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Register a new userGiant oscillator strength in the band-edge light absorption of zincblende, chalcopyrite and kesterite solar cell materials
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In semiconducting solar-cell absorbers, high absorption coefficient (alpha) near the band-edge region is critical to maximize the photocurrent generation and collection. Nevertheless, despite the importance of the band-edge absorption characteristics, the quantitative analysis of the band-edge optical transitions has not been performed. In this study, we have implemented systematic density functional theory (DFT) calculation, focusing on the band-edge oscillator strength of seven practical solar cell absorbers (GaAs, InP, CdTe, CuInSe2, CuGaSe2, Cu2ZnSnSe4, and Cu2ZnSnS4) with zincblende, chalcopyrite and kesterite structures. We find that all these crystals exhibit the giant oscillator strength near the band gap region, revealing the fact that alpha in the band gap region is enhanced significantly by the anomalous high oscillator strength. In high-energy optical transitions, however, the oscillator strength reduces sharply and the absorption properties are determined primarily by the joint density-of-state contribution. Based on DFT results, we show that the giant oscillator strength in the band edge region originates from a unique tetrahedral-bonding structure, with a negligible effect of constituent atoms.
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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.
Exciton problem is solved in the two-dimensional Dirac model with allowance for strong electron-hole attraction. The exciton binding energy is assumed smaller than but comparable to the band gap. The exciton wavefunction is found in the momentum space as a superposition of all four two-particle states including electron and hole states with both positive and negative energies. The matrix element of exciton generation is shown to depend on the additional components of the exciton wavefunction. Both the Coulomb and the Rytova-Keldysh potentials are considered. The dependence of the binding energy on the coupling constant is analyzed for the ground and first excited exciton states. The binding energy and the oscillator strength are studied as functions of the environmental-dependent dielectric constant for real transition metal dichalcogenide monolayers. We demonstrate that the multicomponent nature of the exciton wavefunction is crucial for description of resonant optical properties of two-dimensional Dirac systems.
Solar-energy plays an important role in solving serious environmental problems and meeting high-energy demand. However, the lack of suitable materials hinders further progress of this technology. Here, we present the largest inorganic solar-cell material search to date using density functional theory (DFT) and machine-learning approaches. We calculated the spectroscopic limited maximum efficiency (SLME) using Tran-Blaha modified Becke-Johnson potential for 5097 non-metallic materials and identified 1997 candidates with an SLME higher than 10%, including 934 candidates with suitable convex-hull stability and effective carrier mass. Screening for 2D-layered cases, we found 58 potential materials and performed G0W0 calculations on a subset to estimate the prediction-uncertainty. As the above DFT methods are still computationally expensive, we developed a high accuracy machine learning model to pre-screen efficient materials and applied it to over a million materials. Our results provide a general framework and universal strategy for the design of high-efficiency solar cell materials. The data and tools are publicly distributed at: https://www.ctcms.nist.gov/~knc6/JVASP.html, https://www.ctcms.nist.gov/jarvisml/, https://jarvis.nist.gov/ and https://github.com/usnistgov/jarvis .
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.
In kesterite CZTSSe solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered or double-layered absorber. So far, only segregated pieces of research exist to characterize this double layer, its formation dynamics and its effect on the performance of devices. Here, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the known case of ultrathin CIGS solar cells. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. We comment on the generality of the mechanism proposed, comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide and oxide-based targets. Although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix.
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