In addition to co-evaporation in a vacuum, ZSW is also developing CIGS absorber layers that can be applied in a non-vacuum printing process. Essential steps include researching and manufacturing suitable precursor materials. After they have been coated from a solution, the precursors have to be transformed into the desired chalcopyrite structure as part of a thermal intermediate step. Only once the interaction between the precursor and the conversion step is fully understood and mastered can this method be used to manufacture high-efficiency CIGS solar cells.
In order to manufacture printed CIGS solar cells, a suspension of nano-sized precursor materials is first of all mixed with additives to produce a printable ink. Once this ink has been applied to a substrate, such as molybdenum-coated glass, it is sintered and then converted into polycrystalline CIGS in a selenium atmosphere.
This development is aimed at producing cells not only on glass substrates but also on different flexible substrates. The use of flexible substrates is facilitated by the fact that the high substrate temperatures customary in CIGS production are not used for organic cells. Plastic films (polymers) therefore also become viable as flexible substrates.
An interesting future alternative to CIGS is provided by kesterites with the chemical formula Cu2ZnSn(S,Se)4 (CZTS). They are very similar to the established CIGS material but do not rely on the relatively rare elements indium and gallium, and should therefore be capable of producing cost-effective solar cells. Today's established CIGS production technology could easily be converted to CZTS at a comparable efficiency level. Although the current efficiency level of 12.7% (IBM 2014) is still well below the record set by CIGS, an interesting aspect with CZTS is that different manufacturing methods can be used that vary in terms of the complexity but which are equal in terms of the quality. Thus, similar efficiencies can be achieved both with elaborate vacuum evaporation and with simple solution-based printing methods with a subsequent chemical conversion in a selenium atmosphere.
Consequently, inexpensive coating from the solution is the method of choice at ZSW. The institute managed to set a European record for CZTS with a certified efficiency of 10.3% in 2014. Current research topics include basic research aspects concerned with morphology and interface formation and their impact on solar cell efficiency, with the aim of increasing efficiency and identifying industry-relevant aspects during the development process.
In recent years, a new type of solar cell based on organohalide perovskites has experienced a meteoric rise as a highly efficient new alternative in the thin-film solar cell field. In the last five years, efficiencies have risen by leaps and bounds: in 2016, the current record was set by a solar cell produced by a Korean research institute with a certified efficiency of 22.1% (Research Cell Efficiency Records http://www.nrel.gov/ncpv).
There are several concepts for perovskite solar cells. One relies on the classic structure similar to dye-sensitised solar cells on a mesoporous layer of metal oxides. Another is based on inverted planar solar cells, which are structured analogously to form organic solar cells with the exception that a perovskite layer is used instead of the organic absorber layer. This approach is even more promising, since it requires only moderate processing temperatures.
Despite the high efficiencies, some challenges still need to be overcome on route to industrial maturity: firstly, the stability of the perovskite cells needs to be improved and, secondly, possibilities are still being sought for replacing the lead with environmentally friendly alternatives.
ZSW is examining and testing the various concepts for production, composition and structure of the perovskite solar cells, and is further developing them in terms of industry-relevant aspects.
Organic polymer solar cells are based on a mixture of donor-like and acceptor-like organic semiconductors. Similar to printed CIGS, these materials can be applied using quick and simple printing processes with inks without having to rely on complex and expensive vacuum technologies. They promise a significant reduction in the production costs of thin-film solar modules. Depending on the nature of the donor molecules, a distinction is made between
ZSW is conducting research on various aspects concerned not only with the cell physics, stability, and efficiency improvements but also with upscaling the production processes (for example, using squeegees or slot casting) and developing prototypes. ZSW has been able to achieve efficiencies of up to 7% using solution-processed polymer and oligomer-like solar cells.
It is hoped that the efficiency of polymer solar cells can be increased by optimally leveraging the entire solar spectrum using so-called tandem structures. Here, two single cells with different spectral sensitivities are stacked directly over each other (“in tandem") so that, in the ideal case, the useful cell voltage is doubled for the same current. This should enable efficiencies of up to 15% to be achieved. ZSW is currently cooperating with partners to realise these organic tandem cells.