In the development of materials for enhanced photovoltaic (PV) performance, it is critical to have quantitative knowledge of both their initial performance and their performance over the required 25-year warranted lifetime of the PV system. Lifetime and degradation science, based on an environmental stress and response framework, is being developed to link the intensity and net stress to which materials, components, and systems are exposed to the responses observed and their subsequent degradation and damage accumulation over the lifetime. Induced absorbance to dose (IAD), a metric developed for solar radiation durability studies of solar and environmentally exposed materials, is defined as the rate of photodarkening or photobleaching of a material as a function of radiation dose. Quantitative degradation rates like IAD, determined over a wide range of stress intensities and net stresses, have the potential to predict degradation, failure, and power loss rates in photovoltaic systems over time caused by damage accumulation. Two grades of poly(methyl methacrylate) were exposed and evaluated in two cases of high-intensity ultraviolet exposures. A three- to six-fold increase in photodarkening was observed for one acrylic formulation when exposed to UVA-340 light when compared with concentrated xenon-arc exposure. The other, more highly stabilized acrylic formulation, showed up to three times more photodarkening in the same exposure.
Back-surface acrylic mirrors can be used in low concentration and mirror augmented photovoltaics (LCPV, MAPV) to
increase the irradiance on a module. Back-surface mirrors can spectrally filter incoming solar radiation reducing the
ultraviolet (UV) and infrared (IR) load on the module, while useful radiation is coupled into a module or photovoltaic
cell. Degradation of these mirrors can occur from UV induced photodegradative processes and metallization corrosion.
Environmental stresses such as humidity, thermal cycling and exposure to corrosive substances can cause an increase in scattering, reducing mirror performance. In order to increase the lifetime and durability of back-surface acrylic mirrors a better understanding of the degradation modes is necessary. In a study of acrylic back-surface mirrors for LCPV and MAPV applications, optical properties and bidirectional scattering distribution functions (BSDF) were investigated and correlated to simulated exposure protocols. Formulations of Poly(methyl methacrylate) (PMMA) with differing concentration of UV absorbers were used for the aluminum backsurface acrylic mirrors.
The formulations of aluminum back-surface acrylic mirrors were exposed in a QUV accelerated weathering tester (QLabs) to ASTM G154 Cycle 4. Total and diffuse reflectance spectra were measured for each mirror under exposure
using a diffuse reflectance accessory (DRA) from 180-1800 nm on a Varian Cary 6000i at defined dose intervals. The
total reflectance losses in the 250-400 nm region were greater and diffuse-only reflectance increased for formulations of acrylic mirrors that contained the least amount of UV stabilizer after each dose of QUV exposure.
Acrylic back-surface mirrors were exposed to salt fog corrosion and QUV and were analyzed using BSDF. There was
an increase in scattering from roughening of the mirror surface after exposure to the corrosive environment.
Mirror augmented photovoltaic (MAPV) systems utilize low cost mirrors to couple more light into a
photovoltaic (PV) absorber. By increasing the light absorbed, they are expected to produce less expensive electricity.
As a substrate candidate for back surface reflector mirrors, two grades of PMMA have been exposed to UV stress
from two sources at two intensities for two doses in an effort to see the response of materials under different states of
stress and after exposure to different amounts of total stress. By developing a framework for correlating stresses,
such as short wave ultraviolet radiation, with responses, such as induced absorbance and yellowing, mirror durability
we have made progress in developing lifetime and degradation science using mirror durability as a case study. All of
the samples showed similarities in their degradation characteristics. The UV stress acceleration factor was quantized
as 10.2 in short wave ultraviolet irradiance, and 15.8 in total shortwave UV dose. The effects of UV absorbers in
protecting the polymer from degradation are discussed. Further study into degradation mechanisms will elucidate the
exact phenomena that contribute to these material responses to stress.