Synthesis and Characterization of Wide‐Bandgap Conjugated Polymers Consisting of Same Electron Donor and Different Electron‐Deficient Units and Their Application for Nonfullerene Polymer Solar Cells

Substantial development has been made in nonfullerene small molecule acceptors (NFSMAs) that has resulted in a significant increase in the power conversion efficiency (PCE) of nonfullerene‐based polymer solar cells (PSCs). In order to achieve better compatibility with narrow‐bandgap nonfullerene small molecule acceptors, it is important to design the conjugated polymers with a wide bandgap that has suitable molecular orbital energy levels. Here two donor–acceptor (D–A)‐conjugated copolymers are designed and synthesized with the same thienyl‐substituted benzodithiophene and different acceptors, i.e., poly{(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐(1,3‐bis(2‐octyldodecyl)‐1,3‐dihydro‐2H‐dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2‐d]imidazol‐2‐one‐5,8‐diyl)} (DTBIA, P1) and poly{(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐(2‐(5‐(3‐octyltridecyl)thiophen‐2‐yl)dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2‐d]thiazole‐5,8‐diyl)} (TDTBTA, P2) (and their optical and electrochemical properties are investigated). Both P1 and P2 exhibit similar deeper highest occupied molecular orbital energy level and different lowest unoccupied molecular orbital energy level. Both the copolymers have complementary absorption with a well‐known nonfullerene acceptor ITIC‐F. When blended with a narrow‐bandgap acceptor ITIC‐F, the PSCs based on P1 show a power conversion efficiency of 11.18% with a large open‐circuit voltage of 0.96 V, a Jsc of 16.89 mA cm−2, and a fill factor (FF) of 0.69, which is larger than that for P2 counterpart (PCE = 9.32%, Jsc = 15.88 mA cm−2, Voc = 0.91 V, and FF = 0.645). Moreover, the energy losses for the PSCs based on P1 and P2 are 0.54 and 0.59 eV, respectively. Compared to P2, the P1‐based PSCs show high values of incident photon to current conversion efficiency (IPCE) in the shorter‐wavelength region (absorption of donor copolymer), more balanced hole and electron mobilities, and favorable phase separation with compact π–π stacking distance. After optimization, P1:ITIC‐F‐ and P2:ITIC‐F‐based polymer solar cells show overall power conversion efficiency (PCE) of 11.19% and 9.30%, respectively.