# Recent Developments of Solar Cells from PbS Colloidal Quantum Dots
## 3 PbS Quantum Dot Schottky Solar Cells  
One recent idea to increase Schottky quantum dot solar cells' (QDSCs) power conversion efficiency is to combine a \(p\)-type PbS quantum dot layer with an \(n\)-type metal oxide layer for the front junction [57], or to use inverted Schottky QDSCs with a low-work-function, abstaining from metal oxides [58]. [PERSON] et al. investigated different interfacial electrolytes to fit the electrolyte chemistry to the cell performance [59]. They capped PbS quantum dots (QDs) with oleic acid [58], and passivated them afterwards with CI'. The transparent conductive oxide (TCO) front electrode's work function was lowered by spin-coating the TCO electrode with a few nanometers of polyethylenimine (PEI) or poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), respectively. After adding layers of PbS quantum dots, and MoO\({}_{\mathrm{x}}\) and Au/Ag as a back electrode, the cell was finished. For the different polymer electrolytes used to reduce the work function, the electrical characteristics of the inverted Schottky QDSC changed clearly, as visible in Figure 3. In this way, it was possible to reach an efficiency of 4.5% in an oxide-free QDSC [59]. Using ligand-exchanged PbS quantum dots, [PERSON] et al. reached a more than twice-as-high photoconversion efficiency [60]. The ligand exchange means that carrier mobility in a quantum dot thin film is increased by a chemical reduction of the ligand length [61]. In this case, soaking the PbS quantum dot thin film in dissolved ethanoledithiol (EDT), a typical metal ion ligand, resulted in slightly increased short circuit currents and open circuit voltages, as well as clearly enhanced fill factors [60]. Another way was suggested by [PERSON] et al. [62], who used the inorganic-organic hybrid material CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) (MAPI) for PbS nanoparticle passivation and investigated the resulting electrical properties of Schottky type solar cells built with these core-shell nanoparticles. It should be mentioned that passivation, in general, is a technology step which reduces surface-related effects and emphasizes the intrinsic bulk-electronic properties. Comparing these core-shell nanoparticles with the original PbS nanoparticles capped with oleic acid/oleylamine, as prepared before the ligand exchange (Figure 4), showed clearly improved electric properties of the corresponding Schottky type solar cell with an approximately five-times higher efficiency than without a ligand exchange. The increased open current voltage was explained by the larger barrier height of the core-shell nanoparticles, in addition to trap state passivation in the quantum dot surface by the MAPI shell, while the improved fill factor and photocurrent were attributed to an enhanced charge transport, based on the lower distance between neighboring quantum dots after the ligand exchange [62]. Figure 3: Current density-voltage (J-V) curves of inverted Schottky QDSCs with two different electrolytes. From [59], reprinted with permission. Copyright © Elsevier 2019. Similarly, [PERSON] et al. suggested passivating PbS colloidal quantum dots by hybrid ligands prepared from combinations of oleic amine, octyl-phosphine acid and CdCl\({}_{2}\), and found increased efficiencies in Schottky and p-n junction solar cells, which was attributed to oxidation prevention during device fabrication and shorter chain lengths in the hybrid ligands corresponding to higher hole mobility [63], as also mentioned in [61]. [PERSON] et al. concentrated on a chemical way to \(p\)-dope films of thiol-capped PbS quantum dot, resulting in \(p\)-\(n\) junction solar cells with an improved short circuit current and fill factor, and thus increased efficiency [64].