Designing Donor-Acceptor Small Molecules For Organic Solar Cells With Promising Photovoltaic Parameters

Abstract

Cyclopentadithiophene (CPT) is one of the most efficient electron donating unit and has been reliably used in highly efficient optoelectronic applications because of its unique characteristics of excitation of electrons by fused thiophene ring derivatives, the rigidity and the high flatness of its structure. In order to meet the increasing demand of high-efficiency photovoltaic compounds in modern high-performance applications, we have designed and explored new CPT-based donor-acceptor-donor-acceptor-donor based materials with higher efficiency from d-CDT(PTTh2). Three novel isostructural D1-A-D2-A-D1 based low band gap small molecule donors (SMDs) having one-atom replacement (Flourine, Chlorine and Hydrogen) in acceptor unit of benzothiadiazole (BTZ) were designed and investigated by density functional theory (DFT) simulations. The designed molecules are 7,7'(7H-cyclopenta[1,2-b:3,4-b']dithiophene-2,5-diyl)bis(6fluoro4(5'hexyl[2,2’[2,2' bithiophen]-5yl)benzo[c][1,2,5]thiadiazole) (CPT1), its chlorine analogue 7,7'-(7H-cyclopenta[1,2-b:3,4-b']dithiophene-2,5-diyl)bis(6-chloro4-(5'-hexyl-[2,2'-bithiophen]-5yl)benzo[c][1,2,5]thiadiazole) (CPT2) and their hydrogen analogue 7,7'-(7H cyclopenta[1,2-b:3,4-b']dithiophene-2,5diyl)bis(4-(5'-hexyl-[2,2'bithiophen]-5 yl)benzo[c][1,2,5]thiadiazole) (CPT3). All are based on the strong electron rich unit CPT and electron withdrawing BTZ acceptors. The structures of designed compounds were optimized at DFT/B3LYP/6- 311 g(d,p). The new designed donors CPT1, CPT2, CPT3 showed superior absorption range found at the wavelengths of 715nm,674nm and 705nm respectively as compared to 667nm of reference molecule d-CDT(PTTh2) in chloroform solution. DFT calculations revealed that CPT1,CPT2 and CPT3 transfer their charges efficiently with band gaps 2.0 eV,2.21 eV and 2.1 eV respectively. The calculated open circuit voltages range from 0.55 eV to 0.70 eV, while CPT2 have a significantly higher open circuit voltage than the open circuit voltage of 0.62 of reference molecule d-CDT(PTTh2). Charge transfer mobilities of designed molecules are also superior than d-CDT(PTTh2) because of their lower reorganization energies ranging from 0.00114 to 0.038282 for electron and 0.11101 to 0.44505 for hole. This DFT investigation revealed that tuning of organic compounds can further enhance the efficiency of SMDs in bulk hetero-junction (BHJ) organic solar cells (OSC’s). DFT calculations showed that one atom substitution has a slight but notable effect on band gaps of frontier molecular orbital (FMOs) which affects the absorption ranges, reorganization energies, Voc, FF and PCE of SMDs in BHJ OSC. Our investigations suggest that the designed molecules can work as fine SMDs when coupled with PC71BM as an acceptor in BHJ OSC.