Bilayer Devices Research Articles

Reconfigurable Logic Operations via Gate Controlled Skyrmion Motion in a Nanomagnetic Device

Owing to the topological protection and the ease of efficient manipulation, skyrmions have emerged as potential candidates for carrying information in future memory and logic devices. Here, we have proposed a reconfigurable skyrmion based two-input logic device architecture. Using micromagnetic simulations, we have demonstrated that the device is capable of performing both OR and AND logic gate functionalities in a reconfigurable manner. Different logic functionality of the device is selected by using a current through a nonmagnetic metallic gate, and the resultant Oersted field controls the trajectory of the skyrmion, which in turn determines the logic states. The logic functions are implemented on a ferromagnet/heavy metal bilayer device structure by virtue of several physical effects, such as the spin–orbit torque, skyrmion–edge repulsion, skyrmion–skyrmion topological repulsion, and skyrmion Hall effect. The skyrmion trajectory has been characterized by estimating the skyrmion Hall angle. A wide range of operations by varying the current density, skyrmion velocity, Dzyaloshinskii–Moriya interaction, magnetic anisotropy, and geometrical parameters have been presented in detail. We believe that our spin orbit torque driven logic design will have potential implications for a high-speed and low-power skyrmion based computing architecture.

Diffusion interface layer controlling the acceptor phase of bilayer near-infrared polymer phototransistors with ultrahigh photosensitivity

The narrow bandgap of near-infrared (NIR) polymers is a major barrier to improving the performance of NIR phototransistors. The existing technique for overcoming this barrier is to construct a bilayer device (channel layer/bulk heterojunction (BHJ) layer). However, acceptor phases of the BHJ dissolve into the channel layer and are randomly distributed by the spin-coating method, resulting in turn-on voltages (Vo) and off-state dark currents remaining at a high level. In this work, a diffusion interface layer is formed between the channel layer and BHJ layer after treating the film transfer method (FTM)-based NIR phototransistors with solvent vapor annealing (SVA). The newly formed diffusion interface layer makes it possible to control the acceptor phase distribution. The performance of the FTM-based device improves after SVA. Vo decreases from 26 V to zero, and the dark currents decrease by one order of magnitude. The photosensitivity (Iph/Idark) increases from 22 to 1.7 × 107.

Interfacial Molecular Doping at Donor and Acceptor Interface in Bilayer Organic Solar Cells

Molecular doping is an effective means to tune the optoelectronic properties of organic semiconductors. Despite its versatility, its application in bulk heterojunction (BHJ) organic solar cells (OSCs) is still limited, especially blend‐cast doping. Difficulties in desolving molecular dopants in weakly polar solvents have led to the formation of undesired structures, with relatively high concentrations of dopants affecting the bicontinuous BHJ morphology. Bilayer OSCs are stacked structures with sequential deposition of donor and acceptor films, which make important contributions to the fine tuning of morphology and balanced charge transport. The sequential deposition of thin films in bilayer devices organically facilitates the sequential doping of pure donor polymers by dopants. The sequential doping of the dopant at the interface between the donor and the acceptor can efficiently improve the carrier mobility and the charge transfer between the donor and the acceptor without damaging the quality of the film, thus improving the device performance efficiently. With three different dopants F4TCNQ, F6TCNNQ, and BCF, sequential interfacial doping in bilayer devices is found to be a versatile and effective method to improve device performance. Making this simple doping strategy promising for high‐performance bilayer OSCs, it is evaluated as a promising alternative to BHJ OSCs.

Strengthening the Intermolecular Interaction of Prototypical Semicrystalline Conjugated Polymer Enables Improved Photocurrent Generation at the Heterojunction.

