Enhancing the Sensitivity of Photovoltaic Photodetectors From Benzo[1,2-b:4,5-b’]Dithiophene-Alt-Thienothiadizole Conjugated Polymers via Fluorine Side Chian Engineering

Abstract

Three narrow bandgap alternating conjugated polymers of PBDT-TT, PBDT-2F-TT, and PBDT-4F-TT, which accordingly derived from 2-(2-ethylhexyl)thiophene, 2-(2-ethylhexyl)-3-fluorothiophene and 2-(2-ethylhexyl)-3,4-difluorothiophene flanked benzo[1,2-b:4,5-b’]dithiophene (BDT) and 2,5-bis(4-(ethylhexyl)thiophen-2-yl)thienothiadiazole (TT), are synthesized by Palladium-catalyzed Stille coupling reaction. The absorption, thermal stability, and electrochemical properties of the copolymers are characterized. It is found that, with the increase of the fluorine atoms in the BDT blocks of the copolymers, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, and decomposition temperatures of the copolymers, are successively varied from −5.16 to −5.30 eV, −3.98 to −4.09 eV, and 364 to 384 °C. The specific detectivity (D^*) at 790 nm and linear dynamic range (LDR) of the polymer photovoltaic photodetectors (PPDs) from the PBDT-TT:Y6, PBDT-2F-TT:Y6 and PBDT-4F-TT:Y6 blend films, are ordinally increased from 7.54 × 10¹¹ Jones and 53.5 dB to 1.79 × 10¹² Jones and 87.0 dB, and then up to 4.23 × 10¹² Jones and 88.7 dB under −0.1 V bias voltage. Moreover, the possible reasons for the successive improvements in the performance of the PPDs from the BDT-alt-TT copolymers with the increase of the fluorine atoms in the BDT units are investigated and discussed.