Buffer Layer Research Articles

Fine Tuning the Work Function of ZnO Cathode Buffer Layers in Organic Solar Cells by Phenanthroline Coordination

Fine Tuning the Work Function of ZnO Cathode Buffer Layers in Organic Solar Cells by Phenanthroline Coordination

Study of electrical properties of fullerene based all-small-molecule low donor organic devices with and without cathode buffer layers for photovoltaic application

This article examines electrical properties of fullerene (C70) based all-small-molecule organic photovoltaics with low donor concentration. Enhancements in the properties of devices are observed with the incorporation of lithium fluoride (LiF) and molybdenum trioxide (MoO3) as cathode buffer layers (CBLs). Samples are characterized using field emission-scanning electron microscopy (FESEM), optical absorption spectra, current-voltage (I-V) characteristics in dark and under illumination, and capacitance-frequency (C-ω) measurements. Device with single CBL (LiF) exhibits reduced photoabsorption leading to less saturation photocurrent density (Jsat) and lower maximum exciton generation rate (Gmax), due to non-uniform deposition of LiF in device. In contrast, device having two CBLs (LiF & MoO3) has enhanced the photoabsorption exhibiting high Jsat and Gmax attributed to uniform deposition of MoO3. Moreover, dielectric properties and AC conductivity of devices are also measured confirming the results and these are found to be dependent on the frequency and voltage.

Thermal Transport at the AlN-SiC Interface and Grain Boundary of AlN.

AlN is deposited on silicon carbide (SiC) for high-power electronics; in these devices, AlN acts as both a buffer layer for the growth of the active device and a thermal conductor. However, the mechanism of thermal transport through the AlN-SiC interfaces and through grain boundaries of AlN has not been clearly analyzed, even though AlN forms grain boundaries during the deposition process. The thermal properties of the AlN-SiC interface and the inversion domain boundaries (IDBs) of AlN were examined by a phonon transport model based on a nonequilibrium Green function formalism and first-principles calculations. The interface and grain boundary models were designed, and the thermal resistances (TRs) and origins of TR were examined. The TRs of the AlN-SiC interface and the IDB of AlN are much higher than the TRs of AlN and SiC of relevant thickness. Elemental intermixing and vacancy formation were modeled. The formation of charge-balanced defect of VAl + 3ON is thermodynamically favorable compared to other defects, indicating that ON induces formation of VAl. The charge-balanced defect combining VAl and ON increases the TRs of both AlN-SiC interfaces and AlN grain boundaries because vacancy defects induce larger changes in mass than all other defects, and TRs are proportional to changes in mass. In addition, VAl defects are increased by excess ON, resulting in a continuous increase in TR, and then, the calculated thermal boundary resistance (TBR) of the AlN-SiC interface with increased density of VAl by excess ON reaches the experimental TBR. Therefore, it is expected that the large increase in TR by the formation of VAl + ON would be suppressed by controlling the low O density during synthesis.

Improvement of Single Event Transient Effects for a Novel AlGaN/GaN High Electron-Mobility Transistor with a P-GaN Buried Layer and a Locally Doped Barrier Layer.

In this paper, a novel AlGaN/GaN HEMT structure with a P-GaN buried layer in the buffer layer and a locally doped barrier layer under the gate (PN-HEMT) is proposed to enhance its resistance to single event transient (SET) effects while also overcoming the degradation of other characteristics. The device operation mechanism and characteristics are investigated by TCAD simulation. The results show that the peak electric field and impact ionization at the gate edges are reduced in the PN-HEMT due to the introduced P-GaN buried layer in the buffer layer. This leads to a decrease in the peak drain current (Ipeak) induced by the SET effect and an improvement in the breakdown voltage (BV). Additionally, the locally doped barrier layer provides extra electrons to the channel, resulting in higher saturated drain current (ID,sat) and maximum transconductance (gmax). The Ipeak of the PN-HEMT (1.37 A/mm) is 71.8% lower than that of the conventional AlGaN/GaN HEMT (C-HEMT) (4.85 A/mm) at 0.6 pC/µm. Simultaneously, ID,sat and BV are increased by 21.2% and 63.9%, respectively. Therefore, the PN-HEMT enhances the hardened SET effect of the device without sacrificing other key characteristics of the AlGaN/GaN HEMT.

