Interfacial Buffer Layer Research Articles

Incorporating Solution-Processed Mesoporous WO3 as an Interfacial Cathode Buffer Layer for Photovoltaic Applications.

Dextran-templating hydrothermal synthesis of monoclinic WO3 exhibits excellent specific surface area of ∼110 m2/g and a monomodal pore distribution with an average pore diameter of ∼20 nm. Dextran plays a crucial role in generating porosity on WO3. The role of supporting dextran has been investigated and found to be crucial to tune the surface area, porosity, and morphology. The photoluminescence and X-ray photoelectron spectroscopy studies reveal the existence of oxygen vacancies in substoichiometric WO3, which creates localized defect states in WO3 as synthesized through this templating method. The highly mesoporous WO3 has been further explored as an interfacial cathode buffer layer (CBL) in dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). A significantly enhanced photoconversion efficiency has boosted up the performance of the counter electrode used in traditional DSSC (as platinum) and PSC (as carbon) devices by ∼48 and ∼29%, respectively. The electrochemical impedance and incident photon to current conversion efficiency (IPCE) studies were also analyzed in order to understand the catalytic behavior of the WO3 interfacial CBL for both DSSCs and PSCs, respectively. The much higher surface area of WO3 enables rapid electron hopping mechanism, which further benefits for higher electron mobility, resulting in higher short circuit current. Through this study, we were able to unequivocally establish the importance of buffer layer incorporation, which can further help to integrate the DSSC and PSC devices toward more stable, reliable, and enhanced efficiency-generating devices. In spite of this, using WO3 constitutes an important step toward the efficiency improvement of the devices for futuristic photoelectrochromic or self-powered switchable glazing for low-energy adaptive building integration.

Atomic force microscopy phase imaging of epitaxial graphene films

Dynamic mode atomic force microscopy phase imaging is known to produce distinct contrast between graphene areas of different atomic thickness. But the intrinsic complexity of the processes controlling the tip motion and the phase angle shift excludes its use as an independent technique for a quantitative type of analysis. By investigating the relationship between the phase shift, the tip-surface interaction, and the thickness of the epitaxial graphene areas grown on silicon carbide, we shed light on the origin of such phase contrast, and on the complex energy dissipation processes underlying phase imaging. In particular, we study the behavior of phase shift and energy dissipation when imaging the interfacial buffer layer, single-layer, and bilayer graphene regions as a function of the tip-surface separation and the interaction forces. Finally, we compare these results with those obtained on differently-grown quasi free standing single- and bilayer graphene samples.

Performance improvement of polymer solar cells with binary additives induced morphology optimization and interface modification simultaneously

Active layer morphology optimization and electrode buffer layer interface modification are commonly used strategies in improving the performance of polymer solar cells (PSCs). In this study, we prepared PTB7: PC71BM bulk heterojunction PSCs with 1,8-diiodooctane (DIO) and polyethylene glycol (PEG) additives, and studied the influence of binary additives on exciton dissociation, charge transport and charge extraction. DIO facilitates donor/acceptor phase separation for efficient exciton dissociation and charge transport. The migration of PEG from active layer to the PEDOT:PSS layer improves the crystallinity of PTB7, optimizes charge transport pathway, and enhances the conductivity of PEDOT:PSS layer. With the combined advantages of binary additives in active layer morphology optimization and anode buffer layer modification, the device exhibits a high short-circuit current density of 20.03 mA/cm2 and an improved power conversion efficiency. Binary additive provides a promising method to optimize active layer morphology and improve interfacial buffer layer of PSCs simultaneously.

Buffer layers inhomogeneity and coupling with epitaxial graphene unravelled by Raman scattering and graphene peeling

The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during SiC (0001) sublimation. The covalent bonds between some carbon atoms in this layer and underlying silicon atoms makes it different from epitaxial graphene. We report a systematical and statistical investigation of the BL signature and its coupling with epitaxial graphene by Raman spectroscopy. Three different BLs are studied: bare buffer layer obtained by direct growth (BL0), interfacial buffer layer between graphene and SiC (c-BL1) and the interfacial buffer layer without graphene above (u-BL1). To obtain the latter, we develop a mechanical exfoliation of graphene by removing an epoxy-based resin or nickel layer. The BLs are ordered-like on the whole BL growth temperature range. BL0 Raman signature may vary from sample to sample but forms patches on the same terrace. u-BL1 share similar properties with BL0, albeit with more variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. The signal high frequency side onset upshifts upon graphene coverage, unexplainable by a simple strain effect. Two fine peaks (1235, 1360 cm−1), present for epitaxial monolayer and absent for BL and transferred graphene. These findings point to a coupling between graphene and BL.

