Role of impurities in determining the exciton diffusion length in organic semiconductors

Journal Article · Mon Apr 18 00:00:00 EDT 2016 · Applied Physics Letters

DOI:https://doi.org/10.1063/1.4945688· OSTI ID:22591615

Curtin, Ian J.; Holmes, Russell J. ^[1]; Blaylock, D. Wayne ^[2]

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1. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (United States)
2. Engineering and Process Sciences, Core R&D, The Dow Chemical Company, Midland, Michigan 48674 (United States)

The design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length (L{sub D}). Despite the importance of this parameter, there have been few investigations connecting L{sub D} and materials purity. Here, we investigate L{sub D} for the organic small molecule N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine as native impurities are systematically removed from the material. Thin films deposited from the as-synthesized material yield a value for L{sub D}, as measured by photoluminescence quenching, of (3.9 ± 0.5) nm with a corresponding photoluminescence efficiency (η{sub PL}) of (25 ± 1)% and thin film purity of (97.1 ± 1.2)%, measured by high performance liquid chromatography. After purification by thermal gradient sublimation, the value of L{sub D} is increased to (4.7 ± 0.5) nm with a corresponding η{sub PL} of (33 ± 1)% and purity of (98.3 ± 0.8)%. Interestingly, a similar behavior is also observed as a function of the deposition boat temperature. Films deposited from the purified material at a high temperature give L{sub D} = (5.3 ± 0.8) nm with η{sub PL} = (37 ± 1)% for films with a purity of (99.0 ± 0.3)% purity. Using a model of diffusion by Förster energy transfer, the variation of L{sub D} with purity is predicted as a function of η{sub PL} and is in good agreement with measurements. The removal of impurities acts to decrease the non-radiative exciton decay rate and increase the radiative decay rate, leading to increases in both the diffusivity and exciton lifetime. The results of this work highlight the role of impurities in determining L{sub D}, while also providing insight into the degree of materials purification necessary to achieve optimized exciton transport.

• OSTI ID: 22591615
• Journal Information: Applied Physics Letters, Vol. 108, Issue 16; Other Information: (c) 2016 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0003-6951
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
DEPOSITION
DIFFUSION
DIFFUSION LENGTH
HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY
IMPURITIES
ORGANIC SEMICONDUCTORS
PHOTOLUMINESCENCE
PHOTOVOLTAIC CELLS
PHOTOVOLTAIC EFFECT
PURIFICATION
RADIATIVE DECAY
SUBLIMATION
TEMPERATURE GRADIENTS
THIN FILMS