Transparent backsheets for bifacial photovoltaic (PV) modules: Material characterization and accelerated laboratory testing

Abstract

Interest in bifacial modules has rapidly increased over the past decade due to their ability to generate more power than conventional monofacial photovoltaic (PV) technology as they can absorb sunlight from both sides of the module. Compared to the traditional glass/glass bifacial modules, glass/backsheet modules show many advantages including lighter weight, high light transmittance, and high corrosion resistance. However, research on the weatherability and long-term reliability of transparent backsheet materials and their usage in bifacial modules under service environments is lacking. In this study, accelerated weather testing using the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was conducted to investigate the durability of three fluoropolymer-based transparent backsheets and their laminated coupon counterparts. The transparent backsheets were exposed at 75 C/50% relative humidity (RH), while the coupons were exposed at 65 C/50% RH, both subjected to UV irradiance of approximately 140 W/m 2 for up to 2000 h. Results indicate that a fluoroethylene vinyl ether (FEVE)/polyethylene terephthalate (PET)/ethylene-vinyl acetate (EVA)-based transparent backsheet (CB3) exhibited substantial chemical, optical, thermal, and mechanical changes and ultimate cracking after 1200 h (≈600 MJ/m 2 ). The other two backsheets, polyvinyl fluoride (PVF)/PET/FEVE-based (CB1) and polyvinylidene fluoride (PVDF)/PET/FEVE-based (CB2) backsheets, showed no obvious signs of cracking up to 2000 h of UV exposure(≈1000 MJ/m 2 ). For all three backsheets, the PET core layer demonstrated the greatest material property changes after UV exposure indicating that this layer is the most susceptible to UV degradation. Results indicate that the application of transparent backsheets for bifacial modules is promising. However, proper design of the layers of the backsheets for increasing the stability of PET core layer under UV exposure is critical. This study will provide a scientific basis for material choice and product development for a more reliable bifacial PV technology.

Review of Potential‐Induced Degradation in Bifacial Photovoltaic Modules

“…[91,92] This impacts their chemical and electrical properties and may be accelerated by certain climatic conditions such as salt-mist exposure. [93] PET also demonstrates UV susceptibility [87,94] leading to a yellowing effect. [94] PA-based backsheets are disappearing from the market due to a high tendency to develop cracks [80,89] although PA is fluorine free, which is advantageous for recyclability.…”

“…[93] PET also demonstrates UV susceptibility [87,94] leading to a yellowing effect. [94] PA-based backsheets are disappearing from the market due to a high tendency to develop cracks [80,89] although PA is fluorine free, which is advantageous for recyclability. [84] PVF is very reliable and stable compared to FEVE and PVDF.…”

Chemical and mechanical interfacial degradation in bifacial glass/glass and glass/transparent backsheet photovoltaic modules

“… 9 , 10 , 11 In particular, maintaining high transmittance and mechanical properties over long periods of time remains a challenge in certain backsheet formulations. 10 , 11 …”

“…The long-term durability of transparent backsheets is still under investigation; currently, there are limited field and accelerated weathering data. [9][10][11] In particular, maintaining high transmittance and mechanical properties over long periods of time remains a challenge in certain backsheet formulations. 10,11 Thus, the durability of the module materials in the G/G and G/TB configurations requires further analysis.…”

“…[9][10][11] In particular, maintaining high transmittance and mechanical properties over long periods of time remains a challenge in certain backsheet formulations. 10,11 Thus, the durability of the module materials in the G/G and G/TB configurations requires further analysis. Previous work on conventional G/B with EVA has shown bias-induced failures such as loss of adhesion and sodium ion migration under negative bias 12 and silver migration under positive bias.…”