A new narrow bandgap polyfluorene copolymer containing 2,6-bis-(3-hexyl-thiophen-2-yl)-anthraquinone unit for solar cell applications

We have synthesized a new narrow bandgap alternating polyfluorene copolymer (PFTTDIONE) based on 2,7-dibromo-9,9-dioctylfluorene, 2,5-bis-(tributyl-stannyl)thiophene, and 2,6-bis-(5-bromo-3-hexyl-thiophen-2-yl)-anthraquinone (M1), via a Stille polymerization reaction. The optical properties, electrochemical properties, photovoltaic properties, hole mobility, and AFM morphology of the copolymers were investigated and discussed. The optical bandgap of PFTTDIONE is equal to the electrochemical bandgap (1.90 eV). PFTTDIONE exhibits an extended absorption band in the visible part of the spectrum with an absorption edge close to 650 nm. In order to investigate its photovoltaic properties, polymer solar cells (PSCs) devices based on PFTTDIONE were fabricated with a structure of ITO/PEDOT:PSS/ copolymer:PCBM/LiF/Al under the illumination of AM 1.5 G, 100 mW/cm2. The bulk heterojuction (BHJ) polymer solar cells were fabricated with the conjugated polymer as the electron donor and 6,6-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor. The power conversion efficiency (PCE) of the solar cells based on PFTTDIONE/PCBM (1:2) annealing at 110 °C for 20 min was 1.58% with an open-circuit voltage (Voc) of 0.74 V, Fill Factor of 35.7%, and a short-circuit current (Jsc) of 5.99 mA/cm2.