Influence of citric acid linker molecule on photovoltaic performance of CdS quantum dots-sensitized TiO2 solar cells

Influence of citric acid on the photovoltaic properties of the CdS quantum dot-sensitized TiO2 solar cells (QDSSCs) was studied. Tethering of citric acid molecules with both TiO2 and CdS quantum dots (QDs) was confirmed by Fourier transform infrared spectroscopy technique. High-resolution transmission electron microscopic studies revealed that QDs with average size of ~4.5 nm, were tethered with TiO2 nanoparticles of diameter ~40 nm. Presence of Cd, S, C, Ti and O elements in the composite photoanode and their uniform distribution throughout the photoanode were confirmed by energy dispersive X-ray spectroscopy measurements. QDSSCs fabricated with pristine TiO2 photoanode exhibited a short circuit current density (JSC) of 5.80 mA cm−2 and an overall power conversion efficiency (η) of 1.10%, whereas solar cells made with citric acid-treated, photoanode-exhibited a JSC of 8.20 mA cm−2 with 1.50% efficiency under 100 mW cm−2 (AM 1.5) light illumination. This is an impressive 60% increase in the JSC and ~36% enhancement in the overall power conversion efficiency. Interfacial resistance of QDSSCs is estimated by using electrochemical impedance spectroscopy revealed that citric acid treatment enhanced both the electron injection to the conduction band of the TiO2 from the CdS as well as the overall charge transfer of the device, while decreasing the recombination of the photo-generated electrons with their holes in the electrolyte.