Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer

In the short time since its discovery, perovskite solar cells (PSCs) have attained high power conversion efficiency but their lack of thermal stability remains a barrier to commercialization. Among the experimentally accessible parameter spaces for optimizing performance, identifying an electron transport layer (ETL) that forms a thermally stable interface with perovskite and which is solution-processable at low-temperature will certainly be advantageous. Herein, we developed a mesoporous graphene/polymer composite with these advantages when used as ETL in CH₃NH₃PbI₃ PSCs, and a high efficiency of 13.8% under AM 1.5G solar illumination could be obtained. Due to the high heat transmission coefficient and low isoelectric point of mesoporous graphene-based ETL, the PSC device enjoys good chemical and thermal stability. Our work demonstrates that the mesoporous graphene-based scaffold is a promising ETL candidate for high performance and thermally stable PSCs

Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer

In the short time since its discovery, perovskite solar cells (PSCs) have attained high power conversion efficiency but their lack of thermal stability remains a barrier to commercialization. Among the experimentally accessible parameter spaces for optimizing performance, identifying an electron transport layer (ETL) that forms a thermally stable interface with perovskite and which is solution-processable at low-temperature will certainly be advantageous. Herein, we developed a mesoporous graphene/polymer composite with these advantages when used as ETL in CH₃NH₃PbI₃ PSCs, and a high efficiency of 13.8% under AM 1.5G solar illumination could be obtained. Due to the high heat transmission coefficient and low isoelectric point of mesoporous graphene-based ETL, the PSC device enjoys good chemical and thermal stability. Our work demonstrates that the mesoporous graphene-based scaffold is a promising ETL candidate for high performance and thermally stable PSCs

Molecular Beam Epitaxy of Highly Crystalline MoSe₂ on Hexagonal Boron Nitride

Molybdenum diselenide (MoSe₂) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe₂ cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe₂ on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe₂ film can be grown with electron mobility ∼15 cm²/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe₂ grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry

Elastic Properties of Chemical-Vapor-Deposited Monolayer MoS₂, WS₂, and Their Bilayer Heterostructures

Elastic properties of materials are an important factor in their integration in applications. Chemical vapor deposited (CVD) monolayer semiconductors are proposed as key components in industrial-scale flexible devices and building blocks of two-dimensional (2D) van der Waals heterostructures. However, their mechanical and elastic properties have not been fully characterized. Here we report high 2D elastic moduli of CVD monolayer MoS₂ and WS₂ (∼170 N/m), which is very close to the value of exfoliated MoS₂ monolayers and almost half the value of the strongest material, graphene. The 2D moduli of their bilayer heterostructures are lower than the sum of 2D modulus of each layer but comparable to the corresponding bilayer homostructure, implying similar interactions between the hetero monolayers as between homo monolayers. These results not only provide deep insight into understanding interlayer interactions in 2D van der Waals structures but also potentially allow engineering of their elastic properties as desired

Chemical Vapor Deposition of Large-Size Monolayer MoSe₂ Crystals on Molten Glass

We report the fast growth of high-quality millimeter-size monolayer MoSe₂ crystals on molten glass using an ambient pressure CVD system. We found that the isotropic surface of molten glass suppresses nucleation events and greatly improves the growth of large crystalline domains. Triangular monolayer MoSe₂ crystals with sizes reaching ∼2.5 mm, and with a room-temperature carrier mobility up to ∼95 cm²/(V·s), can be synthesized in 5 min. The method can also be used to synthesize millimeter-size monolayer MoS₂ crystals. Our results demonstrate that “liquid-state” glass is a highly promising substrate for the low-cost growth of high-quality large-size 2D transition metal dichalcogenides (TMDs)

Doping against the Native Propensity of MoS₂: Degenerate Hole Doping by Cation Substitution

