p- ton-Type Conversion and Nonmetal–Metal Transition of Lithium-Inserted Cu₃N Films

Cu 3 N is a semiconductor with a bandgap of ∼1.4 eV, which is ideal for solar energy conversion applications. To obtain high-efficiency Cu 3 N-based solar cells, the fabrication of p - n homojunctions is desirable. Studies on the bipolar doping of Cu 3 N have been initiated very recently. In this study, we demonstrate that lithium-insertion into p -type Cu 3 N films using a soft chemical treatment causes a p - to n -type conversion and nonmetal–metal transition. This lithium insertion was achieved by treating p -type Cu 3 N films with n -butyllithium solutions at moderate temperatures of 313–343 K, such that the lithium atoms diffused into Cu 3 N and occupied the vacant interstitial sites of the crystalline lattice. As a result, the lithium-inserted Cu 3 N (Li x Cu 3 N) films became n -type semiconductors, indicating that the lithium atoms act as electron donors. Moreover, Li x Cu 3 N films with x > 0.05 were found to show metallic electrical conductivity with resistivities of (2–4) × 10 –3 Ω cm. This metallic behavior was also observed in the optical transmittance and reflectance data. The findings of this study allowed us to prepare p -type, n -type, and metallic Cu 3 N-based films. These results may pave the way for the realization of nontoxic wholly Cu 3 N-based homojunction solar cells delivering high performance.