Effect of electron‐withdrawing groups on photovoltaic performance of thiophene‐vinyl‐thiophene derivative and benzochalcogenadiazole based copolymers: A computational study

Abstract

We report a density functional theory study of the effect of electron-withdrawing groups such as -F, -CN, -NO 2 on the geometrical, optoelectronic, intramolecular charge transfer (ICT), and photovoltaic properties of (E)-1,2-bis(5-alkyl-[2,3 0bithiophene]-2 0 -yl)ethene (TVT-T) based donor-acceptor (D-A) copolymers with different acceptor units, that is, benzo[c] [1,2,5]thiadiazole, benzo[c][1,2,5]oxadiazole, and benzo[c][1,2,5]selenadiazole. The computed optical absorption spectra of the designed compounds lie in the visible and near-infrared regions. Of all the studied copolymers, -CN substituted and Se-based compound displays the lowest HOMO-LUMO (E H -L ) gap and optical band gap (E opt ). The exciton binding energy (E b ) is found to be smaller for O-incorporated compounds and -CN substituted copolymer as well, inferring more ICT.The electron-hole coherence concentrated over the D-A units is nearly the same for -CN and -NO 2 substituted compounds, but larger in -F derivatives, indicating weak electronhole coupling in the formers. Comparatively larger dipole moment (6.421 Debye-9.829 Debye) and charge transfer length (D CT ) (1.976 Å-3.122 Å) for -CN derivatives lead to enhanced ICT properties. The designed donors yield good hole mobilities (0.127-6.61 cm 2 V −1 s −1 ) and the predicted power conversion efficiencies are calculated to be as high as~6%-7% for -CN and -NO 2 substituted compounds. K E Y W O R D SDFT, donor-acceptor copolymer, electron-withdrawing groups, Intramolecular charge transfer, power conversion efficiency | INTRODUCTIONOrganic solar cells (OSC) or organic photovoltaics (OPVs) incorporating conjugated polymers (CPs) have been a thrust area of research due to their diverse potential applications. In addition, these materials are envisaged to be highly cost-effective, lightweight, mechanically flexible, environmentally benign and hence are considered as efficacious alternatives to the conventional inorganic solar cell. [1][2][3][4][5] The highest achieved power conversion efficiency (PCE) of bulk heterojunction OPV devices containing CPs as donor materials and fullerene derivatives as acceptor units were reported so far to be over 11%. [6][7][8] Conventional organic materials naturally have band gap greater than 2 eV encompassing the only visible range of the solar spectra (300-650 nm). However, to achieve high PCE, materials need to have a lower band gap as well as deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. A deep HOMO level increases the open circuit voltage

Computational Study on Optoelectronic Properties of Donor-Acceptor Type Small π-Conjugated Molecules for Organic Light-Emitting Diodes (OLEDs) and Nonlinear Optical (NLO) Applications

“…The band gap energies are about 3.16 eV and 2.81 eV for M1 and M2, respectively. These lower values of band gaps with the FMOs distributions demonstrate the presence of a significant intra-molecular charge transfer (ICT) that leads to enhance the electronic properties [37].…”

Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based on Cyclopentadithiophene as Donors in Bulk Heterojunction Organic Solar Cells: DFT-Based Modeling and Calculations

“…The interest of examining the bridge bond length is to get an idea about the interactions among the different building blocks. Where, the shorter bridge bond length leads to stronger intra-molecular interactions and higher charge transfer [44,45]. For the studied compounds, the bridge bond defines the bond C-C between the CPDT donor and BT or DPP acceptor units.…”

Enhancement of air stability and photovoltaic performance in organic solar cells by structural modulation of bis‐amide‐based donor‐acceptor copolymers: A computational insight

“…[ 7–10 ] The most trivial approach to constructing a stronger electron‐deficient acceptor unit is to incorporate electron‐withdrawing groups (such as F, Cl, CN, NO 2 ) onto the conjugate molecular backbone to modulate the optoelectronic behavior. [ 11–14 ] In addition, embedding sp 2 ‐nitrogen onto a conjugated backbone and increasing conjugation length are also possible ways to tune the frontier molecular orbitals (FMOs) for better air stability. [ 15–18 ]…”

“…[7][8][9][10] The most trivial approach to constructing a stronger electron-deficient acceptor unit is to incorporate electron-withdrawing groups (such as F, Cl, CN, NO 2 ) onto the conjugate molecular backbone to modulate the optoelectronic behavior. [11][12][13][14] In addition, embedding sp 2 -nitrogen onto a conjugated backbone and increasing conjugation length are also possible ways to tune the frontier molecular orbitals (FMOs) for better air stability. [15][16][17][18] Although a significant number of D-A copolymers based on bis-imide-functionalized electron acceptor units, such as naphthalene diimide (NDI) and perylene diimide (PDI), are reported for photovoltaic applications, D-A copolymers consisting of bis-amide-functionalized acceptor moieties are relatively less explored.…”