Growth of VO2-ZnS thin film cavity for adaptive thermal emission

Low-weight, passive, thermal-adaptive radiation technologies are needed to maintain an operable temperature for spacecraft while they experience various energy fluxes. In this study, we used a thin film coating with the Fabry–Pérot (FP) effect to enhance emissivity contrast (Δε) between VO2 phase-change states. This coating utilizes a hybrid material architecture that combines VO2 with a mid- and long-wave infrared transparent chalcogenide, zinc sulfide (ZnS), as a cavity spacer layer. We simulated the design parameter space to obtain a theoretical maximum Δε of 0.63 and grew prototype devices. Using x-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR), we determined that an intermediate buffer layer of TiO2 is necessary to execute the crystalline growth of monoclinic VO2 on ZnS. Through temperature-dependent FTIR measurements, our fabricated devices demonstrated FP-cavity enhanced adaptive thermal emittance.

Optimization of Non-Alloyed Backside Ohmic Contacts to N-Face GaN for Fully Vertical GaN-on-Silicon-Based Power Devices.

In the framework of fully vertical GaN-on-Silicon device technology development, we report on the optimization of non-alloyed ohmic contacts on the N-polar n+-doped GaN face backside layer. This evaluation is made possible by using patterned TLMs (Transmission Line Model) through direct laser writing lithography after locally removing the substrate and buffer layers in order to access the n+-doped backside layer. As deposited non-alloyed metal stack on top of N-polar orientation GaN layer after buffer layers removal results in poor ohmic contact quality. To significantly reduce the related specific contact resistance, an HCl treatment is applied prior to metallization under various time and temperature conditions. A 3 min HCl treatment at 70 °C is found to be the optimum condition to achieve thermally stable high ohmic contact quality. To further understand the impact of the wet treatment, SEM (Scanning Electron Microscopy) and XPS (X-ray Photoelectron Spectroscopy) analyses were performed. XPS revealed a decrease in Ga-O concentration after applying the treatment, reflecting the higher oxidation susceptibility of the N-polar face compared to the Ga-polar face, which was used as a reference. SEM images of the treated samples show the formation of pyramids on the N-face after HCl treatment, suggesting specific wet etching planes of the GaN crystal from the N-face. The size of the pyramids is time-dependent; thus, increasing the treatment duration results in larger pyramids, which explains the degradation of ohmic contact quality after prolonged high-temperature HCl treatment.

Single Layer of Crown Ether Enables Efficient, Stable, and Pb Leakage-Free Inverted Perovskite Solar Cells.

Significant challenges in ensuring long-term stability, addressing environmental safety issues, and improving efficiency have hindered the commercialization of inverted Pb-based halide perovskite solar cells (PeSCs). One reasonable approach to addressing these issues is to place an effective buffer layer between the perovskite active layer and the electrode. In this study, we demonstrate the use of crown ether, di-tert-butyl dibenzo-18-crown-6, as a single buffer layer to improve the efficiency, long-term stability, and environmental safety of PeSCs for the first time. The crown ether buffer layer suppressed Ag diffusion from the Ag metal electrodes, thereby improving the performance and lifetime of the device. In addition, it effectively captures Pb ions that may leak into the environment during the whole lifetime of devices, thereby enhancing the environmental safety of PeSCs. Furthermore, PeSCs incorporating crown ethers as buffer layers demonstrated enhanced stability in a nitrogen atmosphere and achieved a high power conversion efficiency of 22.8%. Consequently, this crown ether buffer layer offers an effective and straightforward strategy capable of achieving efficient, stable, and environmentally safe PeSCs.

Optimized dual-band amplified spontaneous emission properties of PFO and BUBD-1 blend films based on AgNPs doped buffer layers

