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Author Curtin, Ian J. ♦ Holmes, Russell J. ♦ Blaylock, D. Wayne
Source United States Department of Energy Office of Scientific and Technical Information
Content type Text
Language English
Subject Keyword 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
Abstract 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.
ISSN 00036951
Educational Use Research
Learning Resource Type Article
Publisher Date 2016-04-18
Publisher Place United States
Journal Applied Physics Letters
Volume Number 108
Issue Number 16


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