Thin-film Devices Research Articles

Thermal Runaway in Thin Film PV: temperature profile modeling

Thermal runaway is a well known phenomena in degradation analysis of the CdS/CdTe thin film solar cells. The thin thickness of such devices can be deteriorated by the temperature fluctuations after a long enough time. We modeled the temperature profile of thin film devices considering the uniform and nonuniform distribution for temperature of the glass substrate and current flowing through TCO layer. In addition, the temperature profile of a hot spot at the edge of the device has been formulated and compared with the real data and fitting functions represented in a few literature. All the above modeling was based on the heat transfer equation considering the importance of thermal resistances. The convection and conduction resistances were introduced in the considered Biot number optimized for PV applications. The modeling presented in this paper can be extended to other solar cell structures or large scale modules. The current study suggests that the thickness and temperature dependencies of the thermal conductivity, convection and conduction resistance might be taken into account and be optimized in thermal runaway issues of thin film devices. The hot spot modeling suggests that the current flowing across the TCO layer can assist the thermal runaway and deformation of neighboring area.

Impact of doping on the mechanical properties of conjugated polymers.

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Dynamically blocking leakage current in molecular tunneling junctions.

Molecular tunneling junctions based on self-assembled monolayers (SAMs) have demonstrated rectifying characteristics at the nanoscale that can hardly be achieved using traditional approaches. However, defects in SAMs result in high leakage when applying bias. The poor performance of molecular diodes compared to silicon or thin-film devices limits their further development. In this study, we show that incorporating "mixed backbones" with flexible-rigid structures into molecular junctions can dynamically block tunneling currents, which is difficult to realize using non-molecular technology. Our idea is achieved by the interaction between interfacial dipole moments and electric field, triggering structured packing in SAMs. Efficient blocking of leakage by more than an order of magnitude leads to a significant enhancement of the rectification ratio to the initial value. The rearrangement of supramolecular structures has also been verified through electrochemistry and electroluminescence measurements. Moreover, the enhanced rectification is extended to various challenging environments, including endurance measurements, bending of electrodes, and rough electrodes, thus demonstrating the feasibility of the dynamic behavior of molecules for practical electronic applications.

Characterization of photocatalytic hybrid TiO2–WOX thin films deposited via co-sputtering

Photocatalytic hybrids of titanium dioxide (TiO2) and tungsten oxide (WOX) can exhibit high photocatalytic activity under weak ultraviolet (UV) and visible-light irradiation. In this study, hybrid TiO2–WOX thin films deposited via co-sputtering are characterized using radiofrequency sputtering at various compositions, and their photocatalytic activity in the degradation of a methylene blue solution is evaluated. X-ray photoelectron spectroscopy measurements show enhanced W(4f) and decreased Ti(2p) signal intensities with increasing WOX content. Further, chemical shifts toward lower binding energies are observed as the compositions of TiO2 and WOX become equivalent. X-ray diffraction analysis reveal decreased anatase TiO2 peaks and monoclinic WOX peaks with the formation of amorphous structures as the compositions of TiO2 and WOX become equivalent. Atomic force microscopy images show homogeneous and flat structures on the surfaces of the hybrid TiO2–WOX thin films. The deposition rate decreases with decreasing WOX sputter power, whereas the optical bandgap decreases with increasing WO3 content. Furthermore, the photocatalytic activity under UV irradiation improves with increasing TiO2 content, whereas that under visible-light irradiation improves with increasing WOX content. This approach can provide the guidelines to fabricate the high-performance photocatalytic materials that can be used in different condensed thin-film devices.

The buffer – Free semitransparent perovskite solar cells with ion-beam sputtered back electrode

Semitransparent perovskite solar cells (PSCs) have the potential for utilization in tandem photovoltaics (PV) and see-through PV for building integration. The passage of light through the thin-film device structure requires transparent contact at the front and back sides of the device. Typically, transparent contacts are made of wide bang-gap conductive oxides (In2O3:SnO2(ITO), etc.) deposited by magnetron sputtering (MS). The use of conventional MS usually requires the addition of a buffer layer to avoid the damage of the charge transporting layers. In this work, the semitransparent perovskite solar cell with p-i-n architecture is fabricated with ITO back electrode deposited directly on C60 electron transporting layer. A comparative analysis was conducted to evaluate the performance and properties of ITO films deposited using ion-beam sputtering (IBS) and the more common MS method. The IBS process was found to eliminate plasma-substrate interaction, resulting in improved deposition on organic layers. Conversely, the MS method resulted in poor output performance and S-shaped IV characteristics of the semitransparent-PSCs. By utilizing IBS fabrication for the back electrode, the relevant efficiency of 12.65% was achieved for the best device. The changes in chemical, optical, and transport properties of the ITO samples fabricated with different methods were investigated. This study represents the first successful utilization of ion-beam sputtering for the fabrication of semitransparent PSCs.

