Engineering of hybrid interfaces in organic photovoltaic devices

Abstract

In this study, we engineer and investigate the interface structure and chemistry at the indium tin oxide (ITO) anode (front-side electrode) as well as at the Mg−Ag cathode (back-side electrode) in metal phthalocyanine (MePc)/C 60 organic solar cells (OSCs). For the front-side electrode, Zn-phthalocyaninetetraphosphonic acid (Zn-PTPA) and Sn-phthalocyanine axially substituted with tartaric acid (Sn-PTA) have been used for the surface termination of ITO coated glass substrates. Both terminations yielded OSCs with higher fill factors and open circuit voltages, thus increasing the power conversion efficiency by 33% and 67%, respectively. A possible influence of a chemisorbed Zn-PTPA on the film growth of the adjacent ZnPc absorber in the vicinity of the hybrid interface is discussed using X-ray reflectivity and near edge X-ray absorption fine structure data. Distinct effects of the Zn-PTPA and Sn-PTA terminations on the electronic properties of the ITO surface were found by X-ray photoelectron spectroscopy (XPS) measurements at the valence band edge. We demonstrate the possibility to engineer the hybrid interface without additional buffer. For the back-side electrode we report the formation of buffer-free charge carrier selective Mg−Ag cathodes, which are applied for bulk heterojunction organic absorbers consisting of copper phthalocyanine (CuPc) donor and fullerene C 60 acceptor materials. The chemical and structural properties of the CuPc:C 60/Mg−Ag interface are investigated by element depth profiling using secondary ion mass spectrometry (SIMS), grazing incidence X-ray diffraction analysis (GI-XRD) and XPS. We demonstrate that an optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag 3Mg. In addition, Mg diffusion into CuPc:C 60 layer is observed. As a result, an interaction between Mg and Cu 2+ with a concurrent change in oxidation state of both metals takes place. However, no formation of MgPc is observed. The findings of this work are discussed against the background of the performance and electrical properties of the corresponding MePc/C 60-based organic solar cells.