An important property of hybrid layered perovskite is the possibility to reduce its dimensionality to provide wider band gap and better stability. In this work, 2D perovskite of the structure ...(PEA)2(MA)n–1PbnBr3n+1 has been sensitized, where PEA is phenyl ethyl‐ammonium, MA is methyl‐ammonium, and using only bromide as the halide. The number of the perovskite layers has been varied (n) from n = 1 through n = ∞. Optical and physical characterization verify the layered structure and the increase in the band gap. The photovoltaic performance shows higher open circuit voltage (Voc) for the quasi 2D perovskite (i.e., n = 40, 50, 60) compared to the 3D perovskite. Voc of 1.3 V without hole transport material (HTM) and Voc of 1.46 V using HTM have been demonstrated, with corresponding efficiency of 6.3% and 8.5%, among the highest reported. The lower mobility and transport in the quasi 2D perovskites have been proved effective to gain high Voc with high efficiency, further supported by ab initio calculations and charge extraction measurements. Bromide is the only halide used in these quasi 2D perovskites, as mixing halides have recently revealed instability of the perovskite structure. These quasi 2D materials are promising candidates for use in optoelectronic applications that simultaneously require high voltage and high efficiency.
A quasi 2D perovskite materials based on bromide, having low exciton binding energy, high absorbance coefficient, and high stability is presented. These materials provide high open circuit voltage and high power conversion efficiency in perovskite solar cells with and without hole transport material.
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•Judgement of the interface stability prior to fabrication.•Simulated XANES curves show a fingerprint of the interface quality.•Bands alignment and charge self-doping of the ...perovskite junctions with GaN and ZnO 0001 and 000-1 surfaces.
Structural degradation is a main problem for the commercialization of the organic lead halide perovskite solar cells. A role of the interfaces with the transparent electrodes, in this process, could be made clear by the combined ab initio calculations and the X-ray absorption measurements. The junctions of MAPbI3(MA = CH3NH3) with pure GaN and ZnO crystals, oriented with the polar 0 0 0 1 and 0 0 0 −1 surfaces, are theoretically investigated by means of the density functional theory (DFT) and simulations of the X-ray absorption near-edge spectroscopy (XANES). Unwanted decomposition of the perovskite, into the layers which are parallel to the interface, coexists with a strong binding of the halide atom at GaN 0 0 0 1 and ZnO 0 0 0 1. In contrast, at the interfaces with the 0 0 0 −1-oriented GaN and ZnO, aforementioned segmentation of the perovskite does not occur, because binding of the halide atom is much weaker. Thus, the adsorption properties of the halide atom at semiconductor surface might be a precondition for a choice of the substrate to grow the stable perovskite film. The I-core K-edge XANES spectra at the interface atoms contain a fingerprint of the iodide adsorption strength.
Abstract
Transition metal dichalcogenide MoS
2
is a two-dimensional material, attracting much attention for next-generation applications thanks to rich functionalities stemming from its crystal ...structure. Many experimental and theoretical works have focused on the spin-orbit interaction which couples the valley and spin degrees of freedom so that the spin-states can be electrically controllable. However, the spin-states of charge carriers and atomic vacancies in devices have not been yet elucidated directly from a microscopic viewpoint. Here, we report the spin-states in thin-film transistors using operando electron spin resonance spectroscopy. We have observed clearly different electron spin resonance signals of the conduction electrons and atomic vacancies, and distinguished the corresponding spin-states from the signals and theoretical calculations, evaluating the gate-voltage dependence and the spin-susceptibility and
g
-factor temperature dependence. This analysis gives deep insight into the MoS
2
magnetism and clearly indicates different spin-scattering mechanisms compared to graphene, which will be useful for improvements of the device characteristics and new applications.
The gapless edge states have been found in a 2D molecular system built with light atoms: C,O,H. This prediction is done on the basis of combined density functional theory (DFT) and tight-binding ...calculations. The system does not exhibit any effect of the spin-orbit coupling (SOC), neither intrinsic nor Rashba type. The band structure and the edge states are tuned with a strength of the p-stacking and O...H interactions. The elementary cell of this noncovalent structure, does not have the 3D inversion or rotational symmetry. Instead, the system transforms via a superposition of two reflections: with respect to the xz and xy mirror planes, both containing the non-periodic direction. This superposition is equivalent to the inversion in the 2D subspace, in which the system is periodic. The energy gap obtained with the DFT method is 0.11 eV, and largely opens (above 1 eV) with the GW and hybrid-DFT approaches. The bands inversion is partial, i.e. the Bloch states are mixed, with the "swapping" and "non-swapping" atomic contributions.
