Energy relaxation in optically excited Si and Ge nanocrystals

The scientific objective of the research presented in this thesis is to explore energy relaxation processes of optically excited Si and Ge nanocrystals. The identification and deeper understanding of unique energy relaxation paths in these materials will open a new window of opportunity for these materials. In particular, in this work a different way of harvesting energy, which is typically lost to heat in conventional photovoltaic devices, is proposed using silicon nanocrystals together with erbium ions. Our findings form the physical basis for a novel hot-carrier photovoltaic architecture which would then comprise (i) a standard Si solar cell with (ii) a non-contacted spectrum shaper in front, converting high-energy photons into IR, and (iii) a low bandgap cell on the back side for efficient harvesting of the IR photons, and would be capable of exceeding the Shockley-Queisser limit. In our investigations, we found that the external quantum yield of the IR photon generation by Er³⁺ ions shows ~ 15-fold enhancement in the high excitation energy range. In this work an important result on the observation of carrier multiplication in Ge NCs is also reported. While carrier multiplication has been reported for nanocrystals of many semiconductors, Ge was missing from that list. Ge NCs also are very interesting for detectors and photovoltaics. Our study shows that carrier multiplication: (i) does take place in Ge NCs (ii) is considerably more efficient than impact excitation in bulk Ge, and (iii) occurs with only a minimal energy loss.