Properties Of Buffer Layer Research Articles

Mitigating Delamination in Perovskite/Silicon Tandem Solar Modules

As perovskite/silicon tandem solar cells head toward industrialization, one emerging challenge relates to the mechanical reliability of these organic–inorganic multilayer devices. Herein, the fracture toughness and interfacial strength of monolithic p–i–n perovskite/silicon tandems are assessed in the context of module integration. While the weakest layer in the tandem stack investigated is found to be C60, used here as electron‐transport layer (interfacial tensile strength of 0.64 MPa), more concerningly, the fracture energy of the C60/tin‐oxide interface is found to be only 1.2 J m−2. The low fracture toughness of perovskite/silicon tandems can encourage crack propagation and large‐scale delamination during processes used for their integration into modules such as cell cutting, interconnection, and vacuum lamination. By improving the tin oxide buffer layer properties and reducing sputtering‐induced internal stress (associated with the transparent top electrode deposition onto the tin the oxide buffer layer), the fracture energy is improved to over 160 J m−2. A second strategy to mitigate delamination due to the low fracture toughness of the cells is tailoring encapsulation and cell processing techniques specifically toward the perovskite/silicon tandem technology. In this work, a critical reliability issue, relevant for any perovskite‐based optoelectronic technology requiring device packaging, is addressed.

Mechanism of improvement of efficiency of Cu2ZnSn(S,Se)4 solar cells by optimization of deposition temperature of CdS buffer layer

The physical properties of CdS buffer layers and the interface quality between CdS layer and Cu2ZnSn(S,Se)4 (CZTSSe) absorber have a significant impact on the performance of CZTSSe solar cells. In the present work, five types of CZTSSe solar cells with conventional structure (denoted as CdS-x-cells, x = 55, 60, 65, 70 and 75) were fabricated by using CdS prepared at deposition temperatures of 55, 60, 65, 70 and 75 ℃, respectively, as buffer layer, and influence mechanism of the deposition temperature of CdS layers on power conversion efficiency (PCE) of the CdS-x-cells was investigated. It is found that the PCE of the CdS-x-cells increases from 8.52% to 10.36% as the deposition temperature rises from 55 to 60 ℃ but falls from 10.36% to 7.14% from 60 to 75 ℃. The analysis on the photovoltaic and electrical parameters indicates that the increase in PCE is predominantly derived from the decreased reverse saturated current density (J0), followed by the decreased series resistance (RS) and increased shunt resistance (RSh). The decreased PCE with the further increasing deposition temperature is mainly contributed from the increased J0, followed by the decreased RSh and photogenerated current density (JL). The influence mechanism of deposition temperature of the CdS layer on PCE is suggested via quantitatively analysis of influence of electrical, optical properties and crystal quality of CdS prepared at various deposition temperatures on JL and electrical parameters of CdS-x-cells. This work is helpful for improvement of PCE of CZTSSe solar cells by modification of properties of buffer layer.

Importance of the Buffer Layer Properties for the Performance of Perovskite/Silicon Tandem Solar Cells

The power conversion efficiency of two-terminal perovskite/silicon tandem solar cells recently surpassed 30%, demonstrating the great potential of this technology. The most common electron-selective contact is a stack of a buckminsterfullerene film, a tin oxide buffer layer, and a transparent conductive oxide. While research is conducted on improving the fullerene/perovskite interface and on removing the buffer layer, less focus is put on tuning the buffer layer properties. In this work, the interplay of the doping densities in the fullerene film and the tin oxide layer as well as the impact of the tin oxide electron affinity on the device performance is investigated using electrical numerical simulations (technology computer-aided design). The simulated power conversion efficiency increases with higher doping densities in the fullerene film and the tin oxide layer, while an increasing conduction band offset between the two materials leads to a decreasing device performance. The results of this work imply that the properties of the buffer layer directly influence the performance of the tandem solar cell. In addition, the importance of measuring various material and interface characteristics in order to derive an improved device understanding is stressed.

