The solar cell utilizing clean and inexhaustible solar energy is expected to be the major domestic energy source in future in view of preventing global warming. One of the most promising candidates is organic thin-film solar cell, which can be produced at a reduced manufacturing cost. We have found that introducing a nano-structured layer (i-layer) where organic semiconductor forms a 3-dimensional p-n junction at the molecular level into p-n junction interface of organic thin-film solar cell expands the practical photovoltaic layer to enhance the efficiency of light utilization. With the p-i-n type organic thin-film solar cell, a power conversion efficiency, 4 %, a world top level, has been achieved. Getting the perspective for upgrading the power conversion efficiency of organic thin-film solar cells in this way, is expected to accelerate the realization of low cost, lightweight and flexible plastic solar cells.