2025 AIChE Annual Meeting

(185h) Precise Control of Semiconducting Polymer Film Thickness for Device Applications

Authors

Adam J. Moulé, University of California, Davis
P3HT and PM6 polymer films were prepared by spin-coating the polymer solution onto a glass substrate at varying angular velocities and times. The UV-Vis absorption spectra and film thickness were measured using a spectrometer and a profilometer. It was observed that slower angular velocities resulted in increased absorbance for both polymers. Additionally, when plotting the measured thickness data against their respective angular velocities, both P3HT and PM6 exhibit exponential decay in film thickness with increasing speed. Conversely, absorption and thickness were determined to be independent of the duration the substrate spends on the spin-coater. We derived an expression from the momentum balance equation that determines the film thickness based on input angular velocity (⍵), which further supports our observations as ⍵ contributes more significantly than time due to its second-order dependence. The expression also provides a relatively accurate model for determining the film thickness for multiple polymers, demonstrating its universality despite different molecular weights and morphologies. Regardless of minor deviations due to factors such as viscosity and evaporation, our basic model allows for the selective control of polymer film thicknesses.

To demonstrate the potential of controlling polymer layer thickness, we have developed planar heterojunction organic photodetectors. These devices are capable of selective wavelength photodetection, with promising applications in areas such as night vision, heat vision, and more recently, biomedical fields like cancer cell detection and the identification of impurities in the bloodstream.

The photodetectors typically employ a blend of two distinct polymers to form small domains within the film. To block undesirable wavelengths, an optical filter is placed on top. However, the use of inorganic materials to produce these filters limits biocompatibility. In our novel approach, we propose using a planar heterojunction structure made of two different polymer films, without the need for domains. This creates a single interface between the polymers, with one polymer acting as the optical filter, making the devices fully organic and better suited for biological applications.

By precisely controlling the thickness of both films, we can enhance or block specific wavelengths through the cavity resonance effect. Therefore, precise control over film thickness is critical for achieving wavelength range selectivity in photodetection. Our work highlights the significance of developing a simple model for controlling polymer film thickness, one that is universally applicable across various semiconducting polymer applications.

Panel 1: Thickness Control Model Fitted to Raw Data

Panel 2: Schematic of Planar Heterojunction Organic Photodetector

Panel 3: SEM Image of Individually Stacked Polymer Film Layers

Panel 4: External Quantum Efficiency of Organic Photodetector Demonstrating Narrow Range Detection of Specific Wavelengths.