The high conversion efficiency has made metal halide perovskite solar cells a real breakthrough in thin film photovoltaic technology in recent years. Here, we introduce a straightforward strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium and guanidinium iodide. We use a triple cation perovskite formulation containing primarily formamidinium and small amounts of cesium and methylammonium. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We apply 2D- solid-state NMR to unravel the atomic-level mechanism of this passivation effect.
Chemical doping of
inorganic–organic hybrid perovskites
is an effective way of improving the performance and operational stability
of perovskite solar cells (PSCs). Here we use 5-ammonium valeric acid
iodide (AVAI) to chemically stabilize the structure of α-FAPbI3. Using solid-state MAS NMR, we demonstrate the atomic-level
interaction between the molecular modulator and the perovskite lattice
and propose a structural model of the stabilized three-dimensional
structure, further aided by density functional theory (DFT) calculations.
We find that one-step deposition of the perovskite in the presence
of AVAI produces highly crystalline films with large, micrometer-sized
grains and enhanced charge-carrier lifetimes, as probed by transient
absorption spectroscopy. As a result, we achieve greatly enhanced
solar cell performance for the optimized AVA-based devices with a
maximum power conversion efficiency (PCE) of 18.94%. The devices retain
90% of the initial efficiency after 300 h under continuous white light
illumination and maximum-power point-tracking measurement.
Scandium nitride ͑001͒ oriented layers have been grown on magnesium oxide ͑001͒ substrates by molecular beam epitaxy using a rf-plasma source and a scandium effusion cell. The Sc/N flux ratio is found to be critical in determining the structural, optical, and electronic properties of the grown epitaxial layers. A distinct transition occurs at the point where the Sc/N flux ratio equals 1, which defines the line between N-rich and Sc-rich growth. Under N-rich conditions, the growth is epitaxial, and the surface morphology is characterized by a densely packed array of square-shaped plateaus and four-faced pyramids with the terraces between steps being atomically smooth. The films are stoichiometric and transparent with a direct optical transition at 2.15 eV. Under Sc-rich conditions, the growth is also epitaxial, but the morphology is dominated by spiral growth mounds. The morphology change is consistent with increased surface diffusion due to a Sc-rich surface. Excess Sc leads to understoichiometric layers with N vacancies which act as donors. The increased carrier density results in an optical reflection edge at 1 eV, absorption below the 2.15 eV band gap, and a drop in electrical resistivity.
ScN͑001͒ 1 ϫ 1 surfaces have been prepared by growing ScN on MgO͑001͒ using radio frequency molecular beam epitaxy. In situ ultrahigh vacuum scanning tunneling spectroscopy indicates that the Fermi level at the surface lies slightly above the Sc 3d conduction band edge, which is attributed to a downward band bending at the surface. In situ scanning tunneling microscopy is used to image the Sc and N atom sublattices. While only one atom ͑Sc͒ appears at small negative bias, both atoms (Sc and N) appear at certain positive sample biases due to the partially ionic nature of the bonding. Charge accumulation near ionized subsurface donors is evident from the long-range topographic distortions at the surface. The combination of tunneling spectroscopy and optical absorption results show that ScN has an indirect bandgap of 0.9± 0.1 eV and a direct transition at 2.15 eV.
Structural and magnetic characterizations of Mn2CrO4 and MnCr2O4 films on MgO(001) and SrTiO3(001) substrates by molecular beam epitaxy J. Appl. Phys. 109, 07D714 (2011); 10.1063/1.3545802Structural characteristics and magnetic properties of λ-MnO 2 films grown by plasma-assisted molecular beam epitaxyThe phase and orientation of manganese nitride grown on MgO͑001͒ using molecular beam epitaxy are shown to be controllable by the manganese/nitrogen flux ratio as well as the substrate temperature. The most N-rich phase, phase ͑MnN͒, is obtained at very low Mn/N flux ratio. At increased Mn/N flux ratio, the next most N-rich phase, the phase ͑Mn 3 N 2 ͒, is obtained having its c axis normal to the surface plane. Further increasing the Mn/N flux ratio, the phase ͑Mn 3 N 2 ͒ having its c axis in the surface plane is obtained. Finally, the phase ͑Mn 4 N͒ is obtained at yet higher Mn/N flux ratio. The structural phase variation with Mn/N flux ratio is due to the kinetic control of the surface chemical composition, which determines the energetically most favorable phase. For a given Mn/N flux ratio, the phase is also found to be a function of the substrate temperature, with the less N-rich phase occurring at the higher substrate temperature. The change of phase with temperature is attributed to the change in the chemical composition resulting from the diffusion of N vacancies. Since the magnetic properties of Mn x N y depend on the phase, the Mn/N flux ratio provides a way of directly controlling the magnetic properties. A phase diagram for molecular beam epitaxial growth is presented.
Alloy formation in ScGaN is explored using rf molecular beam epitaxy over the Sc fraction range x = 0-100%. Optical and structural analysis show separate regimes of growth, namely I) wurtzite-like but having local lattice distortions in the vicinity of the Sc Ga substitutions for small x (x ≤ 0.17), II) a transitional regime for intermediate x, and III) cubic, rocksalt-like for large x (x ≥ 0.54). In regimes I and III, the direct optical transition decreases approximately linearly with increasing x but with an offset over region II. Importantly, it is found that for regime I, an anisotropic lattice expansion occurs with increasing x in which a increases much more than c. These observations support the prediction of Farrer and Bellaiche [Phys. Rev. B 66, 201203-1 (2002)] of a metastable layered hexagonal phase of ScN, denoted h-ScN.
Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].
High quality scandium nitride films have been grown on magnesium oxide ͑001͒ substrates by molecular beam epitaxy using a rf plasma source for nitrogen. Both reflection high energy electron diffraction and x-ray diffraction confirm that these films have ͑001͒-orientation. Atomic force microscopy reveals a surface morphology consisting of large plateaus and pyramids. The plateaus are found to be atomically smooth and have a 1ϫ1 surface structure, as revealed by in situ scanning tunneling microscopy.
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