Origin of the Tunable Carrier Selectivity of Atomic-Layer-Deposited TiOx Nanolayers in Crystalline Silicon Colar Cells

Titanium oxide (TiO_x) nanolayers grown by atomic layer deposition are investigated with respect to their application as carrier selective contacts for crystalline silicon (c-Si) solar cells. Although TiO_x is known to act as an electron contact, in this work the selectivity of TiO_x layers is found to be widely tunable from electron to hole selective depending on deposition conditions, post-deposition treatments, and work function of the metal electrode used. Using TiO_x and an intrinsic hydrogenated amorphous silicon buffer layer, solar cell test structure exhibiting open-circuit voltages (V_oc) as high as 720 and 650 mV are shown for electron and hole selective contacts, respectively. Surface photovoltage and capacitance-voltage measurements reveal that carrier selectivity is correlated with the amount of c-Si band bending induced by TiO_x, which are governed not only by the effective work function difference at the Si/TiO_x interface, but also by the negative fixed charge present in the TiO_x layer. This new finding is in contrast to the previous model for carrier transport where selectivity is determined only by the asymmetric band offsets at the Si/contact interface. It highlights the influence of induced band bending to produce carrier depletion/inversion conditions, and the importance of its selectivity effect in a c-Si absorber.