Influence of the Energy Level Alignment on Charge Transfer and Recombination at the Monolayer-MoS₂/Organic Hybrid Interface

Abstract

Monolayer (ML) transition-metal dichalcogenides (TMDCs) exhibit numerous unique optoelectronic features. This motivates recent efforts to combine TMDCs with organic semiconductors to form heterostructures with tailorable properties that feature the advantages of both materials. Here, we study the photoinduced charge transfer across hybrid interfaces of ML-MoS 2 and a series of organic semiconductors�often used as hole transport materials�where we systematically tune the offsets of the frontier energy levels. Steady-state photoluminescence and ultrafast transient absorption spectroscopy reveal that a larger energy level offset causes a lower efficiency of photoinduced charge transfer but also a longer lifetime of the charge separated state. Both observations are explained in the framework of Marcus' theory of electron transfer. In fact, our observations question direct electron−hole recombination across the hybrid interface as the main decay pathway for photogenerated carriers in the considered systems. Instead, back transfer of holes to ML-MoS 2 is suggested as the key decay channel. Adding a 1 nm LiF interlayer causes a significant slowdown of interfacial carrier recombination while not suppressing free carrier formation. This strategy serves as a guideline for optimizing further hybrid systems toward high-performance ML-TMDC/organic-based optoelectronic devices.

Ab Initio Modeling of Mixed-Dimensional Heterostructures: A Path Forward

“…In the limiting case of highly polarizable substrates (ε s → ∞), the gap is reduced from 2.67 to 2.16 eV [Figure b]. In spite of indications about the importance of vertex corrections for the determination of absolute band energies, , we find the VBM energy of −6.09 eV from substrate-corrected GW to agree well with the experimental value of −6.10 eV, which we obtained by averaging results of several PE measurements on substrate-adsorbed MoS 2 reported in the literature ,,,,− (values in the SI). The band gap of substrate-sustained MoS 2 turns out to be very similar to the band gap given by the HSE functional, although, again, absolute energies from the hybrid functional are too low [Figure b].…”

“…90 In experimental practice, the scenario of isolated adsorbed molecules, i.e., very low coverage, is relatively rare and often does not even produce measurable optical signals. 8 Typically, several organic layers are deposited, 5,6,[8][9][10][15][16][17]91 forming what is more accurately described as hybrid inorganic/organic interfaces rather than adsorbed molecules. We consider an H 2 Pc molecule directly adjacent to a MoS 2 layer but surrounded by other molecules, modeled as a dielectric continuum.…”

“…In such mixed-dimensional heterojunctions, bright excitons with hybrid character have been observed, − giving rise to an charge-separated state immediately upon photoabsorption. More commonly, dynamical charge transfer (CT) takes place in such systems, where the exciton is formed on either side of the interface and subsequently separated by electron or hole migration to the other subsystem. ,− At other interfaces, the entire exciton is transported across the interface, representing a transfer of energy. − Both charge and energy transfer in hybrid heterostructures have been exploited in various prototypical devices. ,− …”

Evidence for Hybrid Inorganic–Organic Transitions at the WS2/Terrylene Interface

“…It is not enough energy for across band gap excitations, leaving only Auger‐type scattering processes with carriers in the WS 2 and energy release via light emission in the near‐infrared, similar to previous studies on inorganic/organic hybrid systems. [ 13,15,60,61 ] Keeping in mind that the electrons are localized in the CB minimum at the K point of the Brillouin zone, Figure 4 indicates that the WS 2 band structure offers only very few final states for Auger‐type scattering processes that could account for the energy loss of 1 eV. In any case, they require large momentum changes.…”