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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) <ref>Vijayalakshmi, R., and V. Rajendran. "Synthesis and Characterization of Nano-TiO2 via Different Methods." Archive of Applied Science Research 4.2 (2012): 1183-190. Web.</ref>, 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, 21nm), and can be redispersed in a chosen of dispersant. 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. <ref>Etgar, L. “Semiconductor Nanocrystals as Light Harvesters in Solar Cells”, Materials, p. 445-455, 2013.</ref> <ref> Nikolenko, L. M., and V. F. Razumov. Colloidal Quantum Dots in Solar Cells. Russian Chemical Reviews 82.5 (2013): 429-48. Print.</ref>
[[Image:CdSe_Eg_tuneability.PNG‎|center|Dependence of Band Gap on nanocrystal size]] <ref>Baskoutas, Sotirois, and Adreas F. Terzis. "Size-dependent Bad Gap of Colloidal Quantum Dots." Journal of Applied Physics 99 (2006): n. pag. Web.</ref>
Movement of electrons generated from the CdSe nanocrystal layer (p-type) to the TiO<sub>2</sub> layer (n-type) is made possible by the induced electric field, created by the depletion layer that results from contact between the two semiconductors (and charge carrier transfer from n-layer to p-layer). Free electrons are then transferred out of the metal oxide and into a transparent electrode (usually Indium Tin Oxide), while holes move in the opposite direction toward a metallic electrode (usually Ag) backing. See the corresponding plot of cell structure and energy levels for a visual representation of process.
[[Image:QD_heterojunction_energy_diagram.PNG‎ ‎|center|Energy level diagram of QD heterojunction consisting of films of TiO<sub>2</sub> QDs, CdSe QDs, and two electrodes ]] <ref>Nikolenko, L. M., and V. F. Razumov. Colloidal Quantum Dots in Solar Cells. Russian Chemical Reviews 82.5 (2013): 429-48. Print.</ref>
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