Novel transparent conducting barriers for photovoltaics

NREL has leveraged its expertise in multifunctional thin-film technologies to develop enabling and inexpensive transparent conductive coatings for energy applications (Figure 1). The design of these films provides an unique complement of performance characteristics including enhanced durability, flexibility, and impermeable and self-healing barriers to environmental contaminants (e.g., water and oxygen). This is especially needed for environmentally sensitive thin-film and organic photovoltaic technologies. In general, the majority of transparent conducting films used in industry today involves 0.5 to 2 micrometers of relatively expensive indium tin oxide (ITO) and/or doped zinc oxide coatings that have 60%-80% transmission in the visible region and resistances from 10 to 500 ohms/sq. However, to reduce materials/processing costs and maintain a high level of conductivity/transparency while enhancing barrier and durability performance, we have developed nanoscale film composites of ultra-thin TCOs (focusing on lower-cost materials that will include doped ZnO) and atomic layer-deposited materials. These composite structures will be a disruptive technology providing the transparency, conductivity, structural integrity (adhesion and fracture toughness), and impermeability needed for the most demanding applications, with processing that is scalable and adaptable for inexpensive, low-temperature manufacturing. Specifically, NREL has demonstrated transparent conducting films with the potential to reduce water/oxygen vapor transport rates by more than five orders of magnitude compared to typical organic materials. In addition, these films have resistances of ~5 ohms/sq. and transmissions over 90%. These transparent conducting barrier films also have significantly enhanced cyclic loading durability and strain tolerance. Finally, despite these substantial improvements, NREL's transparent conducting barrier layers can be integrated with PV devices for two orders of magnitude less cost than ITO. NREL's ultimate goal is to improve the technology and provide water vapor transport rates (WVTRs) lower than 10
^-6
g/m
²
-day-the barrier protection needed to enable organic, some thin-film, and 3rd generation photovoltaics.