Oxygen impurities link bistability and magnetoresistance in organic spin valves

Vertical cross-bar devices based on manganite and cobalt injecting electrodes and metal-quinoline molecular transport layer are known to manifest both magnetoresistance and electrical bistability. The two effects are strongly interwoven, inspiring new device applications such as electrical control of the magnetoresistance and magnetic modulation of bistability. To investigate the full device functionality, we first identify the mechanism responsible for electrical switching by associating the electrical conductivity and the impedance behavior with chemical states of buried layers obtained by in operando photoelectron spectroscopy. These measurements revealed that a significant fraction of oxygen ions migrates under voltage polarity, resulting in a modification of the electronic properties of the organic material and of the oxidation of interfacial layer with ferromagnetic contacts. Variable oxygen doping of the organic molecule represents the key element for correlating bistability and magnetoresistance and our measurements provide the first experimental evidence in favor of the impurity band model describing the spin transport in organic semiconductors in similar devices

Influence des inhomogénéités spatiales des cellules photovoltaïques à hétérojonction de silicium sur leur performances

Dans un contexte où les substrats de silicium monocristallin disponibles dans le commerce permettent des durées de vie de porteurs de charges de plusieurs millisecondes, l’état de passivation des surfaces du silicium dans les cellules solaires à hétérojonction de silicium vient au premier plan pour l’amélioration des performances de ces cellules produites industriellement. Premièrement les défauts de passivation des faces avant et arrière du dispositif ont été étudiés en combinant simulations (Silvaco) et expériences. Ceci a permis la compréhension et la quantification des phénomènes physiques impliqués dans les pertes associées aux défauts de passivation. L’influence de la variation des propriétés des défauts a ensuite été révélée dans les conditions standard de test ainsi que sous différentes illuminations et températures de fonctionnement afin de reproduire les conditions réelles de fonctionnement des cellules. De plus, il est connu que la qualité de passivation des bords des cellules est moindre comparée à celles de ses surfaces avant et arrière. Cela est d’autant plus important avec l’essor des modules photovoltaïques à cellules découpées. Ces cellules étant plus petites le courant les parcourant, et donc les pertes résistives associées, sont réduites. Dans ces cellules, des bords sont générés par les découpes et le ratio périmètre/surface étant élevé, les effets de bords sont exacerbés. Dans un second temps, un code inédit de simulation a donc été développé afin d’étudier ces effets sur les performances des cellules. Cela a permis de comprendre et quantifier les phénomènes et impliqués dans les pertes liées aux bords des cellules entières et coupées. En particulier les performances d’une cellule ont été liées à la vitesse de recombinaison sur ses bords quelles que soient la taille et la géométrie de la cellule. Pour chacune de ces études, des recommandations pratiques pour la réduction de ces pertes sur des cellules produites de façon industrielle ont été formulées.In a context where commercially available monocrystalline silicon substrates allow charge carrier lifetimes of several milliseconds, the passivation state of the silicon surfaces in silicon heterojunction solar cells comes to the forefront for improving the performance of these industrially produced cells. First, the passivation defects of the front and back surfaces of the device have been studied by combining simulations (Silvaco) and experiments. This allowed the understanding and quantification of the physical phenomena involved in the losses associated with passivation defects. The influence of the variation of the defect properties was then revealed under standard test conditions as well as under different illuminations and operating temperatures in order to reproduce the real operating conditions of the cells. Furthermore, it is known that the passivation quality of the cell edges is lower compared to the front and back surfaces. This is all the more important with the rise of photovoltaic modules with cut cells. These cells being smaller, the current flowing through them, and therefore the associated resistive losses, are reduced. In these cells, edges are generated by the cuts and the ratio perimeter/surface being high, the edge effects are exacerbated. In a second step, a novel simulation code was developed to study these effects on the performance of the cells. This allowed to understand and quantify the phenomena involved in the edge losses of whole and cut cells. In particular, the In particular, the performance of a cell was related to the speed of recombination at its edges, regardless of the size and geometry of the cell. For each of these studies, practical recommendations for the reduction of these losses on industrially produced cells have been formulated

Influence of cell edges on the performance of silicon heterojunction solar cells

International audienceFull size silicon heterojunction solar cells reach conversion efficiencies above 25%. However, photoluminescence pictures of such cells (full or cut) reveal a significant recombination activity at the cell edges. Therefore, mitigating recombination at the edges can in principle represent an interesting path to unlock higher cell efficiencies. This challenge is all the more important for cells with a high perimeter/area ratio, as achieved through the cutting of full size cells. For such technologies, the edges resulting from cutting are cleaved while the remaining edges typically feature a gap where TCO is missing to avoid front to back short-circuit. In this paper, we specify the physical mechanisms involved in the edge-induced performance losses for SHJ cells. In light of these results, we provide guidelines for the mitigation of such losses at the full-size and cut cells scale for M6 to M12 sizes such as the reduction of the TCO-free region and the c-Si bulk resistivity. Having a closer look at cut cells, we calculate the cell performance as a function of its size (from half-to sixth-cell), the size of its mother cell (from M6 to M12) and the passivation quality of the cut-edges. Our results emphasize on the interest to develop suitable repassivation schemes for cut cells to improve or even surpass the efficiency of the mother cell

Understanding of the influence of localized surface defectivity properties on the performances of silicon heterojunction cells

