Birgit Kammlander: Time-resolved Photoelectron Spectroscopy of Lead Halide Perovskites

Abstract

Lead halide perovskites are promising for applications such as solar cells or LEDs. Their success partly stems from the tunablity of their properties by varying their composition. For efficient devices, interfaces between the lead halide perovskites and transport materials play a crucial role. However, stability remains a challenge for commercializing these thin film devices. Further development requires understanding the fundamental properties of both the material itself and of interfaces with transport materials, ideally at an atomic level. This includes understanding charge dynamics and chemical changes under external impacts.

In this thesis, the method photoelectron spectroscopy (PES) was used and developed for investigating both model systems (via single crystals) and thin film devices. This includes studying dynamics at timescales from pico- to nanoseconds and seconds to minutes. Different single crystal compositions were studied and compared.

Degradation under X-rays and heat was studied. X-rays induced AX radiolysis, ion migration and metallic lead formation while heat degradation was dominated by AX radiolysis. The interface formation and energy alignment between different single crystal compositions and a potential transport material was characterized.

Electron and ion dynamics were investigated via laser-pump-X-ray-probe PES and simpler laser ON/OFF experiments. Timescales for electron and ion dynamics were determined and discussed, including surface band bending effects.

This work further includes a more applied use of this method on thin film devices. Charge dynamics at different interfaces with transport layers in a quantum dot solar cell were studied and the gold contact was found to be crucial for large and long-lived illumination-induced voltage. Degradation of blue perovskite LEDs under applied bias was followed via hard X-ray PES. Chloride migration into the top transport layer and metallic lead formation were found. Overall, this thesis gives insight into chemical changes and electron dynamics of surfaces and interfaces.