Construction of Layered Structure of Anion-Cations To Tune the Work Function of Aluminum-Doped Zinc Oxide for Inverted Polymer Solar Cells

Suitable work function (WF) of the cathode in polymer solar cells (PSCs) is of essential importance for the efficient electron extraction and collection to boost the power conversion efficiency. Herein, we report a facile and efficient method to tune the surface WF of aluminum-doped zinc oxide (AZO) through building of a definite interfacial dipole, which is realized by the construction of a layered structure of positive and negative ionized species. A cross-linked perylene bisimide (poly-PBI) thin film is deposited onto the AZO surface first, and then it is reduced to the radical anion state (poly-PBI^•-) in an electrochemical cell, using tetraoctylammonium (TOA⁺), a bulky cation, as a counter ion. Owing to the huge volume of TOA⁺, it is absorbed on the surface of the cross-linked PBI^•- thin film through Coulomb force, and thus a definite interface dipole is formed between the two ionized layers. Because of the definite interface dipole, the surface WF of the electrode modified with ionized layers is decreased dramatically to 3.9 eV, which is much lower than that of the electrode modified with the neutral PBI layer (4.5 eV). By using this novel cathode interlayer with a definite interface dipole in PSCs, a significantly increased open-circuit voltage ( V_OC) is obtained. The results indicate that it is a facile and unique method by the construction of a definite interface dipole to tune the surface WF of the electrode for the application in organic electronic devices.