Assistant Professor, Stevens Institute of Technology
Research Interests: Solution-processable materials for renewable energy applications, nanostructured materials, hybrid polymers, crystal engineering
Solution Shearing of Continuously Processed Organic Photovoltaic Active Layers for Morphological Control
We demonstrate the use of solution shearing prior to film deposition as a means to control the morphology of model poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) films. Specifically, P3HT/PCBM solutions in the gel state were subjected to oscillatory and steady torsional shear forces in the nonlinear regime. The sheared solutions were then doctor bladed onto solar cell device platforms. P3HT/PCBM solar cells with pre-sheared active layers displayed up to double the efficiency of those comprising unsheared active layers. Rheological measurements, 2D x-ray diffraction experiments and confocal fluorescence spectroscopy revealed a two-step thermoreversible gelation mechanism involving phase separation between P3HT and the solvent followed by interchain crystallization. Solution shearing of this gel further induced crystallization of and phase separation between P3HT and PCBM, necessary for efficient charge transport during solar energy harvesting. Unlike techniques developed to optimize P3HT/PCBM films deposited via batch spin coating processes, this method of morphology control via solution shearing is compatible with continuous processing methods. The ability to continuously process active layers is critical for advancing organic solar cell technologies towards commercialization.