Hybrid solar cells composed of both organic and inorganic semiconductors have received intense research interests recently. They presumably combine the advantages of both classes of materials. Organic semiconductors are cost-effective since their preparation usually does not require energy-intensive processes such as at high temperature or in high vacuum. In addition, organic semiconductors are usually solution-processable, so they can be fabricated onto flexible substrates by simple techniques such as doctor-blading, spin-coating and inkjet-printing. While inorganic semiconductors are better charge conductors and have broader spectral coverage. Among inorganic semiconductors, colloidal nanocrystals(NCs) are good candidates to combine with organic semiconductor for hybrid cell, since they are solution-processable as well. Currently, the most frequently studied hybrid cell using colloidal NCs is made from composites of CdSe NCs and conductive polymers. Work in the area was pioneered by Greenham, Alivisatos and their coworkers. Significant progress in this direction has been made by improving composite morphology and electron transport via incorporating branched/elongated NCs.
Compared to CdSe NCs, we are more interested in lead-based NCs (PbSe, PbS) since they have absorption in the infrared which accounts for nearly 50% energy of the solar spectrum. Furthermore, it has been recently demonstrated by Klimov and his coworkers that these lead-based NCs are able to achieve multiexciton generation with extremely high efficiency. The idea to harvest the energy in the infrared and the ultraviolet more efficiently by NCs with multiexciton generation has been long proposed by Nozik, and the direct experiment observation by Klimov and his coworkers has open the path to develop solar cells based on multiexciton generation to break the Shockley-Queisser apparent thermodynamic limit.