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Perovskites – Basic Research for High Efficiency Solar Cells

HyPerCells – A joint graduate school of Helmholtz-Zentrum Berlin and University of Potsdam under the direction of Prof. Dr. Dieter Neher (Universität Potsdam) and Dr. Thomas Unold (Helmholtz-Zentrum Berlin).

Thin film solar cells have great potential for employment in large scale sustainable solar energy conversion, out of several reasons. On the one hand, low energy payback times and low cost may be achieved by minimal material and energy usage in thin film solar cell production processes. On the other hand, this technology may allow the design of tandem solar cells by combining semiconductors with complementary absorption, thus significantly raising the conversion efficiency by using a much wider spectral range of the incoming sun light. Tandem cells, or generally multijunction solar cells, can thus be considered as promising photovoltaic device concepts for the future and have already been subject to substantial research efforts in the Berlin-Brandenburg region.

The design of efficient tandem cells is challenging, as it requires the combination of materials with specifically matched optical and optoelectronic properties. In recent years, important achievements with respect to amorphous/microcrystalline silicon-based tandem solar cells and modules were made at the Helmholtz Zentrum Berlin. On the other hand, research work at the UP lead to a comprehensive understanding of the function of organic single and multilayer cells, enabling the substantial improvement of the performance of these cells. This laid the basis for a number of collaborative projects between the UP and the HZB on hybrid tandem solar cells, combining inorganic and organic semiconductors.

During the last two years, exceptional results have been reported for a new type of hybrid solar cells based on so-called organometallic perovskites. Within only a very short research time span, record efficiencies larger than 15% were achieved by several groups in Europe and Asia, with a current record efficiency of 17.9%. These values are considerably larger than the efficiencies of solar cells based on organic materials or amorphous and microcrystalline silicon, which are currently employed in thin film tandem cells.

Despite the notable recent achievements reported for perovskite-based solar cells, many basic material properties including charge transport and recombination processes are not yet understood on a fundamental level for this new material system.  As the best solar performance for these hybrid solar cells have been reported for CH3NH3-PbI3 based perovskites with a band gap of 1.5 eV, it would be highly desirable to be able to deliberately adjust the optical properties such as the band gap, in order to enable perovskite semiconductor-based tandem solar cells with an optimal match of the absorption range to the sun spectrum.  Also, as lead is a toxic element subject to serious restrictions in production processes as well as in products, a partial or complete replacement of this metal by a non-toxic alternative would be advantageous.