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Boron, a nearest-neighbor of carbon, is possibly the second element that can possess free-standing flat monolayer structures, evidenced by recent successful synthesis of single-walled and multiwalled boron nanotubes (MWBNTs). From an extensive structural search using the first-principles particle-swarm optimization (PSO) global algorithm, two boron monolayers (α(1)- and β(1)-sheet) are predicted to be the most stable α- and β-types of boron sheets, respectively. Both boron sheets possess greater cohesive energies than the state-of-the-art two-dimensional boron structures (by more than 60 meV/atom based on density functional theory calculation using PBE0 hybrid functional), that is, the α-sheet previously predicted by Tang and Ismail-Beigi and the g(1/8)- and g(2/15)-sheets (both belonging to the β-type) recently reported by Yakobson and co-workers. Moreover, the PBE0 calculation predicts that the α-sheet is a semiconductor, while the α(1)-, β(1)-, g(1/8)-, and g(2/15)-sheets are all metals. When two α(1) monolayers are stacked on top each other, the bilayer α(1)-sheet remains flat with an optimal interlayer distance of ~3.62 Å, which is close to the measured interlayer distance (~3.2 Å) in MWBNTs.

Phosphorene, a monolayer of black phosphorus, is promising for nanoelectronic applications not only because it is a natural p-type semiconductor but also it possesses a layernumber dependent direct bandgap (in the range of 0.3 eV~1.5 eV). On basis of the density functional theory calculations, we investigate electronic properties of the bilayer phosphorene with different stacking orders. We find that the direct bandgap of the bilayers can vary from 0.78 -1.04 eV with three different stacking orders. In addition, a vertical electric field can further reduce the bandgap down to 0.56 eV (at the field strength 0.5 V/Å). More importantly, we find that when a monolayer of MoS 2 is superimposed with the p-type AA-or AB-stacked bilayer phosphorene, the combined tri-layer can be an effective solar-cell material with type-II heterojunction alignment. The power conversion efficiency is predicted to be ~18% or 16% with AA-or AB-stacked bilayer phosphorene, higher than reported efficiencies of the state-of-the-art trilayer graphene/transition metal dichalcogenide solar cells.

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We perform a comprehensive first-principles study of the electronic properties of phosphorene nanoribbons, phosphorene nanotubes, multilayer phosphorene, and heterobilayers of phosphorene and two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayer. The tensile strain and electric-field effects on electronic properties of low-dimensional phosphorene nanostructures are also investigated. Our calculations show that zigzag phosphorene nanoribbons (z-PNRs) are metals, regardless of the ribbon width while armchair phosphorene nanoribbons (a-PNRs) are semiconductors with indirect bandgaps and the bandgaps are insensitive to variation of the ribbon width. We find that tensile compression (or expansion) strains can reduce (or increase) the bandgap of the a-PNRs while an in-plane electric field can significantly reduce the bandgap of aPNRs, leading to the semiconductor-to-metal transition beyond certain electric field. For single-walled phosphorene nanotubes (SW-PNTs), both armchair and zigzag nanotubes are semiconductors with direct bandgaps. With either tensile strains or transverse electric field, similar behavior of bandgap modulation can arise as that for a-PNRs. It is known that multilayer phosphorene sheets are semiconductors with their bandgaps decreasing with increasing the number of multilayers. In the presence of a vertical electric field, the 2 bandgaps of multilayer phosphorene sheets decrease with increasing the electric field, and the bandgap modulation is more significant with more layers. Lastly, heterobilayers of phosporene with a TMDC (MoS 2 or WS 2 ) monolayer are still semiconductors while their bandgaps can be reduced by applying a vertical electric field as well.

