A non-conjugated polyelectrolyte, poly(4-vinyl N-ethyl pyridinium bromide) (PVPy-EtBr), is applied to polymer solar cells (PSCs) as a cathode buffer layer either inverted or conventional type polymer solar cells to improve the performances. The Kelvin probe microscopy measurement support the formation of favorable interface dipole at the cathode interface, indicating the reduction of a Schottky barrier for an electron collection from the active layer to the cathode treated with electrostatically self-assembly of PVPy-EtBr. Inverted type PSC with PVPy-EtBr as a cathode buffer layer demonstrate the power conversion efficiency (PCE) of 3.49%, which is better than the device without interlayer (PCE = 2.95%). Most increase in the PCE of the devices with interlayer is resulted from enhancement of the J(sc). This is due to that the reduction of a Schottky barrier at the cathode interface. Conventional type PSC with PVPy-EtBr (3.45%) at the cathode interface also exhibit better the PCE compared to that of the device without interlayer (2.88%).