A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells
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99 GENERAL AND MISCELLANEOUS↗
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Blading Phase–Pure Formamidinium–Alloyed Perovskites for High–Efficiency Solar Cells with Low Photovoltage Deficit and Improved Stability
Currently, blade-coated perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs), i.e. greater than 20%, normally employ methylammonium lead tri-iodide with a sub-optimal bandgap. Alloyed perovskites with formamidinium (FA) cation have narrower bandgap and thus enhance device photocurrent. However, FA-alloyed perovskites show low phase stability and high moisture sensitivity. Here, we report that incorporating 0.83 molar percent organic halide salts (OHs) into the perovskite inks enables phase-pure, highly crystalline FA-alloyed perovskites with extraordinary optoelectronic properties. Here, the OH molecules modulate the crystal growth, enhance the phase stability, passivate ionic defects at the surface and/or grain boundaries, and enhance the moisture stability of the perovskite film. We demonstrate a high efficiency of 22.0% under 1 sun illumination for blade-coated PSCs with an open-circuit voltage of 1.18 V, corresponding to a very small voltage deficit of 0.33 V, and significantly improved operational stability with 96% of the initial efficiency retained under one sun illumination for 500 hours.
14 SOLAR ENERGY↗
Stable Potassium Metal Anodes with an All-Aluminum Current Collector through Improved Electrolyte Wetting
This is the first report of successful potassium metal battery anode cycling with an aluminum-based rather than copper-based current collector. Dendrite-free plating/stripping is achieved through improved electrolyte wetting, employing an aluminum-powder-coated aluminum foil “Al@Al,” without any modification of the support surface chemistry or electrolyte additives. The reservoir-free Al@Al half-cell is stable at 1000 cycles (1950 h) at 0.5 mA cm -2 , with 98.9% cycling Coulombic efficiency and 0.085 V overpotential. The pre-potassiated cell is stable through a wide current range, including 130 cycles (2600 min) at 3.0 mA cm -2 , with 0.178 V overpotential. Al@Al is fully wetted by a 4 m potassium bis(fluorosulfonyl)imide-dimethoxyethane electrolyte (θCA = 0°), producing a uniform solid electrolyte interphase (SEI) during the initial galvanostatic formation cycles. On planar aluminum foil with a nearly identical surface oxide, the electrolyte wets poorly (θCA = 52°). This correlates with coarse irregular SEI clumps at formation, 3D potassium islands with further SEI coarsening during plating/stripping, possibly dead potassium metal on stripped surfaces, and rapid failure. The electrochemical stability of Al@Al versus planar Al is not related to differences in potassiophilicity (nearly identical) as obtained from thermal wetting experiments. Additionally, planar Cu foils are also poorly electrolyte-wetted and become dendritic. The key fundamental takeaway is that the incomplete electrolyte wetting of collectors results in early onset of SEI instability and dendrites.
25 ENERGY STORAGE↗
Unraveling the Nature of Excellent Potassium Storage in Small-Molecule Se@Peapod-Like N-Doped Carbon Nanofibers
The potassium-selenium (K-Se) battery is considered as an alternative solution for stationary energy storage because of abundant resource of K. However, the detailed mechanism of the energy storage process is yet to be unraveled. In this work, the findings in probing the working mechanism of the K-ion storage in Se cathode are reported using both experimental and computational approaches. A flexible K-Se battery is prepared by employing the small-molecule Se embedded in freestanding N -doped porous carbon nanofibers thin film (Se@NPCFs) as cathode. The reaction mechanisms are elucidated by identifying the existence of short-chain molecular Se encapsulated inside the microporous host, which transforms to K 2 Se by a two-step conversion reaction via an "all-solid-state" electrochemical process in the carbonate electrolyte system. Through the whole reaction, the generation of polyselenides (K 2 Se n , 3 ≤ n ≤ 8) is effectively suppressed by electrochemical reaction dominated by Se-2 molecules, thus significantly enhancing the utilization of Se and effecting the voltage platform of the K-Se battery. This work offers a practical pathway to optimize the K-Se battery performance through structure engineering and manipulation of selenium chemistry for the formation of selective species and reveal its internal reaction mechanism in the carbonate electrolyte.
