Hybrid organic-inorganic lead-halide perovskites have attracted significant interest for their applications in
optoelectronics, ranging from light-emitting diodes to photovoltaics and X-ray detectors. Due to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore optoelectronic properties of perovskites. Similarly, layered (2D) perovskites traditionally incorporate bulky organic molecules exerting primarily a structural role by acting as an electronically insulating spacer. To explore novel functionalities of 2D perovskites, we use optically and electronically active organic spacers, such as the carbazole-based Cz-Ci spacer where Ci indicates the alkyl chain length. In this talk, I will discuss how the incorporation of this conjugated Pi-core drastically changes the photophysics of 2D perovskites through an enhanced electronic coupling between the inorganic lead-halide and organic layers. I will present spectroscopic evidence of photoinduced interlayer charge transfer through ultrafast transient absorption (TA) spectroscopy. Furthermore, we have demonstrated how the interlayer electronic coupling can be tuned by varying the interlayer distance through Ci, resulting in an intriguing sub-gap charge transfer state in (Cz-C3)2PbI4. Finally, this enhanced coupling translates to larger distances as revealed by improved vertical charge transport relevant to photovoltaic devices.

The second part of my talk will focus on coherent phonons in 2D defective transition metal dichalcogenide (TMD) monolayers. Point defects in TMDs have the potential to induce single photon emission, enhance electro/photocatalysis and enable sensing. Absolute control over these defect-related capabilities requires a full understanding of exciton trapping and recombination mechanisms. Ultrafast TA spectroscopy is the ideal tool to study this as the visible broadband probe provides direct access to both excitonic and defect spectral contributions. Exciton-phonon coupling may be investigated through time-domain and Fourier analysis of the phonon coherences induced by the temporally short pump pulse. I will discuss how these coherences shine light on the (~100 fs) exciton trapping observed in a butyl-lithium treated metal-organic chemical vapor deposited (MOCVD) WS2 monolayer.