Head-to-Head Linkage Containing Dialkoxybithiophene-Based Polymeric Semiconductors for Polymer Solar Cells with Large Open-Circuit Voltages

High degree of polymer backbone planarity is achieved by incorporating intramolecular noncovalent sulfur center dot center dot center dot oxygen interaction, which also affords good solubility by attaching alkoxy chains at the 3,3'-positions of bithiophene. However, the applications of the resulting polymers are plagued by their high lying HOMOs due to the strong electron-donating alkoxy chains, resulting in small open-circuit voltages (V-oc(s)) in polymer solar cells. Herein, a novel head-to-head linkage containing 3,3'-dialkoxy-4,4'-dicyano-2,2'-bithiophene (BTCNOR) is invented. Single-crystal X-ray diffraction shows direct evidence of non covalent sulfur center dot center dot center dot oxygen interaction and coplanar backbone of BTCNOR Very low-lying HOMOs (-5.5 to -5.6 eV) of the corresponding polymers with high degree of backbone planarity and good solubility are achieved by introducing strong electron withdrawing cyano group onto the dialkoxybithiophene. The cyano offsets the effect of the electron-donating alkoxy chain, rendering the new BTCNOR as a weak donor unit. With this approach, polymer solar cells fabricated from BTCNOR-based polymers deliver very large V-oc(s) (0.9-1.0 V). By varying the BTCNOR side chains, the highest power conversion efficiency of 7.13% is obtained. Diverse characterization techniques are performed to elucidate the structure-property correlations of the new BTCNOR-based semiconductors. The results demonstrate that incorporating strong electron-withdrawing groups into the highly electron-rich dialkoxybithiophene can lead to improved optoelectrical property. The study offers a promising materials design strategy for high-performance organic electronics.