Perovskite solar cells have been attracted as new representatives for the third-generation photovoltaic devices. Simple strategies for high efficiency with the long-term stability of solar cells are the challenges for commercial solar cell technology. Another challenge of the development toward industrial scale in perovskite solar cells is the production under the ambient and high humidity. In this sense, we successfully fabricated perovskite solar cells via solution depositions of all layers under ambient air with a relative humidity above 50%. Titanium dioxide (TiO₂) nanoparticles with the roles for efficient charge extraction and electron transportation properties were used as an electron-transporting layer in the cell fabrication. The modification of TiO₂ nanoparticles for energy band adjustment was done by doping with nontoxic cadmium sulfide (CdS) quantum dots. With the variation of CdS concentrations, energy band is not only changeable, but the enhancement of the perovskite solar cells efficiency could be achieved compared with the conventional cells made of pristine-TiO₂ film and TiO₂ nanoparticles.