Anomalous temperature dependence of diode saturation currents in polycrystalline silicon thin-film solar cells on glass

Temperature dependent Suns-V-oc measurements are performed on four types of polycrystalline silicon thin-film solar cells on glass substrates, all of which are made by solid phase crystallization/epitaxy of amorphous silicon from plasma enhanced chemical vapor deposition or e-beam evaporation. Under the two-diode model, the diode saturation currents corresponding to n=1 recombination processes for these polycrystalline silicon p-n junction cells follow an Arrhenius law with activation energies about 0.15-0.18 eV lower than that of single-crystal silicon p-n diodes of 1.206 eV, regardless of whether the cells have an n- or p-type base. This discrepancy manifests itself unambiguously in a reduced temperature sensitivity of the open-circuit voltage in thin-film polycrystalline silicon solar cells compared to single-crystal silicon cells with similar voltages. The physical origin of the lowered activation energy is attributed to subgap levels acting either as minority carrier traps or shallow recombination centers.