The benefit of synthesizing soluble polymers for optoelectronic applications is the flexibility in how these materials are processed. We are studying a variety of casting methods to take polymer and discrete molecular solutions to thin films. In addition, we are studying post-casting treatments to modify the polymer redox properties prior to device integration.
For redox active applications such as electrochromic devices and supercapacitors, we have studied a variety of pre- and post- processing methods to improve film quality and device performance. The use of an alkyl-phosphonic acid surface treatment prior to casting of the polymer film improves its adhesion, while post-processing chemical oxidation can be used to adjust the redox state of the polymer prior to device integration.1
The polymers we design are highly soluble, allowing us to work with a variety of coating methods such as spray coating, spin coating, blade coating, and inkjet printing and evaluate if and how the processing method influences material and device performance.2
While the performance of redox active devices is fairly independent of the processing conditions, the final conformation and film morphology of photoactive organic polymers and discrete molecules is a critical factor for the performance of solid state devices, such as organic photovoltaic devices (OPVs).
We study the dynamics of the ink drying process as the solvent evaporates and the active layer is formed. A wide array of variables control this drying process such as solution formulation, the use of cosolvents, and the temperature of the substrate and solution. For solar cell fabrication, we use primarily spin coating and blade coating that give smooth films and well-defined drying processes, which is critical for generating an optimum phase-separated blend of the p- and n-type materials. Blade coating is amenable to in situ spectroscopy since the coated substrate is stationary allowing us to monitor the optical properties of the blend in real time during the drying process.
Once films dry, we carry out thorough morphological characterization to understand the surface topology with atomic force microscopy and molecular packing within the bulk with x-ray scattering techniques (e.g. GIWAXS). These properties, such as surface roughness, degree of molecular order, and molecular packing distances are often directly correlated with device performance.3
1. Pittelli, S. L.; Shen, D. E.; Österholm, A. M.; Reynolds, J. R. ACS Appl. Mater. Interfaces, 2018, 10, 970-978.
2. Padilla, J.; Österholm, A. M.; Dyer, A. L.; Reynolds, J. R. Sol. Energy Mater. Sol. Cells, 2015, 140, 54-60.
3. Lo, C.K.; Gautam, B. R.; Selter, P.; Zheng, Z.; Oosterhout, S. D.; Constantinou, I.; Knitsch, R.; Wolfe, R. M. W.; Yi, X.; Brédas, J. L.; So, F.; Toney, M. F.; Coropceanu, V.; Hansen, M. R.; Gundogdu, K.; Reynolds, J. R. Chem. Mater. 2018, 30, 2995-3009.