AgxNi1-xO transparent conducting oxide film with various Ag mole ratios were prepared via a low-cost sol-gel method, and p-AgxNi1-xO/n-Si heterojunction diodes (HJDs) were fabricated. The effects of various Ag mole ratios on the structure, optical, and electrical properties of AgxNi1-xO films were systematically investigated, and their device applications in p-AgxNi1-xO/n-Si HJDs were examined. The Ag atoms accumulate at grain boundaries and inhibit the grain growth of AgxNi1-xO films. The pristine NiO film exhibits a transmittance exceeding 90% for the visible wavelengths. However, transmittance decreases with the Ag mole ratio. A transmittance hollow occurs at approximately 410 nm in Ag-doped AgxNi1-xO film, attributing to the absorption of surface-plasma-resonance of Ag nanoparticles. The energy gap increases from 3.58 to 3.75 eV for the AgxNi1-xO films with the Ag mole ratio of 0 and 1.8, respectively. Hall measurement indicated that the AgxNi1-xO film is p-type. The pristine NiO film exhibits a very high resistivity (≧106 ohm-cm). The resistivity significantly decreases from 14.8 to 1.88 × 10−4 ohm-cm for the AgxNi1-xO film with the Ag mole ratio of 0.3 and 1.8, respectively. Simultaneously, the hole concentration significantly increases from 1.46 × 1016 to 8.03 × 1022 cm−3. X-ray photoelectron spectroscopy revealed the Ni+3/Ni+2 ratio increases from 0.73 to 1.73 for the AgxNi1-xO films with the Ag mole ratio of 0 and 1.8, respectively. The increased Ni+3/Ni+2 ratio is responsible for increase in the hole concentration. The fabricated p-AgxNi1-xO/n-Si HJDs with Ag mole ratio of 0.5 exhibits a clear rectifying behavior with series resistance and ideality factor of 0.88 kΩ and 2.8, respectively.

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Conventionally, p-NiO/n-Si (p-n) heterojunction photodiodes (HPDs) exhibit a larger visible response than the ultraviolet response due to the thick Si substrate; hence, it is used as a broadband photodetector with a poor ultraviolet (UV)-to-visible rejection ratio. Herein, an intrinsic NiO (i-NiO) layer is inserted between the p-NiO and the n-Si substrate to fabricate p-NiO/i-NiO/n-Si (p-i-n) HPDs, significantly suppressing leakage current and visible response. Compared with the conventional p-n HPDs, the insertion of the i-NiO layer significantly reduces leakage current by approximately 241 times and enhances the rectification ratio from 13.8 to 3228 for the p-n and p-i-n HPDs. The insertion of an i-NiO layer not only increases the UV-response but also suppresses the visible response. These issues enhance the UV-to-visible rejection ratio from 72.2 in p-n HPDs to 915.3 in p-i-n HPDs. The p-NiO reveals a poorer crystalline structure than the i-NiO film because the Ag dopants accumulate at the grain boundary and inhibit crystalline growth. The Ag diffusion in the Si substrate causes defect states within the Si bandgap, whereas it is retarded by the i-NiO layer in the p-i-n HPDs. The poor crystallinity in the p-NiO and defect states within the Si bandgap contributes to a high leakage current and visible response in p-n HPDs. The p-i-n HPDs demonstrate a higher UV-response due to absorption by the i-NiO layer. Because visible light cannot be absorbed by the i-NiO layer, visible response is suppressed in p-i-n HPDs.