Improved ferroelectric response of pulsed laser deposited BiFeO3-PbTiO3 thin films around morphotropic phase boundary with interfacial PbTiO3 buffer layer

(1 − x)BiFeO3-xPbTiO3 (BF-xPT) is an interesting material for sensing and actuating devices with large polarization near the morphotropic phase boundary (MPB) (x = 0.30) in the bulk form. However, pulsed laser deposition (PLD) grown (BF-xPT) thin films usually show high electrical leakage and, hence, saturated ferroelectric hysteresis loops are only obtained at subzero temperatures. In this article, we report on high room temperature ferroelectric polarization with saturated hysteresis loops in pulsed laser deposited (BF-xPT) polycrystalline thin films of compositions near the MPB with the use of a thin buffer layer of PbTiO3 (PT). The thin films possessed a perovskite structure with excellent crystallinity and exhibit the presence of a monoclinic (Cm) phase (MA-type) for x = 0.20–0.25 and a mixture of a monoclinic (Cm) phase and a tetragonal (P4mm) phase for x = 0.30–0.35 compositions. The thin films with composition x = 0.25 exhibit a monoclinic phase and yield very large room temperature ferroelectric polarization (2Pr > 80μC/cm2), perhaps the highest room temperature ferroelectric polarization and excellent piezoelectric properties in PLD deposited (BF-xPT) thin films of near-MPB composition. Furthermore, the evolution of ferroelectricity with PT content, studied using room temperature Raman spectroscopy, reveals a correlation with lattice dynamics and stereochemical activity of Bi. Piezoforce domain analysis of the thin films reveals that ferroelectric polarization and electrical leakage in the thin films are intricately related to the type of domains present in the samples, viz., 180°, 109°, 90°, and 71° due to differences in the nature of the domain walls.

An inverted ZnO/P3HT:PbS bulk-heterojunction hybrid solar cell with a CdSe quantum dot interface buffer layer.

An inverted bulk-heterojunction (BHJ) hybrid solar cell having the structure ITO/ZnO/P3HT:PbS/Au was prepared under ambient conditions and the device performance was further enhanced by inserting an interface buffer layer of CdSe quantum dots (QDs) between the ZnO and the P3HT:PbS BHJ active layer. The device performance was optimized by controlling the size of the CdSe QDs and the buffer layer thickness. The buffer layer, with an optimum thickness and QD size, has been found to promote charge extraction and reduces interface recombinations, leading to an increased open-circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) and power conversion efficiency (PCE). About 40% increase in PCE from 1.7% to 2.4% was achieved by the introduction of the CdSe QD buffer layer, whose major contribution comes from a 20% increase of VOC.

Optimizing the Performance of CsPbI3-Based Perovskite Solar Cells via Doping a ZnO Electron Transport Layer Coupled with Interface Engineering

HighlightsDevice simulations and first-principle calculations are employed to derive a guideline for the optimization of CsPbI3-based perovskite solar cells (PSCs).The open voltage and power conversion efficiency of the PSCs are, respectively, improved to 1.31 V and 21.06% by simultaneously introducing an ultra-thin TiO2 buffer layer and increasing the doping concentration of the ZnO electron transport layer.The influence of the interfacial buffer layer and doping of the CsPbI3/ZnO interface on PSC performance is discussed.

Efficient and Stable Inverted Quantum Dot Light-Emitting Diodes Enabled by An Inorganic Copper-Doped Tungsten Phosphate Hole-Injection Layer.

Inorganic interfacial buffer layers have widely been employed for efficient and long lifetime optoelectronic devices due to their high carrier mobility and excellent chemical/thermal stability. In this paper, we developed a solution-processed inorganic tungsten phosphate (TPA) as hole injection layer (HIL) in inverted quantum dot light-emitting diodes (QLEDs) achieving a high external quantum efficiency (EQE) of up to ∼20%. Further, the copper ions are doped into tungsten phosphate (Cu:TPA) which leads to an enhancement in hole injection due to increased hole mobility and conductivity of TPA as well as decreased hole injection barrier, enabling better charge balance in QLEDs and lower turn-on voltage from 5 to 2.5 V. Compared with the devices using conventional organic poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) HIL, the half-lifetime of Cu:TPA-based devices is over 3000 h at an initial brightness of 100 cd m-2, almost 5-fold operating lifetime enhancement.

