The morphology of polyfluorene: fullerene blend films for photovoltaic applications
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Polymer photovoltaic systems whose photoactive layer is a blend of a semiconducting polymer with a fullerene derivative in a bulk heterojunction configuration are amongst the most successful organic photovoltaic devices nowadays. The three-dimensional organization in these layers (the morphology) plays a crucial role in the performance of the devices. Detailed characterization of this organization at the nanoscale would provide valuable information for improving future material and architectural design and for device optimization.
In this thesis, the results of morphology studies of blends of several polyfluorene copolymers (APFOs) blended with a fullerene derivative are presented. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was combined with dynamic Secondary Ion Mass Spectrometry (dSIMS) for surface and in-depth characterization of the blend films. NEXAFS was performed using two different electron detection methods, partial (PEY) and total (TEY) electron yield, which provide information from different depth regimes. Quantitative compositional information was obtained by fitting the spectra of the blend films with a linear combination of the spectra of films of the pure components. In blends of APFO3 with PCBM in two different blend ratios (1:1 and 1:4 of polymer:fullerene) NEXAFS data show the existence of compositional gradients in the vertical direction for both blend ratios, with clear polymer enrichment of the free surface. A series of APFOs with systematic changes in the side-chains was studied and it was shown that those small modifications can affect polymer:fullerene interaction and induce vertical phase separation. Polymer-enrichment of the free surface was clearly identified, in accordance with surface energy minimization mechanisms, and a compositional gradient was revealed already in the first few nanometers of the surface of the blend films. dSIMS showed that this vertical phase separation propagates throughout the film. It was possible to determine that as the polar character of the polymer increases, and thus the polymer:fullerene miscibility is improved, the tendency for vertical phase separation becomes stronger.
Place, publisher, year, edition, pages
Karlstad: Karlstad University , 2011. , 47 p.
Karlstad University Studies, ISSN 1403-8099 ; 2011:36
Materials Science, Organic Photovoltaics, Polymer Solar Cells
Research subject Materials Engineering
IdentifiersURN: urn:nbn:se:kau:diva-7950ISBN: 978-91-7063-370-6OAI: oai:DiVA.org:kau-7950DiVA: diva2:430532
2011-09-02, 21D 302, Karlstads universitet, Karlstad, 10:15 (Swedish)
Rensmo, Håkan, Docent
Paper II was not published at the time of the licentiate defence and had the title: NEXAFS spectroscopy study of the surface composition in APFO3:PCBM blend films2011-08-102011-07-112016-08-16Bibliographically approved
List of papers