Organic-2D Material Heterostructures: A Promising Platform for Exciton Condensation and Multiplication

Excitons at the interface of 2D TMDs and molecular semiconductors

Van der Waals heterostructures (vdWHs) of vertically stacked two-dimensional (2D) atomic crystals have been used to elicit intriguing phenomena stemming from strong electronic correlations, magnetic textures, and interlayer excitons spawned at the heterointerface. However, vdWHs comprised of heterointerfaces between these 2D atomic crystal lattices and molecular assemblies are emerging as equally intriguing platforms supporting properties to be harnessed for photovoltaic energy conversion, photodetection, spin-selective charge injection, and quantum emission. In this perspective, we summarize recent research examining exciton dynamics in heterostructures between semiconducting 2D transition metal dichalcogenides and molecular organic semiconductors. We discuss methods for assembly of these heterostructures, the nature of interlayer or charge-transfer excitons at transition-metal dichalcogenide (TMD)-molecule interfaces, explicit exciton transfer between organics and TMDs, and other interfacial phenomena driven by the merger of these two material classes. We also suggest key new research directions extending the remit of these 2D atomic-molecular lattice heterointerfaces into the domains of condensed matter physics, quantum sensing, and energy conversion.

Insulating 6,6-Phenyl-C61-butyric Acid Methyl Ester on Transition-Metal Dichalcogenides: Impact of the Hybrid Materials on the Optical and Electrical Properties

In this study we develop a strategy to insulate 6,6 -Phenyl C61 butyric acid methyl ester (PCBM) on the basal plane of transition metal dichalcogenides (TMDs). Concretely single layers of MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2 and ultrathin MoO2 and WO2 were grown via chemical vapor deposition (CVD). Then, the thiol group of a PCBM modified with cysteine reacts with the chalcogen vacancies on the basal plane of TMDs, yielding PCBM-MoS2, PCBM-MoSe2, PCBM-WS2, PCBM-WSe2, PCBM-WTe2, PCBM-MoO2 and PCBM-WO2. Afterwards, all the hybrid materials were characterized using several techniques, including XPS, Raman spectroscopy, TEM, AFM, and cyclic voltammetry. Furthermore, PCBM causes a unique optical and electrical impact in every TMDs. For MoS2 devices, the conductivity and photoluminescence (PL) emission achieve a remarkable enhancement of 1700 % and 200 % in PCBM-MoS2 hybrids. Similarly, PCBM-MoTe2 hybrids exhibit a 2-fold enhancement in PL emission at 1.1 eV. On the other hand, PCBM-MoSe2, PCBM-WSe2 and PCBM-WS2 hybrids exhibited a new interlayer exciton at 1.29-1.44, 1.7 and 1.37-154 eV along with an enhancement of the photo-response by 2400, 3200 and 600 %, respectively. Additionally, PCBM-WTe2 and PCBM-WO2 showed a modest photo-response, in sharp contrast with pristine WTe2 or WO2 which archive pure metallic character.

Enthalpy-uphill exciton dissociation in organic/2D heterostructures promotes free carrier generation

Despite the large binding energy of charge transfer (CT) excitons in type-II organic/2D heterostructures, it has been demonstrated that free carriers can be generated from CT excitons with a long lifetime. Using a model fluorinated zine phthalocyanine (F8ZnPc)/monolayer-WS2 interface, we find that CT excitons can dissociate spontaneously into free carriers despite it being an enthalpy-uphill process. Specifically, it is observed that CT excitons can gain an energy of 250 meV in 50 ps and dissociate into free carriers without any applied electric field. This observation is surprising because excited electrons typically lose energy to the environment and relax to lower energy states. We hypothesize that this abnormal enthalpy-uphill CT exciton dissociation process is driven by entropy gain. Kinetically, the entropic driving force can also reduce the rate for the reverse process - the conversion of free electron-hole pairs back to CT excitons. Hence, this mechanism can potentially explain the very long carrier lifetime observed in organic/2D heterostructures.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Pseudo-heterostructure and condensation of 1D moiré excitons in twisted phosphorene bilayers

Heterostructures are not expected to form in a single homogeneous material. Here, we show that planar pseudo-heterostructures could emerge in a twisted bilayer of phosphorene (tbP), driving in-plane energy and charge transfer. The formation of moiré superlattices combined with electronic anisotropy in tbPs yields one-dimensional (1D) moiré excitons with long radiative and nonradiative lifetimes, large binding energies, and deep moiré potentials. Low-frequency moiré phonons and dynamic moiré potentials are revealed to be responsible for the in-plane energy/charge transfer and exciton dynamics. The 1D moiré excitons are predicted to exhibit Bose-Einstein condensation at high temperatures and may lead to exotic Tonks-Girardeau Bose gases.

Nanoscale Periodic Trapping Sites for Interlayer Excitons Built by Deformable Molecular Crystal on 2D Crystal

The nanoscale moiré pattern formed at 2D transition-metal dichalcogenide crystal (TMDC) heterostructures provides periodic trapping sites for excitons, which is essential for realizing various exotic phases such as artificial exciton lattices, Bose-Einstein condensates, and exciton insulators. At organic molecule/TMDC heterostructures, similar periodic potentials can be formed via other degrees of freedom. Here, we utilize the structure deformability of a 2D molecular crystal as a degree of freedom to create a periodic nanoscale potential that can trap interlayer excitons (IXs). Specifically, two semiconducting molecules, PTCDI and PTCDA, which possess similar band gaps and ionization potentials but form different lattice structures on MoS₂, are investigated. The PTCDI lattice on MoS₂ is distorted geometrically, which lifts the degeneracy of the two molecules within the crystal's unit cell. The degeneracy lifting results in a spatial variation of the molecular orbital energy, with an amplitude and periodicity of ∼0.2 eV and ∼2 nm, respectively. On the other hand, no such energy variation is observed in PTCDA/MoS₂, where the PTCDA lattice is much less distorted. The periodic variation in molecular orbital energies provides effective trapping sites for IXs. For IXs formed at PTCDI/MoS₂, rapid spatial localization of the electron in the organic layer toward the interface is observed, which demonstrates the effectiveness of these interfacial IX traps.

