Compromising Device Performance Research Articles

Quantum Dot Passivation of Halide Perovskite Films with Reduced Defects, Suppressed Phase Segregation, and Enhanced Stability.

Structural defects are ubiquitous for polycrystalline perovskite films, compromising device performance and stability. Herein, a universal method is developed to overcome this issue by incorporating halide perovskite quantum dots (QDs) into perovskite polycrystalline films. CsPbBr3 QDs are deposited on four types of halide perovskite films (CsPbBr3, CsPbIBr2, CsPbBrI2, and MAPbI3) and the interactions are triggered by annealing. The ions in the CsPbBr3 QDs are released into the thin films to passivate defects, and concurrently the hydrophobic ligands of QDs self‐assemble on the film surfaces and grain boundaries to reduce the defect density and enhance the film stability. For all QD‐treated films, PL emission intensity and carrier lifetime are significantly improved, and surface morphology and composition uniformity are also optimized. Furthermore, after the QD treatment, light‐induced phase segregation and degradation in mixed‐halide perovskite films are suppressed, and the efficiency of mixed‐halide CsPbIBr2 solar cells is remarkably improved to over 11% from 8.7%. Overall, this work provides a general approach to achieving high‐quality halide perovskite films with suppressed phase segregation, reduced defects, and enhanced stability for optoelectronic applications.

Solution processable transition metal oxide ultra-thin films as alternative electron transport and hole blocking layers in dye sensitized solar cells

An electron transporting and hole-blocking layer (ETL) between the fluorine-doped tin oxide (FTO) and mesoporous titanium dioxide interface of photoanode is essential in dye-sensitized solar cells (DSCs) for improving their photovoltaic performance. In the present work, we have examined the potential for further improving device performance by using alternative materials for ETLs. Accordingly, the effect of replacing TiO2 based ETLs with spin coated ultra-thin transition metal oxides (TMOs), molybdenum trioxide (MoO3) and tungsten trioxide (WO3) films has been studied. TMO films of 10 nm thickness have been found to be optimal for obtaining good transparency and charge transport properties, without compromising device performance. Further, devices fabricated with surfactant assisted (SA-) MoO3 ETLs have shown efficiency ∼4%, which is comparable with devices based on conventionally prepared TiO2 ETLs. In addition, a significant improvement in efficiency (15 %) has been observed for DSCs prepared using SA-WO3 films. The overall improvement in device efficiency is attributed to the increase in short-circuit current density without loss of open circuit potential and fill factor. Detailed analyses of results suggest that in addition to surface morphology, transparency and charge transport properties; oxygen vacancies in sub-stoichiometric TMOs also plays an important role in determining the device performance. The findings reveal that TMOs have significant potential to replace TiO2 ETL in DSCs and other third generation solar cell, viz. perovskite and polymer solar cells.

Redox Targeting-Based Thermally Regenerative Electrochemical Cycle Flow Cell for Enhanced Low-Grade Heat Harnessing.

A large amount of low-grade heat (<100°C) is produced in electrical devices and mostly wasted. This type of heat without effective dissipation also causes compromised device performance, reliability, and lifespan. To tackle these issues, a redox targeting (RT)-based flow cell with judiciously designed thermoelectrically active redox materials is demonstrated for the first time for efficient heat-to-electricity conversion through a thermally regenerative electrochemical cycle (TREC). Compared with the conventional TREC systems, the RT-based flow cell not only reveals considerably enhanced thermoelectric efficiency, but the flow of redox fluids also provides a cooling function to the system. In this work, solid material Ni0.2 Co0.8 (OH)2 and redox mediator [Fe(CN)6 ]4-/3- , both of which have negative temperature coefficient and share identical redox potential, are paired via RT-reactions to boost the capacity and meanwhile thermoelectric efficiency of a [Fe(CN)6 ]4-/3- /Zn0/2+ -based flow cell. Upon operating over the TREC cycle, the RT-based flow cell converts heat to electricity at an unprecedented absolute thermoelectric efficiency of 3.61% in the temperature range of 25-55°C.

