# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 3 Quantum Dot Solar Cells
To overcome the limitations posed by Schottky junction cells, planar p-n heterojunction solar cells were developed (middle structure of Figure 4) and are the current front-runner for the highest efficiency devices. Finding the proper band alignment is critical to maximizing the PCE in a heterojunction solar cell. In this regard, ligands play a large role in determining the conduction band (CB), valence band (VB), Fermi energy levels by inducing a dipole moment on the QD surface. [PERSON] et al. [145] compared the effects of 12 different ligands on the energy levels of PbS QDs and concluded that a dipole moment formed at the interface between the ligand and the QD surface combined with the intrinsic dipole moment of different ligands shifts the energy levels of the QD by different magnitudes. Following this, other research groups have also performed similar spectroscopy studies on QDs under inert and ambient conditions to determine the effect of different ligands and processing conditions on the energy levels of PbS QDs [173]. Figure 5 shows a compilation of UPS and XPS measurements of the band positions of 29 different ligands on PbS QDs. Furthermore, density functional theory (DFT) calculations have shown that the lack of an intrinsic dipole moment in iodide ligands lead to deeper energy level shifts. Thiols, with higher intrinsic dipole moments, cancel out the interface dipole moment created by ligand interaction with the QD surface leading to a shallower energy level shift. This has formed the basis for band alignment engineering leading to high-efficiency PV cells [174]. While ligands have played the role of dopants, QD films have also been doped using remote doping techniques. [PERSON] et al. [175] showed that based on the electron affinity and ionization energies, dopants with a deep electron affinity induce a remote flow of electrons from the VBM of  
Figure 4: A compilation of the band diagrams of the three common PbS QDSCs configurations. The leftmost configuration depicts a Schottky junction solar cell where charge carrier separation and extraction occur at the rear junction, between the PbS QD layer and the metal electrode. The middle configuration shows a depleted heterojunction solar cell in which the TiO2 nanoparticles-PbS QD layer interface forms the main junction where charge extraction and separation takes place. The rightmost configuration shows a QD-sensitized device in which the differences in electron affinity between the TiO2 and the PbS QD layer causes charge separation. Reproduced with permission from reference [172], copyright 2010 American Chemical Society. the QD into the LUMO of the dopant to leave behind a p-doped QD (with the reverse energy flow yielding n-doped QDs). In 2010, [PERSON] et al. [172] developed a planar depleted heterojunction configuration for QDSCs after significant work starting with QDs blended with conductive polymers [179; 180]. These devices were the first to break the 5% PCE mark and hence formed a basis for future research. In such a configuration, the heterojunction is formed between the light absorbing QD layer and a wide bandgap n-doped electrode like TiO\({}_{2}\) or zinc oxide (ZnO) which acts as an electron accepting and hole blocking layer as seen in Figure 4. The holes are collected at the back contact using a deep work function metal like gold (Au) while the electrons are collected at the top contact which is typically a transparent conductive oxide (TCO) like ITO or fluorine-doped tin oxide (FTO). The presence of states inside the bandgap in real QDs leads to small minority carrier diffusion lengths in the QD film. This leads to significant difficulty in extracting minority charge carriers formed on photoexcitation in such planar devices, which is tackled by driving the minority carrier transport by the electric field formed by the space charge region (depletion width) at the heterojunction.