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1.
Nanoscale ; 14(26): 9485-9497, 2022 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-35748506

RESUMO

Chemical vapor deposition (CVD) has been established as a versatile route for the large-scale synthesis of transition metal dichalcogenides, such as tungsten disulfide (WS2). Yet, the precursor composition's role on the CVD process remains largely unknown and remains to be explored. Here, we employ Pulsed Laser Deposition (PLD) in a two-stage approach to tune the oxygen content in the tungsten oxide (WO3-x) precursors and demonstrate the presence of oxygen vacancies in the oxide films leads to a more facile conversion from WO3-x to WS2. Using a joint study based on ab initio density functional theory (DFT) calculations and experimental observations, we unravel that the oxygen vacancies in WO3-x can serve as niches through which sulfur atoms enter the lattice and facilitate an efficient conversion into WS2 crystals. By solely modulating the precursor stoichiometry, the photoluminescence emission of WS2 crystals can be significantly enhanced. Atomic resolution scanning transmission electron microscopy imaging (STEM) reveals that tungsten vacancies are the dominant intrinsic defects in mono- and bilayers WS2. Moreover, bi- and multilayer WS2 crystals derived from oxides with a high V0 content exhibit dominant AA'/AB or AA(A…) stacking orientations. The atomic resolution images reveal local strain buildup in bilayer WS2 due to competing effects of complex grain boundaries. Our study provides means to tune the precursor composition to control the lateral growth of TMDs while revealing insights into the different pathways for forming grain boundaries in bilayer WS2.

2.
ACS Appl Mater Interfaces ; 14(12): 14342-14358, 2022 Mar 30.
Artigo em Inglês | MEDLINE | ID: mdl-35297597

RESUMO

Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as the bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high-temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe), and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 °C, we show that increasing the heavily doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by 1 order of magnitude, with final lifetimes above 500 µs uniformly across areas up to 20 cm2. In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.

3.
Sci Rep ; 10(1): 20749, 2020 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-33247169

RESUMO

In this paper, we study the DMSO/thiourea/chloride salt system for synthesis of pure-sulfide [Formula: see text] (CZTS) thin-film solar cells under ambient conditions. We map out the ink constituents and determine the effect of mixing time and filtering. The thermal behavior of the ink is analyzed, and we find that more than 90% of the solvent has evaporated at [Formula: see text]. However, chloride and sulfoxide species are released continually until [Formula: see text], suggesting the advantage of a higher pre-annealing temperature, which is also commonly observed in the spin-coating routines in literature. Another advantage of a higher pre-annealing temperature is that the worm-like pattern in the spin-coated film can be avoided. We hypothesize that this pattern forms as a result of hydrodynamics within the film as it dries, and it causes micro-inhomogeneities in film morphology. Devices were completed in order to finally evaluate the effect of varying thermal exposure during pre-annealing. Contrary to the previous observations, a lower pre-annealing temperature of [Formula: see text] results in the best device efficiency of 4.65%, which to the best of our knowledge is the highest efficiency obtained for a pure-sulfide kesterite made with DMSO. Lower thermal exposure during pre-annealing results in larger grains and a thicker [Formula: see text] layer at the CZTS/Mo interface. Devices completed at higher pre-annealing temperatures display the existence of either a Cu-S secondary phase or an incomplete sulfurization with smaller grains and a fine-grain layer at the back interface.

4.
Sci Rep ; 10(1): 18388, 2020 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-33110088

RESUMO

Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its alloys plays a crucial role in determining the efficiency of the solar cells. Whereas Ag alloying of CZTS has been shown to reduce anti-site defects in the bulk and thus rise the efficiency, the electronic properties at the interface with the CdS buffer layer have not been extensively investigated. In this work, we present a detailed study on the band alignment between n-CdS and p-CZTS upon Ag alloying by depth-profiling ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Our findings indicate that core-level peaks and the valence band edge of CdS exhibit a significant shift to a lower energy (larger than 0.4 eV) upon the etching of the CdS layer, which can be assigned due to band bending and chemical shift induced by a change in the chemical composition across the interface. Using a simplified model based on charge depletion layer conservation, a significantly larger total charge region depletion width was determined in Ag-alloyed CZTS as compared to its undoped counterpart. Our findings reveal a cliff-like band alignment at both CdS/CZTS and CdS/Ag-CZTS heterointerfaces. However, the conduction-band offset decreases by more than 0.1 eV upon Ag alloying of CZTS. The approach demonstrated here enables nanometer-scale depth profiling of the electronic structure of the p-n junction and can be universally applied to study entirely new platforms of oxide/chalcogenide heterostructures for next-generation optoelectronic devices.

5.
ACS Appl Mater Interfaces ; 12(35): 39405-39424, 2020 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-32805807

RESUMO

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered - or double-layered - CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics, and its effect on the performances of devices. In this work, 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 large statistical data set of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 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 well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence and find that the double layer limits the efficiency of our devices to below 7%. 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. Finally, we comment on the generality of the mechanism proposed by 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. We find that 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. We suggest that understanding the limitations imposed by the double-layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

6.
ACS Omega ; 5(18): 10501-10509, 2020 May 12.
Artigo em Inglês | MEDLINE | ID: mdl-32426607

RESUMO

In the last decade, Cu2ZnSnS4 (CZTS) has been a promising earth-abundant, nontoxic candidate material for absorption layers within thin-film solar cells. One major issue preventing this type of solar cells from achieving competitive efficiency is impurity phases and structural defects in the bulk of the absorber; as a four-element compound, the formation of CZTS is highly sensitive to synthesis conditions. The impurity phases and defects differ by the fabrication method, and thus experimental characterization is vital for the successful development of CZTS photovoltaics. In this work, we characterize CZTS nanoparticles obtained by the hot-injection method and a standard N2/S annealing procedure. Phase-pure kesterite CZTS samples in the desired compositional range were characterized by standard means, i.e., Raman spectroscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. However, using synchrotron X-ray diffraction with Rietveld refinement, we show that the as-synthesized nanoparticles consist of a mixture of the tetragonal and the fully disordered cubic sphalerite phase and transform into the tetragonal structure after heat treatment. Sn vacancies are seen in the annealed samples. X-ray total scattering with pair distribution function analysis furthermore suggests the presence of a nanostructured CZTS phase along with a bulk material. Finally, this study compares the benefits of applying synchrotron radiation instead of a standard laboratory X-ray diffraction when characterizing highly complex materials.

7.
RSC Adv ; 8(13): 7152-7158, 2018 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-35540321

RESUMO

The effect of adding LiCl, NaCl, and KCl to Cu2ZnSnS4 (CZTS) nanoparticle thin-film samples annealed in a nitrogen and sulfur atmosphere is reported. We demonstrate that the organic ligand-free nanoparticles previously developed can be used to produce an absorber layer of high quality. The films were Zn-rich and Cu-poor, and no secondary phases except ZnS could be detected within the detection limit of the characterization tools used. Potassium was the most effective alkali metal to enhance grain growth, and resulted in films with a high photoluminescence signal and an optical band gap of 1.43 eV. The alkali metals were introduced in the form of chloride salts, and a significant amount of Cl was detected in the final films, but could be removed in a quick water rinse.

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