Corrosion testing of solar cells: Wear-out degradation behavior

Corrosion is one of the main end-of-life degradation and failure modes in photovoltaic (PV) modules. However, it is a gradual process and can take many years to become a major risk factor because of the slow accumulation of water and acetic acid (from encapsulant ethylene vinyl acetate (EVA) degradation). In this work, an accelerated aging test for acetic acid corrosion was developed to probe wear-out and end-of-life behavior and facilitate screening of new cell, passivation, metallization, and interconnection technologies. In the tests, the top glass and EVA layers were removed from PV modules to expose the solar cells and interconnects. These "opened" modules were then placed in acid baths under varying conditions, including acid concentration, temperature, and elec-trical bias. Three cell technologies were tested, including Al-back surface field (BSF), passivated emitter and rear contact (PERC), and silicon heterojunction (SHJ). For all conditions, the presence of acid accelerated module power loss compared to control tests with water baths. Increased temperature accelerated the rate of degradation by several times. Application of electrical bias led to an initial drop in short circuit current, but these modules eventually outperformed the non-biased modules. In all tests, lead oxides were the primary degradation products detected, and found to accumulate mostly along the printed metallization, especially in proximity to the busbar. SHJ cells with a silver paste-based interconnection outperformed Al-BSF and PERC cells with a solder-based interconnection. The accelerated corrosion test methods can be optimized to match corrosion behavior observed in field modules with greater precision and shorter times than standard damp heat tests, and is espe-cially useful to assess the long-term durability of the continuously increasing variety of corrosion sensitive PV materials and components.

The role of Encapsulants in the long-term performance of advanced Crystalline Silicon Glass-Glass PV modules

Luca Gnocchi

The adoption of crystalline silicon (c-Si) PV modules based on glass-glass (G-G) structures is gaining momentum because of the possibility to manufacture bifacial panels, collecting light from the rear side. Despite not being an optimal material, Ethylene Vinyl Acetate (EVA) has become the dominant encapsulant in the PV industry. In fact, upon exposure to UV at high temperatures in the presence of moisture, EVA generates acetic acid (HAc), leading to the corrosion of the metallic interconnects. This problematic may be even more pronounced in a non-permeable G-G structure, compared to a conventional glass-backsheet (G-BS) design, in which HAc can be partially degassed out through the breathable BS. Nevertheless, some industries would like to continue using EVA in the manufacturing of G-G modules, because of the lower cost, the much longer track record and easier processability of EVA compared to newer alternatives, such as polyolefns (POs). In this work, we try to answer the question whether it is possible to use EVA in the manufacturing of G-G modules. The experimental chapters focus primarily on study the long-term reliability, in particular:1. The sensitivity to water of different c-Si cell technologies (i.e. Aluminum Back Surface Field (Al-BSF), Passivated Emitter Rear Cell (PERC) and Silicon Heterojunction (SHJ)) exposed to damp-heat, where we highlight the good stability of the EVA, and the sensitivity to moisture of the SHJ solar cells.2. We propose a detailed microscopical model explaining the problematic of the SHJ to water. In reality, the presence of moisture (favored by the use of EVA) is only a contributing factor, as clearly indicated by the fact that the degradation in more pronounced in the presence of glass. Here we point at the role of a glass corrosion process and of NaOH in damaging the passivating properties of the cell. Either by reducing the water ingress (with an edge seal or a PO) or by replacing the glass type, the degradation phenomenon is not observed.3. The impact of EVA storage conditions on the lamination quality and ultraviolet (UV) degradation. If good polymer storage and handling practices are carefully respected, the results tend to suggest that EVA can still be a viable solution to encapsulate G-G PV modules, for deployment in geographical zones where the humidity levels are not so high during the year (i.e. temperate climates). On the contrary, if these conditions are not observed, or in the event of module operating in a hot-humid climate, we believe that this may affect the long-term performance of G-G modules encapsulated with EVA.4. An accelerated aging test for acetic acid corrosion (missing in the industry standards) is developed to probe wear-out and end-of-life behavior and facilitate screening of metallization, and interconnection technologies. Corrosion is in fact one of the main end-of-life failure modes in PV modules which use EVA. Here, SHJ cells with a low temperature silver paste-based interconnection outperformed cells with conventional solder-based interconnection (i.e. Al-BSF and PERC). The proposed corrosion test method can be optimized to match corrosion behavior observed in feld modules with shorter times than standard damp heat tests.These results give us a deeper insight on how to manufacture reliable and durable G-G modules, based on different cell technologies, and do not rule out the possibility to use EVA when targeting extended lifetimes of double glass panels.

EPFL2022

Energy Yield and Electricity Management of Thin-Film and Crystalline Silicon Solar Cells

Yannick Samuel Riesen

In the case of high photovoltaic (PV) penetration into the electricity grid, the energy produced by a PV system that is effectively used (useful energy) depends on the energy yield and on how this energy is managed to avoid detrimental effects occurring at high PV injection, e.g. during the midday peak. The overall goal of this thesis is to provide guidelines for maximizing the useful energy of a PV system by quantifying losses incurred during operation at both the solar cell device and the system levels. Solar cells are usually optimized for the standard test conditions (STC). However, the conditions are generally different during operation. This work assesses how solar cell materials and designs can be optimized to maximize the energy yield for specific operating condition. We mainly focus on thin-film silicon solar cells because of their challenging metastable behavior. The temperature dependence of the performance of thin-film amorphous silicon (a-Si:H) and microcrystalline silicon solar cells is thus measured for different deposition parameters and cell designs. We observe that, by tuning the intrinsic layer thickness of a-Si:H cells, the cells with the best (STC) efficiency do not necessarily provide the highest energy output. We also explain the presence of a maximum in the value of the fill factor as a function of temperature. The temperature dependence study is then extended to thin-film silicon multi-junction, crystalline silicon heterojunction (SHJ) and other crystalline silicon solar cells. For thin-film silicon solar cells, spectral effects and degradation or recovery effects due to the metastable character of a-Si:H (due to the Staebler-Wronski effect) significantly impact the energy yield. Based on indoor and outdoor degradation/recovery experiments, we show that it is challenging to describe this metastability with a diode model. However, such a model with a current loss term and an additional temperature dependence for the saturation current and ideality factors accurately reproduces the current-voltage characteristics of a-Si:H solar cells over a wide range of irradiance levels and operating temperatures. On the system level, we model a PV system with local storage to evaluate several strategies to reduce the detrimental midday injection peaks. The impact of such measures on the useful energy is also investigated. We develop a simple control algorithm that minimizes the losses due to a feed-in limit and maximizes self-consumption without the need of a production forecast. We show that heat storage using a boiler or a heat pump performs as well as battery storage. In general, a feed-in limit reduces significantly peak injection but only a relatively small storage capacity is needed to reduce losses (due to this limit). Changes in tilt and orientation of modules also reduce losses resulting from feed-in limits and shrink the winter/summer production ratio by more than a factor of two. We also develop a statistical method that estimates - from loads measured every 15 min - when different electrical appliances in a household are commonly used. This model indicates that about 8% of the total load could be shifted easily to the midday period, thereby reducing the midday injection peak. Finally, we combine device and system aspects to show that varying cell technology (e.g. with different temperature response) has a limited but not negligible impact on system output.

EPFL2016

Energy Yield and Electricity Management of Thin-Film and Crystalline Silicon Solar Cells

Yannick Samuel Riesen

EPFL2016

The role of Encapsulants in the long-term performance of advanced Crystalline Silicon Glass-Glass PV modules

Luca Gnocchi

EPFL2022