In order to increase photocurrents of crystalline silicon solar cells through modification of the solar spectrum, we have investigated the upconversion effect of NaYF4:Er3+ nanocrystals. The upconverting layers of NaYF4:Er3+ nanocrystals were prepared by spin casting and post-soaking processes. The material characterization and the upconversion spectroscopy were carried out as functions of the erbium doping concentration and the film thickness. The effect of the laser density on the optimal ratio of trivalent erbium against trivalent yttrium has also been investigated using power dependent upconversion spectroscopy at various doping ratios. Through this study, optimized upconverting nanocrystals and fabrication methods to minimize the thickness of upconverting films have been investigated and achieved.