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1.
Nat Commun ; 10(1): 16, 2019 01 03.
Artigo em Inglês | MEDLINE | ID: mdl-30604757

RESUMO

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

2.
J Am Chem Soc ; 140(33): 10456-10463, 2018 08 22.
Artigo em Inglês | MEDLINE | ID: mdl-30043607

RESUMO

Low-dimensional organic-inorganic halide perovskites (OIHPs) have attracted intense interest recently for photovoltaic applications, due to their markedly high chemical stability as compared to the widely studied three-dimensional (3D) counterparts. However, low-dimensional OIHPs usually give much lower device performance than the 3D OIHPs. In particular, for the zero-dimensional (0D) OIHPs, it is believed that the strong intrinsic quantum-confinement effects can lead to extremely low carrier motility, which can severely limit the photovoltaic performance. Herein, we predict a new family of 0D perovskite variants that, surprisingly, exhibit outstanding optoelectronic properties. We show that the "atypical" carrier mobility of these new 0D perovskites is attributed to the strong electronic interaction between neighboring octahedrons in the crystal. These findings also suggest a new materials design strategy for resolving the low-performance issue commonly associated with the low-dimensional OIHPs for photovoltaic applications.

3.
Nanoscale ; 10(24): 11314-11319, 2018 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-29897093

RESUMO

Despite their high power conversion efficiency, the commercial applications of hybrid organic-inorganic lead (Pb) halide perovskite based solar cells are still hampered by concerns about the toxicity of Pb and the structural stability in open air. Herein, based on density-functional theory computation, we show that lead-free tin (Sn) and germanium (Ge) based two-dimensional (2D) Ruddlesden-Popper hybrid organic-inorganic perovskites with a thickness of a few unit-cells, BA2MAn-1MnI3n+1 (M = Sn or Ge, n = 2-4), possess desirable electronic, excitonic and light absorption properties, thereby showing promise for photovoltaic and/or photoelectronic applications. In particular, we show that by increasing the layer thickness of the Sn-based 2D perovskites, the bandgap can be lowered towards the optimal range (0.9-1.6 eV) for solar cells. Meanwhile, the exciton binding energy is reduced to a more optimal value. In addition, theoretical assessment indicates that the thermodynamic stability of Sn-/Ge-based 2D perovskites is notably enhanced compared to that of their 3D analogues. These features render the Sn-/Ge-based 2D hybrid perovskites with a thickness of a few tens of unit cells promising lead-free perovskites with much improved structural stabilities for photovoltaic and/or photoelectronic applications.

4.
Phys Chem Chem Phys ; 19(32): 21691-21695, 2017 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-28770937

RESUMO

Double perovskites in the form of A2B'B''X6 (A = Cs, B' = Ag, B'' = Bi) have been reported as potential alternatives to lead-containing organometal trihalide perovskites. However, all double perovskites synthesized to date exhibit indirect bandgaps >1.95 eV, which are undesirable for photovoltaic and optoelectronic applications. Herein, we report a comprehensive computer-aided screening of In- and Ga-based double perovskites for potential photovoltaic applications. To this end, several preconditions are implemented for the screening of optimal candidates, which include structural stability, electronic bandgaps, and optical absorption. Importantly, four In- and Ga-based double perovskites are identified to possess direct bandgaps within the desirable range of 0.9-1.6 eV for photovoltaic applications. Dominant optical absorption of the four double perovskites is found to be in the UV range. The structural and thermal stability of the four double perovskites are examined using both the empirical Goldschmidt ratio and convex-hull calculations. Only Cs2AgInBr6 is predicted to be thermodynamically stable.

5.
Angew Chem Int Ed Engl ; 56(41): 12658-12662, 2017 10 02.
Artigo em Inglês | MEDLINE | ID: mdl-28671739

RESUMO

The alloying behavior between FAPbI3 and CsSnI3 perovskites is studied carefully for the first time, which has led to the realization of single-phase hybrid perovskites of (FAPbI3 )1-x (CsSnI3 )x (0

6.
J Am Chem Soc ; 139(23): 8038-8043, 2017 06 14.
Artigo em Inglês | MEDLINE | ID: mdl-28537073

RESUMO

The power-conversion efficiency (PCE) of lead halide perovskite photovoltaics has reached 22.1% with significantly improved structural stability, thanks to a mixed cation and anion strategy. However, the mixing element strategy has not been widely seen in the design of lead-free perovskites for photovoltaic application. Herein, we report a comprehensive study of a series of lead-free and mixed tin and germanium halide perovskite materials. Most importantly, we predict that RbSn0.5Ge0.5I3 possesses not only a direct bandgap within the optimal range of 0.9-1.6 eV but also a desirable optical absorption spectrum that is comparable to those of the state-of-the-art methylammonium lead iodide perovskites, favorable effective masses for high carrier mobility, as well as a greater resistance to water penetration than the prototypical inorganic-organic lead-containing halide perovskite. If confirmed in the laboratory, this new lead-free inorganic perovskite may offer great promise as an alternative, highly efficient solar absorber material for photovoltaic application.

