Background

Thin film PV technologies face a number of hurdles as they advance towards low-cost goals that are competitive with traditional sources of electricity. The US Department of Energy cost goal for thin films is about $0.33/Wp, which is based on a module efficiency goal of about 15% and module manufacturing costs of about $50/m2. This paper investigates the issues associated with achieving the $50/m2 goal based on opportunities for manufacturing cost reductions. Key areas such as capital costs, deposition rates, layer thickness, materials costs, yields, substrates, and front and back end costs will be examined. Several prior studies support the potential of thin films to reach $50/m2. This paper will examine the necessary process research improvements needed in amorphous silicon, copper indium diselenide, cadmium telluride, and experimental thin film silicon PV technologies to reach this ambitious goal. One major conclusion is that materials costs must be reduced because they will dominate in mature technologies. Another is that module efficiency could be the overriding parameter if different thin films each optimize their manufacturing to a similar level.
Introduction

PV module sales have been growing at an average 20% annually for a number of years. Worldwide module sales in 1998 are estimated to be about 150MWp, or about $600M for modules and over a billion dollars for PV systems. However, these sales are for high-value applications that do not directly compete with commodity electricity available from utilities. For PV to become competitive for energy-significant uses, the cost of modules in $/Wp must fall substantially. Today, PV modules are sold at about $3-$5/Wp. PV systems are priced at about twice this figure. Various studies suggest that system prices near $3/Wp will be needed to open rooftop and other distributed markets in the United States. Indeed, system prices in the range of about $1-$3/Wp for PV systems will be a great benefit in the competition for most energy-significant markets. International markets may be less price sensitive. PV sales will continue to grow for high-value markets while new markets will open up as PV module and system prices fall. This continuum of market growth has been an advantage of PV, because it allows substantial business cash flow well before energy significance is reached.

To improve module prices, progress is needed on three fronts: the performance of the modules (efficiency, or W/m2), their direct manufacturing cost ($/m2), and increased volume production. It is direct manufacturing cost ($/m2) and some effects of volume production that are the focus of this paper.

The simple combination of cost per square meter divided by output per square meter yields the key parameter, $/Wp used as a measure of PV cost-effectiveness. Table 1 shows for thin films some likely future combinations of direct manufacturing cost and efficiency in order to indicate the multiple paths by which low module cost in $/Wp can be approached. The highest cost ($3.3/Wp) represents about where we are today in commercial thin films. The lowest cost, which is a combination of 15% module efficiency with $50/m2 direct manufacturing cost, represents the US Department of Energy's long-term goal for thin film modules. It is about a tenfold reduction, which will require substantial progress. Actually, any combination of cost and performance below about $0.5/Wp would represent a good approximation of this low-cost goal. Despite its importance, the predominant emphasis of this paper will not be on PV module efficiency. This paper will be on direct module manufacturing cost and efficiency will only be of concern in that it must be maintained or enhanced despite strategies that reduce manufacturing cost.

Table 1. Direct Manufacturing Cost ($/m2) and Efficiency Determine Module Cost in $/Wp

$200/m2


$150/m2


$100/m2


$50/m2

$3.3


$2.5


$1.7


$0.8

$2.5


$1.9


$1.35


$0.63

$2


$1.5


$1


$0.5

$1.7


$1.25


$0.83


$0.42

$1.3


$1


$0.67


$0.33

With progress toward low cost, new energy-significant markets should open up for PV. Examples of such markets are: residential and commercial rooftops, building facades, parking lot structures, distributed utility grid support, centralized PV for daytime loads, hybrid systems for decentralized rural electrification, hybrid systems (e.g., with gas-powered fuel cells or microturbines) for distributed power networks, and PV with storage (compressed air) or PV for hydrogen production. By contributing energy on this scale, PV can fulfill its promise to contribute to the betterment of the world's environment (e.g., better air quality and reduced carbon dioxide) while reducing high risks associated with international energy supply tensions resulting from more traditional sources.
Today's Thin Film PV Modules

There is little solid information available about thin film manufacturing costs. Three thin films are commercially available, but only in small quantities (less than 10 MWp/year of the 150 MWp/yr market). As such, details of cost status and cost reduction have been based on estimates and models. Indeed, no thin film technology is nearly optimized for low cost. Therefore, projections are also needed. Despite this, quite a bit can be understood from previous studies and from some detailed information available from certain manufacturers. Table 2 summarizes the commercial status of three thin film technologies covered in this paper. Thin film silicon is also covered but is in early developmental stages at several companies, including AstroPower.