Excited state dynamics in low-dimensional perovskite nanocrystals

Abstract: Two-dimensional (2D) perovskites have emerged as promising building blocks for optoelectronic applications. To fabricate high-performance devices, the relation between the material structures and their function in terms of excited state and charge carrier dynamics need to be well understood. In this thesis, we investigate photophysics of 2D lead halide perovskite with different compositions to reveal the relation between lattice distortion and electronic properties. Firstly, we found that the threshold of the Goldschmidt tolerance factor is relaxed and thereby the range of possible composition for forming stable 2D perovskite is extended compared to 3D perovskite. In addition, lattice distortion is greater when containing large cations inside octahedral cages and the formed 2D perovskite has a larger band gap and higher trap state density. The link between lattice distortion and modification of electronic structure shows a potential approach to designing and developing high-performance PV materials. To further evaluate the influence of local lattice distortion on the electronic properties of 2D perovskite, we analyze fluorescence signals from different facets of samples with different spacers. We found that free carriers dominate in the in-plane facet (IF) while self-trapped excitons (STE) are the main emitters from the facet perpendicular to the 2D layer (PF). The strain accumulated along the 2D layers leads to enhanced carrier-phonon coupling and facilitates STE formation in PF, while in IF the separated flexible spacers contribute to releasing the strain accumulation. To directly characterize the electronic structure at different areas of a 2D perovskite single crystal, electrons emitted from Pb 5d and I 4d core levels are mapped at the edge and the bulk areas by using X-ray photoemission electron microscopy. The observed asymmetric shifts of the emission spectra of 2D perovskite indicate different degrees of lattice distortion at the edge and the bulk areas since the internal strain accumulation is released at the edge area. The different shift in Pb 5d core level emission between edge and bulk areas at 2D perovskites with different layer thickness confirms the contribution of spacers in releasing accumulated strain. In addition, we investigated the ultrafast hot carrier (HC) relaxation dynamics in 2D perovskite single crystals by employing transient absorption (TA) spectroscopy and time-resolved two-photon photoemission (TR-2PPE) spectroscopy. With TR-2PPE, the distribution of hot electrons and their dynamics in the conduction band can be directly visualized. The different cooling rates of HC observed in the two techniques reflect the spatial sensitivity of relaxation dynamics across the 2D perovskite single crystal. We believe the comprehensive study on HC relaxation in 2D perovskite provides an effective approach to compare the potential of different materials in hot carrier solar cell (HCSC) applications and can extend thermoelectric applications based on 2D perovskites. We also investigated the influence of transition metal doping on electronic and phononic features of three-dimensional perovskite by studying the HC relaxation processes in Mn2+-doped and undoped CsPbI3 nanocrystals (NCs). The Mn2+ doping leads to the enlarged phononic gap between longitudinal optical (LO) – acoustic phonons, enhanced carrier-LO phonon coupling strength, and additional Mn orbitals within the original bands of the undoped sample, which are beneficial for establishing a hot quasi-equilibrium to recycle the energy from HC relaxation to reheat cold carriers. The results present a methodology to optimize HC dynamics by element doping and are meaningful for guiding the future development of HCSC applications.