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.