Optimization of structural and electrical properties of graphene-based TiO2 thin film device using Bayesian machine-learning approach

The optimization of laser-induced graphene (LIG) patterning is crucial for achieving desirable electronic properties in flexible electronic devices. In this study, we applied state-of-the-art automated parameter optimization techniques, specifically Bayesian optimization, to enhance the electrical resistance of LIG patterns. By iteratively optimizing the laser power, irradiation time, pressure, and gas type, we achieved minimum LIG resistance within eight batch configurations suggested by the Bayesian optimization (BO) approach. Our method eliminates the reliance on skilled operators, as the initial surrogate models were trained with random parameter evaluations. Notably, our system enables the optimization of material properties even when characterizations are only available outside the experiment loop. Furthermore, the surrogate model provided insights into the underlying mechanisms of LIG growth on quartz substrates. Through partial dependence analysis, we identified relevant physical domains for further investigation, leading to the discovery of negative capacitance and a correlation between structural and electrical properties in LIG. These findings were supported by XPS, Raman, and optical characterizations. Our approach streamlines the experimental design, reducing time and cost while accelerating materials research, and offers human-interpretable conclusions for a deeper understanding of LIG patterning processes.

Effect of Applied Stress on the Interfacial Kinetics, Ionic Conductivity, and Other Phenomena in Thin-Film Solid-State Batteries

Controlled stress is critical to both fabrication and operation of solid-state batteries to ensure pristine, low-impedance interfaces. The effects of externally applied stresses on overall cell performance and degradation due to altered thermodynamics, interfacial kinetics, and bulk transport of the active species have been studied in several prior works. In addition to the applied stack pressure which can vary up to 100s MPa, internal and localized stresses up to a few ~1 GPa can be generated from volume changes during charge and discharge. Electrode volume changes can result in cracking and delamination, causing loss of conductive pathways and active electrode-electrolyte area, and electrolyte fracture in the case of dendrite growth. The extent of these stress-electrochemistry couplings depends on the material and geometry of interfaces that affects the local stress distribution. To decouple some of these stress-potential, stress-kinetics, and stress-transport coupling phenomena, we use a nanoindenter platform to apply controlled uniaxial compressive forces to a thin-film electrochemical devices and batteries1. Interfaces in a thin-film sample can be uniform and planar over large areas, helping us study the effect of stress on the interfacial electrochemistry more directly and with greater quantitative accuracy. In this work we present our observations and analysis on the direct electrochemical-mechanical coupling in thin-film TiO2 and V2O5 metal oxide thin films as the working electrode with the single-ion Li+ conductor solid electrolyte LiPON. Preliminary results show that external uniaxial compressive load affects the electrochemistry observed through cyclic voltammetry and electrochemical impedance spectroscopy (EIS) measurements on thin-film batteries with TiO2 and V2O5 electrodes.

Solution-Processed Robust Multifunctional Memristor of 2D Layered Material Thin Film.

Memristors have gained significant attention recently due to their unique ability to exhibit functionalities for brain-inspired neuromorphic computing. Here, we demonstrate a high-performance multifunctional memristor using a thin film of liquid-phase exfoliated (LPE) 2D MoS2 pinched between two electrodes. Nanoscale inspection of a solution-processed MoS2 thin film using scanning electron and scanning probe microscopies revealed the high-quality and defect-free nature. Systematic current-voltage (I-V) characterizations depict a facile, nonvolatile resistive switching behavior of our 2D MoS2 thin film device with a current On/Off ratio of 103 and energy cost of only a few picojoules. Excellent performance metrics, including at least 103 cycle endurance, 104 s retention, and switching speed down to a few nanoseconds, reflect robust high-performance data storage capability. Charge carriers trapping and detrapping at the sulfur vacancy defect sites in MoS2 nanosheets mainly display the resistive switching property, supported by the impedance analysis and theoretical fitting results. Multifunctionality is leveraged through implementing two-input logic gate operations, edge computation, and crucial adaptive learning via a Pavlov's dogs experiment. Overall, our solution-processed MoS2 memristor has the potential for tremendous future opportunities in integrated circuits and different computing paradigms, including energy-efficient neuromorphic computing hardware in artificial intelligence.