2D Oxides Realized via Confinement Heteroepitaxy

AbstractNovel confinement techniques facilitate the formation of non‐layered 2D materials. Here it is demonstrated that the formation and properties of 2D oxides (GaOx, InOx, SnOx) at the epitaxial graphene (EG)/silicon carbide (SiC) interface is dependent on the EG buffer layer properties prior to element intercalation. Using 2D Ga, it is demonstrated that defects in the EG buffer layer lead to Ga transforming to GaOx with non‐periodic oxygen in a crystalline Ga matrix via air oxidation at room temperature. However, crystalline monolayer GaO2 and bilayer Ga2O3 with ferroelectric wurtzite structure(FE‐WZ') can then be formed via subsequent high‐temperature O2 annealing. Furthermore, the graphene/X/SiC (X = 2D Ga or Ga2O3) junction is tunable from Ohmic to a Schottky or tunnel barrier depending on the interface species. Finally, using vertical transport measurements and electron energy loss spectroscopy analysis, the bandgap of 2D gallium oxide is identified as 6.6 ± 0.6 eV, significantly larger than that of bulk β‐Ga2O3 (≈4.8 eV), suggesting strong quantum confinement effects at the 2D limit. The study presented here is foundational for development of atomic‐scale, vertical 2D/3D heterostructure for applications requiring short transit times, such as GHz and THz devices.

Enhanced Performance of CuIn1-xGxSe2 Solar Cell Through Optimization of Absorber and Buffer Layer Properties Using SCAPS-1D

This study is a follow up to our previously published article on “Numerical Simulation of Copper Indium Gallium Diselenide Solar Cells Using One Dimensional SCAPS Software”. Five more parameters were optimized which are: absorber band gap, absorber electron affinity, buffer layer band gap, buffer layer electron affinity and working temperature using the same simulation tool initially used. When the absorber bandgap was varied between 0.8 eV and 1.6 eV, the efficiency of the solar cell increases until it reached its peak at 27.81%. This occurred at absorber bandgap of 1.4 eV. Other photovoltaic parameters at this optimum value are: Voc of 1.00 V, Jsc of 31.99 mA/cm2 and FF of 87.47 %. On varying the absorber electron affinity from 4.20 eV through 4.55 eV, we obtained an optimum value of 4.45 eV at Voc of 0.82 V, Jsc of 37.96 mA/cm2, FF of 84.99 % and an efficiency of 26.36%. The optimization of buffer bandgap resulted in an optimal value of 3.0 eV, when the buffer bandgap was varied between 1.6 eV and 3.2 eV. The photovoltaic parameters at this optimal value are: Voc of 0.80 V, Jsc of 37.96 mA/cm2, FF of 85.22 % and an efficiency of 25.86%. The effect of buffer electron affinity was studied by varying its value between 4.00 eV and 4.40 eV and its best value was found to be 4.05 eV at photovoltaic parameters with a Voc of 0.82 V, Jsc of 37.96 mA/cm2, FF of 84.98 % and an efficiency of 26.36 %. These optimized values in all parameters were used to simulate a solar cell which resulted to device with performances: Voc of 1.11 V, Jsc of 31.50 mA/cm2, FF of 88.91 % and an efficiency of 31.11 %. On varying the working temperature on the optimized solar cell, the optimized device with its best performance at 270 K with Photovoltaic (PV) values of Voc of 1.15 V, Jsc of 31.55 mA/cm2, FF of 88.64 % and an efficiency of 32.18%. The results obtained were encouraging and can serve as a guide to those involved in practical development of solar cells.

Three‐dimensional thermal modeling and dimensioning design in the nuclear waste repository

AbstractA mathematical model is established by using the cylindrical heat source to simulate the temperature field in the nuclear waste repository. By means of the integral transformation method, the partial differential equations of heat conduction were transformed into ordinary differential equations to obtain the semi‐analytical solutions to temperature change. The radial temperature distribution around the nuclear waste canister was analyzed and compared with other analytical models. The built model was applied to determine the canister spacing with variable tunnel spacing for different type canister panel. Then, the quantitative effects of decay heat power at initial installation and of other parameters on the canister spacing were specified. The results show that the built model can predict the near‐field and far‐field temperature around the waste canister at the same time. The decay heat power at the initial installation and thermal properties of buffer layer and surrounding rock have strong effects on the canister spacing.

Optical and Structural Characterization of Cd-Free Buffer Layers Fabricated by Chemical Bath Deposition

Chemical bath deposition (CBD) is a suitable, inexpensive, and versatile synthesis technique to fabricate different semiconductors under soft conditions. In this study, we deposited Zn(O;OH)S thin films by the CBD method to analyze the effect of the number of thin film layers on structural and optical properties of buffer layers. Thin films were characterized by X-ray diffraction (XRD) and UV-Vis transmittance measurements. Furthermore, we simulated a species distribution diagram for Zn(O;OH)S film generation during the deposition process. The optical results showed that the number of layers determined the optical transmittance of buffer layers, and that the transmittance reduced from 90% (with one layer) to 50% (with four layers) at the visible range of the electromagnetic spectrum. The structural characterization indicated that the coatings were polycrystalline (α-ZnS and β-Zn(OH)2 to four layers). Our results suggest that Zn(O;OH)S thin films could be used as buffer layers to replace CdS thin films as an optical window in thin-film solar cells.