The intra and intermolecular interactions (J- and H-type aggregation) in the conjugated polymer films are found to readily facilitate the electron and hole transport, respectively. However, how those different aggregation types influence the photocurrent generation at the heterojunction is still mysterious, especially for the newly developed semicrystalline conjugated polymers. Here, the prototypical copolymer PM6 is used to tune the relative content of aggregation types with various halogen-free processing solvents. Various measurements reveal that the toluene-processed PM6 film exhibits the increased H-aggregates and crystallinity in the π-π stacking direction compared to its o-xylene- and trimethylbenzene (TMB)-processed counterparts. This is partly resulted from the weak steric effect and good solubility in the PM6 solution prepared with toluene, which strengthens the intermolecular interaction of adjacent polymer segments. After analyzing the photovoltaic properties of PM6/Y6 bilayer devices, the faster charge carrier transport, smaller charge recombination, lower energy losses, and interfacial energetic disorder can be observed in the toluene-processed device, leading to the synergistically improved short-circuit current density (JSC ) and open-circuit voltage (VOC ). These findings indicate the control of the molecular packing structure in terms of aggregation types is a powerful strategy to promote the photocurrent generation process at the conjugated polymer-based heterojunction.

Long-term stable perovskite solar cells prepared by doctor blade coating technology using bilayer structure and non-toxic solvent

The doctor blade coating technology for the perovskite solar cells (PSCs) is studied. The bilayer and triple-layer structures based on CsFAPb(IBr)3 and MAPbI3 layers were designed and fabricated by the doctor blade coating. The conventional highly toxic dimethylformamide (DMF) used in the precursors are replaced by the mixtures of gamma-butyrolactone (GBL) and dimethyl sulfoxide (DMSO). Single cation and mixed-cation perovskite layer with different stacks are also compared. It is found that the as-prepared single cation perovskite (MAPbI3) PSCs have better photovoltaic performance but worse lifetime than those of the mixed-cation perovskite (CsFAPb(IBr)3) PSCs. Additionally, the bi-layer structure PSC has a brighter and smoother surface, higher quantum efficiency, and higher crystallinity. Those factors concerning the MAPbI3/CsFAPb(IBr)3(upper/lower) solar cell to have superior properties and better lifetime under the elevated temperature and humidity than that of the CsFAPb(IBr)3/MAPbI3(upper/lower) solar cell. Moreover, when the MAPbI3 is used as the capping layer of the CsFAPb(IBr)3/MAPbI3 bilayer PCS, its photovoltaic performance is better. Secondary ion mass spectroscopy indicates that MAPbI3 capping layer has better ability to dissolve the bottom layers than that of CsFAPb(IBr)3 due to the reduction of inter-diffusion of molecules and smooth surface during lifetime test. Finally, the MAPbI3/CsFAPb(IBr)3 bilayer device shows the best initial power conversion efficiency (PCE), Voc, and Jsc of 15.4%, 1.01 V, and 20.7 mA/cm2 respectively. The encapsulated PSC keeps 92% of original PCE under ambient environment at room temperature and relative humidity of 40% after 1000 h.

Effect of the Oxygen Composition Control of HfOx Films on Threshold and Memory Switching Characteristics for Hybrid Memory Applications

AbstractConsidering a high‐density vertical X‐point memory array, the film thickness of an active device for the selector and memory needs to be scaled down. Here, the authors propose an AgTe/HfOx‐based hybrid memory device with a film thickness of less than 10 nm by adopting a bilayer (HfO1.8/HfO2) film using precise composition control. Non‐volatile switching characteristics can be obtained by considering the formation of stable nanoscale Ag filaments in the nanoporous region in non‐stoichiometric HfO1.8 deposited via sputtering. By contrast, volatile switching characteristics are achieved by an unstable atomic‐scale Ag path in stoichiometric HfO2 deposited via plasma‐enhanced atomic layer deposition. Based on density functional theory calculations, it is confirmed that the stoichiometry of the HfOx film is the key parameter that controls the switching characteristics. The bilayer (HfO1.8/HfO2) device exhibits excellent hybrid memory characteristics, such as distinct read margin (>1.5 V), high ON/OFF ratio (>106), and extremely low OFF current (≈pA), for vertical X‐point memory applications.

