Layered Dion-Jacobson-Type Chalcogenide Perovskite CsLaM2X7 (M = Ta/Nb; X = S/Se) with a Narrow Band Gap of ∼1 eV as a Promising Rear Cell for All-Perovskite Tandem Solar Cells.

Perovskite-perovskite tandem solar cells have bright prospects to improve the power conversion efficiency (PCE) beyond the Shockley-Queisser (SQ) limit of single-junction solar cells. The star lead-based halide perovskites are well-recognized as suitable candidates for the front cell, thanks to their suitable band gap (∼1.8 eV), strong optical absorption, and high certified PCE. However, the toxicity of lead for the front cell and the lack of a narrow band gap (∼1.1 eV) for the rear cell seriously restrict the development of the two-junction tandem cell. To break through this bottleneck, a novel Dion-Jacobson (DJ)-type (n = 2) chalcogenide perovskite CsLaM2X7 (M = Ta, Nb; X = S, Se) has been found based on the powerful first-principles and advanced many-body perturbation GW calculations. Their excellent electronic, transport, and optical properties can be summarized as follows. (1) They are stable and environmentally friendly lead-free materials. (2) The direct band gap of CsLaTa2Se7 (0.96-1.10 eV) is much smaller than those of lead-based halide perovskites and very suitable for the rear cell in the two-junction tandem cell. (3) The carrier mobility in CsLaTa2Se7 reaches 1.6 × 103 cm2 V-1 s-1 at room temperature. (4) The absorption coefficients (3-5 × 105 cm-1) are 1 order higher than that of Si (104 cm-1). (5) The estimated PCEs of the Cs2Sb2Br8-CsLaTa2Se7 tandem cell (33.3%) and the concentrator solar cell (35.8% in 100 suns) are higher than those of the best recorded GaAs-Si tandem cell (32.8%) and the perovskite-perovskite tandem solar cell (24.8%). These energetic results strongly demonstrate that the novel lead-free chalcogenide perovskites CsLaM2X7 are good candidates for the rear cell of tandem cells.

Optimized band gap and fast interlayer charge transfer in two-dimensional perovskite oxynitride Ba2NbO3N and Sr2NbO3/Ba2NbO3N bonded heterostructure visible-light photocatalysts for overall water splitting.

Searching for promising visible-light photocatalysts for overall water splitting into hydrogen and oxygen is a very challenging task to solve the energy crisis and environment pollution. The widely-used tantalate and niobate perovskite photocatalysts have two drawbacks, i.e., the large energy band gap (∼3.2-4.6 eV) and low electron (hole) mobility 102 (101) cm2 V-1 s-1, which greatly limit their photocatalytic performance. Here, based on the powerful first-principles and accurate GW calculations, we design several novel two-dimensional (2D) Ruddlesden-Popper (RP) type (n = 1) perovskite oxynitrides A2BO3N (A = Ca, Sr, Ba and B = Ta, Nb) and their bonded heterostructures and comprehensively investigate their interlayer coupling, electronic structures, transport and photocatalytic characteristics. We find that 2D A2BO3N oxynitrides have a reduced direct band gap at Γ-point, especially for three-layer (3L) Ba2NbO3N and 1L-Sr2NbO3N/1L-Ba2NbO3N bonded heterostructure with the optimized band gap ∼2.0 eV. Compared with tantalate and niobate perovskite oxides, the electron (hole) mobility increases 1-2 orders of magnitude up to 103-104 (102-103) cm2 V-1 s-1. A fast electron-hole vertical transport across the heterointerface and remarkable electron-hole separation can be realized in 1L-Sr2NbO3N/1L-Ba2NbO3N bonded heterostructure due to its strong interface Ba-O and Sr-O bonds and type-II band offset. Compared with the well-known photocatalysts, such as BiVO4 and MoS2/g-C3N4, an improved optical absorption (8 × 104 cm-1) in A2BO3N is obtained in the visible region. The 2D RP-type perovskite oxynitrides 3L-Ba2NbO3N and 1L-Sr2NbO3N/1L-Ba2NbO3N are powerful visible-light photocatalysts for overall water splitting.

