Collective Effects of Band Offset and Wave Function Dimensionality on Impeding Electron Transfer from 2D to Organic Crystals Kushal Rijal, Fatimah Rudayni, Tika R. Kafle,* and Wai-Lun Chan* Cite This: J. Phys. Chem. Lett. 2020, 11, 7495-7501 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Excited-state electron transfer (ET) across molecules/transition metal dichalcogenide crystal (TMDC) interfaces is a critical process for the functioning of various organic/TMDC hybrid optoelectronic devices. Therefore, it is important to understand the fundamental factors that can facilitate or limit the ET rate. Here it is found that an undesirable combination of the interfacial band offset and the spatial dimensionality of the delocalized electron wave function can significantly slow down the ET process. Specifically, it is found that whereas the ET rate from TMDCs (MoS 2 and WSe 2 ) to fullerenes is relative insensitive to the band offset, the ET rate from TMDCs to perylene molecules can be reduced by an order of magnitude when the band offset is large. For the perylene crystal, the sensitivity of the ET rate on the band offset is explained by the 1D nature of the electronic wave function, which limits the availability of states with the appropriate energy to accept the electron. L ayered 2D crystals such as graphene and transition metal dichalcogenide crystals (TMDC) have electronic proper- ties that are extremely sensitive to E-field and doping, which makes them useful for sensor and transistor applications. 1-3 When sensitized by organic molecules, the light absorption properties can be further tuned and enhanced. 4-7 Whereas many studies on hybrid organic/2D interfaces focus on understanding the doping in 2D crystals induced by organic molecules and the molecule-substrate interaction, 5,8-10 the mechanism of the excited-state electron transfer (ET) between the two materials is less studied. The excited-state ET process is especially important for applications such as photosensors and photovoltaics because their operation relies on the fast and effective transfer of photocarriers between the two materials. Although it is demonstrated that effective ET can occur in a number of organic/2D interfaces, 11-19 different factors that would affect the ET rate have not been studied systematically. At these hybrid interfaces, the ET between the two materials can be asymmetric because 2D materials possess band-like electronic structure, whereas organic molecules have more localized electronic states. Typically, ET can effectively occur if there are plenty of electron-accepting states with energies close to that of the electron-donating state. 20 As illustrated in Figure 1a, this condition can be readily fulfilled for the ET from a molecule to a 2D crystal as long as the molecular state is above the conduction band edge because a continuum of states is available in the 2D crystal to accept the transferred electron. Indeed, we have previously observed that the ET from zinc phthalocyanine (ZnPc) to both monolayer (ML) MoS 2 and bulk MoS 2 can occur in ∼50 fs despite the fact that the energy offset between the conduction band minimum (CBM) of MoS 2 and the S 1 state of ZnPc is different by as much as ∼0.5 eV for the two interfaces. 12,21 On the contrary, the ET from the 2D crystal to the molecule can be more restrictive (Figure 1b) because of the discrete nature of the molecular state, which limits the availability of electron-accepting states with the optimal energy for fast ET. As a result, for the ET from molecules to 2D crystals, the interfacial band offset (E off ) needs to be properly tuned to facilitate the ET. For example, recent simulation works have shown that ET from molecules to 2D crystals can be hindered by a lack of electron-accepting states with the appropriate energy. 22 A molecular state can be hybridized into a band in Received: June 9, 2020 Accepted: August 19, 2020 Published: August 19, 2020 Figure 1. Schematic diagrams illustrating the ET at the organic/2D hybrid interface. (a) ET from organic molecules to 2D crystals is facilitated by a continuum of electron-accepting states in the conduction band (CB) of the 2D crystal. (b) ET from 2D crystals to organic molecules can be hindered by a combination of a large interfacial offset (E off ) and a small bandwidth (W) of the organic crystal. Letter pubs.acs.org/JPCL © 2020 American Chemical Society 7495 https://dx.doi.org/10.1021/acs.jpclett.0c01796 J. Phys. Chem. Lett. 2020, 11, 7495-7501 Downloaded via UNIV OF NEW ENGLAND on October 28, 2020 at 06:13:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.