Regioisomeric Polymer Semiconductors Based on Cyano-Functionalized Dialkoxybithiophenes: Structure–Property Relationship and Photovoltaic Performance

Cyano substitution is vital to the molecular design of polymer semiconductors toward highly efficient organic solar cells. However, how regioselectivity impacts relevant optoelectronic properties in cyano-substituted bithiophene systems remain poorly understood. Three regioisomeric cyano-functionalized dialkoxybithiophenes BT_HH, BT_HT, and BT_TT with head-to-head, head-to-tail, and tail-to-tail linkage, respectively, were synthesized and characterized in this work. The resulting polymer semiconductors (PBDTBTs) based on these building blocks were prepared accordingly. The regiochemistry and property relationships of PBDTBTs were investigated in detail. The BT_HH moiety has a higher torsional barrier than the analogs BT_HT and BT_TT, and the regiochemistry of dialkoxybithiophenes leads to fine modulation in the optoelectronic properties of these polymers, such as optical absorption, band gap, and energy levels of frontier molecular orbitals. Organic field-effect transistors based on PBDTBT_HH had higher hole mobility (4.4 × 10⁻³ cm²/(V·s)) than those (ca. 10⁻⁴ cm²/(V·s)) of the other two polymer analogs. Significantly different short-circuit current densities and fill factors were obtained in polymer solar cells using PBDTBTs as the electron donors. Such difference was probed in greater detail by performing space-charge-limited current mobility, thin-film morphology, and transient photocurrent/photovoltage characterizations. The findings highlight that the BT_HH unit is a promising building block for the construction of polymer donors for high-performance organic photovoltaic cells. Graphical abstract: [Figure not available: see fulltext.].