Artificially MoO ₃ graded ITO anodes for acidic buffer layer free organic photovoltaics

We report characteristics of MoO ₃ graded ITO anodes prepared by a RF/DC graded sputtering for acidic poly(3,4-ethylene dioxylene thiophene):poly(styrene sulfonic acid) (PEDOT:PSS)-free organic solar cells (OSCs). Graded sputtering of the MoO ₃ buffer layer on top of the ITO layer produced MoO ₃ graded ITO anodes with a sheet resistance of 12.67 Ω/square, a resistivity of 2.54 × 10 ^-4 Ω cm, and an optical transmittance of 86.78%, all of which were comparable to a conventional ITO anode. In addition, the MoO ₃ graded ITO electrode showed a greater work function of 4.92 eV than that (4.6 eV) of an ITO anode, which is beneficial for hole extraction from an organic active layer. Due to the high work function of MoO ₃ graded ITO electrodes, the acidic PEDOT:PSS-free OSCs fabricated on the MoO ₃ graded ITO electrode exhibited a power conversion efficiency 3.60% greater than that of a PEDOT:PSS-free OSC on the conventional ITO anode. The successful operation of PEDOT:PSS-free OSCs indicates simpler fabrication steps for cost-effective OSCs and elimination of interfacial reactions caused by the acidic PEDOT:PSS layer for reliable OSCs.