In this work, by introducing Cu dopant, ultra-thin Ag transparent conductive films were prepared on glass and PET substrates without seed layers. The percolation threshold thickness for the Cu doped Ag thin films was as low as 6 nm with a RMS roughness only 0.19 nm. The prepared 6 nm Cu doped Ag thin films, Ag(Cu), had a transmittance of 80% at 550 nm and a sheet resistance of 14.1 Omega sq(-1). The Ag(Cu) thin films revealed excellent thermal, chemical, and mechanical stability. Flexible polymer solar cells using the Ag(Cu) electrode reached a convert efficiency of 7.53% with a very thin PTB7-Th:PC71BM active layer. The proposed ultra-thin alloy transparent conductive films are ease of fabrication and beneficial to light harvesting, which are promising for large-area applications in flexible photovoltaics.