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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 CO 2 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 CO 2 reuse. By a simple electrochemical transfer method, waste bronzes with composition Cu 14 Sn were anodically dissolved and cathodically redeposited under dynamic hydrogen bubble template conditions to yield mesoporous foams with Cu 10 Sn surface composition. The bimetal foam electrodes exhibited high CO 2 electroreduction selectivity toward CO, achieving greater than 85% faradaic efficiency accompanied by a considerable suppression of the competing H 2 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.

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (Δ V oc,nrad ) because of the reported decomposition reactions, an uncontrolled growth of MoSe 2 , or a nonoptimal electrical contact with high recombination. Several intermediate layers (IL), such as MoO 3 , 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 Δ V oc,nrad . In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures (SLG, SLG/Mo, SLG/Al 2 O 3 , and SLG/Mo/Al 2 O 3 ) with a Na-doped kesterite absorber optimized for a device efficiency >10%. The IL of choice is Al 2 O 3 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 Δ V oc,nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al 2 O 3 exhibits higher nonradiative losses up to 350 mV. The anticipated field-effect passivation from Al 2 O 3 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 MoSe 2 remains a suitable back contact for high-efficiency kesterite devices.

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)Se 2 (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.

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.