What determines the performance of metal phthalocyanines (MPc, M = Zn, Cu, Ni, Fe) in organic heterojunction solar cells? A combined experimental and theoretical investigation

An investigation on the MPc series (M = Zn, Cu, Ni, Fe) targeted at a fundamental under-standing of the physical origin of their different performance in organic solar cells is presented. MPc/C-60 based bilayer hetero-junction (HJC) devices were built and analyzed. The crystal structure in thin films of MPc's was explored by high resolution XRPD and AFM measurements and compared to powder data. The absorption spectra of the evaporated MPc films was observed to differ only slightly, while there was a strong fluorescence quenching for M = Cu, Ni, Fe as opposed to ZnPc. The differences in the solid state fluorescence were further explored by high-level quantum mechanical calculations. Thus, the calculations of the lowest excited states show, that there are intermediate states below the Q-band (pi-pi*) excitation for all of the MPc series but ZnPc. It will be discussed, that these states are ligand-metal exchange coupled states, which are treated at the multi-reference configuration interaction (MR-CI) level, in order to account for multi-configurational mixing and double excitations. The implications of these electronic effects on exciton diffusion are discussed and related to device performance. (C) 2009 Elsevier B.V. All rights reserved.