Trends and pathways to high-efficiency perovskite solar cells

The data show band gaps and efficiency levels of various perovskite materials. The efficiency levels for high band gaps fall due to undesired halide segregation effects.

The data show band gaps and efficiency levels of various perovskite materials. The efficiency levels for high band gaps fall due to undesired halide segregation effects. © HZB

Perovskite  solar cells have been the big surprise over the last while: inside of only a few years, their efficiency level has been increased from just under 10 % to fully 22 %. There has never been such rapid progress in a new material for solar cells. Scientists around the world are therefore working on this new class of materials. Eva Unger and Steve Albrecht from Helmholtz-Zentrum Berlin (HZB) have evaluated trends in the advancement of perovskite materials in an invited review article in Journal of Materials Chemistry A. They point out what opportunities exist for advancing this class of materials, combining them with other semiconductors, and where limitations lie. 

Scientists from all over the world are fascinated by perovkite solar cells. Not only, because of the rapid progress in their efficiency levels. In addition, perovskite materials may convert spectral regions of light into electrical energy which can be used by silicon-based solar cells only relatively inefficiently. The combination of the two materials into a tandem solar cell allows better utilisation of the sunlight and hence promises particularly high efficiency levels.

New HZB focus on perovskite

The combination of perovskite and silicon layers into tandem modules is an important new research priority at the Helmholtz-Zentrum Berlin. Two new Helmholtz Young Investigator Groups (YIGs) headed by Dr. Eva Unger and Dr. Steve Albrecht are working on this within the HySPRINT Innovation Lab.
At the invitation of the Journal of Materials Chemistry A, Unger and Albrecht have now compiled a review article for the special edition “Emerging Young Investigators” covering the advancement of this technology for perovskite materials with various absorption regions.

Advantage: variable band gaps

The authors compare a large number of data sets from experiments with perovskite materials of various chemical compositions. One of the advantages of this class of materials for employment in tandem cells is precisely that the chemical composition of perovskites can sharply influence what spectral region of sunlight is absorbed. Variations in the ratio of halogens such as bromine or iodine affect the band gaps and therefore the spectral region of the light to be absorbed. Larger band gaps that allow absorption of the green and blue regions would be needed to perfectly complement silicon cells.

Limitation: Phase segregation

“By compiling all of the pertinent data, we were able to document the improvement in efficiency level over the prior years, but also demonstrate the limitations”, says Unger. In order to achieve the desired higher band gap, bromine as well as iodine atoms must be uniformly incorporated into the crystal lattice. However, many materials exhibit an undesired effect at present when illuminated with light: Areas form in the lattice that are dominated by bromine, and other areas in which iodine is predominantly found. This phase segregation causes the efficiency level to be considerably below the expected theoretical value (see illustration).

Good news for silicon based tandem solar cells

Now the question is whether this effect can be understood and how to go about dealing with it, write the two researchers. There is already good news for silicon-based tandem solar cells: all the materials that would ideally complement silicon known so far appear to be stable over time. This means there are no show-stoppers for the development of tandem perovskite/silicon solar cells as a high-efficiency solar module. 

Published in:J. Mater. Chem. A, 2017, Advance Article
Roadmap and roadblocks for the band gap tunability of metal halide perovskites
E. L. Unger, L. Kegelmann,  K. Suchan,  D. Sörell, L. Kortec  and  S. Albrecht

DOI: 10.1039/C7TA00404D

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