Influence of mixed phases on the optoelectronic properties of Cs-Pb-Br compounds

The current dissertation presents a correlative approach to investigate the influence of phase mixture on the optical properties of Cs–Pb–Br compounds. This material system exhibits three ternary phases, CsPbBr${}_3$, Cs${}_4$PbBr${}_6$ and CsPb${}_2$Br${}_5$, which coexist in Cs–Pb–Br materials when deposited by various methods. The coexistence influences the light emission and by extension can affect the performance of the optoelectronic devices based on these compounds. Motivated by the open debates regarding the green emission from Cs–Pb–Br, I investigated the structure, composition, and optical properties of mixed-phase materials by means of X-ray diffraction, energy-dispersive X-ray spectroscopy, cathodoluminescence (CL) and photoluminescence (PL) spectroscopy. Using these techniques, the spatial phase distribution of Cs–Pb–Br thin films was investigated, with a resolution of below 50 nm. It was found that for three different deposition methods, at least two of the ternary phases coexist. This result agreed very well with ab-initio calculations showing very similar formation enthalpies for the three phases. Secondly, the influence of the phase transformation on the luminescence of CsPbBr${}_3$/CsPb${}_2$Br${}_5$ thin films was investigated. In-situ experiments revealed that phase transformation is preceded by crystal structure transitions of CsPbBr${}_3$ and that it only occurs at elevated temperatures of above 583 K. PL and microscopic, correlative analyses showed that annealing increases the defect density at the CsPbBr${}_3$/CsPb${}_2$Br${}_5$ interface, which contributes to the quenching of the green luminescence. Finally, green luminescent Cs${}_4$PbBr${}_6$ and CsPb${}_2$Br${}_5$ were investigated by means of PL and CL, which revealed embedded green-emitting CsPbBr${}_3$ nanocrystals with enhanced emission yield. Spectral blue and red shifts were measured for Cs${}_4$PbBr${}_6$ and CsPb${}_2$Br${}_5$. These emission shifts were explained by an effective mass approximation model, which considers band-gap energy differences and dielectric mismatch between the embedded nanocrystals and the host material. The experimental-theoretical approach in the present thesis was found to be a valuable tool to investigate the Cs–Pb–Br material system and helped improving the understanding of their optoelectronic properties.