Electrocatalyst Derived from Waste Cu-Sn Bronze for CO2 Conversion into CO.

To sustainably exist within planetary boundaries, we must greatly curtail our extraction of fuels and materials from the Earth. This requires new technologies based on reuse and repurposing of material already available. Electrochemical conversion of CO2 into valuable chemicals and fuels is a promising alternative to deriving them from fossil fuels. But most metals used for electrocatalysis are either endangered or at serious risk of limitation to their future supply. Here, we demonstrate a combined strategy for repurposing of a waste industrial Cu-Sn bronze as a catalyst material precursor and its application toward CO2 reuse. By a simple electrochemical transfer method, waste bronzes with composition Cu14Sn were anodically dissolved and cathodically redeposited under dynamic hydrogen bubble template conditions to yield mesoporous foams with Cu10Sn surface composition. The bimetal foam electrodes exhibited high CO2 electroreduction selectivity toward CO, achieving greater than 85% faradaic efficiency accompanied by a considerable suppression of the competing H2 evolution reaction. The Cu-Sn foam electrodes showed good durability over several hours of continuous electrolysis without any significant change in the composition, morphology, and selectivity for CO as a target product.

Influence of the Rear Interface on Composition and Photoluminescence Yield of CZTSSe Absorbers: A Case for an Al2O3 Intermediate Layer.

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (ΔVoc,nrad) because of the reported decomposition reactions, an uncontrolled growth of MoSe2, or a nonoptimal electrical contact with high recombination. Several intermediate layers (IL), such as MoO3, TiN, and ZnO, have been tested to mitigate these issues, and efficiency improvements have been reported. However, the introduction of IL also triggers other effects such as changes in alkali diffusion, altered morphology, and modifications in the absorber composition, all factors that can also influence ΔVoc,nrad. In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures (SLG, SLG/Mo, SLG/Al2O3, and SLG/Mo/Al2O3) with a Na-doped kesterite absorber optimized for a device efficiency >10%. The IL of choice is Al2O3 because of its reported beneficial effect to reduce the surface recombination velocity at the rear interface of solar cell absorbers. Identical annealing conditions and alkali distribution in the kesterite absorber are preserved, as measured by time-of-flight secondary ion mass spectrometry and energy-dispersive X-ray spectroscopy. The lowest ΔVoc,nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al2O3 exhibits higher nonradiative losses up to 350 mV. The anticipated field-effect passivation from Al2O3 at the rear interface could not be observed for the kesterite absorbers prepared by the two-step process, further confirmed by an additional experiment with air annealing. Our results suggest that Mo with an in situ formed MoSe2 remains a suitable back contact for high-efficiency kesterite devices.

BaZrS3 Chalcogenide Perovskite Thin Films by H2S Sulfurization of Oxide Precursors.

The earth-abundant ternary compound BaZrS3, which crystallizes in the perovskite-type structure, has come into view as a promising candidate for photovoltaic applications. We present the synthesis and characterization of polycrystalline perovskite-type BaZrS3 thin films. BaZrO3 precursor layers were deposited by pulsed laser deposition and sulfurized at various temperatures in an argon-diluted H2S atmosphere. We observe increasing incorporation of sulfur for higher annealing temperatures, accompanied by a red shift of the absorption edge, with a bandgap of Eg = 1.99 eV and a large absorption strength >105 cm-1 obtained for sulfurization temperatures of 1000 °C. X-ray diffraction analysis and SEM indicate enhanced crystallization at the higher annealing temperatures, but no evidence for a crystalline solid solution between the BaZrO3 and BaZrS3 phases is found. The charge carrier sum mobility estimated from optical-pump-terahertz-probe spectroscopy indicates increasing mobilities with increasing sulfurization temperature, reaching maximum values of up to ∼2 cm2 V-1 s-1.

Electronic Structure of the CdS/Cu(In,Ga)Se2 Interface of KF- and RbF-Treated Samples by Kelvin Probe and Photoelectron Yield Spectroscopy.

Ambient-pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments (KF-PDT, RbF-PDT) on the electronic features of Cu(In,Ga)Se2 (CIGSe) thin films and the CdS/CIGSe interface in a CdS thickness series that has been sequentially prepared during the chemical bath deposition (CBD) process depending on the deposition time. We observe distinct features correlated to the CBD-CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum (VBM) of the CIGSe surface is significantly shifted (by 180-620 mV) toward the Fermi level. However, VBM positions at the surface of the CIGSe are still much below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of postdeposition treatment, showing values between 1.46 and 1.58 eV, characteristic for a copper-poor CIGSe surface composition. At the CdS/CIGSe interface, the lowest VBM discontinuity is observed for the RbF-PDT sample. At this interface, a thin layer with a graded band gap is found. We also find that K and Rb act as compensating acceptors in the CdS layer. Detailed energy band diagrams of the CdS/CIGSe heterostructures are proposed.

Water Adsorption Enhances Electrical Conductivity in Transparent P-Type CuI.

CuI has been recently rediscovered as a p-type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a 2-fold increase in the conductivity of CuI after exposure to ambient humidity for 5 h, followed by slight long-term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is maximized at intermediate humidity levels. On the basis of the large intragrain mobility measured by THz spectroscopy, we suggest that hydration of grain boundaries may be beneficial for the overall hole mobility.