Dual-band amplified spontaneous emission (ASE) can be used as a spectral analysis technique to detect specific chemical or biological molecules. In this paper, the dual-band ASE properties of blend films containing a small organic molecule N,N’-(4,4′-(1E,1′E)-2, 2′-(1,4-phenylene) bis(ethene-2,1-diyl)) −bis(4,1-phenylene))-bis(2-ethyl-6-methyl-phenylaniline) (BUBD-1) and a conductive polymer Poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) were investigated and optimized. As the doping concentration of BUBD-1 in the PFO was increased from PFO: 1 wt% BUBD-1 to PFO: 5 wt% BUBD-1, the ASE emission wavelength shifted from single- to dual-band and then back. The dual-band ASE performance of the blend film was also improved by introducing a Poly(methyl methacrylate) (PMMA) buffer layer containing silver nanoparticles (AgNPs), which resulted in a significant decrease in the excitation energy threshold of both ASE peaks and a significant increase in the net gain. Due to the LSPR effect of AgNPs, the ASE thresholds for peaks at 460 nm (PFO) and 490 nm (BUBD-1) in the blend film decreased from 12.24 ± 0.49 μJ/pulse to 6.99 ± 0.28 μJ/pulse and from 12.75 ± 0.51 μJ/pulse to 7.03 ± 0.28 μJ/pulse, respectively. The experimental and theoretical simulation demonstrated that the incomplete energy transfer between PFO and BUBD-1 led to a dual-band ASE effect. In addition, the enhanced light absorption, emission, and scattering caused by LSPR in the AgNPs improved the threshold and gain for dual-band ASE. This provides a possibility of realizing dual-band organic semiconductor lasers.

Ultra-thin perovskite solar cells with high specific power density based on colorless polyimide substrates

Ultra-thin perovskite solar cells (UTPSCs) have garnered significant attention for their high specific power and potential application in space missions. However, the efficiency of ultra-thin solar cells has been constrained by challenges in handling and fabricating them on the fragile ultra-thin substrates, leading to notable performance disparities compared to their rigid counterparts. Here, we present a novel method for fabricating efficient and stable ultra-thin flexible PSCs. By employing a polydimethylsiloxane (PDMS) buffer layer and controlling the imidization temperature of colorless polyimide (CPI), we prepared an ultra-thin CPI substrate with a thickness of 1–3 μm, which can be easily peeled off. This technique provides a heat-resistant ultra-thin substrate with a smooth surface for UTPSC fabrication. Additionally, the ultra-thin substrate exhibits superior thermal conductivity and more uniform temperature distribution compared to conventional flexible substrates. Ultimately, we achieve a 1.5 μm UTPSC with a power conversion efficiency of 22.13 % and an outstanding specific power density of 50 W/g, surpassing most existing solar cells. Remarkably, the UTPSCs demonstrate exceptional mechanical robustness, maintaining performance even under rigorous bending and twisting. Moreover, CPI exhibited better thermal expansion matching with perovskite and demonstrated enhanced stability under low-temperature conditions and thermal cycling, showing potential for space applications. Our approach of preparing ultra-thin CPI substrates offers an effective pathway for fabricating lightweight and highly flexible electronics.

Heterogeneous Nucleation Regulation Amends Unfavorable Crystallization Orientation and Defect Features of Antimony Selenosulfide Film for High-Efficient Planar Solar Cells.

Antimony selenosulfide (Sb2(S,Se)3) has obtained widespread concern for photovoltaic applications as a light absorber due to superior photoelectric features. Accordingly, various deposition technologies have been developed in recent years, especially hydrothermal deposition method, which has achieved a great success. However, device performances are limited with severe carrier recombination, relating to the quality of absorber and interfaces. Herein, bulk and interface defects are simultaneously suppressed by regulating heterogeneous nucleation kinetics with barium dibromide (BaBr2) introduction. In details, the Br adsorbs and dopes on the polar planes of cadmium sulfide (CdS) buffer layer, promoting the exposure of nonpolar planes of CdS, which facilitates the favorable growth of [hk1]-Sb2(S,Se)3 films possessing superior crystallinity and small interface defects. Additionally, the Se/S ratio is increased due to the replacement of S/Se by Br, causing a downshift of the Fermi levels with a benign band alignment and a shallow-level defect. Moreover, Ba2+ is located at grain boundaries by coordination with S and Se ions, passivating grain boundary defects. Consequently, the efficiency is increased from 7.70% to 10.12%. This work opens an avenue towards regulating the heterogeneous nucleation kinetics of Sb2(S,Se)3 film deposited via hydrothermal deposition approach to optimize its crystalline orientation and defect features.

Growth Conditions and Interfacial Misfit Array in SnTe (111) Films Grown on InP (111)A Substrates by Molecular Beam Epitaxy.

