Interactions between cells and titanium surfaces

The interaction between cells and implant materials is determined by the surface structure and/or surface composition of the material. In the past years, titanium and titanium alloys have proved their superiority over other implant materials in many clinical applications. This predominant behaviour is caused by a dense passive oxide layer which forms within milliseconds in oxidizing media. Titanium dioxide layers of 100 nm thickness were produced on the surface of cp-titanium grade 2, and on an experimental alloy of high vanadium content (Ti1.5Al25V) as a harmful control. The layers were produced by thermal and anodic oxidation and by coating by means of the sol-gel process. The resulting oxide layers were characterized with respect of their structure and chemical composition. In cell tests (proliferation, MTT, morphology, actin staining), the reaction of the cells was examined. It was shown that the sol-gel-produced titanium oxide layer is able to shield the cells from toxic alloying elements, with the result that the cell reaction is influenced only by the thin titanium oxide surface layer and not by the composition of the bulk material.     

Efficient Indium‐Doped TiOx Electron Transport Layers for High‐Performance Perovskite Solar Cells and Perovskite‐Silicon Tandems

In addition to a good perovskite light absorbing layer, the hole and electron transport layers play a crucial role in achieving high‐efficiency perovskite solar cells. Here, a simple, one‐step, solution‐based method is introduced for fabricating high quality indium‐doped titanium oxide electron transport layers. It is shown that indium‐doping improves both the conductivity of the transport layer and the band alignment at the ETL/perovskite interface compared to pure TiO₂, boosting the fill‐factor and voltage of perovskite cells. Using the optimized transport layers, a high steady‐state efficiency of 17.9% for CH₃NH₃PbI₃‐based cells and 19.3% for Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃‐based cells is demonstrated, corresponding to absolute efficiency gains of 4.4% and 1.2% respectively compared to TiO₂‐based control cells. In addition, a steady‐state efficiency of 16.6% for a semi‐transparent cell is reported and it is used to achieve a four‐terminal perovskite‐silicon tandem cell with a steady‐state efficiency of 24.5%.     

Enhanced Stability and Thickness‐Independent Oxygen Evolution Electrocatalysis of Heterostructured Anodes with Buried Epitaxial Bilayers

Achieving high oxygen evolution reaction (OER) activity while maintaining performance stability is a key challenge for designing perovskite structure oxide OER catalysts, which are often unstable in alkaline environments transforming into an amorphous phase. While the chemical and structural transformation occurring during electrolysis at the electrolyte–catalyst interface is now regarded as a crucial factor influencing OER activity, here, using La_0.7Sr_0.3CoO_3?δ (LSCO) as an active OER catalyst, the critical influence of buried layers on the oxidation current stability in nanoscopically thin, chemically and structurally evolving, catalyst layers is revealed. The use of epitaxial thin films is demonstrated to engineer both depletion layer widths and chemical stability of the catalyst support structure resulting in heterostructured anodes that maintain facile transport kinetics across the electrolyte–anode interface for atomically thin (2–3 unit cells) LSCO catalyst layers and greatly enhanced oxidation current stability as the perovskite structure OER catalysts chemically and structurally transform. This work opens up an approach to design robust and active heterostructured anodes with dynamically evolving ultrathin OER electrocatalyst layers for future green fuel technologies such as conformal coatings of high‐density 3D anode topologies for water splitting.     

Low Electron Scattering Potentials in High Performance Mg2Si0.45Sn0.55 Based Thermoelectric Solid Solutions with Band Convergence

Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg₂(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ～ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density‐of‐states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.     

