Electrochemical processes for ammonia synthesis could potentially replace the high temperature and pressure conditions of the Haber-Bosch process, with voltage offering a pathway to distributed fertilizer production that leverages the rapidly decreasing cost of renewable electricity. However, nitrogen is an unreactive molecule and the hydrogen evolution reaction presents a major selectivity challenge. An electrode of electrodeposited lithium in tetrahydrofuran solvent overcomes both problems by providing a surface that easily reacts with nitrogen and by limiting the access of protons with a nonaqueous electrolyte. Under these conditions, we measure relatively high faradaic efficiencies (ca. 10 %) and rates (0.1 mA cm À 2 ) toward NH 3 . We observe the development of a solid electrolyte interface layer as well as the accumulation of lithium and lithium-containing species. Detailed DFT studies suggest lithium nitride and hydride to be catalytically active phases given their thermodynamic and kinetic stability relative to metallic lithium under reaction conditions and the fast diffusion of nitrogen in lithium.[a] Dr.
Halide
perovskite/crystalline silicon (c-Si) tandem solar cells
promise power conversion efficiencies beyond the limits of single-junction
cells. However, the local light-matter interactions of the perovskite
material embedded in this pyramidal multijunction configuration, and
the effect on device performance, are not well understood. Here, we
characterize the microscale optoelectronic properties of the perovskite
semiconductor deposited on different c-Si texturing schemes. We find
a strong spatial and spectral dependence of the photoluminescence
(PL) on the geometrical surface constructs, which dominates the underlying
grain-to-grain PL variation found in halide perovskite films. The
PL response is dependent upon the texturing design, with larger pyramids
inducing distinct PL spectra for valleys and pyramids, an effect which
is mitigated with small pyramids. Further, optimized quasi-Fermi level
splittings and PL quantum efficiencies occur when the c-Si large pyramids
have had a secondary smoothing etch. Our results suggest that a holistic
optimization of the texturing is required to maximize light in- and
out-coupling of both absorber layers and there is a fine balance between
the optimal geometrical configuration and optoelectronic performance
that will guide future device designs.
Replacing organic contact layers
with inorganic counterparts, such
as metal oxides, is one strategy for improving long-term device stability
in metal halide perovskite solar cells. Often, the methods used to
deposit metal oxide thin films are incompatible with metal halide
perovskites, creating challenges for the fabrication of contacts above
the perovskite absorber layer. In this study, we utilize a one-step,
solution treatment of the top surface of Cs0.25FA0.75Pb(Br0.20I0.80)3 to create a thin
(∼1 nm) overlayer of lead sulfide (PbS) to protect the underlying
perovskite during subsequent deposition. X-ray characterization of
the surface region shows that the PbS overlayer limits undesirable
changes to the perovskite structure and stoichiometry during atomic
layer deposition (ALD) of SnO2. This protection enables
ALD growth of SnO2 electron contacts on top of the perovskite
without an organic transport layer (e.g., C60), resulting
in a solar cell with a power conversion efficiency of 5.8%. This result
is a marked improvement over devices with ALD SnO2 grown
directly on the perovskite without a PbS overlayer, which produce
no power output. The interface characterization and device results
in this study highlight some of the key challenges associated with
ALD metal oxide growth on perovskite materials and can help inform
the future design of inorganic contact layer deposition in solar photovoltaic
technologies.
The Front Cover shows the idealized lithium‐containing species on an electrocatalyst surface for the reduction of nitrogen to ammonia at atmospheric pressure and temperature. The nitrogen activation occurs in non‐aqueous electrolytes to achieve high faradaic efficiency on a complex reactive surface.More information can be found in the Article by J. A. Schwalbe et al.
How did the collaborationo nt his project start?While many of the senior professors in our collaboration have workedo nn itrogen chemistryf or years, often together,t he formal collaboration startedw hen experimental, computational and characterization efforts converged to bring together researchersf rom Stanford and Denmark Te chnicalU niversity to work on ammonia synthesis at room temperature and atmospheric pressure with ag rant from the Villum Foundation.What do you consider the exciting developments in the field?
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