Air-stable self-powered photodetector based on TaSe2/WS2/TaSe2 asymmetric heterojunction with surface self-passivation.

Two-dimensional (2D) transition metal dichalcogenides are highly suitable for constructing junction photodetectors because of their suspended bond-free surface and adjustable bandgap. Additional stable layers are often used to ensure the stability of photodetectors. Unfortunately, they often increase the complexity of preparation and cause performance degradation of devices. Considering the self-passivation behavior of TaSe2, we designed and fabricated a novel self-powered TaSe2/WS2/TaSe2 asymmetric heterojunction photodetector. The heterojunction photodetector shows excellent photoelectric performance and photovoltaic characteristics, achieving a high responsivity of 292 mA/W, an excellent specific detectivity of 2.43 × 1011 Jones, a considerable external quantum efficiency of 57 %, a large optical switching ratio of 2.6 × 105, a fast rise/decay time of 43/54 µs, a high open-circuit voltage of 0.23 V, and a short-circuit current of 2.28 nA under 633 nm laser irradiation at zero bias. Moreover, the device also shows a favorable optical response to 488 and 532 nm lasers. Notably, it exhibits excellent environmental long-term stability with the performance only decreasing âˆ¼ 5.6 % after exposed to air for 3 months. This study provides a strategy for the development of air-stable self-powered photodetectors based on 2D materials.

High-Performance and Reliable Lead-Free Layered-Perovskite Transistors.

Perovskites have been intensively investigated for their use in solar cells and light-emitting diodes. However, research on their applications in thin-film transistors (TFTs) has drawn less attention despite their high intrinsic charge carrier mobility. In this study, the universal approaches for high-performance and reliable p-channel lead-free phenethylammonium tin iodide TFTs are reported. These include self-passivation for grain boundary by excess phenethylammonium iodide, grain crystallization control by adduct, and iodide vacancy passivation through oxygen treatment. It is found that the grain boundary passivation can increase TFT reproducibility and reliability, and the grain size enlargement can hike the TFT performance, thus, enabling the first perovskite-based complementary inverter demonstration with n-channel indium gallium zinc oxide TFTs. The inverter exhibits a high gain over 30 with an excellent noise margin. This work aims to provide widely applicable and repeatable methods to make the gate more open for intensive efforts toward high-performance printed perovskite TFTs.

Novel Lewis Base Cyclam Self-Passivation of Perovskites without an Anti-Solvent Process for Efficient Light-Emitting Diodes.

Metal halide perovskites have been focused as a candidate applied as a promising luminescent material for next-generation high-quality lighting and high-definition display. However, as perovskite films formed, high density of defects would be produced in solution processing inevitably, leading to low exciton recombination efficiency in light-emitting diodes (LEDs). Herein, a facile and novel self-passivation strategy to inhibit defect formation in perovskite films for constructing high-performance LEDs is developed. For the first time, we introduce 1,4,8,11-tetraazacyclotetradecane (cyclam) in perovskite precursor solution, and it spontaneously passivates defect states of CsPbBr3-based perovskites by coaction between amine and uncoordinated lead ions during spin-coating without an anti-solvent process. Furthermore, as a delocalized system, cyclam also possesses chemical properties that facilitate exciton transportation. The proposed passivation strategy boosts the external quantum efficiency from 1.25% (control device) to 16.24% (cyclam-passivated device). Furthermore, defect passivation is also conductive to reduce LED degradation paths and improve device stability as the extrapolated lifetime (T50) of LEDs at an initial brightness of 100 cd/m2 is increased from 0.9 to 127 h. These findings indicate that the introduction of cyclam is highly effective to enhance the performance of LEDs, and such a strategy in effectively reducing the defects could be also applied in other perovskite-based devices, such as lasers, solar cells, and photodetectors.

Self-Passivation of 2D Ruddlesden-Popper Perovskite by Polytypic Surface PbI2 Encapsulation.

Two-dimensional Ruddlesden-Popper (2D RP) halide perovskites, C2MAn-1PbnI3n+1 (C = bulky ammonium cation; MA = methylammonium) with low n-members (n < 5), have been garnering sensational attention for photovoltaic and optoelectronic applications because of the long carrier diffusion lengths, long-term stability, and tunable bandgap. Yet, the surface modification of 2D RP under kinetic particle irradiation, such as light or electron irradiation, is ambiguous, even though it is imperative to elucidate long-stabilized conversion efficiency. Herein, we present molecular-scale observations of dynamic surface reconstruction of BA2MA2Pb3I10 (n = 3) 2D RP induced by the electron beam. The surface dynamics reveal lateral growth of polytypic PbI2 with 3R, 4H, and 2H structures at the edge and surface of the 2D perovskite, accompanied by simultaneous annihilation at the other edges. Local radiolysis occurs dominantly by the internal energy increase of electron momentum transfer, which triggers a sequential layer-by-layer degradation into PbI2. In situ observation of the polytypic PbI2 growth at the whole surface and edges of 2D RP under electron irradiation elucidates how the outer PbI2 self-passivation can protect inner 2D RP, causing longer operando stability.

Electronic bandgap and edge reconstruction in phosphorene materials.

Single-layer black phosphorus (BP), or phosphorene, is a highly anisotropic two-dimensional elemental material possessing promising semiconductor properties for flexible electronics. However, the direct bandgap of single-layer black phosphorus predicted theoretically has not been directly measured, and the properties of its edges have not been considered in detail. Here we report atomic scale electronic variation related to strain-induced anisotropic deformation of the puckered honeycomb structure of freshly cleaved black phosphorus using a high-resolution scanning tunneling spectroscopy (STS) survey along the light (x) and heavy (y) effective mass directions. Through a combination of STS measurements and first-principles calculations, a model for edge reconstruction is also determined. The reconstruction is shown to self-passivate most dangling bonds by switching the coordination number of phosphorus from 3 to 5 or 3 to 4.