Exciton Dynamics in MoS₂-Pentacene and WSe₂-Pentacene Heterojunctions

Abstract

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS 2 -pentacene and WSe 2 -pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS 2 and WSe 2 monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS 2 to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS 2 -pentacene junctions formed by vapor deposition of pentacene molecules onto MoS 2 on SiO 2 . The B-exciton decay in WSe 2 shows faster hole transfer rates for WSe 2 -pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe 2 also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions.

Exciton Dissociation, Charge Transfer, and Exciton Trapping at the MoS2/Organic Semiconductor Interface

“…A similar dampening is likely taking place in the ANNP/MoS 2 heterostructure. These observations are consistent with previous heterostructures based on MoS 2 and pentacene. , Two new components arise when ANNP is present, namely, hole transfer (τ 2 = 4.51 ± 0.72 ps) from 1L-MoS 2 to ANNP and interlayer recombination (τ 5 = 2.41 ± 0.63 ns) of the hole with an electron in 1L-MoS 2 . The ∼5 ps hole transfer in ANNP/MoS 2 is quicker than the 6.7 and 21 ps reported for pentacene/MoS 2 on quartz and gold substrates, respectively. , Interlayer charge recombination also occurs at a faster rate than pentacene/MoS 2 on quartz (5.13 ns) but not for the same heterojunction on a gold substrate (1.9 ns). ,, There is no observable interaction with the triplet state of ANNP (1.77 eV), which is expected to be below the band gap of 1L-MoS 2 (1.86 eV).…”

“…16,20 Interlayer charge recombination also occurs at a faster rate than pentacene/MoS 2 on quartz (5.13 ns) but not for the same heterojunction on a gold substrate (1.9 ns). 16,20,12 There is no observable interaction with the triplet state of ANNP (1.77 eV 40 ), which is expected to be below the band gap of 1L-MoS 2 (1.86 eV 29 ). Next, both ANNP and 1L-MoS 2 were excited at 400 nm with a fluence of 8.45 μJ/cm 2 and excited carrier density of 1.38 × 10 12 cm −2 (Figure S2d−f).…”

“…Investigations of heterostructures based on 1L-MoS 2 have focused on the interface with pentacene − and tetracene . The interface of pentacene and MoS 2 has shown hole transfer from 1L-MoS 2 to pentacene, respectively, occurring at ∼6.7 and ∼21 ps, when on quartz or gold substrates. Tetracene and pentacene are the most widely studied organic semiconductors in heterostructures consisting of monolayer materials.…”

Hybrid Heterostructures to Generate Long-Lived and Mobile Photocarriers in Graphene

“…We attribute this feature to the charge-transfer excitons formed at F 8 ZnPc/TMD interfaces. Such hybrid charge-transfer excitons with long recombination lifetimes can form at organic/inorganic semiconductor interfaces. − In the regions of our samples with poor contact between graphene and TMD layers, charge transfer to graphene can be inefficient, favoring charge-transfer exciton formation.…”

A roadmap to decipher ultrafast photophysics in two-dimensional nanomaterials

“…in charge transfer studies or to evaluate trapping dynamics. 60,61 The sheer complexity of the resulting ∆A spectra made researchers devise data analysis techniques to deconvolute and quantitatively analyse the time-dependent evolution of these metrics. Throughout the literature, spectral deconvolution approaches have been used to unravel charge carrier decay in a variety of TMDs, including MoS 2 , ReS 2 , and WS 2 .…”