Outdoor Monitoring, Degradation Rate Estimation, and Failure Mode Analysis of Thin Film Amorphous Silicon Solar Cells

Abstract

The global shift towards low-carbon and renewable energy sources, driven by climate concerns associated with fossil fuels, has positioned solar photovoltaics (PV) as a major alternative. With the increasing adoption of PV technologies, it is crucial to study various PV module types. This thesis focuses on the HyET Powerfoil, an amorphous silicon (a-Si) PV module with a polymer encapsulation, offering a lightweight alternative to conventional crystalline silicon (c-Si) PV modules. Due to the new type of polymer encapsulation (TE) and to compare a-Si absorber with its c-Si counterparts, numerous factors must be examined before large-scale outdoor deployment to prevent failure and degradation and assess module reliability.

This research addresses the losses observed in HyET Powerfoil modules, leading to the design of an experiment involving two groups of four modules each to identify and segregate potential degradation at the interfaces. The modules included 2 c-Si glass-encapsulated PV modules, 2 c-Si glass + TE encapsulated modules, 2 HyET modules with glass encapsulation, and 2 HyET Powerfoils. The primary objective was to investigate factors contributing to performance losses especially the short-circuit current (Isc) and soiling compared to traditional glass-encapsulated modules. The study utilized measurements from a monitoring system and a solar simulator, along with regular cleaning. Initial findings indicated significant absorber degradation, with HyET Powerfoils showing a degradation rate of around 17% at the final measurements. Additionally, the TE polymer exhibited substantial losses in transmission due to degradation and soiling.

The thesis also addresses optical degradation in HyET Powerfoil modules, particularly focusing on the TE polymer stack. Using a combination of outdoor exposure tests, WOM ageing, and surface roughness measurements, the study assessed degradation mechanisms. The results indicate that the TE polymer stack experiences significantly higher soiling of around 1% more than glass. This can be attributed to increased surface roughness. Moreover, the adhesive used in the TE polymer stack was identified as a major contributor to optical degradation, leading to increased light scattering of around 5%. These findings suggest that while TE polymers offer certain benefits, their long-term durability under various environmental conditions needs further optimization.

Additionally, an injection-dependent Electroluminescence (EL) predictor was developed to screen for hotspots, based on the IEC 61215 Hotspot Endurance Tests. Defects visible in the EL images were classified into three modes based on defect intensities: Mode A, Mode B, and Mode C. Further testing revealed that only Mode B defects led to hotspot formation due to their localized effects.

In conclusion, while the flexible HyET Powerfoil demonstrates the potential to facilitate high-efficiency modules for lightweight applications, specific areas in the polymer configuration need improvement. This study indicates potential pathways for enhancing the long-term performance of these modules.