Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration N d , grain size d and interface state density N is at the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential V d adjacent to the boundaries, and ranges from \simeq 10^{2} to 10 6 cm . s -1 depending on N is and N d . Under illumination, the population of the interface states is altered considerably from its dark level and as a result, V d decreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing N d . Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities of \simeq 10^{11} cm -2 eV -1 , τ decreases from 10 -6 to 10 -10 s as the grain size is reduced from 1000 to 0.1 µm (for N_{d} = 10^{16} cm -3 ). For a constant grain size, τ decreases with increasing N d . The open-circuit voltage of p-n junction solar cells decreases for \tau \leq 10^{-7} s, whereas that for Schottky barrier cells remains at its maximum value until \tau \lsim 10^{-8} s.