Grain-size dependent carrier compensation in Cu₂O

Cuprous oxide (Cu₂O) is an absorber material for low cost solar cells and various fundamental phenomena have been explained using this material. Phase pure Cu₂O samples with different grain sizes, in the range of 400 µm to 3 mm, were obtained by thermal oxidation of a Cu sheet. The temperature dependent carrier concentration of all samples follows the compensated semiconductor model. In this model, two independent monovalent acceptors (N_A1, N_A2) are assumed with one compensating donor level (N_D) and the carrier concentration is obtained from charge neutrality condition. It is observed that the difference in energy levels between two acceptors is ~80 meV, pertaining to a normal and a split Cu-vacancy in Cu₂O as acceptor defects. Interestingly, the concentration of donor defects increases with increasing grain boundary cross section (Λ_GB), from 5.8×10¹³ cm^-3 for a sample with Λ_GB=0.22x10^-3 µm^-1 to 3.0×10¹⁴ cm^-3 for a sample with Λ_GB=0.45x10^-3 µm^-1. It indicates that the grain boundaries act as source of compensating donor defects in Cu₂O. This study suggests that the increment in grain size of Cu₂O can improve the performance of electronic devices based on it.