Interfacial Buffer Layer Research Articles

The interface modification for GNWs/Si Schottky junction with PEI/PEIE interlayers

Polyethylenimine ethoxylated (PEIE) and polyethyl-enimine (PEI), the two kinds of interface buffer layer, are widely used in the organic light-emitting diodes and solar cells for band alignment adjustment. In this report, we carefully studied the influence of the inserting organic layer on the graphene nanowalls(GNWS)/Si junction quality and the photoresponse of the Schottky devices. We found that thinner layers of PEI could decrease the dark current and improve the photo-to-dark ratio to 105 for n-Si devices. The s-kink effect and degradation of open circuit voltage could be observed for thicker thickness and excessive doping. Relatively, PEIE with stable thin layer not only improve the rectifying characteristics of p-Si devices but also the incident photon conversion efficiency. The maximus IPCE could reach 44% and be adjusted to zero by the reverse bias. The tunneling inhibition for electrons can be alleviated by increasing the barrier height. Our results provide an attractive method to improve the efficiency of pristine GNWs/Si junction with interface doping and passivation.

High dielectric constant and energy density induced by the tunable TiO2 interfacial buffer layer in PVDF nanocomposite contained with core–shell structured TiO2@BaTiO3 nanoparticles

To realize application in high-capacity capacitors and portable electric devices, large energy density is eagerly desired for polymer-based nanocomposite. The core–shell structured nanofillers with inorganic buffer layer are recently supposed to be promising in improving the dielectric property of polymer nanocomposite. In this work, core–shell structured TO@BT nanoparticles with crystalline TiO2 buffer layer coated on BaTiO3 nanoparticle were fabricated via solution method and heat treatment. The thickness of the TO buffer layer can be tailored by modulating the additive amount of the titanate coupling agent in preparation process, and the apparent dielectric properties of nanocomposite are much related to the thickness of the TO layer. The relatively thin TO layer prefer to generate high polarization to increase dielectric constant while the relatively thick TO layer would rather to homogenize field to maintain breakdown strength. Simulation of electric field distribution in the interfacial region reveals the improving effect of the TO buffer layer on the dielectric properties of nanocomposite which accords with the experimental results well. The optimized nanoparticle TO@BT-2 with a mean thickness of 3–5 nm buffer layer of TO is effective in increasing both the ε and Eb in the PVDF composite film. The maximal discharged energy density of 8.78 J/cm3 with high energy efficiency above 0.6 is obtained in TO@BT-2/PVDF nanocomposite with 2.5 vol% loading close to the breakdown strength of 380 kV/mm. The present study demonstrates the approach to optimize the structure of core–shell nanoparticles by modulating buffer layer and provides a new way to further enlarge energy density in polymer nanocomposite.

Solution-processed functionalized reduced graphene oxide-an efficient stable electron buffer layer for high-performance solar cells

Interfacial buffer layer is a primary requirement for highly efficient and stable organic solar cells (OSCs) and has become a significant part of OSC research today. Here, highly dispersed functionalized reduced graphene oxide (FGr) was obtained as a stable electron buffer layer (EBL) for high-performance conventional solar cells. The power conversion efficiency of FGr-based devices (9.47% for PTB7-Th:PC71BM and 4.05% for P3HT:PC71BM) outperforms PFN-based devices (8.94% and 3.52%). Furthermore, the stability of the devices was greatly improved with FGr as EBL compared to PFN, and only dropped 7.4% of its original value after stored for 61 days. The results indicate that solution-processed functionalized reduced graphene oxide is a promising interfacial material and an excellent candidate as electron buffer layer.

Tailoring nucleation process to gain highly oriented ZnO thin film with excellent UV luminescent and electrical performances

We present a novel approach for tuning orientation in ALD deposited ZnO nanofilm by inserting interfacial buffer layer (IBL) at interface between film and substrate, which is found to induce strong orientation and significant improvement in crystalline quality by largely decreasing grain boundary (GB) area through tailoring nucleation process. As a result, the UV emission in ZnO film with IBL is enhanced by over an order of magnitude due to the reduction of surface quenching at GB. Meanwhile, the decrease of electron scattering at GB also helps improve the electrical mobility of the film, leading to great reduction in resistivity.