Microsupercapacitor with a 500 nm gap between MXene/CNT electrodes

Abstract Decreasing the size of portable, wearable, and integrated electronics requires subsequent reduction in the accompanying energy storage devices. To further decrease the size of supercapacitors without compromising device performance, adequate materials are required as well as appropriate device design. Traditionally, carbon nanomaterials have been utilized within high-performance electrochemical energy storage applications, however carbides, specifically two-dimensional (2D) transition metal carbides and or nitrides (MXenes) have shown promise. Herein, a focused-ion-beam (FIB) process is used for patterning of titanium carbide (Ti3C2) carbon nanotube (CNT) electrodes into a finely controlled coplanar interdigitated configuration. The symmetric micro supercapacitors produced have a narrow, 500 nm gap between the electrode fingers and exhibit high areal capacitance of ~317 mF cm−2 at a scan rate of 50 mV s−1 and still retain >30% of their capacitance (~104 mF cm−2) at 100 V s−1. In addition, electron beam lithography and photolithography are utilized to create almost-invisible devices with ultra-small footprints (0.04 mm2 device areas) that can be used for securing electronic components. This study provides a simple and reliable method for fabrication of on-chip symmetric MSCs with coplanar interdigitated electrodes.

Flexible and Green Electronics Manufactured by Origami Folding of Nanosilicate-Reinforced Cellulose Paper.

Today's consumer electronics are made from nonrenewable and toxic components. They are also rigid, bulky, and manufactured in an energy-inefficient manner via CO2-generating routes. Though petroleum-based polymers such as polyethylene terephthalate and polyethylene naphthalate can address the rigidity issue, they have a large carbon footprint and generate harmful waste. Scalable routes for manufacturing electronics that are both flexible and ecofriendly (Fleco) could address the challenges in the field. Ideally, such substrates must incorporate into electronics without compromising device performance. In this work, we demonstrate that a new type of wood-based [nanocellulose (NC)] material made via nanosilicate (NS) reinforcement can yield flexible electronics that can bend and roll without loss of electrical function. Specifically, the NSs interact electrostatically with NC to reinforce thermal and mechanical properties. For instance, films containing 34 wt % of NS displayed an increased young's modulus (1.5 times), thermal stability (290 → 310 °C), and a low coefficient of thermal expansion (40 ppm/K). These films can also easily be separated and renewed into new devices through simple and low-energy processes. Moreover, we used very cheap and environmentally friendly NC from American Value Added Pulping (AVAP) technology, American Process, and therefore, the manufacturing cost of our NS-reinforced NC paper is much cheaper ($0.016 per dm-2) than that of conventional NC-based substrates. Looking forward, the methodology highlighted herein is highly attractive as it can unlock the secrets of Fleco electronics and transform otherwise bulky, rigid, and "difficult-to-process" rigid circuits into more aesthetic and flexible ones while simultaneously bringing relief to an already-overburdened ecosystem.

Dimensionality-Controlled Surface Passivation for Enhancing Performance and Stability of Perovskite Solar Cells via Triethylenetetramine Vapor.

Perovskite solar cells (PSCs) have achieved unprecedented progress in terms of enhancement of power conversion efficiency (PCE). Nevertheless, device stability is still an obstacle to the commercialization of this emerging photovoltaic technology. Though strategies such as compositional management and ligand engineering have been reported to tackle this critical issue, these methods often have drawbacks such as compromised device performance. Herein, we propose an approach combining material dimensionality control and interfacial passivation by a post-device treatment via triethylenetetramine (TETA) vapor to enhance both efficiency and stability of Cs0.05FA0.79MA0.16PbI2.5Br0.5-based PSCs. Results of X-ray diffraction and scanning electron microscopy show the formation of low-dimensional perovskites at the interface between the perovskite film and the hole transporting layer after the TETA vapor treatment. Measurements of the energy level alignment and electrochemical properties by ultraviolet photoelectron spectroscopy and impedance spectra confirm the reduced density of trap states and improved interfacial charge transport. Consequently, TETA-based treatment significantly enhances both efficiency (from 17.07 to 18.03%) and stability (PCE retention from 73.4 to 88.9%) of the PSCs under >65% relative humidity for 1000 h compared to the controlled device without TETA treatment. Furthermore, the TETA vapor also shows an advantageous effect of dramatically improving the performance of PSC devices, which initially had poor performance (from 6.8 to 10.5%) through surface defect passivation.

