  Quantum dot (QD) solids are being widely exploited as a solution-processable
technology to develop photovoltaic, light-emission, and photo-detection
devices. Charge transport in these materials is the result of a compromise
between confinement at the individual QD level and electronic coupling among
the different nanocrystals in the ensemble. While this is commonly achieved by
ligand engineering in colloidal-based systems, ligand-free QD assemblies have
recently emerged as an exciting alternative where nanostructures can be
directly grown into porous matrices with optical quality as well as control
over their connectivity and hence charge transport properties. In this context,
we present a complete photophysical study comprising fluence and
temperature-dependent time-resolved spectroscopy to study carrier dynamics in
ligand-free QD networks with gradually varying degrees of interconnectivity,
which we achieve by changing the average distance between the QDs. Analysis of
the photoluminescence and absorption properties of the QD assemblies, involving
both static and time-resolved measurements, allows us to identify the weight of
the different recombination mechanisms, both radiative and non-radiative, as a
function of QD connectivity. We propose a picture where carrier diffusion,
which is needed for any optoelectronic application and implies inter-particle
transport, gives rise to the exposure of carriers to a larger defect landscape
than in the case of isolated QDs. The use of a broad range of fluences permits
extracting valuable information for applications demanding either low or high
carrier injection levels and highlighting the relevance of a judicious design
to balance recombination and diffusion.
