Regulation of the Polar Groups in n-Type Conjugated Polyelectrolytes as Electron Transfer Layer for Inverted Polymer Solar Cells

Conjugated polyelectrolytes based on n-type backbone (n-CPEs) were recently developed as highly efficient cathode interlayers for polymer solar cells (PSCs), but the relevance between structure and property of n-CPEs has not been fully understood yet. Herein, three new self-doping n-CPEs based on the diketopyrrolo-pyrrole (DPP) alternated fluorene framework were reported. The effect of the number and location of polar groups on the properties of the new n-CPEs has been systematically studied. It can be found that the photoelectric properties of n-CPEs were sensitive to the number and location of polar groups. A tunable work function (WF), interfacial interaction, and conductivity of these n-CPEs can be readily realized by regulating the number and location of polar groups on the electron skeleton. The polar groups directly appending on the electron-withdrawing DPP unit can promote a stronger n-type self-doping and lower WF than the ones attached on the electron-pushing fluorene unit. Increasing the number of the polar groups can further optimize the photoelectric properties of conjugated electrolytes. As a result, upon incorporation of new n-CPEs as cathode buffer layer, the PSC performance was significantly improved to 8.3% for the fullerene system and 10.57% for the non-fullerene system. These results demonstrated that without complex molecular design, simply regulating the number and location of polar groups of the n-CPEs can provide a facile way to develop highly efficient cathode interlayers for high performance polymer solar cells.