Layered LaSrGa3O7‐Based Oxide‐Ion Conductors: Cooperative Transport Mechanisms and Flexible Structures

Novel melilite‐type gallium‐oxides are attracting attention as promising new oxide‐ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic‐scale investigation of the LaSrGa₃O₇‐based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner‐sharing GaO₄ tetrahedra. A major finding is the first indication that oxide‐ion conduction in La_1.54Sr_0.46Ga₃O_7.27 occurs through an interstitialcy or cooperative‐type mechanism involving the concerted knock‐on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.     

Energy Materials: Layered LaSrGa3O7‐Based Oxide‐Ion Conductors: Cooperative Transport Mechanisms and Flexible Structures (Adv. Funct. Mater. 22/2010)

Novel melilite‐type gallium‐oxides are attracting attention as promising new oxide‐ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic‐scale investigation of the LaSrGa₃O₇‐based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner‐sharing GaO₄ tetrahedra. A major finding is the first indication that oxide‐ion conduction in La_1.54Sr_0.46Ga₃O_7.27 occurs through an interstitialcy or cooperative‐type mechanism involving the concerted knock‐on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.     

Atomistic Study of a CaTiO3‐Based Mixed Conductor: Defects,Nanoscale Clusters,and Oxide‐Ion Migration

Mixed oxide‐ion and electronic conductivity can be exploited in dense ceramic membranes for controlled oxygen separation as a means of producing pure oxygen or integrating with catalytic oxidation. Atomistic simulation has been used to probe the energetics of defects, dopant‐vacancy association, nanoscale cluster formation, and oxide‐ion transport in mixed‐conducting CaTiO₃. The most favorable energetics for trivalent dopant substitution on the Ti site are found for Mn³⁺ and Sc³⁺. Dopant‐vacancy association is predicted for pair clusters and neutral trimers. Low binding energies are found for Sc³⁺ in accordance with the high oxide‐ion conductivity of Sc‐doped CaTiO₃. The preferred location for Fe⁴⁺ is in a hexacoordinated site, which supports experimental evidence that Fe⁴⁺ promotes the termination of defect chains and increases disorder. A higher oxide‐ion migration energy for a vacancy mechanism is predicted along a pathway adjacent to an Fe³⁺ ion rather than Fe⁴⁺ and Ti⁴⁺, consistent with the higher observed activation energies for ionic transport in reduced CaTi(Fe)O_3–δ.     

Molecule Charge Transfer Doping for p‐Channel Solution‐Processed Copper Oxide Transistors

The doping of semiconductors plays a critical role in improving the performance of modern electronic devices by precisely controlling the charge carrier density. However, the absence of a stable doping method for p‐type oxide semiconductors has severely restricted the development of metal oxide‐based transparent p–n junctions and complementary circuits. Here, an efficient and stable doping process for p‐type oxide semiconductors by using molecule charge transfer doping with tetrafluoro‐tetracyanoquinodimethane (F₄TCNQ) is reported. The selections of a suitable dopant and geometry play a crucial role in the charge‐transfer doping effect. The insertion of a F₄TCNQ thin dopant film (2–7 nm) between a Au source‐drain electrode and solution‐processed p‐type copper oxide (Cu_xO) film in bottom‐gate top‐contact thin‐film transistors (TFTs) provides a mobility enhancement of over 20‐fold with the desired threshold voltage adjustment. By combining doped p‐type Cu_xO and n‐type indium gallium zinc oxide TFTs, a solution‐processed transparent complementary metal‐oxide semiconductor inverter is demonstrated with a high gain voltage of 50. This novel p‐doping method is expected to accelerate the development of high‐performance and reliable p‐channel oxide transistors and has the potential for widespread applications.     

