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.

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Dr. Erik Ahlswede
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Perovskite solar cells

// Perovskite 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)

// The structure of perovskite solar cells

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.

Example stack structures for standard and inverted perovskite solar cells.
This is how a thin-film layer is fabricated in the lab. Also known as knife coating or blade coating, doctor blading involves spreading the applied solution over the sample substrate with a blade. Besides liquid coatings, researchers are working on another method of depositing perovskites on surfaces areas up to 100 cm² (10 cm x 10 cm) via thermal evaporation.

// Scaling up perovskite solar cells' surface area

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).

// Perovskite solar modules

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.

A perovskite module with 16 monolithically interconnected cells.
Perovskite cells and modules are sealed with suitable encapsulation materials. This is done by guiding contact bands outside through the edge sealing.

// Stability of perovskite solar cells

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.

// Lead-free perovskite solar cells

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.

Perovskites of ABX3 structure contain lead (Pb) as a central element. It may be replaced by neighbouring elements of the periodic system. Currently in focus: tin (Sn).
Tandem solar cells

// Tandem solar cells

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.

Tandem structures: A 2-terminal monolith structure and a 4-terminal structure with electrically separate sub-cells.