# Recent Developments of Solar Cells from PbS Colloidal Quantum Dots
## 5 PbS Quantum Dot Heterojunction Solar Cells  
Plasmonic metal nanoparticles can be used to concentrate incident sunlight by plasmon resonance, in this way increasing the overall absorption of the incident light in a solar cell without increasing the quantum dot film thickness over the carrier diffusion length. [PERSON] et al. used Au and Ag nanoparticles to introduce localized surface plasmon resonance, as well as the scattering of the incident light. By placing the small Au nanoparticles (diameter \(\sim\) 10 nm) at the ZnO coated front layer, followed by the PbS active layer and the larger Ag nanoparticles (diameter \(\sim\) 50 nm), and then the gold contact back electrode, the latter serves mostly as a far-field scatterer, while the Au nanoparticles enable near-field coupling, in this way increasing the efficiency by approx. 25% [82]. [PERSON] et al. concentrates on the problem of UV stability of the usual short organic ligands of PbS quantum dots used for solar cells. They used a CdSe/ZnS quantum dot layer to down-shift luminescence on the back of the bulk heterojunction with ZnO nanowires, thus not only increasing the time-stability of the cells, but also their power conversion efficiency, since the UV photons--which were usually thermalized or absorbed in other layers--were now downshifted to visible light, and thus also contributed to power conversion [83]. [PERSON] et al. optimized planar hybrid heterojunction PbS-polymer solar cells by tuning the band gap of the PbS quantum dots between 0.86 eV and 2.1 eV by varying the synthetization temperature (Figure 8), ligand exchange and an optimized production process with a thinner deposited PbS layer. In this way, the performance could be doubled as compared to similar hybrid heterojunction PbS/organic solar cells [84]. Using a variable band-gap structure in quantum dot heterojunction solar cells was also suggested by [PERSON] et al., who used different quantum dot sizes to align the energy levels of the active layer, resulting in a power conversion efficiency increase of more than 20% [85]. Figure 8: Absorption spectra of PbS quantum dots synthesized at temperatures between 45 \({}^{\circ}\)C and 185 \({}^{\circ}\)C. From [84], reprinted with permission. Copyright © Elsevier 2019. The problem of interfacial deficiency between semiconductor and colloidal PbS quantum dots in ordered bulk heterojunction solar cells was investigated by [PERSON] et al. They produced exactly interpenetrating structures of a ZnO nanowire array and PbS colloidal quantum dots by optimizing nanowire growth orientation and using convective assembly to deposit the quantum dots. The resulting dense packing led to a high power conversion efficiency of nearly 10% [86]. Nano-heterojunctions between CdS nanowires surrounded by PbS nanoparticles were developed by [PERSON] et al. When combining the chemical bath deposition of CdS nanowires with the aforementioned SILAR process for PbS nanoparticle deposition, an increase of the power conversion efficiency by more than one order of magnitude could be reached in corresponding solar cells (Figure 9), which was attributed to the flat band potential, optimized electron lifetime and recombination rate [87]. [PERSON] et al. optimized the ZnO/PbS heterojunction for a colloidal quantum dot solar cell by passivating defects in the sol-gel ZnO at low temperatures by adding polyethylenimine (PEI) to the precursor solution. In this way, a higher crystallization rate was achieved, the ZnO work function was reduced and the heterojunction interface voltage was increased, which was attributed to improved charge carrier separation in the depletion region. In addition, higher electron mobility was found, while the undesired carrier recombination was suppressed. In this way, the power conversion efficiency could be increased by 25%, as compared to pure ZnO/PbS heterojunction solar cells [88]. [PERSON] et al. used perovskite/PbS quantum dot heterojunction films to promote hole extraction and decrease carrier recombination in a perovskite solar cell.