DFT and TD-DFT Study on the Structural, Optoelectronic and Photovoltaic Properties of Chemically Modified Donor-Acceptor Conjugated Oligomers for Organic Solar Cells

Abstract

In this paper, a novel theoretical tool for some new low-band-gap copolymers has been developed on the basis of density functional theory (DFT) quantum chemical calculations to model their optoelectronic properties. We have designed a series of novel double organic D-π-A (electron donor-π-conjugated-acceptor) dyes employed in dye-sensitized solar cells (DSSCs). These copolymers are constituted of (Carbazole-Methylthiophene), benzothiadiazole and thiophene [(Cbz-Mth)-B-T] units essentially as well as their derivatives leading to donor (D)-acceptor (A) structure-types. The cyanoacrylic acid (A) anchoring group leads to more red shift of absorption bands. The optimized structures and optoelectronic properties of these dyes were investigated by using the Density Functional Theory DFT/B3LYP/6-31G (d, p) method and Time Dependant Density Functional Theory (TD/DFT) calculations. Firstly, we studied the insertion of thiophene as spacer unit into [(Cbz-Mth)-B]2-A backbone to reach (Carbazole-Methylthiophene), benzothiadiazole-thiophene-cyanoacrylic acid [(Cbz-Mth)-B-T]2-A copolymer, secondly, thiophene usual unit was replaced by bithiophene and three-thiophene respectively, entity to obtain (Carbazole-Methylthiophene), benzothiadiazole-dithiophene-cyanoacrylic acid [(Cbz-Mth)-B-DT]2-A and (Carbazole-Methylthiophene), benzothiadiazole-Three-Thiophene-cyanoacrylic acid [(Cbz-Mth)-B-TT]2-A copolymers. Later, we examined the insertion effect of bridging by C=C (CN)2 groups on the energy gaps and the electronic properties of the study copolymer [(Cbz-Mth)-B-T-C=C (CN)2-T]2-A, and finally, ethynyl spacer was added to obtain a novel oligomer model, denoted (Carbazole-Methylthiophene)–ethynyl- benzothiadiazole-dithiophene- C=C(CN)2 groups-cyanoacrylic acid [(Cbz-Mth)-E-B-T-C=C(CN)2-T]2-A and to investigate their bridging effect into the main backbone on various properties by examining structural and electronic properties. The calculated geometries indicate that these dyes are all coplanar. In order to predict the band gaps for guiding the synthesis of novel materials with low band gaps, we applied quantum-chemical techniques to calculate the band gaps in several oligomers. The analysis of microelectronic and photonic structure in one dimension program (AMPS-1D) program has been successfully used to study the compounds organic solar cells. The calculated results of these dyes demonstrate that these compounds are then blended with [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) in bulk-heterojunction solar cell.