Intrinsic Electronic Structures and Optical Anisotropy of α- and β-Phase Copper Phthalocyanine Molecular Crystals

Electronic structures and optical anisotropy of α- and β-phase copper phthalocyanine (CuPc) molecular crystals have been systemically investigated by first-principles calculations based on Density Functional Theory (DFT). Both crystals were shown to be small gap organic semiconductors with relatively flat and dispersionless bands. The α-CuPc was a direct band gap semiconductor, whereas the β-CuPc was an indirect band gap semiconductor. The analysis of Partial Density of States (PDOS) showed that the top of valance band was mainly contributed by N 2p and C 2p states; the bottom of the conduction band was mainly contributed by N 2p, C 2p and Cu 3d states. The interband optical properties, such as the complex dielectric function, absorption coefficient and complex refractive index, showed a high degree of anisotropy that can be traced to the unique structures of these molecular crystals. The calculated dielectric function for α-CuPc in the low energy region was consistent with the experiment results proposed in the literature. These calculations provided particular interpretations on electronic structure and optical properties of α- and β-CuPc organic semiconductors that were critical to optoelectronics, which would promote the applications of these materials in semiconductor optoelectronic devices.