25 ENERGY STORAGE↗
Tailored Graphene Micropatterns by Wafer-Scale Direct Transfer for Flexible Chemical Sensor Platform
2D materials, such as graphene, exhibit great potential as functional materials for numerous novel applications due to their excellent properties. The grafting of conventional micropatterning techniques on new types of electronic devices is required to fully utilize the unique nature of graphene. However, the conventional lithography and polymer-supported transfer methods often induce the contamination and damage of the graphene surface due to polymer residues and harsh wet-transfer conditions. In this work, a novel strategy to obtain micropatterned graphene on polymer substrates using a direct curing process is demonstrated. Employing this method, entirely flexible, transparent, well-defined self-activated graphene sensor arrays, capable of gas discrimination without external heating, are fabricated on 4 in. wafer-scale substrates. Finite element method simulations show the potential of this patterning technique to maximize the performance of the sensor devices when the active channels of the 2D material are suspended and nanoscaled. This study contributes considerably to the development of flexible functional electronic devices based on 2D materials.
2D materials, Graphene, Microscale patterning, Che↗
Tunable Stochasticity in an Artificial Spin Network
Metamaterials present the possibility of artificially generating advanced functionalities through engineering of their internal structure. Artificial spin networks, in which a large number of nanoscale magnetic elements are coupled together, are promising metamaterial candidates that enable the control of collective magnetic behavior through tuning of the local interaction between elements. In this work, the motion of magnetic domain-walls in an artificial spin network leads to a tunable stochastic response of the metamaterial, which can be tailored through an external magnetic field and local lattice modifications. This type of tunable stochastic network produces a controllable random response exploiting intrinsic stochasticity within magnetic domain-wall motion at the nanoscale. An iconic demonstration used to illustrate the control of randomness is the Galton board. In this system, multiple balls fall into an array of pegs to generate a bell-shaped curve that can be modified via the array spacing or the tilt of the board. A nanoscale recreation of this experiment using an artificial spin network is employed to demonstrate tunable stochasticity. Furthermore, this type of tunable stochastic network opens new paths toward post-Von Neumann computing architectures such as Bayesian sensing or random neural networks, in which stochasticity is harnessed to efficiently perform complex computational tasks.
Artificial spin network↗
Ultrathin and Ultrasensitive Direct X‐ray Detector Based on Heterojunction Phototransistors
Abstract Most contemporary X‐ray detectors adopt device structures with non/low‐gain energy conversion, such that a fairly thick X‐ray photoconductor or scintillator is required to generate sufficient X‐ray‐induced charges, and thus numerous merits for thin devices, such as mechanical flexibility and high spatial resolution, have to be compromised. This dilemma is overcome by adopting a new high‐gain device concept of a heterojunction X‐ray phototransistor. In contrast to conventional detectors, X‐ray phototransistors allow both electrical gating and photodoping for effective carrier‐density modulation, leading to high photoconductive gain and low noise. As a result, ultrahigh sensitivities of over 10 5 μC Gy air −1 cm −2 with low detection limit are achieved by just using an ≈50 nm thin photoconductor. The employment of ultrathin photoconductors also endows the detectors with superior flexibility and high imaging resolution. This concept offers great promise in realizing well‐balanced detection performance, mechanical flexibility, integration, and cost for next‐generation X‐ray detectors.
Gao, Yuanhong↗
Textured Electrodes: Manipulating Built-In Crystallographic Heterogeneity of Metal Electrodes via Severe Plastic Deformation
Control of crystallography of metal electrodeposit films has recently emerged as a key to achieving long operating lifetimes in next-generation batteries. It is reported that the large crystallographic heterogeneity, e.g., broad orientational distribution, that appears characteristic of commercial metal foils, results in rough morphology upon plating/stripping. On this basis, an accumulative roll bonding (ARB) methodology—a severe plastic deformation process—is developed. Zn metal is used as a first example to interrogate the concept. It is demonstrated that the ARB process is highly effective in achieving uniform crystallographic control on macroscopic materials. After the ARB process, the Zn grains exhibit a strong (002) texture (i.e., [002] Zn //ND). The texture transitions from a classical bipolar pattern to a nonclassical unipolar pattern under large nominal strain eliminate the orientational heterogeneity of the foil. The strongly (002)-textured Zn remarkably improves the plating/stripping performance by nearly two orders of magnitude under practical conditions. The performance improvements are readily scaled to achieve pouch-type full batteries that deliver exceptional reversibility. The ARB process can, in principle, be applied to any metal chemistry to achieve similar crystallographic uniformity, provided the appropriate temperature and accumulated strains are employed. Here this concept is evaluated using commercial Li and Na foils, which, unlike Zn (HCP), are BCC crystals. The simple process for creating strong textures in both hexagonal and cubic metals and illustrating the critical role such built-in crystallography plays underscores opportunities for developing highly reversible thin metal anodes (e.g., hexagonal Zn, Mg, and cubic Li, Na, Ca, Al).
