Simulations of Exciton Diffusion and Trapping in Semicrystalline Morphologies of Poly(3-hexylthiophene)

A multiscale simulation method for exciton diffusion in semicrystalline morphologies of conjugated polymers is described. Simulated exciton migration in single chain regime vastly differs for crystalline and amorphous segments in the thin films of poly(3-hexylthiophene) (P3HT). The methodology relies on atomistic treatment of crystalline and amorphous domains for excitonic coupling calculations, while extracting energy landscapes from experimental optical spectra. Simulated one-dimensional exciton diffusion length (L-D) of P3HT has been found as 23 nm in the g-stacking direction and 19 nm for the interdigitated chain direction. Introduction of energetic disorder decreases L-D by nearly a factor of 2. L-D in pure amorphous domains has been simulated as 5.7 nm with inclusion of energetic disorder. The simulations suggest that diffusion occurs primarily in crystalline material due to higher diffusion length and low trapping probability. Surprisingly, the rate of exciton capture by crystalline domains from amorphous domains is rather significant and has significant implications for efficient photovoltaic operation in the active layer blends of organic solar cells.