Chemical and Electronic Properties of PTB7-Th and ITIC on ZnO Interfaces in Organic Solar Cells

ZnO is commonly used as a cathode interface layer in organic photovoltaics due to its capability to extract electrons from the active layer. In this work, we focused on the electronic structure of the interface between ZnO and an active layer containing poly[[2,60-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene (ITIC). Several electron spectroscopy techniques were used to determine the energy levels of these materials at their interface. The investigations show that the energy levels of PTB7-Th and ITIC were well aligned with ZnO. It is also found that there are two dipoles that form at the PTB7-Th/ZnO interface; the first is formed directly at the PTB7-Th/ZnO interface, whereas a dipole with an opposite polarity is formed within the PTB7-Th positioned further away from the interface. The findings of this work play an important role in the factors influencing the device performance of photovoltaic devices including ZnO as an interface layer.