Diffusion-controlled annealing kinetics of interstitial H in SnO2

SnO2 is a prototypical transparent conducting oxide that finds widespread applications as transparent electrodes, gas sensors, and transparent thin-film devices. Hydrogen impurities in SnO2 give rise to unintentional n-type behavior and unexpected changes to conductivity. Interstitial H (Hi) and H at an oxygen vacancy (HO) are both shallow donors in SnO2. An O–H vibrational line at 3155 cm−1, that can be produced by a thermal anneal at 500 °C followed by a rapid quench, has been assigned to the Hi center and is unstable at room temperature on a timescale of weeks. An IR absorption study of the decay kinetics of the 3155 cm−1 O–H line has been performed. The disappearance of Hi upon annealing has been found to follow second-order kinetics. Measurements of the decay rate for a range of temperatures have determined an activation energy for the diffusion of interstitial H in SnO2. These results provide fundamental information about how unintentional hydrogen impurities and their reactions can change the conductivity of SnO2 device materials in processes as simple as thermal annealing in an inert ambient.

Fabrication and structure analysis of freestanding BaTiO3–CeO2 epitaxial nanocomposite membranes

Vertically aligned nanocomposite films hold the promise of making a significant contribution to the performance enhancement and multifunctional integration of flexible thin-film devices but are hindered by the difficulty of direct epitaxial growth on flexible substrates. In this work, freestanding BaTiO3–CeO2 epitaxial nanocomposite membranes were prepared by using water-soluble Sr3Al2O6 as the sacrificial buffer during film deposition. The membrane's lattice parameters as well as their epitaxial relationship, before and after its removal from the single-crystal substrates, were quantitatively measured and analyzed by combining x-ray diffraction and TEM. In addition, synchrotron soft x-ray absorption spectroscopy reveals the discrepancy in the surface and internal electron states of Ce and Ti via comparative studies in total electron yield mode and transmission mode, respectively. This work exemplified that freestanding membranes provide a convenient platform for (1) the study of in-plane lattice information of epitaxial oxide films via TEM and for (2) the comparative study of surface and internal electron band information of epitaxial oxide films.

The Transient Cooling Performance of a Compact Thin-Film Thermoelectric Cooler with Horizontal Structure

Thermoelectric cooling is an ideal solution for chip heat dissipation due to its characteristics of no refrigerant, no vibration, no moving parts, and easy integration. Compared with a traditional thermoelectric device, a thin-film thermoelectric device significantly improves the cooling density and has tremendous advantages in the temperature control of electronic devices with high-power pulses. In this paper, the transient cooling performance of a compact thin-film thermoelectric cooler with a horizontal structure was studied. A 3D multi-physics field numerical model with the Thomson effect considered was established. And the effects of impulse current, thermoelectric leg length, pulse current imposition time, and the size of the contact thermal resistance on the cooling performance of the device were comprehensively investigated. The results showed that the model achieved an active cooling temperature difference of 25.85 K when an impulse current of 0.26 A was imposed. The longer the length of the thermoelectric leg was, the more unfavorable it was to the chip heat dissipation. Due to the small contact area between different sections of the device, the effect of contact thermal resistance on the cooling performance of the device was moderate.

Abnormal evolution of resistivity and microstructure of annealed Ag nanoparticles/Ag–Mo films

Abstract Ag–Mo films with different thicknesses were prepared on polyimide substrate by magnetron sputtering and annealed at different temperatures. The effects of film thickness and annealing temperatures on the resistivity and microstructure of Ag–Mo alloys were investigated. Results show that many Ag nanoparticles were self-formed on Ag–Mo films’ surface. As the thickness of the Ag–Mo film increased from 163 to 409 nm, there was a significant decrease in its resistivity, dropping from 485.44 to 237.12 μΩ cm. Resistivity of the Ag–Mo film is dependent on the annealing temperature. When temperature rises from room temperature to 180℃, the resistivity decreases from 390.62 to 339.37 μΩ cm, with little change. After annealing above 270℃, a sudden increase of resistivity of the Ag–Mo film was attributed to the growth of Ag particles on the film surface contributing to the increase in surface roughness, which hindered electron transport and caused severe surface scattering. High-resistivity Ag–Mo films are expected to be candidates for high-resistivity thin-film devices.

Wireless Deep Brain Stimulation by Ultrasound-Responsive Molecular Piezoelectric Nanogenerators.