Progress of Cu2O/ZnO oxide heterojunction solar cells

Recent progress of low cost Cu2O/ZnO hetero-junction solar cells is reviewed in this paper. The Cu2O used as an absorbing layer in photovoltaic cells is a direct bandgap semiconductor, exhibiting natural p-type conductivity. The source material of Cu2O-based solar cells is abundant and environmentally friendly. The main device structure of Cu2O/ZnO solar cells presents a planar and nano-wire/nano-rod configuration. The nanostructured Cu2O architecture conduces to charge collection in the device. The planar Cu2O absorbing layer with large grain size, achieved through the thermal oxidation of Cu sheets, exhibits high quality of the Cu2O/ZnO solar cells. The interface buffer layer (like i-ZnO, a-ZTO and Ga2O3) and back surface field (BSF, such as p+-Cu2O) can effectively improve energy band alignment match and increase carrier transport. The Cu2O paired with a 10-nm-thick Ga2O3 layer provides a nearly ideal conduction band offset and thus reduces the interface recombination. The Ga2O3 is a highly suitable buffer layer for enhancing the Voc (Voc value reaches 1.2 V) and conversion efficiency of Cu2O-based solar cells. The p+-Cu2O like N-doped Cu2O (Cu2O:N) and Na-doped Cu2O (Cu2O:Na) can reduce back-contact resistance and create an electron-reflecting back surface field in the Cu2O based solar cells. When a p-type Cu2O: Na acts as an absorbing layer and a zinc-germanium-oxide (Zn1-xGex-O) thin film is used as an n-type layer (buffer), Cu2O hetero-junction solar cell with the device structure MgF2/Al-doped ZnO (ZnO:Al)/Zn0.38Ge0.62-O/Cu2O:Na shows an efficiency of 8.1%. The oxide hetero-junction solar cells have a great potential application in the future photovoltaic field.

Organic solar cells on Al electroded opaque substrates: Assessing the need of ZnO as electron transport layer

Interfacial buffer layers such as electron and hole transporting layers are crucial in maximizing power conversion efficiencies of organic solar cell (OSC) devices. In this manuscript, we studied the role of solution processed ZnO as electron transport layer in devices with Aluminum (Al) cathode on opaque substrates in inverted device configuration where Al and Gold (Au) are used as cathode and anode, ZnO and MoO3 are used as electron and hole transport layers respectively whilst P3HT:PC60BM is used as active layer (ActL). The results show that a thin layer of Aluminum Oxide (Al2O3) present on thermally evaporated Al cathode acts as an electron transport layer (ETL) when ZnO is absent, with surface states present on Al2O3 layer aiding the electron transport to cathode. The presence of ZnO as ETL between Al and active layer leads to a decrease in the efficiency of solar cell from 2.9% to 1.9%, attributed to the trapping of electrons at Al/ZnO interface due to unfavourable band alignment created by the presence of composite Al2O3/ZnO layer, implying that an additional ETL is not required when Al is used as a bottom electrode and could be an added advantage due to reduced processing steps.

Probing the dielectric response of the interfacial buffer layer in epitaxial graphene via optical spectroscopy.

Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Though much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density functional theory (DFT), to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV.

Bimetallic nanocomposite as hole transport co-buffer layer in organic solar cell

Silver–zinc bimetallic nanocomposite (Ag:Zn BiM-NPs) was used as an inter-facial buffer layer in the preparation of thin film organic solar cell (TFOSC). The current investigation focuses on the effect of bimetallic nanoparticles on the performance of TFOSC. A number experiments were conducted by employing Ag:Zn nanocomposite buffer layer of thickness 1 nm at various positions of the device structure. In all cases, we found significant improvement on the power conversion efficiency of the solar cells. It is also noted that the open circuit voltage of the devices are decreasing when Ag:Zn form direct contact with the ITO electrode and without the inclusion of PEDOT:PSS. However, all results show that the introduction of Ag:Zn nanocomposite layer close to PEDOT:PSS could be beneficial to improve the charge transport processes in the preparation of thin film organic solar cell. The Ag:Zn BiM-NPs and the device properties were presented and discussed in terms of optical, electrical and film morphologies of the devices.

Switching characteristics of microscale unipolar Pd/Hf/HfO2/Pd memristors

In this paper, we investigate the switching behavior of nano-thick microscale Pd/Hf/HfO2-10nm/Pd memristors. Several key electrical characteristics such as the forming voltage and the SET/RESET IV parameters are studied as a function of the device size, the hafnium-capping thickness and the environmental temperature. The fabricated stack exhibits a unipolar switching behavior that is characteristic of two interfacial oxide and buffer layers consisting of the same transition metal. The average forming voltage value decreases oppositely to the microscale device area while it increases with the Hf-capping layer thickness. The unipolar memristive switching behavior is found to be polarity-dependent due to diverse contributions of the top Hf-buffer layer and the bottom Pd electrode to the switching mechanism. Joule heating effects involving thermophoresis and diffusion are concluded to be predominantly governing the ON/OFF switching events in the positive polarity. Synergistic electric field effects are found to additionally control the unipolar switching behavior in the negative mode. The respective key roles of the top Hf-buffer layer and the bottom Pd electrode in the asymmetric unipolar memristive switching are elucidated at the molecular level according to well-established transport phenomena reported for oxygen vacancies. Given the importance of heat in controlling the conductance state of the proposed stack, the memristor's electrical sensitivity to changes in the ambient temperature is preliminarily investigated for potential temperature sensing applications.

