A quantum dot device includes: a first electrode and a second electrode facing each other; a quantum dot layer between the first electrode and the second electrode, and an electron auxiliary layer between the quantum dot layer and the second electrode, the electron auxiliary layer including a first nanoparticle and a second nanoparticle which is larger than the first nanoparticle, wherein a work function of the first electrode is greater than a work function of the second electrode, and wherein a difference between a lowest unoccupied molecular orbital energy level of the quantum dot layer and a lowest unoccupied molecular orbital energy level of the electron auxiliary layer is less than about 1.1 electronvolts.