7.
Phys Chem Chem Phys ; 19(9): 6554-6562, 2017 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-28197566

RESUMO

Stanene, a low thermal conductivity two-dimensional (2D) sheet composed of group-IV element Sn, is a prototype material with novel properties such as 2D topological insulating behavior and near-room-temperature quantum Hall effects. Monolayer graphene, on the other hand, possesses unusual thermal properties, but has a zero bandgap. By stacking stanene and graphene monolayers vertically into a hetero-bilayer, an indirect bandgap can be obtained, making the hetero-bilayer a good candidate for special applications. In this work, the in-plane thermal conductivity (κ) and out-of-plane interfacial thermal resistance (R) in the hetero-bilayer are systematically investigated using non-equilibrium molecular dynamics and transient pump-probe methods. Effects of dimension, system temperature and van der Waals coupling strength on the thermal properties are examined. The predicted in-plane thermal conductivity of the graphene/stanene hetero-bilayer is 311.1 W m-1 K-1, higher than most 2D materials such as phosphorene, hexagonal boron nitride (h-BN), MoS2 and MoSe2. Phonon power spectra are recorded for graphene and stanene individually to help the explanation of their κ difference. The inter-layer thermal resistance between graphene and stanene hetero-bilayers is predicted to be 2.13 × 10-7 K m2 W-1, which is on the same order of magnitude as several other 2D bilayer structures.

8.
Nano Lett ; 17(3): 1623-1628, 2017 03 08.
Artigo em Inglês | MEDLINE | ID: mdl-28212486

RESUMO

Two-dimensional materials, such as graphene and monolayer transition metal dichalcogenides, allow the fabrication of multilayer structures without lattice matching restriction. A central issue in developing such artificial materials is to understand and control the interlayer electron transfer process, which plays a key role in harnessing their emergent properties. Recent photoluminescence and transient absorption measurements revealed that the electron transfer in heterobilayers occurs on ultrafast time scales. However, there is still a lack of fundamental understanding on how this process can be so efficient at van der Waals interfaces. Here we show evidence suggesting the coherent nature of such interlayer electron transfer. In a trilayer of MoS2-WS2-MoSe2, electrons excited in MoSe2 transfer to MoS2 in about one picosecond. Surprisingly, these electrons do not populate the middle WS2 layer during this process. Calculations showed the coherent nature of the charge transfer and reproduced the measured electron transfer time. The hole transfer from MoS2 to MoSe2 is also found to be efficient and ultrafast. The separation of electrons and holes extends their lifetimes to more than one nanosecond, suggesting potential applications of such multilayer structures in optoelectronics.

9.
Angew Chem Int Ed Engl ; 55(47): 14723-14727, 2016 11 14.
Artigo em Inglês | MEDLINE | ID: mdl-27766739

RESUMO

Methylamine-induced thin-film transformation at room-temperature is discovered, where a porous, rough, polycrystalline NH4 PbI3 non-perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH3 NH3 PbI3 perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH4 PbI3 -to-CH3 NH3 PbI3 transformation process. The chemical origins of this transformation are studied at various length scales.

10.
Phys Chem Chem Phys ; 18(33): 23174-83, 2016 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-27499005

RESUMO

With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb-I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI3 perovskites in the presence of moisture.

