Green power: Just how to enhance the effectiveness of solar cells

• Dr. Torsten Schwarz, senior scientist in the team "Nanoanalytics and also Interfaces" at the Max-Planck-Institut für Eisenforschung, evaluated just how surface area therapies of chalcogenide semiconductors influence their problem concentrations and also thus their efficiency and which modifications during the manufacturing of solar cells are needed to achieve a much better efficiency.

Along with coworkers primarily from the University of Luxembourg, he released their searchings for in the journals Nature Communications as well as Nano Energy.

"Understanding the nature of flaws, their concentration and mobility during development is crucial for the layout of lots of innovations. We already know that we can influence the homes of semiconductors made use of in photovoltaic or pv applications by refined modifications of their development conditions throughout manufacturing. At the same time, transforming the development problems indicates a differing chemical possibility, which may cause instable interfaces favouring destructive chain reaction. In the case of the chalcogenide semiconductor CuInSe2 (CISe), which we make use of, such detrimental reactions can occur at the surface.", describes Schwarz. The development of CIS thin-films normally passes the phase boundary from Cu-poor to Cu-rich and also back to Cu-poor in order to integrate the remarkable microstructure of Cu-rich films as well as the premium electrical buildings of Cu-poor films to accomplish the highest solar cell performance. Nevertheless, although Cu-rich films show a far better finger print of their optical residential properties, it was till now unknown why these films reveal a worse efficiency than their Cu-poor equivalents. One more unsolved inquiry worries the effect of problem concentrations on the device's efficiency. These issues are caused by surface area therapies throughout the solar cell's production, especially while the cell goes through ambient air, chemical etchants as well as post-deposition treatments, before being shielded by a barrier layer. "Our work discloses that the direct exposure of the CISe surface area to ambient air triggers also at room temperature level the spontaneous formation of In2O3 and also Cu2Se second phases. This response takes place a lot faster in the Cu-rich examples, since it is most likely catalyzed by the really mobile and excess Cu atoms.", mentions Schwarz. A crucial to obtain this result was making use of atom probe tomography to show that domains of just a couple of nanometers in dimension with an unwanted of Cu in fact exist. Cyanide etching is needed to eliminate the destructive Cu2Se, which would or else bring about a failing of the final gadget. Nevertheless, this required etching step does not only eliminate Cu2Se externally however additionally Cu and also Se atoms up to several 10s of nanometer below the surface area. The Cu as well as Se deficiency causes the formation of Cu-Se divacancies problems. These defects trigger a significant loss in open-circuit voltage. They likewise exist in Cu-poor CISe films, but with a much smaller sized focus, and can be removed completely by transferring of a barrier layer. "Specific chemical therapies with team VI aspects, such as O, S, as well as Se can inhabit the anion jobs as well as therefore passivate the divacancies. If we pay an unique attention to those post-treatments, we can increase the efficiency of solar cells.", states Schwarz.

In another paper Schwarz and also his associates from the MPIE and also the University of Luxembourg investigated CuInS2 (CIS) thin-films, which are made use of in tandem solar cells.

The inquiry below was, which various other adjustments can be made to boost the performance if the problems caused by the post-treatment can be overcome. For that reason, the team of researchers has actually investigated the microstructure of these films, especially the composition of issues such as grain borders. "We made use of correlated atom probe tomography as well as electron backscatter diffraction to examine the connection in between the chemistry and also the framework of these flaws. In our work, we show that the grain boundaries in Cu-rich and also Cu-poor films have very in contrast make-ups. While supposed random grain borders in the Cu-poor film show Cu exhaustion and also In enrichment, these grain limits in the Cu-rich film reveal Cu enrichment and In depletion. We were likewise able to confirm this phenomenon for an unique sort of grain borders, the so-called ∑ 9 double boundaries. However much more interesting is that, although both films had been grown on a glass substrate and also, therefore, the same pollutants were presumed randomly grain borders, we discovered only in the Cu-poor example Na and also K contaminations. On the other hand, in the Cu-rich films we located all of a sudden C at random grain boundaries along with Na. Moreover, ∑ 3 double boundaries, revealed no chemical changes whatsoever in both films.", explains Schwarz.

The scientists are now heading to recognize the beginning and the results of post-treatment as well as the passivation of grain boundaries in Cu-rich CIS films on the total performance of solar cells.