Prof. Dr. Steve Albrecht, Technische Universität Berlin

Integrating metal halide perovskite top cells with crystalline silicon or CIGS bottom cells into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential of these cell architectures. To further increase the tandem device performance to a level well above the best single junctions, optical and electrical optimizations as well as a detailed device understanding of this advanced tandem architecture need to be developed. In this talk, Prof. Albrecht will present the recent results on monolithic tandem combinations of perovskite with crystalline silicon and CIGS, as well as tandem relevant aspects of perovskite single junction solar cells that were developed at HZB’s Innovation Lab HySPRINT.

By optimization of the tandem stack optics as well as contact layers an efficiency of 26.0% is realized for perovskite/silicon tandem solar cells and it will be presented how especially the fill factor (FF) behaves under current-mismatch conditions. In strong mismatch the FF of the tandem cell is enhanced which reduces the sensitivity of efficiency to spectral mismatch. Additionally, the introduction of light trapping foils with textured surfaces is presented together with the influence on texture position on lab performance and outdoor energy yield by advanced optical simulations.

The monolithic combination of perovskite and CIGS was highly challenging up to now as the CIGS surface is rather rough. By implementing a conformal hole transport layer formed by metal oxides from atomic layer deposition, an 21.6% efficient monolithic perovskite/CIGS tandem was realized and will be presented. Cross sectional elemental mapping highlights the conformal coverage and absolute photoluminescence of the perovskite and CIGS sub-cells gives insights into the contributions to the tandem open-circuit voltage (Voc).

Recently the YIG EE-NPET has shown that self-assembled monolayers (SAM) could be implemented as robust, effective and conformal hole selective contacts. The implementation of new generation SAM molecules enabled further reduction of non-radiative recombination losses enabling excellent carrier lifetimes above 2 μs and thus Voc’s up to 1.19 V. This enabled an efficiency of 21.2% for p-i-n perovskite single junctions.

Finally, it will be shown how utilization of the SAM molecules and fine tuning of the perovskite band gap in perovskite/silicon tandem solar cells further improved the efficiency to 27.5%. For perovskite/CIGS tandems, the conformal SAM homogeneously covered the rough CIGS surface enabling a certified record efficiency of 23.26%.