Sensing Amorphous/Crystalline Silicon Surface Passivation by Attenuated Total Reflection Infrared Spectroscopy of Amorphous Silicon on Glass

Attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy and effective lifetime measurements have been used to characterize amorphous/crystalline silicon surface passivation in silicon heterojunction solar cells. The comparative studies show a strong link between microstructure factor R* and effective lifetime of amorphous silicon (a-Si:H) passivation layers incorporating an interface buffer layer, which prevents the epitaxial growth. It is demonstrated that thin a-Si:H films deposited on glass can be used as ATR substrates in this case. The obtained results show that a-Si:H films with R* close to 0.1 are required for manufacturing of high-efficiency (>23%) silicon heterojunction solar cells.

Analysis of scaling of thickness of the buffer layer on analog/RF and circuit performance of InAs‐OI‐Si MOSFET using NQS model

AbstractWith use of III‐V material in the channel region of a metal‐oxide‐semiconductor (MOS) transistor, interface buffer layers above and below the channel region are necessary to reduce several sources of scattering of the inversion charge carriers. This work presents a detailed analysis of effect of scaling down thickness of the top buffer layer of a buffered InAs‐OI‐Si metal‐oxide‐semiconductor field effect transistor (MOSFET) on electrostatic integrity, carrier mobility, analog/RF, linearity, and circuit performance using well‐calibrated 2D technology computer‐aided design simulation. We further develop non–quasi‐static small‐signal equivalent circuit model of the device. The model results have been compared with experimental results and good accuracy of our model is demonstrated. Both the model and numerical simulation results have been used in our study. In‐depth analysis of behaviour of several performance parameters at DC and radio frequency (RF) is presented with analytical explanations and numerical simulation results. A common source amplifier is designed and simulated in SPICE framework using the equivalent circuit model. From the study, we find that the quantum well that is formed in the buffer/channel/buffer layers needs to be formed as near to the oxide/semiconductor interface as possible for optimum performance.

Performance and stability improvement of single junction a-Si:H solar cell by interface engineering

In this study, the influence of p/i interface on the stability of amorphous silicon solar cells has been investigated by light-induced degradation study. To peruse this study a series of p-i-n solar cells with and without p/i interface buffer layer have been fabricated using plasma enhanced chemical vapor deposition (PECVD) technique. In the light soaking experiment, the samples were kept under one sun illumination for 1000 h. The degradation results reveal that the stability of the solar cell with buffer layer at p/i interface has significantly improved. It has been found that the solar cell with p/i buffer layer has only 13.42% degradation in power conversion efficiency after 1000 h light soaking. Whereas the cell without any interface treatment, the degradation in efficiency has reached 23.56%. The quantum efficiency and dark current measurement were also performed to establish the correlation between interface degradation and stability of the a-Si:H solar cells. Our extensive light soaking studies indicate that the light-induced degradation in a-Si:H solar cell is not only a bulk or material degradation effect. The p/i interface deterioration also has a major influence on the instability of the cell. Based on this hypothesis, a solar cell designed through proper device engineering has shown improved performance and stability against light soaking.

Indium Tin Oxide–Based Fully Spray‐Coated Inverted Solar Cells with Nontoxic Solvents: The Role of Buffer Layer Interface on Low‐Bandgap Photoactive Layer Performance

In bulk heterojunction solar cells, the morphology of the interfaces between the photoactive layer (PAL) and charge transporting layers during the deposition process plays a key role in achieving high‐efficiency devices. Herein, an inverted fully spray‐coated solar cell fabricated on an indium tin oxide (ITO)‐glass substrate is presented. It is demonstrated that a spray‐coated double electron transporting layer composed of zinc oxide (ZnO) nanoparticles coated with polyethylenimine ethoxylated (PEIE) improves the morphology of the spray‐coated active layer on top of the spray‐coated cathode. Moreover, focusing on the hole transporting layer and anode, the performance obtained using a commercial poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) blend is compared with a high‐conductive anhydrous PEDOT:PSS (A‐PEDOT) mixed with a commercial PEDOT:PSS (CPP‐105D) as transporting layer. By optimizing the spray deposition of all the layers, a fully scalable spray process is used to produce polymer solar cells with ITO/ZnO/PEIE/poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b’]dithiophene‐2,6‐diyl] [3‐fluoro‐2‐ [(2‐ethylhexyl)carbonyl] thieno[3,4‐b]thiophenediyl]] (PTB7): [6,6]‐phenyl‐C70‐butyric‐acid‐methyl‐ester (PC70BM)/CPP:A‐PEDOT structure, achieving a power conversion efficiency (PCE) of 3.6%. Such result is significant if compared to a spray‐coated structure with evaporated anode (MoO3‐Ag). In this case (ITO/ZnO/PEIE/PTB7:PCBM/MoO3‐Ag), a power conversion efficiency of 5.5% is obtained.

