Centimetre-scale perovskite solar cells with fill factors of more than 86 per cent.

Owing to rapid development in their efficiency1 and stability2, perovskite solar cells are at the forefront of emerging photovoltaic technologies. State-of-the-art cells exhibit voltage losses3-8 approaching the theoretical minimum and near-unity internal quantum efficiency9-13, but conversion efficiencies are limited by the fill factor (<83%, below the Shockley-Queisser limit of approximately 90%). This limitation results from non-ideal charge transport between the perovskite absorber and the cell's electrodes5,8,13-16. Reducing the electrical series resistance of charge transport layers is therefore crucial for improving efficiency. Here we introduce a reverse-doping process to fabricate nitrogen-doped titanium oxide electron transport layers with outstanding charge transport performance. By incorporating this charge transport material into perovskite solar cells, we demonstrate 1-cm2 cells with fill factors of >86%, and an average fill factor of 85.3%. We also report a certified steady-state efficiency of 22.6% for a 1-cm2 cell (23.33% ± 0.58% from a reverse current-voltage scan).

Core-shell GaN/AlGaN nanowires grown by selective area epitaxy.

GaN/AlGaN core-shell nanowires with various Al compositions have been grown on GaN nanowire array using selective area metal organic chemical vapor deposition technique. Growth of the AlGaN shell using pure N2 carrier gas resulted in a smooth surface for the nonpolar m-plane sidewalls with superior optical properties, whereas, growth using a mixed N2/H2 carrier gas resulted in a striated surface similar to the commonly observed morphology in the growth of nonpolar III-nitrides. The Al compositions in the AlGaN shells are found to be less than the gas phase input ratio. The systematic reduction in efficiency of Al incorporation in the AlGaN shells with increasing the Al molar flow in the gas phase is attributed to geometric loss, strain-limited Al incorporation, and increased gas phase parasitic reactions. Defect-related luminescence has been observed for AlGaN shells with Al content ≥ 30% and the origin of the defect luminescence has been determined as the (VIII-2ON)1- complex. Microstructural analysis of the AlGaN shells revealed that the dominant defects are partial dislocations. Growth of the nonpolar m-plane AlxGa1-xN/AlyGa1-yN quantum wells on the sidewalls of the GaN nanowires produced arrays with excellent morphology and optical emission, which demonstrated the viability of such a growth scheme for large area efficient ultraviolet LEDs as well as for next generation ultraviolet micro-LEDs.

Evaluating a Dual-Ion Battery with an Antimony-Carbon Composite Anode.

Dual-ion batteries (DIBs) are attracting attention due to their high operating voltage and promise in stationary energy storage applications. Among various anode materials, elements that alloy and dealloy with lithium are assumed to be prospective in bringing higher capacities and increasing the energy density of DIBs. In this work, antimony in the form of a composite with carbon (Sb-C) is evaluated as an anode material for DIB full cells for the first time. The behaviour of graphite||Sb-C cells is assessed in highly concentrated electrolytes in the absence and presence of an electrolyte additive (1 % vinylene carbonate) and in two cell voltage windows (2-4.5 V and 2-4.8 V). Sb-C full cells possess maximum estimated specific energies of 290 Wh/kg (based on electrode masses) and 154 Wh/kg (based on the combined mass of electrodes and active salt). The work expands the knowledge on the operation of DIBs with non-graphitic anodes.

Defect engineering enhances plasmonic-hot electrons exploitation for CO2 reduction over polymeric catalysts.

Defect sites present on the surface of catalysts serve a crucial role in different catalytic processes. Herein, we have investigated defect engineering within a hybrid system composed of "soft" polymer catalysts and "hard" metal nanoparticles, employing the disparity in their thermal expansions. Electron paramagnetic resonance, X-ray photoelectron spectroscopy, and mechanistic studies together reveal the formation of new abundant defects and their synergistic integrability with plasmonic enhancement within the hybrid catalyst. These active defects, co-localized with plasmonic Ag nanoparticles, promote the utilization efficiency of hot electrons generated by local plasmons, thereby enhancing the CO2 photoreduction activity while maintaining the high catalytic selectivity.

Epitaxial Growth of GaAs Nanowires on Synthetic Mica by Metal-Organic Chemical Vapor Deposition.

