Analysis of the photo voltage decay (PVD) method for measuring minority carrier lifetimes inPhyphen;Njunction solar cells

摘要

When the diffusion length of minority carriers becomes comparable or even larger than the thickness of aPhyphen;Njunction solar cell, the characteristic decay of the photogenerated voltage becomes a mixture of contributions with different time constants. The minority carrier recombination lifetime tgr; and the time constantl2/D, wherelis essentially the thickness of the cell andDthe minority carrier diffusion length, determine the signal as a function of time. It is shown that for ordinary solar cells (N+hyphen;Pjunctions), particularly when the diffusion lengthLof the minority carriers is larger than the cell thicknessl, the excess carrier density decays according to exp(minus;t/tgr;minus;pgr;2Dt/4l2), tgr; being the lifetime. Therefore, tgr; can be readily determined by the photo voltage decay (PVD) method onceDandlare known. Foridealbackhyphen;surfacehyphen;field (BSF) cells (N+hyphen;Phyphen;P+junctions) under the same circumstances the excess number density of carriers decays purely exponentially as exp(minus;t/tgr;). However most BSF solar cells are not ideal, possessing an effective surface recombination velocityseffof 100 to 1000 cm/sec at the highhyphen;low junction. Therefore, PVD measurements for BSF cells must be treated with caution and must be supplemented with other nonstationary methods recently developed. These facts are important for a determination of diffusion lengths sincesteadyhyphen;statemethods are notoriously unreliable when the cell thickness is smaller than the diffusion length. Finally a connection will be made with older work on the same subject.