A computational investigation on core-expanded subphthalocyanines

Subazaphenalenephthalocyanines are the derivatives of boron subphthalocyanine chloride with a core-expanded six-membered ring, which have a wide absorption range in the visible region and seem to have great potential to be new photo-electro materials. However, it is still an open question how the core expansion influences the electronic and optical properties of boron subphthalocyanine chloride. In the present work, three subazaphenalenephthalocyanine molecules, having one, two, and three six-membered rings, are explored by using density functional theory. The relaxation energy, electronic structures, UV-Vis spectra, and exciton binding energy of three molecules are calculated, and the open circuit voltages of subazaphenalenephthalocyanine/C60 solar cells are roughly predicted using an empirical equation. The results show that the planarity, the wavelength of absorption peaks, and HOMO energy increase gradually along with the increasing number of expanded rings, while the trends of open circuit voltage and relaxation energy are opposite. We find that the subazaphenalenephthalocyanine molecule with three six-membered rings seems to have great potential to be a new donor material of organic solar cell because it has the smallest exciton binding energy and relaxation energy, and strong absorption strength in the visible region.