On the Use of Reflection Polarized Optical Microscopy for Rapid Comparison of Crystallinity and Phase Segregation of P3HT:PCBM Thin Films

Rapid, nondestructive characterization techniques for evaluating the degree of crystallinity and phase segregation of organic semiconductor blend thin films are highly desired for in-line, automated optoelectronic device fabrication facilities. Here, it is demonstrated that reflection polarized optical microscopy (POM), a simple technique capable of imaging local anisotropy of materials, is capable of determining the relative degree of crystallinity and phase segregation of thin films of polymer:fullerene blends. While previous works on POM of organic semiconductors have largely employed the transmission geometry, it is demonstrated that reflection POM provides 3x greater contrast. The optimal configuration is described to maximize contrast from POM images of polymer:fullerene films, which requires K & ouml;hler illumination and slightly uncrossed polarizers, with an uncrossing angle of +/- 3 degrees. It is quantitatively demonstrated that contrast in POM images directly correlates with 1) the degree of polymer crystallinity and 2) the degree of phase segregation between polymer and fullerene domains. The origin of the bright and dark domains in POM is identified as arising from symmetry-broken liquid crystalline phases (i.e., dark conglomerates), and it is proven that they have no correlation with surface topography. The use of reflection POM as a rapid diagnostic tool for automated device fabrication facilities is discussed. The use of reflection polarized optical microscopy (POM) for imaging changes in crystallinity and phase segregation of (poly(3-hexylthiophene-2,5-diyl)):([6,6]-phenyl-C61-butyric acid methyl ester) (P3HT:PCBM) thin films is demonstrated. Methods to reduce aberrations, including the use of proper K & ouml;hler illumination, are discussed and demonstrated to maximize contrast. Bright and dark domains in POM images of P3HT:PCBM are identified as originating from dark conglomerate liquid crystalline states. image