Almost all thin film PV devices have a great deal in common. They attempt to minimize material costs by using ultrathin semiconductors to convert sunlight to electricity; they attempt to reach adequate sunlight-to-electricity conversion efficiencies; and they require excellent outdoor reliability. In this sense, thin films are a direct response to the high materials costs of wafer silicon PV modules.
However, thin film modules share most functional aspects with wafer silicon modules. That is, they require top and bottom protection from the outdoor environment, so that they can last outdoors about thirty years. They need top and bottom contacts, bus bars, and a connection to an external circuit to carry away current. They need ways to connect the cells together to provide the right balance of voltage and current. They need some sort of mounting scheme, or at least the ability to be mounted if none is explicitly included. They may need edge seals and edge protection, such as an edge delete. All in all, there is a great deal that thin film modules have in common with crystalline silicon modules; and with each other, no matter what the semiconductor converter is. These commonalities can be called the balance of module, or BOM. Importantly, they turn out to be a significant portion of module cost. (See tables below for particulars.)
The presence of substantial BOM means that: (i) reaching the highest possible efficiencies and (ii) reducing the manufacturing costs of the active elements of thin film modules are essential if they are to be competitive in the PV marketplace – not to mention (the real goal of all PV development), to be able to be competitive with fossil fuels. Cost reductions must come from reducing semiconductor materials costs, energy costs, capital costs, maintenance costs; and intending to use the largest substrate areas possible with easily connected cells to allow for the fullest automation. This must all be done while also optimizing conversion efficiencies, yield, and stability.
This simple comparison with crystalline silicon sets the stage for what follows (a comparison among thin films in greater depth) and is a caution against the idea that all thin films are automatically cheaper at the system level than wafer x-Si technologies. Such a system cost advantage can be tenuous or absent, and only a full exploitation of the potential advantages of thin films in real designs can lead to success in the marketplace; and more importantly, to a cost low enough to meet climate change and fossil fuel depletion needs on a global scale. Fortunately, several thin films (CdTe and a-Si products) are already competitive in the marketplace, so many challenges have been overcome.
Thin films share enough costs in common (BOM from encapsulation, contacting, and other shared costs; balance of system costs extrinsic to them) that efficiency is a very important parameter for defraying these shared costs (Von Roedern 12005).
