Long-lived hot carriers, coherent spin transport, and the role of surfaces in lead halide perovskites

• Applicant: Professor Dr. Thomas Fauster, since 5/2020
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 395604916

The development of high-efficiency lead-halide perovskite solar cells in recent years poses a major breakthrough in the field of second generation photovoltaics and a step towards a third generation. First generation solar cells are made from crystalline silicon. They are available as commercial products with lifetimes lasting decades and efficiencies around 26%. Their fabrication, however, requires a significant investment of energy and capital, and their maximum efficiency is limited to 32%. Second generation photovoltaics are produced as thin films at a potentially reduced cost using smaller amounts of material with less strict requirements to crystallinity. Mechanical flexibility opens¬¬ up additional fields of application. Thin-film solar cells made from lead-halide perovskite use abundant materials and are processed at low temperature to reduce their financial and energetic cost. They have reached efficiencies around 22% at the laboratory scale. Research for commercial production is already underway. Moreover, initial research demonstrated the potential of lead halide perovskites for third generation photovoltaics. In this future generation of photovoltaics, novel concepts will be used to achieve efficiencies beyond the fundamental limit of 32% which applies to the first two generations.Large-scale applications of lead halide perovskite solar cells are limited by the contained toxic lead and problems with long-term stability. This project hence follows a complementary approach: The unique properties of lead halide perovskites on the atomic scale will be investigated to derive general design principles for high-efficiency solar cell materials. Despite intensifying basic research since 2013, it remains unclear to date what physical mechanism make lead halide perovskite solar cells so exceptional. Together with collaboration partners I found several relevant effects that can contribute to a high solar-cell efficiency: Due to the Rashba effect, electrons of opposite spin propagate differently through lead halide perovskites. Spin is a property of electrons that is not yet made use of in todays conventional devices. In addition, lead halide perovskites preserve the high energy of the blue spectral components in sunlight a thousand times longer than more traditional materials like silicon. This may also be a result of the Rashba effect. Alternatively, the formation of large polarons was proposed. Polarons are distortions of the crystal structure which might stabilize the electronic energy.Within this project, these effects will be quantified using state-of-the-art time-resolved spectroscopies. Their influence on device performance will be investigated by transport measurements performed in parallel. On the one hand, the results may open up new fields of application for lead halide perovskites. On the other, they will help to identify new materials for high-efficiency solar cells based on similar working principles.

• DFG Programme: Research Grants
• Major Instrumentation: Upgrade of a laser system
• Instrumentation Group: 5700 Festkörper-Laser
• Cooperation Partners: Professor Dr. Michael Bauer; Professor Dr. Christoph J. Brabec; Professor Dr. Martin Eckstein; Professor Dr. Martin Hundhausen; Professorin Dr. Janina Maultzsch; Professor Dr. Heiko B. Weber
• Ehemaliger Antragsteller: Dr. Daniel Niesner, until 4/2020