Degradation Of Passivation Research Articles

The Impact of Different Hydrogen Configurations on Light- and Elevated-Temperature- Induced Degradation

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measurements. Degradation conditions were light soaking at 77 °C and 1 sun-equivalent illumination intensity and dark anneal at 175 °C. The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B- or P-doped). We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp. This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and to a smaller degree the concentration of boron-hydrogen (BH) pairs. A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si. In p-type Si, the LeTID extent was found to scale with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , suggesting a direct link between both. The temporal evolution of BH pairs, LeTID degradation/regeneration, and surface degradation depends on wafer thickness and the cooling ramp of the fast-firing process. Based upon these findings, we formulate a theory of the hydrogen-related mechanism behind LeTID: Hydrogen originating from H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> moves between different temporary traps. First, hydrogen binds to LeTID precursors and acceptor atoms in the silicon bulk, later moving toward the surface. This leads first to the LeTID degradation and regeneration and then to the degradation of surface passivation.

Unraveling the passivation mechanisms of c-Si/SiOx/poly-Si contacts

As one of the prospective crystalline silicon (c-Si) technologies, tunnel oxide passivating contact (TOPCon) solar cells have recently attracted much attention in the photovoltaic community. However, the deficit of open-circuit voltage due to the imperfect passivation performance together with the insufficient understanding of the passivation mechanism, especially for the key structures of TOPCon devices ( i.e. , poly-Si/SiO x /c-Si), is a crucial factor to limit the improvement of device efficiency. To unlock the full passivation potential of this type of poly-Si/SiO x /c-Si contact, we report a systematic investigation by combining experiments and simulations. The related results can be summarized: 1) the best passivation can be obtained by optimizing SiO x quality, tailoring the in-diffusion profile, and performing hydrogenation; 2) if SiO x quality is good enough, high-level passivation can be expected even without impurity doping; 3) although the deepened in-diffusion could partially alleviate the degradation of the poor surface passivation, it would also reduce the passivation performance owing to the increased Auger recombination inherent to the high impurity doping; 4) the presence of pinholes would degrade the passivation performance, especially for the cases with the good surface passivation, a shallow in-diffusion and a low doping concentration of poly-Si, where the electric-field effect was minimized. To further confirm these conclusions, recombination distributions and the corresponding current profiles of the minority carriers were reviewed. This work clarifies the passivation mechanism of c-Si/SiO x /poly-Si contact in detail and provides effective guidance for further promoting the passivation level and the efficiencies of TOPCon devices. • An experiment and complementary simulations were designed to confirm the passivation properties of c-Si/SiO x /poly-Si contacts. • Annealing temperature, hydrogenation and SiO x quality were simultaneously regulated to promote passivation quality. • The underlying factors including poly-Si thickness and pinholes to affect the passivation performance were investigated in detail. • Passivation mechanisms were clarified by addressing recombination distributions and current profiles of the minority carriers.

Poly-Si thickness and temperature dependent oxide disruption induced by penetration of the interfacial oxide in (p) poly-Si/SiOx passivating contacts

Passivating contacts based on polycrystalline silicon on an interfacial oxide (poly-Si/SiO x ) require a fine-tuned in-diffused dopant profile at the surface of the crystalline silicon wafer. In particular, a strong dopant in-diffusion causes excessive recombination. This study contributes to reveal the yet unclear mechanism of this degradation of passivation. Boron in-diffusion from co-sputtered (p) poly-Si layers through the interfacial oxide is investigated for several layer thicknesses and plateau temperatures during the solid phase crystallization. It is shown that for higher temperatures and unexpectedly also for thicker layers the observed recombination current density increases up to a total loss of passivation, accompanied by a physical interfacial oxide disruption, which is not observed for intrinsic reference layers. This behavior is supported by simple numerical calculations. The recombination current densities are transferred into the actual surface recombination velocity which in turn reveals an exponential increase towards higher boron concentrations at the interface between the oxide and the crystalline Si base. It is concluded that rather dopant penetration of the interfacial oxide is responsible for the degradation of passivation than the actual boron concentration at the interface itself as reported before in literature. • Co-sputtered (p) poly-Si/SiO x passivating contacts degrade towards thicker poly-Si and higher annealing temperatures. • The identified degradation mechanism is a physical disruption of the interfacial oxide. • The degradation is rather driven by boron penetration than by the actual boron concentration at the interface. • Modelling of the actual surface recombination velocity reveals an exponential relation to the interface boron concentration.