Zinc oxide and indium-gallium-zinc-oxide bi-layer synaptic device with highly linear long-term potentiation and depression characteristics

The electrical properties, resistive switching behavior, and long-term potentiation/depression (LTP/LTD) in a single indium-gallium-zinc-oxide (IGZO) and bi-layer IGZO/ZnO (ZnO: zinc oxide) memristors were investigated for synapse application. The use of the oxide bi-layer memristors, in particular, improved electrical properties such as stability, memristor reliability, and an increase in synaptic weight states. The set voltage of bi-layer IGZO/ZnO memristors was 0.9 V, and the reset voltage was around − 0.7 V, resulting in a low-operating voltage for neuromorphic systems. The oxygen vacancies in the X-ray photoelectron spectroscopy analysis played a role in the modulation of the high-resistance state (HRS) (oxygen-deficient) and the low-resistance state (oxygen-rich) region. The VRESET of the bi-layer IGZO/ZnO memristors was lower than that of a single IGZO, which implied that oxygen-vacancy filaments could be easily ruptured due to the higher oxygen vacancy peak HRS layer. The nonlinearity of the LTP and LTD characteristics in a bi-layer IGZO/ZnO memristor was 6.77% and 11.49%, respectively, compared to those of 20.03% and 51.1% in a single IGZO memristor, respectively. Therefore, the extra ZnO layer in the bi-layer memristor with IGZO was potentially significant and essential to achieve a small set voltage and a reset voltage, and the switching behavior to form the conductive path.

Biohybrid Variable-Stiffness Soft Actuators that Self-Create Bone.

Inspired by the dynamic process of initial bone development, in which a soft tissue turns into a solid load-bearing structure, the fabrication, optimization, and characterization of bioinduced variable-stiffness actuators that can morph in various shapes and change their properties from soft to rigid are hereby presented. Bilayer devices are prepared by combining the electromechanically active properties of polypyrrole with the compliant behavior of alginate gels that are uniquely functionalized with cell-derived plasma membrane nanofragments (PMNFs), previously shown to mineralize within 2 days, which promotes the mineralization in the gel layer to achieve the soft to stiff change by growing their own bone. The mineralized actuator shows an evident frozen state compared to the movement before mineralization. Next, patterned devices show programmed directional and fixated morphing. These variable-stiffness devices can wrap around and, after the PMNF-induced mineralization in and on the gel layer, adhere and integrate onto bone tissue. The developed biohybrid variable-stiffness actuators can be used in soft (micro-)robotics and as potential tools for bone repair or bone tissue engineering.

A feasibility method of constructing bilayer device for enhanced regulation in the mid-to-far IR region

A dual-layer structure combining the properties of two polymers is demonstrated to be efficient in strengthening the mid-to-far infrared modulation. Prior to the assembly of bilayer devices, the infrared adjustment capability of single-layer devices poly(3,4-ethylenedioxythiophene) (PEDOT)、poly(pyrrole) (PPY) and poly(aniline) (PANI) with different polymerization charges is scrutinized. The results exhibit that the optimal polymerization charge affording the highest reflectance variation is inconsistent between the mid-wavelength and long-wavelength bands in PEDOT and PPY devices. However, through the arrangement of deposition sequence and relative thickness of PEDOT and PPY coatings, a device with the reflectance contrast of 0.662 in the mid-wavelength and 0.281 in the long-wavelength bands was obtained utilizing a bilayer film PEDOT-0.2C/PPY-0.3C. Further FTIR and UV–Vis spectra show that the bilayer films are successfully formed and both polymers play a role in the electrochemical process. Different morphologies of bilayer films with reverse positions observed by SEM images account for the correlation between the infrared regulation performance and deposition sequence.

