Importance of molecular alignment for organic photovoltaic devices

We study the effect of molecular alignment on the performance of organic photovoltaic devices (OPV) by using two porphyrin molecules of etioporphyrin-I (HOX) and octaethylporphyrin (HOEP) with similar chemical structures. Due to the minor differences in their peripheral substituents, the two porphyrin molecules form crystalline films of different molecular alignment. This has been verified by broader and redshift in the UV-vis absorption spectra as well as the grazing incidence x-ray diffraction of the films. OPV devices fabricated with these films show different performance. Stronger pi-pi stacking of HOX molecules exhibited better charge transport and photovoltaic performance than that of HOEP molecules. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3502598]