MAPbI3 (MA = methylammonium, CH3NH3) perovskite solar cells are efficient but not stable. Some reasons for this fact originate from the interface effects, unsaturated bonds and defects. Four types of ...defects are studied: (i) dehydrogenated MA (CH3NH2), (ii) MA vacancies, (iii) halide vacancies, and (iv) Zn or Ga vacancies. X-ray absorption near-edge spectroscopy (XANES) is a sensitive tool for interface characterization when it is combined with computationally simulated spectra for specific atomic structures. This work shows how the simulated XANES of the MA N-core K-edge of MAPbI3 differ for various defected interfaces with the ZnO and GaN wurtzite crystals. Two polar orientations, (0001) and (000−1), of the semiconductors are chosen, and both surfaces are terminated with the metal atoms. Each defect leaves behind its unique fingerprint in their XANES. The most pronounced change in the spectrum is caused by the deprotonated MA, which is a product of the perovskite degradation due to moisture.
Lead halide perovskites (LHPs) with the formula ABX3 are very efficient materials for optoelectronic devices: solar cells, light-emitting diodes, lasers, photodetectors, γ-detectors, and field effect ...transistors. But they are very fragile and sensitive to humidity, temperature, and operating conditions such as voltage bias and light. The reason is their weakly ionic structure with twice lower atomic ionicities than those in oxides. Photostability is difficult to achieve in LHP, due to the nature of electronic states that originate mainly from the halide anions in the valence band and B cations in the conduction band; hence, the charge transfer lowers the ionicity at the B–X bond. To date, the A cations used have been optically inactive and only served as one-electron donors for the charge balanced BX3 frame. We propose a mechanism of photoexcitation acting between two different A cations, namely SLi3 and SH3, while the inorganic frame remains unchanged under illumination preventing B–X bond breakage and halide migration. Demand for a specific crystal localization of the optical excitations is a new factor for achieving photostability, in addition to a control of the ionic radii, defects, morphology, and surface and interface stabilization.
We theoretically investigate lead iodide perovskites of general formula APbI
3
for a series of metallic cations (namely Cs
+
, Rb
+
, K
+
, Na
+
and Li
+
) by means of density functional theory, the ...GW method and the Bethe-Salpeter equation including spin-orbit coupling. We demonstrate that the low-energy edges (up to 1.3 eV) of the absorption spectra are dominated by weakly bound excitons, with binding energies
E
b
of ∼ 30-80 meV, and the corresponding intensities increase as metallic cations become lighter. The middle parts of the spectra (1.8-2.4 eV), on the other hand, contain optical dipole transitions comprising more confined excitons (
E
b
∼ 150-200 meV) located at PbI
3
. These parts of the spectra correspond to the optical-gain wavelengths which are experimentally achieved in optically pumped perovskite lasers. Finally, the higher energy parts, from about 2.8 eV (LiPbI
3
) to 4.3 eV (CsPbI
3
), contain optical transitions with very confined excitons (
E
b
∼ 220-290 meV) located at halide atoms and the empty states of the metallic cations.
In lead halide perovskites, the role of inorganic A cations is leading in the middle part of the absorption spectrum where the excitonic binding energies of dipole transitions are much stronger than those involving PbI
3
bands only.
Halide perovskites are widely used as components of electronic and optoelectronic devices such as solar cells, light-emitting diodes (LEDs), optically pumped lasers, field-effect transistors, ...photodetectors, and γ-detectors. Despite this wide range of applications, the construction of an electrically pumped perovskite laser remains challenging. In this paper, we numerically justify that mixing two perovskite compounds with different halide elements can lead to optical properties suitable for electrical pumping. As a reference, the chosen model material was CsPbBr3, whose performance as a part of lasers has been widely recognised, with some Br atoms substituted by I at specific sites. In particular, a strong enhancement of the low-energy absorption peaks has been obtained using the
Bethe-Salpeter equation. Based on these results, we propose specific architectures of ordered doping that could be realised by epitaxial growth. Efficient light emission from the bottom of the conduction band is expected.
Full inorganic perovskites display their potential to function as stable photovoltaic materials better than the hybrid organic–inorganic perovskites. However, to date, the cesium lead iodide ...perovskite, which displays a promising absorbance range, has suffered from low stability, which degrades to a nonactive photovoltaic phase rapidly. In this work, we show that the black phase of cesium lead iodide can be stabilized when the perovskite dimensionality is reduced. X-ray diffraction, absorbance, and scanning electron microscopy were used to follow the degradation process of various dimensionalities under room conditions and 1 sun illumination. When comparing the effect on the stability and photostability of cesium lead iodide with linear or aromatic barrier molecules, the aromatic barrier molecule displays better photostability for over 700 h without degradation under continuous 1 sun illumination than does the linear barrier molecule. Theoretical calculations show that the addition of the barrier molecule makes a different charge distribution over the perovskite structure, which stabilizes the CsPbI3 black phase. This work provides the possibility to use the CsPbI3 perovskite as a stable photovoltaic material in solar cells.
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