Enhancement of photoluminescence and hole mobility in 1- to 5-layer InSe due to the top valence-band inversion: strain effect.

Recently, two-dimensional (2D) few-layer InSe nanosheets have become one of the most interesting materials due to their excellent electron transport, wide bandgap tunability and good metal contact. However, their low photoluminescence (PL) efficiency and hole mobility seriously restrict their application in 2D InSe-based nano-devices. Here, by exerting a suitable compressive strain, a remarkable modification for the electronic structure and the optical and transport properties of 1- to 5-layer InSe has been confirmed by powerful GW-BSE calculations. Both top valence band inversion and indirect-to-direct bandgap transition are induced; the light polarization is reversed from E||c to E⊥c; and the PL intensity and hole mobility are enhanced greatly. Surprisingly, under 6% compressive strain, the light emission of monolayer InSe with E⊥c is allowed at 2.58 eV, which has never been observed previously. Meanwhile, for the 2D few-layer InSe, the PL with E⊥c polarization increases over 10 times in intensity and has a blue-shift at about 0.6-0.7 eV, and the hole mobility increases two orders of magnitude up to 103 cm2 V-1 s-1, as high as electron mobility. The strained few-layer InSe are thus a promising candidate for future 2D electronic and optoelectronic nano-devices.

Modulation of electronic and magnetic properties in InSe nanoribbons: edge effect.

Quite recently, the two-dimensional (2D) InSe nanosheet has become a hot material with great promise for advanced functional nano-devices. In this work, for the first time, we perform first-principles calculations on the structural, electronic, magnetic and transport properties of 1D InSe nanoribbons with/without hydrogen or halogen saturation. We find that armchair ribbons, with various edges and distortions, are all nonmagnetic semiconductors, with a direct bandgap of 1.3 (1.4) eV for bare (H-saturated) ribbons, and have the same high electron mobility of about 103 cm2V-1s-1 as the 2D InSe nanosheet. Zigzag InSe nanoribbons exhibit metallic behavior and diverse intrinsic ferromagnetic properties, with the magnetic moment of 0.5-0.7 µ B per unit cell, especially for their single-edge spin polarization. The edge spin orientation, mainly dominated by the unpaired electrons of the edge atoms, depends sensitively on the edge chirality. Hydrogen or halogen saturation can effectively recover the structural distortion, and modulate the electronic and magnetic properties. The binding energy calculations show that the stability of InSe nanoribbons is analogous to that of graphene and better than in 2D InSe nanosheets. These InSe nanoribbons, with novel electronic and magnetic properties, are thus very promising for use in electronic, spintronic and magnetoresistive nano-devices.

Enhancement of hole mobility in InSe monolayer via an InSe and black phosphorus heterostructure.

To enhance the low hole mobility (∼40 cm2 V-1 s-1) of InSe monolayer, a novel two-dimensional (2D) van der Waals heterostructure made of InSe and black phosphorus (BP) monolayers with high hole mobility (∼103 cm2 V-1 s-1) has been constructed and its structural and electronic properties are investigated using first-principles calculations. We find that the InSe/BP heterostructure exhibits a direct band gap of 1.39 eV and type-II band alignment with electrons (holes) located in the InSe (BP) layer. The band offsets of InSe and BP are 0.78 eV for the conduction band minimum and 0.86 eV for the valence band maximum, respectively. Surprisingly, the hole mobility in the InSe/BP heterostructure exceeds 104 cm2 V-1 s-1, which is one order of magnitude larger than the hole mobility of BP and three orders larger than that of the InSe monolayer. The electron mobility is also increased to 3 × 103 cm2 V-1 s-1. The physical reason has been analyzed deeply, and a universal method is proposed to improve the carrier mobility of 2D materials by forming heterostructures with them and other 2D materials with complementary properties. The InSe/BP heterostructure can thus be widely used in nanoscale InSe-based field-effect transistors, photodetectors and photovoltaic devices due to its type-II band alignment and high carrier mobility.