Layered transition metal dichalcogenides (TMDs) draw much attention as the key semiconducting material for two-dimensional electrical, optoelectronic, and spintronic devices. For most of these applications, both <i>n</i>- and <i>p</i>-type materials are needed to form junctions and support bipolar carrier conduction. However, typically only one type of doping is stable for a particular TMD. For example, molybdenum disulfide (MoS₂) is natively an <i>n</i>-type presumably due to omnipresent electron-donating sulfur vacancies, and stable/controllable <i>p</i>-type doping has not been achieved. The lack of <i>p</i>-type doping hampers the development of charge-splitting <i>p</i>–<i>n</i> junctions of MoS₂, as well as limits carrier conduction to spin-degenerate conduction bands instead of the more interesting, spin-polarized valence bands. Traditionally, extrinsic <i>p</i>-type doping in TMDs has been approached with surface adsorption or intercalation of electron-accepting molecules. However, practically stable doping requires substitution of host atoms with dopants where the doping is secured by covalent bonding. In this work, we demonstrate stable <i>p</i>-type conduction in MoS₂ by substitutional niobium (Nb) doping, leading to a degenerate hole density of ∼3 × 10¹⁹ cm^–3. Structural and X-ray techniques reveal that the Nb atoms are indeed substitutionally incorporated into MoS₂ by replacing the Mo cations in the host lattice. van der Waals <i>p</i>–<i>n</i> homojunctions based on vertically stacked MoS₂ layers are fabricated, which enable gate-tunable current rectification. A wide range of microelectronic, optoelectronic, and spintronic devices can be envisioned from the demonstrated substitutional bipolar doping of MoS₂. From the miscibility of dopants with the host, it is also expected that the synthesis technique demonstrated here can be generally extended to other TMDs for doping against their native unipolar propensity

Dense Electron System from Gate-Controlled Surface Metal–Insulator Transition

Two-dimensional electron systems offer enormous opportunities for science discoveries and technological innovations. Here we report a dense electron system on the surface of single-crystal vanadium dioxide nanobeam via electrolyte gating. The overall conductance of the nanobeam increases by nearly 100 times at a gate voltage of 3 V. A series of experiments were carried out which rule out electrochemical reaction, impurity doping, and oxygen vacancy diffusion as the dominant mechanism for the conductance modulation. A surface insulator-to-metal transition is electrostatically triggered, thereby collapsing the bandgap and unleashing an extremely high density of free electrons from the original valence band within a depth self-limited by the energetics of the system. The dense surface electron system can be reversibly tuned by the gating electric field, which provides direct evidence of the electron correlation driving mechanism of the phase transition in VO₂. It also offers a new material platform for implementing Mott transistor and novel sensors and investigating low-dimensional correlated electron behavior

Achieving Ultrafast Hole Transfer at the Monolayer MoS₂ and CH₃NH₃PbI₃ Perovskite Interface by Defect Engineering

The performance of a photovoltaic device is strongly dependent on the light harvesting properties of the absorber layer as well as the charge separation at the donor/acceptor interfaces. Atomically thin two-dimensional transition metal dichalcogenides (2-D TMDCs) exhibit strong light–matter interaction, large optical conductivity, and high electron mobility; thus they can be highly promising materials for next-generation ultrathin solar cells and optoelectronics. However, the short optical absorption path inherent in such atomically thin layers limits practical applications. A heterostructure geometry comprising 2-D TMDCs (<i>e</i>.<i>g</i>., MoS₂) and a strongly absorbing material with long electron–hole diffusion lengths such as methylammonium lead halide perovskites (CH₃NH₃PbI₃) may overcome this constraint to some extent, provided the charge transfer at the heterostructure interface is not hampered by their band offsets. Herein, we demonstrate that the intrinsic band offset at the CH₃NH₃PbI₃/MoS₂ interface can be overcome by creating sulfur vacancies in MoS₂ using a mild plasma treatment; ultrafast hole transfer from CH₃NH₃PbI₃ to MoS₂ occurs within 320 fs with 83% efficiency following photoexcitation. Importantly, our work highlights the feasibility of applying defect-engineered 2-D TMDCs as charge-extraction layers in perovskite-based optoelectronic devices

Mo-Terminated Edge Reconstructions in Nanoporous Molybdenum Disulfide Film

The catalytic and magnetic properties of molybdenum disulfide (MoS₂) are significantly enhanced by the presence of edge sites. One way to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. However, the large-scale bottom-up synthesis of a porous 2D MoS₂ film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. Here, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS₂ films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS₂ films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated edges leads to excellent activity for the hydrogen evolution reaction

Valley Polarization of Trions and Magnetoresistance in Heterostructures of MoS₂ and Yttrium Iron Garnet

Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin-up (-down) electrons are distributed in a +K (−K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many-body effects in a heterostructure geometry comprising a MoS₂ monolayer and YIG. High-order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ∼10¹³ cm^–2, resulting in a large splitting of ∼40 meV between trions and excitons. The trions exhibit a high circular polarization of ∼80% under optical pumping by circularly polarized light at ∼1.96 eV; it is confirmed experimentally that both phonon scattering and electron–hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS₂ monolayer was observed, and a giant MR ratio of ∼30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS₂/CoFe₂O₄ heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates