Structured interfaces in perovskite-silicon tandem solar cells

This thesis explores the effects of interface structuring in perovskite-silicon tandem solar cells on their optical behavior and overall performance, based on both simulation models and experimental validation. It focuses on three primary aspects: Optimal structure geometry and configuration is explored through simulation. A nanostructuring process and its interaction with the perovskite-silicon tandem solar cell processing route is developed and tested. The impact of structuring on standard optical measurement methods is studied using photolumi-nescence imaging as an application example.
For the first aspect, rigorous coupled wave analysis simulations, verified against experimental data, are utilized to probe a large variation of nanostructure geometries at the different tandem solar cell interfaces. The largest performance improvement is expected with the utilization of nanostructures at both the air-perovskite and perovskite-silicon interfaces, with a short-circuit current increase of 2.0 mA/cm2 predicted. The interface between air and perov-skite is identified as the largest contributor to this effect due to the large refractive index con-trast and spectral bandwidth arriving at this interface. Based on a near-field analysis, a limit for the structure height at the perovskite-silicon interface is derived, above which it reduces the performance of the upper interface nanostructure. A broad range of nanostructures, varying in geometry and size, are explored and compared to larger structure sizes. From this sampling, structure height and (to a lesser extent) period are identified as key attributes, while remaining structure geometry parameters appear to be only secondary contributors.
As a second aspect, the practical challenges of implementing a nanostructure into the silicon bottom solar cell are investigated. The method of nanoimprint lithography is adapted for this application, with developments regarding cleanroom compatibility, stamp material and suitability for non-polished wafers made. With this method, a nanostructure at the perovskite-silicon interface is introduced. Although optical performance gains are consistent with the simulation model, challenges regarding electrical performance retention remain. The need for further refinement of the nanostructuring process is highlighted and pathways for this are identified.
Lastly, the effects of perovskite-silicon tandem solar cell structure on photoluminescence (PL) measurements are investigated. Both simulation and experimental data show that variations in perovskite thickness and silicon geometry can lead to shifts in PL peak positions of up to 20 meV, primarily due to reabsorption of photons. This has significant implications for interpreting PL data in structured tandem solar cells, as such shifts could be misinterpreted as changes in material properties rather than a result of structural variations. The work contributes a deeper understanding of how structural modifications at the interface can affect PL measurements, as well as to how the effects observed could impact other measurement methodologies.
In conclusion, this dissertation for the first time presents a quantitative, experimentally vali-dated comparison of multiple structure geometries and concepts, allowing for the extraction of general design rules. Additionally, key challenges in fabrication and performance measurement are clearly identified. Comparisons with fabrication and measurement methods beyond the specific technologies discussed in this work have been drawn and connection points for future research are provided.