(Magic Dopant) Amphoteric Behavior of a Redox‐Active Transition Metal Ion in a Perovskite Lattice: New Insights on the Lattice Site Occupation of Manganese in SrTiO3

It is demonstrated that a transition metal redox‐active ion can exhibit amphoteric dopant substitution in the SrTiO₃ perovskite lattice. In stoichiometric SrTiO₃, the manganese dopant is preferably accommodated through isovalent substitution as Mn²⁺ on the strontium site and as Mn⁴⁺ on the titanium site. Previous studies have suggested that either type of substitution is possible for compositions with tailored Sr/Ti stoichiometry. Using electron paramagnetic resonance (EPR) spectroscopy, the site occupancy of dilute concentrations of manganese is investigated in SrTiO₃ as a function of the Sr/Ti ratio. The tuned Sr/Ti ratio can be used to manipulate the nature of the manganese substitution, and it is shown that Sr‐rich compositions (Sr/Ti > 1.001) processed in air result in B‐site isovalent doping. For B‐site substituted manganese ions, a new EPR signal for aliovalent Mn²⁺ is observed in compositions annealed under reducing atmosphere. The concentration of oxygen vacancies observed with EPR is also shown to depend on the Sr/Ti stoichiometry. With improved control over the site of substitution and valence state, doping with a transition metal redox‐active ion may facilitate the ability to engineer new electronic functionality into the perovskite lattice.     

The Influence of A‐Site Rare Earth Ion Size in Controlling the Curie Temperature of Ba1 − xRExTi1 − x/4O3

The defect structures for Gd‐ and La‐doping on the A‐site of BaTiO₃ with the creation of Ti vacancies are reported. The rare‐earth cations cluster together around the vacancy. The local relaxations caused by the defect cluster lead to distortion and tilting of nearby TiO₆ octahedra and this is responsible for the lowering of the Curie temperature with increasing dopant concentration. Larger distortions to the octahedra are observed for La‐doping and this is the proposed origin of the greater decrease in T_c as observed by experimental results associated with La‐ compared to Gd‐doped samples.     

Structural and Electrical Properties of Atomic Layer Deposited Al‐Doped ZnO Films

Structural and electrical properties of Al‐doped ZnO (AZO) films deposited by atomic layer deposition (ALD) are investigated to study the extrinsic doping mechanism of a transparent conducting oxide. ALD‐AZO films exhibit a unique layer‐by‐layer structure consisting of a ZnO matrix and Al₂O₃ dopant layers, as determined by transmission electron microscopy analysis. In these layered AZO films, a single Al₂O₃ dopant layer deposited during one ALD cycle could provide ≈4.5 × 10¹³ cm^?2 free electrons to the ZnO. The effective field model for doping is suggested to explain the decrease in the carrier concentration of ALD‐AZO films when the interval between the Al₂O₃ layers is reduced to less than ≈2.6 nm (>3.4 at% Al). By correlating the electrical and structural properties, an extrinsic doping mechanism of ALD‐AZO films is proposed in which the incorporated Al atoms take oxygen from the ZnO matrix and form doubly charged donors, such as oxygen vacancies or zinc interstitials.     

Enhanced Ionic Conductivity in Ce0.8Sm0.2O1.9: Unique Effect of Calcium Co‐doping

In order to identify new oxide ion‐conducting materials in the ceria family of oxides, the unique effect of co‐doping is explored and a novel series of Ce_0.8Sm_0.2–xCa_xO_2–δ compositions is identified that have enhanced properties compared to the single‐doped Ce_0.8Sm_0.2O_1.9 and Ce_0.8Ca_0.2O_1.9 compositions. Moreover, the superior characteristics of the co‐doped Ce_0.8Sm_0.2–xCa_xO_2–δ powders prepared by the mixed‐fuel process aid in obtaining 98 % dense ceramics upon sintering at 1200 °C for 6 h. Though a linear increase in conductivity is observed by replacing Sm with Ca, the composition with the maximum amount of Ca and the minimum amount of Sm exhibits a significant improvement in properties compared to the rest in the series. The composition Ce_0.80Sm_0.05Ca_0.15O_2–δ exhibits a conductivity as high as 1.22 × 10^–1 S cm^–1 at 700 °C with minimum activation energy (0.56 eV) and a superior chemical stability to reduction compared to any of the hitherto known (CaSm) compositions. The absence of Ce^III, confirmed both from X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy, strongly suggests that the observed increase in conductivity is solely due to the oxide ion conductivity and not due to the partial electronic contribution arising from the presence of Ce^III and Ce^IV. To conclude, the experimental results on the Ce_0.8Sm_0.2–xCa_xO_2–δ series underscore the unique effect of calcium co‐doping in identifying a cost‐effective new composition, with a remarkably high conductivity and enhanced chemical stability to reduction, for technological applications.     