A Solution‐Processed All‐Perovskite Memory with Dual‐Band Light Response and Tri‐Mode Operation

AbstractIntegrating multiple semiconductors with distinct physical properties is a practical design strategy for realizing novel optoelectronic devices with unprecedented functionalities. In this work, a photonic resistive switching (RS) memory is demonstrated based on solution‐processed bilayers of strontium titanate (SrTiO3 or STO) quantum dots (QDs) and all‐inorganic halide perovskite CsPbBr3 (CPB) with an Ag/STO/CPB/Au architecture. Compared with the single‐layer STO or CPB RS device, the double‐layer device shows considerably improved RS performance with a high switching ratio over 105, an endurance of 3000 cycles, and a retention time longer than 2 × 104 s. The formation of heterojunction between STO and CPB significantly enhances the high resistance state, and the separation of the active silver electrode and the CPB layer contributes to the long‐term stability. More importantly, the photonic RS device exhibits UV–visible dual‐band response due to the photogating effect and the light‐induced modification of the heterojunction barrier. Last, tri‐mode operation, i.e., photodetector, memory, and photomemory, is demonstrated via tailoring the light and electric stimuli. This bilayer device architecture provides a unique approach toward enhancing the performance of photoresponsive data‐storage devices.

A programmable bilayer hydrogel actuator based on the asymmetric distribution of crystalline regions.

A novel strategy to fabricate bilayer hydrogel actuators based on the asymmetric distribution of crystalline regions across the bilayer structures was proposed. By employing PVA polymer chains into an alkali solvent-derived chitosan hydrogel matrix, chitosan/PVA hybrid bilayer hydrogels with both excellent responsive bending and mechanical properties were obtained as pH-controlled manipulators. In the design, the chitosan/PVA hydrogels upon treatment with freeze-thawing cycles were taken as the first monolayer, where excessive crystalline regions appeared. The original chitosan/PVA hydrogel as the second monolayer was then integrated into one bilayer device through the chemical-crosslinking of epichlorohydrin at the interface. The results showed that the resultant chitosan/PVA bilayer hydrogel actuator with a weight ratio of 3 : 1 displayed better sensitivity upon exposure to stimuli. The actuation behaviors are strongly dependent on experimental parameters such as the pH, PVA content and the chemical-crosslinking density. It is proposed that the driving force originates from the asymmetric distribution of crystalline regions, thus resulting in differential swelling ratios between the monolayers. In addition, programmable 3D shape transformations were achieved by using the bilayer hydrogel with designed 2D geometric patterns, and the tailored gripper-like hydrogel actuator can successfully capture and transport the cargo. Moreover, this actuation behavior can be erased and re-written on demand under certain conditions. Taking advantage of this universal strategy, more attractive actuators derived from synthetic or natural polymers in combination with PVA are highly expected, which can be used as smart soft robots in various fields such as manipulators, grippers, and cantilever sensors.

Performance optimization of single graded CIGS absorber and buffer layers for high efficiency: A numerical approach

We present a numerical simulation based study of single graded Cu(In,Ga)Se2 (Copper Indium Gallium Diselenide) thin film solar cell. In this work, initially a basic CIGS single graded cell structure is optimized in terms of thickness, band-gap and doping concentration. CdS is kept as the buffer layer, which is widely used for high efficiency CIGS solar cells. In the next step, CdS is replaced with ZnMgO as the buffer layer in order to exploit its greater photon absorption ability due to its higher band-gap which further enhances the cell efficiency. A thorough analysis is carried out on the solar cell parameters open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and quantum efficiency (η) of the photovoltaic cell structure. An intermediate layer of p-type MoS2 is inserted in between the single graded CIGS absorber layers. The objective is to limit the unintentional Ga inter diffusion and maintain the desired grading during the high temperature annealing for the absorber preparation. The power conversion efficiency of the bilayer device structure with Ga fraction x=(0.31) of the top absorber layer along with Ga fraction y=(0.25) of the bottom absorber layer exhibits an improved efficiency from 24.02% (CdS as the buffer layer) to 25.37% (ZnMgO as buffer layer). An excellent power efficiency of η = 26.78% is reported after adding the intermediate layer of p-type MoS2 and optimizing its thickness and the carrier concentration.

