There are several different cell architectures that are currently being used in Colloidal Quantum Dot (QCD) solar cell research; they include quantum dot sensitized cells, Schottky Junction cells, and a variety of heterojunction based cell designs. Current research efficiencies for QCD solar cells are around 5%.
For this application, the solar cell will use a semiconductor-semiconductor heterojunction as it's effective p-n junction. Titanium Dioxide is a metal oxide that is commonly used as a transparent film with n-type semiconductor properties due to oxygen vacancies in its lattice and the presence of negatively charged charge carriers. This film material is intentionally chosen with a high band gap (~3.5eV, depending on nanoparticle size and preparation technique), and subsequently allows the majority of photons to pass through into the layer of colloidal CdSe nanocrystals. TiO<sub>2</sub> nanoparticles are available for purchase from chemical companies (for example Sigma Aldrich, ~1.13$/gram). Once the majority of incoming photons pass through the metal oxide layer, they are absorbed by the thin film of CdSe nanocrystals, with a lower (and much easier to excite) band gap. The thickness of these layers is typically 100-300nm. See corresponding plot of the CdSe nanocrystal band gap as a function of size.
[[Image:CdSe_Eg_tuneability.PNG|center|Dependence of Band Gap on nanocrystal size]]