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Top1. Introduction
In today’s industry, the most common type of solar photovoltaic (PV) cell is fabricated from either crystalline silicon or thin-film materials (P. F. Carcia, R. S. McLean, & S. Hegedus, 2010). The rigid construction of Si solar cells hampers their economic integration into residential and commercial buildings; however, thin film solar cell technologies may prove to be most appropriate with respect to cost and ease of manufacture, and it is anticipated that the next generation of photovoltaic devices will be based entirely on thin film technologies. These cells are based on the material CuIn1-xGaxSe2 (CIGS) as the absorber layer (p-type) and they are the most efficient cells at present (Igalson & Urbaniak, 2005). The key weakness of these cells is their moisture sensitivity. This is critical problem if this technology is to meet the requirements of international standard IEC61646, which requires that all PV modules survive 1000 hours in an environment of 85 Cº and 85% relative humidity(IEC61646-2, 2008). Cost effective encapsulation facilitating stable outdoor performance for more than 20 years is still a challenge. The only cost-effective encapsulation possibility for long term stability available at the moment is to use rigid glass, where all benefits of flexibility and lightweight disappear (BrÃ, 2009). Therefore, a robust, transparent flexible encapsulation method for CIGS PV cells is needed. Meeting these requirements is a major concern for the manufacture of thin film CIGS cells.
1.1. Flexible PV Modules
Flexible solar modules comprise four functional layer groupings as shown in Figure 1. The main focus of the investigation in this paper is the barrier layer which is incorporated in the encapsulation layers. This layer is typically formed from a planarised Polyethylene Naphthalate (PEN) sheet with an amorphous Al2O3 barrier coating < 50 nm thick. The Al2O3 barrier coating is produced by Atomic Layer Deposition (ALD). This technique is a controlled layer-by-layer deposition that enables the deposition of thin, smooth, and highly conformal films with atomic layer precision and typical thickness ranges between 1 and 100 nm (S. M. George, 2009).
Figure 1. Schematic of the flexible PV module (Courtesy of Flisom, Switzerland)
2. Polyethylene-Naphthalate And Aluminum-Oxide Properties
Polymer materials such as polyethylene naphthalate (PEN) consist of long straight-chain polymer molecules with weak chemical interaction between the chains. The molecular building block for PEN chains is shown in Figure 2. Therefore, small molecules such as water vapor and oxygen can diffuse around the material chains. Although polymers have strong covalent bonds (short range) along the chains, they have weaker (long range) bonding between chains (Ayache, Beaunier, Boumendil, Ehret, & Laub, 2010).
Figure 2. Structure of Polyethylene Naphthalate -PEN (Hansen, Myers, & Osakada, 1998)