# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 3 Quantum Dot Solar Cells  
A better understanding of the physics of QDs and the development of efficient methods of QD synthesis paved the way for their implementation in light harvesting devices (photodiodes and solar cells) and light emitting devices (Lasers and LEDs). For light-harvesting devices, PbS has emerged as the most promising candidate among the different QD materials due to its large light absorption coefficient, large Bohr radius, and facile band gap tunability [148]. The design or structure of a QDSC dictates many of its performance parameters [149; 150; 151; 39; 40; 152; 39]. Three structures are shown in Figure 4. The rightmost structure of Figure 4 shows QD sensitized solar cells (based on dye-sensitized solar cells [152]) that were first fabricated using an n-type mesoporous TiO\({}_{2}\) (m-TiO\({}_{2}\)) sensitized with PbS QDs. The first report from [PERSON] et al. [153] described photoconduction in a device with a bandgap of \(\approx\)1.85 eV where the QD energy bands aligned favorably with that of the cathode enabling carrier extraction from the QDs. Adding to this work, [PERSON] et al. [154] developed PV cells showing a power conversion efficiency (PCE) of 3% by sandwiching the above setup between tin-doped indium oxide (ITO) and gold. However, the disadvantages of the mesoporous structure for sensitized cells with liquid electrolytes pushed QDSCs towards planar solid-state configurations. Nonetheless, research on sensitized solar cells continued with the development of a technique known as successive ion layer adsorption and reaction (SILAR) to synthesize PbS QDs in situ on m-TiO\({}_{2}\). This technique involves dipping a photoelectrode (TiO\({}_{2}\)) coated substrate successively in precursor solutions containing the Pb\({}^{2+}\) and S\({}^{2-}\) ions to get PbS QDs. This deposition method, known to pack a higher density of PbS QDs in the pores of m-TiO\({}_{2}\), promotes higher light absorption and charge extraction at the interface between TiO\({}_{2}\) and PbS QDs and hence has improved device performance. All solid-state sensitized cells, without any polysulphide electrolyte, have also been fabricated using TiO\({}_{2}\) NRs similar to a bulk heterojunction used by organic photovoltaics [155]. This has opened interesting avenues of research to achieve higher performance QD-sensitized cells [156; 157; 158; 159]. Other notable achievements include devices with Hg\({}^{2+}\) doped PbS QDs made through the SILAR method with exceptionally high photocurrents of 30 mA/cm\({}^{2}\)[160] and devices using PbS/CdS core/shell QDs that exhibit PCEs greater than 7% [161]. We limit our discussion to Pb-based QDs; however, there is a burgeoning area of research using other QD materials as the sensitizers with over 12% PCE using Zn-Cu-In-Se (ZCISe) QDs [162; 163; 164; 165]. To overcome the shortcomings of a QD-sensitized configuration, planar junction devices were developed, starting with a Schottky junction. A Schottky junction QDSC uses the work function difference between the QDs and a metal contact to generate a photovoltage that extracts charges as shown in the leftmost structure of Figure 4. Prior work [166; 167; 168; 169] has shown the presence of a Schottky junction at the QD/metal contact interface leading to charge separation at the rear boundary which introduces a limitation to this configuration if the diffusion length is not long enough for charges to reach the junction. The increased probability of recombination of charge carriers excited farther away from the junction by shorter wavelength photons reduces the produced photocurrent and limits the thickness of the device to below the point where not all the photons greater than the bandgap are absorbed. Additionally, Schottky junctions have a practical limit on the maximum open circuit voltage (VOC). The maximum VOC is approximately 1/2 the bandgap; limited mainly because of the high reverse saturation current and photocurrent limited by the standard AM1.5 irradiance (higher VOC is possible under concentrated light) [170; 171].