Exponential optical absorption edge in PbS quantum dot-ligand systems on single crystal rutile-TiO₂ revealed by photoacoustic and absorbance spectroscopies

Abstract

The photovoltaic properties of quantum dot (QD) sensitized solar cells (QDSCs) depend significantly on the surface modification applied to the QDs and on the nanostructured interface between the QDs and the electrode surface. In the development of QDSCs with spatially ordered QD arrays, linking molecular ligands with the QDs (QD-ligands) can lead to the realization of novel QDSCs. The ligand shell around the QDs mediates the electron and energy transfer processes that underpin their use in QDSC applications. The dependence of the photovoltaic properties on the interparticle distance (QD spacing) can also be evaluated by applying different sizes of molecular ligands. The present study focuses on specific attention to the exponential optical absorption edge (often termed Urbach tail) in PbS QD-ligand systems with different QD spacing adsorbed on rutile-TiO2 (R-TiO2) substrates with different crystal orientations. It is essential to accurately characterize QD-ligands on electrode surfaces with different crystal orientations, not only for scientific studies, but to further optimize the growth conditions and processes in order to design and fabricate advanced QDSCs. Photoacoustic (PA) and conventional absorbance (Abs) spectroscopies were applied to determine the optical absorption and nonradiative relaxation properties. There is a discrepancy between the PA and Abs spectra especially in the Urbach tail region. As the Urbach tail states are related to the absorbed photon energy lost in the form of heat generated by nonradiative relaxation, therefore characterization of the Urbach tail is important and essential for QDSC applications. Characterization of the Urbach tail and the heat generated by nonradiative relaxation of PbS QD-ligand systems by combined PA and Abs spectroscopies showed that the characteristics depend strongly on the crystal orientation of the R-TiO2 substrate, the QD spacing, and the free energy change.