Alkoxy-Substituted Anthrabisthiadiazole–Benzodithiophene-Based Wide-Band-Gap Semiconducting Polymers for High-Performance Non-Fullerene Solar Cells

Abstract

Three wide-band-gap donor−acceptor (D−A) type polymers, PBA-TX, PBA-T2, PBA-T3, and PBA-T4, based on alkoxy-substituted anthrabisthiadiazole (ATz) and benzodithiophene (BDT) structures, were synthesized and characterized for the development of high-performance semiconducting polymers in non-fullerene organic photovoltaics (OPVs). The synthesized polymers PBA-TX possess deep highest occupied molecular orbital (HOMO) energy levels of −5.4−5.5 eV and a relatively wide band gap of 1.8 eV, attributed to the introduction of the alkoxysubstituted ATz core with a weak electron-acceptor character into polymer backbones. Additionally, the bulky alkylthienyl side chains on the BDT core provide sufficient solubility and suppress the formation of lamellar structures, enabling the construction of a suitable face-on orientation in the solid state and high miscibility with representative low-band-gap small-molecular acceptor (SMA) Y6. The halogenation of the β-position of alkylthienyl groups on the BDT core further reduced the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels. Solar cells fabricated with halogen atom-substituted polymers PBA-T3 (chlorine) and PBA-T4 (fluorine) exhibited a higher open-circuit voltage (V oc ) than cells based on the nonhalogenated polymer PBA-T2. Interestingly, all PBA-TX/Y6 blended films formed almost the same molecular order and phase separation structures. As a result, all fabricated solar cells based on PBA-TX/Y6 systems showed a similar power conversion efficiency (PCE) of around 8−9%. One possible reason for the small differences observed may be attributed to the presence of an ATz framework in the polymer backbone, as its rigid and π-extended manner can provide sufficient strong intermolecular interaction. The subtle halogenation effect here proves advantageous for crafting halogen-free wide-band-gap semiconducting polymers in non-fullerene organic photovoltaics (OPVs), steering clear of intricate synthetic pathways.

Conjugated polymers with near-infrared (NIR) optical absorption: structural design considerations and applications in organic electronics

“…Next, ring fusion and π-extension ( 17–20 ) are additional strategies for rigidifying or increasing the extent of π-orbital overlap within the π-conjugated system of the CP, which in turn narrows the bandgap and facilitates charge transport. 65–67 In the case of ring-fusion for 11 this often invokes the functionalization of naphthalene and anthracene analogues and derivatives. However, extended fused ring monomers can also be synthesized from simpler molecular building blocks ( 6 , 7 , and 12 ) via linear or convergent synthetic pathways.…”