Prospects of Colloidal Nanocrystals for Electronic - Computer Science

416 Chemical Reviews, 2010, Vol. 110, No. 1 Talapin et al.
Figure 32. Current transient signals used to extract (a) the hole
mobility (CELIV, 80 000 V s -1 ramp rate) and (b) the electron
mobility (time-of-flight, under 40 V bias). The inset shows the bias
dependence of the transit time (ttr). The slope of the data above 15
V was used to determine the mobility. The dashed red lines
represent in (a) the different current density levels and in (b) the
least-squares fits to the data. Reprinted with permission from ref
272. Copyright 2008 American Institute of Physics.
much greater than the absorption depth at the laser wavelength.
A typical time-of-flight transient is shown in Figure
32b. The intersection time of the transient asymptotes was
used to determine the transit time.The bias dependence of
the transit time is shown in the inset of Figure 32b. In the
fully depleted regime, the electron mobility (µe) was found
to be (2 ( 1) × 10 -4 cm 2 V -1 s -1 . 272
Table 2 summarizes the electron and hole mobilities
reported for different semiconductor NC films. One can see
a large spread in the reported data. It could be explained by
lack of commonly accepted measurement protocolts and
sample preparation techniques. So far, we can only conclude
that higher doping levels and shorter surface ligands result
in larger carrier mobility. Lead chalcogenide NCs show
higher carrier mobilities than do II-VI NCs. On the other
hand, understanding the effects of particle size and shape
on the mobility of charge carriers will require additional
experimental studies.
The recombination-limited lifetime of carriers (τrec) in the
NC solids was measured using the decay of the open-circuit
voltage (Voc) following illumination turn off, so-called
transient open circuit voltage decay (OCVD). For these
measurements, ITO/NCs/metal devices were illuminated with
a pulse modulated 975 nm diode laser. Immediately after
laser turn off, the initial slope of the decay curve (dVoc/dt)
is related to τrec. 272,381 Measured lifetimes for PbS and PbSe
NC solids showed that the lifetime decreased steadily from
1 ms at low light intensities to ∼10 µs at high light
intensities. 272,367
Carriers in the depletion region are separated via the action
of the built-in field resultant from the metal-semiconductor,
or Schottky, junction (Figure 30b). Their drift length (Ldrift)
is given by
L drift ) µτ rec φ built-in
W depl
where µ is the carrier mobility. For PbSe NCs cross-linked
with 1,4-benzenedithiol, Ldrift was estimated as 8.5 µm for
electrons and 14.5 µm for holes. 367
In the quasi-neutral region of NC solid, charge transport
occurs mainly through diffusion, and the carrier diffusion
length (Ldiff) may be obtained from
L diff ) � µτ rec k B T
e
The calculated minority carrier (electrons) diffusion length
for 1,4-benzenedithiol linked PbSe NCs was about 220 nm, 367
which allows a substantial fraction of the minority carriers
to diffuse out of the neutral region in NC-based solar cells
and photoderectors.
6.4. Sintering Metal and Semiconductor
Nanoparticles into Continuous Films
(23)
(24)
Nanoparticles can be deposited onto a substrate by spincoating
or inkjet printing a colloidal solution and sintered
into a continuous conductive film during subsequent annealing
step. Coalescence of individual nanoparticles yields
polycrystalline solids with grains considerably larger than
the initial nanoparticles. 388 Sintering individual NCs is
facilitated by significant reduction of the melting temperature
with decreasing particle size. 389,390 Size-dependent meltingpoint
depression is a general effect observed across a broad
class of materials, including technologically important metals
and semiconductors (Figure 33). The reduction of the melting
point can be described by the effect of surface tension that
plays an increasingly important role when particle size
decreases below 10 nm. 390 Changes in the melting temperature
(Tm) of the particle of size Rsol as compared to that of
the bulk (Tb) are given by: 389,390
T m - T b )
2Tm LfusionFsolRsol[ γsol - γliq( F 2
/3
sol
Fliq) where Lfusion is the molar latent heat of fusion, and γ and F
are the surface tension and density of solid (sol) and liquid
(liq) phases, respectively. The size-dependent suppression
]
(25)