Tin telluride (SnTe) is an IV-VI semiconductor with a topological crystalline insulator band structure, high thermoelectric performance, and in-plane ferroelectricity. Despite its many applications, there has been little work focused on understanding the growth mechanisms of SnTe thin films. In this manuscript, we investigate the molecular beam epitaxy synthesis of SnTe (111) thin films on InP (111)A substrates. We explore the effect of substrate temperature, Te/Sn flux ratio, and growth rate on the film quality. Using a substrate temperature of 340 °C, a Te/Sn flux ratio of 3, and a growth rate of 0.48 Å/s, fully coalesced and single crystalline SnTe (111) epitaxial layers with X-ray rocking curve full-width-at-half-maxima of 0.09° and root-mean-square surface roughness as low as 0.2 nm have been obtained. Despite the 7.5% lattice mismatch between the SnTe (111) film and the InP (111)A substrate, reciprocal space mapping indicates that the 15 nm SnTe layer is fully relaxed. We show that a periodic interfacial misfit (IMF) dislocation array forms at the SnTe/InP heterointerface, where each IMF dislocation is separated by 14 InP lattice sites/13 SnTe lattice sites, providing rapid strain relaxation and yielding the high quality SnTe layer. This is the first report of an IMF array forming in a rock-salt on zinc-blende material system and at an IV-VI on III-V heterointerface, and highlights the potential for SnTe as a buffer layer for epitaxial telluride film growth. This work represents an important milestone in enabling the heterointegration between IV-VI and III-V semiconductors to create multifunctional devices.

Boosting the performance of SnSe-based solar cells through electron and hole transport layers: A qualitative study and perspectives

The SnSe compound is garnering considerable interest as a potential solar absorber for developing highly efficient thin-film solar cells. Using one experimental report as a foundation and employing the one-dimensional solar cell capacitance simulator (SCAPS-1D), we examined the elements that impact the efficiency of solar cells utilizing tin selenide as the base material. The base structure consists of Anode/SnSe/CdS/i-ZnO/ZnO:Al/Cathode. A 0.5 μm thick SnSe film demonstrated an efficiency of 2.51 %. In the initial phase, we optimized the device efficiency to 18.65 % through parameter adjustments, utilizing toxic CdS as a buffer layer. In the subsequent phase, earth-abundant and non-toxic Zn1-xMgxO, SnO2, and TiO2 alternatives were explored as electron transport materials (ETL) to replace CdS. Zn1-xMgxO (with x = 0.1875) exhibited the highest efficiency at 19.03 %. In the final phase, various hole transport materials (HTL) were studied to enhance the SnSe-based solar cell's performance. Among the HTL materials investigated, NiO yielded the best efficiency of 20.59 % when using Zn1-xMgxO (with x = 0.1875) as a buffer layer.

Magnetic Domain and Structural Defects Size in Ultrathin Films

Herein, a model is proposed for measuring the structural defects size r0 in an ultrathin magnetic layer with perpendicular magnetic anisotropy. Based on the observations of magnetic domains in Ta/Pt/Co/Pt ultrathin films, using polar magneto‐optical Kerr effect microscopy and measurements of their magnetic anisotropies, the correlation between magnetic domains size D and structural defects size r0, as well as the defects concentration parameter αK, which designates the degree of pinning, has been modeled. The average r0 value found is high in the sample with unannealed buffer layers and considerably decreases with annealing. It is 6.17 nm with unannealed Ta/Pt buffer layers, 1.06 nm in sample with Ta/Pt buffer layers annealed at 423 K, and 0.49 nm in that with buffer layers annealed at 573 K. The significant drop of r0 is in good agreement with the high depinning noted with buffer layers annealing in recent work.

