Semi-conductive nano-particles also known as Quantum Dots (QDs) absorbs light in solar cells. The characteristics of QDs that are needed for transferring the absorbed light’s energy is its “conduction and valence bands of the QDs permit electron injection and hole transportation” through to the metal oxide and metal layers, which the QDs is between (Etgar 448). The amount of light absorbed by the Quantum dots depends on its thickness if it “is too thick, the collection of photogenerated charge carriers is incomplete, while too-thin QD layers show poor light harvesting” (E. 448). The QDs size also plays a factor in its performance when “open circuit voltage, fill factor and photocurrent decrease with increasing the QD size; however, inner-particle electron transfer is facilitated in films made of the larger QDs” (E.448). Electrons and holes move faster “by one or two orders of magnitude with an increase in QD diameter” (E.448). These solar cells don’t rely on single p-n junction design, but uses tandem cells or multi-junction solar cells with a “stack of p-n junctions of low-dimensional semiconductor structures” (E. 446). The p-n junction stacks can have different Eg, thus covering a very wide range of the solar spectrum, thus increasing the theoretical energy conversion efficiency from 31% to 66%.