Functionalized Graphite Platelets and Lead Sulfide Quantum Dots Enhance Solar Conversion Capability of a Titanium Dioxide/Cadmium Sulfide Assembly

A photoactive electrode comprising lead sulfide (PbS) and cadmium sulfide (CdS) quantum dots (QDs) and functionalized graphite platelets (FGPs) was prepared by assembling them onto titanium dioxide (TiO2), which functioned as the wide band gap semiconducting scaffold. The QDs were cumulatively capable of harvesting portions of visible and infrared regions of solar spectrum, and FGP served as electron conduit. Graphite platelets (GPs) were noncovalently functionalized using 1-pyrenecarboxylic acid (PCA) to yield FGP. The insertion of PCA between GP layers to yield few-layer graphene or FGP was confirmed by high-resolution transmission electron microscopy and Raman and X-ray photoelectron spectroscopic analyses. Fluorescence quenching, emission decay analyses, and energetics of the TiO2/FGP/PbS/CdS electrode demonstrated excited electron deactivation via a cascade mechanism. Photoexcited electrons propagate from PbS to CdS to TiO2 and to the external circuit through FGP, which had a suitably poised Fermi level at 4.52 eV. The role of FGP in working as an efficient electron acceptor and PbS as a red wavelength absorbing layer was evidenced in the form of enhanced external and internal quantum efficiencies achieved for the TiO2/FGP/PbS/CdS electrode over the entire solar spectrum compared with the TiO2/CdS electrode. This was accomplished using cells with S-n(2-)/S2- as the redox couple and a multiwalled carbon nanotube-based counter electrode. The best overall power conversion efficiency of the TiO2/FGP/PbS/CdS photoanode-based cell is 3.82%, which is greater by 54% compared with that of the TiO2/CdS cell. Our studies demonstrate the prowess of using a near-infrared absorber like PbS and an electron acceptor like FGP in realizing remarkable improvements in solar-cell performance metrics.