Article pubs.acs.org/JPCC
Simulations of Exciton Diffusion and Trapping in Semicrystalline Morphologies of Poly(3-hexylthiophene) Josiah A. Bjorgaard† and Muhammet E. Köse*,‡ †
Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States TUBITAK Marmara Research Center, Gebze, Kocaeli 41470, Turkey
‡
S Supporting Information *
ABSTRACT: 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 (LD) of P3HT has been found as 23 nm in the π-stacking direction and 19 nm for the interdigitated chain direction. Introduction of energetic disorder decreases LD by nearly a factor of 2. LD 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.
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INTRODUCTION The process of exciton diffusion in thin films of π-conjugated organic materials is relevant to a variety of technological applications including organic solar cells, light emitting diodes, and sensors.1 These applications involve thin films, which can be semicrystalline in nature. Using chemically modified polymers, it has been shown that increasing crystallinity in representative thin films increases exciton diffusion length (LD).2 The effect of microscopic properties on LD in semicrystalline thin films is difficult to examine experimentally or computationally because of the time range and length scales involved. Understanding the effects of microscopic properties such as semicrystalline morphology on the kinetics of energy transfer could assist efforts to tune LD. Controlling thin-film morphology could allow for tuning of LD for specific applications. In light emitting diodes, decreasing the diffusion length of excitons can lead to increased device efficiency by reducing loss pathways such as exciton−exciton annihilation.3 In organic solar cells, increasing LD can increase device efficiency by increasing the yield of charge separation due to increased probability that an exciton reaches a domain boundary where charge separation is facilitated.4 Predictive computational studies can provide insight into the properties of these morphologies. In thin films of conjugated polymers and molecular materials, low-energy excitons are often localized to molecules or segments of conjugated polymer chains.5−7 In poly(3hexylthiophene) (P3HT), this effectively creates subunits of the polymer backbone, which act as single chromophores.8 In thin films, these subunits are packed together in crystalline or amorphous domains. This gives rise to hopping type transport as excitons hop from one site to another. This discrete picture © 2014 American Chemical Society
of chromophores is used in the present simulations to justify separation of P3HT chains into small segments in order to simplify the calculation of electronic coupling integrals. The model described here is performed in the limit of excitons localized to small (
