Ultra Accelerated Aging and Characterization of Thin-Film Materials with Concentrating Light.

The materials’ sectors have experienced the most advancement in years to provide solutions to our societies' ever-increasing demands. Therefore, significant engineering tests for products and materials have been developed, like thermal-stability and predicted lifetime as a function of exposure to temperature, humidity, and UV radiation. However, there is still a deficiency in the understanding and testing when it comes to the degradation effects driven by the combined effect of solar light intensity and exposure duration. To cover this gap, a small number of accelerated aging methods based on concentrating natural or artificial light have been developed with the mature ones to have reached the standardization level and the rest still in low readiness levels. The former methods utilize concentration ratios of a few tenths of suns and focus on paints, plastics and other commercial materials for outdoor applications. The latter methods focus on solar energy-absorbing materials with concentration ratios that might exceed 1,500 suns. Our group has produced published work in both directions.

Noteworthy, all the above methods require specialized equipment, weeks or months of testing, and a large number of samples to produce reliable results. Additionally, many methods are based on indirect degradation mechanisms by inducing very large variations of temperature and humidity. However, they neglect the impact of solar radiation and that most materials and products will operate between low temperature and humidity ranges. Moreover, most of them provide a single degradation data point (often just a pass or fail) per sample and exposure test, which implies using many samples to extract reliable and long-term degradation predictions. A few of the materials that depend on such accelerated aging methods are paints, plastics, coatings, window films, and solar cells with very large market sizes and predicted growths. For example, the global paints and coatings market size was around USD 150 billion (2019), around USD 570 billion for plastics, and USD 10 billion for window films; all three of these markets are expected to grow by ~4-5% per year from 2020-2027.

To address the above technical limitations, we developed a method which combines in one apparatus concentrating artificial light with continuous optical characterization. The apparatus can be used to investigate the light-induced degradation of (research and commercial-grade materials) by conducting accelerated aging tests efficiently under fully controlled conditions (acceleration ratio, temperature, humidity, pressure, and atmosphere composition). In parallel, it continuously collects and analyzes the light transmitted through the sample and its temperature (optionally the evolved gases). The transmitted light provides online insights as simple as the material's aggregated light properties and as complex as its composition. Until now, successful aging tests, using our High Flux Solar Simulator and several lab-components, on new and standard materials proved the concept and the above claims. Here we present our results from the first two campaigns with materials used in organic photovoltaics (OPV) and alkyd paints.

Poly-3-hexylthiophene (P3HT) is one of the many polymers in the field of OPV and has the largest set of degradation data available in the literature; thus, one of the model systems for degradation benchmarking. Combination of P3HT with phenyl-C61-Butyric-Acid-Methyl Ester (PCBM) in a bulk heterojunction OPV cell (P3HT:PCBM) has dominated the research for a decade. Qualitatively, two degradation mechanisms were observed on the continuous UV-Vis spectrograms for both materials when exposed to various intensities of concentrated light. For P3HT a chemical transformation captured as the blue-shift at ~554nm and the overall decomposition of the material captured with the gradual decrease of the absorbance. These results agree with the observations in literature suggesting that both mechanisms could be attributed to the photo-oxidation of P3HT. Indeed, conducting the experiments in an inert Argon atmosphere showed a similar behavior but with significantly lower rate. In comparison with P3HT, P3HT:PCBM proved significantly more stable when exposed in the same conditions; as expected.

Coating materials such as paints and varnishes are mixtures of several (often numerous) components (binding media, pigments, solvents and fillers). Since binding media are not simple mixtures but rather complex polymers, degradation mechanisms are more complicated, because the initial components have reacted to form new compounds (polymers). Past studies on the photo degradation kinetics of Alkyd paints were able to simulate a year of sunlight exposure in 1,008 hours with the use of a commercial light aging device. The estimated degradations were estimated with the use of FTIR-ATR at around 48% to 65%. On the other hand, we were able to simulate the same degradation (~50% also checked with FTIR-ATR) in just 0.5 hours i.e. circa 2,000 times reduction in the experimental time. In addition, the degradation mechanisms were monitored continuously using the transmitted light which yielded a similar overall degradation (55% - 60%).