Doping leading to low electrical resistivity in electrodeposited thin films of Cu2O is a straightforward requirement for the construction of efficient electronic and energy devices. Here, Bi (7 at. %) doped Cu2O layers were deposited electrochemically onto Si(100) single-crystal substrates from aqueous solutions containing bismuth nitrate and cupric sulfate. X-ray photoelectron spectroscopy shows that Bi ions in a Cu2O lattice have an oxidation valence of 3+ and glancing angle X-ray diffraction measurements indicated no presence of secondary phases. The reduction in the electrical resistivity from undoped to Bi-doped Cu2O is of 4 and 2 orders of magnitude for electrical measurements at 230 and 300 K, respectively. From variations in the lattice parameter and the refractive index, the electrical resistivity decrease is addressed to an increase in the density of Cu vacancies. Density functional theory (DFT) calculations supported the experimental findings. The DFT results showed that in a 6% Bi doped Cu2O cell, the formation of Cu vacancies is more favorable than in an undoped Cu2O one. Moreover, from DFT data was observed that there is an increase (decrease) of the Cu2O band gap (activation energy) for 6% Bi doping, which is consistent with the experimental results.
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During current vs. voltage measurements of undoped samples (deposited at pH 9.0) two types of electrical behaviors were observed. One totally insulating, with no current flowing through the thin film even for high electrical applied potential, and another very conductive with calculated resistivity comparable with the n-Si(100) one. The first behavior is explained by the high resistivity of the Cu2O thin film deposited at pH 9.0. It is expected an electrical resistivity enhancement of at least 2 orders of magnitude by reducing the pH from 10.0 to 9.0, as found in Ref. 21, and this increase can put the resistance of our samples outside equipment's work range. The second behavior is attributed to pinholes or copper filaments, which strongly reduce the resistance of the system.