Solution‐Processed Ternary Oxides as Carrier Transport/Injection Layers in Optoelectronics

With the remarkable progress in solution‐processed optoelectronics, high performance is required of the carrier transport/injection layer. Ternary oxides containing a variety of crystal structures, and adjustable composition that results in tunable optical and electrical properties, are one of the promising class of candidates to fulfill the requirements of carrier transport/injection layers for high‐performance and stable optoelectronic devices. Solution‐processed ternary oxides have seen considerable progress in recent decades, due to their advantages in the quest to design low‐cost, high‐performance, large‐scale, and stable optoelectronic devices. Herein, the recent advances of solution‐processed ternary oxides are reviewed. The first section consists of a brief introduction to the topic. In the following section, the fundamentals of the effect of tuning ternary oxide composition are summarized. Section three briefly reviews the synthesis approaches for preparing ternary oxides. Section four discusses the recent progress of solution‐processed ternary oxide as carrier transport/injection layer in optoelectronic devices (such as organic solar cells, perovskite solar cells, organic light emitting diodes, etc.). In this section, the impact of controlling ternary oxide composition on device performance and stability is highlighted. Finally, a brief summary and an outlook are given.     

Building Electron/Proton Nanohighways for Full Utilization of Water Splitting Catalysts

Low electron/proton conductivities of electrochemical catalysts, especially earth‐abundant nonprecious metal catalysts, severely limit their ability to satisfy the triple‐phase boundary (TPB) theory, resulting in extremely low catalyst utilization and insufficient efficiency in energy devices. Here, an innovative electrode design strategy is proposed to build electron/proton transport nanohighways to ensure that the whole electrode meets the TPB, therefore significantly promoting enhance oxygen evolution reactions and catalyst utilizations. It is discovered that easily accessible/tunable mesoporous Au nanolayers (AuNLs) not only increase the electrode conductivity by more than 4000 times but also enable the proton transport through straight mesopores within the Debye length. The catalyst layer design with AuNLs and ultralow catalyst loading (≈0.1 mg cm^?2) augments reaction sites from 1D to 2D, resulting in an 18‐fold improvement in mass activities. Furthermore, using microscale visualization and unique coplanar‐electrode electrolyzers, the relationship between the conductivity and the reaction site is revealed, allowing for the discovery of the conductivity‐determining and Debye‐length‐determining regions for water splitting. These findings and strategies provide a novel electrode design (catalyst layer + functional sublayer + ion exchange membrane) with a sufficient electron/proton transport path for high‐efficiency electrochemical energy conversion devices.     

Impact of Hole Transport Layer Surface Properties on the Morphology of a Polymer‐Fullerene Bulk Heterojunction

Investigations on the impact of interfacial modification on organic optoelectronic device performance often attribute the improved device performance to the optoelectronic properties of the modifier. A critical assumption of such conclusions is that the organic active layer deposited on top of the modified surface (interface) remains unaltered. Here the validity of this assumption is investigated by examining the impact of substrate surface properties on the morphology of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐[6,6]C₆₁ (P3HT:PCBM) bulk‐heterojunction (BHJ). A set of four nickel oxide and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layers (HTL) with contrasting surface properties and performance in organic photovoltaic (OPV) devices is studied. Differences in vertical composition variation and structural morphologies are observed across the samples, but only in the near‐interface region of <～20 nm. Near‐interface differences in morphology are most closely correlated with surface polarity and surface roughness of the HTL. Surface polarity is more influenced by surface composition than surface roughness and crystal structure. These findings corroborate the previously mentioned conclusions that the differences in device performance observed in solar cells employing these HTLs are dominated by the electronic properties of the HTL/organic photoactive active layer interface and not by unintentional alteration in the BHJ active layer morphology.     

Tailoring the Membrane‐Electrode Interface in PEM Fuel Cells: A Review and Perspective on Novel Engineering Approaches

The interface between the catalyst layer (CL) and the polymer electrolyte membrane (PEM) in a fuel cell has substantial impact on its electrochemical performance. In consequence, there have been growing research activities to engineer this interface to improve the performance of polymer electrolyte membrane fuel cells (PEMFCs). This review summarizes these novel approaches and compares the various techniques. Based on available fuel cell data in the literature, a quantitative comparison of relative improvements due to a micro‐ and nano‐engineered PEM|CL interface is provided. This allows several conclusions: First, regardless of the applied method, a re‐engineering of the PEM|CL interface leads to an improvement of power‐determining parameters, such as mass transport resistances. The latter has hitherto not been clearly connected to the PEM|CL interface and is an important piece of information for future fuel cell development. Second, for patterned membrane surfaces, feature sizes of about 1–10 µm on the membrane surface seem to result in the most significant power density improvement. Third, an engineered PEMCL interface can contribute to extend the fuel cell durability due to enhanced adhesion and contact between the two layers. With this, novel membrane electrode assemblies (MEAs) can be designed that enable significantly higher power densities compared conventional 2D‐layer MEAs.     

