Publication date: 27th June 2014
Semiconductor nanocrystals are of great promise to develop highly efficient photovoltaic devices. This is caused in particular by the possibility of exciting two or more electrons over the band gap after absorption of only a single photon, via a process known as Carrier Multiplication (CM). One particularly important aspect of carrier multiplication is the rate of hot carrier cooling, which occurs in competition with CM. So far cooling rates have been investigated for charges with a relatively small excess energy. The cooling rates relevant to CM however are those at high energy above the band edge. In addition, spectroscopic studies into carrier cooling have not distinguished between electron and hole cooling. Here we present an ultrafast White-Light Transient Absorption study of charge signatures at high excess energy. We distinguish between electron and hole cooling via addition of electron acceptors. The determined cooling rates have direct relevance for the efficiency of CM and lead to more insight into the electronic structure of PbSe quantum dots.