# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 3 Quantum Dot Solar Cells
The primary area of improvement that researchers have focused on has been the improvement of the diffusion length of minority carriers in PbS QDs. While solid-state ligand exchange techniques have been shown to effectively replace the oleate ligands attached to the as-prepared QD surface, it does not fully protect the QD surface and a certain percentage of trap states remain [198, 199]--some of which could arise from fusing of QDs during the ligand exchange forming dimers that naturally have lower energy levels in the bandgap of the other QDs [181, 200]. As a result, solution phase passivation techniques were adopted. First, a post-synthesis treatment to improve the surface passivation with metal halides was used [201]. It was observed that this process replaced some oleate ligands while retaining enough to maintain the stability of the QDs in the non-polar solvent (usually octane). Due to their small size, halides have shown very good passivating capabilities and have been extensively used in solution phase passivation techniques. Cadmium chloride--CdCl\({}_{2}\) (provided Cd\({}^{2+}\) and halide passivation of the S\({}^{2-}\) and Pb\({}^{2+}\) vacancies) [201], TBAI [202, 203], molecular iodine (I\({}_{2}\)) [181], methyl ammonium iodide (MAI, a milder I source) [182] and Trimethylsilyl chloride (TMS-Cl in n-hexylamine and n-octylamine) [137] are a few examples of the ligands that have been used effectively in solution phase ligand passivation of PbS QDSCs. The TMS-Cl passivation of QDs has also resulted in a record diffusion length of \(\approx\)230 nm arising from the partial fusion of neighboring QDs. Additionally, it was observed that of these techniques, MAI's ability to controllably strip oleate ligands and subsequently passivate the QD surface well with I\({}_{2}\) has led to QDSCs with PCEs as high as 10.6% [182] and that QDSC devices made using I\({}_{2}\) ligand passivation were found to be the most air-stable [203]. While all the above devices involved developing the QD absorber by a layer-by-layer (LBL) deposition method coupled with consecutive solid-state ligand exchange and rinsing steps, a single step deposition method involving complete ligand exchange through solution-phase ligand exchange was explored to simplify device fabrication. One of the major hurdles in such a single step method was finding the best solvent that could maintain the stability of the QDs capped with shorter ligands in solution and at the same time evaporate in such a way that leads to smooth, uniform, and crack-free films. [PERSON] and co-workers [204], at a similar time as the [PERSON] group [205, 206], developed a strategy involving a solution-phase exchange of the oleate ligands from the PbS QDs in octane with iodide ligands present in a polar solvent like \(N\),\(N\)-dimethylformamide (DMF). The immiscibility of the two solvents allows the easy separation of the two phases formed and the I\({}^{-}\)-capped QDs dissolved in DMF are then washed and dispersed in n-butylamine with a very high concentration (\(\approx\)200 mg/mL). This CQD ink is then deposited in a single step (as opposed to LbL deposition) to obtain QD films of the required thickness and uniformity. While MAI, sodium iodide (NaI) [204] and PbI\({}_{2}\)[207] have been used as iodine sources, a mixture of PbI\({}_{2}\)/PbBr\({}_{2}\)/ammonium acetate (NH\({}_{4}\)Ac) has shown the best results leading to PbS QDSCs with PCEs of 11.28% [193] and 11.6% [192]. In addition to new passivation techniques to improve the diffusion length in PbS QD films, researchers have also worked on increasing the width of the depletion region in the absorber layer to improve drift-based carrier transport. In PbS QDSCs with a bilayer configuration like the one developed by [PERSON] et al. [174], the existence of 2 junctions, the n+/n heterojunction between ZnO (or TiO\({}_{2}\)) and the n/p quantum junction between I\({}^{-}\) capped and EDT capped PbS (also called the hole transport layer, HTL) makes it interesting to optimize the device architecture.