Wafer-scale pulsed laser deposition of ITO for solar cells: reduced damage vs. interfacial resistance

Transparent conducting oxides (TCOs) used in solar cells must be optimized to achieve minimum parasitic absorption losses while providing sufficient lateral conductivity. Low contact resistance with the adjacent device layers and low damage to the substrate during deposition of the TCO are also important requirements to ensure high solar cell efficiencies. Pulsed laser deposition (PLD) has been proposed as an alternative low-damage TCO deposition technique on top of sensitive layers and interfaces in organic and perovskite solar cells but is yet to be studied for the more mature silicon technology. Focusing on the PLD deposition pressure as the key parameter to reduce damage, we developed tin-doped indium oxide (ITO) with a sheet resistance of 60 Ω □−1 at different pressures and implemented it in silicon heterojunction (SHJ) solar cells. Buffer-free semi-transparent perovskite cells with the same PLD ITO electrodes were also fabricated for comparison. While in the perovskite cells increased ITO deposition pressure leads to an improved open circuit voltage and fill factor indicative of damage reduction, SHJ cells with PLD ITO at all conditions maintained a high passivation quality, but increased pressures lead to high series resistance. Transmission electron microscopy and time-of-flight secondary ion mass spectrometry confirmed the formation of a parasitic SiOx at the ITO/a-Si:H interface of the SHJ cell causing a transport barrier. The optimized ITO films with the highest carrier density were able to obtain >21% SHJ efficiency with 75 nm-thick PLD ITO. Moreover, reducing the ITO thickness to ∼45 nm and using TiOx for optical compensation enables fabrication of SHJ devices with reduced indium consumption and efficiencies of >22%

