# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 3 Quantum Dot Solar Cells
Techniques like substitutional doping of the ZnO layer by incorporating indium [191] or magnesium [186; 190] into the ZnO lattice in sol-gel-based ZnO, the chloride passivation of oxygen vacancies in ZnO NC films by treating the film with sodium chloride (NaCl) [192] and the soaking of ZnO NC films in ultraviolet (UV) light to release gaseous molecules possibly adsorbed onto the ZnO NC surface [208] have demonstrated that increasing the n-doping in the n+ layer has indeed resulted in an improved performance of PbS QDSCs. Analogously, increased p-type doping of the HTL via molecular doping [18] and sodium hydrosulphide (NaHS) treatment [209] has also shown to increase the depletion width in the absorber layer in addition to improving band bending at the interface during the maximum power point condition leading to high fill factors (FF) and high PCE solar cells. Introducing extra electron transport layers (ETLs) and HTLs [196; 210; 211; 212; 213] at these interfaces have also been reported to improve device performance. ETLs are chosen such that they passivate the surface defects present in the metal oxide layer and also promote favorable band bending at the interface between the metal oxide layer and the n type QD films that improves the built-in and open circuit voltage. These ETLs can be self-assembled monolayers (SAMs) formed by treating the metal oxide layer with organic solutions like tetrahydrofuran [184] and aminobenzoic acid [197] or other intermediate buffer layers made of materials like carbon QDs [212], CdSe QDs [214], mixed nanocrystals [188], magnesium-doped ZnO [215] or other organic buffers [216] that have given rise to devices with PCEs >9%. While all these ETLs are used in conjunction with a metal oxide layer, recently, CdS has also been successfully used as an n-type electrode instead of a metal oxide to make a heterojunction QDSC with a PCE of 8% [217]. Despite the high performance of heterojunction solar cell, there are some inherent disadvantages associated with such a configuration. The misalignment of bands at the junction could potentially lead to a loss in open circuit voltage and improper charge injection into the n-type window layer for QD films with a smaller bandgap [218]. Thus, the ideal QDSC configuration would be to have a specific bandgap tuned PbS QD film with p- and n-type doping on opposite side of the p-n junction that has the entire p-n junction absorbing light to produce photo-generated carriers. Such quantum junction solar cells (QJSCs) have also been developed with p-type doping being achieved through physical deposition of the oleate capped PbS layer in air followed by tetramethylmoroxide (TMAOH) treatment and air annealing. On top of the p-type layer, n-type doping through iodide treatment inside an N\({}_{2}\) filled glovebox [181; 218] completes the p-n junction. Further modifications through the addition of an n+ layer in between the iodide treated film and the metal contact [219] and replacing the iodide ligand-exchanged n-type layer with an n-type CQD ink as discussed earlier have resulted in performance enhancement leading to PCE as high as 8% [203]. Similar doping strategies using S\({}^{2-}\) have shown promise creating n-type HgS QDs for photodetectors [220]. Though PbS has been the most explored QD material in QDSCs, theoretically, PbSe shows better promise to overcome the Shockley-Queisser limit for solar cell devices. PbSe's larger Bohr exciton radius improves quantum confinement of charge carriers in the QDs which in turn increases the probability of carrier multiplication as evidenced by the work of [PERSON] et al. [79] and [PERSON] et al. [80]. Similar to PbS QDSC devices, PbSe QDSC devices were first fabricated in Schottky junction configuration [5; 167; 221] following which the heterojunction configuration was adopted. In this configuration, devices with PCEgreater than 5% have been achieved through a combination of effective PbSe QD synthesis protocols and different solid-state ligand exchange treatments. [PERSON] et al. [PERSON] et al.