Hot Electron Transfer From Semiconductor Nanocrystals

In conventional semiconductor solar cells, absorption of photons with energies greater than the semiconductor band gap generate “hot” charge carriers that quickly “cool” before all of their energy can be captured – a process that limits device efficiency. Semiconductor nanocrystals (or quantum dots) have been touted as promising materials for photovoltaics because discretization of their electronic energy levels can slow down this cooling process, which might enable the extraction of photogenerated charge carriers before their excess energy is converted to heat.

In this talk, I will demonstrate sub-50 fs electron transfer from hot energy levels of PbSe nanocrystals to delocalized conduction band sates of TiO₂. In order to make these measurements, we developed the use of optical second harmonic generation for femtosecond time-resolved studies of interfacial charge separation. I will discuss the information we obtain from this technique as well as the effect of temperature, nanocrystal size, and surface chemistry. Additionally, I will show how ultrafast electron transfer excites coherent vibration of the first layer of TiO₂ surface atoms, whose collective atomic motions can be followed in real time.