Metal Oxide Electron Research Articles

Enhanced Performance of Inverted Non-Fullerene Organic Solar Cells by Using Metal Oxide Electron- and Hole-Selective Layers with Process Temperature ≤150 °C.

In this work, an efficient inverted organic solar cell (OSC) based on the non-fullerene PBDB-T:IT-M blend system is demonstrated by using an aqueous solution processed ZnO electron-selective layer with the whole process temperature ≤150 °C and a thermally evaporated MoO3 hole-selective layer The ZnO selective layer is deposited by aqueous solution and prepared in a low-temperature process, so that it can be compatible with the roll-to-roll process. The proposed device achieves an enhanced power conversion efficiency (PCE) of 9.33% compared with the device based on the high-temperature sol-gel-processed ZnO selective layer, which achieves a PCE of 8.62%. The inverted device also shows good stability, keeping more than 82% of its initial PCE after being stored under ambient air conditions and a humidity of around 40% without any encapsulation for 240 h. The results show the potential for the fabrication of efficient non-fullerene OSCs with low-temperature metal oxide selective layers.

Fabrication of Fe2O3@TiO2 core–shell nanospheres as anode materials for lithium-ion batteries

Metal oxide electronics materials possess splendid application prospect in lithium ions battery. In this work, Fe2O3@TiO2 nanospheres are synthesized via a facile chemical co-precipitation method combined with hydrolysis method. Field-emission scanning electron microscopy, transmission electron microscopy reveals that the as-prepared Fe2O3@TiO2 is composed of TiO2 as a rigid nanoshell and Fe2O3 as a core. It is found that the TiO2 shell is effective for improving the electrical conductivity and structural stability. This novel core–shell structure showed enhanced electrochemical properties is mainly attributed to the inert TiO2 preventing the Fe2O3 nanoparticles from pulverization and aggregation and the synergistic effects between the Fe2O3 core and TiO2 shell.

Size-dependent single electron transfer and semi-metal-to-insulator transitions in molecular metal oxide electronics

All-inorganic self-arranged molecular transition metal oxide hyperstructures based on polyoxometalate molecules (POMs) are fabricated and tested as electronically tunable components in emerging electronic devices. POM hyperstructures reveal great potential as charging nodes of tunable charging level for molecular memories and as enhancers of interfacial electron/hole injection for photovoltaic stacks. STM, UPS, UV–vis spectroscopy and AFM measurements show that this functionality stems from the films’ ability to structurally tune their HOMO–LUMO levels and electron localization length at room temperature. By adapting POM nanocluster size in solution, self-doping and current modulation of four orders of magnitude is monitored on a single nanocluster on SiO2 at voltages as low as 3 Volt. Structurally driven insulator-to-semi-metal transitions and size-dependent current regulation through single electron tunneling are demonstrated and examined with respect to the stereochemical and electronic structure of the molecular entities. This extends the value of self-assembly as a tool for correlation length and electronic properties tuning and demonstrate POM hyperstructures’ plausibility for on-chip molecular electronics operative at room temperature.

Electronic Devices Based on Oxide Thin Films Fabricated by Fiber-to-Film Process

Technical development for thin-film fabrication is essential for emerging metal-oxide (MO) electronics. Although impressive progress has been achieved in fabricating MO thin films, the challenges still remain. Here, we report a versatile and general thermal-induced nanomelting technique for fabricating MO thin films from the fiber networks, briefly called fiber-to-film (FTF) process. The high quality of the FTF-processed MO thin films was confirmed by various investigations. The FTF process is generally applicable to numerous technologically relevant MO thin films, including semiconducting thin films (e.g., In2O3, InZnO, and InZrZnO), conducting thin films (e.g., InSnO), and insulating thin films (e.g., AlO x). By optimizing the fabrication process, In2O3/AlO x thin-film transistors (TFTs) were successfully integrated by fully FTF processes. High-performance TFT was achieved with an average mobility of ∼25 cm2/(Vs), an on/off current ratio of ∼107, a threshold voltage of ∼1 V, and a device yield of 100%. As a proof of concept, one-transistor-driven pixel circuit was constructed, which exhibited high controllability over the light-emitting diodes. Logic gates based on fully FTF-processed In2O3/AlO x TFTs were further realized, which exhibited good dynamic logic responses and voltage amplification by a factor of ∼4. The FTF technique presented here offers great potential in large-area and low-cost manufacturing for flexible oxide electronics.

