# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 3 Quantum Dot Solar Cells
(2018) and [PERSON] et al. [PERSON] et al. (2019) devised cation exchange reactions to synthesize PbSe from CdSe and ZnSe, respectively, which resulted in QDSC devices with PCE greater than 5% and superior air-stability due to possible surface passivation by a metal and halide ions. While the insulating oleate ligands in as-prepared PbSe QD films were replaced with shorter and more conductive EDT in early PbSe QDSC devices, [PERSON] et al. 's [PERSON] et al. (2018) work showed that solid state ligand exchange with TBAI ligands provided the best results by comparing the effect of TBAI, EDT, MPA and cetyltrimethylammonium bromide (CTAB) on device performance. By adopting techniques used in PbS QDSCs, the performance of PbSe QDSCs has also improved in recent times. [PERSON] et al. [PERSON] et al. (2019) fabricated air-stable devices with a PCE \(\approx\)6% using the bi-layer configuration, while [PERSON] et al. [PERSON] et al. (2018) added an additional CsPbBr\({}_{3}\) HTL in between the MPA exchanged layer and the back contact to improve hole transfer and consequently the device FF and PCE. Using a PbSe CQD ink with Pb\({}_{2}\) surface passivation similar to that made by [PERSON] and co-workers [PERSON] and co-workers [PERSON] et al. (2018), [PERSON] et al. [PERSON] et al. (2018) were able to fabricate high-efficiency QDSC devices through a single step deposition method as opposed to the LBL deposition process through spin/dip coating. The [PERSON] group's unique halide passivation technique of the PbSe QDs using Cs-based perovskite NCs has helped achieve a PCE of 8.2% [PERSON] et al. (2018) which currently is the world record efficiency for PbSe QDSC devices. Figure 6 plots the PCE and V\({}_{\text{OC}}\) of all PbS and PbSe QDSCs as a function of the QD bandgap that we could find in the literature. Figure 6a shows the majority of the devices use QDs around 1.3 eV (the theoretical optimum bandgap for a single junction solar cell). However, one of the biggest strengths of QDs is their tunable bandgap, which could be used more to complement other existing technologies. For instance, using QDs in combination with commercial Si or CdTe cells could boost their efficiency in a tandem configuration (Ple  
a favorable band bending for both electrons and holes. While Au is predominantly used for this purpose, a combination of stepwise decreasing work function metals like Au/Ag or MoO\({}_{\rm x}\)/Au/Ag or AZO/Ag (AZO is aluminum-doped zinc oxide) has also been adopted by a few research groups. Other than this, a few others have also adopted the use of inorganic materials like molybdenum oxide (MoO\({}_{\rm x}\)) [231; 232], or other organic materials like poly(3-hexylthiophene-2,5-diyl) (P3 HT) [233], 2\(\prime\),7,7\(\prime\)-tetrakis(\(N\),\(N\)-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) and bis-triphenylamine (BTPA-4) [211] as hole selective layers to solve this problem. Despite favorable results with MoO\({}_{\rm x}\) under inert conditions, [PERSON] et al. [174] showed that the devices with MoO\({}_{\rm x}\) lacked air-stability due to MoO\({}_{\rm x}\)'s possible oxidation that results in the lowering of its work function and thus unfavorable band bending at the interface. While different configurations seemed to have worked for different devices, a systematic study of any possible device performance enhancement by optimizing this interface has not yet been studied. Other record-setting PbS devices include those developed by [PERSON] et al. [192] with a PCE of 11.6% based on a CQD ink and [PERSON] et al. [195] with a PCE of 11.21% based on solid-state ligand exchange. Though in recent years, perovskite-based QDs have also been fabricated based on cesium lead iodide (CsPbI\({}_{3}\)) [234; 235] one of which developed at the National Renewable Energy Laboratories (NREL) currently holds the world record for QD-based solar cells with PCE of 13.23% [236].