Photovoltaics International Papers

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.

The rapid expansion of high volume manufacturing to meet growing demand in recent years has highlighted the development of increasingly higher throughput machines, especially in the critical bottleneck process of module assembly, specifically characterised by tabbing and stringing steps. Significant productivity improvements have come about with the development of integrated, highly-automated tabber and stringers from a range of equipment vendors. However, module assembly remains the most expensive step in conventional c-Si cell production. Equipment suppliers are also challenged to meet the evolving demands of processing thinner wafers and to address overall production cost reduction strategies while meeting yield/throughput goals that are seen as a significant enabler of reducing the cost per watt.

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.

News of credit crunch woes filtering down the lines over the past few months has instilled a sense of frugality in all industry sectors. While the credit crisis has indeed affected the PV industry, German banks, investors and creditors have claimed that the financing of small PV systems in the private sector seems not to be endangered. Downstream players are still optimistic, but the upstream sector is anticipating severe damage as a result of the economic situation. An interactive workshop-style discussion, hosted by the German market researcher EuPD Research and its consulting division 360Consult, invited top-level executives to contribute their experiences of the current financial situation, as discussed 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.

The U.S. residential solar market is poised for growth. For solar companies seeking to capitalize on the growth potential of this market, the keys to success will be sales volume and operating efficiency. Solar employee purchase programs (solar EPPs), which have been initiated by companies as diverse as SunPower, REC Solar, and SolarCity, represent a new and potentially important channel for increasing sales and improving sales efficiency. Driving these programs are increasing corporate sustainability initiatives and growth in voluntary employee benefit offerings, especially employee purchase programs and green benefits. This article provides an introduction to solar employee purchase programs, analysis of the business ecosystem, and discussion of an example program. It is based on the industry’s first report to identify and analyze this emerging trend, which was published by AltaTerra Research in November 2008 [1].

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 any solar cell process, the metallization step is critical as it often sets conditions and limitations for the other process steps. The main metallization technique used today in Si solar cell production is screen-printing of metallic pastes, namely Ag pastes for the front side, Al pastes for most of the rear side, and Ag or Ag-Al pastes for the solder pads at the rear. While these techniques are clearly robust and convenient, they have limitations. Therefore alternatives are being investigated. A technique that is presently finding its way into production is two-step metallization with Ag plating. Another more radical approach is to avoid printing altogether, instead using some kind of ablation followed by plating. For the rear, the full Al-BSF is being replaced by dielectric passivation and local Al-alloyed contacts. Back-contacted cells are increasingly being introduced in production, and they pose very specific challenges to metallization. For the sustainability of Si photovoltaics, it is crucial that the future metallization solutions only make use of abundantly available and non-toxic materials.

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.