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The process of graphene transfer generally involves a polymer support. Thermal annealing of graphene monolayers transferred from Cu to SiO 2 /Si substrate has been investigated to remove polymethyl methacrylate (PMMA) residue. The results show that a clean graphene surface without any deterioration is difficult to obtain by conventional thermal annealing method owing to the reaction between the graphene and the carbon by-products from the residual polymers. Although a higher annealing temperature facilitates removal of the polymer residue on graphene surface, it may also increase the amount of amorphous carbon. In this work, we examine various annealing conditions to remove the polymer residue. Annealing under H 2 ambience is effective in removing pendant functional groups in the polymer, yet increases the amorphous carbon also. In case of Ar ambience with high temperature, the polymer residue is efficiently removed without amorphous carbon.

In this work, we report the efficiency enhancement of a GaAs solar cell by using a luminescent down-shifting (LDS) layer consisting of (CdSe)ZnS quantum dots (QDs). The calculated conversion efficiency shows strong dependence on the luminescence quantum efficiency (LQE) and concentration of QDs in the LDS layer as well as the difference in external quantum efficiency (EQE) of the solar cell in the absorption and emission regions of the QD. Although the irradiance of the modified spectrum by the LDS layer was reduced with increases in QD concentration, the emission range of QDs in the spectrum was intensified. So the optimum parameters need to be calculated carefully. When we measured the GaAs solar cell with the LDS layer, it showed increased current density and consequently improved efficiency. The highest performance difference between the GaAs solar cell efficiencies with and without the LDS layer was over 2.8%, although a crystalline silicon solar cell below the LDS layer showed negative change in efficiency owing to its small EQE difference.

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