Effects of Intermolecular Distance on the Absorption Spectra of Organic Semiconductors

Organic semiconductors have several advantages over conventional inorganic semiconductors. The optical properties of π-conjugated organic molecules are important in determining the performance efficiencies of photovoltaic cells, optoelectronic devices and organic thin-film transistors and lasers.  This work examined the variation of absorption spectra of three organic semiconductors (perylene, pentacene, and sexithienyl) with their geometries and intermolecular distances between the dimer molecules. Quantum mechanical calculations using the PM3 semi-empirical approximation were used to obtain the optimal geometry of the semiconductors, and ZINDO/S was used to determine absorption spectra.  The study revealed distinct absorption spectra for each molecule, with specific absorption edges of 499.295 nm (2.49 eV), 506 nm (2.41 eV), and 12449.90 nm (0.1 eV) identified for pentacene, perylene, and sexithienyl momomers, respectively. The significance of intermolecular distance in influencing the absorption spectra of the materials investigated was revealed, indicating a systematic blue shift at certain distances and a red shift at others. For Perylene, a red shift in the absorption edge was observed between the intermolecular distances of 1.5 Å - 3 Å. A blue shift occurred as the separation distance increased to 3.5 Å and 4 Å. A similar trend was observed for Pentacene in which a read shift was noticed between 1.5 Å – 2.5 Å. A blue shift occurred as the intermolecular (separation distance) was raised to 3 Å and 3.5 Å. A different trend was observed for Sexithienyl, where a blue shift in the absorption edge was seen between 1.5 Å and 2.5 Å.  A read shift was observed between 3 Å and 4 Å.  It was observed that the peak absorptions occur at an intermolecular distance of 1.5 Å for both perylene and pentacene and at 2.5 Å for sexithienyl. The minimum absorptions occur at an intermolecular distance of 2.5 Å for both perylene and pentacene and at 4.0 Å for sexithienyl.