Towards Ge-based electronic devices: Increased longevity of alkanethiol-passivated Ge(100) in low humidity environments

Germanium is a critically important material for future complementary metal-oxide-semiconductor devices, however, to maximise its potential it is necessary to develop a robust passivation process that prevents Ge re-oxidation for a queue time of 24 h. Self-assembled monolayers (SAMs) of alkanethiols on Ge have previously been shown to inhibit oxidation; however, re-oxidation eventually occurs when exposed to ambient conditions. Herein, it is shown that humidity plays a key role in the degradation of the SAM, ultimately resulting in re-oxidation. To demonstrate this, thiol-passivated Ge(100) surfaces are exposed to controlled humidity environments with different levels of relative humidity (RH). The rate of re-oxidation of the Ge surfaces are tracked using X-ray photoelectron spectroscopy and water contact angle analysis to discern what role RH plays in the re-oxidation of the Ge and the degradation of the SAM passivation. Atomic force microscopy data is presented to show that humidity-mediated re-oxidation of the Ge has little or no impact on the route mean square roughness of those surfaces. Finally, atomistic modelling of thiol-SAM passivated Ge in the presence of water molecules has been studied using first principles density functional theory in order to simulate experimental conditions and to understand the atomic level processes that determine stability in hydrophilic and hydrophobic configurations.

Damp‐heat induced degradation in photovoltaic modules manufactured with passivated emitter and rear contact solar cells

AbstractCorrosion is one of the main PV module failure mechanisms, as it can cause severe electrical performance degradation in PV modules exposed to hot and humid environments. Moisture penetrating a photovoltaic (PV) module may react with the metallic components causing corrosion. In addition, acetic acid which is produced by hydrolysis of ethylene vinyl acetate (EVA), the most common encapsulant, may further degrade metallic components. Corrosion is one of the main PV module failure mechanisms, as it can cause severe electrical performance degradation in PV modules exposed to hot and humid environments. The specific chemical reactions involved in the corrosion mechanisms for the different components are well understood. However, which of these causes the most serious degradation in the field, and therefore, most severe power loss is unknown. Moreover, the severity of corrosion in the absence of acetic acid is not yet well researched. This work distinguished between the front and rear side corrosion mechanisms and identified the different electrical signatures observed due to them. The experiment included damp‐heat (DH) conditioning of single‐cell mini‐modules, containing passivated emitter and rear contact (PERC) solar cells, laminated with a polyethylene terephthalate (PET) based backsheet. Furthermore, half‐encapsulated PERC PV cells were tested, with either the front or the rear side exposed. Electrical and material characterisation were conducted for the investigation of the sample degradation, and the performance decrease, related to the degradation of the rear surface passivation, was examined.

The Mechanism of Surface Passivation Degradation in SiO2/SiNx Stack Under Light and Elevated Temperature

In this article, a significant degradation of the surface passivation was observed in Czochralski silicon (Cz-Si) lifetime samples using effective lifetime measurements during 1 sun illuminated treatment at 130 °C–175 °C. The samples are passivated by plasma-enhanced chemical vapor deposited silicon nitride (SiNx) and thermally grown silicon oxide (SiO2). Samples showed strong variations in the kinetics of the surface passivation degradation depending on the passivation stacks, phosphorus diffusion, and bulk doping. Recovery of the surface passivation was only observed on samples passivated by a single silicon nitride layer. To further investigate the observed behavior, corona charging capacitance-voltage measurements were performed. It is shown that surface degradation arises from a combination of changes in interface defect density ( Dit ) and fixed charge density ( Qf ). We show that two separate degradation channels contribute to the degradation in surface passivation, where the fixed charge density continuously declines, while the interface defect density increases to a maximum and subsequently decreases. The decrease in interface defect density leads to a recovery of surface passivation.

