Grain boundary effects in poly‐Si as determined by low frequency capacitance and conductance data

The dependence of MIS capacitance (C_M) and conductance (G_M) on dc bias (U) at various frequencies (20 Hz to 50 kHz) is measured for MIS devices on the grain, grain boundary, and multiple grain boundaries for p‐type silicon. Significant differences in C_M–U and G_M–U are seen when comparing devices with and without grain boundaries. The effective interface state density (N_ss) is calculated from the above data. At mid‐gap, N_ss = 1.5 × 10¹¹ cm⁻² eV⁻¹ without a grain boundary, whereas N_ss = 4 to 8 × 10¹¹ cm⁻² eV⁻¹ when grain boundaries are present. This can account for the 50 mV reduced U_oc on solar cells made from polycrystalline Si. This technique is also a useful tool for evaluating effects of grain boundary passivation.