Topics of the R&D activities at AEG-TELEFUNKEN are shown in Fig. 2: • development and qualification of 100 gim thick solar cells • thickness reduction to 50 gm • 5x5 cm2 high efficiency cells. Samples of 100 and 50 gm thick solar cells have an electrical BOL conversion efficiency of 13%-14%. Their relative power degradation is typically smaller when they are subjected to the geostationary particle environment. Both cell types are equipped with a back-surface-field (BSF) and an additional back-side-reflector (BSR). The BSF characteristic makes the electrical conversion efficiency nearly independent from the cell’s thickness. The back-side-reflector reduces the operation temperature of the cell by approximately 10°C. It makes the cells more sensitive to IR-light and increases the operation efficiency (efficiency at lower temperature) by about 5% relative. As can be seen from Fig. 2 the development plan follows two lines. Based on the actual technology with 200 gim thick high-efficiency-cells (HEC), the power/weight ratio shall be increased in two steps: the 2x4 cm2 HEC with a thickness of 100 gun shall be space qualified in 1981 to satisfy the requirements of future multi-kW- communication satellites. The efforts will continue with the development of ultrathin 50 gim HEC’s. A cost reduction in cell fabrication and array manufacture shall be achieved by large area, 5x5 cm2 solar cells. The cost-effectiveness of this type of cell has been demonstrated in terrestrial photovoltaics and shall be utilized for space programs. The goal of the current development program is to transfer the 2x4 cm2 HEC technology to the 5x5 cm2 cells by introducing • multifinger-grids • shallow n/p-junction • BSR and BSF technology • reduced cell thickness. The presentation of the present technological status and current development activities has shown that several elements of the SPS solar array requirements are the subject of running R&D programs. A lightweight blanket technology with weldable, large area and thin solar cells will be available in the near future. However, there remain two severe problem areas which need more attention in the future in order to make the SPS feasible. These problem areas are • end-of-life conversion efficiency • mass-production and cost and shall be discussed in the next sections. CONVERSION EFFICIENCY The reference system is based on a solar cell efficiency of 17.3% at the end of life after 30 years. Present production cells have a typical BOL efficiency of about 13%, and approximately 40% of the initial power will be degraded in the geostationary particle environment. Maximum efficiencies obtained in the laboratory with 50 gim thick cells are approximately 14%. Since the theoretical efficiency limit is approximately 22% for silicon solar cells it can be expected that a further improvement of cell efficiency is not unlikely. Whether or not the required 17% can be achieved within the next 10-20 years is speculation. A review of the last 15 years in solar cell research shows that the cell
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