cells on board the LIPS III satellite. We also include recent significant developments in other countries, notably in Japan. Background Most of the early research on InP solar cells was concerned with their possible use in the terrestrial environment [7]. Since no radiation damage data emanated from the terrestrial program, there was little or no interest in developing these cells for use in space. However, in 1984, it was demonstrated in Japan that InP solar cells had radiation resistance under 1 MeV electron irradiation which was significantly greater than that exhibited by either Si or GaAs [8, 9]. Subsequent to this, the superior radiation resistance of InP was demonstrated under proton irradiation [10]. In addition, it was oberved that radiation damage in InP could be annealed at low temperature and under the influence of light [3, 8], The major problems encountered at this relatively early stage of development were the low efficiencies obtained and the relatively high cost of the InP wafers. To date, significant progress has been made in achieving relatively high efficiencies and a modest beginning has been made to solve the problem of cost. Cell Research and Development The major emphasis in InP solar cell research since the discovery of its high radiation resistance has been largely concerned with increasing cell efficiency and determining its properties in laboratory radiation environments. Figure 1 shows the progress attained in increasing cell efficiency since 1984. As one can see from the figure, the highest AMO total area efficiency achieved has risen from 13% in 1984 to 18.8% by the end of 1987 [11]. The progress toward higher efficiency has followed from increased experience and refinement of processing techniques. In 1984, the best cells were processed in Japan by closed tube diffusion [12], Over the course of this program, other processes were investigated. These principally included open tube diffusion, liquid phase epitaxy and metal organic chemical vapor deposition (MOCVD) [13, 14, 15]. The geometrical configuration of the best cell, which attained an efficiency of 18.8%, is shown in Fig. 2 [15]. In attaining this efficiency, an addition to the MOCVD process entailed formation of the silicon doped w-region by ion-implantation [15]. Noting that AMO efficiencies of over 21% are predicted by computer modeling calculations [5], the highest efficiency achieved represents an encouraging step towards this goal. A comparison of achieved and predicted efficiencies for InP, GaAs and Si is shown in Fig. 3, where the solid line represents theoretical AMO efficiencies due to an earlier calculation by Loferski [16], From the figure, the predicted maximum efficiencies for GaAs, InP and Si are 23% 22% and 19%, respectively. Both silicon and GaAs efficiencies achieved to date are close to their maxima [17, 18], while for InP there is obviously more room for improvement. It should be noted that there have been approximately 30 years of R&D on silicon solar cells and 19 years on GaAs cells. Since only six years have been expended on InP space solar cell research, it is anticipated that with continued research InP solar cells will achieve efficiencies much closer to the maximum value shown. The two efficiencies shown for silicon require further comment. The highest efficiency shown refers to a cell processed from low resistivity silicon which degrades excessively under irradiation [2]. On the other hand, the lower efficiency shown for silicon refers to efficiencies achievable in production for higher resistivity silicon solar cells. These latter cells exhibit greater inherent radiation
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