Polarized Light Microscopy as a Rapid, Non-Destructive Evaluation Method for Qualitative Evaluation of Morphology in Organic Semiconductor Thin-Films

While preparation of thin-films of organic semiconductors <i>via</i> solution-based processing techniques is relatively straight-forward, understanding the morphology of these thin-films at different length scales, from micro- to nano-, which is critical in designing high-efficiency devices, can be fairly complex. High-efficiency organic photovoltaics (OPVs) are fabricated from blending two or more organic semiconductor components. This mixing process must be fine-tuned based on the ratio of different components, solvents utilized, and morphology-controlling additives, to obtain uniform thin-films at micro- and nano-scales. To characterize the uniformity, researchers must often turn to time-consuming or destructive techniques, such as atomic force microscopy (AFM) or scanning electron microscopy (SEM), which are not amenable to high-throughput, automated device fabrication facilities.<br/><br/>Polarized light microscopy (PLM) is a relatively unexplored technique for the characterization of organic semiconductor thin-films. PLM uses a set of orthogonal polarizers in the excitation and collection paths. As such, most of the incident light is filtered out from the image, and only the local optical birefringence is imaged, providing enhanced contrast compared to standard bright-field light microscopy. Organic semiconductors tend to have high degrees of anisotropy based on their molecular orientations and local degrees of crystallinity. Thus, PLM is a powerful imaging technique to qualitatively evaluate the morphology of organic semiconducting thin-films. Because it is a form of light microscopy, it is non-destructive, has a wide-field of view, and has potential to be employed in automated device fabrication facilities.<br/><br/>In this work, we employ PLM to evaluate the quality of thin-films of the stable conjugated polymer, PCDTBT. PCDTBT thin-films are an example of polymer thin-films that are challenging to prepare due to the neat polymer’s poor solubility in typical organic solvents, its high viscosity, and its ease of aggregation. We optimized PCDTBT thin-films by varying the solvent, molecular weight, heating times and temperatures, and filtering conditions. Using PLM, we rapidly evaluated the quality of PCDTBT thin-films with various preparation conditions to find optimal conditions for uniform thin-films with low degrees of aggregation. Based on these optimal conditions, we fabricated PCDTBT hole-only devices in a metal-insulator-metal Schottky photodiode geometry for thick (200 nm) and thin (80 nm) PCDTBT layers. We extracted the Schottky barrier height and hole mobility of PCDTBT from current-voltage measurements and drift-diffusion simulations, respectively.<br/><br/>We will discuss the potential of PLM to interpret the nanoscale morphology and crystallinity of several other common organic donor-acceptor blends.