Phosphonate self-assembled monolayers as organic linkers in solid-state quantum dot sensetized solar cells

We have employed X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) spectroscopy, infrared (IR) spectroscopy, water contact angle (WCA) measurements, ellipsometry, and electrical measurements to study the effects of self-assembled monolayers (SAMs) with phosphonic acid headgroups on the bonding and performance of cadmium sulfide (CdS) solid-state quantum dot sensitized solar cells (QDSSCs). ~2 to ~6 nm size CdS quantum dots (QDs) were grown on the SAM-passivated TiO₂ surfaces by successive ionic layer adsorption and reaction (SILAR). Our results show differences in the bonding of the CdS QDs at the TiO₂ surfaces with a SAM linker. Moreover, our data indicate that presence of a SAM increases the CdS uptake on TiO₂ as well as the performance of the resulting devices. Importantly, we observe ~2 times higher power conversion efficiencies in the devices with a SAM compared to those that lack a SAM.