Organic Bilayer Photovoltaics for Efficient Indoor Light Harvesting

AbstractIndoor organic photovoltaics (OPVs) are a potential niche application for organic semiconductors due to their strong and well‐matched absorption with the emission of indoor lighting. However, due to extremely low photocurrent generation, the device parameters critical for efficient indoor OPVs differ from those under 1 Sun conditions. Herein, these critical device parameters—recombination loss and shunt resistance (Rsh)—are identified and it is demonstrated that bilayer OPVs are suitable for indoor PV applications. Compared to bulk‐heterojunction (BHJ), the open‐circuit voltage loss of bilayer devices under low light intensities is much smaller, consistent with a larger surface photovoltage response, indicating suppressed recombination losses. The bilayer devices show a higher fill factor at low light intensities, resulting from high Rsh afforded by the ideal interfacial contacts between the photoactive and the charge transport layers. The high Rsh enables bilayer devices to perform well without a light‐soaking process. Finally, the charge carriers are extracted rapidly in bilayers, which are attributed to strongly suppressed trap states possibly induced by isolated domains and non‐ideal interfacial contacts in BHJs. This study highlights the excellent suitability of bilayer OPVs for indoor applications and demonstrates the importance of device architecture and interfacial structures for efficient indoor OPVs.

Low power multilevel resistive switching in titanium oxide-based RRAM devices by interface engineering

The stochastic nature of conductive filament formation and dissolution always leads to large fluctuations of key device parameters that hinder the practical applications of resistive random-access memories (RRAMs). Here, we report a simple bilayer oxide-based device structure of Al/TiOx/TiOy/FTO (x < y) employed to address this variability issue and improve the overall performance of the memory device. The bipolar resistive switching performance remarkably improved in these bilayer devices with lower forming voltage (~1 V), set/reset voltages of 0.4/-0.6 V, a programming current of 10 μA, an enlarged ON/OFF ratio (>103), longer retention (>103 s), and better uniformity as compared to the control Al/TiOy/FTO device. Moreover, the modulation of TiOx layer thickness enables tunability of switching voltages, currents, and the resistance window. In addition, reliable and reproducible multiple resistance states, more specifically different low resistance states, can be achieved in bilayer devices by controlling the programming current down to 10 μA, which is in contrast to the binary states of the control devices. These improved switching parameters with multilevel storage capability in bilayer devices are attributed to the incorporation of an oxygen vacancy-rich interfacial layer, namely TiOx, which serves as an oxygen vacancy reservoir and facilitates effective conductive filament formation inside the switching layer. Furthermore, the thermodynamics involving the dielectric constant and Gibb's free energy of oxide formation at the Al/TiOx and TiOx/TiOy interfaces play a pivotal role in the associated switching mechanism. The spatial variability of the operating voltage across these devices is found to be as low as 8%. These findings pave the way for low power, low cost, and high density data storage applications.

Polariton Decay in Donor-Acceptor Cavity Systems.

Enhanced delocalization is beneficial for absorbing molecules in organic solar cells, and in particular bilayer devices, where excitons face small diffusion lengths as a barrier to reaching the charge-generating donor-acceptor interface. As hybrid light-matter states, polaritons offer exceptional delocalization which could be used to improve the efficiency of bilayer organic photovoltaics. Polariton delocalization can aid in delivering excitons to the donor-acceptor interface, but the subsequent charge transfer event must compete with the fast decay of the polariton. To evaluate the viability of polaritons as tools to improve bilayer organic solar cells, we studied the decay of the lower polariton in three cavity systems: a donor only, a donor-acceptor bilayer, and a donor-acceptor blend. Using several spectroscopic techniques, we identified an additional decay pathway through charge transfer for the polariton in the bilayer cavity, demonstrating charge transfer from the polariton is fast enough to outcompete the decay to the ground state.