Modification of elastic deformations and analysis of structural and optical changes in Ar+-implanted AlN/GaN superlattices

The effects of built-in deformation and stress relaxation on the structural and optical properties for a 21-period AlN/GaN superlattice (SL) after implantation with argon ions have been studied. Upon implantation, the satellites’ peaks in the X-ray diffraction spectra shift toward the smaller angles which indicate decreasing of tensile deformation in SL by the value of Δc/c = 0.28%. The SIMS analysis showed that the sputtering rate for the initial sample decreases with depth, which is explained by the effect of varying deformation fields in SL and a faster etching of SL layers with a larger deformation magnitude. The change in the sputtering rate of individual layers of SL after implantation also correlates well with the magnitude of deformation. Implantation of Ar+ ions into SL leads to relaxation of the system and more homogeneous distribution of bond energy in individual layers of superlattice. The Raman spectra of the implanted SL are characterized with a low-frequency shoulder in the most intense band of the GaN layer E2 (high), which testifies partial stress relaxation in SL. Thus, based on a comprehensive study of the initial and implanted SL, it has been concluded that point defects that occur during implantation activate the process of dislocations nucleation in the superlattice/GaN buffer layer interface, which reduces the compressive stresses in the GaN layers of SL.

Improved Interface and Electrical Properties by Inserting an Ultrathin SiO2 Buffer Layer in the Al2O3/Si Heterojunction

AbstractAn ultrathin SiO2 interfacial buffer layer is formed using the nitric acid oxidation of Si (NAOS) method to improve the interface and electrical properties of Al2O3/Si, and its effect on the leakage current and interfacial states is analyzed. The leakage current density of the Al2O3/Si sample (8.1 × 10−9 A cm−2) due to the formation of low‐density SiOx layer during the atomic layer deposition (ALD) process, decreases by approximately two orders of magnitude when SiO2 buffer layer is inserted using the NAOS method (1.1 × 10−11 A cm−2), and further decreases after post‐metallization annealing (PMA) (1.4 × 10−12 A cm−2). Based on these results, the influence of interfacial defect states is analyzed. The equilibrium density of defect sites (Nd) and fixed charge density (Nf) are both reduced after NAOS and then further decreased by PMA treatment. The interface state density (Dit) at 0.11 eV decreases about one order of magnitude from 2.5 × 1012 to 7.3 × 1011 atoms eV−1 cm−2 after NAOS, and to 3.0 × 1010 atoms eV−1 cm−2 after PMA. Consequently, it is demonstrated that the high defect density of the Al2O3/Si interface is drastically reduced by fabricating ultrathin high density SiO2 buffer layer, and the insulating properties are improved.

Control of the metal/WS2 contact properties using 2-dimensional buffer layers.

Transition metal dichalcogenides (TMDC) have recently attracted much attention as a promising platform for the realization of 2-dimensional (2D) electronic devices. One of the major challenges for their wide-scale application is the control of the potential barrier at the metal/TMDC junction. Using conductive atomic force microscopy (c-AFM) we have investigated modifications of the Schottky barrier height (SBH) across a Pt/WS2 junction by the introduction of thin buffer layers of graphene and MoSe2. While graphene greatly reduces the contact resistance in both bias directions, thin layers of MoSe2 lower the Schottky barrier and leave the rectifying properties of the junction intact. We have studied the dependence of the transport properties on the thickness of the graphene and MoSe2 buffer layers. In both cases, the charge transport characteristics can be tailored by varying the buffer layer thickness. The edge of single layer graphene is observed to form an ohmic contact to the underlying WSe2 substrate. This study demonstrates that the introduction of atomically thin MoSe2 and graphene buffer layers is a feasible and elegant method to control the Schottky barrier when contacting TMDCs. The results are important for the fabrication of devices utilizing 2D materials.

