Development of Lithium-Doped Radiation-Resistant Solar Cells

In the middle 1960s it was discovered that the addition of lithium to n-base silicon solar cells resulted in what appeared to be annealing of radiation-induced defects. For the past five years JPL has been intimately involved in an effort to exploit this phenomenon in order to develop a highly radiation-resistant, high-efficiency silicon solar cell. This paper discusses the results of the investigations carried out which represent major achievements with respect to attaining this goal. Lithium-doped solar cells have now been fabricated with initial lot efficiencies averaging 11.9!? in an air mass zero (AMO) solar simulator and a maximum observed efficiency of 12.8%. The best lithium-doped solar cells are approximately 15% higher in maximum power than state-of-the-art n—p cells after moderate to high fluences of 1-MeV electrons and after 6-7 months exposure to low flux (approx 10 e/cm/day) irradiation by a Sr beta source, which approximates the electron spectrum and flux associated with near-Earth space. Furthermore, lithium-doped cells were found to degrade at a rate only one tenth that of state-of-the-art n-p cells under 28-MeV electron irradiation. Excellent progress has been made in quantitative predictions of post-irradiation current—voltage characteristics as a function of cell design by means of capacitance—voltage measurements, and this information has been used to achieve further improvements in lithium-doped cell design. Major improvements in cell processing have also been achieved, resulting in higher cell efficiency and greater reproducibility. JPL TECHNICAL REPORT 32-1574 vii Development of Lithium-Doped Radiation-Resistant