Abstract
Low-dimensional metal halides have outstanding optoelectronic properties that make them promising candidates for light-emitting and photovoltaic applications. Recently, lead-free copper halides are drawing growing attentions due to their high photoluminescence quantum yields (PLQYs), generally from self-trapped exciton (STE) emissions. However, the factors influencing formation and loss of emissive species in these materials are still not fully understood. Additionally, quasi-2D structures have been engineered in tin halide perovskite solar cells to improve stability and achieve high efficiency. Nevertheless, the excited state characteristics and their relationship to the structural features remain unclear.
The thesis presents a comprehensive study on the dynamics and characteristics of STEs in the low dimensional copper halides, including one-dimensional (1D) CsCu2I3 and zero-dimensional (0D) Cs3Cu2X5 (X=Br, I). The results show that the slower formation of self-trapped excitons in the 1D structure is influenced by the large number of phonons released during exciton self-trapping, and this process is barrierless. Furthermore, for the 1D structure, non-radiative recombination of STEs can occur through the band intersection of STE state and ground state, causing an intrinsic loss of PLQY. Furthermore, 0D Cs3Cu2Br5 shows dual emissions from intrinsic STEs and extrinsic defect-trapped excitons at low temperature, with no indication of transition observed between them.
In addition, the study of the quasi-2D tin perovskite film using electroabsorption (EA) spectroscopy reveals that the photoinduced excitons are more ordered and delocalized in the quasi-2D film compared to those in its 3D counterpart. The results indicate that the studied quasi-2D tin perovskite has significantly improved crystal order and reduced defects. The delocalized excitons and improved structural order contribute to an over 5-fold increase in carrier lifetime and significantly improved efficiency of the solar cell.
The presented findings shed light on the excited state characteristics of low-dimensional metal halides and their impacts on optoelectronic properties.