Japan's National Institute of Materials Science (NIMS) prototyped a solar cell using boron nitride (BN), which is transparent to visible light.
High hopes are placed on BN as a material to produce a semiconductor with a wide band gap that enables ultraviolet lasers and transparent transistors. But, thus far, it has been difficult to dope an impurity, which is necessary to make a semiconductor, into BN.
This time, NIMS used a proprietary process known as the "laser mixing/plasma CVD method" to dope an impurity into a high density BN thin film and succeeded in producing a BN/Si heterojunction diode. A solar cell prototyped by using the diode showed a generating efficiency of about 2%.
Based on the laser mixing/plasma CVD method, a p-type BN semiconductor was obtained by doping a Si impurity while synthesizing a high density BN that has an interatomic bonding similar to that in diamonds (the sp3 bonding). With this technique, a solar cell made of a heterojunction diode, which is composed of a Si substrate (n-type) and BN (p-type) formed on the substrate, can be produced in one step, according to the institute.
Furthermore, micro cones covering the surface of the thin film can improve the light absorption efficiency because the cones reduce the reflection of sunlight, the institute said.
NIMS plans to commercialize the BN/Si solar cell, targeting applications such as unmanned observation systems and devices used in space, which have particularly severe requirements for durability, reliability and weatherability. Also, it is planning to develop an n-type BN semiconductor, in addition to the p-type, to produce a homojunction diode, which is made entirely of BN.
The use of BN homodiode makes it possible to develop solar cells that are transparent to visible light. Therefore, on-board sunroof power generating systems for vehicles, which can be used in combination with rechargeable batteries, and solar cells attachable to sunglasses, windows, etc may be developed, the institute said.
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