Implantable neural stimulation devices are becoming prevalent in bioelectronic medicine for the precise treatment of various clinical diseases. Nevertheless, the limited lifespan and buckling size of the implanted devices remain significant obstacles for chronic clinical application. In this study, we developed an ultrasound-driven battery-free neurostimulator based on a high-performance mini-sized nanogenerator and demonstrated its successful application for the deep-brain-stimulation (DBS) therapy of Parkinson's disease in a rat model. This soft piezoelectric-triboelectric hybrid nanogenerators (PTNG) are made of porous thin-films of molecular piezoelectric materials, which have great advantages of facile, scalable, low-temperature, and flexible processing. Without any bucky accessory control circuits, the subcutaneously implanted soft PTNG can function as a wirelessly powered neurostimulator, allowing for the adjustment of stimulation parameters through external programmable ultrasound pulses. This DBS electroceutical application of energy-harvesting thin-film devices based on molecular piezoelectric materials provides valuable insight into the development of a soft high-performance bioelectronic device.

Levitation force measurement and characteristics analysis of double sided high temperature superconducting thin film device

In this paper, the levitation characteristics of double sided high temperature superconducting (HTS) thin film device is proposed and measured. Based on the unique advantages such as, passive self-stable levitation, higher payload, lower energy consumption, less maintenance cost, etc., HTS-magnetic levitation (maglev) technology has been applied in the traditional fields (rail transit and energy storage fields) and novel fields (astrophysics, space station microgravity, vibration isolation fields). As the key core components, HTS device should meet the requirements of volume, weight and cost, to improve the system operation efficiency. Thus, considering the low profile, miniaturization and lightweight characteristics of the double sided YBa2Cu3O7-x (YBCO) thin film, we measured its levitation and relaxation characteristics. Meanwhile, we also designed the YBCO thin film with meander-lines, to explore the levitation characteristic evolution mechanism. It can be found from the experimental results that, the single piece of YBCO thin film with/without meander-lines can offer the levitation force more than 400 mN and 800 mN, respectively. The signature of this work is that the double sided YBCO thin film has the potential to be employed as the core device in the special application fields (such as astrophysics and space station), to supply the sufficient micro levitation force in limited volume and weight.

Large Room-Temperature Magnetoresistance in a High-Spin Donor-Acceptor Conjugated Polymer.

Open-shell conjugated polymers (CPs) offer new opportunities for the development of emerging technologies that utilize the spin degree of freedom. Their light-element composition, weak spin-orbit coupling, synthetic modularity, high chemical stability, and solution-processability offer attributes that are unavailable from other semiconducting materials. However, developing an understanding of how electronic structure correlates with emerging transport phenomena remains central to their application. Here, the first connections between molecular, electronic, and solid-state transport in a high-spin donor-acceptor CP, poly(4-(4-(3,5-didodecylbenzylidene)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-2-yl)-6,7-dimethyl-[1,2,5]-thiadiazolo[3,4-g]quinoxaline), are provided. At low temperatures (T < 180 K), a giant negative magnetoresistance (MR) is achieved in a thin-film device with a value of -98% at 10 K, which surpasses the performance of all other organic materials. The thermal depopulation of the high-spin manifold and negative MR decrease as temperature increases and at T > 180 K, the MR becomes positive with a relatively large MR of 13.5% at room temperature. Variable temperature electron paramagnetic resonance spectroscopy and magnetic susceptibility measurements demonstrate that modulation of both the sign and magnitude of the MR correlates with the electronic and spin structure of the CP. These results indicate that donor-acceptor CPs with open-shell and high-spin ground states offer new opportunities for emerging spin-based applications.

Anisotropic Superconducting Nb2 CTx MXene Processed by Atomic Exchange at the Wafer Scale.

Superconductivty has recently been induced in MXenes through surface modification. However, the previous reports have mostly been based on powders or cold-pressed pellets, with no known reports on the intrinsic superconsucting properties of MXenes at the nanoale. Here, it is developed a high-temperature atomic exchange process in NH3 atmosphere which induces superconductivity in either singleflakes or thin filmsof Nb2 CTx MXene. The exchange process between nitrogen atoms and fluorine, carbon, and oxygen atoms in the MXene lattice and related structural adjustments are studied using both experiments and density functional theory. Using either single-flake or thin-film devices, an anisotropic magnetic response of the 2D superconducting transformation has beensuccessfully revealed. The anisotropic superconductivity is further demonstrated using superconducting thin films uniformly deposited over a 4 in. wafers, which opens up the possibility of scalable MXene-based superconducting devices.

