PhD defence by Olivér Nagy

Abstract

Metal-halide perovskites are a new class of semiconductors that can be manufactured from solution at low temperatures, which makes them promising for next-generation solar cells. While perovskite solar cells have reached high efficiencies, their performance and long-term stability are still limited by defects, material disorder, and incomplete understanding of how electrical charges move and recombine inside the films. This PhD thesis investigates the optoelectronic properties of perovskite thin films with photovoltaic applications in mind, with an emphasis on connecting thin-film processing to charge-transport behavior.

The main experimental approach is terahertz (THz) pump–probe spectroscopy, a contact-free technique that measures the ultrafast photoconductivity of a material immediately after it absorbs light. By tracking how conductivity evolves in time and frequency, the thesis quantifies how quickly charges become mobile, how strongly they are scattered, and how disorder affects their motion in polycrystalline perovskite films. The results reveal clear signatures of carrier backscattering and localization—transport limitations that can reduce the ability of charges to reach contacts in real devices. These findings are then linked to microstructure and processing history, providing physically grounded explanations for why seemingly high-quality films can still show suppressed electrical response.

In parallel, the thesis evaluates strategies for controlled crystallization and defect passivation, including approaches relevant to tin-based perovskites as a lead-free alternative. These treatments modify morphology, crystallinity, and preferred orientation, and the accompanying THz measurements show how such changes translate into improved transport characteristics. Finally, the thesis contributes practical measurement advances by developing and validating streamlined signal-processing and acquisition workflows for ultrafast experiments, enabling robust data collection with simpler instrumentation.

Overall, the thesis provides experimentally supported guidelines for designing perovskite films with improved charge transport and reduced loss pathways—key requirements for more efficient and stable perovskite solar cells. The methods and workflows developed here can also support faster screening of emerging semiconductor materials where understanding ultrafast charge dynamics is essential.

Supervisors

• Principal supervisor: Associate Professor, Binbin Zhou, Department of Electrical and Photonics Engineering, DTU
• Co-supervisor: Professor, Peter Uhd Jepsen, Department of Electrical and Photonics Engineering, DTU
• Co-supervisor: Associate Professor, Edmund Kelleher, Department of Electrical and Photonics Engineering, DTU

Evaluation Board

• Examiner: Professor József András Fülöp, ELI-ALPS Research Institute, HU
• Examiner: Professor Lyubov Titova, Worcester Polytechnic Institute, USA

Chairman

• Associate Professor, Lars Søgaard Rishøj, Department of Electrical and Photonics Engineering, DTU

Master of the Ceremony

• TBA, Department of Electrical and Photonics Engineering, DTU