Two stage solar cell for ground
A tandem solar cell enables a high energy yield of over 30%. A new Empa process makes it possible to produce the cells cheaply for the first time.
What is good for double-blade razors also applies to solar cells: two work steps are more thorough. If you place two solar cells on top of each other, one of which is semi-transparent, then a greater proportion of the light energy can be converted into electricity. Until now, this complex technology has been used primarily in space travel. So-called tandem cells were too expensive for mass production. An Empa team led by Stephan Bücheler and Ayodhya N. Tiwari from the Laboratory for Thin Films and Photovoltaics has now succeeded in producing a low-cost tandem solar cell that can be applied to flexible plastic films. An important milestone towards mass production of highly efficient solar cells has thus been reached.
The highlight of the new process: The researchers produce the additional solar cell layer in a low-temperature process at just 50 degrees Celsius. This promises to be an energy- and cost-saving production step for future manufacturing processes. Right off the bat, the tandem solar cell achieved an efficiency of 20.5 percent in converting light into electricity. This puts it on a par with the best flexible solar cells produced in the world to date. Yet its potential is far from exhausted, as the Empa researchers emphasize.
Molecular footballs as a base
The key to the double success was the development of a semi-transparent solar cell made of methyl ammonium lead iodide, which is deposited in the form of tiny perovskite crystals. A substance with the abbreviation PCBM (phenyl-C61-butyric acid methyl ester) serves as the substrate for the perovskite. Each molecule of PCBM contains 61 carbon atoms, which are linked together in the form of a football. The perovskite is vapor-deposited "lukewarm", so to speak, onto this football layer. This magic crystal absorbs UV rays and the blue portion of visible light and converts them into electricity. Red light and infrared radiation, however, are allowed to pass through the crystal. This allows the researchers to arrange another solar cell under the semi-transparent perovskite cell, which converts the remaining light into electricity.
Use sunlight spectrum
The Empa researchers are using a CIGS cell (copper indium gallium diselenide) as the bottom layer of the tandem solar cell - a technology that the team has already been researching for years. Small-scale production for flexible solar cells based on the CIGS cells is already underway.
