Optimization of doping and interfaces in semiconductors : a two case study of BiCuOS and ZnO

Le domaine émergeant de l’électronique imprimée nécessite de nouveaux matériaux peu coûteux et non toxiques pour réaliser de nombreux systèmes tels que des circuits logiques, des capteurs, des affichages, des thermoélectriques ou même du photovoltaïque sur substrat souple. Il s’agit aussi d’optimiser le fonctionnement de couches granulaires de semiconducteurs de type n et p.BiCuOS a été identifié comme un semiconducteur de type p possédant des propriétés intéressantes. Cependant, sa forte sous stoechiométrie en cuivre induit un dopage de type p trop élevé qui nuit à ses propriétés semiconductrices. De plus, comme la plupart des composés à base de chalcogénures, BiCuOS se décompose lors d’un frittage, et ne peut être densifié thermiquement. Dans le but d’optimiser des couches minces de BiCuOS, des solutions doivent ainsi être trouvées pour i) réduire le taux de dopage ; ii) obtenir de bonnes mobilités dans des couche peu denses. De nombreuses substitutions chimiques ont été essayées telles que celle du soufre par l’iode et celle du cuivre par l’argent. Ces substitutions ont permis de réduire fortement le taux de porteurs de charge. D’autre part, nous avons étudié l’effet du greffage de molécules à la surface des grains de semiconducteurs sur la conduction électronique. Des molécules conjuguées (acides téréphtaliques substitués) et des polymères dérivés des polythiophènes ont été adsorbés à la surface d’un semiconducteur modèle de type n, ZnO. L’amélioration du transfert électronique intergranulaire a été expliquée par le saut des électrons au travers de la LUMO de ces molécules.L’élaboration d’encres de particules semiconductrices stabilisées par de telles molécules a permis la fabrication par voie liquide de jonctions diodes p-n ZnO/BiCuOS avec de bonnes performances malgré l’absence de propriétés photovoltaïques. Plus largement, ce travail est une contribution à la mise en forme de nouveaux systèmes d’électronique hybride par voie de chimie douce, dont le développement permettrait la commercialisation de technologies plus respectueuses de l’environnement.The emerging domain of printed electronics requires new cheap and non-toxic materials for applications such as logic devices, sensors, displays, thermoelectric and photovoltaic devices. It also requires optimizing the conduction in granular semiconductors. BiCuOS has been identified as a promising p-type semiconductor for such applications. However, its high copper under-stoichiometry, induces an important p-type doping, which is detrimental for its use as a photovoltaic absorber. Moreover, like all chalcogenide based materials, it shows a poor chemical stability during sintering, thermal treatment necessary to enhance transport properties. In order to optimize its properties, solutions must be found i) to control the doping content, ii) to obtain good charge carrier mobilities in thin films. On the one hand, we have explored different kinds of substitutions such as iodine for sulfur or silver for copper, which successfully enabled to strongly reduce the charge carrier density. On the other hand, we have studied the effect of grafting conjugated molecules (terephthalic acid and polythiophene derivatives) onto the surface of a model n-type semiconductor (ZnO) to study their effect on the intergranular transport. Electronic transfer improvement occurs by transfer though a lowered energy barrier formed by the LUMO of the molecules. The formulation of optimized inks using these molecules as additives allowed the thin film deposition of p-n diodes formed with ZnO/BiCuOS. Although no photovoltaic effect has been detected yet, the p-n junctions showed high nonlinear properties and are strongly photosensitive. With this work, we have participated to the elaboration of new sulfides and hybrid interfaces systems for the improvement of semiconductor devices. The development of such hybrid electronic devices through soft chemistry method is a valuable step towards the commercialization of sustainable technologies

Oxygen Diffusion and Surface Exchange Coefficients Measurements under High Pressure: Comparative Behaviour of Oxygen Deficient vs. Over-stoichiometric Air Electrode Materials

International audienceMixed ionic electronic conductors (MIECs) oxides are used as electrode materials for solid oxide cell (SOC) application, as they combine high electronic conductivity as well as high oxygen diffusivity and oxygen surface exchange coefficients. The ionic transport properties can be directly determined thanks to the isotopic exchange depth profiling (IEDP) method. To date, the reported measurements have been performed at ambient pressure and below. However, for a higher efficiency of hydrogen production at the system level, it is envisaged to operate the cell between 10 to 60 bar. To characterize the MIEC oxides properties in such conditions, an innovative setup able to operate up to a total pressure of 50 bar and 900 °C has been developed. The main goal of this study was to compare the behaviour of two types of reference materials: the oxygen deficient La-Sr-Fe-Co perovskites, and the overstoechiometric lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). Diffusion and surface exchange coefficients obtained under 6.3 bar of oxygen are measured and their evolution discussed in light of the change in oxygen stoichiometries. This analysis allows better understanding of the dependency of the surface exchange coefficient with the oxygen partial pressure