Tuning moiré excitons in Janus heterobilayers for high-temperature Bose-Einstein condensation

Using first-principles calculations, we predict that moiré excitons in twisted Janus heterobilayers could realize tunable and high-temperature Bose-Einstein condensation (BEC). The electric dipole in the Janus heterobilayers leads to charge-transfer interlayer and intralayer moiré excitons with exceptionally long lifetimes, in the absence of spacer layers. The electric dipole is also expected to enhance exciton-exciton repulsions at high exciton densities and can modulate moiré potentials that trap excitons for their condensation. The key parameters for exciton condensation, including exciton Bohr radius, binding energy, effective mass, and critical Mott density, are examined as a function of the twist angle. Last, exciton phase diagrams for the Janus heterobilayers are constructed from which one can estimate the BEC (>100 K) and superfluid (~30 K) transition temperatures. In addition to indirect interlayer excitons, we find that direct intralayer excitons can also condense at high temperatures, consistent with experiments.

Twisting Enabled Charge Transfer Excitons in Epitaxially Fused Quantum Dot Molecules

A heterojunction with type-II band alignment has long been considered as a prerequisite to realize charge transfer (CT) excitons which are highly appealing for exploration of quantum many-body phenomena, such as excitonic Bose-Einstein condensation and superfluidity. Herein, we have shown CT excitons can be activated via twisting in epitaxially fused heterodimer quantum dot (QD) molecules with quasi type-II band alignment, and even in QD homodimer molecules, therefore breaking the constraint of band alignment. The enabling power of twisting has been revealed. It modulates the orbital spatial localization toward charge separation that is mandatory for CT excitons. Meanwhile, it manifests an effective band offset that counterbalances the distinct many-body effects felt by excitons of different nature, thus ensuring the successful generation of CT excitons. The present work extends the realm of twistroincs into zero-dimensional materials and opens a novel pathway of manipulating the properties of QD materials and closely related molecular systems.

Enhanced Electron Transfer and Spin Flip through Spin-Orbital Couplings in Organic/Inorganic Heterojunctions: A Nonadiabatic Surface Hopping Simulation

The circumstances of transferred electrons across organic/inorganic interfaces have attracted intensive interest because of the distinctive electronic structure properties of those two components. Leveraging ab initio nonadiabatic molecular dynamics methods in conjunction with spin dynamics induced by spin-orbital couplings (SOCs), this study reports two competitive channels during photoinduced dynamical processes in the prototypical ZnPc/monolayer MoS₂ heterojunction. Interestingly, the electron-transfer and relaxation processes occur simultaneously because of the enhancement of electron-phonon couplings and expansion of dynamical pathways by SOCs, suggesting that the electron-transfer rate and relaxation processes can be tuned by SOCs, hence yielding the performance promotion of photovoltaic and photocatalytic devices. Additionally, approximately half of the transferred electrons flip their spin within 1.6 ps because of strong SOCs in MoS₂, achieving great agreement with experimental measurements. This investigation provides instructive perspectives for designing novel devices and applications based on organic/inorganic heterojunctions, demonstrating the importance of spin dynamics simulations in exploring sophisticated photoinduced processes in materials.

Two-dimensional transition metal dichalcogenides as an emerging platform for singlet fission solar cells

Singlet fission, a rapid exciton doubling process via inverse Auger recombination, is recognized as one of the most practical and feasible means for overcoming the Shockley-Queisser limit. Singlet fission solar cells are generally developed by integrating photon downconversion organic semiconductors into conventional photovoltaic devices to break the maximum photovoltaic response of the host semiconductors by virtue of extra triplet excitons. In this regard, proper matching of two different semiconductors and heterointerface engineering are both crucial for highly efficient singlet fission solar cells. Therefore, the aim of this study is to review the prerequisite conditions for efficient triplet transfer at the heterointerfaces and thus highlight the robust spin and valley degrees of freedom of transition metal dichalcogenides with the ultimate goal of stimulating research into next-generation singlet fission solar cells.

Quasiparticle electronic structure of phthalocyanine:TMD interfaces from first-principles GW

Interfaces formed between monolayer transition metal dichalcogenides and (metallo)phthalocyanine molecules are promising in energy applications and provide a platform for studying mixed-dimensional molecule-semiconductor heterostructures in general. An accurate characterization of the frontier energy level alignment at these interfaces is key in the fundamental understanding of the charge transfer dynamics between the two photon absorbers. Here, we employ the first-principles substrate screening GW approach to quantitatively characterize the quasiparticle electronic structure of a series of interfaces: metal-free phthalocyanine (H₂Pc) adsorbed on monolayer MX₂ (M = Mo, W; X = S, Se) and zinc phthalocyanine (ZnPc) adsorbed on MoX₂ (X = S, Se). Furthermore, we reveal the dielectric screening effect of the commonly used α-quartz (SiO₂) substrate on the H₂Pc:MoS₂ interface using the dielectric embedding GW approach. Our calculations furnish a systematic set of GW results for these interfaces, providing the structure-property relationship across a series of similar systems and benchmarks for future experimental and theoretical studies.