We report an increase in open-circuit voltage (V_(oc)) by inserting an MoO_3 layer on ITO substrate to improve built-in potential of organic solar cells (OSCs). In the OSCs using 5,10,15,20-tetraphenylporphyrine (H_2TPP) as p-type material and C_(60) as n-type material, the V_(oc) effectively increased from 0.57 to 0.97V with increasing MoO_3 thickness. The obtained highest V_(oc) (0.97V) is consistent with the theoretical value estimated from the energy difference between the LUMO (-4.50eV) of C_(60) and the HOMO (-5.50eV) of H_2TPP layer. Importantly, the enhancement in the V_(oc) was achieved without affecting the short-circuit current density (J_(sc)) and the fill-factor (FF). Thus, the power conversion efficiency of the device increased linearly from 1.24% to 1.88%. We also demonstrated that a MoO_3 buffer layer enhances the stability of OSCs after photo-irradiation. We have investigated the stability of OSCs using H_2TPP and N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine as p-type layer. Both devices with MoO_3 layer showed improved stability. These results clearly suggest that the interface between ITO and p-type layer affects device stability.
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