Dependence of Exciton Diffusion Length and Diffusion Coefficient on Photophysical Parameters in Bulk Heterojunction Organic Solar Cells

Abstract

Recently, the dependence of exciton diffusion length $$ (L_{D} ) $$ on some photophysical parameters of organic solids has been experimentally demonstrated, however no systematic theoretical analysis of this phenomenon has been carried out. We have conducted a theoretical study by using the Forster resonance energy transfer and Dexter carrier transfer mechanisms together with the Einstein–Smoluchowski diffusion equation to derive analytical models for the diffusion lengths $$ (L_{D} ) $$ and diffusion coefficients $$ (D) $$ of singlet $$ (S) $$ and triplet $$ (T) $$ excitons in organic solids as functions of spectral overlap integral $$ (J) $$ , photoluminescence (PL) quantum yield $$ (\phi_{D} ) $$ , dipole moment $$ (\mu_{T} ) $$ and refractive index $$ (n) $$ of the photoactive material. The exciton diffusion lengths and diffusion coefficients in some selected organic solids were calculated, and we found that the singlet exciton diffusion length $$ (L_{D}^{S} ) $$ increases with $$ \phi_{D} $$ and J, and decreases with n. Also, the triplet exciton diffusion length $$ (L_{D}^{T} ) $$ increases with $$ \phi_{D} $$ and decreases with $$ \mu_{T} $$ . These may be achieved through doping the organic solids into broad optical energy gap host materials as observed in previous experiments. The calculated exciton diffusion lengths are compared with experimental values and a reasonably good agreement is found between them. The results presented are expected to provide insight relevant to the synthesis of new organic solids for fabrication of bulk heterojunction organic solar cells characterized by better power conversion efficiency.