Transient Dielectric Function of Squaraine Thin Films and ITO Electrodes

Using ultrafast spectroscopic ellipsometry, we aim to determine the transient dielectric function of organic and inorganic semiconducting thin films. We pick two sample systems that differ in their electronic signature, namely bound excitons versus free Drude electrons: an organic semiconductor type consisting of solution-processed quadrupolar anilino squaraine and an inorganic transparent electrode consisting of sputter-coated indium tin oxide (ITO). We have experienced that with the type of squaraine used so far (n-alkyl SQs), the trade-off between low pump power to avoid damage and the strength of the signal was only reasonable. Therefore, we want to focus on a different type of squaraine (ProSQ-C16) in this continued study, which promises a stronger signal, i.e. several tens of milli-OD instead of only a few milli-OD. ITO electrodes are characterized by their transparency over the entire visible spectrum and at the same time show a high electrical conductivity due to free charge carriers with a characteristic absorption in the NIR. By strong pumping in the NIR range we intend to push the absorption edge into the visible to probe a significant change in the transient dielectric function. We expect that ITO layers with different conductivity show correspondingly different dynamic responses.