11.
J Phys Chem Lett ; 7(17): 3395-400, 2016 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-27522866

RESUMO

Two-dimensional (2D) monolayer nanomaterials can be exploited as the thinnest membrane with distinct differential sieving properties for proton isotopes. Motivated from the experimental evidence of differential sieving proton isotopes through graphene and hexagonal boron nitrate (h-BN) monolayer, we compute the kinetic barrier of isotope H(+) and D(+) permeation through model graphene and h-BN fragments at the MP2/6-31++G(d,p) level of theory. On the basis of the ratio of tunneling reaction rate constant, the isotope separation ratio of H(+)/D(+) and H(+)/T(+) is predicted to be ∼12 and 37, respectively. The tunneling reaction rate constant can be estimated from the zero-point-energy computed at the transition state for the proton isotope permeation though the 2D model systems. We show that the presence of Stone-Wales (55-77) defect in the model graphene fragment can significantly lower the proton permeation barrier by 0.55 eV. With the defect, the ratio of tunneling reaction rate constant of H(+)/D(+) is increased to ∼25. In addition to model graphene and h-BN, we have examined proton permeation capability of α-boron monolayer. We compute the tunneling reaction pathway for H(+) through α-boron monolayer using both the climbing nudged elastic band (c-NEB) method and the scanning-path method. Both methods suggest that α-boron monolayer entails a relatively low barrier of ∼0.20 eV for H(+) permeation, much lower than that of the model graphene and h-BN fragments. Our studies provide molecular-level insights into the differential permeation of proton isotopes through 2D materials. The methods can be extended to examine isotope separation capability of other 2D materials as well.

12.
Phys Chem Chem Phys ; 17(37): 24438-45, 2015 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-26339695

RESUMO

Osmapentalyne cations synthesized recently show remarkable optical properties, such as near-infrared emission, unusual large Stokes shift and aggregation-enhanced emission. Here, the mechanisms behind those novel optical behaviors are revealed from the combined molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations. The results demonstrate that the large Stokes shift in the gas phase comes from a photoexcitation-induced deformation of the osmium plane, whereas in solution it corresponds to the variation of osmium ring symmetry. Although the central chromophore ring dominates the absorption and emission processes, the protecting groups PPh3 join the emission. As osmapentalyne cations are aggregated together in solution, the radical distribution functions of their mass-central distances display several peaks immersed in a broad envelope due to different aggregation pathways. However, the chromophore centers are protected by the PPh3 groups, the aggregation structures do not affect the Stokes shift too much, and the calculated aggregate-enhanced emission is consistent with experimental measurements.

13.
Phys Chem Chem Phys ; 17(27): 17679-87, 2015 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-26081196

RESUMO

Recently, solar cells with hybrid organic-inorganic lead halide perovskites have achieved a great success and their power conversion efficiency reaches about 17.9%. For practical applications, one has to avoid the toxicology issue of lead, to develop lead-free perovskite solar cells by using metal substitution. It has been shown that tin is one of possible candidates as a replacement for lead. Herein, a step-by-step protocol based on the first-principles calculations is performed to investigate the geometrical and electronic properties of mixed Sn and Pb perovskite MAPbxSn1-xI3 with different crystal symmetries. At first, a GGA functional with the inclusion of the van der Waals interaction, vdW-DF3, is used to optimize the geometries and it reproduces closely the unit cell volume. Then, a more accurate hybrid functional PBE0 combined with the spin-orbit coupling effect is used to perform electronic-structure calculations. The calculated results reveal that the band gaps of MAPbxSn1-xI3 are sensitive to the ratio of Sn/Pb, and are proportional to the x component, consistent with the previous reports. Further investigations show that the crystal symmetry can also modify the band gap in an order of Pnma > I4cm > P4mm at x = 0.5. The random rotation of organic cations, which makes the band alignments in the compounds, facilitates the separation and transfer of holes and electrons. Interestingly, the computed binding energies of the unrelaxed exciton have the same trend as band gaps, which decreases with decreasing x, the binding energies of MAPb0.5Sn0.5I3 also decrease as the crystal symmetry decreases, implying a faster exciton dissociation with lower x and lower symmetry at an ambient temperature.

14.
ACS Appl Mater Interfaces ; 6(15): 12885-92, 2014 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-24964379

RESUMO

We present a step-by-step theoretical protocol based on the first-principles methods to reveal the insight into the origin of the high photocatalytic activity achieved by the mixed-phase TiO2, consisting of anatase and rutile. The interfacial geometries, density of states, charge densities, optical absorption spectrum, electrostatic potential, and band offsets have been calculated. The most stable mixed-phase structures have been identified, the interfacial tensile strain-dependent electronic structures have been observed, and the energy level diagram of band alignment has been given. We find that the geometrical reconstruction around the interfacial area has a negligible influence on the light absorption of the heterojunction and the interfacial sites seem not to dominantly contribute to the band-edge states. For the most stable heterojunction, the calculated valence-band maximum and conduction-band minimum of rutile, respectively, lie 0.52 and 0.22 eV above those of anatase, which agrees well with the experimental measurements and other theoretical predications. The good match of band energies to reaction requirements, large driving force for the charge immigration across the interface, and the difference of electrostatic potentials around the interface successfully explain the high photocatalytic activity achieved by the mixed-phase TiO2.

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