An Integrated, Multipart Experiment: Synthesis, Characterization, and Application of CdS and CdSe Quantum Dots as Sensitizers in Solar Cells

Quantum dots (QDs) are useful for demonstrating the particle-in-a-box (PIB) model utilized in quantum chemistry, and can readily be applied to a discussion of both thermodynamics and kinetics in an undergraduate laboratory setting. Modifications of existing synthetic procedures were used to create QDs of different sizes and compositions (CdS passivated with polymer, and CdSe passivated with oleic acid/ trioctylphosphine). These were investigated by spectroscopy, to which standard 3D PIB mathematical models were applied to determine their effective size. The data were compared to those from other methods for students to see the validity of the PIB model. For CdSe QDs, an empirical formula was applied to the spectroscopic data. In the case of CdS, the synthesized QDs were studied with X-ray diffraction, from which one can also estimate the size of the QDs. Finally, the QDs were utilized as the light-harvesting layer in photovoltaic cells by attachment to a layer of surface-modified titania (TiO2) nanoparticles on conductive glass, and the surface chemistry tested via water contact-angle measurements. The photoresponse of these cells was measured using basic electrochemistry equipment for a selection of QDs, and these results were considered in relation to the light source used for excitation (CdS QDs absorb UV light, and a voltage was only measurable upon exposure to UV light). Students are able to synthesize, characterize, and apply their materials to a functional purpose. Ultimately, students drafted reports in the form of an ACS-style communication, allowing for a tie-in of typical lab reports to real-world journal publications.