Towards integrated PV applications: development of lightweight silicon heterojunction solar modules and their damp-heat and UV stability

Engineering notes

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Chinese explanation / 中文解读

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Original abstract

Lightweight solar modules are essential for diversifying the solar energy supply due to their lower weight compared with traditional glass solar modules. This opens up vast new scenarios for solar modules, especially for roofs with low load capacity, and vehicleintegrated photovoltaic (VIPV), significantly boosting the capacity of renewable energy. Silicon heterojunction (SHJ) solar cells have been recognized as one of the most advanced technologies for improving solar power generation. They can achieve a high power conversion efficiency (η) owing to their excellent surface passivation to minimize surface recombination, resulting in a higher open-circuit voltage (Voc) and fill factor (FF) compared to other crystalline silicon (c-Si)-based solar cell technologies, such as the aluminum back-surface field (Al-BSF) solar cell and passivated emitter rear contact (PERC) solar cell. Additionally, SHJ solar cells are highly compatible with integrated photovoltaic (PV) applications due to their thinner wafer thickness and symmetrical structure, which ensures identical stress distribution across the front and rear sides. Integrated PV typically operates under limited ventilation conditions, and the low power-temperature coefficient of SHJ solar cells results in significantly reduced power degradation in high-temperature environments. This makes them particularly advantageous in such scenarios. Therefore, a combination of SHJ solar cell technology with a lightweight solar module configuration would be a good attempt. However, several challenges are associated with the development and deployment of lightweight SHJ solar modules. On the one hand, SHJ solar cells are more sensitive to moisture than other solar cell technologies, such as PERC solar cell technology, because the amorphous/crystalline interface is susceptible to moisture, due to the oxidation of the hydrogenated amorphous silicon (a-Si:H) passivation layer. Different transparent conductive oxides (TCOs), such as indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO), have varying resistance to moisture. The SHJ solar cells are degraded by moisture ingress with increasing temperature, causing damp-heat-induced degradation (DHID), which decreases the solar module’s power output in the long run. Ultraviolet (UV) radiation has been identified as another critical reliability issue for SHJ solar cells and modules. This poses a serious concern when SHJ solar modules are operating in the field, particularly for lightweight solar module configurations, where flexible polymer-based front sheets may provide reduced shielding against UV radiation. On the other hand, a reliable encapsulation solution is currently only available for glass/glass solar module structures, and it is not yet ready for front sheet/back sheet solar module structures. It is crucial to address these challenges to develop and commercialize lightweight SHJ solar modules that meet the required performance and durability standards for diverse applications. This thesis aims to develop high-performance lightweight solar modules based on SHJ solar cells, with a particular focus on improving resistance to damp-heat (DH) and mitigating the effects of UV radiation. In the first part of this work, the DH stability of the specific lightweight SHJ minimodules based on ITO or AZO films was investigated through an accelerated DH aging test. The efficiency of the AZO module decreased by 58.57%rel, which was dominated by a large increase in series resistance (Rs) and a significant decrease in FF. It is identifiedthat the AZO film is more vulnerable to moisture than the ITO film under damp and heat conditions, which is attributed to the hydrophilic property of the AZO film. The AZO film was damaged by the combined effects of moisture ingress and delamination of the interconnection foil. The poor adhesion of the AZO layer to the interconnection foil accelerates the penetration of moisture, leading to severe degradation of the AZO layer. Consequently, moisture has a greater chance of percolating through the damaged AZO layer into the a-Si:H passivation layer, causing passivation degradation, which leads to an increase in recombination, resulting in a decrease in Voc of the solar modules. Therefore, a capping layer strategy was proposed to improve the moisture resistance of the AZOincorporated solar module. Notably, capping the AZO with an ITO layer reduced the efficiency loss of the ITO/AZO/ITO solar module to 18.92%rel, suggesting that the ITO capping layer could enhance moisture resistance and improve DH stability. Thus, it is expected to be a promising protective