Interplay Between Structure, Stoichiometry, and Electron Transfer Dynamics in SILAR-based Quantum Dot-Sensitized Oxides

We quantify the rate and efficiency of picosecond electron transfer (ET) from PbS nanocrystals, grown by successive ionic layer adsorption and reaction (SILAR), into a mesoporous SnO2 support. Successive SILAR deposition steps allow for stoichiometry- and size-variation of the QDs, characterized using transmission electron microscopy. Whereas for sulfur-rich (p-type) QD surfaces substantial electron trapping at the QD surface occurs, for lead-rich (n-type) QD surfaces, the QD trapping channel is suppressed and the ET efficiency is boosted. The ET efficiency increase achieved by lead-rich QD surfaces is found to be QD-size dependent, increasing linearly with QD surface area. On the other hand, ET rates are found to be independent of both QD size and surface stoichiometry, suggesting that the donor–acceptor energetics (constituting the driving force for ET) are fixed due to Fermi level pinning at the QD/oxide interface. Implications of our results for QD-sensitized solar cell design are discussed.This work has been financially supported by the Max Planck Society. H.W. is a recipient of a fellowship of the Graduate School Materials Science in Mainz (MAINZ) funded through the German Research Foundation in the Excellence Initiative (GSC 266). I.B. is grateful to the Materials Science Doctoral program of the Universitat d’Alacant (UA) for the award of a travel grant. The UA team acknowledges the financial support from the Spanish Ministry of Economy and Competitiveness through project MAT2012-37676 (FONDOS FEDER)