Novel Design Approach of Extended Gate-On-Source Based Charge-Plasma Vertical-Nanowire TFET: Proposal and Extensive Analysis

While quenching the thirst of device scaling, it is utmost necessary not to compromise device performance. Thus, the scaling of tunnel devices is much efficient as compared to Field-Effect Transistors. A dopingless Nanowire (NW) Tunnel Field Effect Transistor (TFET) is proposed and analyzed in the submitted work. Intrinsic silicon is used as a base material for the NWTFET structure. The required doping for the proper working of TFET is induced by implementing the Charge-Plasmatechnique. The vertical NWTFET structure is scaled down to smaller dimensions to show the possibility and consistent performance of Nano-TFET devices. The proposed structure provides a better solution for the requirement of stringent conditions. The scaled-down source/drain length is kept equal to the Debye length to compensate for the need of abrupt junction at the source-channel interface. Several device parameters and analog characteristics are calculated to understand the proposed device behavior at different operating biases. The proposed deviceswitches with a steep subthreshold slope while maintaining ultra-low OFF-state current. The subthreshold slope is lowest for silicon-based TFET devices ever reported. The linearity parameters are calculated to check the effect of noise introduced within the device due to scaled-down dimensions. The proposeddevice is optimized by using Dual-Metal-Gate and Gate-On-Source technique. The achieved drain current is approx. 10 μA/μm for gate bias equal to 1 V.

Enhanced Stability and Optimized Morphology Induced by Electric‐Field‐Assisted Annealing of Bulk Heterojunction Solar Cells

The utility of electric fields during the bulk heterojunction (BHJ) film drying process for tuning the morphology and stability of the device is demonstrated. An external electric‐field‐assisted annealing (EFTA) treatment is used to engineer the stability of amorphous donor polymer‐based BHJs without compromising device performance. Residual additive in the device post fabrication is a major source of degradation. Thermal annealing of an active layer effectively removes residual additive, which in case of amorphous polymer donor‐based BHJs, however, leads to unfavorable changes in the morphology. The detrimental effect of thermal annealing in amorphous donor polymer‐based solar cells is mitigated by the presence of an electric field during the drying stage. The complete removal of additive is ensured by this treatment procedure and leads to improved packing and a rigid morphology. The structural stability is reflected in the performance parameters monitored over the long term and electrical noise measurements. The magnitude and polarity of the applied electric field are observed to control the vertical distribution of donor and acceptor components.

Procedure for the Transfer of Polymer Films Onto Porous Substrates with Minimized Defects.

The fabrication of devices containing thin film composite membranes necessitates the transfer of these films onto the surfaces of arbitrary support substrates. Accomplishing this transfer in a highly controlled, mechanized, and reproducible manner can eliminate the creation of macroscale defect structures (e.g., tears, cracks, and wrinkles) within the thin film that compromise device performance and the usable area per sample. Here, we describe a general protocol for the highly controlled and mechanized transfer of a polymeric thin film onto an arbitrary porous support substrate for eventual use as a water filtration membrane device. Specifically, we fabricate a block copolymer (BCP) thin film on top of a sacrificial, water-soluble poly(acrylic acid) (PAA) layer and silicon wafer substrate. We then utilize a custom-designed, 3D-printed transfer tool and drain chamber system to deposit, lift-off, and transfer the BCP thin film onto the center of a porous anodized aluminum oxide (AAO) support disc. The transferred BCP thin film is shown to be consistently placed onto the center of the support surface due to the guidance of the meniscus formed between the water and the 3D-printed plastic drain chamber. We also compare our mechanized transfer-processed thin films to those that have been transferred by hand with the use of tweezers. Optical inspection and image analysis of the transferred thin films from the mechanized process confirm that little-to-no macroscale inhomogeneities or plastic deformations are produced, as compared to the multitude of tears and wrinkles produced from manual transfer by hand. Our results suggest that the proposed strategy for thin film transfer can reduce defects when compared to other methods across many systems and applications.

Enabling High-Performance Heterogeneous Integration via Interface Standards, IP Reuse, and Modular Design

Abstract DARPA is leading a new thrust to leverage mainstream semiconductor design approaches to enable the rapid and cost-effective integration of heterogeneous device technologies. This represents a leap ahead beyond the monolithic silicon approach that has served the semiconductor industry well, but which now creates prohibitive cost and design issues at leading-edge nodes, as well as performance constraints without the benefits of broad device technology options. DARPA's Common Heterogeneous Integration and IP Reuse Strategies (CHIPS) program will develop interface standards, IP reuse methodologies, and modular design approaches with the goal of making heterogeneous integration as straightforward as printed circuit board design and assembly, without compromising device performance. An overview of the program's vision, goals, and progress to date is presented here.