Synthesis of WOn -WX2 (n=2.7, 2.9; X=S, Se) Heterostructures for Highly Efficient Green Quantum Dot Light-Emitting Diodes.

Preparation of two-dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WOn -WX2 (n=2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WOn nanomaterials. The WOn -WX2 heterostructures are composed of WO2.9 nanoparticles (NPs) or WO2.7 nanowires (NWs) grown together with single- or few-layer WX2 nanosheets (NSs). As a proof-of-concept application, the WOn -WX2 heterostructures are used as the anode interfacial buffer layer for green quantum dot light-emitting diodes (QLEDs). The QLED prepared with WO2.9 NP-WSe2 NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.

Synthesis of WOn‐WX2 (n=2.7, 2.9; X=S, Se) Heterostructures for Highly Efficient Green Quantum Dot Light‐Emitting Diodes

AbstractPreparation of two‐dimensional (2D) heterostructures is important not only fundamentally, but also technologically for applications in electronics and optoelectronics. Herein, we report a facile colloidal method for the synthesis of WOn‐WX2 (n=2.7, 2.9; X=S, Se) heterostructures by sulfurization or selenization of WOn nanomaterials. The WOn‐WX2 heterostructures are composed of WO2.9 nanoparticles (NPs) or WO2.7 nanowires (NWs) grown together with single‐ or few‐layer WX2 nanosheets (NSs). As a proof‐of‐concept application, the WOn‐WX2 heterostructures are used as the anode interfacial buffer layer for green quantum dot light‐emitting diodes (QLEDs). The QLED prepared with WO2.9 NP‐WSe2 NS heterostructures achieves external quantum efficiency (EQE) of 8.53 %. To our knowledge, this is the highest efficiency in the reported green QLEDs using inorganic materials as the hole injection layer.

Performance enhancement of polymer solar cells by incorporating Ag nanoparticles at an indium tin oxide/MoO3 buffer layer interface

In the present study, the plasmonic effects of silver nanoparticles (Ag NPs) have been utilized to improve the performance of polymer solar cells. Plasmonic polymer solar cells were fabricated by incorporating Ag NPs at the indium tin oxide (ITO)/MoO3 buffer layer interface. The size of the Ag NPs has been optimized to improve the device performance. The optimized device with 75 nm sized Ag NPs shows 26% enhancement in the power conversion efficiency (PCE) compared with the reference device. The short circuit current density mainly contributes to the efficiency enhancement compared with other device parameters. The diffuse reflectance measurements indicate that there is significant enhancement in the optical absorption of active layers when Ag NPs are incorporated into the devices. The scattering cross section of Ag NPs and electric field intensity distribution around the nanoparticles were modeled by finite-difference-time-domain simulations. The experimental results and theoretical simulations revealed that both far-field scattering and near-field concentration of light contributed significantly to the PCE enhancement of the devices.

Research progress of cathode buffer layers of the polymer solar cells

Polymer solar cells (PSCs) have attracted great attention in the clean energy utilization due to their advantages of light weight, low cost and easy fabrication. Recently, the power conversion efficiency (PCE) of the PSCs has been improved gradually. The PCE of the single junction PSCs have been increased over 11%, and the interface buffer layers have become the key factors to significantly influence the device performance. In this review, we summaried the recent progress on the cathode buffer layers (CBLs) of the PSCs. The commonly used CBLs have been introduced from two types: inorganic and organic materials. The inorganic materials include metal oxides, metals and metal compounds. The organic materials include polymers, fullerene derivatives and n-type organic semiconductors. We have also investigated the influences on the device photovoltaic performances by introducing their electric properties, optical properties, energy levels, surface morphology and interface contacts. Finally, we have given a perspective on PSCs.

High-resistivity sol-gel ITO thin film as an interfacial buffer layer for bulk heterojunction organic solar cells

Bulk heterojunction organic solar cells have been fabricated by inserting a high-resistivity sol-gel ITO buffer layer between an ITO anode and a PEDOT:PSS hole injection layer. The performance of the devices with the sol-gel ITO atop the ITO anodes treated by conventional annealing at 500 °C for 1 h and rapid thermal process (RTP) at 800 °C for 20 and 30 s was compared. The best power conversion efficiency of 3.5% was achieved for the device with the 15-nm-thick sol-gel ITO treated with RTP at 800 °C for 30 s, as compared with 2.7% of the standard device under an illumination of AM 1.5. In addition, the short circuit current of the device was significantly increased by 42.7%. The observed enhancement of the short circuit current can be attributed an interfacial energy step created by the high-resistivity sol-gel ITO between the ITO anode and the PEDOT:PSS.

