As
´there is plenty of room at the bottom´ prediction, exploiting the very tiny
building blocks of unique mechanical, optical and electrical properties for
various applications have been a global trend and an active research arena of
huge investment for more than a decade. [1] Nanotechnology is a trillion dollar
worth market being explored for applications in materials, pharmaceuticals,
electronics, chemical manufacturing, aerospace science and others. Of all other
hurdles, energy has come the number one global challenge of 14 TW a year as we
approached the peak oil (Hubbert´s peak) and the end of the oil age on top of
which sustainability and environmental issues urge alternative and renewable
power sources to take stage. [2]
Promising
enough, the clean solar- energy of the future, is showing enormous
breakthroughs a century after the demonstration of the first 1% solar cell. [3]
Depending on the materials and configurations, they can be generally classified
as inorganic, organic or hybrid under which a list of state-of-the art
technologies are systematically grouped as first, second, third and next
generation solar cells.
Efficiency,
cost and stability determine the niche (competitiveness) of the devices
converting light into electricity in the global market. Theoretical predictions
showed that semiconducting nanocrystals could be promising in terms of efficiency
that significantly exceeds the schockley-casler limit () and the ease of
solution processing. Consequently, quantum dots, nanorods, nanowires, tetrapods
and nanocrystals of different heterostructures has been incorporated in
Schottky, dye-sensitized and bulk heterojunction solar cell configurations
(Fig. 1). However, these types of solar cells are not yet efficient enough to
show up commercially, best efficiency reported being 6%, 5.4% and 3.2% for
architectures shown in the schemes below, respectively. [4] A major limitation
is the electron mobility impeded by organic passivating ligands on the nanocrystals,
which insulate the charge transport in the interpenetrating network. This
report covers the syntheses of high quality CdSe quantum dots and tetrapods and
their respective treatments to enhance electron mobility. Later is discussed
some aspect of the optical and electrical device characterization with respect
to J-V properties, IPCE, CE and TPV and TAS results.
II) Syntheses and Characterizations
Optimized routes for CdSe quantum
dot and tetrapod preparation were studied at different growth times and
temperatures. Optical, TEM and XRD characterizations of typical samples were
conducted. New synthesis methods for hollow nanoparticles and ODE-Se
mediated long tetrapods were tried on
the basis of collaboration.
CdSe quantum dots
The well-known rapid and hot injection synthesis was
followed in all cases with a modified recipe. Briefly, 1 mM of 99.99% CdO (128.4 mg) was mixed with
6 mM of oleic acid (2 mL) and 20 mL 1-octadecene in a four-necked flask. The
mixture was degased for few minutes and heated up with a reflux until the
solution turns clear (usually around 240 oC). The Se precursor was
prepared by weighing out 118 mg of Se powder in a glove bag and dissolving in 1
mL TOP (90%) under sonication for 5 minutes in a small vial. As soon as the
Cd-oleate turns clear, the temperature was adjusted for injection and the argón
flow and heat mantle set so as to control the growth temperature which in most
cases was few degrees lower than the injection temperature. In a typical synthesis where 0.34 mL of TOPSe
was injected at 250 oC, the particles were grown around 245 oC
for two minutes after which the heat mantle was quickly removed and argón flow
increased to cool the sulution fast. The cleaning was done with acetone three
times at 4000 rpm for 1hr, 30 and 10 minutes. The resulting precipitate was
dried and redispersed in chloroform for characterization.
Fig. 2
Synthesis set up
Quantum
dots of CdSe were synthesized at 190 oC and 250 oC
injection temperatures at different growth times of 20 sec, 1, 2 and 5 minutes.
Fig. 2 and 3 show the normalized absorbance and photoluminescence spectra of
the as-synthesized CdSe nanocrystals at respective injection temperatures. At
lower injection temperatures, the growth was relatively slower providing the
smallest quantum dots as predicted in Table 1 by the following empirical
relation.
where
D is the diameter of the nanocrystal and λ
is the first excitonic peak (nm). The quantum dots show a stock´s shift about 24 nm.
Typical
to these syntheses is the existence of numbers of other excitonic peaks which
are favorable and an indication of the kind of crystallinity exibited. For
example, the second excitonic peak for a zinc blend structure forms a shoulder
which is characteristic to quantum dots. This could be evidenced from the XRD
data shown in Fig. x later. These several band features which are found to be
growth time and temperature dependent.
The
TEM images taken with a high contrast electron microscope show a cluster of
smaller nanocrystals of 2-3 nm in size.
The
photoluminescence efficiency of the quantum dots were also measured by
calculating the quantum yield as in equation 2 below. The fluorescence quantum
yield (Φ) is the ratio of photons absorbed to photons emitted through
fluorescence. In other words the quantum yield gives the probability of the
excited state being deactivated by fluorescence rather than by another,
non-radiative mechanism such as internal conversion and vibrational relaxation.
