# Recent Developments of Solar Cells from PbS Colloidal Quantum Dots
## 5 PbS Quantum Dot Heterojunction Solar Cells
In comparison with cells without PbS as a hole transporting material, the efficiency could nearly be doubled, and the solar cells showed much higher stability [89]. The interface between TiO\({}_{2}\) and PbS in planar heterojunction solar cells was the focus of a study by [PERSON] et al. [90]. To suppress surface defects in the electron transport layer between the active PbS layer and the FTO substrate through a TiO\({}_{2}\) layer, thermo-stable hydroxyl radicals were pre-implanted on the TiO\({}_{2}\) nanoparticle surface, in this way creating an electron transport layer, eliminating interband traps by passivating the intrinsic oxygen-deficient defects and matching electron affinity and work function. Thus, undesired interface recombination was suppressed, and power conversion efficiency was strongly increased [90]. [PERSON] et al. improved the interface between the active layer and the anode of a heterojunction quantum dot solar cell by introducing a graphene oxide layer between the PbS active layer and Au anode. By post-annealing, the graphene oxide layer was partly reduced to graphene, worked as a  
Figure 9: Production process of CdS nanowires on FTO (fluorine-doped tin oxide) glass, SILAR deposition of PbS nanoparticles and final solar cell assembly. From [87], reprinted with permission. Copyright © Elsevier 2019.  
hole-transport layer and significantly reduced the defects at the PbS/Au interface, finally resulting in an increase in efficiency of 12.9% [91]. [PERSON] et al. prepared bulk heterojunction organic solar cells based on the electron-donating organic semiconductor P3 HT-poly(3-hexylthiophene) and the electron accepting fullerene PCBM [6,6]-phenyl-C61-butyric acid methyl ester with various ratios of the inorganic semiconductor PbS. Here, the PbS nanoparticles decreased the series resistance, and thus improved the power conversion efficiency of these solar cells [92]. Since traditional PbS quantum dot solar cell structures require high-temperature fabrication of an inorganic buffer layer, an inverted structure was proposed by some groups [93; 94]. [PERSON] et al. built an effective p-n heterojunction in an inverted colloidal quantum dot solar cell by using NiO for the \(p\)-type layer and PbS colloidal quantum dots with iodide ligands for the \(n\)-type layer. Due to the large depletion region, a large photocurrent was achieved. At the same time, interface carrier recombination was prohibited by an additional slightly doped \(p\)-type quantum dot layer with EDT as ligands, which improved the device voltage, leading to strongly enhanced power conversion efficiency [95]. Spin-coated colloidal PbS quantum dot films were ligand-exchanged to methylammonium iodide (MAI) and used in depleted heterojunction solar cells with TiO\({}_{2}\) as a semiconductor, resulting in a more than doubled power conversion efficiency, as compared to common oleic-acid passivated PbS quantum dot films [96]. [PERSON] et al. used ligand exchange to 3-mercaptoethanolpropionic acid, thioglycolic acid and 4-mercaptoethanolzoic acid to examine the significant impact of ligand structures on the photovoltaic performance of quantum dot heterojunction solar cells [97]. [PERSON] et al. and [PERSON] et al. used different ligands and quantum dot dimensions to vary the quantum dot energy levels [98; 99]. To avoid undesired charge recombination between ZnO nanowires and PbS quantum dots in a depleted bulk heterojunction solar cell, [PERSON] et al. inserted an ultrathin Al\({}_{2}\)O\({}_{3}\) layer between these materials, which clearly increased open-circuit voltage, as well as the short-circuit current and thus the power conversion efficiency [100]. [PERSON] et al. reduced interfacial recombination between these materials by a nanocrystal buffer layer, mixing ZnO nanocrystals with PbS quantum dots (Figure 10), in this way improving the charge collection efficiency and thus the overall power conversion efficiency [101]. [PERSON] et al.