Theoretical Study of the Electronic Structure and Properties of Alternating Donor-Acceptor Couples of Carbazole-Based Compounds for Advanced Organic Light-Emitting Diodes (OLED)

Abstract

Generally, it is difficult to generate a high-performance pure blue emission organic light-emitting diode (OLED). That is because the intrinsically wide band-gap makes it hard to inject charges into the emitting layer in such devices. To solve the problem, carbazole derivatives have been widely used because they have more thermal stability, a good hole transporting property, more electron rich (p-type) material, and higher photoconductivity. In the present work, novel copolymers containing donor-acceptor-acceptor-donor (D-A-A-D) blue compounds used for OLEDs were investigated. The theory of the geometrical and electronic properties of N-ethylcarbazole (ECz) as donor molecule (D) coupled to a series of 6 acceptor molecules (A) for advanced OLEDs were investigated. The acceptors were thiazole (TZ), thiadiazole (TD), thienopyrazine (TPZ), thienothiadiazole (TTD), benzothiadiazole (BTD), and thiadiazolothienopyrazine (TDTP). The ground state structure of the copolymers were studied using Density Functional Theory (DFT) at B3LYP/6-31G(d) level. Molecular orbital analysis study indicated 3 investigated copolymers (ECz-diTZ-ECz, ECz-diTD-ECz, ECz-diBTD-ECz) have efficient bipolar charge transport properties for both electron and hole injection to the TiO2 conduction band (4.8 eV). In addition, the excited states electronic properties were calculated using Time-Dependent Density Functional Theory (TD-DFT) at the same level. Among these investigated copolymer ECz-diTZ-ECz and ECz-diTD-ECz showed the maximum absorption wavelengths (λabs) with blue emitting at 429 and 431 nm, respectively. The results suggested that selected D-A-A-D copolymers can improve the electron- and hole- transporting abilities of the devices. Therefore, the designed copolymers would be a promising material for future development of light-emitting diodes, electrochromic windows, photovoltaic cells, and photorefractive materials.