PMMA as an Additive for Nanostructured TiO₂ Thin Films for Heterojunction Visible-Blind Photodetectors

Abstract

This study reports on high-quality, crystalline, nanostructured TiO 2 thin films with large-area coverage prepared by the sol−gel process using PMMA (poly(methyl methacrylate)) as an additive for applications in visible-blind photodetection. The Si/TiO 2 p−n junction photodiodes, fabricated through an inexpensive yet simple process with the addition of PMMA, exhibit superior characteristics, making them excellent candidates for visible-blind photodetectors operational in the UV and NIR ranges. The addition of PMMA to the solution significantly enhances the crystalline and optical properties, as well as the nanostructure morphology of the TiO 2 films. It is demonstrated that the nanostructure morphology, refractive index, and bandgap of TiO 2 can be precisely tailored by controlling the TiO 2 sol/PMMA volume ratio. A valence band offset as large as 2.8 ± 0.20 eV is measured for the TiO 2 /Si heterointerface, ensuring hole blockage through the film and electron-selective transport. Application of the TiO 2 nanostructured thin films in the p-Si/n-TiO 2 junction photodiodes reveals an excellent rectification ratio of ∼2 × 10 4 with a leakage current as low as ∼10 −10 A and a superior phototo-dark current ratio of ∼8000 under 325 nm illumination. Under reverse bias conditions, the photodiodes exhibit a reasonably high photoresponsivity (>1 A/W) with a high detectivity of ∼1 × 10 13 Jones. Moreover, a response time of ∼200 ms is achieved using these photodiodes.