Influence of Electrode Surface Composition and Energetics on Small-Molecule Organic Solar Cell Performance: Polar versus Nonpolar Donors on Indium Tin Oxide Contacts

Abstract

We present a systematic study of the band-edge energy offsets and shifts in local vacuum levels for indium tin oxide (ITO)/donor heterojunctions, using vacuum-deposited chloroindium phthalocyanine (ClInPc), titanyl phthalocyanine (TiOPc), pentacene (PEN), and copper phthalocyanine (CuPc) donor layers. We include a comparison of the performance of ITO/donor/C60 planar heterojunction (PHJ) organic solar cells (OPVs) as a function of activation and modification of the ITO contact. UV photoemission spectroscopy (UPS) was used to characterize the interfacial region between these donors and ITO to infer the ordering of these first deposited 1–2 monolayers as a function of ITO activation and modification. For the polar donors ClInPc and TiOPc, deposited on air plasma (AP) treated, high work function ITO (φeff ≈ 5.1–5.2 eV), shifts in local vacuum level observed during deposition of the first two monolayers of donor molecules suggest that the halo–metal or oxo–metal bond points (on average) toward the oxide surfa...