Advanced Functional Materials Solutions to Engineering the Neural Interface

Advanced Functional Materials Solutions to Engineering the Neural Interface

Understanding charge trapping/detrapping at the zinc oxide (ZnO)/MAPbI3 perovskite interface in the dark and under illumination using a ZnO/perovskite/ZnO test platform.

We fabricated a zinc oxide (ZnO)/methylammonium lead iodide (MAPbI3) perovskite/ZnO field effect transistor (FET) test platform device through which ZnO/perovskite interfacial contact properties can be probed in the dark and under illumination. Using pulsed laser deposition, highly conductive (0.014 Ω cm) ZnO source and drain electrodes were fabricated allowing for the investigation of the interfacial charge transfer properties through current-voltage characteristics of a ZnO/perovskite/ZnO FET. With a bottom-contact FET device, gate voltage dependent current hysteresis in the drain current-gate voltage curves was probed at low temperature to minimize the effect of ion migration on electronic charge transport in the perovskite layer. Under illumination, importantly, ZnO/perovskite electrical contact properties were significantly altered due to electronic energy barrier change at the interface arising from the detrapping of electrons from the ZnO/perovskite interface, resulting in an enhanced dark current and a suppressed photocurrent. The origin of current hysteresis in the ZnO/perovskite/ZnO FET device is discussed relating it to interfacial charging/discharging associated with ultraviolet (UV)-induced oxygen adsorption/desorption. The results presented herein demonstrate that interfacial electronic properties at the donor (perovskite)/acceptor (ZnO) interface can be altered by photoinduced carrier trapping/detrapping, providing insights that UV-induced persistent photoconduction in transition metal oxide electron transport layers including ZnO may be contributing to the current hysteresis observed in the perovskite photovoltaic devices.

Quantitative Evidence for Lanthanide-Oxygen Orbital Mixing in CeO2, PrO2, and TbO2.

Understanding the nature of covalent (band-like) vs ionic (atomic-like) electrons in metal oxides continues to be at the forefront of research in the physical sciences. In particular, the development of a coherent and quantitative model of bonding and electronic structure for the lanthanide dioxides, LnO2 (Ln = Ce, Pr, and Tb), has remained a considerable challenge for both experiment and theory. Herein, relative changes in mixing between the O 2p orbitals and the Ln 4f and 5d orbitals in LnO2 are evaluated quantitatively using O K-edge X-ray absorption spectroscopy (XAS) obtained with a scanning transmission X-ray microscope and density functional theory (DFT) calculations. For each LnO2, the results reveal significant amounts of Ln 5d and O 2p mixing in the orbitals of t2g (σ-bonding) and eg (π-bonding) symmetry. The remarkable agreement between experiment and theory also shows that significant mixing with the O 2p orbitals occurs in a band derived from the 4f orbitals of a2u symmetry (σ-bonding) for each compound. However, a large increase in orbital mixing is observed for PrO2 that is ascribed to a unique interaction derived from the 4f orbitals of t1u symmetry (σ- and π-bonding). O K-edge XAS and DFT results are compared with complementary L3-edge and M5,4-edge XAS measurements and configuration interaction calculations, which shows that each spectroscopic approach provides evidence for ground state O 2p and Ln 4f orbital mixing despite inducing very different core-hole potentials in the final state.

Linker Length-Dependent Electron-Injection Dynamics of Trimesitylporphyrins on SnO2 Films

Electron-injection dynamics of dye-sensitized photoelectrochemical cells depend on the length of the linker connecting the molecular photosensitizer to the metal oxide electron acceptor. However, systematic studies of the effect of chromophore–oxide distance are scarce. Here we present the synthesis, characterization, spectroscopy, and computational modeling of electron-injection dynamics from free-base trimesitylporphyrins with varying linker lengths into tin(IV) oxide (SnO2). In each system, the porphyrin core and metal oxide film remain the same while only the linker binding the porphyrin to the carboxylate anchor group is varied. A length range spanning 8.5–17.2 A is studied by employing phenylene, biphenylene, terphenylene, and benzanilide groups as the linker. We find a clear correlation between linker length and injection rates, providing insights that will be exploited in the optimization of dye-sensitized photoelectrochemical cells.

