# Recent Developments of Solar Cells from PbS Colloidal Quantum Dots
## 4 PbS Quantum Dot Sensitized Solar Cells  
Light harvesting properties of metal oxides like TiO\({}_{2}\) or ZnO, which are typically applied as semiconducting layers in DSSCs, can be enhanced by decorating them with low bandgap metal sulfides. One of the techniques used for this purpose is the so-called pseudo-successive ionic layer absorption and reaction (p-SILAR). [PERSON] et al. described in their most recent study how PbS and CdS quantum dots were deposited on TiO\({}_{2}\) nanoparticles, performing a H\({}_{2}\)S treatment of the TiO\({}_{2}\) nanoparticles in a rotary reactor. Using rhodamine B degradation as a measure, they showed the positive influence of the H\({}_{2}\)S treatment for the p-SILAR process [68]. [PERSON] et al. improved the p-SILAR technique by a wet chemical modification, as depicted in Figure 6, applying a fluorination treatment of PbS and CdS nanoparticles during centrifugation, resulting in an increased deposition on ZnO nanoparticles and thus a significant increase of the photocurrent density of corresponding solar cells [69]. With common SILAR, [PERSON] et al. realized a synchronous PbS and CdS quantum dot deposition on TiO\({}_{2}\), in this way avoiding the excessive growth of PbS quantum dots in TiO\({}_{2}\) mesopores and increasing high-density defect passivation on the PbS surface, both leading to a strong increase of solar cell efficiency [70]. Similarly, [PERSON] et al. reached epitaxial nucleation of PbS quantum dots on a planar TiO\({}_{2}\)(100) surface in rutile modi  
Figure 6: Comparison of the normal (upper row) and the modified p-SILAR process (lower row). From [69], reprinted with permission. Copyright © Elsevier 2016. [PERSON] et al. applied SnO\({}_{2}\) instead of TiO\({}_{2}\) for quantum dot-sensitized solar cells. They used the aforementioned ligand exchange to reach an improved charge transfer to the sensitized substrate by using short-chain multidentate hydrophilic ligands instead of the common hydrophobic oleic acid ligands, in order to increase electronic coupling to the substrate and at the same time avoid degradation of the PbS-sensitized SnO\({}_{2}\) single crystals in the electrolyte [72]. On the base of ZnO/ZnSe core/shell nanorod arrays decorated with PbS quantum dots by SILAR, [PERSON] prepared solar cells with a maximum efficiency of 0.88%, concentrating on the core/shell structure and the nanorod array density rather than optimizing the PbS decoration [73]. Another technique was used by [PERSON] et al., who deposited colloidal quantum dots by electrophoretic deposition on TiO\({}_{2}\), increasing the quantum dot loading by optimizing the solvent, as visible in Figure 7. The clear difference in absorption resulted in also suggesting dichloromethane (DCM) as a possible solvent for PbS and other quantum dots [74]. Electrophoretic deposition was also used by [PERSON] et al. to develop nanocomposite-doped photo-anodes with PbS quantum dots as a sensitizer, showing that the incorporation of TiO\({}_{2}\)/Au nanocomposites, the optimization of the quantum dot deposition temperature and the postdeposition annealing of the sensitized photo-anode improved the power conversion efficiency [75]. [PERSON] et al. applied PbS colloidal quantum dots as sensitizers on mesoporous TiO\({}_{2}\)/CsPbBr\({}_{3}\) perovskite interfaces in solar cells by spin-coating, in this way approximately doubling the power conversion efficiency in comparison with the pristine device [76]. PbS quantum dots were synthesized inside ferritin protein shells by [PERSON] et al., which protected them from photocorrosion, resulting in quantum dot sensitized solar cells with mesoporous TiO\({}_{2}\), giving efficiencies of around 0.29% when a drop casting method was used [77]. [PERSON] et al. concentrated on the colloidal PbS quantum dots themselves.