Establishing Monomer vs. Aggregate Composition of Squaraine-Based Organic Photovoltaics Using Atomic Force Microscopy

Organic photovoltaics (OPV) offer exciting possibilities for energy production and in optimizing OPV devices, a greater understanding of the organic solar cells' morphology is needed. Structural order is thought to improve charge mobility and energy transfer. Here we present a method for determining the monomer vs. aggregate composition for squaraine-based thin films using measurements of each film’s absorptivity, thickness, and area. Population can be measured through absorption measurement, provided that the extinction coefficient of the species in question (aggregate or monomer) and the path length for the light through the sample are known. Therefore the ability to measure populations depends on an accurate values for the film thickness, typically 50-150 nm. We employ atomic force microscopy (AFM) to measure film thickness and assess two techniques for removing thin-film material to establish accurate heights. Both techniques yield consistent height data and we discuss their merits. AFM measurements yield average thickness measurements for each type of film with an uncertainty under 17% and area measurements with an uncertainty under 3%. These low uncertainties are crucial as they allow for the monomer to aggregate population to be measured with a high level of precision.