Publication date: 1st July 2014
The successive ionic layer adsorption and reaction (SILAR) method represents a promising low-cost solution process approach to develop QD sensitized oxides to be exploited for solar energy conversion. The QD sensitization of a mesoporous oxide matrix by SILAR is achieved by repeated cycles of 4 successive dipping steps of an oxide film into beakers containing: (i) a cation solution (ii) pure solvent to remove the excess of unbound cations, (iii) an anion solution and (iv) pure solvent to remove the excess of unbound anions. This process – from (i) to (iv) – is termed one SILAR cycle. Complete SILAR cycles provide QDs which are necessarily anion rich, while half cycle treatments (ending with step (ii)) allow generating QDs that are cation rich; This atomic control on QD surface chemistry provides a test bed system for exploring how QD stoichiometry affects ET dynamics. Exploiting optical pump-THz probe (OPTP) measurements, we study the effect of QD stoichiometry on the ET efficiency on PbS QDs directly nucleated by SILAR onto oxide matrices. QD to oxide electron transfer efficiency is maximized in lead rich quantum dots (terminated by half SILAR cycle). This effect can be traced to atomic surface passivation of QDs provided by lead cations - in perfect agreement with theoretical calculations1. The passivation efficiency (PE) effect is found to be QD size dependent (absolute QD sizes are obtained using HRTEM), particularly PE is found to increase linearly with QD surface area (a manifestation of the kinetic competition between electron trapping at the QD surface and ET to the oxide). Finally, we demonstrate that the improvement in ET efficiency for lead rich QDs (monitored by OPTP on QD/oxide electrodes) is directly and quantitatively correlated to the increase of photocurrent in QD sensitized photovoltaic devices, highlighting the relevance of these results for solar cell optimization.
[1]Donghun Kim et al. PRL 110, 196802 (2013)
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