Warren Cai

Research Interests

Reliability and Degradation of Optics and Encapsulants and Concentrated Photovoltaics

High efficiency multijunction solar cells in terrestrial concentrator photovoltaic (CPV) modules are becoming an increasingly cost effective and viable option in utility scale power generation. As with other utility scale photovoltaics, CPV modules need to guarantee operational lifetimes of greater than 25 years. The reliability of optical elements in CPV modules poses a unique materials challenge due to increased UV irradiance and enhanced temperature cycling associated with concentrated solar flux. The polymeric and thin film materials used in the optical elements are especially susceptible to UV damage, diurnal temperature cycling and active chemical species from the environment. Understanding the underlying mechanisms of materials degradation under elevated stress conditions is critical for commercialization of CPV technology and can offer unique insights into degradation modes in similar materials used in other photovoltaic modules. 

Fig. 1. Plot of responses from CPV and lens manufacturers on the most critical interfaces for study. Survey conducted by Dr. David Miller at NREL as part of DOE PREDICTS program. 

To determine the critical interfaces and materials for study, 13 CPV module and optics manufacturers were surveyed (Fig 1). The most critical interface for CPV reliability was determined to be the junction between the secondary optical element (SOE) and the anti-reflective (AR) coating (Fig. 2a) of the CPV cell. The SOE, typically a silicon oxide homogenizer, is adhered to the surface of the AR coating, typically Al₂O₃, with a silicone adhesive. During operation, the silicone adhesive degrades, which leads to the failure of the joint and ends the operational lifetime of the CPV module.

We investigated the effects of photo-degradation on the silicone adhesive and the adjacent interfaces with Al₂O₃ and silicon oxide. Two thermally cured silicone elastomer adhesives (Dow-Corning Sylgard 184 and NuSil LS2-6140) were examined. Specimens were aged both in an environmental chamber under UV illumination (Fig. 2b) and in an 1100x outdoor concentrator (Fig. 2c) to simulate operating conditions.

Figure 2. The SOE to AR interface (a) is most critical for CPV reliability. The silicone adhesive used at the interface is exposed in an environmental chamber under UV (b) and in an outdoor concentrator (c). 

Mechanical tests were carried out in the single cantilever beam configuration to characterize the adhesion of the silicone material after various aging treatments. We showed that during operation, silicone adhesives used in optical elements for CPV modules experience significant degradation in adhesive properties within the first months of operation. The cause for the decrease in adhesion energy was attributed to the photo-chemical reaction that increased crosslink density within the bulk silicone.  We studied the parameters that govern the underlying degradation process and developed models to predict the adhesive strength of the silicone elastomer. With these insights, we can better predict the operational lifetime of a CPV module and the bankability of CPV technology as a whole.

Increasing Adhesion Energy of Organic/Inorganic Interface Through Nanoscale Structures

The development of a cheap, flexible, and reliable barrier film technology is critical for the integration and commercialization of flexible photovoltaics and organic electronics devices. A promising emerging barrier technology utilizes multiple thin films of alternating organic/inorganic layers that is amenable to roll-to-roll processing while maintaining the desired ultra-low diffusion barrier properties. In operation, the barrier films are subject to weathering effects such as diurnal temperature cycling, moisture and chemical erosion, and UV degradation. The organic/inorganic interfaces are highly susceptible to damage from these degradation sources and limit the operational lifetime of the barrier film. 

Fig. 3. Schematic for constructing nanoscale structures at an organic/inorganic interface.

We developed a novel method to increase the organic/inorganic interfacial adhesion strength in a model system of poly (methyl methacrylate) (PMMA) and silicon oxide through interfacial patterning. An array of nanoscale patterns is etched into the PMMA through the use of nanosphere lithography. A schematic of the overall process is shown in Figure 2. A thin layer of silicon oxide is conformally deposited onto the patterned PMMA through plasma enhanced chemical vapor deposition. The patterned interface exhibit an order of magnitude increase in adhesion strength over that of a non-patterned interface. The increase in adhesion between the organic and inorganic layer can be attributed to the pullout of the nanoscale structures from the PMMA matrix. 

Awards

• Stanford Graduate Fellowship (2011-2014)