Technical Papers

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.

The principal paths to cost reduction for the photovoltaics industry are increasing the efficiency of solar cells and the power density of modules, together with the reduction of the specific consumption of silicon. Following the slowdown in the ever-increasing growth of the PV market earlier this year, and the reduction in the market cost of polysilicon, wafer producers and most cell producers moved back to the 180µm generation substrates. It may take some time for manufacturers to tackle the technological issues that need to be addressed in order to successfully decrease wafer thickness further. In this article, some of the issues related to the production of thinner and thinner cells are outlined and discussed.

In order to stimulate the economy and create jobs, the bill includes over US$6 billion in loan guarantees for renewable energy projects, solar in particular. Industry representatives have estimated that the bill will create 67,000 jobs in the solar power sector this year and a total of 119,000 jobs over the next two years.

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.

Crystalline silicon solar cell fabrication involves many wet chemical process steps. Like most processes in solar cell manufacturing, many of these wet chemical processes were transferred from the semiconductor industry. In contrast to microchip fabrication with maximum throughputs of 100 wafers/hour, state-of-the-art solar cell equipment relies on several 1000 wafers/hour. Furthermore, specific processes have been developed for the texturisation of the wafer surface. Therefore, there is a need for dedicated methods of characterization of these wet chemical processes. Fraunhofer ISE has developed several analytical methods such as titration, ion chromatography and near infrared (NIR) spectroscopy for the complete analysis of the chemical composition of wet chemical processes baths. These methods were compared considering the inline/online capability, measurement cycle and running costs, with the result that NIR spectroscopy was identified as a complex but very powerful tool for process characterization, as outlined in this paper.

Glass has been playing an ever more important role in photovoltaics, and with the increasing demand for solar modules, the glass industry will be pushed even more to the fore. As a result, the photovoltaics industry is fast becoming a field of business of increasing importance for some of the glass industry’s sectors. Mechanical engineering companies around the world are working to meet the demands of the solar industry, with the tremendous potential of glass, especially in the thin-film sector, at the epicentre of this effort. This paper presents the beneficial properties of glass for use in the photovoltaics industry, and the material’s potential for future applications.

Invented in their high efficiency version in the early 1990s, dye-sensitised solar cells (DSCs) entered the global market in 2007 with the first commercial modules based on this versatile, hybrid (organic-inorganic) technology. The 6-7% efficiency of the first modules is a result of their good performance in diffuse light conditions, allowing for the production of electricity both under cloudy conditions and indoors. These low-cost solar cells are manufactured by highly productive roll-to-roll printing methods over rigid or flexible substrates affording modules coloured in widely different tones. These attributes render DSC a photovoltaic technology particularly well suited for BIPV applications and for electrification in developing countries, as discussed in this paper.

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.

Formation of the pn-junction for charge carrier separation is one of the key processes of a modern high-volume solar cell production. In silicon wafer-based solar cell technology this is achieved by diffusion of phosphorus atoms in boron pre-doped wafers forming a sub-micron shallow n-type emitter in a 200µm-thick p-type base. In this contribution we discuss both the characteristics of emitter doping profiles and the diffusion process itself as required for optimal solar cell conversion efficiencies. In addition we give an overview on state-of-the-art industrial diffusion technologies and conclude with a brief outlook on their evolution.

This review is based on primary research of global solar cell and thin-film manufacturing companies that are either currently manufacturing, expanding manufacturing, building facilities for manufacturing or progressing towards establishment of manufacturing facilities.