Mixed‐Conducting Perovskites as Cathode Materials for Protonic Ceramic Fuel Cells: Understanding the Trends in Proton Uptake

The proton uptake of 18 compositions in the perovskite family (Ba,Sr,La)(Fe,Co,Zn,Y)O_3‐δ, perovskites, which are potential cathode materials for protonic ceramic fuel cells (PCFCs), is investigated by thermogravimetry. Hydration enthalpies and entropies are derived, and the doping trends are explored. The uptake is found to be largely determined by the basicity of the oxide ions. Partial substitution of Zn on the B‐site strongly enhances proton uptake, while Co substitution has the opposite effect. The proton concentration in Ba_0.95La_0.05Fe_0.8Zn_0.2O_3‐δ is found to be 10% per formula unit at 250 °C, 5.5% at 400 °C, and 2.3% at 500 °C, which are the highest values reported so far for a mixed‐conducting perovskite exhibiting hole, proton, and oxygen vacancy transport. A comprehensive set of thermodynamic data for proton uptake in (Ba,Sr,La)(Fe,Co,Zn,Y)O_3‐δ is determined. Defect interactions between protons and holes partially delocalized from the B‐site transition metal to the adjacent oxide ions decrease the proton uptake. From these results, guidelines for the optimization of PCFC cathode materials are derived.     

Chemical Control of Correlated Metals as Transparent Conductors

Correlated metallic transition metal oxides offer a route to thin film transparent conductors that is distinct from the degenerate doping of broadband wide gap semiconductors. In a correlated metal transparent conductor, interelectron repulsion shifts the plasma frequency out of the visible region to enhance optical transmission, while the high carrier density of a metal retains sufficient conductivity. By exploiting control of the filling, position, and width of the bands derived from the B site transition metal in ABO₃ perovskite oxide films, it is shown that pulsed laser deposition‐grown films of cubic SrMoO₃ and orthorhombic CaMoO₃ based on the second transition series cation 4d² Mo⁴⁺ have superior transparent conductor properties to those of the first transition series 3d¹ V⁴⁺‐based SrVO₃. The increased carrier concentration offered by the greater bandfilling in the molybdates gives higher conductivity while retaining sufficient correlation to keep the plasma edge below the visible region. The reduced binding energy of the n=4 frontier orbitals in the second transition series materials shifts the energies of oxide 2p to metal nd transitions into the near‐ultraviolet to enhance visible transparency. The A site size‐driven rotation of MoO₆ octahedra in CaMoO₃ optimizes the balance between plasma frequency and conductivity for transparent conductor performance.     

Magnetism and Phase Formation in the Candidate Dilute Magnetic Semiconductor System In2 − xCrxO3: Bulk Materials are Dilute Paramagnets

Well‐characterized bulk materials in the candidate dilute magnetic semiconductor system In_{2 − x}Cr_xO₃ are prepared for 0 ≤ x < 0.15, with cation site preferences in the bixbyite structure identified by diffraction methods. Small ferromagnetic moments are observed; their size (<10⁻² µ_B/dopant ion) is not consistent with bulk ferromagnetism. The resulting bulk materials display dilute paramagnetic behaviour, with all of the moment expected per Cr³⁺ cation dopant being involved in this paramagnetic response.     

