Charge Generation and Selective Separation at PbS Quantum Dot Metal Oxide Interfaces

Charge separation and transfer at the interface between layers of oleic acid capped PbS quantum dots QDs and Titanium and Indium Tin oxide TiO2 and ITO films were investigated by surface photovoltage SPV measurements. Photoluminescence PL measurements were performed in order to check for excitonic transitions and determine the QD band gaps. The QDs diameter of 4.2 nm and 5.0 nm were estimated by using the PL band gaps and the theoretical equation derived by Wang et al. [J. Chem. Phys. 87 1987 7315]. The SPV spectra of the PbS QDs TiO2 system reveal a positive charge on the PbS film surface and show three distinguished regions which demonstrate i the charge separation across QDs, ii the electron injection from QDs into TiO2 and iii the fundamental absorption in TiO2. The on set of the electron injection depends on the QD size QD band gap it shifts to lower photon energies for lower QD dimensions for higher QD band gaps . Thus, a better conduction band alignment is achieved in the latter case. In contrast to PbS QDs TiO2, the SPV spectra of the PbS QDs ITO structure reveal the negative charge on PbS surface. Moreover, the charge transfer at this interface is not observed. Instead, the SPV peculiarities in the photon energy range 1.4 3.0 eV point out to trapped holes on the ITO surface state