The invention provides an organic solar cell, including: a substrate having a first electrode formed thereon; a hole transport layer overlying the first electrode; a metal layer having a first pattern in the hole transport layer; a photoactive layer, including: a first organic semiconductor film having a second pattern complementary to the first pattern and overlying the metal layer and the hole transport layer; a second organic semiconductor film having a first pattern substantially aligned to the first pattern of the metal layer and overlying the first organic semiconductor film, wherein the first organic semiconductor film and the second organic semiconductor film have opposite conductive types; a second electrode overlying the photoactive layer. The invention further provides a method for forming the organic solar cell.