The Photonic Research Institute (PRI) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has 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 based on organic semiconductor to construct a p-i-n junction expands the photovoltaic conversion layer to enhance the efficiency of light utilization. With the p-i-n type organic thin-film solar cell, an energy conversion efficiency, 4 %, a world top level, has been achieved under the condition of simulated solar radiation of AM1.5G. It is expected that this feat will accelerate the implementation of plastic film solar cell characterized by lightweight and flexibility.
The organic thin-film solar cell, of which practical use as low cost, flexible solar cells is pursued for, is a solid-state solar cell based on the same principle as the widely used silicon solar cell, and has a long history of R&D longer than 30 years. However, its energy conversion efficiency has been rather poor, and upgrading the energy conversion efficiency provides is the most significant hurdle for enjoying the practical application.
The solid-state solar cells, either organic or inorganic, are based on the photovoltaic effect of p-n junction. As the photovoltaic conversion layer of p-n junction in organic semiconductor is as thin as a few nanometers, the efficiency of light utilization is so poor with the conventional simple layered solar cell that adequate photocurrent could not be derived. For this reason, upgrading of light utilization efficiency by expanding the photovoltaic conversion layer has been regarded as the key factor for improving the energy conversion efficiency of organic thin-film solar cells.
The PRI-AIST has 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 to construct multiple p-n junctions expands the photovoltaic conversion layer.
For n- and p-type organic semiconductors, fullerene (C60) and zinc phthalocyanine (ZnPc), respectively, are used, and the preparation of p-i-n junction organic thin-film solar cell by introducing a nano-structured layer with mixed ZnPc and C60 (ZnPc: C60 = i-layer) into p-n junction interface composed of ZnPc and C60, has boosted the energy conversion efficiency to about 4 %. This value is at the top level among the organic thin-film solar cells as evaluated under the simulated solar radiation of AM1.5G. While the newly developed organic thin-film solar cells have a lot of light input left still untapped owing to the thinness of photovoltaic layer, it is expected that further enhancement of light utilization efficiency by building tandem structures will make it possible to improve the energy conversion efficiency substantially. Getting the perspective for upgrading the energy conversion efficiency of organic thin-film solar cells in this way, is expected to accelerate the realization of plastic film solar cells markedly.
Photo. Currently predominant silicon solar cell (left) and plastic film solar cell (right)