Role of Ionic Functional Groups on Ion Transport at Perovskite Interfaces

Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current–voltage (I–V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. It is shown that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I–V hysteresis in perovskite‐based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups are utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial‐adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I–V hysteresis. Moreover, the ion adsorbing properties of this interface are exploited for perovskite‐based memristors. This fundamental study of I–V hysteresis in perovskite‐based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.     

Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers

We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work‐function of the hole collecting indium‐tin oxide (ITO) contact, modified with a solution‐processed nickel oxide (NiO_x) hole‐transport layer (HTL), is matched to the ionization potential of the donor material in a bulk‐heterojunction solar cell. Addition of the NiO_x HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiO_x films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiO_x HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiO_x HTL is shown to provide superior contact properties by increasing the ITO/NiO_x contact work‐function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiO_x interface with the poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothidiazole) (PCDTBT):[6,6]‐phenyl‐C61 butyric acid methyl ester PCBM and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC₇₀BM) active layer. A high work‐function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.     

Carbon‐Tailored Semimetal MoP as an Efficient Hydrogen Evolution Electrocatalyst in Both Alkaline and Acid Media

The electrolysis processes such as hydrogen evolution reaction (HER) require high efficient catalysts with robust surface stability. A high conductivity is also necessary to speed up the charge transport between the catalyst and the electrolyte. Recently, the observation of exceedingly high conductivity in the topological semimetal MoP, has provided a model catalyst to investigate the correlation between the electrical transport and the electrocatalytic activity for the HER. Thus, MoP is encapsulated in a Mo, P codoped carbon layer (MoP@C). This composite material exhibits outstanding HER performance, with an extremely low overpotential of 49 mV at a current density of 10 mA cm^?2 and a Tafel slope of 54 mV dec^?1 in an alkaline medium. In addition, electron transport analysis indicates that MoP exhibits high conductivity and mobility due to the existence of triple‐point fermions and a complex Fermi surface. Furthermore, the presence of P? C and Mo? C bonds at the interface between the carbon layer and the MoP particles modulates the band structure of MoP@C and facilitates fast electron transfer, accumulation, and subsequent delocalization, which are in turn responsible for the excellent HER activity.     

Hole Transporting Bilayers for Efficient Micrometer-Thick Perovskite Solar Cells

Achieving high efficiencies in halide perovskite solar cells with thicknesses >1 µm is necessary for developing perovskite-Si tandem cells based on small pyramidal structures. To achieve this goal, not only is the perovskite layer quality to be optimized but also the properties of the charge-transport layers must be tuned to reduce charge-collection losses. The transport layers provide a non-ohmic resistance that modulates the Fermi-level splitting inside the perovskite absorber. The finite conductivity of the transport layers can lead to losses in the fill factor (FF) and short-circuit current, even at infinite charge-carrier mobility in the absorber layer. These losses notably scale with the absorber layer thickness, which implies that higher-conductivity transport layers are required for thicker perovskite absorbers. One strategy to improve charge collection and thereby FFs in thick inverted perovskite solar cells is to use bilayers of hole-transport layers. In this study, the combination of poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] with self-assembled monolayers provides the best photovoltaic performance in single-junction devices.     

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride‐trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect.     

