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The primary cell wall is highly hydrated in its native state, yet many structural studies have been conducted on dried samples. Here, we use grazing-incidence wide-angle X-ray scattering (GIWAXS) with a humidity chamber, which enhances scattering and the signal-to-noise ratio while keeping outer onion epidermal peels hydrated, to examine cell wall properties. GIWAXS of hydrated and dried onion reveals that the cellulose ([Formula: see text]) lattice spacing decreases slightly upon drying, while the (200) lattice parameters are unchanged. Additionally, the ([Formula: see text]) diffraction intensity increases relative to (200). Density functional theory models of hydrated and dry cellulose microfibrils corroborate changes in crystalline properties upon drying. GIWAXS also reveals a peak that we attribute to pectin chain aggregation. We speculate that dehydration perturbs the hydrogen bonding network within cellulose crystals and collapses the pectin network without affecting the lateral distribution of pectin chain aggregates.

The direct current bias for photovoltaic (PV) modules interconnected in series‐strings may include both high voltage negative ("HV−") and positive ("HV+") polarity with respect to the electrical ground. Multiple degradation modes, resulting in quantifiable optical loss, were found to occur during HV−/HV+ sequential stress, including corrosion of the external glass surface, encapsulant delamination (at its interfaces with the glass and the PV cell), internal haze formation (resulting from a chemical interaction between the glass and the encapsulant), corrosion and migration of the gridlines, and corrosion of the silicon nitride (SixNy) antireflective coating on the cell. The effects of these separate modes were examined using monolithic (e.g., glass or PV cell) and laminated‐coupon (glass/encapsulant/glass or glass/encapsulant/cell/encapsulant/backsheet) specimens. Characterizations during and after unbiased accelerated testing at 85°C/85% relative humidity included spectrophotometry, optical microscopy, electron microscopy, and ellipsometry. For some module components (i.e., the glass and the SixNy coating), the optical performance was determined through iterative analysis of empirical measurements. Concentrating on just their spectral effect, a novel model was then developed to estimate the transfer of light to the PV cell and the return of light from the PV module with simultaneous degradation mechanisms, which was compared with a mini‐module previously subjected to HV−/HV+ stress. The model suggests that one third of the current loss observed for the mini‐module can be attributed to the optical degradation of the packaging materials. The dominant degradation modes include encapsulant delamination and corrosion of the SixNy coating. Recommendations are given so that the optical model may be improved relative to accelerated testing and validated relative to field aging. [ABSTRACT FROM AUTHOR]

The backsheet layer of a solar module provides a safety and environmental barrier to the high voltages running through the photovoltaic (PV) cells and electrical contacts within the core of the module. However, in the past decade, backsheet cracking has become one of the most observed failure modes in PV module field surveys. In this work, the degradation of polyvinylidene fluoride (PVDF)‐based backsheets is explored. Backsheet samples are either exposed to accelerated laboratory aging (UV light, heat, and moisture) or collected from the field. Fourier transform infrared and Raman spectroscopy, fragmentation testing, atomic force and scanning electron microscopy, and small‐angle neutron scattering are combined to develop an understanding of how chemistry and microstructure evolve during aging. Chemical degradation, surface pitting, polymer phase changes, and anisotropic polymer domains are all observed in aged backsheet samples. The results provide insight into the degradation mechanisms that lead to cracking and field failure of PVDF‐based backsheets. The comparison of aged PVDF‐based backsheets helps to lay the groundwork for limiting polymer‐based failure modes in PV modules. [ABSTRACT FROM AUTHOR]

Degradation from ultraviolet (UV) radiation has become prevalent in the front of solar cells due to the introduction of UV‐transmitting encapsulants in photovoltaic (PV) module construction. Here, we examine UV‐induced degradation (UVID) in various commercial, unencapsulated crystalline silicon cell technologies, including bifacial silicon heterojunction (HJ), interdigitated back contact (IBC), passivated emitter and rear contact (PERC), and passivated emitter rear totally diffused (PERT) solar cells. We performed UV exposure tests using UVA‐340 fluorescent lamps at 1.24 W·m−2 (at 340 nm) and 45°C through 4.02 MJ·m−2 (2000 h). Our results showed that modern cell architectures are more vulnerable to UVID, leading to a significant power decrease (−3.6% on average; −11.8% maximum) compared with the conventional aluminum back surface field (Al‐BSF) cells (<−1% on average). The power degradation is largely caused by the decrease in short‐circuit current and open‐circuit voltage. A greater power decrease is observed in bifacial cells with rear‐side exposure compared with those with front‐side exposure, indicating that the rear side is more susceptible to UV damage. Secondary ion mass spectroscopy (SIMS) confirmed an increase in hydrogen concentration near the Si/passivation interface in HJ and IBC cells after UV exposure; the excess of hydrogen could result in hydrogen‐induced degradation and subsequently cause higher recombination losses. Additionally, surface oxidation and hot‐carrier damage were identified in PERT cells. Using a spectral‐based analysis, we obtained an acceleration factor of 5× between unpackaged cells (containing a silicon nitride antireflective coating on the front) in the UV test and an encapsulated module (with the front glass and encapsulant blocking 90% of the UV at 294 nm and 353 nm, respectively) in outdoor conditions. From the analytical calculations, we show that a UV‐blocking encapsulant can reduce UV transmission in the module by an additional factor of ~50. [ABSTRACT FROM AUTHOR]

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/m2 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/m2). 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/m2). 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. [ABSTRACT FROM AUTHOR]