Fabrication of YSZ ceramic thin films with sol-gel method for mixed potential-type zirconia-based NO2 sensor

In this work, yttria-stabilized zirconia (YSZ) thin films were prepared by sol-gel and spin-coating methods, which are simple and inexpensive, with good film quality and excellent electrical conductivity. Using this film, a YSZ film mixed-potential type NO2 gas sensor with NiO as the sensitive electrode material was fabricated. The films had the best dielectric properties when spin-coated four times, and the sensors had the highest response (48 mV–100 ppm NO2) as well as the best sensitivity (14.8 mV/decade for 5–20 ppm NO2 and 42 mV/decade for 20–500 ppm NO2). The YSZ thin-film gas sensor operates at a lower temperature, with an optimum operating temperature of 550 °C, and the power consumption of the sensor has been reduced by approximately 46 % or more compared to conventional bulk YSZ NO2 gas sensors. This provides a very effective direction for the low-power design of high-temperature devices such as the YSZ gas sensor. In addition, thin-film devices are more conducive to integrating sensors into microelectronic circuits, greatly improving the application scenarios for such devices. This sensor has excellent repeatability, very good selectivity, and outstanding long-term stability over 30 days of continuous measurement. In conclusion, based on the excellent sensing performance and low power consumption of the YSZ thin-film gas sensor, the sensor has a promising future for in-situ monitoring of NOx.

Atomic layer deposition of magnetic thin films: Basic processes, engineering efforts, and road forward

Atomic layer deposition (ALD) is known as a key enabler of the continuous advances in device engineering for microelectronics. For instance, the state-of-the-art transistor technology depends entirely on ALD-grown high-κ materials. Another application branch where ALD could potentially play a similar important role in future is the magnetic thin film devices. Spin-based devices based on high-quality magnetic thin films are anticipated to provide high-efficiency operations with low power consumption. The strict quality demands the magnetic thin films must fulfill in the next-generation applications form the strong bases for the efforts to implement ALD in this application area. In this first comprehensive review on the topic, our aim is to provide an insightful account of the ALD processes so far developed for magnetic materials and to highlight the application-relevant magnetic properties of the thus fabricated thin films. Moreover, we discuss the various innovative engineering efforts made toward different multi-layered and nanostructured composite materials and complex architectures uniquely enabled by the sophisticated self-terminated film-growth mechanism of ALD. The review is finished with a brief outlook toward the future prospects and challenges in the field.

All layers patterned conical nanostructured thin-film silicon solar cells for light-trapping efficiency improvement

Thin-film silicon solar cells (TSSC) has received great attention due to its advantages of low cost and eco-friendly. However, traditional single-layer patterned solar cells (SPSC) still fall short in light-trapping efficiency. This article presents an all layers patterned (ALP) conical nanostructured TSSC to enhance the low absorption caused by the thin absorption layers. The Finite-Difference Time-Domain result shows that a photocurrent density up to 41.27 mA/cm2 can be obtained for the structure, which is 31.39% higher than that of the SPSC. An electrical optimization simulation of doping concentration was carried out on the parameters of the optically optimal structure of the model. The power conversion efficiency is 17.15%, which is 1.72 times higher than that of the planar structure. These results demonstrate a success for the potential and prospect of the fully patterned nanostructures in thin-film photovoltaic devices.

Self-sacrifice PbI2 seed layer-assisted crystal growth for improved perovskite thin films for efficient and stable transportation layer-free photodetectors

Recently, the growth of improved perovskite thin films with high crystallinity and low defects has been an important research subject owing to their critical roles in achieving efficient and stable optoelectronic devices. In this work, we introduced a self-sacrifice PbI2 seed layer to prepare improved CH3NH3PbI3 thin films with high crystallinity and reduced defects. It showed that the PbI2 seed layer at the buried interface acted as nuclei and modulated the crystal growth of CH3NH3PbI3, which induced the vertically upward growth of CH3NH3PbI3, resulting in increased sizes of grains and reduced grain boundaries. Further observations verified the suppression of defect and trap states in the obtained CH3NH3PbI3 thin films. On this basis, efficient and stable transportation layer-free Au/CH3NH3PbI3/indium tin oxide photodetectors have been demonstrated, where the responsivity and detectivity are as high as 0.54 A/W and 7.5 × 1012 Jones, respectively, at the bias of 0 V. It is supposed that the self-sacrifice PbI2 seed layer-assisted crystal growth method will find applications in fabricating high-quality and stable perovskite thin film devices in the future.