36 MATERIALS SCIENCE↗
From p- to n-Type Mixed Conduction in Isoindigo-Based Polymers through Molecular Design
Organic mixed ionic and electronic conductors are of significant interest for bioelectronic applications. Here, we use three different isoindigoid building blocks to obtain polymeric mixed conductors with vastly different structural and electronic properties which can be further fine-tuned through the choice of comonomer unit. We show how careful design of the isoindigoid scaffold can afford highly planar polymer structures with high degrees of electronic delocalization, while subtle structural modifications can control the dominant charge carrier (hole or electron) when probed in organic electrochemical transistors. We employ a combination of experimental and computational techniques to probe electrochemical, structural and mixed ionic and electronic properties of the polymer series which in turn allows us to derive important structure-property relations for this promising class of materials in the context of organic bioelectronics. Ultimately, we use these findings to outline robust molecular design strategies for isoindigo-based mixed conductors that can support efficient p-type, n-type and ambipolar transistor operation in an aqueous environment.
36 MATERIALS SCIENCE↗
Polymerized Small Molecular Acceptor with Branched Side Chains for All Polymer Solar Cells with Efficiency over 16.7%
The power conversion efficiencies (PCEs) of small molecule acceptor (SMA)-based organic solar cells have already exceeded 18%. However, the development of polymer acceptors still lags far behind their SMA counterparts mainly due to the lack of efficient polymer acceptors. In this work, a series of polymer acceptors named PY-X (with X being the branched alkyl chain) are designed and synthesized by employing the same central core with the SMA L8-BO but with different branched alkyl chains on the pyrrole motif. It is found that the molecular packing of SMA-HD featuring 2-hexyldecyl side chain used in the synthesis of PY-HD is similar to L8-BO, in which the branched alkyl chains lead to condensed and high-order molecular assembly in SMA-HD molecules. When combined with PM6, PY-HD-based all polymer solar cell (all-PSC) exhibits a high PCE of 16.41%, representing the highest efficiency for the binary all-PSCs. Moreover, the side-chain modification on the pyrrole site position further improves the performance of the all-PSCs, and the PY-DT-based device delivers a new record high efficiency of 16.76% (certified as 16.3%). The work provides new insights for understanding the structure–property relationship of polymer acceptors and paves a feasible avenue to develop efficient conjugated polymer acceptors.
14 SOLAR ENERGY↗
Origin of the 2D Electron Gas at the SrTiO 3 Surface
Bulk SrTiO 3 is a well-known band insulator and the most common substrate used in the field of complex oxide heterostructures. Its surface and interface with other oxides, however, have demonstrated a variety of remarkable behaviors distinct from those expected. In this work, using a suite of in situ techniques to monitor both the atomic and electronic structures of the SrTiO 3 (001) surface prior to and during growth, the disappearance and re-appearance of a 2D electron gas (2DEG) is observed after the completion of each SrO and TiO 2 monolayer, respectively. The 2DEG is identified with the TiO 2 double layer present at the initial SrTiO 3 surface, which gives rise to a surface potential and mobile electrons due to vacancies within the TiO 2-x adlayer. Much like the electronic reconstruction discovered in other systems, two atomic planes are required, here supplied by the double layer. The combined in situ scattering/spectroscopy findings resolve a number of longstanding issues associated with complex oxide interfaces, facilitating the employment of atomic-scale defect engineering in oxide electronics.
2D electron gas↗
Enhancing the Reversibility of Lattice Oxygen Redox Through Modulated Transition Metal–Oxygen Covalency for Layered Battery Electrodes
Utilizing reversible lattice oxygen redox (OR) in battery electrodes is an essential strategy to overcome the capacity limitation set by conventional transition metal redox. However, lattice OR reactions are often accompanied with irreversible oxygen oxidation, leading to local structural transformations and voltage/capacity fading. Herein, it is proposed that the reversibility of lattice OR can be remarkably improved through modulating transition metal-oxygen covalency for layered electrode of Na-ion batteries. By developing a novel layered P2-Na 0.6 Mg 0.15 Mn 0.7 Cu 0.15 O 2 electrode, it is demonstrated that the highly electronegative Cu dopants can improve the lattice OR reversibility to 95% compared to 73% for Cu-free counterpart, as directly quantified through high-efficiency mapping of resonant inelastic X-ray scattering. Crucially, the large energetic overlap between Cu 3d and O 2p states dictates the rigidity of oxygen framework, which effectively mitigates the structural distortion of local oxygen environment upon (de)sodiation and leads to the enhanced lattice OR reversibility. The electrode also exhibits a completely solid-solution reaction with an ultralow volume change of only 0.45% and a reversible metal migration upon cycling, which together ensure the improved electrochemical performance. Finally, these results emphasize the critical role of transition metal-oxygen covalency for enhancing the reversibility of lattice OR toward high-capacity electrodes employing OR chemistry.
