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Issue 10, 2012
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Full electronic structure across a polymer heterojunction solar cell

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We correlate the morphology and energy level alignment of bilayer structures comprising the donor poly(3-hexylthiophene) (P3HT) and the acceptor polyfluorene copolymer poly(9,9′dialklylfluorene-alt-4,7-bis(2,5-thiendiyl)-2,1,3-benzothiadiazole) (PFTBTT) with the performance of these bilayers in organic photovoltaic cells (OPVCs). The conducting polymer poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) was used as the bottom electrode and Ca as the top electrode. Ultraviolet photoelectron spectroscopy (UPS) revealed that notable interface dipoles occur at all interfaces across the OPVC structure, highlighting that vacuum level alignment cannot reliably be used to estimate the electronic properties for device design. Particularly the effective electrode work function values (after contact formation with the conjugated polymers) differ significantly from those of the pristine electrode materials. Chemical reactions between PEDT:PSS and P3HT on the one hand and Ca and PFTBTT on the other hand are identified as cause for the measured interface dipoles. The vacuum level shift between P3HT and PFTBTT is related to mutual energy level pinning at gap states. Annealing induced morphological changes at the P3HT/PFTBTT interface increased the efficiency of OPVCs, while the electronic structure was not affected by thermal treatment.

Graphical abstract: Full electronic structure across a polymer heterojunction solar cell

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Supplementary files

Article information

03 Oct 2011
15 Nov 2011
First published
03 Jan 2012

J. Mater. Chem., 2012,22, 4418-4424
Article type

Full electronic structure across a polymer heterojunction solar cell

J. Frisch, M. Schubert, E. Preis, J. P. Rabe, D. Neher, U. Scherf and N. Koch, J. Mater. Chem., 2012, 22, 4418
DOI: 10.1039/C1JM14968G

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