Toward Understanding of the Li-Ion Migration Pathways in the Lithium Aluminum Sulfides Li₃AlS₃ and Li_4.3AlS_3.3Cl_0.7 via ^6,7Li Solid-State Nuclear Magnetic Resonance Spectroscopy

[Image: see text] Li-containing materials providing fast ion transport pathways are fundamental in Li solid electrolytes and the future of all-solid-state batteries. Understanding these pathways, which usually benefit from structural disorder and cation/anion substitution, is paramount for further developments in next-generation Li solid electrolytes. Here, we exploit a range of variable temperature (6)Li and (7)Li nuclear magnetic resonance approaches to determine Li-ion mobility pathways, quantify Li-ion jump rates, and subsequently identify the limiting factors for Li-ion diffusion in Li(3)AlS(3) and chlorine-doped analogue Li(4.3)AlS(3.3)Cl(0.7). Static (7)Li NMR line narrowing spectra of Li(3)AlS(3) show the existence of both mobile and immobile Li ions, with the latter limiting long-range translational ion diffusion, while in Li(4.3)AlS(3.3)Cl(0.7), a single type of fast-moving ion is present and responsible for the higher conductivity of this phase. (6)Li–(6)Li exchange spectroscopy spectra of Li(3)AlS(3) reveal that the slower moving ions hop between non-equivalent Li positions in different structural layers. The absence of the immobile ions in Li(4.3)AlS(3.3)Cl(0.7), as revealed from (7)Li line narrowing experiments, suggests an increased rate of ion exchange between the layers in this phase compared with Li(3)AlS(3). Detailed analysis of spin–lattice relaxation data allows extraction of Li-ion jump rates that are significantly increased for the doped material and identify Li mobility pathways in both materials to be three-dimensional. The identification of factors limiting long-range translational Li diffusion and understanding the effects of structural modification (such as anion substitution) on Li-ion mobility provide a framework for the further development of more highly conductive Li solid electrolytes

Li4.3AlS3.3Cl0.7: A Sulfide-Chloride Lithium Ion Conductor with a Highly Disordered Structure

Mixed anion materials and anion doping are very promising strategies to improve solid-state electrolyte properties by enabling an optimized balance between good electrochemical stability and high ionic conductivity. In this work, we present the discovery of a novel lithium aluminum sulfide-chloride phase. The structure is strongly affected by the presence of chloride anions on the sulfur site, as this stabilizes a higher symmetry phase presenting a large degree of cationic site disorder, as well as disordered octahedral lithium vacancies, in comparison with Li-Al-S ternaries. The effect of disorder on the lithium conductivity properties was assessed by a combined experimental-theoretical approach. In particular, the conductivity is increased by a factor 103 compared to the pure sulfide phases. Although it remains moderate (10−6 S·cm-1), Ab Initio Molecular Dynamics and Maximum Entropy (applied to neutron diffraction data) methods show that disorder leads to a 3D diffusion pathway, where Li atoms move thanks to a concerted mechanism. An understanding of the structure-property relationships is developed to determine the limiting factor governing lithium ion conductivity. This analysis, added to the strong step forward obtained in the determination of the dimensionality of diffusion paves the way for accessing even higher conductivity in materials comprising an hcp anion arrangement

A systematic ab initio study of vacancy formation energy, diffusivity, and ionic conductivity of Ln2NiO4+δ (Ln=La, Nd, Pr)

This study systematically investigates the vacancy formation energy, diffusivity, and ionic conductivity of the Ln2NiO4+δ (Ln=La, Nd, Pr, and δ=0.125) compound using the ab initio approach. Specifically, the impact of thermal expansion on the oxygen transport properties is considered, using a combination of quasi-harmonic approximation (QHA) and a linear regression model to study and reproduce the temperature-dependent properties of Ln2NiO4+δ. Overall, the predictions are in excellent agreement with previous theoretical studies in the literature. It is confirmed that the ionic transport properties of the Ln2NiO4+δ are not dominated by oxygen vacancy diffusion due to the high vacancy formation energy. Additionally, the interstitialc