capping layer for AZO-incorporated SHJ solar cells and modules. Furthermore, a comparison of lightweight AZO solar modules with two different interconnection foils (IF1 and IF2) revealed that the adhesion between the polymer interconnection foil and the AZO layer is essential for the DH stability of the lightweight AZO solar modules. In the second part of this work, lightweight SHJ mini-modules were fabricated using SHJ solar cells with different encapsulation materials and architectures. These solar modules have a low area density (∼ 2 kg/m2), which is about eight times lighter than traditional glass-based solar modules, while preserving a high power-to-weight ratio (∼ 70 W/kg). The DH stability of the lightweight SHJ mini-modules encapsulated with various encapsulants was investigated through a 1000 h accelerated DH aging test. A comprehensive analysis of lightweight solar module degradation was carried out, with a focus on the optoelectronic properties of solar modules, as well as the chemical properties of the polymer encapsulants. The efficiency loss in lightweight SHJ solar modules following the DH test varied significantly, ranging from 3.22%rel to 54.06%rel, with the extent of the loss depending strongly on the type of encapsulation material used. The increase in Rs is the main cause of the decrease in solar module efficiency. The degradation mechanisms specific to each solar module have also been revealed. The solar module encapsulated with EPE (EVA/POE/EVA) showed the greatest efficiency degradation due to a significant increase in Rs. This can be attributed to two main factors: wires detaching from the ribbons due to solder joint failure, and interconnection failure of wires detaching from the fingers. The thermoplastic polyolefin (TPO)-encapsulated solar module was found to have the lowest degradation efficiency of 3.22%rel after 1000 h of DH due to its low water permeability and high thermal stability. This indicated that TPO is a promising encapsulant for moisture-sensitive solar cells. Based on Arrhenius-based modeling, 1000 h of DH testing at 85 ○C, 85% RH is estimated to correspond to approximately 1.1 years of exposure under typical field conditions for the ethylene tetrafluoroethylene (ETFE)/polyolefin-based aluminum back sheet (Al-bs) lightweight SHJ solar module with TPO encapsulants, during which about 5%rel efficiency degradation was observed. Additionally, a degradation of the downshifting (DS) effect was also found in solar modules encapsulated with downshifting encapsulants. An optimized, damp-heat-stable, lightweight SHJ solar module was successfully fabricated by using polyethylene terephthalate (PET)-based material instead of ETFE as the front sheet, polyolefin-based aluminum as the back sheet, and TPO as the encapsulant. The efficiency degradation of the solar module was only 0.47%rel after 1000 h of the DH test. Its DH stability is almost identical to that of the glass/back sheet solar module. In the third part of this work, a comprehensive investigation of the UV-induced degradation (UVID) behavior of lightweight SHJ solar modules utilizing different types of encapsulants with different UV transmittance: UV-blocking, UV-transmitting, and UV-downshifting is presented. After indoor UV exposure of 120 kWh/m2, equivalent to30 months of outdoor exposure in Jülich, Germany, solar modules incorporating these encapsulants exhibited relative efficiency losses of 2.17%, 9.25%, and 6.15%, respectively. The decrease in efficiency was mainly attributed to a reduction in the FF of the solar modules, accompanied by a diminished pseudo fill factor (pFF). Based on detailed FF and pFF loss analyses, we found that pFF loss was the major cause of FF loss, which is attributed to the deterioration of the passivation properties due to UV radiation. And the influence of Rs-related FF losses increased, which is attributed to the deterioration of the interconnection foil rather than the UV radiation itself. Additionally, while downshifting encapsulants helped mitigate UV damage, we observed a diminished DS effect in lightweight solar module configurations, potentially due to photo-oxidation. Utiliza- tion efficiency of DS decreases from around 34%abs to 21%abs after 120 kWh/m2 of UV exposure. Therefore, a novel encapsulation architecture combining UV-downshifting and UV-blocking encapsulants was proposed to ensure the UV utilization and stability of lightweight SHJ solar modules. Solar modules featuring this innovative dual-layer structure preserved over 98% of their initial performance after UV exposure, demonstrating a promising new approach for enhancing UV stability. The comprehensive investigation provides substantial insights into the degradation mechanism of lightweight SHJ solar modules under UV exposure and offers practical strategies in the progress of improving their durability and performance.

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