Time‐Resolved Investigation of Transient Field Effect Passivation States during Potential‐Induced Degradation and Recovery of Bifacial Silicon Solar Cells

Various types of potential‐induced degradation (PID) with fundamentally different physical root causes have been observed recently ranging from shunting (PID‐s), corrosion (PID‐c) to degradation of surface passivation (PID‐p). Herein, PID‐p is investigated for the first time at the rear side of bifacial silicon solar cells with enhanced time‐resolution during degradation and recovery under simultaneous illumination. The tests reveal fast transient changes of rear‐side short‐circuit current within minutes and thus allow to understand the degradation process related to PID‐p. The short‐circuit current with rear‐side illumination first decreases significantly by more than 80 % resulting in an intermediate degraded state. Subsequently, it increases to a regenerated state. Under reversed bias, a total recovery of the cell parameters is observed and confirmed by I–V measurements under standard testing conditions (STCs). The underlying transient PID‐p degradation and recovery mechanism is well described by a PC‐1D simulation of the field‐induced band bending at the rear surface. Finally, a qualitative model involving the migration of mobile charged species and their impact on the charge equilibrium in the field effect passivation layer is presented. Thus, time‐resolved analysis of transient PID effects can serve as a clear characteristic for PID of polarization type.

Exceptional Surface Passivation Arising from Bis(trifluoromethanesulfonyl)-Based Solutions

The surface properties of many inorganic electronic materials (e.g., MoS2, WSe2, Si) can be substantially modified by treatment with the superacid bis(trifluoromethane)sulfonimide (TFSI). Here we find more generally that solutions based on molecules with trifluoromethanesulfonyl groups, including TFSI, give rise to excellent room temperature surface passivation, with the common factor being the presence of CF3SO2 groups and not the solution’s acidity. The mechanism of passivation comprises two effects: (i) chemical passivation; and (ii) field effect passivation from a negatively charged thin film likely to be physically adsorbed by the surface. Degradation of surface passivation is caused by de-adsorption of the thin film from the surface, and occurs slowly in air and rapidly upon vacuum exposure. The air stability of the passivation is enhanced by the presence of droplets at the surface which act to protect the properties of the film. The finding that nonacidic solutions can provide excellent electrical passivation at room temperature opens up the possibility of using them on materials more sensitive to an acidic environment.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

In situ Photoluminescence Study of Plasma Effects on Passivation of Crystalline Silicon Coated with Aluminum Oxide

A degradation of crystalline silicon surface passivation provided by aluminum oxide (Al2O3) is generally observed after plasma processes, e.g., deposition of amorphous silicon nitride. To minimize such detrimental effect, a better understanding of the interaction between plasma species and the Al2O3 layer is required. Using in situ photoluminescence, the passivation quality of as‐deposited and annealed crystalline silicon wafers coated with Al2O3 grown by atomic layer deposition is characterized in real time during argon‐hydrogen plasma exposure. The photoluminescence intensity of as‐deposited samples instantly steps up after the plasma ignition, and then it gradually decreases as a function of plasma exposure time. However, only degradation of photoluminescence signal can be found if the samples are annealed prior to the plasma treatment. The interaction between vacuum UV light from plasma and different types of chemical bonds in the Al2O3 layer is proposed to explain the obtained results. Understanding the mechanisms and root cause leading to different behavior between as‐deposited and annealed samples under plasma exposure is a first step toward redesigning the process flow for better surface passivation.

