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···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 ocs) 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 noncovalent sulfur···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 ocs (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. [ABSTRACT FROM AUTHOR]