Perovskite and Brownmillerite as catalyst support materials

Ce projet est dédié à la recherche industrielle pour le développement de systèmes catalytiques innovants tels que le contrôle des émissions de véhicules. L'Europe connait actuellement une forte dépendance au niveau de l'importation de certains éléments utilisés comme support de catalyseur (oxyde de Cérium), nous souhaitons nous concentrer sur des éléments plus facilement disponibles tels que Ca, Fe, Mn, Sr, Cu... tout en essayant de garder le mécanisme catalytique bien connu de l'oxyde de cérium. Pour ce faire, nous avons sélectionné des conducteurs en oxygène de la famille des brownmillerites comme matériaux supports. Ceux-ci présentent des lacunes en oxygènes ayant un impact bénéfique sur leur activité catalytique pour les réactions d'oxydations. Il est aussi prévu de regarder les interactions entre métaux nobles et support conducteurs en oxygène pour une application de dépollution des gaz. Les réactions modèles étudiées au début de ce projet seront l'oxydation du CO ainsi que le stockage et la réduction des NOx. Les brownmillerites peuvent être vues comme des oxydes de type pérovskite avec un défaut en oxygène. Les brownmillerites ont une structure anisotropique avec un enchainement de lacines d'oxygènes-1D apportant une augmentation de l'activité catalytique. Ces browmillerites sont bien connues pour présenter une mobilité de l'oxygène à basse température. La présence de défauts tels que des liaisons anti-phase peut significativement diminuer la diffusion de l'oxygène. CaFeO2.5 riche en défauts, connu pour être une phase stœchiométrique peut être oxyder dans de "douces" conditions en CaFeO3 alors que l'oxydation d'un CaFeO2.5 ordinaire requiert des conditions extrêmes (1100°C et plusieurs GPa de pression en oxygène). Ainsi, introduire un nombre élevé de défauts dans la structure cristalline semble être une manière prometteuse de transformer des phases stoechiométriques en réservoir à oxygène. Les matériaux obtenus alors ayant des capacité de stockage et d'amélioration des réactions d'oxydations à température très modérée. Le mécanisme mis en jeu est comparable à celui de la capacité de stockage en oxygène des cérines dopées et offre donc un vrai potentiel catalytique. Au cours de ce projet CaFeO2.5 sera premièrement étudié mais nous étendrons l'étude avec des dopages (Cu, Mn, W) et une autre composition sera aussi étudiée : SrFeO2.5; Concernant le support nous souhaitons obtenir : -une grande dispersion du métal noble dans la matrice -une grande mobilité de l'oxygène à température modérée -une grande surface spécifique Obtenir ces trois caractéristiques simultanément est actuellement un challenge pour les brownmillérites. Pour ce faire, nous souhaitons étudier différentes voies de synthèse. Une grande partie du projet sera dédiée aux caractérisations des matériaux avec des analyses structurales et spectroscopiques incluant de l'échange isotopique pour l'étude de la mobilité en oxygène. Ces études permettront une meilleure compréhension des propriétés des matériaux en relation avec leur activité catalytique. Les matériaux les plus prometteurs à l'issue de cette étude seront synthétisés à l'échelle du pilote par un processus d'électro-fusion.The present project is dedicated to industrial research for the development of innovative catalytic systems for air purification, such as those used for the control of road vehicle emission (three way converter, TWC). In the context of Europe’s dependency on imports of some critical elements currently used as catalyst support (e.g. cerium oxide), we focus on more available elements such as Ca, Fe, Mn, Sr, Cu… by keeping the well-understood mechanisms governing the catalytic activity of cerium oxide in mind. As such, we choose oxygen ion conductors of the Brownmillerite family as support material, because it has been reported that lattice oxygen atoms have a beneficial impact on the catalytic activity of oxidation reactions. Next to the pure support material, also the interaction of a noble metal with the oxygen ion conductive support for the efficient removal of gas phase pollutants will be studied. In terms of catalytic reactions, the oxidation of CO, and the storage and reduction of NOx will be the primary metrics. In this project, oxygen ion conductors of the Brownmillerite family are chosen as support material. Brownmillerites can be regarded as oxygen-deficient perovskite type oxides. The Brownmillerite type structure is anisotropic with 1D-oxygen vacancy channels providing a catalytically enhanced surface/interface structure. Brownmillerites are known to reveal oxygen ion mobility down to ambient temperature. The presence of extended defects as anti-phase boundaries can significantly decrease the activation energy for oxygen diffusion. Defect-rich CaFeO2.5, which is traditionally known to be a stoichiometric line-phase, can be oxidized under mild conditions to CaFeO3, while the oxidation of ordinary CaFeO2.5 usually requires extreme reaction conditions, i.e. 1100°C and several GPa oxygen partial pressure. Thus, introducing a high concentration of defects seems to be a promising concept to transform even traditionally known stoichiometric line-phases to become a kind of oxygen sponge and behave as oxygen storage/buffer compound at very moderate temperatures. This mechanism is thus comparable to the oxygen storage capacity of doped cerium oxide, and offers a true potential for application in catalysis. Consequently, the Brownmillerite CaFeO2.5 will be a first candidate to study due to its known oxygen ion conductivity properties, however, also doping with other elements (e.g. Cu, Mn, W) and other compositions (e.g. SrFeO2.5) will be investigated. For the support material, we will attempt to achieve (i)- a high degree of dispersion of the noble metal into the matrix, (ii)- a high oxygen mobility at moderate temperatures (e.g. by introducing defects) and (iii)- a high surface area, which we anticipate to be key aspects for achieving high catalytic activity. To date, it is still a challenge to achieve these goals simultaneously for Brownmillerites. As a result, in this project, several synthesis routes are foreseen. More straightforward synthesis routes, such as citrate- EDTA gel methods and spray pyrolysis, will be investigated alongside with more advanced synthetic approaches such and hard-templating routes. This multitude of possibilities allows for an easy adaption of a synthesis route to the material under study. A major part of the project will be dedicated to the detailed characterization of the materials involving large scale facilities for structure analysis and spectroscopy (in-situ studies), including oxygen isotope exchange reactions to trace the oxygen ion mobility. These studies will allow for a detailed understanding of the materials properties in relation to its catalytic activity. The most promising materials will be synthesized on a pilot-scale using electrofusion. This technique is well-established by the industrial partner and is extremely suitable for the synthesis of reduced powders, such as CaFeO2.5

Etude de conducteurs d'oxygène type pérovskites et brownmillérites comme support catalytiques