Sensing amorphous/crystalline silicon surface passivation by attenuated total reflection infrared spectroscopy of amorphous silicon on glass -=SUP=-*-=/SUP=-

AbstractAttenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy and effective lifetime measurements have been used to characterize amorphous/crystalline silicon surface passivation in silicon heterojunction solar cells. The comparative studies show a strong link between microstructure factor R * and effective lifetime of amorphous silicon ( a -Si:H) passivation layers incorporating an interface buffer layer, which prevents the epitaxial growth. It is demonstrated that thin a -Si:H films deposited on glass can be used as ATR substrates in this case. The obtained results show that a -Si:H films with R * close to 0.1 are required for manufacturing of high-efficiency (>23%) silicon heterojunction solar cells.

New insights on the chemistry of plasma-enhanced atomic layer deposition of indium oxysulfide thin films and their use as buffer layers in Cu(In,Ga)Se2 thin film solar cell

A comparative chemical analysis of InxSy and In2(S,O)3 thin films grown by atomic layer deposition (ALD) and plasma-enhanced ALD, respectively, was performed to understand the challenges and issues related to the assistance of plasma, especially for the implementation of these films as ultrathin (<50 nm) interfacial buffer layers in copper indium gallium diselenide (CIGS) solar cells. The films were synthesized using indium acetylacetonate [In(acac)3], hydrogen sulfide, and an Ar/O2 plasma as indium, sulfur, and oxygen precursors. Film growth mechanisms and chemistries were studied using gas phase measurements by quadrupole mass spectrometry and x-ray photoelectron spectroscopy for surface and in-depth characterizations. Distinctive signatures of thermal and plasma processes on the overall compositions of the films were evidenced, which were further discussed and explained. Added to this, the impact of the plasma on the underlying substrate, using silicon as a reference, was further investigated to identify its modification. This extensive study has led to a readjustment of the deposition conditions of In2(O,S)3 thin films and allowed promising implementation as buffer layers in CIGS solar cells.

Interface Engineering of BCP Buffer Layers in Planar Heterojunction Perovskite Solar Cells With NiOx Hole Transporting Layers

Bathocuproine (BCP) was first employed as the interface buffer layers between the PCBM and Ag layers to improve the performance of Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3 perovskite solar cells based on NiOx hole transporting layers. The effect of different evaporation times on the performance of solar cells at the same evaporation temperature (100℃) was carefully investigated. The optimum device efficiency of 14.47% was obtained by evaporating BCP for 36 s, suggesting an enhancement of 14% as compared to the cell without BCP modification.

Controlling the thin interfacial buffer layer for improving the reliability of the Ta/Ta2O5/Pt resistive switching memory

Thin metal interfacial layers (1-nm-thick Ti and Hf) were exploited as the appropriate interfacial layer for forming an interfacial (buffer) layer between the 10-nm-thick Ta2O5 resistance switching layer and the Ta electrode to enhance the switching cycle endurance and uniformity. The thin metal interfacial layers were in-situ oxidized to TiOx (x < 2) and HfO2 layers, respectively, during the ion beam sputter deposition on the Ta2O5 layer. Compared with the devices with no interfacial layers, the switching uniformity was improved for both interfacial layers, with Ti showing the greatest improvement. The switching cycle endurance was largely degraded for the HfO2 interfacial layer, whereas the TiOx interfacial layer greatly improved in such aspects. The appropriate level of Ti-O bond energy and an excessively high Hf-O bond energy were suggested as the main reasons for such a critical difference.

Comparative Study of Reliability of Ferroelectric and Anti-Ferroelectric Memories

With the discovery of the ferroelectric (FE) properties within HfO2, the scaling gap between state-of-the-art technology nodes and non-volatile memories based on FE materials can be bridged. In addition to non-volatility, new memory concepts should guarantee sufficient endurance and operation stability. However, in contrast to optimized perovskite based FEs, binary oxide based FE memories still show changes in the memory window (MW) followed by either hard breakdown or closure of the MW. Recently, we have shown that anti-FE (AFE) materials exhibit very stable and significantly higher endurance with respect to the FE counterparts. Inspired by the robustness and remarkable cycling performance of the AFE materials, we analyze the remaining reliability aspects of these devices. By characterizing the pure film properties of capacitor stacks and switching performance when integrated into devices, we compare and investigate temperature stability, imprint, retention, and variability of both FE and AFE memories. We investigate if the lower energetic barrier to be overcome together with partial switching and lower switching induced stress are responsible for the higher endurance of the AFE with respect to the FE based memories. By utilizing charge trapping and charge pumping tests together with leakage current spectroscopy in combination with comprehensive modeling we check that assumption. Moreover, we identify the interfacial buffer layer as the weakest link of these devices.