Lowered work function by solution processable bilayer cathode buffers in inverted organic photovoltaic and parameters of the equivalent circuit

In this paper, we report solution processable cathode buffers composed polyethyleneimine (PEI) and C60 fullerene or PCBM for OPV. A PEI layer was formed on an ITO surface by dip coating, followed by the immobilization of C60 or PCBM on ITO/PEI surface by the chemical reaction between an aliphatic amine and fullerene. They were used for cathode buffers in OPVs and the device performance was improved compared with the device without the buffer. The improvement can be attributed to the lowered work function (WF) by the dipole formed by PEI and ITO. The parameters in the equivalent circuit with a single diode tells us that the buffers act as electric bilayer lowering the effective voltage on the diode.