Investigation of cubic single crystal HfO2/Si interface under in-plane biaxial tensile strain

HfO2 is a potential dielectric for metal-oxide-semiconductor(MOS) devices. Cubic single crystal HfO2 films have been successfully deposited onto Si substrates by interface buffer layer, so in-plane biaxial tensile strain inevitably exists in HfO2. However, impact of the strain on HfO2/Si interface has not been investigated. In this letter, the interface behaviors of HfO2/Si under the strain are investigated by first principles calculations. We creatively realize band alignment of HfO2/Si interface under the strain. Formation energies and transition levels of oxygen interstitial (Oi) and oxygen vacancy (VO) in HfO2 are assessed. We investigate how they will affect the performance of devices.

Local epitaxy from the silicon substrate in silicon–rich SiC during Si–nanocrystals formation

This paper reports on the structural evolution of the silicon–rich SiC/c–Si interface upon 1100°C, 30min thermal treatment, which is typically used when fabricating c–Si based devices that incorporate Si–nanocrystals in an SiC matrix. The onset of local interfacial epitaxy is revealed and analyzed. The inhibitory effect of a thin SiC interfacial buffer layer is reported. The investigation is based on Transmission Electron Microscopy and UV–Visible Reflectance spectroscopy. We show that a minimum thickness is required for the SiC buffer layer to preserve the presence and quality of the interface. The investigation also indicates that the epitaxial regrowth occurs in competition with random nucleation, which turns out to be the dominant crystallization mechanism in silicon rich SiC. A value 3÷10nm is given for the diffusion distance of carbon in silicon upon 1100°C, 30min thermal treatment, which confirms previously reported results. The investigation shows that qualitative as well as quantitative structural information can be rapidly and routinely obtained by careful analysis of UV–Visible Reflectance spectral data.

Study of ITO-free roll-to-roll compatible polymer solar cells using the one-step doctor blading technique

The performance of simple one-step doctor-blading ITO-free inverted polymer solar cells is primarily influenced by the inherent interfacial buffer layer stratification purity rather than the fine donor/acceptor phase separation.

Formation of graphene/SiC/AlN multilayers synthesized by pulsed laser deposition on Si(110) substrates

We have grown aluminum nitride (AlN) films on Si(110) substrates by pulsed laser deposition (PLD), and investigated the effects of laser power on the crystallinity and surface morphology of the AlN films. First, we epitaxially grew a fairly flat, high-quality AlN film, which contained no rotation domains, onto the Si(110) substrate in a well-lattice-matched relationship. Secondly, we formed a SiC interfacial buffer layer on the AlN film to grow a high-quality 3C-SiC film on the SiC buffer layer by PLD, which gave rise to a 3C-SiC(111)3×3 surface. The root-mean-square-roughness value of the SiC film was smaller than the previously reported values of SiC/AlN multilayers on Si(100) and Si(111) substrates. Thirdly, we grew graphene by annealing the SiC film at a high temperature in an ultra-high vacuum. It was demonstrated that the qualified graphene layer without rotation domains was grown on the SiC film. The formation of voids and the outdiffusion of Al and N atoms from the AlN film were successfully suppressed during the high-temperature annealing.

AC-Impedance Spectroscopic Analysis on the Charge Transport in CVD-Grown Graphene Devices with Chemically Modified Substrates.

A comprehensive study for the effect of interfacial buffer layers on the electrical transport behavior in CVD-grown graphene based devices has been performed by ac-impedance spectroscopy (IS) analysis. We examine the effects of the trap charges at graphene/SiO2 interface on the total capacitance by introducing self-assembled monolayers (SAMs). Furthermore, the charge transports in the polycrystalline graphene are characterized through the temperature-dependent IS measurement, which can be explained by the potential barrier model. The frequency-dependent conduction reveals that the conductivity of graphene is related with the mobility, which is limited by the scattering caused by charged adsorbates on SiO2 surface.

Enhancement of Ga-doped zinc oxide film properties and deposition rate by multiple deposition using atmosphere pressure plasma jet

Multilayer film with an interfacial buffer layer made of different material has been show to improve the film properties. However, it is not well understood if similar benefits may be achieved by multiple deposition of GZO without the addition of new interfacial materials. Here we prepared single-, double-, and triple-deposited GZO films on glass substrate (preheated to 180 °C) by atmosphere pressure plasma jet (APPJ). We found that the triple-deposited film exhibits overall enhancement over the single-deposited film in deposition rate (110 nm/min) and the key properties, such as resistivity (9.4 × 10−4 Ω cm), crystallite size (16 nm), carrier concentration, mobility, and band gap. In particular, the resistivity and deposition rate are significantly improved by 33% and 35%, respectively. The results are attributed to the mitigation of lattice-mismatch between the film and the glass substrate by the bottom thin GZO film. The findings may also be applied to other deposition methods with moving nozzle or moving stage such as spray pyrolysis.