New Benzo[1,2- d:4,5- d′]bis([1,2,3]thiadiazole) (iso-BBT)-Based Polymers for Application in Transistors and Solar Cells

In this paper we report the rationale and implementation of a new building block for organic electronics based on 4,8-di(2-thienyl)-benzo[1,2-d:4,5-d′]bis([1,2,3]thiadiazole) (iso-BBT-T2) and realization of two alkyl-functionalized iso-BBT-tetrathiophene (T4) alternating copolymers (P1 with alkyl = 2DT and P2 with alkyl = 2DH). Compared to the previously investigated small molecules/polymers based on the conventional 4,8-di(2-thienyl)-benzo[l,2-c:4,5-c′]bis[ l,2,5]thiadiazole (BBT-T2), the use of the iso-BBT heterocycle widens the polymer band gap to a region (∼1.4 eV) compatible for use in single junction solar cells. The influence of iso-BBT vs BBT structural variation on the molecular structure, electronic characteristics, and optical properties was accessed by DFT computations, single-crystal determination, optical absorption, and electrochemical measurements. In-plane charge transport for P1 and P2 was investigated in an organic thin-film transistor (OTFT) structure demonstrating hole mobilities approaching 1 cm² V^-1 s^-1 and further enhanced by off-center spin-coating method to 1.32 cm² V^-1 s^-1. Using PC₆₁BM as acceptor, a remarkable PCE of 10.28% was achieved for P1 along with a current density > 20 mA/cm². The substantial PCEs of these devices, despite the relatively narrow donor energy gap, is due to retention of high open circuit voltages (V_oc > 0.8 V) as the result of the small energy loss (E_loss < 0.6 eV). Atomic force microscopy, transmission electron microscopy, and X-ray diffraction characterization further support the solar cell trends and rationalize structure-property correlations. These results demonstrate that iso-BBT-T2-based polymers are promising candidates for both organic electronics and photonic applications.