Is carrier mobility a limiting factor for charge transfer in TiO₂/Si devices? A study by transient reflectance spectroscopy

TiO2 coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO2/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO2/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiO2, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO2 and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 x 10(4) cm s(-1) for TiO2 heat-treated at 300 degrees C, and even faster for amorphous TiO2 if the interfacial SiO.. layer was removed using HF before TiO2 deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO2 away from the TiO2/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 degrees C to 550 degrees C has only little impact on the CT time but leads to reduced carrier lifetime of <3 ns in TiO2 due to back recombination via the interfacial SiO2, which is detrimental to TiO2/Si device performance.