Example 4: metal-dielectric solar reflector - Helicon Thin Film Systems, Ltd

THIN FILM SERVICES / APPLICATION AREA

protected high-reliability metal-dielectric coatings

Example: high-reflectance coatings for harsh conditions

MATERIALS / APPLICATION AREA: Multi-layer, metal-dielectric and metal-semiconductor, thin film designs are required in a very wide, disparate range of applications in virtually every sector: optical, energy, renewables, semiconductor, aerospace, medical, etc. Challenges can arise in the arena of coating performance needed in new products, wherein typically the disparate material properties involved must cooperate to simultaneously provide optimum performance attributes on several different metrics, which can range from control of charge traps at metal-dielectric interfaces, to the film and interfacial properties required to minimize degradation (corrosion, abrasion) in thermal and chemical environmental cycling.

EXAMPLE APPLICATION: Helicon has extensive history in multiple thin film challenges within the high-concentration solar applications for electricity generation, electrolysis, and thermo-chemical applications, all of which hold the promise of providing much more efficient generation for a lower "levelized cost of energy" - and further, more efficient forms of distributed generation - relative to the currently matured industry of non-concentrated photovoltaic panels.

However, the primary cost hurdle in the various applications of high-concentration solar applications is the cost of the optics that will allow efficient collection and concentration. Central to this hurdle is the materials technology incorporated into the thin-film reflective surface, where exposed first-surface mirrors are generally accepted as prerequisite for highest efficiency in both generation and cost. This reality places the onus of developing large optics, and associated secondary optical surfaces, for these energy-harvesting applications; and, most centrally, in the arena of encapsulated, first-surface reflectors.

Silver-dielectric multilayers: In such high-concentration applications, silver can provide a significant efficiency improvement, in its spectral reflectance, over aluminum; however, silver also incurs an entirely more challenging undertaking in the development of large-area applications that will withstand long-term out-door environments. While earlier dielectric multilayer encapsulants on silver have proven themselves for less demanding indoor (or under-dome) applications, the "jury is still out" on their ultimate

ability to prove cost-effective production and use in the far more demanding out-door applications (hence, the vast majority of outdoor solar concentrators still only utilize aluminum-based reflectors); such demanding applications exemplified by concentrated solar applications; e.g, Concentrated Photo Voltaics (CPV), concentrated solar-thermal applications - widely referred to as Concentrated Solar Power (CSP) - or hybrid, co-generating solar systems such as concentrated photovoltaics combined with solar-thermal (variously, CPV-ST, etc.).

Figure: Accelerated reliability tests on silver multilayer coatings developed and fabricated in Helicon facility. produced, high-reliability, sputtered multilayer silver, after accelerated environmental testing

Low-energy ion-assisted deposition using local Kaufman-type/end-hall source configurations is viable for small-area, high-cost substrates in laser and telecom applications; however, large-area, high-rate production processes demand use of highly scalable processes, preferably in more scalable forms of energetically-assisted plasma sputtering and plasma/electron-assisted evaporation. Our work in various protected silver applications, using economically viable production methods for large-scale optical surfaces has produced verified environmentally stable performance that has, in side-by-side reliability tests, completely out-performed other large-scale commercial plasma-assisted sputtering processes that utilize microwave, ECR-plasma and electron-beam assistance, and is on par with the best results obtained by the most expensive, but impractical to scale, ion-beam/ion-assist processes.

Silver (or aluminum) will provide fairly different spectral reflectances as a function of the specific optical design of the dielectric layers deposited on its exposed (reflecting) surface. The reflectance will typically be tailored to the desired application of the reflector (e.g., maximizing efficiency of a specific multijunction photovoltaic (MJPV) system, or, astronomical imaging in a specific spectral region of interest, or, etc). The measured reflectance will also depend significantly on the measurement method - angle-of-incidence, polarization, use of an integration sphere, etc. These optical properties of a specific dielectric stack on silver can be accurately predicted from first principles when aided by published spectral information of both the bare silver and the indices of the overlying dielectric materials. Modern commercial software such as MacLeod will perform this function quite well.

However, what is not remotely so deterministic and predictable, relative to attaining a pre-modeled optical characteristic of such deposited metal-dielectric multilayers, is the process development for achieving environmental reliability of such coatings under long-term exposure to aggressive environments (see Figure). Helicon has conducted extensive work in this process-intensive area of reliable, high-reflectivity and spectrally selective coatings for large-area solar applications, with results of this work being utilized in present-day products.