DFT and TD-DFT studies on the electronic and optical properties of linear π-conjugated cyclopentadithiophene (CPDT) dimer for efficient blue OLED

Abstract

We present a comprehensive theoretical study on the optical and electronic properties of the organic conjugated dimer containing the 4H-cyclopenta[2,1-b:3,4-b’]dithiophen-4-one, as monomer unit, suitable for blue organic light emitting diodes (OLEDs). Ground and excited states properties of the dimer, denoted hereafter as CPDT, were theoretically computed using the Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) at the level of B3LYP/6-311g(d,p) in acetonitrile, respectively. The UV–vis absorption spectrum of CPDT exhibits peaks in the range 250–450 nm and small optical band gap of 2.38 eV. The excited CPDT dimer shows emission in the deep blue domain (∼420 nm). The well resolved vibronic spectra of absorption and emission line profiles with good overlap have separated by the smaller Stokes shift (47 nm, 0.37 eV).Additionally, the two optical bands appeared at 672 nm (UV–vis) and 827 nm (PL) have been attributed to the formation of an intra-molecular charge transfer (ICT) state. Regarding the calculated ionization potential (IP), electronic affinity (EA) and the reorganization energies, the compound presents high performance of charge mobility with the International Commission on Illumination coordinates of (x,y) = (0.16,0.04). The data are interpreted using molecular electrostatic potential surface and the CIE color coordinates.In the three layers based OLED [ITO/NPD/CPDT/Alq3/LiF:Al], the electrical response was evaluated based on the current-voltage (I-V) characteristics. The threshold voltage for luminescence is about 4.6 V at which the carriers appear to reach the CPDT light emitting layer.