A question sometimes posed to researchers in silicon is: Why are you still working on silicon? How much more do we need to learn? Not only is Si a relatively simple element, it is one of the most common in the earth's crust. This element has been very extensively studied. One reason we keep studying silicon is because it is not just an element, it is also an industry. The microelectronics business has a massive economic impact on our lives. From cars to washing machines, almost all consumer goods contain silicon microprocessors in some form. Our daily lives depend on the stuff. Figure 1 shows the predominance of Si in a breakdown of the microelectronics industry. In 1994 microelectronics has been a $1 × 1011-per year industry worldwide, and it is still growing rapidly. CMOS, or complementary metal-oxide-semiconductor transistors, dominate this business, some being bipolar. The entire compound semiconductor industry is a small part of this picture, composing 2% of the business (although $2 billion is not bad for a “niche” market). The $100 billion business built around silicon makes it the most important element in world economics, although there is some evidence that C and O may also be important for the support of our daily lives. One thing, however, is clear-the economic need to understand silicon and everything it does is tremendous.

A variety of performance indicators have been employed by the photovoltaic (PV) community to rate the performance of PV cells, modules, and arrays. The PV power conversion efficiency under a set of standard reporting conditions serves as a basis for meaningful comparisons of PV performance, within and among PV technologies. PV efficiency is defined as 100 times the maximum PV electrical power density produced divided by the incident light power at a reference PV temperature, total irradiance, and spectral irradiance. Standard reporting conditions commonly used by the PV community are summarized in Table I.

The direct and global reference spectra were generated by a comprehensive Monte-Carlo computer model and actually integrate to 768 W m−2 and 964 W m−2. The terrestrial PV community has arbitrarily assigned a 1-sun total irradiance of 1,000 W m−2 for the global and direct reference spectra. The term “direct” in Table I refers to the direct-normal (5° field of view about the sun) component of the global spectral irradiance distribution. The term “global” in Table I refers to the spectral irradiance distribution on a 37-tilted south-facing surface with a solar zenith angle of 48.2° (AM1.5). The terms AM1 or AM1.5 are often used to refer to standard spectra, but the relative optical air mass (AM) is a geometrical quantity and can be obtained by taking the secant of the angle between the sun and the zenith.