Spin dynamics in organic-inorganic lead halide perovskites

• Main Author: Lim, Melvin Jia Wei
• Other Authors: Sum Tze Chien
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Science
• Online Access: http://hdl.handle.net/10356/74805
• Institution: Nanyang Technological University

Organic-inorganic hybrid lead halide perovskites are a new class of revolutionary materials that have great potential for several applications in optoelectronics due to their properties including facile processing, tunable bandgap and long charge carrier diffusion lengths. Their excellent optoelectronic properties have led them to be incorporated into light-harvesting (photovoltaics) and light-emitting (light emitting diodes, lasers, etc.) applications with great efficiencies, amongst several others. Recently, various novel spin phenomena have been demonstrated in this material including tunable spin-selective optical Stark effects, large Rashba splitting and the ability to optically generate spin polarised carriers easily. These novel spin phenomena have been thought to result from the strong SOC present in the system. However, such strong SOC also limits the spin lifetime of organic-inorganic hybrid lead halide perovskites to a few picoseconds, which is detrimental for spintronics devices. One of the main aims of this work is hence to investigate whether we can overcome this limitation of short spin lifetimes by for example, transmitting the spin information at even shorter timescales before the spin dephases. Herein, we employed various ultrafast and steady state spectroscopy techniques to study the general spectral features as well as the spin related dynamics of the various sub-categories of lead halide perovskites. In the 3D perovskite CH3NH3PbI3, we deduced that the primary photogenerated species were free carriers due to its weak exciton binding energy. For the 2D and Ruddlesden-Popper perovskites with dimensionality n = 2 & n = 4, we found that their primary photogenerated species were excitons due to large exciton binding energy. In addition, we verified the existence of a funnelling mechanism in the Ruddlesden-Popper perovskites. Following, we studied the spin dynamics in these perovskites using spin-selective transient absorption spectroscopy. Using a simple model, we were able to determine the electron and hole spin lifetimes in the 3D perovskite and verified that they were only a few picoseconds, in a far shorter timescale compared to carrier recombination which is in the nanosecond timescale. For the Ruddlesden-Popper system, we constructed a similar model for excitons and successfully modelled the spin dynamics. We found that in the n = 2 & n = 4 systems, exciton relaxation occurs primarily in the direct mechanism, where both the spins of the hole and electron flip simultaneously. Of particular interest is that the spin polarisation of the Ruddlesden-Popper perovskites are preserved much better than in the 3D perovskite. We attributed this to the presence of an ultrafast spin funnelling mechanism in which excitons can funnel down multiple bands, bypassing momentum scattering processes greatly and preserving their spin information. Our findings demonstrated the possibility of transmitting spin information before the spin relaxes in the system, hence overcoming the short spin lifetimes in organic-inorganic hybrid lead halide perovskites and validating the viability of using this material in spintronics applications.