25 ENERGY STORAGE↗
Electro-Thermal Characterization of Dynamical VO 2 Memristors via Local Activity Modeling
We report translating the surging interest in neuromorphic electronic components, such as those based on nonlinearities near Mott transitions, into large-scale commercial deployment faces steep challenges in the current lack of means to identify and design key material parameters. These issues are exemplified by the difficulties in connecting measurable material properties to device behavior via circuit element models. Here, the principle of local activity is used to build a model of VO 2 /SiN Mott threshold switches by sequentially accounting for constraints from a minimal set of quasistatic and dynamic electrical and high-spatial-resolution thermal data obtained via in situ thermoreflectance mapping. By combining independent data sets for devices with varying dimensions, the model is distilled to measurable material properties, and device scaling laws are established. The model can accurately predict electrical and thermal conductivities and capacitances and locally active dynamics (especially persistent spiking self-oscillations). The systematic procedure by which this model is developed has been a missing link in predictively connecting neuromorphic device behavior with their underlying material properties, and should enable rapid screening of material candidates before employing expensive manufacturing processes and testing procedures.
36 MATERIALS SCIENCE↗
Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive
Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Finally, owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).
14 SOLAR ENERGY↗
Electrical Manipulation of Topological Phases in a Quantum Anomalous Hall Insulator
Quantum anomalous Hall phases arising from the inverted band topology in magnetically doped topological insulators have emerged as an important subject of research for quantization at zero magnetic fields. Though necessary for practical implementation, sophisticated electrical control of molecular beam epitaxy (MBE)-grown quantum anomalous Hall matter have been stymied by growth and fabrication challenges. Here, in this study, a novel procedure is demonstrated, employing a combination of thin-film deposition and 2D material stacking techniques, to create dual-gated devices of the MBE-grown quantum anomalous Hall insulator, Cr-doped (Bi,Sb) 2 Te 3 . In these devices, orthogonal control over the field-induced charge density and the electric displacement field is demonstrated. A thorough examination of material responses to tuning along each control axis is presented, realizing magnetic property control along the former and a novel capability to manipulate the surface exchange gap along the latter. Through electrically addressing the exchange gap, the capabilities to either strengthen the quantum anomalous Hall state or suppress it entirely and drive a topological phase transition to a trivial state are demonstrated. The experimental result is explained using first principle theoretical calculations, and establishes a practical route for in situ control of quantum anomalous Hall states and topology.
36 MATERIALS SCIENCE↗
Triplet Fusion Upconversion for Photocuring 3D-Printed Particle-Reinforced Composite Networks
High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle-reinforced composite networks since deep light penetration of short-wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet–triplet annihilation upconversion (TTA-UC) is employed for curing opaque hydrogel composites created by direct-ink-write (DIW) 3D printing. TTA-UC converts low energy red light (λ max = 660 nm) for deep penetration into higher-energy blue light to initiate free radical polymerizations within opaque objects. Further, as proof-of-principle, hydrogels containing up to 15 wt.% TiO 2 filler particles and doped with TTA-UC chromophores are readily cured with red light, while composites without the chromophores and TiO 2 loadings as little as 1–2 wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D-printed composite polymer networks.
36 MATERIALS SCIENCE↗
Conversion of Classical Light Emission from a Nanoparticle‐Strained WSe 2 Monolayer into Quantum Light Emission via Electron Beam Irradiation
Abstract Solid‐state single photon emitters (SPEs) within atomically thin transition metal dichalcogenides (TMDs) have recently attracted interest as scalable quantum light sources for quantum photonic technologies. Among TMDs, WSe 2 monolayers (MLs) are promising for the deterministic fabrication and engineering of SPEs using local strain fields. The ability to reliably produce isolatable SPEs in WSe 2 is currently impeded by the presence of numerous spectrally overlapping states that occur at strained locations. Here nanoparticle (NP) arrays with precisely defined positions and sizes are employed to deterministically create strain fields in WSe 2 MLs, thus enabling the systematic investigation and control of SPE formation. Using this platform, electron beam irradiation at NP‐strained locations transforms spectrally overlapped sub‐bandgap emission states into isolatable, anti‐bunched quantum emitters. The dependence of the emission spectra of WSe 2 MLs as a function of strain magnitude and exposure time to electron beam irradiation is quantified and provides insight into the mechanism for SPE production. Excitons selectively funnel through strongly coupled sub‐bandgap states introduced by electron beam irradiation, which suppresses spectrally overlapping emission pathways and leads to measurable anti‐bunched behavior. The findings provide a strategy to generate isolatable SPEs in 2D materials with a well‐defined energy range.
2D material↗