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Reliability of charge carrier recombination data determined with charge extraction methods
Univ Potsdam, Germany.
Univ Potsdam, Germany.
Univ Potsdam, Germany.
Univ Potsdam, Germany.
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2019 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 126, no 20, article id 205501Article in journal (Refereed) Published
Abstract [en]

Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k(2) = 1.7 x 10(-17)m(3) s(-1). BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness. Published under license by AIP Publishing.

Place, publisher, year, edition, pages
AMER INST PHYSICS , 2019. Vol. 126, no 20, article id 205501
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URN: urn:nbn:se:liu:diva-163044DOI: 10.1063/1.5129037ISI: 000504006000029OAI: oai:DiVA.org:liu-163044DiVA, id: diva2:1384247
Note

Funding Agencies|German Ministry of Science and Education [FKZ 13N13719]; STW/NWOTechnologiestichting STWNetherlands Organization for Scientific Research (NWO) [VIDI 13476]; National Natural Science Foundation of China (NNSFC)National Natural Science Foundation of China [21875286]

Available from: 2020-01-09 Created: 2020-01-09 Last updated: 2020-01-09

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Zhang, HuotianGao, Feng
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Biomolecular and Organic ElectronicsFaculty of Science & Engineering
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