Young Investigator Group Perowskite Tandem Solar Cells

The Young Investigator Group Perowskite Tandem Solar Cells is conducting on the organic-inorganic perovskite materials, which are one of the biggest surprises in solar cell research. In just six years, the efficiency of perovskite solar cells has increased five-fold; moreover, perovskite solar cells can be manufactured from solution and be cost-effectively printed on large areas in the future.

Perovskite with silicon: good team but difficult to combine

Hybrid organic-inorganic perovskites can absorb light in the visible region of the sun spectrum and effectively convert it to electricity when being utilized in solar cells. It is therefore useful to combine perovskite top-cells with silicon bottom-cells which primarily harvest near-infrared light. Thus, tandem solar cells enable to efficiently convert a broad range of the sun spectrum into electricity. Nevertheless, the fabrication of these monolithic tandem cells is very complex. To exploit the high efficiency potential that is predicted for this fascinating tandem architecture, the two bottom cells have to generate equal amounts of electric currents. Thus, fine-tuning of the perovskite bandgap is necessary here which, however, may result in photo-instabilities of the perovskite composition. Besides, the thicknesses of the various layers in a tandem solar cell need to be optimized in order to achieve lowest reflection losses. Consequently, 3D optical simulations are performed to understand and improve the light management in the solar cell device, also by including light trapping from textured surfaces. To combine both sub-cells into a monolithic stack, an effective interconnection layer is required that provides low electrical losses at highest transmission. Additionally, highest performance is only reached when both sub-cells show a high open circuit voltage that, especially for the perovskite top-cell, is determined by charge recombination losses at the interfaces to the contact materials. Accordingly, understanding and suppressing non-radiative recombination helps to further improve the efficiency of this important tandem technology to above 30%.