Critical View on Buffer Layer Formation and Monolayer Graphene Properties in High-Temperature Sublimation

In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 ∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.

Surface properties of buffer layers affect the performance of PM6:Y6–based organic photovoltaics

In this study we used p-type [poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and nickel oxide] and n-type (sol–gel and nanoparticle zinc oxides) materials as transporting layers for PM6:Y6–based organic photovoltaic (OPV) devices. These solution-processed transporting layers exhibited various surface energies, morphologies, and roughnesses that affected the subsequent growth of PM6:Y6 blend films. Using atomic force microscopy and grazing-incidence wide-angle X-ray spectroscopy, we observed various PM6:Y6 blend film morphologies on the different substrates. Furthermore, the surface-induced PM6:Y6 blend morphology influenced the mechanism of carrier recombination and, thereby, affected the device performance. Among our tested systems, the PEDOT:PSS–based devices exhibited superior surface properties, possessed larger phase-segregated domains, and featured strong molecular packing, all of which resulted in OPVs displaying power conversion efficiencies (PCEs) of up to 11.5%. When incorporating 1-chloronaphthalene as an additive during solution processing, the molecular packing was enhanced, thereby improving the PCE of the resulting devices from 11.5 to 13.4%. Devices incorporating ZnO (sol–gel) displayed performance comparable with that of the PEDOT:PSS–based devices, but exhibited superior air stability without encapsulation (25 °C, 40% humidity). • We have evaluated PM6:Y6 OPV devices featuring various transporting layers. • The champion OPV device displayed a PCE of 13.4 ± 0.23%. • The unencapsulated ZnO devices possessed good air-stability, retaining 90% of their PCEs after storage in air for over 190 h.

Cd-Free Zn(O,S) as Alternative Buffer Layer for Chalcogenide and Kesterite Based Thin Films Solar Cells: A Review.

Cd is categorized as a toxic material with restricted use in electronics as there are inherent problems of treating waste and convincing consumers that it is properly sealed inside without any threat of precarious leaks. Apart from toxicity, band-gap of CdS is about 2.40-2.50 eV, which results significant photon loss in short-wavelength range which restricts the overall performance of solar cells. Thin film of Zn(O,S) is a favorable contender to substitute CdS thin film as buffer layer for CuInGaSe₂ (CIGS), CuInGa(S,Se)₂ (CIGSSe), Cu₂ZnSn(S,Se)₄ (CZTSSe) Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSnS₄ (CZTS) thin film absorber material based photovoltaic due to it made from earth abundant, low cost, non-toxic materials and its ability to improve the efficiency of chalcogenide and kesterite based photovoltaic due to wider band-gap which results in reduction of absorption loss compared to CdS. In this review, apart from mentioning various deposition technique for Zn(O,S) thin films, changes in various properties i.e., optical, morphological, and opto-electrical properties of Zn(O,S) thin film deposited using various methods utilized for fabricating solar cell based on CIGS, CIGSSe, CZTS, CZTSe and CZTSSe thin films, the material has been evaluated for all the properties of buffer layer (high transparency for incident light, good conduction band lineup with absorber material, low interface recombination, high resistivity and good device stability).

Mitigating early fracture of amorphous metallic thin films on flexible substrates by tuning substrate roughness and buffer layer properties

Ductility mismatch between brittle amorphous thin films and flexible polymer substrate often leads to limited elongation and early failure of the system. A combined experimental and computational study was carried out to investigate the roles of buffer layer material type, thickness, and substrate roughness on the fracture toughness of the amorphous thin film/polymer system. Experimentally, about 1 μm thick amorphous aluminum-manganese thin films were deposited on polyimide substrates and tested in tension. The reliability of such systems was found to be strongly influenced by the film/substrate interface adhesion. Several strategies to improve the adhesion of the interface were conducted, including roughening the surface of the substrate, and adding a buffer layer with a desired mechanical properties and thickness. Computationally, finite element simulations of the same system were carried out under tension. It was found that by introducing substrate roughness and adding a Cr buffer layer of 75 nm, the interface toughness of the film/substrate can be increased by almost twenty times. The results of the present work may shed light on the interfacial engineering strategies for improving reliability of future flexible electronics.

