Effect of Cyano Substitution on Conjugated Polymers for Bulk Heterojunction Solar Cells

The design of polymer structures has played a vital role in improving the efficiency of organic solar cells (OSCs). A common approach to increase solar cell efficiency is to add a specific substituent to the polymer backbone; yet, the cyano substituent, a strong electron-withdrawing group, has not received much attention as such substituents with conjugated polymers. In this study, we systematically explore the impact of cyano substitution on conjugated polymers by varying the amount of cyano groups, from zero to two per repeat unit, on the benzotriazole acceptor moiety of our polymers. We find that cyano substitution effectively decreases the energy levels of the polymer, leading to high VOC values; however, the impact of additional cyano groups offer diminishing returns. Moreover, cyano substitution also decreases the band gap of the polymers, inducing a shift of polymer absorption to longer wavelength. Along with the optical and electrochemical differences, the cyano effect was also thoroughly characterized by changes in morphology, charge transfer state energy, charge carrier density, lifetime, and nongeminate recombination rate. The champion polymer (efficiency of 8.6%) has a single cyano substituent, monoCNTAZ, which affords high VOC, JSC, and FF due to deep energy level, red-shifted absorption, and efficient charge transport properties, respectively. However, further additions of cyano groups degrade the performance. Overall, this work highlights the benefits and limitations of cyano functionalization on conjugated polymers for OSCs.