The epitaxial growth of III-V nanowires with excellent optoelectronic properties on low-cost, light-weight, and flexible substrates is a key step for the design and engineering of future optoelectronic devices. In our study, GaAs nanowires were grown on synthetic mica, a two-dimensional layered material, via vapor-liquid-solid growth using metal-organic chemical vapor deposition. The effect of basic epitaxial growth parameters such as temperature and V/III ratio on the vertical yield of the nanowires is investigated. A vertical yield of over 60% is achieved at an optimum growth temperature of 400 °C and a V/III ratio 18. The structural properties of the nanowires are investigated using various techniques including scanning electron microscopy, high-resolution transmission electron microscopy, and high-angle annular dark-field imaging. The vertical nanowires grown at a low temperature and a high V/III ratio are found to have a zincblende phase with a [111] B polarity. The optical properties are investigated by photoluminescence (PL) and time-resolved PL measurements. First-principles electronic structure calculations within the framework of density functional theory elucidate the van der Waals nature of the nanowire/mica interface. Our results also show that these nanowires can be easily lifted off the bulk 2D mica template, providing a pathway for flexible nanowire devices.

Nanoscale localized contacts for high fill factors in polymer-passivated perovskite solar cells.

Polymer passivation layers can improve the open-circuit voltage of perovskite solar cells when inserted at the perovskite-charge transport layer interfaces. Unfortunately, many such layers are poor conductors, leading to a trade-off between passivation quality (voltage) and series resistance (fill factor, FF). Here, we introduce a nanopatterned electron transport layer that overcomes this trade-off by modifying the spatial distribution of the passivation layer to form nanoscale localized charge transport pathways through an otherwise passivated interface, thereby providing both effective passivation and excellent charge extraction. By combining the nanopatterned electron transport layer with a dopant-free hole transport layer, we achieved a certified power conversion efficiency of 21.6% for a 1-square-centimeter cell with FF of 0.839, and demonstrate an encapsulated cell that retains ~91.7% of its initial efficiency after 1000 hours of damp heat exposure.

Lead-free (Ag,K)NbO3 materials for high-performance explosive energy conversion.

Explosive energy conversion materials with extremely rapid response times have broad and growing applications in energy, medical, defense, and mining areas. Research into the underlying mechanisms and the search for new candidate materials in this field are so limited that environment-unfriendly Pb(Zr,Ti)O3 still dominates after half a century. Here, we report the discovery of a previously undiscovered, lead-free (Ag0.935K0.065)NbO3 material, which possesses a record-high energy storage density of 5.401 J/g, enabling a pulse current ~ 22 A within 1.8 microseconds. It also exhibits excellent temperature stability up to 150°C. Various in situ experimental and theoretical investigations reveal the mechanism underlying this explosive energy conversion can be attributed to a pressure-induced octahedral tilt change from a - a - c + to a - a - c -/a - a - c +, in accordance with an irreversible pressure-driven ferroelectric-antiferroelectric phase transition. This work provides a high performance alternative to Pb(Zr,Ti)O3 and also guidance for the further development of new materials and devices for explosive energy conversion.

Introduction of TiO2 in CuI for Its Improved Performance as a p-Type Transparent Conductor.

The challenges of making high-performance, low-temperature processed, p-type transparent conductors (TCs) have been the main bottleneck for the development of flexible transparent electronics. Though a few p-type transparent conducting oxides (TCOs) have shown promising results, they need high processing temperature to achieve the required conductivity which makes them unsuitable for organic and flexible electronic applications. Copper iodide is a wide band gap p-type semiconductor that can be heavily doped at low temperature (<100 °C) to achieve conductivity comparable or higher than many of the well-established p-type TCOs. However, as-processed CuI loses its transparency and conductivity with time in an ambient condition which makes them unsuitable for long-term applications. Herein, we propose CuI-TiO2 composite thin films as a replacement of pure CuI. We show that the introduction of TiO2 in CuI makes it more stable in ambient conditions while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO2 composite thin films has been performed to understand the reasons for improved conductivity, transparency, and stability of CuI-TiO2 samples in comparison to pure CuI samples. The enhanced conductivity in CuI-TiO2 stems from the highly conductive space-charge layer formation at the CuI-TiO2 interface, whereas the improved transparency is due to reduced CuI grain growth mobility in the presence of TiO2. The improved stability of CuI-TiO2 in comparison to pure CuI is a result of inhibited recrystallization and grain growth, reduced loss of iodine, and limited oxidation of the CuI phase in the presence of TiO2. For optimized fraction of TiO2, an average transparency of ∼78% (in 450-800 nm region) and a resistivity of 14 mΩ·cm are achieved, while maintaining a relatively high mobility of ∼3.5 cm2 V-1 s-1 with hole concentration reaching as high as 1.3 × 1020 cm-3. Most importantly, this work opens up the possibility to design a new range of p-type transparent conducting materials using the CuI/insulator composite system such as CuI/SiO2, CuI/Al2O3, CuI/SiNx, and so forth.

