In 2008, the world annual production of photovoltaic (PV) cells reached more than 7.9 GW p (W p , peak power under standard test conditions) [1], and the average annual growth rate in PV cell production over the last decade has been more than 40%. Yet the electrical power generated by all PV systems around the world has been estimated to be less than 0.1% of the total world electricity generation [1]. Nevertheless, the strong growth in PV cell production is expected to continue for many years. Crystalline silicon PV cells, with over 60 years of development, have the longest production history and now account for the largest share of production, comprising up to 90% of all the solar cells produced in 2008 [1]. Silicon is safe for the environment and one of the most abundant resources on Earth, representing 26% of crustal material. Th e abundance and safety of silicon as a resource grants the silicon solar cell a prominent position among all the various kinds of solar cells in the PV industry. World annual PV cell production of 100 GW p is expected to be achieved by around 2020, and the silicon PV cell is the most viable candidate to meet this demand from the point of view of suitability for large-volume production.Th e crystalline silicon PV cell is one of many silicon-based semiconductor devices. Th e PV cell is essentially a diode with a semiconductor structure (Figure 1), and in the early years of solar cell production, many technologies for crystalline silicon cells were proposed on the basis of silicon semiconductor devices. Th e synergy of technologies and equipment developed for other silicon-based semiconductor devices, such as large-scale integrated circuits and the many diff erent kinds of silicon semiconductor applications, with those developed for PV cells supported progress in both fi elds. Process technologies such as photolithography helped to increase energy conversion effi ciency in solar cells, and mass-production technologies such as wire-saw slicing of silicon ingots developed for the PV industry were also readily applicable to other silicon-based semiconductor devices. However, the value of a PV cell per unit area is much lower than that for other silicon-based semiconductor devices. Production technologies such as silver-paste screen printing and fi ring for contact formation are therefore needed to lower the cost and increase the volume of production for crystalline silicon solar cells. To achieve parity with existing mains grid electricity prices, known as 'grid parity', lower material and process costs are as important as higher solar cell effi ciencies. Th e realization

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Ultrathin silicon (Si) solar cells for space application were fabricated on an experimental basis and the electrical characteristics were investigated for three kinds of cells (Black, BSFR and Conventional cells). Under 135.3 mW/cm2 (AMO) illumination, ultrathin Black cells showed 67.7 mW output, which is equal to 89% output of 280 µm Black cells. The power to mass ratio of bare ultrathin Black cells was 3.6 times high compared with 280 jum thick Black cells. 1 MeV electron irradiation test was carried out to evaluate the radiation resistance. Ultrathin cells showed superior radiation resistance compared with that of 280 µm thick cells, and it was comparable to that of a GaAs solar cell which had been recognized as a radiation resistive cell. Our experiments suggest that the ultrathin solar cells have high potential to be used for space application.

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