Radiation Damage in Silicon P-I-N Lithium-Drift Solar Cells

The effect of electron, proton and neutron irradiation of silicon lithium-drift P-I-N diodes is given in terms of optical and x-ray response changes. All diodes used for this study were fabricated from high oxygen content Czochralski silicon. The long term compensation stability exhibited by these diodes is attributed to the decreased effective lithium ion mobility in this material. The junction capacitance changes as a result of irradiation indicate that, in addition to the usual minority carrier lifetime degradation, there is also a redistribution of the lithium ions within the drifted region. This redistribution has the form of an increased lithium gradient at the junction, resulting in a narrower depletion width. The twofold effect of a reduction in active volume together with a degradation of carrier lifetime explains the experimentally observed result that the lithium-drift P-I-N diodes are more susceptible to radiation damage than the P-N structure. The experimental data also indicate, however, that for operation in a non-nuclear environment the P-I-N structure can be made more efficient than the P-N structure. This is particularly true for photons having energies near the cutoff energy of 1.1 ev or above 100 kev where photoelectric cross sections are insignificant and compton processes predominate.