Investigation of the light-soaking effect in organic solar cells using dielectric permittivity and electric modulus approaches

We demonstrate the influence of a light-soaking treatment on current-voltage characteristics, external quantum efficiency and dielectric response of an encapsulated P3HT:PCBM bulk heterojunction inverted organic solar cell. Furthermore, we apply dielectric permittivity and electric modulus approaches to identify relaxation peaks observed in this device. We discuss previously reported explanations of the light-soaking effect on the basis of our experimental results, showing that the light-soaking effect originates from dipole formation at the ZnO/P3HT:PCBM interface. In addition, we find that metal oxide hole and electron selective layers prevent the P3HT:PCBM molecules' reorientations and, thus, enhance its thermal stability. This type of experimental observations are expected to provide new prospective on the fundamental aspect of elementary charge transfer modifications in organic solar cells.

Thin Films of Tin Oxide Nanosheets Used as the Electron Transporting Layer for Improved Performance and Ambient Stability of Perovskite Photovoltaics

Inorganic semiconducting metal oxide electron transporting layers (ETLs) with tailored nanostructures have the potential to yield efficient perovskite solar cells (PSCs) with improved stability and reduced hysteresis. Here, a simple and facile hydrothermal protocol is demonstrated for the growth of crystalline SnO2 nanosheet (SNS) thin films with excellent optical and photoelectrical properties. When applied as ETLs, the SNS enhances light harvesting, boosts charge collection, reduces hysteresis and improves the ambient stability. Moreover, optimization of the interfacial properties between the SNS and the perovskite layer by the introduction of a C60 interlayer results in better energy level alignment, decreasing charge recombination and thus prolonging the electron lifetime and improving the open‐circuit voltage (Voc). Therefore, the efficient conventional n‐i‐p PSCs based on C60‐modified SNS ETLs have almost hysteresis‐free behavior, with a champion power conversion efficiency, PCE, of 18.31% and a stabilized PCE over 18.00%. The SNS‐based PSC exhibits respectable ambient stability retaining over 90% of its initial efficiency after being stored in air at room temperature for 500 h without encapsulation. Such robust SNS ETLs pave the way to low‐cost and stable PSCs with high efficiency, less hysteresis, and long‐term stability.

Genome Scale Mutational Analysis of Geobacter sulfurreducens Reveals Distinct Molecular Mechanisms for Respiration and Sensing of Poised Electrodes versus Fe(III) Oxides

ABSTRACTGeobacter sulfurreducens generates electrical current by coupling intracellular oxidation of organic acids to the reduction of proteins on the cell surface that are able to interface with electrodes. This ability is attributed to the bacterium's capacity to respire other extracellular electron acceptors that require contact, such as insoluble metal oxides. To directly investigate the genetic basis of electrode-based respiration, we constructed Geobacter sulfurreducens transposon-insertion sequencing (Tn-Seq) libraries for growth, with soluble fumarate or an electrode as the electron acceptor. Libraries with >33,000 unique insertions and an average of 9 insertions/kb allowed an assessment of each gene's fitness in a single experiment. Mutations in 1,214 different genomic features impaired growth with fumarate, and the significance of 270 genes unresolved by annotation due to the presence of one or more functional homologs was determined. Tn-Seq analysis of −0.1 V versus standard hydrogen electrode (SHE) electrode-grown cells identified mutations in a subset of genes encoding cytochromes, processing systems for proline-rich proteins, sensory networks, extracellular structures, polysaccharides, and metabolic enzymes that caused at least a 50% reduction in apparent growth rate. Scarless deletion mutants of select genes identified via Tn-Seq revealed a new putative porin-cytochrome conduit complex (extABCD) crucial for growth with electrodes, which was not required for Fe(III) oxide reduction. In addition, four mutants lacking components of a putative methyl-accepting chemotaxis–cyclic dinucleotide sensing network (esnABCD) were defective in electrode colonization but grew normally with Fe(III) oxides. These results suggest that G. sulfurreducens possesses distinct mechanisms for recognition, colonization, and reduction of electrodes compared to Fe(III) oxides.IMPORTANCE Since metal oxide electron acceptors are insoluble, one hypothesis is that cells sense and reduce metals using the same molecular mechanisms used to form biofilms on electrodes and produce electricity. However, by simultaneously comparing thousands of Geobacter sulfurreducens transposon mutants undergoing electrode-dependent respiration, we discovered new cytochromes and chemosensory proteins supporting growth with electrodes that are not required for metal respiration. This supports an emerging model where G. sulfurreducens recognizes surfaces and forms conductive biofilms using mechanisms distinct from those used for growth with metal oxides. These findings provide a possible explanation for studies that correlate electricity generation with syntrophic interspecies electron transfer by Geobacter and reveal many previously unrecognized targets for engineering this useful capability in other organisms.

