Enhanced Exciton Diffusion using Phosphor Sensitization @ University of Minnesota

Author(s): Wade Luhman, Russell Holmes- University of Minnesota

Graduate student Wade Luhman working with Professor Holmes is examining approaches to overcome the exciton diffusion bottleneck in organic solar cells. In contrast to the morphology-based approaches often employed, Luhman is instead using energy transfer via a phosphorescent sensitizer to populate the long-lived triplet exciton state of a fluorescent donor material, permitting an increase in L_D and active layer thickness. Here, the donor layer consists of an N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD) host doped with the phosphorescent guest fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃), with a thin acceptor layer of C₆₀. The enhancement in donor L_D relies on a multi-step process beginning with the absorption of light and formation of singlet excitons on the NPD host. Photogenerated excitons are transferred to the singlet state of the Ir(ppy)₃ guest by Förster transfer, followed by rapid intersystem crossing to the triplet state. A final transfer occurs from the triplet level of Ir(ppy)₃ to that of NPD. With the photogenerated excited state occupying the NPD triplet, a larger L_D is possible due to the long lifetime of the triplet relative to the singlet. An increase in the NPD L_D from (6.5±0.3) nm to (11.8±0.6) nm is extracted from measurements of the external quantum efficiency for donor layers containing 5 wt.% Ir(ppy)₃. This enhancement leads to a ~80% improvement in the power conversion efficiency relative to devices containing an undoped donor layer. This approach is of interest since it allows for a decoupling of the functions of optical absorption and exciton diffusion, potentially broadening the scope of materials suitable for use in OPVs. 

 

 

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