Assessing Monomer and Aggregate Populations in Squaraine-Based Organic Solar Cells

Higher efficiencies of organic photovoltaic (OPV) devices have often been correlated with structural order within the solid state active layer of the device. Structural order is thought to improve charge mobility and energy transfer since both rely on effective orbital overlap between molecules. Nevertheless, questions remain about the role of electronic aggregates since they may act as energy traps for excited state species after photoexcitation, thereby leading to reduced overall efficiency. We aim to demonstrate how OPV efficiency is indeed related to aggregate (vs. monomer) populations through quantitative measurement. Population can be measured through absorption measurements and with a knowledge of the extinction coefficient of the species in question, along with the path length for the light through the sample. Experimentally, we will focus on the thickness measurements for a set of squaraine films. Squaraines are interesting because their efficiencies are significantly impacted by the exact geometry of aggregation, which can be controlled through chemical tuning. We will therefore use AFM thickness measurements coupled with spectroscopic techniques to confirm the importance of aggregation for molecular design strategy and fully optimized OPV devices.