In addition to co-evaporation in a vacuum, ZSW is also developing 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 solar cells.
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.