Interface formation and chemical reaction between poly(9,9-di-n-hexylfluorenyl-2,7-vinylene) and Ca electrode

Abstract

Conjugated polymer films (poly(9,9-di-n-hexylfluorenyl-2,7-vinylene), PDHFV) are prepared using spin-coating techniques. Synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) are employed to investigate the interface formation and chemical reaction during in situ thermal evaporation of calcium on PDHFV films. In the initial 1.9 ML Ca deposition, the migration of bulk-absorbed oxygen to the surface occurs in response to the deposition of Ca onto PDHFV, and reacts with Ca atoms to form a layer of metal oxide at the Ca/PDHFV interface; meanwhile, Ca strongly reacts with C atoms of PDHFV accompanying immediate charge transfer, which is evidenced by increasing FWHM of C 1s peak and sharp decrease in work function values. All the above should attribute to the chemical reaction between Ca atoms and vinylene double bond. As the coverage of Ca increases from 1.9 ML to 5.4 ML, the main interactions occur between Ca and C atoms at phenyl site, accompanying with the moving of C 1s peak to high binding energy side. In the whole deposition course, no electron injection barrier and obvious gap states are found. By comparing with Ca/poly(9,9-dioctylfluorene) interface, we conclude that, the induction of vinylene double bond during initial deposition results in a stronger interface reaction and effective elimination of gap states, which is beneficial to improving the electroluminescence properties and the stability of interface.