International audienceThe industrial fabrication process of silicon heterojunction (SHJ) solar cells can induce locally depassivated regions (so-called defectivity) because of transportation steps (contact with belts, trays, etc.) or simply the environment (presence of particles at the wafer surfaces before thin film deposition). This surface passivation spatial heterogeneity is gaining interest as it may hinder the SHJ efficiency improvements allowed by incremental process step optimizations. In this paper, an experimentally supported simulation study is conducted to understand how the local a-Si:H/c-Si interface depassivation loss impacts the overall cell performance. The defectivity-induced cell performance drop due to depassivated regions was attributed to a bias-dependent minority carrier current flow towards the depassivated region, which is shown to affect all current-voltage (I(V)) parameters, and in particular the fill factor. Simulation was used further in order to understand how the defectivity properties (spatial distribution, localization and size) impact the induced performance losses. In the light of all results, we propose ways to mitigate the defectivity influence on the cell performances

Magnetic depth profiling of the Co/C60 interface through soft X-ray resonant magnetic reflectivity

We have probed the structural and magnetic properties of a ferromagnetic/organic interface constituted by a polycrystalline Co layer deposited on a fullerene thin film through resonant soft X-ray reflectivity measurements. The fitting analysis of the reflectivity indicates the formation of a sharp interface with limited intermixing and a null remanent magnetization in a 3c1 nm thick region of the Co film at the interface with C60. This information contributes to elucidate the role of organic\u2013inorganic interfaces in the charge and spin transport inside organic spintronic devices

SiOxNy:B layers for ex-situ doping of hole-selective poly silicon contacts: A passivation study

International audiencePassivating the contacts of crystalline silicon (c-Si) solar cells with a polycrystalline silicon layer (poly-Si) on a thin oxide (SiOx) film allows to decrease the recombination current at the metal/c-Si interface. In this study, an ex-situ doping method of poly-Si is proposed, involving a SiOxNy:B layer as a dopant source. In this study, we compare the properties (crystallinity of the deposited layer, doping profile and surface passivation properties) of the resulting ex-situ doped poly-Si(B) layer with our in-situ doped reference

Evolution of the surface passivation mechanism during the fabrication of ex-situ doped poly-Si(B)/SiOx passivating contacts for high-efficiency c-Si solar cells

International audiencePassivating the contacts of crystalline silicon (c-Si) solar cells (SC) with a poly-crystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) is currently sparking interest for reducing carrier recombination at the interface between the metal electrode and the c-Si substrate. However, due to the interrelation between different mechanisms at play, a comprehensive understanding of the surface passivation provided by the poly-Si/SiOx contact in the final SC has not been achieved yet. In the present work, we report on an original ex-situ doping process of the poly-Si layer through the deposition of a B-rich dielectric layer followed by an annealing step to diffuse B dopants in the layer. We propose an in-depth investigation of the passivation scheme of the resulting B-doped poly-Si/SiOx contact by first comparing the surface passivation provided by ex-situ doped and intrinsic poly-Si/SiOx contacts at different steps of the fabrication process. The excellent surface passivation properties obtained with the ex-situ doped poly-Si(B) contact (iVoc = 733 mV and J0 = 6.1 fA cm−2) attests to the good quality of this contact. We then propose further STEM, ECV and ToF-SIMS characterizations to assess: i) the evolution of the microstructure and B-doping profile through ex-situ doping and ii) the diffusion profile of hydrogen in the poly-Si contact. Our results show a gradual filling of the poly-Si layer with active B dopants with increasing annealing temperature (Ta), which strengthens the field-effect passivation and enables an iVoc increase after annealing up to 800 °C. We also observe a diffusion of O from the SiON:B doping layer to the interfacial SiOx layer during annealing, that likely enhances the passivation stability of our ex-situ doped poly-Si contact with increasing Ta. Finally, we conclude that the mechanism dominating the surface passivation changes during the fabrication process of the poly-Si/SiOx contact from field-effect passivation after annealing (performed for B-diffusion in the contact) to chemical passivation after following hydrogenation of the samples (performed by depositing a H-rich silicon nitride layer

Overview of key results achieved in H2020 HighLite project helping to raise the EU PV industries' competitiveness

The EU crystalline silicon (c-Si) PV manufacturing industry has faced strong foreign competition in the last decade. To strive in this competitive environment and differentiate itself from the competition, the EU c-Si PV manufacturing industry needs to (1) focus on highly performing c-Si PV technologies, (2) include sustainability by design, and (3) develop differentiated PV module designs for a broad range of PV applications to tap into rapidly growing existing and new markets. This is precisely the aim of the 3.5 years long H2020 funded HighLite project, which started in October 2019 under the work program LC-SC3-RES-15-2019: Increase the competitiveness of the EU PV manufacturing industry. To achieve this goal, the HighLite project focuses on bringing two advanced PV module designs and the related manufacturing solutions to higher technology readiness levels (TRL). The first module design aims to combine the benefits of n-type silicon heterojunction (SHJ) cells (high efficiency and bifaciality potential, improved sustainability, rapidly growing supply chain in the EU) with the ones of shingle assembly (higher packing density, improved modularity, and excellent aesthetics). The second module design is based on the assembly of low-cost industrial interdigitated back-contact (IBC) cells cut in half or smaller, which is interesting to improve module efficiencies and increase modularity (key for application in buildings, vehicles, etc.). This contribution provides an overview of the key results achieved so far by the HighLite project partners and discusses their relevance to help raise the EU PV industries' competitiveness. We report on promising high-efficiency industrial cell results (24.1% SHJ cell with a shingle layout and 23.9% IBC cell with passivated contacts), novel approaches for high-throughput laser cutting and edge re-passivation, module designs for BAPV, BIPV, and VIPV applications passing extended testing, and first 1-year outdoor monitoring results compared with benchmark products.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Photovoltaic Materials and Device