The role of plasma-enhanced atomic layer deposition growth of AlGaN ternary alloys on c-planar sapphire substrates and the preparation of quantum dot-sensitized solar cells are explored in this work. The interface between the film and the substrate as well as the band gap of AlGaN ternary alloys during atomic layer deposition is dependent on Al component. At high Al fraction, there appears a good interface between the AlGaN alloy film and the substrate, however, the interface becomes rough when the Al fraction is reduced. The AlGaN alloy prepared by atomic layer deposition has a high band gap, which is related to the oxygen content within the film. Subsequently, CdSe/AlGaN/ZnS and CdSe/ZnS/AlGaN structured cells are prepared and analyzed for quantum dot solar cells from AlGaN films with an AlN/GaN cycle ratio of 1∶1. It is found that AlGaN can modify and passivate quantum dots and TiO₂, which can wrap and protect the structure of TiO₂ and CdSe quantum dot, thus avoiding the recombination of photo-generated carriers. This modification effect is also reflected in the improvement of open-circuit voltage, short-circuit current, filling factor and photovoltaic conversion efficiency of quantum dot solar cells. These factors are discussed in this work, trying to modify carrier transport characteristics of AlGaN films prepared by atomic layer deposition.