Degradation of Surface Passivation and Bulk in p-Type Monocrystalline Silicon Wafers at Elevated Temperature

The surface passivation quality and the bulk lifetime of boron-doped p-type Czochralski silicon wafers were studied in response to dark annealing at 175 °C, using in situ effective lifetime measurements. We investigated non-fired and fired silicon nitride (SiN x ), aluminum oxide (AlO x ) capped with SiN x , and thermally-grown silicon oxide (SiO2). Modulation in surface passivation quality and bulk lifetime was detected only in cases where hydrogen is assumed to be released into the silicon wafer from the dielectric (AlO x /SiN x stack and fired SiN x layer). Interestingly, the degradation of both the surface and the bulk were followed by a recovery. It is also interesting to note that the changes in the surface and the bulk seem to be related, as the surface degradation starts when the initial bulk degradation ends. This study indicates a possible involvement of hydrogen in both the degradation and the recovery processes. The evolution of the effective lifetime as a function of time is similar to that reported for carrier-induced degradation in multicrystalline wafers; however, occurring on a different time scale. Hence, these findings may also be valuable for investigation of other degradation mechanisms in different silicon materials.

N-type silicon solar cell with rear tunnel oxide combined with rear screen-printed electrodes

In this article, we describe an n-type silicon solar cell with a conversion efficiency of 21.9% on a 156 mm pseudosquare Czochralski (Cz) wafer with a rear tunnel oxide-passivated structure and rear screen-printed electrodes. The thermal degradation of the rear structure during electrode firing was solved by adding a SiNx film on the rear surface. Not only the SiNx film but also the tunnel oxide structure was chemically etched using fire-through screen-printed electrodes, so that the electrodes contacted directly to the bulk silicon. A phosphorus-diffused layer appeared at the bulk silicon immediately below the tunnel oxide, and this shallowly and densely diffused layer reduced the contact resistivity between the electrodes and the bulk silicon layer. Degradation of rear passivation caused by etching on the tunnel oxide structure was improved by adding a diffused layer using POCl3 before chemical oxidation. The diffused layer using POCl3 was deep and had a low phosphorus concentration, thus the field effect worked successfully while Auger recombination was not enhanced.

Experimental Study on Calendaristic Degradation and Self-Discharge of 3.4 Ah Lithium-Sulfur Pouch Cells

Lithium-Sulfur (Li-S) batteries represent one of possible development pathways of Lithium based batteries, which are expected to provide benefits of high volumetric energy density, various safety features and low production cost 1. Already nowadays, one can find demonstrated applications, which heavily depend on unique attributes of Li-S batteries, e.g. high-altitude long-endurance unmanned aerial vehicles capable to endure for an extended periods of time in air (currently proofed for 14 day without landing) 2. However, wide commercialization of Li-S batteries is still hindered mainly by their limited performance and short lifetime 3. Both, cycling and storage, influence the Li-S battery lifetime. The degradation is often assigned to the irreversible relocation of polysulfides, which results into the corrosion of lithium metal anode, degradation of sulfur cathode and surface passivation of both electrodes 4. These degradation aspects were studied and discussed in literature into a certain degree; however, as pointed out in Ref. 3, some observations are possible to be done only at commercial-size cells, instead at coin cells, which determines the further focus of this work on a larger format cells, specifically pouch cells. The Li-S pouch cells were previously studied for an effect of cycling degradation on sulfur cathodes 5 and lithium metal anodes 6. The current study brings an experimental investigation of a capacity fade and a self-discharge with a periodic one month checking for storage of Li-S cells at various conditions of state-of-charge and temperature. The methodology presented in Ref. 7 is used for evaluation of the battery performance during the degradation in terms of capacity, resistance, power capability and shuttle current. M. Wild, L. O’Neill, T. Zhang, R. Purkayastha, G. Minton, M. Marinescu, and G. J. Offer, Energy Environ. Sci., 8, 3477–3494 (2015).http://defence.airbus.com/portfolio/uav/zephyr/, accessed at 20th November 2017.T. Cleaver, P. Kovacik, M. Marinescu, T. Zhang, and G. Offer, J. Electrochem. Soc., 165, A6029–A6033 (2018).S.-H. Chung and A. Manthiram, ACS Energy Lett., 1056–1061 (2017).S.-E. Cheon, S.-S. Choi, J.-S. Han, Y.-S. Choi, B.-H. Jung, and H. S. Lim, J. Electrochem. Soc., 151, A2067 (2004).X. B. Cheng, C. Yan, J. Q. Huang, P. Li, L. Zhu, L. Zhao, Y. Zhang, W. Zhu, S. T. Yang, and Q. Zhang, Energy Storage Mater., 6, 18–25 (2017).V. Knap, D. I. Stroe, R. Purkayastha, S. Walus, D. J. Auger, A. Fotouhi, and K. Propp, ECS Trans., 77, 479–490 (2017). Figure 1

