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We investigated the characteristics of excimer laser-annealed polycrystalline silicon−germanium (poly-Si1−xGex) thin film and thin-film transistor (TFT). The Ge concentration was increased from 0% to 12.3% using a SiH4 and GeH4 gas mixture, and a Si1−xGex thin film was crystallized using different excimer laser densities. We found that the optimum energy density to obtain maximum grain size depends on the Ge content in the poly-Si1−xGex thin film; we also confirmed that the grain size of the poly-Si1−xGex thin film is more sensitive to energy density than the poly-Si thin film. The maximum grain size of the poly-Si1−xGex film was 387.3 nm for a Ge content of 5.1% at the energy density of 420 mJ/cm2. Poly-Si1−xGex TFT with different Ge concentrations was fabricated, and their structural characteristics were analyzed using Raman spectroscopy and atomic force microscopy. The results showed that, as the Ge concentration increased, the electrical characteristics, such as on current and sub-threshold swing, were deteriorated. The electrical characteristics were simulated by varying the density of states in the poly-Si1−xGex. From this density of states (DOS), the defect state distribution connected with Ge concentration could be identified and used as the basic starting point for further analyses of the poly-Si1−xGex TFTs.

We report the effects of surface passivation by depositing a hydrogenated amorphous silicon (a-Si:H) layer on the electrical characteristics of low temperature polycrystalline silicon thin film transistors (LTPS TFTs). The intrinsic a-Si:H layer was optimized by hydrogen dilution and its structural and electrical characteristics were investigated. The a-Si:H layer in the transition region between a-Si:H and µc-Si:H resulted in superior device characteristics. Using a-Si:H passivation layer, the field-effect mobility of the LTPS TFT was increased by 78.4% compared with conventional LTPS TFT. Moreover, the leakage current measured at VGS of 5 V was suppressed because the defect sites at the poly-Si grain boundaries were well passivated. Our passivation layer, which allows thorough control of the crystallinity and passivation-quality, should be considered as a candidate for high performance LTPS TFTs.

The band structure characteristics of a copper indium gallium sulfur selenide (Cu(In1–xGax)SeS, CIGS) solar cell incorporating a cadmium-free zinc sulfide (ZnS) buffer layer were investigated using technology computer-aided design simulations. Considering the optical/electrical properties that depend on the Ga content, we numerically demonstrated that the front gradient bandgap enhanced the electron movement over the band-offset of the ZnS interface barrier, and the back gradient bandgap generated a back side field, improving electron transport in the CIGS layer; in addition, the short circuit current density (JSC) and open circuit voltage (VOC) improved. The simulation demonstrated that the conversion efficiency of a double graded bandgap cell is higher than with uniform or normal/reverse gradient cells, and VOC strongly correlated with the average bandgap in the space charge region (SCR) of CIGS. After selecting VOC from the SCR, we optimized the band structure of the CIGS cell with a Cd-free ZnS buffer by evaluating JSC and the fill factor. We demonstrated that the cell efficiency of the fabricated cell was more than 15%, which agrees well with the simulated results. Our numerical method can be used to design high-conversion efficiency CIGS cells with a gradient band structure and Cd-free buffer layer.