Title

Investigation of light absorption and transport properties of organic materials for solar cell applications: Final report

College

College of Science

Department/Unit

Chemistry

Document Type

Archival Material/Manuscript

Publication Date

2015

Abstract

The electronic and optical properties of various substituted poly[N-9' - heptadecany1-2,7-carbazole-alt-5,5-(4',7' -di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) conjugated oligomers have been investigated by density functional theory (DFT) and time-independent density functional theory (TD-DFT). Various electron-withdrawing and electron-donating groups were attached to the 5' position of the benzothiadiazole unit in the PCDTBT oligomers. Optimizations of the substituted PCDTBT (PCDTBT-X) oligomers (n = 1 to 4 units) were performed using DFT at the B3LYP/6-31G(d) level of theory. Single-point energy calculations were performed using DFT at the B3LYP/6-3111G(d,p) level of theory to calculate for the HOMO energy (EHOMO), LUMO energy (ELUMO), and fundamental gap (Egap). On the other hand, the first excited singlet state (Eopt) were calculated using TD-DFT, also at the B3LYP/-311G(d,p) level. Exciton binding energies (EB) were then estimated from the difference between Egap and Eopt. Results show that both electron-withdrawing and electron-donating groups are capable of lowering the EGap of PCDTBT. Electron-withdrawing groups generally lower the LUMO energy level, while electron-donating groups primarily increase the HOMO energy level. Several properties of PCDTBT-X which may affect the solar cell properties were then calculated and compared with the calculated values for PCDTBT. From the calculated properties, PCDTBT-F showed improvement in the EGap, open-circuit voltage (VOC), ionization potential (IP), and electron binding energy (EB) which may lead to solar cells with improved power conversion efficiency (PCE)compared to PCDTBT. low-cost devices [1-5]. Recent developments in organic polymer solar cell devices have led to power conversion efficiencies (PCE) close to 10% [6-8]. However, with Si-based commercial solar cells reaching 25% [9,10], significant improvement in material and device engineering must be made in order for organic polymer solar cells to be commercially attractive. Therefore, in order to synthesize new materials that may eventually lead to higher solar cell device performance, the study of organic polymer material design and intrinsic properties is indispensable. The purpose of this project is to investigate the effects of substitution of various electron-withdrawing and electron-donating substituents on the electronic and optical properties of poly[N-9'-heptadecanyl1-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT), a high-performance donor material in OPVs [16], using quantum-mechanical calculations based on DFT and TD-DFT. The hydrogen in the 5' position of PCDTBT was replaced with several substituents: acetyl (-COCH3), amino (-NH2), isopropyl (-CH(CH3)2), cyano (-CN), dimethylamino (-N(CH3)2), ethenyl (-CH=CH2), fluoro (-F), hydroxy (-OH), methyl (-CH3), nitro (-NO2), trifluoromethy (-CF3), and methoxy (-COCH3). The HOMO energy (EHOMO), LUMO energy (ELUMO), and energy gap (EGap) were calculated from DFT calculations. The first excited singlet state, (EOpt) was calculated by TD-DFT calculations. The solar cell characteristics of the new PCDTBT derivatives were then predicted and compared with the calculated values for PCDTBT. This study also aims to provide new insights and propose new designs of organic polymer materials for OPVs with improved solar cell properties.

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Disciplines

Chemistry

Note

"URCO New PHD grant, project no.: 35N1TAY14-3TAY14."

Keywords

Biopolymers—Absorption and adsorption; Biopolymers—Transport properties; Solar cells

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