Effect of the Interfacial Energy Landscape on Photoinduced Charge Generation at the ZnPc/MoS2 Interface

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/jacs.9b05893

Growing Ultra-flat Organic Films on Graphene with a Face-on Stacking via Moderate Molecule-Substrate Interaction

The electronic properties of small molecule organic crystals depend heavily on the molecular orientation. For multi-layer organic photovoltaics, it is desirable for the molecules to have a face-on orientation in order to enhance the out-of-plane transport properties. However, it is challenging to grow well-ordered and smooth films with a face-on stacking on conventional substrates such as metals and oxides. In this work, metal-phthalocyanine molecules is used as a model system to demonstrate that two-dimensional crystals such as graphene can serve as a template for growing high quality, ultra-flat organic films with a face-on orientation. Furthermore, the molecule-substrate interaction is varied systematically from strong to weak interaction regime with the interaction strength characterized by ultrafast electron transfer measurements. We find that in order to achieve the optimum orientation and morphology, the molecule-substrate interaction needs to be strong enough to ensure a face-on stacking while it needs to be weak enough to avoid film roughening

A Multidimensional View of Charge Transfer Excitons at Organic Donor–Acceptor Interfaces

How tightly bound charge transfer (CT) excitons dissociate at organic donor–acceptor interfaces has been a long-standing question in the organic photovoltaics community. Recently, it has been proposed that exciton delocalization reduces the exciton binding energy and promotes exciton dissociation. In order to understand this mechanism, it is critical to resolve the evolution of the exciton’s binding energy and coherent size with femtosecond time resolution. However, because the coherent size is just a few nanometers, it presents a major experimental challenge to capture the CT process simultaneously in the energy, spatial, and temporal domains. In this work, the challenge is overcome by using time-resolved photoemission spectroscopy. The spatial size and electronic energy of a manifold of CT states are resolved at the zinc phthalocyanine (ZnPc)–fullerene (C₆₀) donor–acceptor interface. It is found that CT at the interface first populates delocalized CT excitons with a coherent size of 4 nm. Then, this delocalized CT exciton relaxes in energy to produce CT states with delocalization sizes in the range of 1–3 nm. While the CT process from ZnPc to C₆₀ occurs in about 150 fs after photoexcitation, the localization and energy relaxation occur in 2 ps. The multidimensional view on how CT excitons evolve in time, space, and energy provides key information to understand the exciton dissociation mechanism and to design nanostructures for effective charge separation

Observation of an Ultrafast Exciton Hopping Channel in Organic Semiconducting Crystals

One of the major challenges in using organic semiconductors for photovoltaics is their extremely short exciton diffusion length. Recently, a number of studies have shown that the exciton transport range within the first few picoseconds after photoexcitation can be comparable to the exciton’s diffusion length over its entire lifetime. The origin of this fast transport channel is often attributed to the large spatial coherent size of the exciton right after photoexcitation. Here we observe an ultrafast exciton hopping channel in titanyl phthalocyanine crystals even though the exciton coherent size is a few times smaller than the transport range. This channel operates only within the first few picoseconds after photoexcitation and has a hopping rate that is an order of magnitude faster than the typical Förster resonance energy transfer rate. Resonant Raman spectroscopy shows that the optically excited exciton is strongly mixed with the macrocycle vibrational mode of the phthalocyanine molecules. A hypothesis involving vibronic coherence is proposed to explain the observed transport behavior

Graphene Field-Effect Transistor as a High-Throughput Platform to Probe Charge Separation at Donor–Acceptor Interfaces

In organic and low-dimensional materials, electrons and holes are bound together to form excitons. Effective exciton dissociation at interfaces is essential for applications such as photovoltaics and photosensing. Here, we present an interface-sensitive, time-resolved method that utilizes graphene field effect transistor as an electric-field sensor to measure the charge separation dynamics and yield at donor–acceptor interfaces. Compared to other interface-sensitive spectroscopy techniques, our method has a much reduced measurement time and can be easily adapted to different material interfaces. Hence, it can be used as a high throughput screening tool to evaluate the charge separation efficiency in a large number of systems. By using zinc phthalocyanine/fullerene interface, we demonstrate how this method can be used to quantify the charge separation dynamics and yield at a typical organic donor–acceptor interface

Charge Transfer Exciton and Spin Flipping at Organic–Transition-Metal Dichalcogenide Interfaces

Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)–molybdenum disulfide (MoS₂) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS₂ in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1–100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet–triplet transition is feasible because of the large singlet–triplet splitting in organic materials and the strong spin–orbit coupling in TMD crystals. The back electron transfer would reduce the yield of free carrier generation at the heterojunction if it is not avoided. On the other hand, the spin-selective back electron transfer could be used to manipulate electron spin in hybrid electronic devices