Balancing High Open Circuit Voltage over 1.0 V and High Short Circuit Current in Benzodithiophene‐Based Polymer Solar Cells with Low Energy Loss: A Synergistic Effect of Fluorination and Alkylthiolation

Abstract Based on the most recently significant progress within the last one year in organic photovoltaic research from either alkylthiolation or fluorination on benzo[1,2‐b:4,5‐b′]dithiophene moiety for high efficiency polymer solar cells (PSCs), two novel simultaneously fluorinated and alkylthiolated benzo[1,2‐b:4,5‐b′] dithiophene (BDT)‐based donor–acceptor (D–A) polymers, poly(4,8‐bis(5′‐((2″‐ethylhexyl)thio)‐4′‐fluorothiophen‐2′‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐2′‐ethylhexyl‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate (PBDTT‐SF‐TT) and poly(4,8‐bis(5′‐((2″‐ethylhexyl)thio)‐4′‐fluorothiophen‐2′‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)‐alt‐1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (PBDTT‐SF‐BDD), namely, via an advantageous and synthetically economic route for the key monomer are reported herein. Synergistic effects of fluorination and alkylthiolation on BDT moieties are discussed in detail, which is based on the superior balance between high V_oc and large J_sc when PBDTT‐SF‐TT/PC₇₁BM and PBDTT‐SF‐BDD/PC₇₁BM solar cells present their high V_oc as 1.00 and 0.97 V (associated with their deep highest occupied molecular orbital level of −5.54 and −5.61 eV), a moderately high J_sc of 14.79 and 14.70 mA cm⁻², and thus result a high power conversion efficiency of 9.07% and 9.72%, respectively. Meanwhile, for PBDTT‐SF‐TT, a very low energy loss of 0.59 eV is pronounced, leading to the promisingly high voltage, and furthermore performance study and morphological results declare an additive‐free PSC from PBDTT‐SF‐TT, which is beneficial to practical applications.