An easily prepared carbon quantum dots and employment for inverted organic photovoltaic devices

In this paper, the performance enhancement of organic solar cells based on PCDTBT:PC71BM and P3HT:PC60BM are demonstrated via employing carbon quantum dots (CQD) to modify the metal oxide electron transport layer. The incorporated CQD with carboxylic acid (–COOH) groups could induce self-assembled monolayer (SAM) with TiO2 buffer layer, which lowered the energy barrier for electron transfer and reduced the inherent incompatibility between the metal oxide and organic active layers. Further investigation shows that SAM of TiO2 with CQD can improve the photo-induced exciton dissociation and charge transfer, leading to a low electron accumulation in charge transport layer and a reduced interface recombination loss.

Cadmium-doped flexible perovskite solar cells with a low-cost and low-temperature-processed CdS electron transport layer

Hybrid perovskite solar cells (PSCs) are promising candidates in exploring high performance flexible photovoltaics, where a low-temperature-processed metal oxide electron transfer layer (ETL) is highly preferable.

All‐Carbon Thin‐Film Transistors as a Step Towards Flexible and Transparent Electronics

Carbon nanotubes and graphene have attracted great attention for potential applications in flexible and transparent electronics, owing to their exceptional properties including very high electrical conductivity, optical transmittance, mechanical strength and flexibility. Thin films of carbon nanotubes or graphene are promising for fabricating high‐quality semiconducting channels and transparent conductive films, as electrodes and interconnects in thin‐film transistors and integrated circuits, which allows the construction of all‐carbon electronics. This article reviews the main approaches to and important findings in the fabrication of flexible and transparent electrodes and channels for thin‐film transistors, and highlights recent achievements of flexible and transparent all‐carbon thin‐film transistors, in terms of design, fabrication and performance evaluation. This all‐carbon strategy opens up a way to achieve extremely flexible, transparent and stretchable electronics with relatively low cost and fast fabrication techniques, compared to the traditional rigid silicon, metal and metal oxide electronics.

(Invited) Recent Progresses in Solution-Processed Hybrid Perovskite Devices in Photovoltaics and Optoelectronics

