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 a series of solution-processed homologous quadrupolar anilino squaraine dyes (n-alkyl SQs) and an inorganic transparent electrode consisting of sputter-coated indium tin oxide (ITO). The organic n-alkyl SQs show distinct "double humb" spectral signatures in the green to red spectral region, which are attributed to a hybrid excitonic state due to aggregation in thin films with uniaxial anisotropy. Knowledge of the static dielectric function and results of transient absorption studies promise comprehensible dynamic changes in the dielectric function. 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. Ellipsometry data can be modeled with a Drude-Lorentz model that describes both the optical and electrical (DC conductivity) properties of polycrystalline but isotropic ITO layers. With optical pumping in the near infrared, we expect a strong influence on the free charge carriers to see significant time-dependent changes in conductivity, which can be rated by a change in the transient dielectric function.