An Amidine‐Type n‐Dopant for Solution‐Processed Field‐Effect Transistors and Perovskite Solar Cells

This study reports an effective amidine‐type n‐dopant of 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene (DBU) that can universally dope electron acceptors, including PC₆₁BM, N2200, and ITIC, by mixing the dopant with the acceptors in organic solvents or exposing the acceptor films in the dopant vapor. The doping mechanism is due to its strong electron‐donating property that is also confirmed via the chemical reduction of PEDOT:PSS (yielding color change). The DBU doping considerably increases the electrical conductivity and shifts the Fermi levels up of the PC₆₁BM films. When the DBU‐doped PC₆₁BM is used as an electron‐transporting layer in perovskite solar cells, the n‐doping removes the “S‐shape” of J–V characteristics, which leads to the fill factor enhancement from 0.54 to 0.76. Furthermore, the DBU doping can effectively lower the threshold voltage and enhance the electron mobility of PC₆₁BM‐based n‐channel field‐effect transistors. These results show that the DBU can be a promising n‐dopant for solution‐processed electronics.     

Characterization of the heavily (non-degenerate) boron-doped SiSiO2 interface

The influence of doping, crystallographic orientation and oxide thickness on the parameters of the boron-doped SiSiO₂ interface up to 3 × 10¹⁸ cm⁻³ for oxides thermally grown in a wet O₂ ambient is investigated. The fundamental surface recombination velocity, which is proportional to the product of the density of the effective recombination centers and of the associated capture cross section is nearly constant in the range of doping studied. The fixed-oxide charge density increases with doping concentration; this is probably due to the increasing importance of boron atoms segregating in the oxide and/or to the excess silicon interstitials increasing in density with dopant concentration.     

Bismuth Doping–Induced Stable Seebeck Effect Based on MAPbI3 Polycrystalline Thin Films

In this article, the thermoelectric properties of a Bi‐doped CH₃NH₃PbI₃ (MAPbI₃) perovskite thin film are studied. Bi‐doped MAPbI₃ thin film samples are fabricated, and it is found that Bi doping could greatly enhance the stability and thermoelectric properties of MAPbI₃. The Bi dopant located at the grain boundaries to modify the carrier channel near grain boundaries, which is observed via scanning electron microscopy and atomic force microscopy, efficiently reduces ion migration and facilitates charge transport. In addition, the Bi dopant can also passivate the defects in bulk MAPbI₃, increasing the polarization effect of MAPbI₃ which is demonstrated by the capacitance‐frequency measurement, thus greatly enhancing the mobility of Bi‐doped MAPbI₃. In addition, Bi‐doped MAPbI₃ leads to grain size reduction; the small size effect not only effectively hinders the MAPbI₃'s crystal phase transition from the tetragonal phase to the cubic phase, but it could also make the structure of MAPbI₃ more stable. Especially, the Seebeck voltage variation of Bi‐doped perovskite was less than that of the undoped one, meaning Bi doping would lead to a much more stable state in MAPbI₃ thin films. The results show that Bi‐doped MAPbI₃ is a promising approach to develop high stable thermoelectric and photovoltaic properties in organic–inorganic hybrid perovskite materials.     

