Publication date: 1st April 2013
The understanding of interfacial electron transfer (ET) processes from semiconductor quantum dots (QDs) to metal oxides is of paramount importance in quantum dot sensitized solar cell (QDSSC) technology. Recent progress in the field has established efficiencies beyond 5% for QDSSCs based on the direct nucleation of QDs onto oxide matrices by successive ionic layer absorption and reaction(SILAR). However, many aspects of ET processes in these systems have remained poorly understood, hampering further device optimization. Here, we report quantitative ET dynamics on samples consisting of PbS QDs nucleated onto a SnO2 matrix, using sub-picosecond time-resolved optical pump-terahertz probe spectroscopy. We find that (i) ET ultrafast (~6ps) timescales and (ii) remarkably, the ET rate is independent of the number of SILAR cycles. These findings can be explained by either ET taking place in the strong coupling regime (barrierless ET) or by considering that the number of SILAR cycles affects the QD population rather than the size distribution. Preliminary HR-TEM data combined with optical reflectance measurements point to only very limited variation of the QD size with cycle number, so that the latter scenario is the most plausible. In addition, we present the effect of atomic and molecular passivisation of nucleated SILAR QDs on the ET dynamics. These simple post-nucleation treatments are shown to boost the measured photocurrent beyond 300%. Our findings thus provide new avenues for the further optimization of QDSSCs based on SILAR growth.
1. Lee, J.W.; Son, D.Y.; Ahn, T.K.; Shin, H.W.; Kim, I.Y.; Hwang, S.J.; Ko, M.J.; Sul, S.; Han, H.; Park, N.G. Sci. Rep. 2013, 3. 2. Santra, P.K.; Kamat, P.V. Journal of the American Chemical Society 2012, 134, (5), 2508-2511.
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