ZSW is currently conducting research on perovskite solar cells. They have several attractive qualities, especially low-cost manufacturing and high efficiencies. They also combine very well with other materials to create tandem solar cells.
A new type of solar cell based on organo-halide perovskites has risen to prominence in recent years as a remarkably efficient alternative for thin-film solar cells. Efficiencies have indeed soared to new heights during this time. A Korean research institute just set a certified record in 2020 with a solar cell’s efficiency topping the 25-percent mark. (Research cell efficiency records, NREL, https://www.nrel.gov/pv/cell-efficiency.html)
Perovskite solar cells come in various guises. The classic design features a mesoporous layer of metal oxides with a structure much like that of dye-sensitized solar cells. Another variant is the inverted planar solar cell. It is similar to organic solar cells, but has a perovskite layer instead of the organic absorber layer. This design looks to have a brighter future because the required processing temperatures are moderate.
Some challenges have yet to be surmounted on this new technology’s path to manufacturing maturity. The stability of the perovskite cells has to be improved and ways are being sought to replace lead with ecofriendly alternatives.
ZSW is investigating and testing various concepts for perovskite solar cell layer stack structures and their manufacturing, advancing the state of the art with industrial applications in mind.
One priority is to develop semi-transparent cells and tandem structures in combination with CIGS solar cells in order to make the most of the solar spectrum and further increase efficiency.
Very high efficiencies have been achieved for perovskite solar cells, albeit mainly on small surface areas without industrial-grade coating methods. ZSW is transitioning coating processes from spin coating to industrial-scale doctor blading and slot-die coating, and has already managed to scale up the surface area from 2.25 cm2 (1.5 cm x 1.5 cm) to 81 cm2 (9 cm x 9 cm).
Drawing on many years’ experience researching the technology and engineering of CIGS modules, ZSW possesses the laser structuring, interconnection, measurement equipment, and skills needed to manufacture modules. Its researchers have already produced working 10 x 10 cm perovskite modules and now aim to make full-fledged perovskite CIGS tandem modules alongside standalone (semi-transparent) perovskite modules.
An important technical challenge arises because the currently employed layers are susceptible to moisture and other environmental influences. ZSW researchers have set out to tackle this challenge by conducting accelerated ageing tests in climate chambers and outdoors to identify suitable encapsulation materials.
Classic perovskite solar cells contain lead and so far only this family of compound semiconductors has demonstrated the band structure, stability and quality necessary to achieve high efficiencies. However, if solar cell technology is to be sustainable it has to do without toxic elements such as lead. Keen to rise to this towering technology challenge, ZSW is currently exploring the potential for perovskite layers based on tin.
Tandem solar cells can use the solar spectrum far more efficiently than conventional single solar cells and thus achieve higher efficiencies, potentially in the range of 30 percent. They consist of at least two sub-cells with different absorption spectra related to their energetic band gaps.
Modifying perovskite solar cells’ band gap is easily done by varying their composition. This way, perovskite sub-cells can be tailored for a perfect fit when manufacturing perovskite-silicon, perovskite-CIGS, or perovskite-perovskite tandem solar cells.
ZSW’s R&D is focused on the production of perovskite-CIGS tandem cells with two approaches. One is to deposit cells directly on top of one another in a monolithic array with the standard two-terminal or 2T design featuring a positive and negative terminal each. The other is to produce each cell individually and stack them in electrically separate four-terminal or 4T units. Several projects (PERCISTAND, CAPITANO) are underway to this end.