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Register a new userUniversal rules for visible-light absorption in hybrid perovskite materials
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A variety of organic-inorganic hybrid perovskites (APbX3) consisting of mixed center cations [A = CH3NH3+, HC(NH2)2+, Cs+] with different PbX3- cages (X = I, Br, Cl) have been developed to realize high-efficiency solar cells. Nevertheless, clear understanding for the effects of A and X on the optical transition has been lacking. Here, we present universal rules that allow the unified interpretation of the optical absorption in various hybrid perovskites. In particular, we find that the influence of the A-site cation on the light absorption is rather significant and the absorption coefficient (alpha) reduces to half when CH3NH3+ is replaced with HC(NH2)2+ in the APbI3 system. Our density functional theory (DFT) calculations reproduce all of the fine absorption features observed in HC(NH2)2PbI3 and CH3NH3PbBr3, allowing the unique assignment of the interband transitions in the Brillouin zone. In contrast to general understanding that the A-site cation involves weakly in the optical process, our theoretical calculations reveal that the center cation plays a critical role in the interband transition and the absorption strength in the visible region is modified by the strong A-X interaction. Furthermore, our systematic analyses show that the variation of the absorption spectrum with X can be described simply by the well-known sum rule. The universal rules established in this study explain the large reduction of alpha in HC(NH2)2PbI3 and predict CsPbI3 as the highest alpha material.
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ZnO/GaN alloys exhibit exceptional photocatalyst applications owing to the flexibly tunable band gaps that cover a wide range of the solar spectrum, and thus have attracted extensive attentions over the past few years. In this study, first-principles calculations were employed to investigate structural stabilities and electronic properties of (1-100) and (11-20) ZnO/GaN heterostructured nanofilms. The effects of nanofilm thickness and GaN ratio were explored. It was found that all studied heterostructured nanofilms were less stable than the corresponding pure ZnO film but more stable than pure GaN one, exhibiting a much thicker film with better stability. Electronic band structures displayed that both two types of (1-100) and (11-20) heterostructured nanofilms were semiconductors with band gaps strongly depending on the GaN ratios as well as the thicknesses. Of particular interesting is that the band gaps decreased firstly, and then increased with the increasing GaN ratio. Furthermore, electronic contribution to the valence band maximum and the conduction band minimum, and optical absorption were discussed. Our results of ZnO/GaN heterostructured nanofilms with spatial separation of electrons and holes, and flexibly tunable band gaps hold great promise for applications in visible-photovoltaic field.
In materials science and engineering, one is typically searching for materials that exhibit exceptional performance for a certain function, and the number of these materials is extremely small. Thus, statistically speaking, we are interested in the identification of *rare phenomena*, and the scientific discovery typically resembles the proverbial hunt for the needle in a haystack.
Semiconductor compounds are widely used for water splitting applications, where photo-generated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr$_{1-x}$NbO$_3$, 0.03 < x < 0.20) have shown competitive photocatalytic efficiency, opening up the material space available for finding optimizing performance in water-splitting applications. The origin of the visible light absorption in these powders was reported to be due to an interband transition and the charge carrier separation was proposed to be due to the high carrier mobility of this material. In the current work we have prepared epitaxial thin films of Sr$_{0.94}$NbO$_{3+{delta}}$ and found that the bandgap of this material is ~4.1 eV, which is very large. Surprisingly the carrier density of the conducting phase reaches 10$^{22}$ cm$^{-3}$, which is only one order smaller than that of elemental metals and the carrier mobility is only 2.47 cm$^2$/(V$cdot$s). Contrary to earlier reports, the visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon resonance, arising from the large carrier density, instead of an interband transition. Excitation of the plasmonic resonance results in a multifold enhancement of the lifetime of charge carriers. Thus we propose that the hot charge carriers generated from decay of plasmons produced by optical absorption is responsible for the water splitting efficiency of this material under visible light irradiation.
Tail state formation in solar cell absorbers leads to a detrimental effect on solar cell performance. Nevertheless, the characterization of the band tailing in experimental semiconductor crystals is generally difficult. In this article, to determine the tail state generation in various solar cell materials, we have developed a quite general theoretical scheme in which the experimental Urbach energy is compared with the absorption edge energy derived from density functional theory (DFT) calculation. For this purpose, the absorption spectra of solar cell materials, including CdTe, CuInSe2 (CISe), CuGaSe2 (CGSe), Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS) and hybrid perovskites, have been calculated by DFT particularly using very-high-density k meshes. As a result, we find that the tail state formation is negligible in CdTe, CISe, CGSe and hybrid perovskite polycrystals. However, coevaporated CZTSe and CZTS layers exhibit very large Urbach energies, which are far larger than the theoretical counterparts. Based on DFT analysis results, we conclude that the quite large tail state formation observed in the CZTSe and CZTS originates from extensive cation disordering. In particular, even a slight cation substitution is found to generate unusual band fluctuation in CZT(S)Se. In contrast, CH3NH3PbI3 hybrid perovskite shows the sharpest absorption edge theoretically, which agrees with experiment.
We derive a dielectric-dependent hybrid functional which accurately describes the electronic properties of heterogeneous interfaces and surfaces, as well as those of three- and two-dimensional bulk solids. The functional, which does not contain any adjustable parameter, is a generalization of self-consistent hybrid functionals introduced for homogeneous solids, where the screened Coulomb interaction is defined using a spatially varying, local dielectric function. The latter is determined self-consistently using density functional calculations in finite electric fields. We present results for the band gaps and dielectric constants of 3D and 2D bulk materials, and band offsets for interfaces, showing an accuracy comparable to that of GW calculations.
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