# Recent Developments of Solar Cells from PbS Colloidal Quantum Dots
## 2 Solar Cells with PbS Quantum Dots--The Principle Function  
With regards to solar cells, the most commercially important technology is based on silicon. There are, however, other technologies, such as those based on perovskites [30], which typically use an active layer from a perovskite material, i.e., a material with the crystal structure ABX\({}_{3}\), with A denoting a monovalent cation, B a metal cation and X a halide anion [31]. This active layer is usually embedded between two electrodes, one of which has to be transparent to allow photons to reach the active layer, a hole transport layer on the one side and a hole-blocking layer on the opposite side, in this way defining the orientation of the current flow. Organic solar cells, also named polymer solar cells, also embed the active parts between two electrodes. Usually, the hole transport layer is coated on the anode and the electron transport layer on the cathode. The active layer between them contains electron donors and acceptors [32]. Finally, dye-sensitized solar cells (DSSCs) are of growing interest. While the efficiencies reachable by them are still low, especially when using non-toxic and low-cost materials [33; 34; 35], they can be expected to be highly useful in large-scale applications where overall efficiency is less important than the cost-benefit ratio [36]. As the aforementioned types of solar cells, DSSCs contain two conductive electrodes surrounding the other layers. Usually, a semiconductor like TiO\({}_{2}\) or ZnO is applied on the top electrode and dyed with a natural dye, or a more efficient--usually toxic--one. A catalyst is applied on the counter electrode, supporting electrons from an external circuit reaching the active layers of the DSSC. Between a dyed semiconductor and catalyst, an electrolyte fills the gap, usually based on iodine/triiodide or similar redox couples [37]. Figure 1: Pb-terminated truncated octahedral PbS quantum dot, depicting Pb cations, S anions and organic ligands. From [12], reprinted with permission. Copyright © Wiley 2016. Generally, quantum dot solar cells enable multi-exciton generation, making such solar cells in principle more efficient than common thin-film or crystal-based ones [38; 39; 40]. PbS and other quantum dots can be applied in diverse forms of solar cells, as depicted in Figure 2[41]. In Schottky junction solar cells, a depletion region is formed due to the band bending between a metal and a p-type semiconductor, the latter possibly being formed by colloidal semiconducting quantum dots. In this Schottky barrier, electron extraction from the device is favored, while holes are withdrawn [42]. Typically, such Schottky solar cells and other solar cell types harvesting light by PbS quantum dots absorb especially near-infrared wavelengths [43]. Larger voltages than with Schottky type solar cells can be reached with depleted heterojunction solar cells [41]. With PbS quantum dots, relatively large efficiencies can be reached [44] thanks to electrons flowing towards the TiO\({}_{2}\) layer, while the hole transfer in the opposite direction is prohibited, thus obtaining a high efficiency of carrier separation [45]. Heterojunctions between inorganic and organic parts can also be built with nanocrystals as sensitizers, in combination with a hole transporting polymer [46]. The high expectations due to theoretically high efficiencies could not always be fulfilled, possibly due to reduced contact between both materials [47; 48], inefficient charge separation [49] or charge injection at the often used PbS/TiO\({}_{2}\) interface [50]. As a special heterojunction cell, bulk heterojunction polymer solar cells typically combine electron-donating conjugated polymers with electron-accepting fullerenes. Here, PbS belongs to diverse possible semiconductors which can be added as quantum dots in the original polymer/fullerene composite for light harvesting [51], and sometimes also for electron transport instead of the fullerenes [52].