Passivation of electronic defects at the surface and grain boundaries of perovskite materials has become one of the most important strategies to suppress charge recombination in both polycrystalline and singlecrystalline perovskite solar cells. Although many passivation molecules have been reported, it remains very unclear regarding the passivation mechanisms of various functional groups. Here, we systematically engineer the structures of passivation molecular functional groups, including carboxyl, amine, isopropyl, phenethyl, and tert-butylphenethyl groups, and study their passivation capability to perovskites. It reveals the carboxyl and amine groups would heal charged defects via electrostatic interactions, and the neutral iodine related defects can be reduced by the aromatic structures. The judicious control of the interaction between perovskite and molecules can further realize grain boundary passivation, including those that are deep toward substrates. Understanding of the underlining mechanisms allows us to design a new passivation molecule, D-4-tert-butylphenylalanine, yielding high-performance p-i-structure solar cells with a stabilized efficiency of 21.4%. The open-circuit voltage (V OC ) of a device with an optical bandgap of 1.57 eV for the perovskite layer reaches 1.23 V, corresponding to a record small V OC deficit of 0.34 V. Our findings provide a guidance for future design of new passivation molecules to realize multiple facets applications in perovskite electronics.

In this paper, we describe the fabrication and characterization of solution-processed CH 3 NH 3 PbI 3 photodetectors that combine a high photoconductive gain with a broad spectral response, ranging from the UV to the NIR. Benefi tting from the trapped-hole-induced electron injection, the CH 3 NH 3 PbI 3 photodetector works as a photodiode in the dark and shows large photoconductive gain under illumination. The maximum device gain reached 489 ± 6 at a very low driving voltage of −1 V.The devices studied here have a layered inverted structure ( Figure 1 a) where indium tin oxide (ITO) is the cathode, CH 3 NH 3 PbI 3 is the active layer, 4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]-biphenyl (TPD-Si 2 ) serves as the hole transporting/electron blocking layer, molybdenum trioxide (MoO 3 ) is used for anode work function modifi cation, and silver (Ag) as the anode. The CH 3 NH 3 PbI 3 layers were prepared by thermal-annealing induced interdiffusion of the two perovskite precursors (PbI 2 , CH 3 NH 3 I) by way of a method that has recently been developed in our group to fabricate very effi cient solar cells with high yield. [ 8 ] In short, lead iodide (PbI 2 ) fi lms were deposited fi rst on ITO/glass substrates by spin-coating. A second layer consisting of methylammonium iodide (CH 3 NH 3 I) (hereafter MAI) was then spin-coated on top of the dried PbI 2 fi lm, followed by thermal annealing at 105 °C for 60 min. Scanning electron microscopy (SEM) measurements ( Figure S1, Supporting Information) showed that the interdiffusion method allows the preparation of CH 3 NH 3 PbI 3 fi lms on ITO that are continuous and uniform, which is particularly important for obtaining leakage-free photodetectors. The absorption curve in Figure 1 b shows that the CH 3 NH 3 PbI 3 fi lms have a broad absorption spectrum that ranges from 300 nm (UV) to 800 nm (NIR).The photo-and dark-current densities-voltage ( J -V ) curves of CH 3 NH 3 PbI 3 photodetectors (Figure 1 c) show a transition from a diode-rectifying behavior in the dark, to photoconduction under illumination. Under illumination, both the forward and the reverse bias currents increased dramatically, with the reverse bias current increasing more sharply than the forward bias current. The rectifying effect completely disappeared, and an ohmic conduction behavior (symmetrical photocurrent with respect to the y -axis) was observed when exposing the device to white light irradiation (10 mW cm −2 ). For comparison, the J -V curve of a high performance solar cell (where exactly the same CH 3 NH 3 PbI 3 fabrication procedure was applied) with almost 100% external quantum effi ciency under the same light intensity is also shown in Figure 1 c. The reference photovoltaic (PV) device has a structure of ITO/PEDOT:PSS/ MAPbI 3 /[6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) (20 nm)/C 60 (20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (8 nm)/aluminium (Al) (100 nm). [ 8 ] Although Weak light sensing in the ultraviolet (UV), visible, and nearinfrared (NIR) range has a wide ...