Defect Engineering toward Highly Efficient and Stable Perovskite Solar Cells

AbstractMetal halide perovskite solar cells are emerging candidates amongst the next‐generation thin‐film photovoltaic devices with extremely low fabrication cost and high power conversion efficiency. Defects (both in the bulk material and at the interfaces) are recognized as one of the most fundamental reasons for the compromised device performance and long‐term stability of perovskite solar cells. In this review article, the authors analyze the possible origins of the defects formation in metal halide perovskites, followed by the rationalization of various approaches being utilized to reduce the density of defects. The authors demonstrate that defect engineering, including adding dopants in the precursor solutions, interface passivation, or other physical treatments (thermal or light stress), is an essential way to further boost the device performance and enhance their long‐term stability. The authors note that although the exact mechanisms of defect elimination in some approaches are yet to be elucidated, the research on defect engineering is expected to have enormous impact on next wave of device performance optimization of metal halide perovskite solar cells toward Shockley–Queisser limit.

Sputter-deposited magneto-optical garnet for all-mode (transverse electric/transverse magnetic) Faraday rotators

Abstract

Hybrid reflections from multiple x-ray scattering in epitaxial bismuth telluride topological insulator films

Epitaxial films of bismuth telluride topological insulators have received increasing attention due to their potential applications in spintronic and quantum computation. One of the most important properties of epitaxial films is the presence of interface defects due to the lateral lattice mismatch since electrically active defects can drastically compromise device performance. By describing hybrid reflections in hexagonal bismuth telluride films on cubic substrates, in-plane lattice mismatches were characterized with accuracy at least 20 times better than using other X-ray diffraction methods, providing clear evidence of 0.007% lateral lattice mismatch, consistent with stress relaxation associated with van der Waals gaps in the film structure.

Nano-Localized Thermal Analysis and Mapping of Surface and Sub-Surface Thermal Properties Using Scanning Thermal Microscopy (SThM).

Determining and acting on thermo-physical properties at the nanoscale is essential for understanding/managing heat distribution in micro/nanostructured materials and miniaturized devices. Adequate thermal nano-characterization techniques are required to address thermal issues compromising device performance. Scanning thermal microscopy (SThM) is a probing and acting technique based on atomic force microscopy using a nano-probe designed to act as a thermometer and resistive heater, achieving high spatial resolution. Enabling direct observation and mapping of thermal properties such as thermal conductivity, SThM is becoming a powerful tool with a critical role in several fields, from material science to device thermal management. We present an overview of the different thermal probes, followed by the contribution of SThM in three currently significant research topics. First, in thermal conductivity contrast studies of graphene monolayers deposited on different substrates, SThM proves itself a reliable technique to clarify the intriguing thermal properties of graphene, which is considered an important contributor to improve the performance of downscaled devices and materials. Second, SThM's ability to perform sub-surface imaging is highlighted by thermal conductivity contrast analysis of polymeric composites. Finally, an approach to induce and study local structural transitions in ferromagnetic shape memory alloy Ni-Mn-Ga thin films using localized nano-thermal analysis is presented.

High-Voltage Generation in CMOS Photovoltaic Devices by Localized Substrate Removal

This letter reports a localized substrate removal (LSR) process for on-chip electrical isolation to promote the generation of high voltages by backside-illuminated complementary metal–oxide–semiconductor (CMOS) photovoltaic (PV) devices. The proposed four-cell-cascaded CMOS PV module provides an open-circuit voltage of 2.05 V, a short-circuit current of 2.6 mA, and a conversion efficiency of 11% under 40-mW input laser intensity. The proposed LSR approach can also be used for the stacking of additional on-chip PV cells to generate even more voltage without the need for additional processing steps and without compromising device performance. High-voltage CMOS PV modules could be used in voltage-driven microelectronic circuits (>1.8 V) or current-driven light sources (>1 mA) for a variety of applications.

Non‐Conjugated Flexible Linkers in Semiconducting Polymers: A Pathway to Improved Processability without Compromising Device Performance

Semiconducting polymers, in contrast to inorganic silicon, are solution processable and can potentially be printed cost efficiently on flexible large‐area substrates. However to do so it is of paramount importance to formulate the polymeric semiconductors into inks with specific viscosities. Herein, the synthesis of a new highly soluble isoindigo monomer and its incorporation into low bandgap semiconducting polymers is presented. Non‐conjugated flexible linkers are introduced into the conjugated backbone in order to modulate the materials processability. The viscoelastic properties of the new polymers are studied in detail by means of rheometry and dynamical mechanical analysis. The solution viscosity is directly proportional to the content of non‐conjugated linkers in the polymer backbone. In organic field‐effect transistors maximum hole mobilities of 1.7 cm2 V−1 s−1 are achieved with the new polymers. Due to the enhanced solubility all‐polymer solar cells are fabricated by solution shearing, reaching power conversion efficiency values of 3.7%.

