Substrate-dependent electronic structure of an organic heterojunction

Abstract

This work focuses on organic-organic heterojunctions (OOHs) formed between two small-molecule, low band-gap, semiconductors, tris{2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}$[3,4\text{\ensuremath{-}}b,$$h,n]\text{\ensuremath{-}}1,4,5,$$8,9,12$ $\ensuremath{-}\text{hexaaza}\ensuremath{-}\text{triphenylene}$ (THAP), and copper phthalocyanine (CuPc). The organic layers are deposited on various substrates, and the energy level alignment between them is investigated by ultraviolet photoemission spectroscopy. The electronic structure of the OOH is found to depend on the work function of the organic underlayer predeposited on the different substrates. The vacuum level offset between THAP and CuPc, which consists of the sum of the interface dipole and the molecular level shift, varies from $0.26\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The interface dipole between the two organic films linearly changes with the work function of the organic underlayer.