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Proceedings Paper

Influence of solvents and substrates on the morphology and the performance of low-bandgap polyfluorene: PCBM photovoltaic devices
Author(s): Cecilia M. Björström; Svante Nilsson; Kjell O. Magnusson; Ellen Moons; Andrzej Bernasik; Jakub Rysz; Andrzej Budkowski; Fengling Zhang; Olle Inganäs; Mats R. Andersson
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Paper Abstract

Spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as the active material in polymer photovoltaic cells. Such blends are known for their tendency to phase separate during film formation. Tuning the morphology of the blend in a controlled way is one possible road towards higher efficiency. We studied the effect of adding chlorobenzene to chloroform-based blend solutions before spin-coating on the conversion efficiency of APFO-3:PCBM photodiodes, and related that to the lateral and vertical morphology of thin films of the blend. The lateral morphology is imaged by atomic force microscopy (AFM) and the vertical compositional profile is obtained by dynamic secondary ion mass spectrometry (SIMS). The profiles reveal compositional variations consisting of multilayers of alternating polymer-rich and PCBM-rich domains in the blend film spin-coated from chloroform. The vertical compositional variations are caused by surface-directed spinodal waves and are frozen in during the rapid evaporation of a highly volatile solvent. With addition of the low-vapour pressure solvent chlorobenzene, a more homogeneous vertical composition is found. The conversion efficiency for solar cells of this blend was found to be optimal for chloroform:chlorobenzene mixtures with a volume-ratio of 80:1. We have also investigated the role of the substrate on the morphology. We found that blend films spin-coated from chloroform solutions on PEDOT:PSS-coated ITO show a similar compositional structure as the films on silicon, and that changing the substrate from silicon to gold only affects the vertical phase separation in a region close to the substrate interface.

Paper Details

Date Published: 20 April 2006
PDF: 9 pages
Proc. SPIE 6192, Organic Optoelectronics and Photonics II, 61921X (20 April 2006); doi: 10.1117/12.662857
Show Author Affiliations
Cecilia M. Björström, Karlstad Univ. (Sweden)
Svante Nilsson, Karlstad Univ. (Sweden)
Kjell O. Magnusson, Karlstad Univ. (Sweden)
Ellen Moons, Karlstad Univ. (Sweden)
Andrzej Bernasik, AGH-Univ. of Science and Technology (Poland)
Jakub Rysz, Jagiellonian Univ. (Poland)
Andrzej Budkowski, Jagiellonian Univ. (Poland)
Fengling Zhang, Linköping Univ. (Sweden)
Olle Inganäs, Linköping Univ. (Sweden)
Mats R. Andersson, Chalmers Univ. of Technology (Sweden)

Published in SPIE Proceedings Vol. 6192:
Organic Optoelectronics and Photonics II
Paul L. Heremans; Michele Muccini; Eric A. Meulenkamp, Editor(s)

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