Organo-lead trihalide perovskite compounds, represented by CH3NH3PbI3, exhibit many rare functions as narrow bandgap semiconductors (Fig. 1). In 2006, we have first employed CH3NH3PbBr3 and CH3NH3PbI3 as the sensitizer of electrochemical solar cell. 1a In 2008 this method was applied to make a first solid-state perovskite solar cell with carbon-polymer composite as a hole transport materialin.1c Recent rapid progress of perovskite-based photovoltaics (PV) has enabled power conversion efficiency (PCE) to reach 2２% (Fig. 2) . Our group has studied simple solution process for perovskite absorber preparation in ambient air conditions and achieved efficiency beyond 17%.2 Here, photocurrent-voltage (J-V) behavior of the perovskite PV cell was investigated to clarify the origin(s) of its hysteretic performance by focusing on the influence of interfacial structures.3 Perovskite cells can be fabricated by low temperature processes. Lightweight flexible photovoltaic cells will see enormous applications in power devices. We developed plastic film perovskite cells (PCE >13%), fabricated on ITO-PEN film by using brookite TiO2 as mesoporous layer, to demonstrate its high mechanical durability against repeated bending. As metal oxide electron collectors, ZnO/SnO2 composite films prepared at temperatures <120oC enabled PCE of perovskite cell exceeding 15%.4 Use of SnO2 collector is a new trend in perovskite PV in terms of high voltage and improvement of cell lifetime. The heat stability of perovskite device is an important issue for industrial applications. It is improved by using formamidinium (HC(NH2)2) that replaces CH3NH3. We made HC(NH2)2PbI3-based solar cells on low-temperature mesoporous ZnO. They exhibit relatively high stability for long time preservation with PCE as high as 16% (Fig. 3 left). HC(NH2)2PbI3 solar cell prepared on thin TiO2 layer exhibits high open circuit voltage (Voc)>1.1V with PEC >17% (Fig. 3 right). Enormous potential of perovskite-based device is not only expected for power devices but also for high performance optical sensing devices. Organic lead trihalide perovskite can work as a high function semiconductor photodiode. We found that the gain of CH3NH3PbI3-induced diode photocurrent can reach a high level of the order of 103, showing excellent light-switching and current amplifying performance (Fig. 4)5. Such rare functions of the perovskite devices provide a lot of rooms to explore their versatile applications in photovoltaics and optoelectronics. REFERENCES 1. a) A. Kojima, et al. #397, 210th ECS Meeting, 2006. b) ibid, #352, 212th ECS Meeting, 2007. c). ibid, PRiME 2008, #27, Honolulu,2008.2. T. Miyasaka, Chem. Lett., 44, 720 (2015).3. A. K. Jena, A. Kulkarni, M. Ikegami, T. Miyasaka, J . Power Sources, 309, 1-10(2016).4. J. Song, E. Zheng, X.-F. Wang, W. Tian, T. Miyasaka, Solar Ener . Mater . S olar Cells, 144, 623(2016).5. H.-W. Chen, N. Sakai, A. K. Jena, Y. Sanehira, M. Ikegami, K.-C. Ho, T. Miyasaka, J. Phys. Chem. Lett., 6, 1773(2015) Figure 1

N-type metal-oxide electron transport layer for mesoscopic perovskite solar cells

To meet the challenge of continuously increasing global energy demands, organic-inorganic halide based perovskite solar cells (PSCs) have garnered great attention from the photovoltaic research community for their low cost and high efficiency. The efficiency of perovskite-based mesoscopic solar cells increases rapidly, from 3.8% in 2009 to 22.1% in 2016. N-type metal-oxide electron transport layer, as one of the important components in mesoscopic PSCs (MPSCs), acts as not only a scaffold layer for the growth of perovskite crystals, but also a layer to supply transfer pathways for electrons injected from perovskites. In this review, we discussed recent published reports of MPSCs with the focus on n-type metal-oxide electron transport layer in MPSCs. The scaffold materials, scaffold nanostructure, and scaffold/perovskite interface engineering are considered, and the effects of these modifications of scaffolds on the performance of MPSCs are summarized in this review.

Structure and ferromagnetic instability of the oxygen-deficientSrTiO3surface

SrTiO$_3$ (STO) is the substrate of choice to grow oxide thin-films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and, thus, a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural, electronic and magnetic properties of the oxygen-deficient STO surface. We find that the surface oxygen vacancies order in periodic arrays giving rise to surface magnetic moments and a quasi two-dimensional electron gas in the occupied Ti 3-d orbitals. The surface confinement, the oxygen-vacancy ordering, and the octahedra distortions give rise to spin-polarized $t_{2g}$ dispersive sub-bands; their energy split near the Brillouin zone center acts as an effective Zeeman term, which, when we turn on a Rashba interaction, produces bands with momentum-spin correlations similar to those recently discovered on oxygen deficient STO surface.

