Structure of P3HT crystals, thin films, and solutions by UV/Vis spectral analysis

Böckmann M, Schemme T, de Jong D H, Denz C, Heuer A, Doltsinis NL

Abstract

Optical absorption spectra of poly(3-hexylthiophene) (P3HT) are calculated in solution, spin-coated thinfilms, and the bulk crystal using a multiscale simulation approach. The structure of the amorphous thinfilm is obtained from coarse grained molecular dynamics (MD) simulations and subsequent back-mapping onto an atomistic force field representation. The absorption spectra are computed usingTDDFT by statistically averaging over an ensemble of molecules taken from the MD simulations.Experimental UV/Vis spectra of spin-coated thin films and solutions are recorded with varying ratios of‘good'versus‘poor' solvent. The theoretical approach is able to faithfully predict the spectral position inthe various phases and offers fundamental insight into the cause of any spectral shifts. The position ofthe main absorption peak is found to be chiefly determined by the level of torsion between thethiophene rings inside each molecule, while intermolecular effects are less important. Hence, opticalabsorption spectra hold valuable clues about the microscopic structure of disordered P3HT phases.