Device Simulation of 25.9% Efficient ZnOxNy${\rm ZnO}_x{\rm N}_y$/Si Tandem Solar Cell

AbstractThe novel, high electron mobility material has been investigated theoretically as an absorber in a two‐terminal tandem solar cell. In addition to its high mobility, can attain sufficiently low carrier concentration to enable ‐junctions, and has a tunable bandgap around the 1.7 eV range. It is therefore suitable for pairing with a Si‐based bottom cell. In addition to the layer, the tandem cell consists of a emitter and a Si heterojunction bottom cell. A buffer layer is introduced between the emitter and absorber in the top cell to mediate a large valence band offset that resulted in a poor fill factor, . A buffer layer bandgap of 1.5 eV gave the highest power conversion efficiency (PCE). The objective is to estimate the optimal performance of in a tandem solar cell. The dependence of current–voltage (–) characteristics on thickness, mobility and carrier concentration in the layer is evaluated, and found to yield maximum performance with 0.35 , 250 Vs–1 and , respectively. Using these conditions, the – parameters of the device under AM1.5 illumination are short circuit current density, mA , open circuit voltage, V, and . With this, it is reported on, to the best of the knowledge, the first device simulation based on .

Studies on InZnMgO amorphous buffer layer for Cu(In,Ga)(S,Se)2 solar cell

This study explores the effectiveness of an amorphous buffer layer, specifically Indium Zinc Magnesium Oxide (IZMO), as an alternative buffer in Copper Indium Gallium Sulfide Selenide (CIGSSe) solar cells. The findings reveal a significant impact on efficiency through precise adjustment of the Mg/(In+Zn+Mg) ratio (MIZM) from 0 to 0.23. The bandgap exhibits a consistent increase with an ascending Mg/(In+Zn+Mg) ratio, transitioning from 3.42 eV to 3.63 eV for IZMO prepared with Ar and from 3.18 eV to 3.53 eV for IZMO prepared with an Ar+O2 gas mixture, respectively. This rise is attributed to the augmentation of the conduction band minimum (EC) of IZMO resulting from the addition of MgO. Moreover, an increase in the Mg/(In+Zn+Mg) ratio correlates with improved conversion efficiency, escalating from 6.31 % to 9.19 %. Notably, the open-circuit voltage experiences a rise from 0.430 V to 0.520 V. This is attributed to the heightened EC of IZMO due to MgO addition, which mitigates recombination between the light-absorbing layer and the buffer layer, consequently elevating the open circuit voltage. The addition of MgO also enhances the resistance of the buffer layer, contributing to an increase in shunt resistance and a subsequent decrease in leakage current. Conversely, IZMO introduced with O2 exhibits inferior performance akin to IZO, attributable to substantial sputter damage induced by O2 introduction.

ZnO/Gra/Si structure to improve photoelectric properties

To enhance the interface bonding and optoelectronic properties of ZnO/Si, we employed graphene (Gra) as a buffer layer to mitigate lattice mismatch. Density functional theory (DFT) was utilized to analyze the impact of graphene insertion on the interface structure and optoelectronic properties of ZnO/Si. Our findings indicate strong covalent bonds within the ZnO/Si interface, whereas the ZnO/Gra/Si interface exhibits van der Waals interactions. Additionally, the incorporation of graphene shifts the valence band of ZnO/Gra/Si closer to the conduction band, significantly improving its conductivity. Moreover, ZnO/Gra/Si demonstrates a 74 % increase in visible light utilization compared to ZnO/Si, highlighting the substantial potential of this sandwich structure in solar cell applications.

Preparation and sodium storage properties of CoFe2O4 composite S N co-doped RGO

The application value of iron-base binary oxide in sodium-ion batteries has been widely concerned by researchers, and CoFe2O4 has a better development prospect because of its cost efficiency. However, the CoFe2O4 will produce volume expansion during the reaction, resulting in the collapse of the material structure. This paper created the ground-breaking CFO/SNRGO nanocomposite structure by using a microwave approach. This intricate arrangement entails CoFe2O4 nanoparticles firmly adhering to S and N co-doped graphene hybrid nanosheets. The prepared CFO/SNRGO nanocomposite showcased remarkable electrochemical capabilities. Through the uniform integration of CoFe2O4 nanoparticles onto the SNRGO hybrid nanosheets, we managed to minimize the stacking between SNRGO nanosheets and reduce the size of the CoFe2O4 nanoparticles. Additionally, the SNRGO serves as a buffer layer, allowing it to adjust to variations in volume and stop the CoFe2O4 nanoparticles from collapsing while Na+ is being embedded or removed. This cooperative amalgamation of SNRGO and CoFe2O4 contributes to the outstanding electrochemical performance observed in the CFO/SNRGO nanocomposites. After 200 cycles at an electrical concentration of 0.05 A g−1, the CFO/SNRGO nanotechnology demonstrated a notable 357.3 mAh g−1 bidirectional ability. and sustained excellent rate performance. Even when subjected to an increased electrical concentration of 1.5 A g−1, the reversible capacity remained steady at 174 mAh g−1. These impressive outcomes are due to the robust coupling between the CoFe2O4 nanoparticles and SNRGO within the nanocomposites.

