Temperature dependent relaxation of separated charge carriers at CdSe-QD / ITO interfaces

One and 5 monolayers of CdSe quantum dots with fixed diameter were deposited on ITO substrates by dip coating and investigated by transient surface photovoltage (SPV) at temperatures up to 250 C. The SPV transients were excited with laser pulses (duration time 5 ns) and measured in vacuum at times up to 0.2 s. SPV transients arose within the laser pulse and could be well fitted with one (one monolayer of CdSe-QDs) or two (5 monolayers of CdSe-QDs) stretched exponentials. The parameters of the stretched exponentials changed depending on defect generation during heating as well as on thermal activation processes during heating and cooling. During cooling, the mean relaxation times of both processes were thermally activated with an activation energy of 0.9 eV. Defect generation strongly affected charge separation and relaxation within the first monolayer at the CdSe-QD/ITO interface and between the first monolayer of CdSe-QDs and following CdSe-QD layers.