Here, we report inverted small molecular organic solar cells (SMOSCs) using doped window layer as an optical spacer and a hole transport layer. Optical spacer was used to optimize the optical field distribution inside the active layer, generating more charges from the incident light. Optical field distribution was calculated by transfer matrix formalism (TMF) simulation using refractive indices of each material. N,N,N’,N’-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) is an attractive material for optical spacer because of its large band gap about 3.3 eV. With a doping method, we could increase not only conductivity of MeO-TPD by several orders of magnitude but also hole mobility of the device. Consequently, we achieved power conversion efficiency of 4.15 % with optimized thickness of an optical spacer. Moreover, stability of inverted devices was significantly enhanced compared to that of conventional devices.