# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 3 Quantum Dot Solar Cells
The QD absorber layer thickness in such a case is limited to the sum of the depletion width and diffusion length, which ensures that the charge carrier formed farthest away from the heterojunction can diffuse until it reaches the space charge region and then be transported to the other side of the junction by the electric field. In the following years, this depleted heterojunction QDSC configuration has undergone several modifications leading to solar cell PCE exceeding 10% [181; 182; 183; 184; 185; 186; 187; 188; 189; 190; 191; 192; 193; 194; 195; 196; 197]. The following paragraphs will detail the development of high-efficiency heterojunction QDSCs. In the beginning of the depleted heterojunction QDSC development, short, organic ligands like 1,2-ethanedithiol (EDT) and 3-mercaptoethanolropic acid were exchanged with the insulating oleate ligands due to their ability to p-dope the absorber layer and allow better electronic coupling between neighboring QDs in films. While EDT was predominantly used to make such devices, [PERSON] et al. [128] showed that MPA ligand-exchanged PbS QD films performed much better than EDT  
Figure 5: A compilation of the effect of 29 different QD surface passivating ligand species on the energy levels of PbS QDs with a bandgap of \(\approx\)1.3 eV from literature [176; 177; 144; 178; 174; 176]. It is observed that more electronegative species push the energy levels deeper due to their higher affinity to hold electrons inside the QD by establishing a strong electric field around the QD. Samples prepared under ambient conditions are represented by the superscript “_**_” while other superscripts like “+” [144], “_**_” [174], “_**+**+**” [178], “_**+**+**” [176] differentiate the energy level values obtained in different literature cited for similar ligand treatments. The red, black and green lines represent the CB minimum, Fermi level and the VB minimum respectively. Details are available in the Supplementary Materials. ligand-exchanged films due to its ability to passivate the QD surface better and pack QDs closer than with EDT ligands leading to higher carrier mobilities. Following the rise in usage of MPA, in 2014, [PERSON] et al. [174] developed an approach that involved engineering the band alignment of the PbS QDs based on the interaction of QD films with two different ligands. The PbS QDSCs fabricated with the method from [PERSON] et al. showed a PCE of 8.55% that thereafter became the norm in depleted heterojunction QDSCs. In this approach, each deposited layer of the oleate-capped PbS QD film in the entire stack was ligand-exchanged with tetrabutylammonium iodide (TBAI) except the final two layers that were instead ligand-exchanged with EDT--similar to what [PERSON] et al. [79] observed using EDT and hydrazine in order to observe carrier multiplication in EQE measurements. The deeper energy levels of iodide-capped PbS QD films align with the shallower energy levels of the EDT-capped PbS QD films to create an energetic barrier for electrons. With this alignment, electrons flow from the EDT-capped layers to the iodide-capped layers resulting in favorable band bending forming an electron blocking layer to prevent electron recombination at the rear metal contact. Using the same configuration, other research groups have obtained similar results using other ligand combinations with functionalities like PbI\({}_{2}\) and MPA [144], 1-ethyl-3-methylimidazolium iodide (EMII) and EDT [178, 187, 188] and 1-propyl-2,3-dimethyl-imidazolium iodide (PDMII) and 1,3-propane dithiol (PDT) [183, 184]. Using PbI\({}_{2}\) and mercaptopropionic (MPA) ligands, [PERSON] et al. [144] reported solar cells with thickness optimized at over 500 nm, almost double the thickness of other record-setting devices, due to favorable band alignments between the layers on the cell and improved surface passivation of PbI\({}_{2}\) over other halide treatments. The reproducibility and stability of such treatments then allowed for further optimization of other components in the device structure and fabrication procedures.