Progress and Design Concerns of Nanostructured Solar Energy Harvesting Devices.

Integrating devices with nanostructures is considered a promising strategy to improve the performance of solar energy harvesting devices such as photovoltaic (PV) devices and photo-electrochemical (PEC) solar water splitting devices. Extensive efforts have been exerted to improve the power conversion efficiencies (PCE) of such devices by utilizing novel nanostructures to revolutionize device structural designs. The thicknesses of light absorber and material consumption can be substantially reduced because of light trapping with nanostructures. Meanwhile, the utilization of nanostructures can also result in more effective carrier collection by shortening the photogenerated carrier collection path length. Nevertheless, performance optimization of nanostructured solar energy harvesting devices requires a rational design of various aspects of the nanostructures, such as their shape, aspect ratio, periodicity, etc. Without this, the utilization of nanostructures can lead to compromised device performance as the incorporation of these structures can result in defects and additional carrier recombination. The design guidelines of solar energy harvesting devices are summarized, including thin film non-uniformity on nanostructures, surface recombination, parasitic absorption, and the importance of uniform distribution of photo-generated carriers. A systematic view of the design concerns will assist better understanding of device physics and benefit the fabrication of high performance devices in the future.

A low-cost non-toxic post-growth activation step for CdTe solar cells.

Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technology. With laboratory efficiencies approaching 20 per cent, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per watt (ref. 2) and to minimize the environmental impact. A central part of the manufacturing process involves doping the polycrystalline thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface and to passivate the grain boundaries, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per gram) and a water-soluble source of toxic cadmium ions, presenting a risk to both operators and the environment during manufacture. Here we demonstrate that solar cells prepared using MgCl2, which is non-toxic and costs less than a cent per gram, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9×10(14)cm(-3)) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concentrations of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of electrically active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell production.

Embracing structural nonidealities and asymmetries in two-dimensional nanomechanical resonators.

Mechanical exfoliation is a convenient and effective approach to deriving two-dimensional (2D) nanodevices from layered materials; but it is also generally perceived as unpreferred as it often yields devices with structural irregularities and nonidealities. Here we show that such nonidealities can lead to new and engineerable features that should be embraced and exploited. We measure and analyze high frequency nanomechanical resonators based on exfoliated 2D molybdenum disulfide (MoS2) structures, and focus on investigating the effects of structural nonidealities and asymmetries on device characteristics and performance. In high and very high frequency (HF/VHF) vibrating MoS2 devices based on diaphragms of ~2−5 μm in size, structural nonidealities in shape, boundary, and geometric symmetry all appear not to compromise device performance, but lead to robust devices exhibiting new multimode resonances with characteristics that are inaccessible in their ‘ideal’ counterparts. These results reveal that the seemingly irregular and nonideal 2D structures can be exploited and engineered for new designs and functions.

A graphene solution to conductivity mismatch: Spin injection from ferromagnetic metal/graphene tunnel contacts into silicon

Spin-injection into silicon from a ferromagnetic metal requires a solution to the conductivity mismatch. Oxide tunnel barriers such as MgO, Al2O3, or SiO2 are typically used to solve this problem, but often include defects and must be several monolayers thick to avoid pinholes. At these thicknesses, the overall tunnel-barrier becomes highly resistive, preventing these junctions to be used in devices based on local magnetoresistance. Besides providing a spin dependent interface resistance, these barriers also prevent metal ions from diffusing into silicon, which would severely compromise device performance. Here, we show that we can lower the junction resistance by 2–3 orders of magnitude when using a single layer of graphene as the tunnel barrier rather than SiO2 or Al2O3. Hanle measurements show that the spin lifetime is independent of the tunnel barrier material (graphene, Al2O3, SiO2), demonstrating that the lifetime measured is not dominated by some characteristics of the tunnel barrier. The graphene provides a highly uniform barrier, with well-controlled thickness and minimal defect and trapped charge density, while successfully circumventing the conductivity mismatch between a ferromagnetic metal and Si and preventing metal ion diffusion from the FM contact.