The present project is dedicated to industrial research for the development of innovative catalytic systems for air purification, such as those used for the control of road vehicle emission (three way converter, TWC). In the context of Europe’s dependency on imports of some critical elements currently used as catalyst support (e.g. cerium oxide), we focus on more available elements such as Ca, Fe, Mn, Sr, Cu… by keeping the well-understood mechanisms governing the catalytic activity of cerium oxide in mind. As such, we choose oxygen ion conductors of the Brownmillerite family as support material, because it has been reported that lattice oxygen atoms have a beneficial impact on the catalytic activity of oxidation reactions. Next to the pure support material, also the interaction of a noble metal with the oxygen ion conductive support for the efficient removal of gas phase pollutants will be studied. In terms of catalytic reactions, the oxidation of CO, and the storage and reduction of NOx will be the primary metrics. In this project, oxygen ion conductors of the Brownmillerite family are chosen as support material. Brownmillerites can be regarded as oxygen-deficient perovskite type oxides. The Brownmillerite type structure is anisotropic with 1D-oxygen vacancy channels providing a catalytically enhanced surface/interface structure. Brownmillerites are known to reveal oxygen ion mobility down to ambient temperature. The presence of extended defects as anti-phase boundaries can significantly decrease the activation energy for oxygen diffusion. Defect-rich CaFeO2.5, which is traditionally known to be a stoichiometric line-phase, can be oxidized under mild conditions to CaFeO3, while the oxidation of ordinary CaFeO2.5 usually requires extreme reaction conditions, i.e. 1100°C and several GPa oxygen partial pressure. Thus, introducing a high concentration of defects seems to be a promising concept to transform even traditionally known stoichiometric line-phases to become a kind of oxygen sponge and behave as oxygen storage/buffer compound at very moderate temperatures. This mechanism is thus comparable to the oxygen storage capacity of doped cerium oxide, and offers a true potential for application in catalysis. Consequently, the Brownmillerite CaFeO2.5 will be a first candidate to study due to its known oxygen ion conductivity properties, however, also doping with other elements (e.g. Cu, Mn, W) and other compositions (e.g. SrFeO2.5) will be investigated. For the support material, we will attempt to achieve (i)- a high degree of dispersion of the noble metal into the matrix, (ii)- a high oxygen mobility at moderate temperatures (e.g. by introducing defects) and (iii)- a high surface area, which we anticipate to be key aspects for achieving high catalytic activity. To date, it is still a challenge to achieve these goals simultaneously for Brownmillerites. As a result, in this project, several synthesis routes are foreseen. More straightforward synthesis routes, such as citrate- EDTA gel methods and spray pyrolysis, will be investigated alongside with more advanced synthetic approaches such and hard-templating routes. This multitude of possibilities allows for an easy adaption of a synthesis route to the material under study. A major part of the project will be dedicated to the detailed characterization of the materials involving large scale facilities for structure analysis and spectroscopy (in-situ studies), including oxygen isotope exchange reactions to trace the oxygen ion mobility. These studies will allow for a detailed understanding of the materials properties in relation to its catalytic activity. The most promising materials will be synthesized on a pilot-scale using electrofusion. This technique is well-established by the industrial partner and is extremely suitable for the synthesis of reduced powders, such as CaFeO2.5.Ce projet est dédié à la recherche industrielle pour le développement de systèmes catalytiques innovants tels que le contrôle des émissions de véhicules. L'Europe connait actuellement une forte dépendance au niveau de l'importation de certains éléments utilisés comme support de catalyseur (oxyde de Cérium), nous souhaitons nous concentrer sur des éléments plus facilement disponibles tels que Ca, Fe, Mn, Sr, Cu... tout en essayant de garder le mécanisme catalytique bien connu de l'oxyde de cérium. Pour ce faire, nous avons sélectionné des conducteurs en oxygène de la famille des brownmillerites comme matériaux supports. Ceux-ci présentent des lacunes en oxygènes ayant un impact bénéfique sur leur activité catalytique pour les réactions d'oxydations. Il est aussi prévu de regarder les interactions entre métaux nobles et support conducteurs en oxygène pour une application de dépollution des gaz. Les réactions modèles étudiées au début de ce projet seront l'oxydation du CO ainsi que le stockage et la réduction des NOx. Les brownmillerites peuvent être vues comme des oxydes de type pérovskite avec un défaut en oxygène. Les brownmillerites ont une structure anisotropique avec un enchainement de lacines d'oxygènes-1D apportant une augmentation de l'activité catalytique. Ces browmillerites sont bien connues pour présenter une mobilité de l'oxygène à basse température. La présence de défauts tels que des liaisons anti-phase peut significativement diminuer la diffusion de l'oxygène. CaFeO2.5 riche en défauts, connu pour être une phase stœchiométrique peut être oxyder dans de "douces" conditions en CaFeO3 alors que l'oxydation d'un CaFeO2.5 ordinaire requiert des conditions extrêmes (1100°C et plusieurs GPa de pression en oxygène). Ainsi, introduire un nombre élevé de défauts dans la structure cristalline semble être une manière prometteuse de transformer des phases stoechiométriques en réservoir à oxygène. Les matériaux obtenus alors ayant des capacité de stockage et d'amélioration des réactions d'oxydations à température très modérée. Le mécanisme mis en jeu est comparable à celui de la capacité de stockage en oxygène des cérines dopées et offre donc un vrai potentiel catalytique. Au cours de ce projet CaFeO2.5 sera premièrement étudié mais nous étendrons l'étude avec des dopages (Cu, Mn, W) et une autre composition sera aussi étudiée : SrFeO2.5; Concernant le support nous souhaitons obtenir : -une grande dispersion du métal noble dans la matrice -une grande mobilité de l'oxygène à température modérée -une grande surface spécifique Obtenir ces trois caractéristiques simultanément est actuellement un challenge pour les brownmillérites. Pour ce faire, nous souhaitons étudier différentes voies de synthèse. Une grande partie du projet sera dédiée aux caractérisations des matériaux avec des analyses structurales et spectroscopiques incluant de l'échange isotopique pour l'étude de la mobilité en oxygène. Ces études permettront une meilleure compréhension des propriétés des matériaux en relation avec leur activité catalytique. Les matériaux les plus prometteurs à l'issue de cette étude seront synthétisés à l'échelle du pilote par un processus d'électro-fusion

