Solvent Anneal of P3HT Polymer for Crystallinity Control in Organophotovoltaics

The application for this research is to stabilize and control the dispersion of nanoparticles for use in organophotovoltaic cells. To do this, it is necessary to control the crystallinity of the photoactive polymer (P3HT).  The research question then becomes can we remove crystals from a spin-coated thin film of P3HT post-deposition by solvent annealing?  Solvent annealing is commonly used to grow crystals; but as in all equilibrium processes, the reverse can also occur.  We seek to find the conditions under which the equilibrium position favors amorphous P3HT.  Some early observations by Dadmun et al. at University of Tennessee – Knoxville indicate that solvent annealing preferentially dissipates small crystals in the thin film.  At various times or temperatures we may be able to extend this effect even to complete disappearance of the crystals.  We propose an apparatus similar to the classic “diffusion through a stagnant gas film” problem to control the concentration of vapor which the film is exposed to.  This consists of a graduated cylinder with pure chlorobenzene vaporizing from the bottom at steady state, with an inert gas blowing over the top of the cylinder. The partial pressure of the solvent will have a well-defined concentration profile, and lowering the spin-coated P3HT glass sample into the cylinder the concentration at different depths will be known.  We hypothesize that at longer times (>5 minutes) the crystals will begin to disappear.  The relative crystallinity will be measured using a Rigaku IV X-ray diffractometer at low incidence angles (source angle ~ 0.2 o).