Transition Metal-Oxide Free Perovskite Solar Cells Enabled by a New Organic Charge Transport Layer.

Various electron and hole transport layers have been used to develop high-efficiency perovskite solar cells. To achieve low-temperature solution processing of perovskite solar cells, organic n-type materials are employed to replace the metal oxide electron transport layer (ETL). Although PCBM (phenyl-C61-butyric acid methyl ester) has been widely used for this application, its morphological instability in films (i.e., aggregation) is detrimental. Herein, we demonstrate the synthesis of a new fullerene derivative (isobenzofulvene-C60-epoxide, IBF-Ep) that serves as an electron transporting material for methylammonium mixed lead halide-based perovskite (CH3NH3PbI(3-x)Cl(x)) solar cells, both in the normal and inverted device configurations. We demonstrate that IBF-Ep has superior morphological stability compared to the conventional acceptor, PCBM. IBF-Ep provides higher photovoltaic device performance as compared to PCBM (6.9% vs 2.5% in the normal and 9.0% vs 5.3% in the inverted device configuration). Moreover, IBF-Ep devices show superior tolerance to high humidity (90%) in air. By reaching power conversion efficiencies up to 9.0% for the inverted devices with IBF-Ep as the ETL, we demonstrate the potential of this new material as an alternative to metal oxides for perovskite solar cells processed in air.

Exploring Graphene Quantum Dots/TiO2 interface in photoelectrochemical reactions: Solar to fuel conversion

Photocarrier (e−/h+) generation at low dimension graphene quantum dots offers multifunctional applications including bioimaging, optoelectronics and energy conversion devices. In this context, graphene quantum dots onto metal oxide electron transport layer finds great deal of attention in solar light driven photoelectrochemical (PEC) hydrogen fuel generation. The merits of combining tailored optical properties of the graphene quantum dots sensitizer with the transport properties of the host wide band gap one dimensional nanostructured semiconductor provide a platform for high charge collection which promotes catalytic proton reduction into fuel generation at PEC cells. However, understanding the underlying mechanism of photocarrier transfer characteristics at graphene quantum dots/metal oxide interface during operation is often difficult as graphene quantum dots may have a dual role as sensitizer and catalyst. Therefore, exploring photocarrier generation and injection at graphene quantum dot/metal oxide heterointerfaces in contact with hole scavenging electrolyte afford a new pathway in developing graphene quantum dots based photoelectrochemical fuel generation systems. In this work, we demonstrate direct assembly of surface modified graphene quantum dots (∼2nm particle size) onto TiO2 hollow nanowire (∼3μm in length and ∼100 to 250nm in diameter) by electrostatic attraction and examine the photocarrier accumulation and recombination processes leading to device operation. Optical characterization reveals that GQDs absorbed light photons at visible light wavelength up to 600nm. Hybrid TiO2-GQDs heterostructures show a photocurrent enhancement of ∼70% for water oxidation compared to pristine TiO2 using sacrificial-free electrolyte, which is further validated by incident photon to current efficiency. Additionally, the charge accumulation processes and charge transfer characteristics are investigated by electrochemical impedance spectroscopy. These results provide the platform to understand the insights of graphene quantum dots/metal oxide interfaces in PEC reactions and discuss the feasibility of graphene quantum dots in wide range of electrochemical and photoelectrochemical based fuel conversion devices.

Efficient organic solar cells using copper(I) iodide (CuI) hole transport layers

We report the fabrication of high power conversion efficiency (PCE) polymer/fullerene bulk heterojunction (BHJ) photovoltaic cells using solution-processed Copper (I) Iodide (CuI) as hole transport layer (HTL). Our devices exhibit a PCE value of ∼5.5% which is equivalent to that obtained for control devices based on the commonly used conductive polymer poly(3,4-ethylenedioxythiophene): polystyrenesulfonate as HTL. Inverted cells with PCE &amp;gt;3% were also demonstrated using solution-processed metal oxide electron transport layers, with a CuI HTL evaporated on top of the BHJ. The high optical transparency and suitable energetics of CuI make it attractive for application in a range of inexpensive large-area optoelectronic devices.