ITO Top-Electrodes via Industrial-Scale PLD for Efficient Buffer-Layer-Free Semitransparent Perovskite Solar Cells

The deposition of transparent conductive oxides (TCO) usually employs harsh conditions that are frequently harmful to soft/organic underlayers. Herein, successful use of an industrial pulsed laser deposition (PLD) tool to directly deposit indium tin oxide (ITO) films on semitransparent vacuum-deposited perovskite solar cells without damage to the device stack is demonstrated. The morphological, electronic, and optical properties of the PLD deposited ITO films are optimized. A direct relation between the PLD chamber pressure and the solar cell performance is obtained. The semitransparent perovskite solar cells prepared exclusively by vacuum-assisted techniques had fill factors of 78% and exceeded 18% in power conversion efficiencies. This demonstrates that the direct deposition of TCO-based top electrodes without protective buffer layers is possible and leads to efficient devices

Scalable Pulsed Laser Deposition of Transparent Rear Electrode for Perovskite Solar Cells

Sputtered transparent conducting oxides (TCOs) are widely accepted transparent electrodes for several types of high-efficiency solar cells. However, the different sputtering yield of atoms makes stoichiometric transfer of target material challenging for multi-compounds. Additionally, the high kinetic energies of the arriving species may damage sensitive functional layers beneath. Conversely, pulsed laser deposition (PLD) is operated at higher deposition pressures promoting thermalization of particles. This leads to stoichiometric transfer and additionally reduces the kinetic energy of ablated species. Despite these advantages, PLD is rarely used within the photovoltaic community due to concerns about low deposition rates and the scalability of the technique. In this study, wafer-scale (4-inch) PLD of high-mobility Zr-doped In2O3 (IZrO) TCO for solar cells is demonstrated. IZrO films are grown at room temperature with deposition rate on par with RF-sputtering (>4 nm min−1). As-deposited IZrO films are mostly amorphous and exhibit excellent optoelectronic properties after solid phase crystallization at <200 °C. 100-nm thick films feature a sheet resistance of 21 Ω◻−1 with electron mobilities ≈70 cm2 V−1s−1. PLD-grown IZrO is applied as rear electrode in efficient semi-transparent halide perovskite solar cells leading to the improved stabilized maximum power point efficiency (15.1%) as compared to the cells with sputtered ITO electrodes (11.9%)

Three-dimensional in situ imaging of single-grain growth in polycrystalline In2O3:Zr films

Strain and interactions at grain boundaries during solid-phase crystallization are known to play a significant role in the functional properties of polycrystalline materials. However, elucidating three-dimensional nanoscale grain morphology, kinetics, and strain under realistic conditions is challenging. Here, we image a single-grain growth during the amorphous-to-polycrystalline transition in technologically relevant transparent conductive oxide film of In2O3:Zr with in situ Bragg coherent X-ray diffraction imaging and transmission electron microscopy. We find that the Johnson-Mehl-Avrami-Kolmogorov theory, which describes the average kinetics of polycrystalline films growth, can be applied to the single grains as well. The quantitative analysis stems directly from imaging results. We elucidate the interface-controlled nature of the single-grain growth in thin films and reveal the surface strains which may be a driving force for anisotropic crystallization rates. Our results bring in situ imaging with coherent X-rays towards understanding and controlling the crystallization processes of transparent conductive oxides and other polycrystalline materials at the nanoscale

Correlated Metals Transparent Conductors with High UV to Visible Transparency on Amorphous Substrates

Correlated metals with high carrier density and strongly correlated electron effects provide an alternative route to achieve transparent conducting materials, different from the conventional degenerately doped wide-bandgap transparent conducting oxides (TCO). The extremely low electrical resistivity and high optical transparency in the ultraviolet-visible spectral range shown in 4d correlated metals present an advantage over conventional TCOs. However, most of the 4d correlated metals are grown epitaxially on single crystal substrates. Here, it has been shown that Ca2Nb3O10 nanosheets with different buffer layers promote the growth of high-quality 4d2 SrMoO3 films on fused silica substrates, overcoming the use of expensive and size-limited single-crystal substrates. The room temperature electrical resistivity of SrMoO3 is as low as 61 µΩ cm, the lowest reported value on amorphous transparent substrates to date, without compromising its high optical transmittance. 4d1 correlated metal SrNbO3 on Ca2Nb3O10 nanosheets also exhibits similarly high optical transmittance but a higher room temperature resistivity of 174 µΩ cm. These findings facilitate the use of highly conducting and transparent 4d correlated metals not only as TCOs on technologically relevant substrates for the applications in the ultraviolet-visible spectral range but also as electrodes for other oxide-based thin film technologies