Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Apart from the geometrical morphology of nanocrystals (NCs), the surface chemistry has also shown significant effects on the physical and chemical properties of NCs. Colloidal quantum dots (QDs) are generally capped with long-chain, electrically insulating ligands which are introduced during the synthetic procedures. Replacement of these molecules with shorter chain ligands has been shown necessary to decrease the inter-dot spacing and improve the electron transfer in QD thin films. Although a number of ligands have been applied for the surface passivation of PbS QDs, the effects of the surface ligand on the photovoltaic properties and exciton dynamics have not been fully understood in quantum dot solar cells (QDSCs). The impact of the ligand chain length and scaffold on the photovoltaic properties and exciton dynamics in PbS QDSCs is investigated by ultrafast transient absorption (TA) spectroscopy, temperature-dependent photoluminescence (PL) and transient photovoltage decay measurements in this work. It was observed the inter-dot spacing was narrowed in PbS QD films as the decrease of ligand chain length, resulting the enhancement of exciton coupling and electron transfer. Although the shortest ligand thioglycolic acid (TGA) leads to the smallest inter-dot spacing, it does not give the highest photocurrent in QDSCs due to its acidity and ligand-induced defects on QD surfaces. It was found that 3-mercaptopropanoic acid (MPA) gave the highest photocurrent of 15.3 mA/cm2 and power conversion efficiency of 2.64%. Another interesting thing is that as the capping ligand on PbS QDs was replaced by conducting conjugated molecules, the exciton coupling was enhanced but the fabricated solar cell was deteriorated. Our findings in this work are useful for the deeper understanding on the effect of surface ligands on QDs and would shed light on the further improvement of QDSCs.
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