Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films with Solar Cell Performance

The structure of titanyl phthalocyanine (TiOPc) thin films is correlated with photovoltaic properties of planar heterojunction solar cells by pairing different TiOPc polymorph donor layers with C-60 as an acceptor. Solvent annealing and the insertion of two different templating layers, namely 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and CuI, prove to be effective methods to control the TiOPc thin film structure. The crystal phase of TiOPc thin films was identified by combining X-ray reflectivity (XRR) measurements with spectroscopic techniques, including absorption and micro-Raman measurements. Implementation of a donor layer with an absorption spectrum extending into the near-infrared (NIR) led to solar cells with external quantum efficiencies (EQEs) above 27% from lambda = 600 - 890 nm, with the best device yielding a power conversion efficiency (PCE) of 2.6%. Our results highlight the need to understand the relationship between processing parameters and thin film structure, as these have important consequences on device performance.