Different nanocrystal systems for carrier multiplication

Abstract

Carrier multiplication is an important process which can enhance the efficiency of photovoltaic and electronic devices. To investigate this process, diverse experimental setups can be used; the thesis considers some particular artifacts which negatively impact the accuracy of the measurements, and then proposes solutions to eliminate them. In silicon nanocrystals, the carrier multiplication efficiency can be determined from the excitation energy dependence of the ensemble photoluminescence quantum yield; that is found to be strongly influenced by the fabrication parameters of the samples. In the low energy excitation region, the initial growth of photoluminescence quantum yield is shown to originate from parasitic absorption. In the high energy excitation regime, the competition between space-separated-quantum cutting and carrier trapping at defect levels leads to diversity of photoluminescence quantum yield dependences. As phosphor and boron are co-doped into Si nanocrystals, the optical properties of these nanocrystals are changed due to the existence of impurity levels inside the band gap. In these nanocrystals the mechanism of carrier multiplication is modeled by a combination of impact ionization of impurity levels and as well as band states. In the last part of the thesis production of intrinsic and N-doped carbon nanodots by rapid pyrolysis of organic precursors is presented and its advantages are discussed