Instability of Dielectric Surface Passivation Quality at Elevated Temperature and Illumination

AbstractHydrogenated silicon nitride and aluminum oxide passivation layers were deposited on boron doped floatzone silicon wafers that underwent a high temperature firing step. The passivation quality was monitored during thermal treatment at 75°C, 150°C and 250°C in darkness or under illumination. It was found that the passivation quality of the specific layers under investigation is far from stable in the course of time showing both deterioration and improvement features on a time scale of minutes to weeks. Furthermore, it was found that these changes occur in both darkness and under illumination, whereupon (stronger) illumination accelerates the changes. Via corona charging and capacitance voltage experiments it could be shown that the observed changes in the short term are mainly caused by changes in the chemical passivation quality

Finite Element Simulation of the Local Al/Si Contact Formation

The formation of local Al/Si contacts, as found in PERC type cells, was evaluated by finite element simulation based on a model of Si diffusion in the molten Al layer as well as dissolution and recrystallization of Si at the liquid/solid interface. In accordance with experimental observations the simulations reproduce the characteristic lateral spread of Si in the Al layer, the different cavity shapes (ranging from round to ω-shaped cavities) as well as the development of the Al-doped recrystallized region which forms the high-low junction to the lower doped substrate responsible for the back surface field passivation.publishe

Application of the Concept of Lifetime‐Equivalent Defect Density in Defect Systems Comprising a Multitude of Defect Species

The specific injection dependence of either excess carrier lifetime or lifetime‐equivalent defect density generally allows for the identification of a single defect species and the analysis of its properties. However, the presence of a multitude of defect species noticeably complicates this endeavor. A temporally different activation/deactivation dynamic of different defect species, as encountered in the context of light‐induced degradation phenomena in crystalline silicon solar cells, in combination with the distinct injection dependence of specific defect species can help to distinguish the involved defect species. Within this simulation‐based contribution, it is shown how injection‐dependent information can be used to reveal the presence of a second defect species in the context of light‐induced degradation and to distinguish clearly whether the formation of deep‐level defects in the bulk or a deterioration in the surface passivation quality is responsible for an observed degradation of effective excess carrier lifetime.publishe

Towards a high throughput solution for boron-oxygen related regeneration

Boron-oxygen related defects are a serious problem limiting the efficiency of solar cells based on boron-doped silicon. But even though the regeneration (a.k.a. permanent deactivation) of BO-related defects was introduced in 2006, it is still limited mainly to laboratory use and almost always illumination is used to induce the regeneration process. In this contribution external biasing/current injection is used to trigger the regeneration effect. The regeneration process itself is carried out in a stack of solar cells optionally separated by conductive spacers. In this configuration a multitude of solar cells can be treated simultaneously. In addition to inducing the regeneration process the external biasing brings along electrical losses and thus heats up the stack. This selfheating can be used to reach and/or hold the temperature required for the regeneration process. For high current densities an active regulation of temperature is necessary. The working principle could be successfully proven

Temperature induced degradation of the contact resistance of Ag-screen printed p-type silicon solar cells

This contribution targets on the stability of screen printed silver front contacts on n-type emitters of crystalline silicon solar cells during thermal treatment steps in the range of 200-300°C which could be of importance for e.g. some dielectric layers or Regeneration. As measurements of the contact resistance taken by TLM technique reveal, the contact resistance may seriously increase within the thermal treatment steps giving rise to a significant degradation of the fill factor of the solar cell. Furthermore it is found that the TLM technique itself has an influence on the measurements

Kinetics of the boron-oxygen related defect in theory and experiment

The formation of boron-oxygen complexes in boron-doped crystalline silicon can lead to a severe reduction in the minority charge carrier lifetime. This strongly influences, e.g., solar cell efficiencies if the material is used for photovoltaic application. Recent investigations have shown that a recovery of the carrier lifetime can be achieved by a subsequent thermally enhanced reaction induced by charge carriers. A model of the reaction dynamics of the boron-oxygen complex by means of rate equations is presented in this paper. Following a mathematical description of the reactions involved, the consequences based on the calculations are presented and allow a prediction of the observable electrical parameters. The fundamental agreement with measured data is proven experimentally for different phenomena

Boron-oxygen related defects in Cz-silicon solar cells degradation, regeneration and beyond

This contribution addresses the stability of Cz-Si solar cells subsequent to the so called Regeneration process which virtually eliminates the losses due to the well known boron-oxygen related defect. The stability of the regenerated state is vital for the industrial application of the Regeneration process. As recent experiments have shown, at least one sample was instable after the completed Degradation-Regeneration cycle at elevated temperatures. However, this instability was not found for other samples investigated. Furthermore, different possible interpretations of this instability are discussed

Upgrade Technologien für die Silizium-Photovoltaik (Upgrade Si-PV) : Teilprojekt der Universität Konstanz : B-O Regeneration auf Zellebene mittels modifiziertem Feuerofen (DEMO). Schlussbericht, Projektlaufzeit: 01.01.2016 - 30.06.2019

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Solarmodul auf der Sonnenbank

Sind Solarmodule Licht und Wärme ausgesetzt, nimmt ihr Wirkungsgrad ab. Bei höherer Beleuchtungsstärke und Temperatur erholen sie sich jedoch wieder. Daher müssen Siliziumwafer nach der Herstellung ausgewogen bestrahlt werden, um unter Betriebsbedingugen stabil zu arbeiten.publishe

On the influence of advection cooling during degradation and regeneration of Boron-Oxygen defects using high intensities

For the industrial application of the regeneration treatment to mitigate boron-oxygen related lightinduced degradation (BO-LID), the process has to work in just a few seconds. Fortunately, the regeneration process (including both degradation and the actual regeneration phase) can be sped up by both higher charge carrier injection and temperature to fulfill this requirement. Unfortunately, there is an upper temperature limit due to reverse reactions, and higher injection comes at the price of unwanted heating. Hence, it is on the one hand of utmost importance to find measures to keep temperature below a certain threshold while increasing injection by illumination. On the other hand, a quick heat-up into a certain temperature range is required to bring process times down to the second range. Within this contribution, it is shown that temperature and illumination intensity can be decoupled to a certain degree thanks to advection cooling, and how a smart control of high intensity laser illumination and advection cooling can further reduce process times.publishe