The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) generally possess a type-II band alignment that facilitates the formation of interlayer excitons between constituent monolayers. Manipulation of the interlayer excitons in TMD vdW heterostructures holds great promise for the development of excitonic integrated circuits that serve as the counterpart of electronic integrated circuits, which allows the photons and excitons to transform into each other and thus bridges optical communication and signal processing at the integrated circuit. As a consequence, numerous studies have been carried out to obtain deep insight into the physical properties of interlayer excitons, including revealing their ultrafast formation, long population recombination lifetimes, and intriguing spin-valley dynamics. These outstanding properties ensure interlayer excitons with good transport characteristics, and may pave the way for their potential applications in efficient excitonic devices based on TMD vdW heterostructures. At present, a systematic and comprehensive overview of interlayer exciton formation, relaxation, transport, and potential applications is still lacking. In this review, we give a comprehensive description and discussion of these frontier topics for interlayer excitons in TMD vdW heterostructures to provide valuable guidance for researchers in this field.

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attractive for applications in a wide range of optoelectronic devices, due to their tremendous interesting physical properties. However, the photoluminescence quantum yield (PLQY) of TMDCs has been found to be too low, due to abundant defects and strong many-body effect. Here, we present a direct physical vapor growth of WO3–WS2 bilayer heterostructures, with WO3 monolayer domains attached on the surface of large-size WS2 monolayers. Optical characterizations revealed that the PLQY of the as-grown WO3–WS2 heterostructures can reach up to 11.6%, which is 2 orders of magnitude higher than that of WS2 monolayers by the physical vapor deposition growth method (PVD-WS2) and about 13-times higher than that of mechanical exfoliated WS2 (ME-WS2) monolayers, representing the highest PLQY reported for direct growth TMDCs materials so far. The PL enhancement mechanism has been well investigated by time-resolved optical measurements. The fabrication of WO3–WS2 heterostructures with ultrahigh PLQY provides an efficient approach for the development of highly efficient 2D integrated photonic applications.

Two-dimensional (2D) atomic layered semiconductor (e.g., transition metal dichalcogenides, TMDCs) heterostructures display diverse novel interfacial carrier properties and have potential applications in constructing next generation highly compact electronics and optoelectronics devices. However, the optoelectronic performance of this kind of semiconductor heterostructures has difficulty reaching the expectations of practical applications, due to the intrinsic weak optical absorption of the atomic-thick component layers. Here, combining the extraordinary optoelectronic properties of quantum-confined organic–inorganic hybrid perovskite (PVK), we design an ultrathin PVK/TMDC vertical semiconductor heterostructure configuration and realize the controlled vapor-phase growth of highly crystalline few-nanometer-thick PVK layers on TMDCs monolayers. The achieved ultrathin PVKs show strong thickness-induced quantum confinement effect, and simultaneously form band alignment-engineered heterointerfaces with the underlying TMDCs, resulting in highly efficient interfacial charge separation and transport. Electrical devices constructed with the as-grown ultrathin PVK/WS2 heterostructures show ambipolar transport originating from p-type PVK and n-type WS2, and exhibit outstanding optoelectronic characteristics, with the optimized response time and photoresponsivity reaching 64 μs and 11174.2 A/W, respectively, both of which are 4 orders of magnitude better than the heterostructures with a thick PVK layer, and also represent the best among all previously reported 2D layered semiconductor heterostructures. This work provides opportunities for 2D vertical semiconductor heterostructures via incorporating ultrathin PVK layers in high-performance integrated optoelectronics.

With unique valley‐dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second‐harmonic generation (SHG) in 2D TMDCs monolayers has shown valley‐dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion‐symmetry‐breaking (3R‐like phase) TMDCs (WSe2, WS2, MoS2) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular‐polarization‐resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley‐dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.

potential applications in integrated photo nics for onchip optical interconnects, gas sensing, and imaging applications. [4][5][6] Thus far, almost all the reported works are limited to the visible region to near infrared (nearIR) waveband. Pushing the laser wavelength of van der Waals crystalbased nanoemitters further into IR spectral range is of paramount signifi cance for sensing and free space telecom munication technologies. In light of the intriguing prospect, black phosphorus (BP), in recent years, has emerged as a promising material for IRoriented nano photonic applications. This is driven by the facts that BP possesses a layer dependent direct bandgap character, greatly different from other 2D transi tion metal dichalcogenide materials. [7,8] By increasing the thickness of BP from monolayer to bulk, the band gap of BP decreases from ≈1.75 eV to ≈0.3 eV, span ning almost all the near to midIR (MIR) spectral region. [8] More importantly, the unique crystalline structure of BP enables strong inplane anisotropy of both elec trical and optical properties, which can be harnessed for novel physical effects. [9][10][11][12] The high carrier mobility has already demonstrated BP's superiority in a wealth of nanoelectronics devices. [13][14][15][16][17] All these merits render BP an ideal building block for potential applications in integrated electronics and Van der Waals layered semiconductor materials own unique physical properties and have attracted intense interest in developing high-performance electronic and photonic devices. Among them, black phosphorus (BP) is distinct for its layer number-tuned direct band gap which spans from near-to mid-infrared (MIR) waveband. In addition, the puckered honey comb crystal lattice endows the material with highly linear-polarized emission and marked anisotropy in carrier transportation. These unique material properties render BP as an intriguing and promising building block for constructing midinfrared-ranged coherent light sources. Here, a room temperature surfaceemitting MIR laser based on single crystalline BP nanosheets coupled with a distributed Bragg reflector cavity is reported. MIR stimulated emission at 3611 nm is achieved with a near-unity linear polarization, which exhibits robust thermal stability up to 360 K. Most importantly, the lasing wavelength can be tuned from 3425 to 4068 nm by varying the cavity length via thickness control of BP layer. The demonstrated highly polarized lasing output and wavelength-tunable capacity of the proposed device scheme in MIR spectral range opens up promising opportunities for a broad array of applications in polarization-resolved IR imaging, range-finding, and free space quantum communications.

The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI2 layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS2 and WSe2. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.

Nanowire (NW) lasers operating in the near-infrared spectral range are of significant technological importance for applications in telecommunications, sensing, and medical diagnostics. So far, lasing within this spectral range has been achieved using GaAs/AlGaAs, GaAs/GaAsP, and InGaAs/GaAs core/shell NWs. Another promising III–V material, not yet explored in its lasing capacity, is the dilute nitride GaNAs. In this work, we demonstrate, for the first time, optically pumped lasing from the GaNAs shell of a single GaAs/GaNAs core/shell NW. The characteristic “S”-shaped pump power dependence of the lasing intensity, with the concomitant line width narrowing, is observed, which yields a threshold gain, g th , of 3300 cm–1 and a spontaneous emission coupling factor, β, of 0.045. The dominant lasing peak is identified to arise from the HE21b cavity mode, as determined from its pronounced emission polarization along the NW axis combined with theoretical calculations of lasing threshold for guided modes inside the nanowire. Even without intentional passivation of the NW surface, the lasing emission can be sustained up to 150 K. This is facilitated by the improved surface quality due to nitrogen incorporation, which partly suppresses the surface-related nonradiative recombination centers via nitridation. Our work therefore represents the first step toward development of room-temperature infrared NW lasers based on dilute nitrides with extended tunability in the lasing wavelength.