Material Properties RAD layers have a polycrystalline texture similar to that of most ribbons except the WEB ribbon [18], They are essentially flat except at grain boundaries where grooves a few micrometers deep may occur. The grains elongated in the pull direction and continuous down to the substrate are separated by quasi-vertical boundaries. The grain width is typically in the millimetre range. In most cases, the grains have a common < 110> direction close to the normal to the surface; intragrain defects are primarily [111] vertical twin boundaries aligned with the pull direction and dislocations heterogeneously distributed. Bulk and planar defects adversely affect the conversion efficiency of the devices in different ways. The recombination rate of minority-carriers large at grain boundaries is small and eventually null at twin boundaries. Grain boundaries degrade the opencircuit voltage (Roc) of the devices by introducing short-circuit resistances in the diode region whereas they are virtually ineffective on the photocurrent density (Jph) [19], Thus, Jph in RAD layers is primarily dominated by the minority-carrier diffusion length, Ln, between twin boundaries. Ln is limited there by both the presence of dislocation lines and a large density of distributed minority-carrier traps. The performances of solar-n+/p/p+ homojunctions - made on 1 to 2Q.cm p-type RAD layers by conventional techniques can be dramatically improved by means of low-temperature annealings and/or hydrogen passivation treatments [20, 21]. The best AR-coated cell thus treated had an AMI conversion efficiency of 15.5% with the following characteristics: Jph = ?>'l mA/cm2, Foc = 560 mV and fill factor, FF=0.75. Adaptability of the RAD Process for Space Cell Applications Thus far, the capability of the RAD process has been deliberately oriented towards the low-cost needs of terrestrial photovoltaics. However, the process presents intrinsic and unique properties which appear to be compatible with large-scale space applications. First, it can produce in a continuous operation flat sheets about 50 pm thick and yield large-area solar cells - typically 5 X 5 cm and more if required. Second, its low- cost background may not be considerably amended if exploited for space applications. Third, it may be well adapted for the growth of gallium-doped sheets owing to the relatively large values of the effective segregation coefficients of impurities in this process [15]. The actual drawbacks of this ribbon approach are in the insufficient performances of the solar cells. However, there exists a considerable potential for a substantial improvement of these performances which has hardly been exploited so far. An immediate improvement should follow from: • the optimization of the cell structure with the realization of e.g., an efficient back-surface field (BSF structure); • the reduction of the impurity content, especially heavy metals, in the carbon ribbon by means of a more efficient purification in chlorine gas. Other large improvements are expected from the control and optimization of processes, still on a research or developmental stage, which tend to neutralize the activity of recombining centres, e.g.: • low-temperature annealings; • hydrogen passivation; • impurity gettering.
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