Impact of anionic ordering on the iron site distribution and valence states in oxyfluoride Sr2FeO3+xF1–x (x = 0.08, 0.2) with a layered perovskite network

Sr2FeO3F, an oxyfluoride compound with an n = 1 Ruddlesden–Popper structure, was identified as a potential interesting mixed ionic and electronic conductor (MIEC). The phase can be synthesized under a range of different pO2 atmospheres, leading to various degrees of fluorine for oxygen substitution and Fe4+ content. A structural investigation and thorough comparison of both argon- and air-synthesized compounds were performed by combining high-resolution X-ray and electron diffraction, high-resolution scanning transmission electron microscopy, Mössbauer spectroscopy, and DFT calculations. While the argon-synthesized phase shows a well-behaved O/F ordered structure, this study revealed that oxidation leads to averaged large-scale anionic disorder on the apical site. In the more oxidized Sr2FeO3.2F0.8 oxyfluoride, containing 20% of Fe4+, two different Fe positions can be identified with a 32%/68% occupancy (P4/nmm space group). This originates due to the presence of antiphase boundaries between ordered domains within the grains. Relations between site distortion and valence states as well as stability of apical anionic sites (O vs F) are discussed. This study paves the way for further studies on both ionic and electronic transport properties of Sr2FeO3.2F0.8 and its use in MIEC-based devices, such as solid oxide fuel cells

Electronic Band Structure Engineering and Enhanced Thermoelectric Transport Properties in Pb-Doped BiCuOS Oxysulfide

International audienceIn this paper, Bi1-xPbxCuOS samples (0 <= x <= 0.05) have been synthesized with a simple and scalable ball-milling process, followed by a reactive Spark Plasma Sintering. Our results highlight that, Pb for Bi substitution increases the charge carriers concentration by more than 2 orders of magnitude from 1.4 x 10(17) cm(-3) to 2.6 x 10(19) cm(-3) for x = 0 and x = 0.05, respectively. As a result, the electrical resistivity is divided by more than 50 at room temperature and the Seebeck coefficient drops from 707 mu V K-1 to 265 mu V K-1 where our experimental results are supported with density functional theory (DFT) calculations. Electronic structure calculations show that, just below the top of the valence band, several other bands are present and may contribute to the transport properties with appropriate tuning of the heavy-light valence band and the position of the Fermi level. Pb doping increases the number of holes pockets and several band degeneracies appear around the Fermi level, leading to a drastic enhancement of the power factor up to 0.2 mW m(-1) K-2 at 700 K. This is 5 times higher than the value of the pristine compound. The intrinsically low thermal conductivity of 0.7 W m(-1) K-1 at 700 K is interpreted on the basis of vibrational properties calculations within the Density Functional Perturbation Theory (DFPT) approach. It indicates that soft acoustic modes along the Gamma-Z direction suggest weak interatomic bonding between the layers and possible strong anharmonicity. The power factor being enhanced with a minimal impact on the thermal conductivity, the figure of merit ZT reaches 0.2 at 700 K for x = 0.05. To the best of our knowledge, it is considered the best reported value among the family of oxysulfides

Computationally Guided Discovery of the Sulfide Li3AlS3 in the Li-Al-S Phase Field: Structure and Lithium Conductivity

With the goal of finding new lithium solid electrolytes by a combined computational-experimental method, the exploration of the Li-Al-O-S phase field resulted in the discovery of a new sulphide Li3AlS3. The structure of the new phase was determined through an approach combining synchrotron X-ray and neutron diffraction with 6Li and 27Al magic angle spinning nuclear magnetic resonance spectroscopy, and revealed a highly ordered cationic polyhedral network within a sulphide anion hcp-type sublattice. The originality of the structure relies on the presence of Al2S6 repeating dimer units consisting of two edge-shared Al tetrahedra. We find that, in this structure type consisting of alternating tetrahedral layers with Li-only polyhedra layers, the formation of these dimers is constrained by the Al/S ratio of 1/3. Moreover, by comparing this structure to similar phases such as Li5AlS4 and Li4.4Al0.2Ge0.3S4 ((Al+Ge)/S = 1/4), we discovered that the AlS4 dimers not only influence atomic displacements and Li polyhedral distortions, but also determine the overall Li polyhedral arrangement within the hcp lattice, leading to the presence of highly ordered vacancies in both the tetrahedral and Li-only layer. AC-impedance measurements revealed a low lithium mobility, which is strongly impacted by the presence of ordered vacancies. Finally, a composition-structure-property relationship understanding was developed to explain the extent of lithium mobility in this structure type