Interface engineering and multi-modal characterization of co-evaporated metal halide perovskite films for efficient perovskite-silicon tandem solar cells

The increasing global energy demand necessitates the rapid development of high-efficiency and scalable renewable energy technologies. Perovskite–silicon tandem solar cells (PSTs) have recently reached certified efficiencies of 34.9%, surpassing the single-junction efficiency limit and underscoring their industrial potential. To enable large-scale deployment, up-scalable deposition methods for metal halide perovskites (MHPs) are essential. Among these, co-evaporation offers conformal coverage of textured silicon, solvent-free processing, and precise thickness control. Despite these advantages, the performance of PSTs with co-evaporated MHP films currently lags behind state-of-the-art solution-processed devices. This thesis addresses the challenges of co-evaporated MHP growth by developing strategies for controlled nucleation and buried interface engineering. We first systematically investigate the impact of PbI2, PbCl2, CsI and CsCl seed layers on the morphology, composition, and optoelectronic properties of co-evaporated FA0.8Cs0.2PbI2.7Br0.3 films. While all seed layers reduce band gap variability compared to reference samples, CsCl and CsI promote organic precursor incorporation, yielding large apparent grains and enhanced photoluminescence. In contrast, PbI2 and PbCl2 lead to poorer film morphology and degraded optoelectronic quality. CsCl emerged as the optimal seed layer, producing films with the targeted ~1.68 eV band gap for PSTs. Single-junction devices with CsCl reach 19.6% efficiency with improved stability, establishing CsCl as an effective tool for reproducible film quality. We then investigate the influence of inhomogeneities in MeO-2PACz self-assembled hole transport layers. Using infrared scattering-type scanning near-field optical microscopy (IR s-SNOM) and, for the first time, demonstrating nanoscale X-ray photoemission electron microscopy (XPEEM) on ultra-thin (5–20 nm) MHP films and buried interfaces, we reveal that insufficient MeO-2PACz coverage hinders organic precursor incorporation and promotes interfacial PbI2 formation. These effects were particularly pronounced on textured silicon, where MeO-2PACz accumulates in pyramid valleys. Incorporating a CsCl seed layer mitigates these inhomogeneities, resulting in more uniform buried interfaces and suppression of interfacial PbI2. Applied to PSTs, CsCl improved fill factor and open-circuit voltage, yielding a certified tandem efficiency of 29.7%, the highest reported for devices with fully vacuum-processed MHP absorbers. Finally, we probe nano-scale inhomogeneities in 20 nm thick co-evaporated MHP films grown on reference, CsCl seeded and substrate washed with ethanol, revealing inhomogeneous film formation and elevated cesium- and carbon-rich domains deficient in iodine and lead. Residual CsCl forms islands that facilitate early perovskite phase formation, as confirmed by cathodoluminescence measurements. These findings highlight the complex growth dynamics of mixed-cation, mixed-halide co-evaporated MHPs and the crucial role of seed layer engineering in controlling nucleation pathways. Together, these results demonstrate that seed layer optimization and interface control are central to advancing co-evaporated MHPs toward scalable tandem applications. By linking nanoscale growth phenomena to macroscopic device performance, this work provides mechanistic insights and practical strategies for evidence-based optimization of buried interfaces in co-evaporation, ultimately advancing the pathway to industrial deployment of PSTs.

Author dc:creator

• Škorjanc, Viktor

Advisors dc:contributor.advisor

• Albrecht, Steve
• Rech, Bernd

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-25728

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/26895