Crystal Chemistry, Optical-Electronic Properties, and Electronic Structure of Cd1- xIn2+2 x/3S4 Compounds (0 ≤ x ≤ 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells.

CdIn2S4 and In2S3 compounds were both previously studied as buffer layers in CIGS-based thin-film solar cells, each of them exhibiting advantages and disadvantages. Thus, we naturally embarked on the study of the CdIn2S4-In2S3 system, and a series of Cd1- xIn2+2 x/3S4 (0 ≤ x ≤ 1) materials were prepared and characterized. Our results show that two solid solutions exist. The aliovalent substitution of cadmium(II) by indium(III) induces a structural transition at x ≈ 0.7 from cubic spinel Fd3̅ m to tetragonal spinel I41/ amd that is related to an ordering of cadmium vacancies. Despite this transition, the variation of optical gap is continuous and decreases from 2.34 to 2.11 eV going from CdIn2S4 to In2S3 while all compounds retain an n-type behavior. In contrast with the Al xIn2-xS3 solid solution, no saturation of the gap is observed. Moreover, XPS analyses indicate a difference between surface and volume composition of the grains for Cd-poor compounds. The use of Cd1- xIn2+2 x/3S4 compounds could be a good alternative to CdIn2S4 and In2S3 to improve CIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.

The stability domain of the selenide kesterite photovoltaic materials and NMR investigation of the Cu/Zn disorder in Cu2ZnSnSe4 (CZTSe).

Bulk compounds, prepared via the ceramic route, related to Cu2ZnSnSe4 (CZTSe), a material considered for use in photovoltaic devices, were investigated using NMR spectroscopy, electron-probe microanalyses and X-ray diffraction. These materials adopt the kesterite structure regardless of the Cu and Zn contents. It is also shown that the stability domain of the copper-poor quaternary phases is wider for selenide derivatives than for sulphides. Finally, the Cu/Zn disorder level in CZTSe is found to be higher when the samples are quenched, which is reminiscent of the behaviour of the parent sulphide compounds CZTS.

Solid-state NMR and Raman spectroscopy to address the local structure of defects and the tricky issue of the Cu/Zn disorder in Cu-poor, Zn-rich CZTS materials.

The material Cu2ZnSn(S,Se)4 (CZTS) offers a promising indium-free alternative to Cu(In,Ga)Se2 for the absorber layer in thin-film solar cells. It is known that the highest solar energy conversion efficiencies are reached for Cu-poor, Zn-rich CZTS compositions and that too much disorder at the Cu and Zn sites can have a negative impact on the device performance. In this article, we investigate the structures of [VCu + ZnCu] A-type and [2ZnCu + ZnSn] B-type defect complexes and their impact on the long-range Cu/Zn disorder. To that end, we use (119)Sn, (65)Cu, and (67)Zn solid-state NMR and Raman spectroscopy to characterize powdered CZTS samples. For both A- and B-type substitutions, our NMR investigations demonstrate the clustering of the complexes. Moreover, we show that (A+B)-type compounds should be considered as A-type and B-type compounds, since no interaction seems to exist between [VCu + ZnCu] and [2ZnCu + ZnSn] defect complexes. In addition, it is worth noting that [2ZnCu + ZnSn] complexes have only a minor impact on the level of disorder at the Cu and Zn sites. In contrast, [VCu + ZnCu] complexes seem to restrain the random distribution of Cu at the Zn site and of Zn at the Cu site; i.e., the long-range Cu/Zn disorder. Raman characterization of the CZTS samples was also conducted. The Q = I287/I303 and the newly introduced Q' = I338/(I366 + I374) ratios determined from Raman spectra collected at 785 nm turn out to be very sensitive to the level of Cu/Zn disorder. Moreover, they can be used to differentiate the nature of the substitution in slow-cooled materials.

X-ray resonant single-crystal diffraction technique, a powerful tool to investigate the kesterite structure of the photovoltaic Cu2ZnSnS4 compound.

Cu/Zn disorder in the kesterite Cu2ZnSnS4 derivatives used for thin film based solar cells is an important issue for photovoltaic performances. Unfortunately, Cu and Zn cannot be distinguished by conventional laboratory X-ray diffraction. This paper reports on a resonant diffraction investigation of a Cu2ZnSnS4 single crystal from a quenched powdered sample. The full disorder of Cu and Zn in the z = 1/4 atomic plane is shown. The structure, namely disordered kesterite, is then described in the I42m space group.

Multinuclear (67Zn, 119Sn and 65Cu) NMR spectroscopy--an ideal technique to probe the cationic ordering in Cu2ZnSnS4 photovoltaic materials.

For the very first time, (67)Zn, (119)Sn and (65)Cu NMR investigations have been carried out on Cu2ZnSnS4 derivatives (CZTS) for photovoltaic applications. NMR spectroscopy is shown to be sensitive enough to probe the Cu/Zn disorder within the kesterite structure of the studied compounds. In addition, reference spectra of Cu2ZnSnS4 are provided, and experimental (67)Zn and (65)Cu parameters are compared with ab initio calculations.