A Printable Organic Electron Transport Layer for Low‐Temperature‐Processed,Hysteresis‐Free,and Stable Planar Perovskite Solar Cells

Despite recent breakthroughs in power conversion efficiencies (PCEs), which have resulted in PCEs exceeding 22%, perovskite solar cells (PSCs) still face serious drawbacks in terms of their printability, reliability, and stability. The most efficient PSC architecture, which is based on titanium dioxide as an electron transport layer, requires an extremely high‐temperature sintering process (≈500 °C), reveals hysterical discrepancies in the device measurement, and suffers from performance degradation under light illumination. These drawbacks hamper the practical development of PSCs fabricated via a printing process on flexible plastic substrates. Herein, an innovative method to fabricate low‐temperature‐processed, hysteresis‐free, and stable PSCs with a large area up to 1 cm² is demonstrated using a versatile organic nanocomposite that combines an electron acceptor and a surface modifier. This nanocomposite forms an ideal, self‐organized electron transport layer (ETL) via a spontaneous vertical phase separation, which leads to hysteresis‐free, planar heterojunction PSCs with stabilized PCEs of over 18%. In addition, the organic nanocomposite concept is successfully applied to the printing process, resulting in a PCE of over 17% in PSCs with printed ETLs.     

Simple,Robust, and Going More Efficient: Recent Advance on Electron Transport Layer‐Free Perovskite Solar Cells

Perovskite solar cells (PSCs) have shown great potential for photovoltaic applications with their unprecedented power conversion efficiency advancement. Such devices generally have a complex structure design with high temperature processed TiO₂ as the electron transport layer (ETL). Further careful design of device configuration to fully tap the potentials of perovskite materials is expected. Particularly, for the practical application of PSCs, it is crucial to simplify their device structures thus the associated manufacturing process and cost while maintaining their efficiency to be comparable with the conventional devices. But how simple is simple? ETL‐free PSCs promise the simplest structured, thus simple manufacturing processes and low cost large area PSCs in practical applications. They can also help the further exploration of the great potential of perovskite materials and understanding the working principle of PSCs. Within this review, the evolution of the PSC is outlined by discussing the recent advances in the simplification of device configuration and processes for cost effective, highly efficient, and robust PSCs, with a focus on ETL‐free PSCs. Their advancement, key issues, working mechanism, existing problems, and future performance enhancements. This review aims to promote the future development of low cost and robust ETL‐free PSCs toward more efficient power output.     

3D Holey‐Graphene Architecture Expedites Ion Transport Kinetics to Push the OER Performance

The kinetics process of heterogeneous catalysis involves several steps including adsorption, diffusion, and surface chemical reactions. Current studies generally aim at increasing active site amount and improving intrinsic activity. However, the ion diffusion kinetics at the electrode/electrolyte interface as a bottleneck has been rarely directly addressed. Here, a 3D holey‐graphene framework is demonstrated as a catalyst‐loading platform, with nanoscale holes that can be elaborately tuned via facile aqueous‐phase chemical etching. This enables the ions to be efficiently transported to deeply buried active sites to mitigate their insufficient supply. With systematical electrochemical investigations tuned by varied pore structures, a series of models from a simplified equivalent circuit to complicate realistic one are proposed to figure out the modulation rules of weakened electrochemical diffusion domination and identify the ion transport resistance as well. Moreover, given the inevitable negative effect on the conductivity of graphene skeleton by introducing nanoscale holes, the balance between the outside ion transport and the inside charge transport of electrode is highlighted. Such a protocol represents a synergistic modulation of catalytic performance from both the supply side (reactive ion transport) and the consuming side (active site), and provides striking information for the precise design of catalyst electrodes toward further pushing the oxygen evolution reaction performance limit.     