Degradation of Surface Passivation on Crystalline Silicon and Its Impact on Light-Induced Degradation Experiments

A decrease of surface passivation quality is observed in FZ, Cz, and mc-Si lifetime samples during light-induced degradation (LID) treatments. It is shown that this degradation occurs not only in samples with single SiN x :H layers but also when using layer stacks consisting of SiO x /SiN x :H or AlO x :H/SiN x :H. Time-resolved calculation of the surface saturation current density J 0 s is shown to be a reliable method to separate changes in the bulk and at the surface of samples during LID treatments. The impact of the observed changes in passivation quality on the outcome and interpretation of LID experiments aiming at changes in the bulk of Cz or mc-Si is investigated and discussed.

UV radiation hardness of photovoltaic modules featuring crystalline Si solar cells with AlOx/p+‐type Si and SiNy/n+‐type Si interfaces

We report on the UV radiation hardness of photovoltaic modules with bifacial n‐type Passivated Emitter and Rear Totally diffused crystalline Si cells that are embedded in an encapsulation polymer with enhanced UV transparency. Modules with front junction cells featuring an AlOx/p+‐type Si passivation interface at the illuminated side are stable for a UV irradiation dose of 598 kWh m−2. In contrast, irradiating modules with back junction cells featuring an a‐SiNy/n+‐type Si passivation interface at the illuminated side reduces the output power by 15%. The quantum efficiency of the a‐SiNy‐passivated module degrades in the spectral range between 300 and 1000 nm, which we ascribe to a degradation of the surface passivation. Modeling the experimental data shows that photons with an energy above 3.4 eV contribute to the degradation effect and enhance the front surface recombination current density by a factor of 15.

Facile and Thermally-Stable Al2O3 Passivation by Using In-Situ TiO2 as a Capping Layer for Boron Emitter of N-Type Silicon

Al2O3/TiO2 stacks formed by atomic layer deposition are known to provide a high level of passivation for boron-doped silicon. In previous works, the TiO2 layer was deposited on a pre-annealed Al2O3 layer, however this stack showed passivation degradation after post-deposition annealing. This work presents an alternative to using the as-deposited Al2O3 for the Al2O3/TiO2 stack, which shows no degradation of passivation after post-deposition annealing up to 400 °C. This approach simplifies the processing, allowing continuous layer deposition, and eliminates the undesirable vacuum breaking. This simplified processing leads to better thermal stability of the Al2O3/TiO2 stacks and a low emitter saturation current density. In order to understand the underlying mechanism of surface passivation, an investigation on the effect of thermal SiO2 on the passivation of the Al2O3/TiO2 stack was performed, which indicates that the TiO2 capping layer enhances the field-effect passivation for both the Si/Al2O3 and SiO2/Al2O3 structures.

UV-induced degradation of PERC solar modules with UV-transparent encapsulation materials

In this paper, we report on the stability of p-type passivated emitter and rear cells under ultraviolet (UV) exposure with various silicone nitride passivation coatings and embedded in different encapsulation polymers. Our results reveal that UV transparent polymers can result in a module power loss of up to 6% under a UV irradiation dose of 497 kWh/m2. We show that the degradation in power is caused by a reduction in open circuit voltage. This loss is related to an increased recombination in the cell, which we ascribe to a degradation of the surface passivation. With ray tracing simulations, we determine the number of photons reaching the passivation interface. Assuming that all photons with energies above 3.5 eV de-passivate the interface is in agreement with our experimental results. Copyright © 2017 John Wiley & Sons, Ltd.

