Carrier dynamics in emergent light-emitting nanomaterials

Abstract

The discovery of quantum confinement effects in nanoscale semiconductor materials has thrown them into the spotlight for optoelectronic applications, particularly in photovoltaics and light-emitting diodes. A comprehensive understanding of the ultrafast carrier transition and recombination dynamics is imperative for developing efficient devices that align with the color coordinates specified by the newly established International Telecommunication Union (ITU) Recommendation BT.2020 (Rec. 2020) standard. Nevertheless, fundamental mechanisms governing carrier transition and recombination processes remain obscured. In this thesis, we investigate the carrier dynamics in the emergent layered perovskites and InP nanocrystal tetrapods light-emitting nanomaterial systems using ultrafast laser spectroscopy. In Chapter 3, we explore the role of organic spacer cations in non-radiative Auger re-combination (AR) processes within layered perovskites, employing fluence- and temperature-dependent spectroscopy techniques. By utilizing phenyl-based spacer cations with varying alkyl chain lengths, we found an enhanced AR threshold with increasing chain length. Further investigation reveals exciton-phonon coupling as a determining factor governing the dynamics of AR, while trap/film morphology shows a secondary influence on the AR rate. Extending our findings in Chapter 4, we explore the practical implications of layered perovskites. We investigate the impact of spacer cations and the n-domain distribution of layered perovskites on the efficiency drop in light-emitting diodes. Fluence-dependent time-resolved photoluminescence unveils that thin films containing higher n-domains exhibit reduced AR rates due to smaller exciton binding energy. The direct correlation between AR rate and efficiency roll-off highlights the pivotal role of AR in efficiency drop, while contributions of other factors are negligible. In Chapter 5, we probe the carrier transition and recombination dynamics of InP nano-crystal (NC) tetrapods, another budding III-V semiconductor material system for light-emitting devices. Electronic structures of InP NCs are characterized by transient absorption (TA) spectra with varying excitation energies. TA kinetics reveal slow hole transition in InP NCs with longer arm lengths. Ultrafast intraband carrier transition processes are further elucidated via two-dimensional electronic spectroscopy (2DES). A detailed analysis using 2DES uncovers that hot carrier cooling is slower in InP NCs with longer arm lengths, while hot carrier thermalization rates are comparable regardless of the arm lengths. This work elucidates the factors controlling carrier recombination dynamics arising from many-body interactions, ultrafast carrier transition dynamics in a coupled excitonic system, and their influences on optoelectronic devices. The investigations conducted herein provide a profound comprehension of ultrafast carrier dynamics within semiconductor light-emitting materials, thereby paving the way for material property tuning to achieve highly efficient optoelectronic devices.