We examine exciton diffusion in a triplet-sensitized organic photovoltaic cell, where transport occurs via the long-lived triplet state of a fluorescent electron donor. While the triplet state is optically dark, it is populated via sensitization by a guest species capable of intersystem crossing. Here, the host material is metal-free phthalocyanine (H2Pc), and the triplet-sensitizing guest is copper phthalocyanine (CuPc). Optical excitation of H2Pc leads to the generation of singlet excitons which rapidly undergo energy transfer to CuPc. Excitons on CuPc undergo intersystem crossing to the triplet state followed by energy transfer back to the H2Pc triplet state. The exciton diffusion length (LD) is extracted using an internal quantum efficiency ratio methodology that permits accurate device-based measurements of exciton transport even in the presence of geminate recombination losses. The donor layer LD varies with composition with a maximum LD of (13.4 ± 1.6 nm) observed at 20 vol % CuPc, an almost 60% increase over the case of the mobile H2Pc singlet. Despite this increase, further improvements may be possible as the neat-film H2Pc triplet LD is estimated to exceed (20.7 ± 5.0) nm.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (NSF) Program in Solid-State and Materials Chemistry DMR-1708177. R.J.H. would like to acknowledge support from a Leverhulme Trust Visiting Professorship at the University of Cambridge and a Visiting Fellowship at Clare Hall, University of Cambridge. The authors thank Dr. D. Wayne Blaylock at the Dow Chemical Company for synthesizing NPD.
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