Analysis of depletion-region collection in GaInNAs solar cells

We provide qualitative insight into depletion-region collection in GaInNAs cells to (1) understand the effect of diffusion length L on the QE; and (2) describe the magnitude of L required to get adequate current from the cell. We use Wolf´s equations for the QE including a drift field E, and model E as being equal to the junction built-in voltage distributed uniformly across the depletion region. This allows us to calculate the QE as a function of L and depletion width W_D. We show that if L is sufficiently small, increasing W_D can actually decrease the QE. To determine how long L needs to be in a practical GaInNAs junction, we calculate from the QE the short-circuit current density as a function of W_D and L. This allows us to estimate that L_ambipolar needs to be greater than roughly 1 μm in order to obtain enough photocurrent for the 4-junction application, giving guidance to the experimental effort to develop such cells.