Interface properties of CoPc and CoPcF16 on graphene/nickel: influence of germanium intercalation

Photoelectron spectroscopy was used to investigate electronic interface properties and interactions of the organic semiconductors CoPc and CoPcF16 on graphene/nickel based substrates. Additional focus was put on the influence of germanium intercalation of graphene/nickel. The presented results demonstrate that germanium can decouple graphene from nickel and in this manner restore its buffer layer properties. No charge transfer from the substrate to the organic layer is observed in the germanium intercalated case, while interface related peaks in the Co 2p core level spectra indicate such charge transfer on graphene/nickel. Strong interface dipoles are found for CoPcF16 on graphene/nickel and on germanium intercalated graphene/nickel. Fluorine Auger parameters have been measured, and the results provide evidence for polarization and charge transfer screening effects of different amounts at the unlike film-substrate interfaces. The various contributions to the observed shifts are discussed.

Eliminating light soaking effect of inverted polymer solar cells functionalized with a conjugated macroelectrolyte electron-collecting interlayer

To overcome the light soaking and hysteresis characteristics in indium tin oxides (ITO)/poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) based bulk-heterojunction polymer solar cells, a smart gate dielectrics nano-interfacial layer was incorporated by spin-coating an ultrathin polyetherimide (PEI) between ITO and PFN electron extraction layer. The ultrathin PEI micromolecule can effectively fill the surface defects of ITO and surmount electron transport barrier upon forming attached dipoles. The passivation surface decreased the total interfacial electron localized state and electron transfer barrier, which could re-modulate the interfacial property of buffer layer and facilitate electron collection. The additional insulating layer can influence the photoconductivity response and consequently restrain photocurrent hysteresis phenomena, leading to the improved power conversion efficiency. This finding would be helpful to achieve large-scale and low-cost polymer solar cells without light soaking effect.

Structures and properties of buffer layers with nanometer thickness for PBCMO/YBCO epitaxial multilayers

Insulator buffer layer materials, PrBa2(Cu1-xMx)3O7-δ (PBCMO), with M = Al and Ga with different portions were synthesized to improve better buffer-layer materials for fabricating superconducting devices from YBa2Cu3O7-δ (YBCO) superconductor. Neutron and X-ray diffraction data reveal that the structures of PBCMO and YBCO are the same and their lattice parameters are well matched, while the resistivity shows that PBCMO have much higher resistivity than PrBa2Cu3O7 (PBCO). Epitaxial multilayers of PBCMO/YBCO were fabricated by laser ablation to explore the effect of PBCMO layer thickness on YBCO superconducting layers. Superconductivity was not found above 4.2 K for one unit cell thickness of YBCO. As the thickness of PBCMO was varied with constant thickness of YBCO films, the transition temperature of the multilayers remains unchanged even though the PBCMO layer is made as thin as the unit cell of 12.5Å. The difference indicates that PBCMO is better buffer layer and is adequate to insulate YBCO films in the multilayers and to cut off the superconducting coupling between YBCO layers.

Influence of AlN Growth Temperature on the Electrical Properties of Buffer Layers for GaN HEMTs on Silicon

Despite the fact that a lower growth temperature is generally considered as a drawback for achieving high crystal quality, the necessity to reduce the nucleation temperature of AlN on Silicon has permitted Molecular Beam Epitaxy (MBE) to demonstrate high performance for GaN transistors operating at high frequency. Compared to Metal‐Organic Vapor Phase Epitaxy (MOVPE), the control of the interface between the AlN nucleation layer and the substrate is easier and the reduced growth temperature allows to obtain a more electrically resistive interface while keeping good crystal quality. Moreover, it is shown that further reducing the growth temperature within the nucleation and stress mitigating layers has a noticeable impact on the lateral and vertical buffer leakage currents. At the same time the buffer of MBE grown HEMT structures exhibits low RF propagation losses (below 0.5 dB mm−1 up to 70 GHz). Also, results obtained with structures regrown by MOVPE on MBE AlN‐on‐Si templates confirm that the thermal budget is critical for the resistivity of AlN/Si. On the other hand, the insertion of a 1.5 μm thick Al0.05Ga0.95N layer within a 2 μm HEMT structure significantly improves the vertical breakdown voltage up to 740 V permitting to compare favorably with MOVPE epilayers with similar total thickness.

Effect of template post-annealing on Y(Dy)BaCuO nucleation on CeO2 buffered metallic tapes

Substrate engineering is very significant in the synthesis of the high-temperature superconductor (HTS) coated conductor. Here we design and synthesize several distinct and stable Cerium oxide (CeO2) surface reconstructions which are used to grow epitaxial films of the HTS YBa2Cu3O7-δ (YBCO). To identify the influence of annealing and post-annealing surroundings on the nature of nucleation centers, including Ar/5%H2, humid Ar/5%H2 and O2 in high temperature annealing process, we study the well-controlled structure, surface morphology, crystal constants and surface redox processes of the ceria buffers by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and field-emission scanning electronic microscopy (FE-SEM), respectively. The ceria film post-annealed under humid Ar/5%H2 gas shows the best buffer layer properties. Furthermore, the film absorbs more oxygen ions, which appears to contribute to oxygenation of superconductor film. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.