Understanding the Role of CdTe in Polycrystalline CdSexTe1–x/CdTe‐Graded Bilayer Photovoltaic Devices

Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te1–x /CdTe‐graded bilayer device has led to a device efficiency over 19%. A CdSe x Te1–x absorber would increase the short‐circuit current due to its lower bandgap but at the expense of open‐circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short‐circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. Herein, a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first‐principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te1–x .

Implementation of a reservoir computing system using the short-term effects of Pt/HfO2/TaOx/TiN memristors with self-rectification

Given the limitations of von Neumann computing systems, we propose a high-performance reservoir computing system as an alternative. These systems operate as neural networks that store the states of the input signal and require a readout layer for data processing and learning. The advantage of this system is that training only takes place at the readout layer leading to good energy efficiency and low power consumption. In this paper, we implement a memristor-based hardware reservoir computing system using HfO 2 /TaO x bilayer based memristor that can imitate the short-term memory effects. We first characterize the volatility and record the self-rectification I-V curves of the HfO 2 /TaO x bilayer device. We also investigate the transient characteristics in terms of the interval required between pulse stimulation to return its initial state. In terms of transmitting information, 4 bits is a significant unit size because at least 4 bits are required to represent a single-digit number. Motivated by this, we successfully implemented a binary 4-bit code ranging from [0 0 0 0] to [1 1 1 1] in the fabricated memristor that can be used as the input signal to a reservoir layer.

Gate-Defined Quantum Confinement in CVD 2D WS2.

Temperature-dependent transport measurements areperformed on the same set of chemical vapor deposition (CVD)-grown WS2 single- and bilayer devices before and after atomic layer deposition (ALD) of HfO2 . This isolates the influence of HfO2 deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS2 and ALD-grown HfO2 . The high dielectric constant and low leakage current enabled by HfO2 allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications.

Estimation of front and back junctions of CZTSe:Ge solar cells by combined modulus and impedance spectroscopy

We have fabricated and investigated CTSe:Ge bi-layer devices by modulus spectroscopy combined with impedance spectroscopy to understand the electrical properties of hetero- and back-contact junctions. The equivalent circuit model is developed to decouple the properties of each junction at different frequency ranges. Modeling and numerical simulations with the in-house MATLAB modeling suites connected to external simulators, such as Sentaurus TCAD and LEVM/LEVMW software are conducted to estimate the impact of each junction component to modulus and impedance spectroscopy with a parallel equivalent circuit combination of resistance and capacitance components. The fabricated devices show doping concentration of 6.4 × 1015/cm3, and the conversion efficiency is 8.1% based on capacitance–voltage and current–voltage characterization. From the characterization and modeling, the built-in potential of the hetero-junction is estimated as 801 meV, which is ∼10% smaller than the ideal case from TCAD, 894 meV. Conversely, the built-in potential of the back contact junction is 428 meV. The total apparent built-in potential is estimated to be 373 meV from the Mott–Schottky equation.

Phase-resolved electrical detection of hybrid magnonic devices.

We demonstrate the electrical detection of magnon-magnon hybrid dynamics in yttrium iron garnet/permalloy (YIG/Py) thin film bilayer devices. Direct microwave current injection through the conductive Py layer excites the hybrid dynamics consisting of the uniform mode of Py and the first standing spin wave (n = 1) mode of YIG, which are coupled via interfacial exchange. Both the two hybrid modes, with Py or YIG dominated excitations, can be detected via the spin rectification signals from the conductive Py layer, providing phase resolution of the coupled dynamics. The phase characterization is also applied to a nonlocally excited Py device, revealing the additional phase shift due to the perpendicular Oersted field. Our results provide a device platform for exploring hybrid magnonic dynamics and probing their phases, which are crucial for implementing coherent information processing with magnon excitations.