2D Mapping of Chemical and Field Effect Passivation of Al2O3 on Silicon Substrates

Fixed charge and interface defect densities are the critical material parameters for silicon surface passivation. These parameters are measured with high spatial resolution on 150 mm wafers using a novel method called ‘BiasMDP’. In this method the effective carrier lifetime is determined by means of microwave detected photoconductivity while a bias voltage is applied to an electrode on top of the passivation layer. The measured carrier lifetime strongly reduces when the external bias voltage compensates the electric field of the fixed charges. Based on this minimum lifetime fixed charge and interface defect densities are calculated. The sensitivity of BiasMDP is demonstrated on a silicon wafer with Al2O3 passivation, which is processed with thickness gradient. A continuous shift of flat band voltage is measured across the wafer. This shift linearly correlates with oxide thickness as predicted by theory. Furthermore, an Al2O3 passivation stack with local HfO2 interface layer is characterized. The 2D map of fixed charge density shows a variation between 0.5 x 1012 cm-2 and 4.0 x 1012 cm-2 of negative polarity. This inhomogeneity is attributed to the local presence or not presence of the HfO2 interface layer. Finally, a wafer with local surface damage and inhomogeneous carrier lifetime distribution is measured with BiasMDP. The measurement shows that the inhomogeneity is caused by degradation of chemical passivation. BiasMDP is shown to be a powerful method for detecting inhomogeneities of passivation layers.

Damp and dry heat degradation of thermal oxide passivation of p+ silicon

Thermal SiO2 passivates both moderately and heavily doped silicon surfaces irrespective of the dopant type, which is advantageous in high-efficiency solar cell designs. Commercial photovoltaic cells are submitted to accelerated ageing tests, such as damp-heat exposure, to ensure they maintain their performance for at least 20 yr. We find damp-heat exposure causes a severe and rapid degradation of thermal SiO2 passivation on p+ silicon surfaces. The reaction is so severe that the diffused-region recombination in the degraded state is limited by the diffusion of minority carriers to the Si–SiO2 interface not the density of interface defects Dit. Certainly, this effect renders the thermal-oxide passivation useless if employed on a solar cell. To study the cause of the degradation, we also test the effects of storage in dry heat and room ambient conditions. Examination of the rate of degradation in the tested storage conditions in comparison with modelled diffusion of moisture in SiO2, we find a significant correlation between the time dependent J0e and moisture supplied to the interface, leading us to the conclusion that moisture ingression and subsequent reaction at the SiO2–Si interface are the cause of both damp-heat and room- ambient degradation.

Direct Metallization Local Al-Back Surface Field for High Efficiency Screen Printed Crystalline Silicon Solar Cells

In this paper, we present a detailed study on the local back contact (LBC) formation of rear-surface-passivated silicon solar cells, where both the LBC opening and metallization are realized by one-step alloying of a dot of fine pattern screen-printed aluminum paste with the silicon substrate. Based on energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM) characterizations, we suggest that the aluminum distribution and the silicon concentration determine the local-back-surface-field (Al-p+) layer thickness, resistivity of the Al-p+ and hence the quality of the Al-p+ formation. The highest penetration of silicon concentration of 78.17% in aluminum resulted in the formation of a 5 microm-deep Al-p+ layer, and the minimum LBC resistivity of 0.92 x 10-6 omega cm2. The degradation of the rear-surface passivation due to high temperature of the LBC formation process can be fully recovered by forming gas annealing (FGA) at temperature and hydrogen content of 450 degrees C and 15%, respectively. The application of the optimized LBC of rear-surface-passivated by a dot of fine pattern screen(-) printed aluminum paste resulted in efficiency of up to 19.98% for the p-type czochralski (CZ) silicon wafers with 10.24 cm2 cell size at 649 mV open circuit voltage. By FGA for rear-surface passivation recovery, efficiencies up to 20.35% with a V(OC) of 662 mV, FF of 82%, and J(SC) of 37.5 mA/cm2 were demonstrated.