title: Chemical Analysis of the Interface in Bulk-Heterojunction Solar Cells by X-ray Photoelectron Spectroscopy Depth Profiling.
abstract: Despite the wide use of blends combining an organic p-type polymer and molecular fullerene-based electron acceptor, the proper characterization of such bulk heterojunction materials is still challenging. To highlight structure-to-function relations and improve the device performance, advanced tools and strategies need to be developed to characterize composition and interfaces with sufficient accuracy. In this work, high-resolution X-ray photoelectron spectroscopy (XPS) is combined with very low energy argon ion beam sputtering to perform a nondestructive depth profile chemical analysis on full Al/P3HT:PCBM/PEDOT:PSS/ITO (P3HT, poly(3-hexylthiophene); PCBM, [6,6]-phenyl-C61-butyric acid methyl ester; PEDOT, poly(3,4-ethylenedioxythiophene; PSS, polystyrenesulfonate; ITO, indium tin oxide) bulk-heterojunction solar cell device stacks. Key information, such as P3HT and PCBM composition profiles and Al-PCBM chemical bonding, are deduced in this basic device structure. The interface chemical analysis allows us to evidence, with unprecedented accuracy, the inhomogeneous distribution of PCBM, characterized by a strong segregation toward the top metal electrode. The chemical analysis high-resolution spectra allows us to reconstruct P3HT/PCBM ratio through the active layer depth and correlate with the device deposition protocol and performance. Results evidence an inhomogeneous P3HT/PCBM ratio and poorly controllable PCBM migration, which possibly explains the limited light-to-power conversion efficiency in this basic device structure. The work illustrates the high potential of XPS depth profile analysis for studying such organic/inorganic device stacks.
authors.affiliation: Research Center in the Physics of Matter and Radiation (PMR), Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), University of Namur , B-5000 Namur, Belgium.
authors.forename: Yan
authors.identifier: http://orcid.org/0000-0002-6826-6142
authors.initials: Y
authors.lastname: Busby
authors.affiliation: Institut für Physik, Institut für Chemie & IRIS Adlershof, Humboldt-Universität zu Berlin , Brook-Taylor-Straße 6, 12489 Berlin, Germany.
authors.forename: Emil J W
authors.initials: EJ
authors.lastname: List-Kratochvil
authors.affiliation: Research Center in the Physics of Matter and Radiation (PMR), Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), University of Namur , B-5000 Namur, Belgium.
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Pireaux
keywords: P3HT:PCBM; XPS; bulk heterojunction; chemical analysis; depth profile; low-energy ion beams; solar cells