Semiconductor nanocrystals have risen as interesting excitonic materials in photovoltaics and light-emitting diodes. Depending on the device architecture, the quantum dots may have to perform optically and electronically (e.g. absorption and charge transport in a solar cell, charge transport and emission in an LED), or only optically (e.g. lumophores for luminescent concentration). Because of their small size, nanocrystals have the potential to be virtually defect-free in their interior. The surface of the crystal however is a source of defects, which become more critical the smaller the nanocrystal. Indeed, much work focuses on passivating nanocrystal surfaces to get rid of such defects. While the effect of these defects is often observed in device performance, the direct characterization of these defects remains a challenge, which affects the ability to correlate systematically processing to structure and properties.
I will first show how an ultra-sensitive absorption spectroscopy (photothermal deflection spectroscopy-PDS) can be used on model materials to correlate surface stoichiometry and passivation to deep defects and Urbach tails. We will use a series of well-defined PbS nanoparticles to directly observe emergence of deep states due to degradation and the slope of the Urbach tail. IN order to do so, we are able to measure absorption over an unprecedented 5 orders of magnitude in these materials. We will also show that states not predicted by theory are observed in the smallest particles. We will also relate particle size to the slope of the Urbach tail.
In the second part of the talk I will show how combining PDS with emission spectroscopy allows to measure the quantum yield (QY) of very luminescent particles (CdS/CdSe core/shell quantum dots) used as lumophores in concentrators with unprecedented accuracy. In order to obtain the high concentration factors needed for the practical application of these devices, the lumophores’ QY must approach 100%. We are able to measure QYs higher than 99% with 0.5% or better accuracy. This accuracy is needed in designing materials for luminescent concentrators as any non-radiative loss decreases the concentration factor.