Collective Effects of Band Offset and Wave Function Dimensionality on Impeding Electron Transfer from 2D to Organic Crystals

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.

Type‐I Energy Level Alignment at the PTCDA—Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement

“…[15] Also recently, an experimental study of the PTCDA/ML-MoS 2 interface claimed a huge energy offset of 1.18 eV between the HOMO level of PTCDA and the VBM of ML-MoS 2 , implying a pronounced type-II level situation, and accordingly efficient interfacial electron transfer subsequent to optical excitation was concluded on. [16] In this latter work, however, the peak maximum of the HOMO photoemission feature was used to construct the energy level diagram, instead of the low binding energy HOMO manifold onset, which is the relevant energy for charge carrier transfer. [17,18] The above examples show that the electronic structure and energy level alignment between organic molecules and ML-TMDCs are of utmost importance, as they govern groundand excited-state CT interactions, which lie at the heart of the functionality of a device.…”

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

“…Organic bandwidth also varies with thickness. Thicker molecular layers create a greater density of states for donating or accepting electrons, which can significantly influence charge transfer. , It should be emphasized that these effects are not captured in simple energy level alignment diagrams because these models are typically based on isolated molecules and thus do not include the influence of molecular adlayer thickness.…”

Extracting Electrons from Delocalized Excitons by Flattening the Energetic Pathway for Charge Separation

“…Here, we use our recently developed time-resolved graphene field effect transistor (TR-GFET) technique to probe the dynamics and yield of free carrier generation in prototype trilayer structures consisting of vacuum-deposited small molecule films. Previously, our time-resolved photoemission studies found that hot CT excitons in these films have an electron delocalization size of ∼3–5 nm. ,− Here, the TR-GFET technique , selectively probes free charges generated by the CS process. By probing the concentration of separated charges as a function of time, we can independently measure the initial CS yield and the recombination rate of separated carriers, and we can distinguish effects from each of those processes on the overall photocurrent.…”

“…Hence, the trilayer structure is effective only when the two interfaces are close to each other. From our previous time-resolved photoemission experiments, ,, we know that the electron delocalization size of CT excitons in these organic films is in the range of 3–5 nm. The optimal thickness of the middle (F 8 ZnPc) layer coincides with the electron delocalization size of CT excitons.…”