Thin Film

This paper, the third in a series covering cost of ownership (COO) studies for photovoltaics [1], examines the need for metallization of silicon-based solar cells and how it has evolved over the past few years. The technologies and techniques that are being developed for this part of cell manufacturing in the foreseeable future are also discussed. The paper will conclude with a COO case study using the DEK Solar PV3000 as an example.

Until the year 2002, wafer-based crystalline silicon solar cells were almost exclusively the solar cell technology used for large-scale power plants. Since then, steady growth in the market share for thin-film technologies has been observed, although crystalline silicon technology still remains the most important solar cell technology used in large-scale PV power plants. The market share of thin-film modules, especially CdTe modules, has been continuously increasing in recent years, most notably in the German market. However, other countries like Spain, the USA, Italy and France have seen some large-scale CdTe-based modules being installed in power plants recently.

Outside of the challenges of fabricating state-of-the-art photovoltaic devices, further care must be taken to package them such that they can withstand environmental conditions for an accepted lifetime of 20-plus years. Moisture ingress is a big adversary to hermetic packaging. The diffusion of water through barriers and edge seals can be minimized by careful choice of materials and package/barrier architecture. However, at present, there exist no solutions for extremely water-sensitive materials for flexible applications. Presented in the following is a review of the physics of permeation, the means of measuring permeation, current architectural strategies for semi-hermetic packages, and a brief evaluation of some common encapsulant materials.

Among the various thin-film solar module options available, CIGS is especially interesting as it exhibits the highest efficiency potential. These chalcopyrite-based solar cells are manufactured on glass or flexible substrates using various thin-film coating methods for each layer. The central CIGS absorber layer is deposited by co-evaporation, selenization of elemental layers, and other methods. In order to achieve highest quality and reproducibility, the absorber properties must be properly monitored and characterized. In this contribution we shed some light on the most important analysis methods used for CIGS solar cell research, development, and production such as x-ray fluorescence, surface analysis, and Raman spectroscopy.

At First Solar’s corporate headquarters in Tempe, Arizona, a morale-boosting slogan adorns posters stuck to the outside of cubicle partitions: “MILESTONE MADE! TEN ONE ONE.” That’s “Ten,” for 10 years in business - at least in the company’s First Solar incarnation. The original firm Glasstech Solar, led by visionary Harold McMaster, actually set up shop in 1984, then became Solar Cells, Inc. in 1992, which begat the present entity in 1999. The middle “One” stands for the gigawatt’s worth of panels produced in the solar module factories in Ohio, Germany, and Malaysia - as well as the annual production capacity that will be ramped by the end of 2009. The final “One” stands for perhaps the biggest accomplishment of all - the dollar-per-manufactured-watt standard beaten by two cents by First Solar in the final quarter of 2008, a cost that has since shrunk to 93 cents per watt in the first quarter of 2009. But then, “Ten/One/0.93” doesn’t quite have the same ring.

A variety of thin-film technologies are now entering a volume manufacturing phase. The benchmark has already been set by First Solar, Inc. in its conversion efficiencies, volume ramp and lowest cost-per-watt in the PV industry. Large-area thin-film deposition is a critical process step, dictating cell performance, reliability and manufacturing throughput. However, adoption of thin-film solar cells has been limited in the past by relatively complex and costly manufacturing processes. The advent of rotating cylindrical magnetrons for sputtering is demonstrating the potential to significantly reduce thin-film manufacturing costs. In this paper we discuss the basics of the technology and the developments taking place with some of the leading suppliers of sputtering target technology for the PV industry.

Despite the low-cost, high-efficiency, radical form factor promise of many thin-film photovoltaic technologies, scaling these materials to large-volume production has presented a wide array of challenges. Because of the recent polysilicon shortage, an incredible amount of resources have been focused on this goal and many thin-film alternatives are now available. One of the most intriguing of these materials, copper indium gallium diselenide (CIGS), has great potential to reset the thin-film market and make new applications cost effective and viable. CIGS technology is differentiated from competing PV materials by a combination of factors. The manufacturing cost of thin-film cells can be very inexpensive since they require few raw materials and can be made with an efficient, scalable roll-to-roll process. CIGS has been established as the most efficient thin-film technology in converting sunlight into electricity. A flexible substrate will ultimately enable energy and building-integrated applications beyond the capability of rigid, heavier PV products.

Transparent conducting oxides (TCOs) are a special class of materials that can simultaneously be both optically transparent and electrically conducting and, as such, are a critical component in most thin-film photovoltaics. TCOs are generally based on a limited class of metal oxide semiconductors such In2O3, ZnO and SnO2, which are transparent due to their large band gap energy and can also tolerate very high electronic doping concentrations to yield conductivities of 1000S/cm or higher. However, these thee basic TCOs alone do not meet the TCO performance needs of emerging PV and other applications.

In recent years, a new generation of solar electric products has emerged from the lab into the global market: thin-film technologies that employ approximately 1% of the active, expensive photovoltaic material used by standard crystalline-silicon cells. Through a combination of cost advantages and new product applications, CdTe, a-Si and CIGS thin-film PV have the potential to foster a paradigm shift toward distributed electricity generation at cost parity with other forms of energy. But until recently, the photoactive compound has not had a reliable, rapid manufacturing process that could scale effectively to multi-megawatt-scale volume production and provide significant amounts of electricity at the point of use. This article describes a novel process, known as field-assisted simultaneous synthesis and transfer (FASST) printing, a manufacturing approach that enables the rapid printing of microscale CIGS films with p- and n-type nanodomains that are critical for achieving the highest efficiencies possible.

Laser-based tools have become increasingly visible within R&D labs, pilot production lines, and as the preferred technology used by many turnkey suppliers. As equipment types however, relatively little is known about the differences in the laser-based tools used for solar applications within each of the c-Si and thin-film segments. This paper explains the key components of a laser-based tool, and how they are adapting to meet the demands from next-generation production line equipment required by the solar industry.