Papers by Author: Ching Cherng Wu

Abstract: We have demonstrated structural and electronic properties of a series solar energy crystals Cu(Al_xIn_1-x)S₂ (0<=x<=1) by using measurement techniques of X-ray diffraction, polarized thermoreflectance (PTR), and X-ray photoelectron spectroscopy (XPS). Single crystals of Cu(Al_xIn_1-x)S₂ (0<=x<=1) (0 and E ^ polarizations. The PTR spectra clearly showed that the energy value of D increases with the increase of Al content x in the Cu(Al_xIn_1-x)S₂ (0<=x<=1) series due to the enhanced strain in the lattice. The composition-dependent crystal-field-splitting energies can be evaluated and determined to be D(x)= (10±2)+( 139±5)×x meV. Based on the experimental analyses, the crystal structure and valence-band structure of the Cu(Al_xIn_1-x)S₂ (0<=x<=1) (0<=x<=1) series are thus realized.

Abstract: In this study, Bi2Te3 was selected as a matrix material and an immiscible semiconductor material CdTe with a wider energy band gap was precipitated to form microstructures in it. For this purpose, a series of two-component composites (Bi2Te3)1-x/(CdTe)x, with x = 0.01, 0.05, 0.1, 0.15, 0.2 were prepared by melting, high speed rocking and quenching technique. The composition and microstructure of these two-component composites were examined by X-ray and SEM. Thermoelectric properties including Seebeck coefficient, electric resistivity, and thermal conductivity were measured from 30 K to 400 K. The compositional and temperature dependence of the microstructure and thermoelectric properties of (Bi2Te3)1-x/(CdTe)x composites were discussed. It is found that (Bi2Te3)0.99/(CdTe)0.01 exhibits a better thermoelectric performance than that of Bi2Te3 at elevated temperatures. The figure of merit (ZT) of (Bi2Te3)0.99/(CdTe)0.01 is about 0.64 at 400 K.

Abstract: Electronic structure of solar-energy related crystals of CuInS2 and CuAlS2 has been characterized using thermoreflectance (TR) measurement in the energy range between 1.25 and 6 eV. The TR measurements were carried out at room (~300 K, RT) and low (~30 K, LT) temperatures. A lot of interband transition features including band-edge excitons and higher-lying interband transitions were simultaneously detected in the low-temperature TR spectra of CuInS2 and CuAlS2. The energies of band-edge excitonic transitions at LT (RT) were analysed and determined to be =1.545 (1.535) and =1.554 eV (1.545 eV) for CuInS2, and =3.514 (3.486), =3.549 (3.522), and =3.666 eV (3.64 eV) for CuAlS2, respectively. The band-edge transitions of the and excitons are originated from the sulfur pp transitions in CuInS2 and CuAlS2 separated by crystal-field splitting. Several high-lying interband transitions were detected in the TR spectra of CuInS2 and CuAlS2 at LT and RT. Transition origins for the high-lying interband transitions are evaluated. The dependence of electronic band structure in between the CuInS2 and CuAlS2 is analysed and discussed.