Nonaromatic Green‐Solvent‐Processable,Dopant‐Free,and Lead‐Capturable Hole Transport Polymers in Perovskite Solar Cells with High Efficiency

With the recent developments in the efficiency of perovskite solar cells (PSCs), diverse functionalities are necessary for next‐generation charge‐transport layers. Specifically, the hole‐transport layer (HTL) in the various synthesized materials modified with functional groups is explored. A novel donor–acceptor type polymer, alkoxy‐PTEG, composed of benzo[1,2‐b:4,5:b′]dithiophene and tetraethylene glycol (TEG)‐substituted 2,1,3‐benzothiadiazole is reported. The alkoxy‐PTEG exhibits high solubility even in nonaromatic solvents, such as 3‐methylcyclohexanone (3‐MC), and can prevent possible lead leakage via chelation. The optical and electronic properties of alkoxy‐PTEG are thoroughly analyzed. Finally, a dopant‐free alkoxy‐PTEG device processed with 3‐MC exhibits 19.9% efficiency and a device with 2‐methyl anisole, which is a reported aromatic food additive, exhibits 21.2% efficiency in a tin oxide planar structure. The PSC device shows 88% stability after 30 d at ambient conditions (40–50% relative humidity and room temperature). In addition, nuclear magnetic resonance reveals that TEG groups can chelate lead ions with moderate strength (K_binding = 2.76), and this strength is considered to be nondestructive to the perovskite lattice to prevent lead leakage. This is the first report to consider lead leakage and provide solutions to reduce this problem.     

Inorganic and Hybrid Interfacial Materials for Organic and Perovskite Solar Cells

As organic solar cells (OSCs) and perovskite solar cells (PVSCs) move closer to commercialization, further efforts toward optimizing both cell efficiency and stability are needed. As interfaces strongly affect device performance and degradation processes, interfacial engineering by employing various materials as hole transport layers (HTLs) and electron transport layers (ETLs) has been a very active field of research in OSCs and PVSCs. Among them, inorganic materials exhibit significant advantages in promoting device performance due to their excellent charge transporting properties and intrinsic thermal and chemical robustness. In this review, an extensive overview is provided of inorganic semiconductors such as copper‐based ones with emphasis on copper iodide and copper thiocyanate, transition metal chalcogenides, nitrides and carbides as well as hybrid materials based on these inorganic compounds that have been recently employed as HTLs and ETLs in OSCs and PVSCs. Following a short discussion of the main optoelectronic and physical properties that interfacial materials used as HTLs and ETLs should possess, the functionalities of the aforementioned materials as interfacial, charge transport, layers in OSCs and PVSCs are discussed in depth. It is concluded by providing guidelines for further developments that could significantly extend the implementation of these materials in solar cells.     

Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high‐temperature preparation of metal oxides as a charge transport layer in most state‐of‐the‐art PSCs has been becoming a big stumbling block for future low‐temperature and large‐scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low‐temperature‐processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all‐solution‐processed PSCs, we propose a novel n‐type sulfur‐containing small molecule hexaazatrinaphtho[2,3‐c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10^?2 cm² V^?1 s^?1 as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via S? I or S? Pb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n‐type organic small molecules for high‐performance PSCs.     

Graded Bandgap Perovskite with Intrinsic n–p Homojunction Expands Photon Harvesting Range and Enables All Transport Layer‐Free Perovskite Solar Cells

Organic–inorganic halide perovskites are promising materials for next‐generation photovoltaic device due to their attractive photoelectrical properties such as strong light absorption, high carrier mobility, and tunable bandgap. Generally, perovskite solar cells require carrier transport layers (CTL) to provide a built‐in electric field and reduce the recombination rate. However, the construction of suitable electron‐ and hole‐transport layers is not cost effective, impairing the commercial application of the devices. An n–p perovskite homojunction absorber with a graded bandgap is developed by introducing a three‐step dynamic spin‐coating strategy and variable valence Sn elements. The bandgap of the perovskite absorber is gradually manipulated from 1.53 eV (the bottom) to 1.27 eV (the top). The electronic behavior is also transformed from n‐type (excess PbI₂, the bottom) to p‐type (Sn vacancy, the top) in a very short distance (50 nm). This designed perovskite homojunction electronic structure not only expands the light harvesting range from 800 to 970 nm which provides potential to break the PCE limits, but also promotes oriented carrier transportation and weakens the dependence on CTL. The demonstrated asymmetrical active layer shows a brand‐new approach to simplify the device structure and boost the performance of CTL‐free perovskite solar cells.