Performance optimization of a novel perovskite solar cell with power conversion efficiency exceeding 37% based on methylammonium tin iodide

The development of highly efficient lead-free solar cells is essential for sustainable energy production in the face of depleting fossil fuel resources and the negative effects of climate change. Perovskite solar cells (PSCs) containing lead pose considerable environmental and public health hazards, in addition to thermal stability and longevity challenges. Here, a novel lead-free solar cell design of the configuration, ITO/PC61BM/CH3NH3SnI3/PEDOT:PSS/Mo, is investigated for improved light harvesting capabilities, enhanced device performance, and better operational efficiency under various temperature conditions. The optimal thickness of the light-absorbing layer, CH3NH3SnI3, was found to be 1000 nm for maximum quantum efficiency (QE). Further, the temperature tolerance of the solar cell was evaluated using Mott-Schottky (MS) capacitance analysis and showed that the model cell retains about 95% of its power at 400 K, demonstrating excellent thermal stability and robust performance. The solar cell also shows promising electrical output parameters, including a short-circuit current density (Jsc) of 34.84 mA/cm², open-circuit voltage (Voc) of 1.5226 V, Fill factor (FF) of 71.04%, and an impressive power conversion efficiency (PCE) of 37.66% at 300 K. The effect of buffer layers such as CdS, ZnS, ZnSe, and V2O5 on the electrical outcomes of the model cell structure has been critically examined. Additionally, parasitic resistances and doping characteristics on the operational performance of the cell have been explored in detail. This work therefore, provides remarkable insights in the field of solar energy harvesting, offering potential sustainable energy generation solutions, supporting de-carbonization of the environment and climate change mitigation efforts towards an energy sustainable future.

Nitride Lithium-ion Conductors with Enhanced Oxidative Stability

It is desirable to develop solid electrolytes that have both excellent reductive stability against lithium metal and oxidative stability against high-voltage cathodes. However, no inorganic superionic conductors reported thus far satisfy these criteria. Nitrides exhibit intrinsically superior stability against reduction but are often readily oxidized at voltages as low as 0.6 V. In this article, we investigated all nitride-based compounds to search for materials with improved oxidative stabilities over 2.0 V while retaining their intrinsic stability against Li metal. We found two compounds, LiPN2 and Li2CN2, with high oxidative stability > 2.0 V and low vacancy migration energies. Using fine-tuned CHGNet machine-learning interatomic potential, we found that upon introducing aliovalent dopants to introduce vacancies in Li2CN2, the dopant and vacancy strongly anchor with each other to result in trapped vacancies, which lowers ionic conductivity. In contrast, vacancies and dopants have minimal interactions in LiPN2, resulting in a high ionic conductivity. These two compounds were synthesized, but their ionic conductivities were not successfully measured because of the challenges in densification. With improved processing conditions, these compounds may serve as anode-side separators in dual-separator-type all-solid-state batteries or anode buffer layer materials interfaced with lithium metal.

Electrochemical sintering of lithium metal constrained by buffer layer in anode-free all-solid-state batteries

Anode-free all-solid-state lithium-metal batteries hold promise in energy density. However, they often suffer from rapid capacity decay, and the underlying mechanisms remain unclear, especially at high current densities. Here, failure mechanism is studied systematically for garnet solid electrolyte in Li anode-free all-solid-state batteries. By combining morphology evolution analysis and quantitative assessments, the decreased Coulombic efficiency for solid electrolyte at high current densities is attributed to large amounts of lump-shaped lithium deposits via the electrochemical sintering. The lump-shaped lithium deposits encapsulate numerous voids and solid electrolyte interphase leading to irreversible capacities. Furthermore, serious pulverization of lithium deposits is observed after long cycles. An addition of polydimethylsiloxane membrane is attempted to constrain electrochemical sintering of lithium and compact lithium metal deposits, leading to prolonged cycling performance.