or produced by vapor deposition with a very low CO₂ footprint. Small defects that occur in the process only have a minor impact on their optoelectronic properties.”

The potential advantages of tasks like SuPerTandem are important, as a result of the upper the effectivity, the inexpensive solar energy will likely be on the finish of the day. “The cell itself only accounts for less than 20% of the cost of a PV system,” says Fu. “The remaining 80% is accounted for by the cables, the inverters, the junction box and, of course, the labor involved in installation.”

If the effectivity of the person cells is elevated, a smaller—and subsequently extra inexpensive—PV system is enough for a similar electrical energy output. Skinny-film cells made from perovskite can be produced on light-weight versatile movies as an alternative of heavy, inflexible glass plates like silicon cells. This implies they can be utilized in extra places, for example on automobile roofs or on buildings with a low load-bearing capability.

From the lab to the roof

This potential of perovskite photo voltaic cells should now be exploited. Along with SuPerTandem, Fan Fu’s group can also be engaged on two Swiss tasks. Empa researchers are working to raised perceive the elemental properties and challenges of perovskite photo voltaic cells that contribute to their effectivity and stability. And in a mission with the Swiss Federal Workplace of Power (SFOE), they’re placing their present data into follow by scaling up the tandem cells already developed at Empa.

What else do we have to do to make sure that the black and blue rectangles on our roofs are joined by reddish perovskite movies? “First of all, we have to scale up the perovskite cells from the current prototypes of a few centimeters to industrial sizes,” says Fu. The cells, that are nonetheless considerably fragile, additionally must be successfully protected against the climate.

Fan Fu is optimistic that each will likely be achieved within the subsequent 5 to 10 years. “We are making good progress, and there is a lot of interest from industry,” says the scientist. “Researchers have only been working on perovskite-based solar cells for just under 15 years. For comparison, research into silicon cells has been going on for almost 70 years.”