Solar cell "cloth" that can generate electricity even under room light

In order to achieve a "1,000 yen stick-type sensor" capable of efficiently generating electricity in a dimly lit room, the development of a flexible solar cell is in progress. Thin-film solar cells manufactured using organic semiconductors that can be reduced in cost will be made into microfine fibers of the order of nm (10-9m) and then woven into "cloths". It plans to introduce this solar cell into the plant factory. Director of the Ookayama Research Center of the Green Sensor Network Research Institute of NMEMS Technology Research Institute of Japan, Akiko Minami, gave a speech titled "Development of Nanofiber Independent Power Supply in Sensor Network System" and introduced the development of this solar cell.

The goal of solar cells currently under development is for sensor network end-uses, to achieve a self-contained power supply that requires no replacement after one installation, and no need to replace the battery. For the sensor network terminal power supply, the currently used button-type battery is also a candidate, but it must be replaced after reaching a certain lifetime.

At the same time, Tanioka is developing solar cells that can generate enough electricity even under the light of the room. According to reports, the general indoor illumination is 1000 lux before the window, 400 lux on the table, and 100 lux under the table. Tanioka's goal is to develop solar cells that can drive sensor network terminals as long as they reach the 400 lux of light on the table (Figure 1).

Figure 1 Development of a power generation element that can function in an environment with 400 lux of desktop illumination.

Figure 2 The conversion efficiency of organic thin-film solar cells is higher in indoor illumination environments.

Suitable for sensor network terminal use environment characteristics

In order to provide solar cells with the characteristics suitable for a type of sensor-based sensor network terminal, Tanioka uses polymer materials in organic semiconductors.

Solar cells using organic semiconductors are characterized by using a resin substrate instead of a silicon substrate, so that the solar cells can be made light, thin, and soft. If you can paste on the object, you can achieve a free layout. Moreover, even if external force is applied during and after installation, it is not easily damaged. This type of solar cell is not only thin, but also simplifies the mounting jig, so it does not take up space when installed.

With organic semiconductors, production costs can also be easily reduced. Because it is not necessary to use expensive, bulky vacuum manufacturing equipment, it is only necessary to apply a polymer material and form a thin film semiconductor under air conditions. Moreover, reliability and longevity can also fully meet the demand. Because it can be provided with a power storage function, it can also stably supply power in an environment where the required illumination is not achieved.

Solar cells using organic semiconductors have achieved power generation efficiency that is not inferior to that of amorphous (polycrystalline) silicon solar cells used in residential and industrial applications. In the currently disclosed products, the highest level of efficiency of organic semiconductor solar cells is 12%. In indoor illumination conditions below 1000 lux, the efficiency is higher than that of amorphous silicon solar cells (Figure 2). In an indoor environment, organic thin film solar cells are also very advantageous in terms of efficiency.

Convert light energy into electricity efficiently

This time, the organic thin-film solar cell uses a bulk heterojunction in which the contact area between the p-type semiconductor and the n-type semiconductor is large and power generation efficiency can be improved. In addition, a structure in which the p-type semiconductor is nanofiber-like and the other portion is filled with an n-type semiconductor ( FIG. 3 ).

In this way, the electronic transmission loss of the existing bulk heterojunction can be reduced. When the existing bulk heterostructure is used, many of the electrons that are generated after being irradiated with light will not be converted into electrons that generate energy. This is because there are isolated areas inside the structure where the electrons generated cannot reach the electrode. This time, the solar cell made the p-type semiconductor fibrous and formed a path through which the electrons generated at the contact surface can reach the electrode, thereby reducing the transmission loss.

The goal of this development is to achieve more than twice the output power of existing solar cells, that is, 150 μW or more, and a conversion efficiency of more than 7%. The battery size is less than 10cm2 (Figure 4).

Figure 3 uses a structure that is easy to increase efficiency.

Figure 4 Development goals.

First applied to plant factories and then expanded to residential and clothing uses

Fig. 5 shows a method of manufacturing an organic semiconductor using a fiber structure.

An organic semiconductor using a nano-fine fiber structure is produced by applying a high voltage to a solution in which a polymer is dissolved and then spinning the fiber. This method is called electrospinning (Figure 5).

In the future, Fukuoka intends to spin or weave fibrous organic thin-film solar cells made by this method into cloths (textiles) and use it as a flat-shaped power source that can be folded freely. Although organic solar cells using a resin as a substrate can also be bent, the solar cells woven into a fabric can not only be bent, but also can be folded like an ordinary fabric, and occupy a small space when taken up and put.

Tanioka said that "(this kind of solar cell) has the advantage that it is a collection of fibrous elements, so it is not only more reliable as a power source, but also can ideally introduce indoor illumination from multiple directions. Light" (Figure 6).

At present, Gugang is using fibrous solar cell prototypes about 5cm wide and 2cm thick for verification. The future will be installed on the sensor network terminal for plant factories. It is estimated that cloth solar cells will be developed in the future and its use is expanded to curtains, wallpapers, carpets and clothing (Figure 7). (Special Contributor: Kato Shinichi)

Figure 6 introduces light in all directions.

Figure 7 uses will expand to curtains, wallpaper, carpets, clothing.