Switchable Crystal Phase and Orientation of Evaporated Zinc Phthalocyanine Films for Efficient Organic Photovoltaics

In photovoltaic cells, increasing the light absorption coefficient enhances the photocurrent and quantum efficiency of the cell. Here, we report an efficient approach that enhances light absorption by changing the crystal phase of an evaporated ZnPc film on indium tin oxide (ITO) substrates. Under thermal treatment (>200 degrees C), the crystal phase of ZnPc changed from metastable alpha-ZnPc to stable beta-ZnPc. We tested and compared the effects of this phase transition on the donor film of organic photovoltaic (OPV) cells. After treating the beta-ZnPc film at 220 degrees C, the short-circuit current of the OPV was improved from 2.58 to 3.40 mA/cm(2), and the quantum efficiency was improved from 650 to 750 nm. Moreover, on a copper iodide (CuI)-ITO substrate, the ZnPc molecules were precisely controlled in the lying-down orientation. With their lying-down orientations, the light absorbance capacities of the metastable aZnPc phase and stable beta-ZnPc phase (annealed at 300 degrees C) were 1.3- and 1.5-fold higher, respectively, than that of metastable alpha-ZnPc standing upright on the ITO substrate. This work provides a fundamental understanding of the crystal phases and orientation changes of the ZnPc molecules, enabling maximization of the light absorption coefficient and the design of efficient small-molecule OPV cells.