Buffer Layer Engineering for High ( $\geq 10^{\mathrm {13}}$ cm $^{\mathrm {-2}}$ ) 2-DEG Density in ZnO-Based Heterostructures

In this paper, we report on the prospect of achieving very high values ( $\geq 10^{13}$ cm $^{-2}$ ) of 2-D electron gas (2-DEG) density in ZnO-based heterostructures through buffer layer engineering. A physics-based analytical model is developed and utilized to demonstrate up to $25\times $ higher 2-DEG values in MgZnO/CdZnO as compared with that in the MgZnO/ZnO heterostructure at lower Mg composition of 0.10 in barrier layer. It is shown that a lower spontaneous polarization in buffer layer due to more electronegative Cd and higher lattice constant of CdZnO, which introduces tensile piezoelectric strain in the barrier layer, favorably add up, and increase polarization difference at barrier-buffer interface, which eventually enhances 2-DEG density. This paper demonstrates new opportunities to effectively utilize buffer layer properties to significantly improve the 2-DEG sheet density (~ $4 \times 10^{\mathrm { {13}}}$ cm $^{\mathrm { {-2}}}$ ) in ZnO heterostructures.

Influence of AlN buffer layer on growth of AlGaN by HVPE

To the end of improvement of layer morphology and crystalline perfection of thick AlGaN layers grown by hydride vapor phase epitaxy (HVPE) as well as for the suppression of crystallite formation during growth, the impact of AlN buffer layer properties on AlGaN growth was investigated. While the surface morphology of 500 nm thick AlN layer improves with higher V/III ratio toward 50 and lower growth temperatures of 1020 °C, its use as a buffer layer for thick AlGaN layers leads to strong degradation of AlGaN surface and even to formation of different crystallites during growth. Best surface morphology and crystalline perfection of thick AlGaN layers was observed for the growth on an AlN buffer layer with a higher crystal quality disregarding its 3D morphology and high surface roughness. Best results were achieved at medium V/III ratio of about 10–20 and AlN buffer growth temperature of 1060 °C.

Analysis of Effect of Zn(O,S) Buffer Layer Properties on CZTS Solar Cell Performance Using AMPS**Supported by the Guiding Project of Strategic Emerging Industries of Fujian Provincial Department of Science and Technology under Grant No 2015H0010, the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure of Shanghai Institute of Ceramics of Chinese Academy of Sciences under

The Cu2ZnSnS4 (CZTS)-based solar cell is numerically simulated by a one-dimensional solar cell simulation software analysis of microelectronic and photonic structures (AMPS-1D). The device structure used in the simulation is Al/ZnO:Al/nZn(O,S)/pCZTS/Mo. The primary motivation of this simulation work is to optimize the composition in the ZnO1−xSx buffer layer, which would yield higher conversion efficiency. By varying S/(S+O) ratio x, the conduction band offset (CBO) at CZTS/Zn(O,S) interface can range from −0.23 eV to 1.06 eV if the full range of the ratio is considered. The optimal CBO of 0.23 eV can be achieved when the ZnO1−xSx buffer has an S/(S+O) ratio of 0.6. The solar cell efficiency first increases with increasing sulfur content and then decreases abruptly for x > 0.6, which reaches the highest value of 17.55% by our proposed optimal sulfur content x = 0.6. Our results provide guidance in dealing with the ZnO1−xSx buffer layer deposition for high efficiency CZTS solar cells.

High-efficiency Cu(In,Ga)Se2 solar cells

Recent progress in the development of high-efficiency solar cells based on Cu(In,Ga)Se2 at our institute is reviewed. The post-deposition treatment with alkali elements (PDT) has been found to improve device quality, mostly through reduced recombination, with the effect of increasing the open-circuit voltage. At the same time, PDT is shown to improve initial growth properties of the chemical-bath-deposited buffer layer, so that this layer may be made thinner and the corresponding losses are reduced. Further optimization by replacing the non-doped ZnO resistive layer by (Zn,Mg)O enables gains in photocurrent for the ultraviolet region. A certified efficiency of 22.0% with a CdS/(Zn,Mg)O buffer/resistive layer combination is presented here. Furthermore, the effect of band gap grading with gallium in the absorber layer is explored with respect to the buffer layer properties by means of device simulation.