Exploring the Chemical Interaction between Diiodooctane and PEDOT-PSS Electrode for Metal Electrode-Free Nonfullerene Organic Solar Cells.

Metal electrode-free organic solar cells with a printable top electrode are attractive in realizing the low cost of photovoltaics. Interaction between the printable electrode and the active layer is critical to the device performance. In this work, we report the chemical interaction between the printable polymer electrode poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and the typically used additive of 1,8-dioodooctane (DIO) in the active layer. DIO can be converted to hydrogen iodide (HI) under the acidic condition of PEDOT:PSS, and the HI chemically reduces the PEDOT:PSS with the appearance of an absorbance band at 800-1100 nm. The generation of I2 is verified by the color change of starch. The reaction results in a decrease of its work function that hinders efficient hole collection. A strategy is proposed to circumvent the detrimental interaction by inserting an ultrathin (15 nm) active layer without DIO between the initial active layer and the PEDOT:PSS electrode. A power conversion efficiency of 10.1% is achieved for the metal electrode-free nonfullerene organic solar cells.