Revealing nanoscale phase separation in small-molecule photovoltaic blends by plasmonic contrast in the TEM

Good prospects of organic solar cells (OSC) are dampened by relatively poor transport properties in the organic donor-acceptor (D-A) absorber system. A widely recognized key to overcome this drawback is the ability to control its D-A morphology at the bulk heterojunction (BHJ) on the nanometer scale. An essential prerequisite for this is to characterize the BHJ of the carbonaceous materials combination. However, due to their similarity, particularly if not crystalline, only poor-contrast images are achieved by conventional transmission electron microscopy (TEM). We apply energy filtered transmission electron microscopy spectrum imaging (EFTEM SI) to study local phase separation with chemical sensitivity in a co-evaporated zinc phthalocyanine (ZnPc) (D) and C-60 (A) blend layer. As image contrast we exploit the significant difference between the plasmon energies of the pure materials (23 and 26 eV for ZnPc and C-60, respectively) as recorded by electron energy loss spectroscopy. In order to fully understand its structure and morphology, on the one hand conventional TEM analysis allows to identify suitably oriented crystalline phases on the basis of lattice fringes (here only C-60). On the other hand the EFTEM SI plasmon map discloses the complete lateral distribution of the two molecular phases independent of crystallinity. (C) 2012 Elsevier B. V. All rights reserved.