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Thin Film Electronic Devices with Conductive and Transparent Gas and Moisture Permeation Barriers

National Renewable Energy Laboratory

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Technology Marketing Summary

Transparent conducting (TC) materials are extensively used in electronics and electronic device applications. Presently, the most commercially used material involves indium-tin oxides (ITO) that have an unequalled combination of transparency and conductivity. In general, the conductivity and transparency are inversely related in the most common forms of transparent conductors (i.e. transparent conducting oxides) and ITO provides an optimum balance between the two. However, since indium is a byproduct of zinc mining, it is relatively rare and thus expensive. Furthermore, the use of thinner layers and repeated stresses (that induce cracks due to the brittle nature of the TCs) typically results in substantial increases in resistivity. In addition, while doped ZnO and other TC materials are a viable, less expensive alternative, process sensitivities and lower conductivity/transparency performance issues have limited their commercial acceptance for many applications.

To overcome some of the limitations of the TC materials, it has been demonstrated that the use of a thin (~7-10 nm) film of metal like silver, gold, or copper in conjunction with transparent conducting oxides provide enhanced conductivity while maintaining the required transparency. The key is that the metal-TCO integration provides higher transmission than the thin metal film by itself and higher conductivity than the TC material by itself. The end result is a set of thinner TC layers (i.e. less material) that have the same or enhanced conductivity and/or transmission. However, due to the islanding issues associated with metal deposition on TCO materials, the non-conformal coating decreases electrical conductivity such that ~7 to 10 nm of metal is needed to achieve the required conductivity.

In addition to needing materials to provide transparency and conductivity, in many electronic devices substantial cost is also expended to incorporate barrier coatings to prevent oxygen, water, and/or other environmental contaminants from reaching sensitive electronic components. While relatively thick metal coatings are used to provide protection in some packaging, this is not transparent and in general only improves the permeation barrier by a factor of ~ 1000. For some applications, permeation rates for water/oxygen need to be ~10-6 g/m2-day (e.g. organic based electronic devices such as organic light emitting diodes for displays or organic photovoltaics). As a point of comparison, typical permeation rates of water through 4mil PET is ~10 g/m2-day at standard test conditions of 38°C and 90% relative humidity. Layers of inorganic coatings (e.g. oxides and nitrides with the most common being aluminum oxide, silicon oxide, and silicon nitride) are often used in conjunction with a plastic substrate to reduce the permeation by a factor of 10-1000. The permeation drops as the inorganic coating thickness increases, but reaches a lower limit for film thicknesses of a few hundred to perhaps 2 thousand Angstroms. The lower limit for permeation is associated with the inevitable presence of film defects, such as pinholes, cracks, scratches, etc. This has been found to be true for a wide range of deposition techniques (e.g. physical vapor deposition by evaporation or sputtering, and plasma enhanced chemical vapor deposition) and materials. In an attempt to disconnect the pinholes and cracks, layers of organic materials were incorporated, but subsequent research has revealed that due to the relatively thick geometry of these layers, the water/oxygen transport is simply delayed, and the overall permeation rate is not reduced. Thus, the diffusion rate through these multiple layer configurations is approximately the same as through a single inorganic layer of sufficient thickness.

The most successful permeation barrier approach devised so far addresses the issue of propagation along pinholes and cracks by constructing a conformal pinhole-free inorganic oxide coating using a technique like atomic layer deposition (ALD) in conjunction with inorganic film(s) deposited using more conventional methods. In this case, the set of depositions has been demonstrated to reduce the permeation of water/oxygen to ~ 10-5 g/m2-day.

 

Description

Scientists at the National Renewable Energy Laboratory (NREL) have improved upon previous state-of-the-art transparent conductor and water/oxygen permeation barrier technologies by using a conformal metal coating in conjunction with inorganic coatings to create a light transmitting conducting thin film stack. The thin conformal metal coating provides substantially higher electrical conductivity while providing a barrier to environmental contamination propagation, especially along pinholes/cracks. A crack propagation inhibition substantially increases fracture toughness and if a non-noble metal is used, a self-healing film forms an oxide permeation barrier at the exact location where water/oxygen is propagating through the coatings.

 

Benefits
  • Enhanced transmission and electrical conductivity
  • Thinner layers than typically needed by TCOs alone
  • Enhanced permeation barrier to water, oxygen, or other environmental contaminants
  • Enhanced fracture toughness from single or cyclic loading
  • Self-healing capabilities that form improved permeation barriers

 

Applications and Industries
  • Opto-electrical devices
  • Solar cells
  • LCDs
  • LEDs
  • Circuits

 

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 8,609,994
Patent
8,609,994
Thin film electronic devices with conductive and transparent gas and moisture permeation barriers
A thin film stack (100, 200) is provided for use in electronic devices such as photovoltaic devices. The stack (100, 200) may be integrated with a substrate (110) such as a light transmitting/transmissive layer. An electrical conductor layer (120, 220) is formed on a surface of the substrate (110) or device layer such as a transparent conducting (TC) material layer (120, 220) with pin holes or defects (224) caused by manufacturing. The stack (100) includes a thin film (130, 230) of metal that acts as a barrier for environmental contaminants (226, 228). The metal thin film (130, 230) is deposited on the conductor layer (120, 220) and formed from a self-healing metal such as a metal that forms self-terminating oxides. A permeation plug or block (236) is formed in or adjacent to the thin film (130, 230) of metal at or proximate to the pin holes (224) to block further permeation of contaminants through the pin holes (224).
National Renewable Energy Laboratory 12/17/2013
Issued
Patent 9,093,661
Patent
9,093,661
Thin film electronic devices with conductive and transparent gas and moisture permeation barriers
Thin film electronic devices (or stacks integrated with a substrate) that include a permeation barrier formed of a thin layer of metal that provides a light transmitting and electrically conductive layer, wherein the electrical conductive layer is formed on a surface of the substrate or device layer such as a transparent conducting material layer with pin holes or defects caused by manufacturing and the thin layer of metal is deposited on the conductive layer and formed from a self-healing metal that forms self-terminating oxides. A permeation plug or block is formed in or adjacent to the thin film of metal at or proximate to the pin holes to block further permeation of contaminants through the pin holes.
National Renewable Energy Laboratory 07/28/2015
Issued
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 08-54DevelopmentAvailable04/20/201604/20/2016

Contact NREL About This Technology

To: Erin Beaumont<erin.beaumont@nrel.gov>