Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effectremain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon [1]. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO2 facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.
[1] J.B. Sambur, T. Novet, B. A. Parkinson "Multiple Exciton Collection in a Sensitized Photovoltaic System", Science, Vol. 330, pp. 63 - 66.
Tom Novet is form Voxtel, and is a visiting researcher in Ethan Minot's group.
http://www.voxtel-inc.com/