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Advanced Development of Flexible Substrates for PV

Flexible photovoltaics (PV) have the potential to reduce the cost per Watt of solar energy and improve lifetime performance of solar modules. The conformable properties open up new market opportunities such as building integrated PV (BIPV) and portable PV. However, for flexible PV to be a viable renewable energy option and to compete with more established solar cell technologies, requires reductions in manufacturing costs, improvements in performance, and greater reliability.

There’s more than meets the eye when it comes to reaching these goals. PV components, like any complex device structure, ultimately require a stable, dependable supply chain to support their evolution and continued growth. FlexTech Alliance has identified technology gaps and been actively involved in developing many underlying elements to support the growth of a robust supply chain for materials and manufacturing process equipment to serve the PV and other related thin-film electronic component and device market segments.

Substrates

The use of flexible substrates for PV components has many advantages over traditional substrates such as silicon. They are more lightweight, conformable to non-planar structures, and have the potential for longer lifetimes and to be less expensive to manufacture.

Flexible substrates have been a priority of the FlexTech Alliance technical program for a number of years as evidenced by the initiation of R&D programs with multiple manufacturers. For instance, a FlexTech Alliance funded project with Akron Polymer Systems (APS) helped develop for commercialization a clear, colorless, polyimide, flexible substrate that can be used for manufacturing PV devices that require high temperature processing.

APS is performing chemical structural modifications to optimize the high temperature, optical, and mechanical properties of its polymers. These properties may also be enhanced through the incorporation of inorganic additives. Currently in pilot production, the objective of the modification work is to obtain a polymer film product that meets all the technical and marketing requirements for various electronic devices and components produced in a high-temperature manufacturing process on a flexible substrate.


Figure 1. Steps in OPV manufacturing. SOURCE: Solarmer Energy

Also in process is a FlexTech Alliance funded project with Corning to develop commercially viable methods for continuous printed electronic components manufacturing on flexible glass substrates. Beneficial characteristics of glass include inherent barrier properties for device hermeticity, low surface roughness, compatibility with device processing chemicals, high-temperature capability for high quality thin-film deposition and minimal distortion with applied stress.

Corning’s goal is to demonstrate working devices made by roll-to-roll printing technologies. It will rely on selected organic PV devices to evaluate program progress and demonstrate the benefits of flexible glass substrates. The general substrate design and process methods identified will be applicable to a wide range of applications in addition to thin-film photovoltaics such as sensors, energy harvesting and storage, displays, and solid-state lighting.

Active layers

Organic PVs (OPV) based on conjugated polymers are ideal for truly clean and affordable energy generation. However, the efficiency of OPVs is still low compared to their inorganic counterparts. The primary hindrance has been the lack of requisite polymers with the required properties for a high efficiency device, such as strong and broad absorption, high carrier mobility, and appropriate energy levels. To solve these problems, Solarmer Energy, with funding from FlexTech Alliance, has designed and synthesized new electron donor polymers with smaller band gap, suitable energy level, and higher mobility, based on Benzo[1,2-b:4,5-b’]dithiophene (BDT) and dithieno[3,2-b:2’,3’-d]silole (DTS) units.


Figure 2. Protection with laminated ultra-barrier film.2-Dyad laminated TFE CIGS solar cells; >90% of original efficiency after 2000hrs at 85°C/85% RH. SOURCE: Vitex Systems

When the Solarmer project was initiated, the world record for OPV efficiency was ~5%. During the project, Solarmer was able to successfully develop high quality donor polymers enabling several NREL certified world records in OPV efficiencies ranging from 6.8% to a high of 8.51%. In addition, Solarmer developed an all-solution process for a flexible OPV cell based on both commercially available P3HT: PCBM and Solarmer’s proprietary polymer (S7) OPV system, in which both the hole transport layer (HTL) and conductive paste have been optimized at least for P3HT:PCBM systems (Fig. 1). They were also able to successfully demonstrate bench-top and pilot line fabrication of OPV modules from a P3HT:PCBM system.

Barriers and encapsulants

Lifetime performance and efficiency of solar modules is greatly affected by the quality of barrier layers and encapsulants. Recognizing the need for improvements in barrier technology, we have identified and funded multiple development projects that will enable the progression of low cost, high-volume flexible PV manufacturing.

Early advancements in barrier layer performance were achieved through a FlexTech Alliance funded project with Vitex Systems. Coatings were developed with a level of barrier performance not previously obtained in a thin-film barrier construction. Using a patented polymeric deposition process, extremely low WVTR values – ≤ 10-6 – are possible utilizing a dyad approach wherein an inorganic thin-film layer is coupled together with a planarizing polymer layer (Fig. 2). Other advantages of the resultant process include low temperature processing, a vacuum-compatible process, and compatibility with roll-to-roll manufacturing or printing and laminating.

Currently under development with help from a FlexTech Alliance grant is Cambridge NanoTech’s high-speed atomic layer deposition (ALD) system. When completed, the system will enable the manufacture of large-area, thin-film coated flexible substrates for use in organic electronics, solar cells, biomedical devices, and displays. To accelerate the ALD process, the engineering team at Cambridge NanoTech is focusing on cycle-time reduction by means of a high-speed precursor delivery and extraction mechanism.

The high-speed ALD system is targeted to operate at the high volumes necessary for commercially viable roll-to-roll manufacturing. ALD is an ideal coating technology for flexible PV because of its perfect, conformal, ultra-thin films that are scalable to large-area substrates. ALD simultaneously offers excellent thickness uniformity, film density, step coverage, interface quality, and low temperature processing, making ALD beneficial for both roll-to-roll flexible substrates and rigid substrates.

Conclusion

Incremental improvements throughout the materials and manufacturing equipment supply chain are paving the way for flexible PV to reach its full potential. Utilizing a collaborative community of individuals, universities and leading technology companies, FlexTech Alliance has identified critical gaps in capability, funded technical research, and directed advanced developments to expand and strengthen the flexible PV infrastructure. Innovative flexible substrates, more efficient active layers and reliable barrier materials and encapsulants will all contribute to having solar energy outperform traditional power production on a financial basis and trigger true economies of scale in solar device manufacturing.

The author would like to thank Mike J. Idicavage, PhD, independent contributor, and Stacy Oresman, Director of Technology at FlexTech Alliance.

Denise Rael is Marketing Manager, FlexTech Alliance, 3081 Zanker Road., San Jose, CA 95134 USA; ph.: 408-577-1300; email denise.rael@flextech.org

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