The effect of ligands on photochemical charge transfer in quantum dot (QD) films is of great interest for applications in photovoltaics and LEDs. Here surface photovoltage spectroscopy (SPS) was used to study the intrinsic charge transfer properties and surface states of 130 ±30 nm thick films of thiol, amine, carboxylic acid supported CdSe QDs on indium tin oxide (ITO) in the absence of an external bias or electrolyte. Based on absorbance and photoluminescence spectra the quantum dots films have a 2.0 eV band gap. On ITO (Ef= -4.8 eV), the QD films give positive or negative photovoltage signals (-120 to +350 mV) under sub band gap and super band gap excitation (0.1-0.3 mW cm-2), depending on the ligands. The fact that photovoltage forms reversibly and depend strongly on light intensity suggests that voltage formation mainly involves charge carrier polarization in a space charge layer at the ITO-QD interface. The experimental photovoltage values are also fond to correlate well with the LUMO energies of the CdSe QDs, obtained from the electrochemical reduction potential in tetra-n-butylammonium hexafluorophosphate electrolyte at unadjusted pH. This confirms that the built-in potential of the ITO-QD Schottky contacts is controlled by electronic properties of the ligands. The findings shed new light on the factors controlling photochemical charge separation in films of ligand-stabilized CdSe QDs.

Effect of ligands on photochemical charge transfer in CdSe quantum dot films – A surface photovoltage spectroscopy study

253rd National Meeting (2017)

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