  We simulated electron transport across a binary nanocrystal solid (BNS) of
PbSe NCs with diameters of 6.5nm and 5.1nm. We used our Hierarchical
Nanoparticle Transport Simulator HINTS to model the transport in these BNSs.
The mobility exhibits a minimum at a Large-NC-fraction f_LNC=0.25. The mobility
minimum is deep at T=80K and partially smoothed at T=300K. We explain this
minimum as follows. As the LNC fraction f_LNC starts growing from zero, the few
LNCs act as traps for the electrons traversing the BNS because their relevant
energy level is lower. Therefore, increasing the f_LNC concentration of these
traps decreases the mobility. As increasing f_LNC reaches the percolation
threshold f_LNC=f_p, the LNCs form sample-spanning networks that enable
electrons to traverse the entire BNS via these percolating LNC networks.
Transport through the growing percolating LNC networks drives the rapid growth
of the mobility as f_LNC grows past f_p. Therefore, the electron mobility
exhibits a pronounced minimum as a function of f_LNC, centered at f_LNC=f_p.
The position of the mobility minimum shifts to larger LNC fractions as the
electron density increases. We have studied the trends of this mobility minimum
with temperature, electron density, charging energy, ligand length, and
disorder. We account for the trends by a "renormalized trap model", in which
capturing an electron renormalizes a deep LNC trap into a shallow trap or a
kinetic obstacle, depending on the charging energy. We verified this physical
picture by constructing and analyzing heat maps of the mobile electrons in the
BNS.
