**5.5. Depleted heterojunction quantum dots solar cells**

In depleted heterojunction colloidal quantum dot solar cells as detailed in ref. [24] a nano‐ structured wide bandgap semiconductor such as TiO2 and quantum dot film are sandwiched between conductive transparent electrode (glass coated with Tin Oxide SnO2F) and metallic (such as gold) coated electrode (see Figure 17-a).Figure 17-b illustrate the energy band diagram. Although TiO2 has low carrier density (~1016 cm-3) compared to metal, a depletion region in the cells forms due to charge transfer to QD film. And because of high electron density in metal (~ 1022 cm-3), the depletion is negligible on its side of the cell. Depleted heterojunction cell overcome the disadvantages encountered with Schottky cell.

**Figure 16.** Shows how photo-generated carriers from quantum dots injected in conducting polymers. From [31].

In their work Grätzel and coworkers [24] have reported that the resultant depleted hetero‐ junction solar cells offer power conversion efficiency of 5.1% at AM1.5 illumination. The devices were capable of harvesting broadband of the solar spectrum as a result of employing infrared-bandgap size-effect-tuned PbS CQD.
