Thin air-plasma-treated alkali fluoride layers for improved hole extraction in copper phthalocyanine/C-70-based solar cells

TitleThin air-plasma-treated alkali fluoride layers for improved hole extraction in copper phthalocyanine/C-70-based solar cells
Publication TypeJournal Article
Year of Publication2012
AuthorsXiao T, Cui WP, Cai M, Liu R, Anderegg JW, Shinar J, Shinar R
Journal TitleJournal of Photonics for Energy
Volume2
Pages021006
Date Published03
Type of ArticleArticle
ISBN Number1947-7988
Accession NumberWOS:000309362300006
Keywordsalkali fluorides, buffer, conjugated polymer, CuPc, ELECTRON INJECTION, extraction, film, hole, LAYER, lif, LIGHT-EMITTING-DIODES, organic photovoltaic cells, performance, photoelectron-spectroscopy, small-molecule organic solar cells, titanium-oxide
Abstract

Alkali fluorides, mostly LiF and CsF, are well-known to improve electron injection/extraction in organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). They are also utilized, though to a lesser extent, for hole injection in OLEDs. Here we demonstrate a new role for such fluorides in enhancing OSCs' hole extraction. We show that an ultrathin air-plasma-treated alkali fluoride layer between the indium tin oxide (ITO) anode and the active layer in copper phthalocyanine (CuPc)/C-70-based OSCs increases the short circuit current by up to similar to 17% for cells with LiF and similar to 7% for cells with NaF or CsF. The effects of the fluoride layer thickness and treatment duration were evaluated, as were OSCs with oxidized and plasma-treated Li and UV-ozone treated LiF. Measurements included current voltage, absorption, external quantum efficiency (EQE), atomic force microscopy, and x-ray photoelectron spectroscopy, which showed the presence of alkali atoms F and O at the treated ITO/fluoride surface. The EQE of optimized devices with LiF increased at wavelengths >560 nm, exceeding the absorption increase. Overall, the results indicate that the improved performance is due largely to enhanced hole extraction, possibly related to improved energy-level alignment at the fluorinated ITO/CuPc interface, reduced OSC series resistance, and in the case of LiF, improved absorption. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.JPE.2.021006]

URL<Go to ISI>://WOS:000309362300006
DOI10.1117/1.jpe.2.021006
Alternate JournalJ. Photonics Energy