Doping Rules and Doping Prototypes in A2BO4 Spinel Oxides

A₂BO₄ spinels constitute one of the largest groups of oxides, with potential applications in many areas of technology, including (transparent) conducting layers in solar cells. However, the electrical properties of most spinel oxides remain unknown and poorly controlled. Indeed, a significant bottleneck hindering widespread use of spinels as advanced electronic materials is the lack of understanding of the key defects rendering them as p‐type or n‐type conductors. By applying first‐principles defect calculations to a large number of spinel oxides the major trends controlling their dopability are uncovered. Anti‐site defects are the main source of electrical conductivity in these compounds. The trends in anti‐sites transition levels are systemized, revealing fundamental “doping rules”, so as to guide practical doping of these oxides. Four distinct doping types (DTs) emerge from a high‐throughput screening of a large number of spinel oxides: i) donor above acceptor, both are in the gap, i.e., both are electrically active and compensated (DT‐1), ii) acceptor above donor, and only acceptor is in the gap, i.e., only acceptor is electrically active (DT‐2), iii) acceptor above donor, and only donor is in the gap, i.e., only donor is electrically active (DT3), and iv) acceptor above donor in the gap, i.e., both donor and acceptor are electrically active, but not compensated (DT‐4). Donors and acceptors in DT‐1 materials compensate each other to a varying degree, and external doping is limited due to Fermi level pinning. Acceptors in DT‐2 and donors in DT‐3 are uncompensated and may ionize and create holes or electrons, and external doping can further enhance their concentration. Donor and acceptor in DT‐4 materials do not compensate each other, and when the net concentration of carriers is small due to deep levels, it can be enhanced by external doping.     

New Apatite‐Type Oxide Ion Conductor,Bi2La8[(GeO4)6]O3: Structure,Properties, and Direct Imaging of Low‐Level Interstitial Oxygen Atoms Using Aberration‐Corrected Scanning Transmission Electron Microscopy

The new solid electrolyte Bi₂La₈[(GeO₄)₆]O₃ is prepared and characterized by variable‐temperature synchrotron X‐ray and neutron diffraction, aberration‐corrected scanning transmission electron microscopy, and physical property measurements (impedance spectroscopy and second harmonic generation). The material is a triclinic variant of the apatite structure type and owes its ionic conductivity to the presence of oxide ion interstitials. A combination of annular bright‐field scanning transmission electron microscopy experiments and frozen‐phonon multislice simulations enables direct imaging of the crucial interstitial oxygen atoms present at a level of 8 out of 1030 electrons per formula unit of the material, and crystallographically disordered, in the unit cell. Scanning transmission electron microscopy also leads to a direct observation of the local departures from the centrosymmetric average structure determined by diffraction. As no second harmonic generation signal is observed, these displacements are non‐cooperative on the longer length scales probed by optical methods.     

Comparative Study of the N‐Type Doping Efficiency in Solution‐processed Fullerenes and Fullerene Derivatives

Molecular doping of organic semiconductors and devices represents an enabling technology for a range of emerging optoelectronic applications. Although p‐type doping has been demonstrated in a number of organic semiconductors, efficient n‐type doping has proven to be particularly challenging. Here, n‐type doping of solution‐processed C₆₀, C₇₀, [60]PCBM, [70]PCBM and indene‐C₆₀ bis‐adduct by 1H‐benzimidazole (N‐DMBI) is reported. The doping efficiency for each system is assessed using field‐effect measurements performed under inert atmosphere at room temperature in combination with optical absorption spectroscopy and atomic force microscopy. The highest doping efficiency is observed for C₆₀ and C₇₀ and electron mobilities up to ≈2 cm²/Vs are obtained. Unlike in substituted fullerenes‐based transistors where the electron mobility is found to be inversely proportional to N‐DMBI concentration, C₆₀ and C₇₀ devices exhibit a characteristic mobility increase by approximately an order of magnitude with increasing dopant concentration up to 1 mol%. Doping also appears to significantly affect the bias stability of the transistors. The work contributes towards understanding of the molecular doping mechanism in fullerene‐based semiconductors and outlines a simple and highly efficient approach that enables significant improvement in device performance through facile chemical doping.     

n‐Doping of a Low‐Electron‐Affinity Polymer Used as an Electron‐Transport Layer in Organic Light‐Emitting Diodes

n‐Doping electron‐transport layers (ETLs) increases their conductivity and improves electron injection into organic light‐emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n‐doping has been notoriously more difficult for these materials. In this work, n‐doping of the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) is demonstrated via solution processing, using the air‐stable n‐dopant (pentamethylcyclopentadienyl)(1,3,5‐trimethylbenzene)ruthenium dimer [RuCp*Mes]₂. Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes]₂, the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n‐doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. The [RuCp*Mes]₂‐doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.