The role of spatial size and orientation of electronic wavefunction in exciton dissociation at van der Waals interfaces

Abstract

Organic photovoltaic (OPV) devices are environmental-friendly, lightweight, flexible and inexpensive. However, one of the setbacks for commercial application is its relative low performance in solar to electrical energy conversion compared to inorganic counterparts such as Si solar cells. Unlike typical inorganic materials in which free carriers are generated directly by the light absorption, excitons, Coulombic bound electron-hole pairs, are created upon light absorption in OPV materials. The performance of OPVs depends on how effective the bound charge transfer (CT) exciton, an exciton with its electron and hole spatially separated by the donor-acceptor interface, can dissociate to generate free charge carriers. In this thesis, the roles of the orientation of the delocalized electron wavefunction and the interfacial energy landscape in the exciton dissociation (ED) process are studied in order to understand mechanisms that control the ED efficiency. A number of donor–acceptor interfaces including organic/organic and organic/transition metal dichalcogenides (TMDs) interfaces with different molecular orientations were prepared, and exciton dynamics at these interfaces were probed. We employed time-resolved photoemission spectroscopy to measure the CT exciton dynamics, by which we were able to track the temporal evolution of the energy and the size of CT excitons. Our results on the organic-organic donor-acceptor interfaces show that the relative orientation of the delocalized electron and hole wavefunction within the CT exciton plays an important role in determining whether free carriers can be generated effectively from the CT exciton. Energy uphill, spontaneous exciton dissociation (SED) was observed on the few picosecond (ps) timescale at the zinc phthalocyanine (ZnPc)/fluorinated zinc phthalocyanine (F8ZnPc) interface with a face-on molecular orientation, at which both the electron and hole wavefunctions delocalize in the direction perpendicular to the interface. By contrast, cooling of hot CT excitons to lower energy bound CT excitons (cold excitons) was observed at the ZnPc/fullerene (C60) interface with an edge-on ZnPc orientation, at which the hole wavefunction in the CT exciton delocalizes in a direction parallel to the interface. The difference in the CT exciton dynamics suggests that free charges can be generated more effectively at the ZnPc/F8ZnPc interface with a face-on orientation. In addition, two very similar organic-TMD interfaces (ZnPc/bulk-MoS2 and ZnPc/monolayer (ML) MoS2) were studied and distinctly different CT exciton dynamics were observed. At the ZnPc/bulk-MoS2 interface, after the formation of the CT exciton, back electron transfer occurs which results in the formation of triplet excitons in the ZnPc. On the other hand, at the ZnPc/ML MoS2 interface, free carriers are generated effectively from CT excitons. This difference in the CT exciton dynamics is explained by the difference in the extent of the interfacial band bending found at the two interfaces. Overall, our study demonstrates that whether free carriers can be generated from the CT exciton depends sensitively on the local energy landscape around the interface and the electron delocalization within the CT exciton at the nanoscale. Understanding how the interfacial structure would affect the temporal evolution of the CT exciton is important for designing interfaces for effective charge generation.