Hydrogenation of Phosphorus-Doped Polycrystalline Silicon Films for Passivating Contact Solar Cells.

We characterize and discuss the impact of hydrogenation on the performance of phosphorus-doped polycrystalline silicon (poly-Si) films for passivating contact solar cells. Combining various characterization techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, low-temperature photoluminescence spectroscopy, quasi-steady-state photoconductance, and Fourier-transform infrared spectroscopy, we demonstrate that the hydrogen content inside the doped poly-Si layers can be manipulated to improve the quality of the passivating contact structures. After the hydrogenation process of poly-Si layers fabricated under different conditions, the effective lifetime and the implied open circuit voltage are improved for all investigated samples (up to 4.75 ms and 728 mV on 1 Ω cm n-type Si substrates). Notably, samples with very low initial passivation qualities show a dramatic improvement from 350 µs to 2.7 ms and from 668 to 722 mV.

Photovoltaic Effect of a Ferroelectric-Luminescent Heterostructure under Infrared Light Illumination.

In this report, a ferroelectric-luminescent heterostructure is designed to convert infrared light into electric power. We use BiFeO3 (BFO) as the ferroelectric layer and Y2O3:Yb,Tm (YOT) as the upconversion layer. Different from conventional ferroelectric materials, this heterostructure exhibits switchable and stable photovoltaic effects under 980 nm illumination, whose energy is much lower than the band gap of BFO. The energy transfer mechanism in this heterostructure is therefore studied carefully. It is found that a highly efficient nonradiative energy transfer process from YOT to BFO plays a critical role in achieving the below-band-gap photon-excited photovoltaic effects in this heterostructure. Our results also indicate that by introducing asymmetric electrodes, both the photovoltage and photocurrent are further enhanced when the built-in field and the depolarization field are aligned. The construction of ferroelectric-luminescent heterostructure is consequently proposed as a promising route to enhance the photovoltaic effects of ferroelectric materials by extending the absorption of the solar spectrum.

Above-Band Gap Photoinduced Stabilization of Engineered Ferroelectric Domains.

The effect of above-band gap photons on the domains of the BiFeO3 (BFO) thin film was investigated via piezoresponse force microscopy and Kelvin probe force microscopy. It is found that under above-band gap illumination, the relaxation time of the polarization state was significantly extended, while the effective polarizing voltage for the pristine domains was reduced. We propose that this photoinduced domain stabilization can be attributed to the interaction between photogenerated surface charges and domains. Importantly, a similar phenomenon is observed in other ferroelectric (FE) materials with an internal electric field once they are illuminated by above-band gap light, indicating that this photoinduced stabilization is potentially universal rather than specific to BFO. Thus, this study will not only contribute to the knowledge of photovoltaic (PV) phenomena but also provide a new route to promote the stability of PV and FE materials.

Uranium(III)-carbon multiple bonding supported by arene δ-bonding in mixed-valence hexauranium nanometre-scale rings.

Despite the fact that non-aqueous uranium chemistry is over 60 years old, most polarised-covalent uranium-element multiple bonds involve formal uranium oxidation states IV, V, and VI. The paucity of uranium(III) congeners is because, in common with metal-ligand multiple bonding generally, such linkages involve strongly donating, charge-loaded ligands that bind best to electron-poor metals and inherently promote disproportionation of uranium(III). Here, we report the synthesis of hexauranium-methanediide nanometre-scale rings. Combined experimental and computational studies suggest overall the presence of formal uranium(III) and (IV) ions, though electron delocalisation in this Kramers system cannot be definitively ruled out, and the resulting polarised-covalent U = C bonds are supported by iodide and δ-bonded arene bridges. The arenes provide reservoirs that accommodate charge, thus avoiding inter-electronic repulsion that would destabilise these low oxidation state metal-ligand multiple bonds. Using arenes as electronic buffers could constitute a general synthetic strategy by which to stabilise otherwise inherently unstable metal-ligand linkages.