efficiencies depend on the production method and the price that the customer is willing to pay for the delivered cells. For one thing, the yield of acceptable cells usually decreases as the efficiencies required by the customer increases. Hence, the efficiencies quoted for production cells are usually less than those achieved in a research environment. Performance in Space 1. The LIPS III Flight Experiment The LIPS III spacecraft was launched, in the spring of 1987, into a nearly circular 60+ degree orbit whose altitude was 1100 km. ‘LIPS’ is an acronym for ‘living plume shield’. The main purpose of the shield lies in protecting the satellite’s primary payload from rocket engine plumes. Since one surface of the plume shield is unaffected by the plumes, a variety of solar cell experiments were mounted on the unexposed surface. Participants include groups from the USA, England, France and West Germany. Further details are contained in publications by the Naval Research Laboratory which is responsible for the satellite [25, 26]. The planned mission lifetime is three years; however, it is hoped that this can be extended to five years. There are two types of InP cells on board the satellite. One experiment, under the aegis of NASA Lewis, contains a four-cell module composed of monolithic n/p InP solar cells. The second experiment, by the Royal Aircraft Establishment, Farnborough, Hants, UK, includes ITO/ InP cells produced by the research group at Newcastle upon Tyne Polytechnic. At present, we have no details from the RAE flight experiment. Hence, we confine ourselves to discussion of the NASA Lewis experiment. The n/p homojunction cells used in our experiment were fabricated at the Renselaer Polytechnic Institute using the open tube diffusion process [13]. Fifteen cells were received, on extremely short notice, with AMO efficiencies ranging from 11.4 to 14.3%. The two highest efficiency cells were retained as standards while two four-cell modules were assembled by Spectrolab [27]. One module was placed on board the satellite, the other being retained as backup. A photograph of the completed four-cell module is shown in Fig. 6. A platinum resistance thermometer was attached to a fifth cell which was used solely for temperature sensing. During more than one year in orbit, module temperatures varied between 1° and 34°C. No 1 MeV damage equivalence data exists for InP. However, a rough estimate concerning the effects of the space radiation environment can be obtained by using 1 MeV damage equivalents for Si [28]. Noting that the present LIPS InP cells are covered with 12 mils of CMX microsheet cover glass, the 1 MeV electron damage equivalent fluence for silicon is 3.5 X 1013/cm2 per year for the LIPS orbit [28], Further details of the Lewis module are contained in Ref. 27. Summaries of flight data obtained after more than one year in orbit are shown in Figs 7 and 8. The plot of short circuit current versus time in orbit (Fig. 7) shows essentially no degradation in this cell parameter after 370 days. The voltage-current data shown in Fig. 8 is more complex. It is noted that cell maximum power on this curve occurs at approximately 0.7 V. Examination of the curves indicates no degradation in Pmm when compared to pre-flight simulator measurements at Lewis. However, the currents at voltages below the maximum power point are uniformly low with respect to the pre-flight simulation. In addition, significant variation in the data occurs near the maximum power point. Further analysis is required to determine if the variation is influenced by the data acquisition system or is inherent in the cells.
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