Energy Conversion Efficiency of World Top Level with Organic Thin-Film Solar Cell
- A Great Leap Forward to Implementation of Plastic Film Solar Cell -

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.

The solar photovoltaic power generation utilizing clean and inexhaustible solar energy is expected to be the major domestic energy source in future in view of preventing global warming. The power generation cost with prevailing silicon solar cells is currently about 3 times as high as that of household power price, and about 6 times that of industrial power price. For spreading the photovoltaic power generation, it is essential to reduce the cost of solar cells, and it has been argued what type of solar cells are to be put to practical use for this purpose. One of the most promising candidates is organic thin-film solar cell, which can be produced at a reduced cost by printing on flexible plastic substrate. R&D works on the solar cells of this category have been actively pursued in the US and European countries, since the beginning of the 21^st century.

There are two types of semiconductors depending upon the electric charge of carriers: p-type with current carried by positive holes (positive charge), and n-type by conducting electrons (negative charge). Among organic materials, there are a number of semiconductors of either type. The concept of using organic semiconductors in place of silicon or other inorganic semiconductors to develop low cost film solar cells has come up with since earlier, and R&D efforts have been made already since the latter half of 1970s. However, its energy conversion efficiency has hardly exceeded 1 % or so, holding up the progress, and a breakthrough enhancement of energy conversion efficiency has been longed for.

Meanwhile, R&D of organic electroluminescent (EL) device, of similar structure as organic thin-film solar cell has been started from the latter half of 1980s, a little behind the latter. Nevertheless, the organic EL device has already been put to practical application, and a lot of findings have been accumulated on the organic semiconductor devices throughout the process. In particular, it has been noted that fullerene (C₆₀) spotlighted because of soccer ball-like molecule behaves as n-type organic semiconductor of excellent performance, meeting the requirements for reviving the organic thin-film solar cells.

However, the combination of n-type organic semiconductor, fullerene, with typical p-type organic semiconductor, phthalocyanine, to construct an organic version of p-n heterojunction solar cells did not result in sizable improvement of energy conversion efficiency. This may be attributed to the inadequate thickness of photovoltaic conversion layer formed at the p-n junction interface of the organic semiconductor, a few molecules thick or a few nanometers, 3 orders of magnitude thinner that that of silicon solar cells. Most of solar radiation passes through the conversion layer, and only a very small part of incident light energy is used for the photovoltaic conversion.