Abstract:
All-polymer solar cells (all-PSCs), which utilize conjugated polymers as both electron donor and electron acceptor, have received great attention for the great advantages of tunability of energy levels of the polymer acceptors, improved solar light absorption and enhanced mechanical/thermal stability. The investigations on the relationships between chemical structures and properties of polymer electron acceptors are very important for the design of efficient polymer acceptor materials and the development of all-PSCs. A polymer acceptor, P-BN-fIID, was synthesized by alternating copolymerization of boron-nitrogen coordination bond bridged thiophene-thiazole unit (BNTT) and fluorinated isoindigo unit (fIID). The relationships between the chemical structure and the photophysical, electrochemical properties as well as the molecular stacking behavior were investigated. According to the electrochemical data, P-BN-fIID showed the lowest unoccupied molecular orbital and the highest occupied molecular orbital energy levels (
ELUMO/
EHOMO) of −3.90/−5.86 eV. In the absorption spectrum, it exhibited the absorption peak at 720 nm. Compared with the reference polymer P-BN-IID based on isoindigo unit (IID) and BNTT unit, P-BN-fIID had a lower
ELUMO by 0.1 eV and a red-shifted absorption by 25 nm. Moreover, P-BN-fIID displayed improved crystallinity with the dense molecular packing, thus leading to the enhanced electron mobility of 3.54 × 10
−5 cm
2 /(V·s). All-PSCs device based on the blend of P-BN-fIID and a conventional polymer donor J61 exhibited a power conversion efficiency of 2.83%. Results suggest that fluorine atom can effectively tune the electronic properties and crystallinity of polymer acceptors and thus improve the photovoltaic performance.