Novel wide bandgap benzodithiophene-based polymer donors with electron-withdrawing indolin-2-one side chains for efficient organic solar cells with high open circuit voltage

Two new π-conjugated polymers with donor backbone and π-conjugated indolin-2-one side chains, PBDTTI and PBDTTIF, are designed and synthesized as wide bandgap donors for non-fullerene acceptor-based organic solar cells (OSCs). The monomers containing electron accepting indolin-2-one side chains, (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-1-methylindolin-2-one (M1) and (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-5-fluoro-1-methylindolin-2-one (M2), can be easily synthesized via Knoevenagel condensation between 2,5-dibromothiophene-3-carbaldehyde and 2-oxindole or 5-fluoro-2-oxindole, respectively. Stille coupling polymerization of the electron donating benzodithiophene (BDT)-containing monomer 1,1′-[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl]bis [1,1,1-trimethylstannane] and M1 or M2 produced PBDTTI or PBDTTIF, respectively. The strong electron accepting π-conjugated 1-methylindolin-2-one and 5-fluoro-1-methylindolin-2-one side chains can achieve low-lying HOMO energy levels of −5.59 eV for PBDTTI and −5.60 eV for PBDTTIF, which is beneficial for realizing high open circuit voltage (V_OC) of the resulting OSCs. On the other hand, since the electron acceptor units are on the side chains, the polymer backbone containing only electron donor units could maintain wide bandgaps of 1.91 eV and 1.89 eV for PBDTTI and PBDTTIF, respectively. When PBDTTI and PBDTTIF were used as donors and a small bandgap non-full acceptor ITIC as an acceptor, the resulting OSCs devices achieved V_OC of 0.97 and 1.00 V, short circuit current densities (J_SC) of 15.60 and 13.70 mAcm^-2, and fill factors (FF) of 0.60 and 0.59, resulting in power conversion efficiencies of 8.00 and 7.70%, respectively.