Development of energy materials based on understanding and controlling their properties in nano scale
–from crystal growth to device design–
Applying academic and knowledge of conventional metallurgical thermodynamics to advanced materials including semiconductors, searching environmentally harmonious new materials and working to solve energy and environmental problems from the viewpoint of material science through construction of new device structure I will.
Bulk crystal growth of semiconductor material
Semiconductor materials in actual devices are often in the form of thin films, but in order to approach the essence of the physical properties of materials, it is necessary to evaluate physical properties using bulk crystals. Our laboratory attempts to bulk crystal growth of various compound semiconductors and attempts to clarify the physical properties of new materials. Especially, with respect to crystal growth of phosphide and sulfide with high vapor pressure, we have created our own crystal growth furnace and we are doing crystal growth using not only liquid phase but also gas phase. A state diagram (phase diagram) is a useful tool in considering crystal growth conditions. When the state diagram is not reported, we are working on the development of bulk crystals of new materials by experimentally creating state diagrams themselves.[Phys. Stat. Sol. a, (2017); Jpn. J. Appl. Phys., (2016); Phys. Stat. Sol. c, (2015)] The left figure is a bulk crystal of ZnSnP2 obtained by the flux method. Since this system is a peritectic reaction, it is difficult to obtain single phase crystals by ordinary liquid phase solidification, and information on the Zn-Sn-P system state diagram is useful for determining the production conditions.
Thermodynamics of semiconductor materials
Even in semiconductor materials, it is important to understand phenomena from the viewpoint of thermodynamics as well as metals and alloys. It is also known that a compound semiconductor ZnSnP2 having a chalcopyrite structure has a zincblende structure, and its band gap changes from 1.66 eV to 1.25 eV It was suggested. The structure of both of them is the difference between whether Zn and Sn are regularly arranged or randomly arranged and can be understood as so-called rule irregular transformation. So far we have only reported by computer simulation, but we have adopted a long range order degree as an indicator of disorder of atomic arrangement and fabricated crystals having various order degrees, so that we can change the band gap to the order degree It was shown that it can be sorted out. Furthermore, we demonstrated that the band gap can be controlled by regularity control. [J. Phys. Chem. C, (2017); IEEE 42nd PVSC, (2016)] The figure on the left shows the relationship between the long range ordering degree and the band gap in ZnSnP2. By changing the degree of regularity, you can see that the band gap is continuously changing. We also demonstrate restoration of regularity by heat treatment and band gap change.
Development of deposition process of multi-component compound semiconductor
Compound semiconductors have a larger light absorption coefficient than silicon, so they can be thinned, and compound semiconductor solar cells are expected from the viewpoint of low cost and resource saving.
Establishment of thin film fabrication process is indispensable for practical application of compound semiconductor solar cells. In our group, we are working on establishing various film forming methods centering on phosphide semiconductors and sulfide semiconductors.
We are investigating processes suitable for target compound such as phosphorization method using proprietary phosphorus vapor pressure control method, vapor deposition method using vapor pressure difference of metal and compound, method using mutual diffusion.
In addition, we actively utilize thermodynamics and chemical potential diagrams to clarify the mechanism of formation of compounds and also apply it to the construction of new processes. [ACS Appl. Energy Mater., (2018); J. Cryst. Growth, (2017); Thin Solid Films, (2015)] The figure on the left is a schematic diagram of the phosphorylation process mechanism of ZnSnP2. In this system, Zn is phosphorylated earlier than Sn thermodynamically. This can be understood from the consideration using the chemical potential diagram.
Understanding and control of interface structure in energy generation device
There are various hetero (hetero phase) interfaces in energy generation devices such as solar cells. In order to improve the performance of the device, it is necessary to optimize the structure and characteristics at the interface between materials, in addition to controlling the physical properties of the various materials that make up the device. For example, in a solar cell device, it is necessary to select a material that properly connects the level of the bottom of the conduction band at the interface between the two materials (light absorption layer material and buffer layer material) that make up the pn junction.
The group will clarify device design guidelines by clarifying these level relationships using methods such as photoelectron spectroscopy. In addition, the interface between the metal serving as an electrode and the light absorbing layer material is also important.
It must be designed to be an ohmic junction at this interface. In addition to choosing the right material, we will analyze the reaction at the interface based on thermodynamics, leading to higher performance of the device. [J Mater. Chem. C, (2017); ACS Appl. Mater. Inter., (2017)] The left figure shows the current-voltage characteristics of the junction between Zn3P2 and Al or Ag. The work function of Al and Ag is almost the same, but the behavior of the current-voltage characteristics of both are different.
This is due to the fact that compound semiconductors are formed as an intermediate layer in Al, but not in Ag.
Creation of new energy generating device
One way to solve these energy problems from the material science standpoint is to develop energy generating devices such as solar cells and thermoelectric elements that actively use solar energy and factory waste heat. We are developing new materials aiming at these device applications and are proposing new device structures for high energy conversion efficiency. All of the above-mentioned creation of new materials based on state diagrams and their process construction, physical property control based on thermodynamics, optimization of heterointerface structure and the like all lead to the creation of devices, We are conducting research while feeding back characteristics. [Sol. Energy Mater. Sol. Cells, (2018); Phys. Stat. Sol. a, (2016)] New solar cell using bulk crystal of ZnSnP2 and its current density - voltage characteristics. It is the world's first power generation demonstration.