High-Performance Photovoltaic Effect with Electrically Balanced Charge Carriers in Black Phosphorus and WS 2

2D van der Waals materials are promising for various electronic and optoelectronic devices because of their thickness-dependent mobility and tunable bandgap. Recently, heterojunction structures based on 2D van der Waals materials have exhibited their potential for photovoltaic applications as ultrathin p–n diodes. In this study, the photovoltaic effect of a multilayer black phosphorus (BP)/WS2 p–n heterojunction device is demonstrated under the 405 nm laser illumination and the AM 1.5 solar spectrum. The diode-like characteristics and photovoltaic effect rely on balance between charge carriers in the heterojunction device, by showing the highest performance at the balance position. The gate-tunable heterojunction device shows a high current rectification of 103 and an external quantum efficiency of 4.4% under the 405 nm laser illumination, and a photovoltaic efficiency of 4.6% under the AM 1.5 solar spectrum. This work suggests that the balancing of the charge carriers in the 2D heterojunction p–n diode is highly prioritized to fabricate high-performance photovoltaic device. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1

Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures

The possibility of hybridizing collective electronic motion with mid-infrared light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement1, 2, 3, 4, 5 and tailored nanophotonics6, 7, 8. Graphene9, 10 and its heterostructures11, 12, 13, 14 have attracted particular attention because the absence of an energy gap allows plasmon polaritons to be tuned continuously. Here, we introduce black phosphorus15, 16, 17, 18, 19 as a promising new material in surface polaritonics that features key advantages for ultrafast switching. Unlike graphene, black phosphorus is a van der Waals bonded semiconductor, which enables high-contrast interband excitation of electron–hole pairs by ultrashort near-infrared pulses. Here, we design a SiO2/black phosphorus/SiO2 heterostructure in which the surface phonon modes of the SiO2 layers hybridize with surface plasmon modes in black phosphorus that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO2, the hybrid interface polariton assumes surface-phonon-like properties, with a well-defined frequency and momentum and excellent coherence. During the lifetime of the photogenerated electron–hole plasma, coherent hybrid polariton waves can be launched by a broadband mid-infrared pulse coupled to the tip of a scattering-type scanning near-field optical microscopy set-up. The scattered radiation allows us to trace the new hybrid mode in time, energy and space. We find that the surface mode can be activated within ∼50 fs and disappears within 5 ps, as the electron–hole pairs in black phosphorus recombine. The excellent switching contrast and switching speed, the coherence properties and the constant wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices