Publication

Investigation and Mitigation of Moisture- and Potential-Induced Degradation Mechanisms of Silicon Heterojunction Solar Cells and Modules

Olatz Arriaga Arruti
2023
EPFL thesis
Abstract

Photovoltaic (PV) technology is one of the most promising renewable energy sources. The PV market is dominated by crystalline silicon (c-Si) based technologies, thus ensuring their long-term performance is of paramount importance for manufacturers, investors and customers. In this thesis, we focus on investigating the reliability, especially the sensitivity to moisture and high voltages, of silicon heterojunction (SHJ) technology. It is expected to be one of the main players in the near future, particularly in Europe. We not only study the root cause of these two degradation mechanisms but also provide strategies to prevent them at the module and cell levels.First, we performed a literature review on the reported performance loss rates (PLR) of SHJ modules installed in the field. The data indicates a median PLR of 0.56 %/year, which falls in line with conventional c-Si technologies. We then researched the indoor data referring to accelerated ageing tests and determined that SHJ technology is sensitive to three factors: moisture (i.e. damp heat (DH)-induced degradation), high voltages (i.e. potential-induced degradation (PID)) and UV exposure. We nonetheless established that with the right module configuration, SHJ solar cells can reach service lifetimes of 35+ years. Next, we focused on two of the conditions SHJ is sensitive to: moisture and PID. We discovered that these are interlinked in SHJ cells encapsulated in a glass/glass (G/G) configuration with ethylene vinyl acetate (EVA) as an encapsulating material. We propose, for the first time, a multi-factorial microscopic model unique to SHJ cells, in which degradation occurs at two different levels. First, the high moisture in the module corrodes the glass, creating sodium hydroxide (NaOH) molecules that can diffuse through the encapsulant (i.e. EVA) and reach the SHJ cell. The sodium (Na+) ions and hydroxyl (OH-) groups can then get adsorbed in the grain boundaries of the transparent conductive oxide (TCO), and increase grain boundary scattering. Second, the application of a high negative bias amplifies the previous mechanisms and enhances the conventional drift of Na+ through the EVA to the cell and into the passivating layers. The diffusion of such ions can create recombination centres, destroying the passivation of the solar cell. We propose three mitigation strategies at the module level: 1) the use of high-volume resistivity and low water vapour transmission rate (WVTR) encapsulants (e.g. polyolefin elastomers (POE) and ionomer); 2) the use of an edge seal in G/G laminates encapsulated with EVA to prevent moisture ingress; and 3) the combination of front-side POE with a rear-side EVA along with a (transparent) backsheet with low permeability.We then investigated approaches to prevent PID at the cell level. We show that PID can be diminished to a certain point when capping layers - acting as barriers against diffusion of ionic species - are deposited on top of the ITO. The deposition of capping layers demonstrated that PID can be mitigated to some extent and, DH-induced degradation, completely prevented.Finally, we compared the stability of SHJ to passivated rear emitter and contact (PERC) and tunnel-oxide passivating contact (TOPCon) solar cell technologies. These showed more stability in DH conditions, but a higher sensitivity to PID at the same time. We show that degradation does not depend on water ingress, thus we dissuade the usage of EVA with these technologies.

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Related concepts (32)
Crystalline silicon
Crystalline silicon or (c-Si) Is the crystalline forms of silicon, either polycrystalline silicon (poly-Si, consisting of small crystals), or monocrystalline silicon (mono-Si, a continuous crystal). Crystalline silicon is the dominant semiconducting material used in photovoltaic technology for the production of solar cells. These cells are assembled into solar panels as part of a photovoltaic system to generate solar power from sunlight. In electronics, crystalline silicon is typically the monocrystalline form of silicon, and is used for producing microchips.
Solar cell
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices are often the electrical building blocks of photovoltaic modules, known colloquially as solar panels.
Solar-cell efficiency
Solar-cell efficiency refers to the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity by the solar cell. The efficiency of the solar cells used in a photovoltaic system, in combination with latitude and climate, determines the annual energy output of the system. For example, a solar panel with 20% efficiency and an area of 1 m2 will produce 200 kWh/yr at Standard Test Conditions if exposed to the Standard Test Condition solar irradiance value of 1000 W/m2 for 2.
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