This has been done following a guide to recording fluorescence quantum yields
using the comparative method of Grabolle et al., which involves the use of well
characterised standard samples with known ΦS values based on suitable
procedures and achievable uncertainities. Rhodamine 6G was used as standard in
these studies. The corresponding fluorescence quantum yield values are provided
in Table 1. Compared to literature values (0.3-0.85), fluorescence is the
determining deactivation mechanism in the nanocrystal.
Fig. 2 Optical characterization of CdSe
QDs synthesized at a) 190 oC/170
oC, b) 250 oC/230 oC and C) 300 oC/280 oC injection and
crystal growth temperatures
Table 1 Typical characteristics of CdSe coloidal
quantum dots in 2 minute growth time
Temperature (oC)
|
1st exciton peak (nm)
|
PL max peak (nm)
|
FWHM (nm)
|
Size (nm)
|
Φ (%)
|
190/170
|
534
|
2.8
|
|||
250/230
|
544
|
569
|
30.6
|
3.1
|
*37
|
300/280
|
558
|
585
|
38.21
|
3.4
|
The
nanocrystals show a broader PL peak at lower growth times indicating surface defect
patterns in the dynamic nucleation and growth reactions. At the shortest growth
time, blue nanocrystals of higher energy bandgap as much as xxeV were
synthesized at 190/170 oC.
Tetrapods
Similar
to the quantum dot synthesis, 1mM of 99.99% CdO (128.4 mg) was mixed with 6 mM
of oleic acid (2 mL) and 20 mL 1-octadecene in a four-necked flask. The mixture
was degased for few minutes and heated up with a reflux until the solution
turns clear (usually around 250 oC). The Se precursor was prepared by weighing
out 39 mg of Se powder in a glove bag and dissolving in 1.5 mL TOP (90%) under
sonication for 5 minutes in a small vial. The TOPSe was mixed with 36 mg of
CTAB in 2 mL of toluene. As soon as the Cd-oleate turns clear, the temperature
was adjusted for injection. In a typical
synthesis where 3.5 mL of TOPSe was injected at 190 oC, the particles were
grown at 170 oC for two minutes after which the heat mantle was quickly removed
and argón flow increased to cool the sulution fast. Short arm tetrapods were
synthesized using 103 mg of CdO and 32 mg of Se poder. The resulting solution
was divided into four 40mL vials in which around 2 mL of chloroform and
methanol mixture was added prior to centrifuging with approximately 20 mL of
acetone for 20 minutes. Additional cleaning was done only with acetone two
times at 4000rpm 10 minutes each. The resulting precipitate was dried and
redispersed in chloroform or hexane for characterization.
As
in the case of the CdSe quantum dots, a matrix of temperature and time
controlled syntheses has been carried out to find the best experimental
conditions that significantly affect the nanocrystal growth. Based on an
optimized recipe, reproducibly shape-controlled tetrapods were prepared.
The
absorption spectra of typical tetrapods is characterized by a sharp first
excitonic peak followed by a shoulder a valley at lower wave length. Peak
counts as much as eight were common in these syntheses while observing the
second excitonic peak coresponding to a wurtzite structure of the tetrapods. As
shown in Fig. 3, the peak broadens at higher growth temperature and time conditions
which could be an indication of Ostwald ripening- the evolution of an
inhomogenous structure over time.
The
flourescence of these semiconducting materials was uniquely sharper than its
quantum dot counter parts. A characteristic PL is sharp, with minimum
FWHM(about 20 nm), symetrical and with high emission. At lower growth times,
the surface trap states are predominant because of a dynamic nucleation and
growth reaction leaving incomplete covalent bonds. This can be seen from the
lagging tail on the PL peak at higher wave length.
However,
the choice of the experimental conditions depends on what arm length and
thickness of the tetrapods is needed. In particular, long arm tetrapods are
synthesized at 190 oC while the short arm tetrapods are prepared at 300 oC with
a modified recipe.
X-ray
powder diffraction is also crucial in studying the crystalinity of the
tetrapods. This has been reported by several authors. Fig. 5 shows XRD of the
as-synthesized quantum dots and tetrapods.
Fig. 4 Optical properties of tetrapods
sybthesized a) 190 oC/170 oC c b) TEM, 190 oC/170 oC/2 min simple c)250 oC/230
oC d) 300 oC/280 oC
Table 2 Optical characteristics of CdSe
TPs
Temperature (oC)
|
1st exciton peak (nm)
|
PL max peak (nm)
|
FWHM (nm)
|
Arm length (nm)
|
Φ (%)
|
190/170
|
533
|
549
|
19.9
|
||
250/230
|
566
|
582
|
20.4
|
*1.3
|
|
300/280
|
614
|
627
|
21.9
|
Fig. 5 XRD
of CdSe nanocrystals of QDs (green)
tetrapods (red) sythesized at 250 oC for 2
min
TOP-free tetrapods
Normally,
tetrapods are synthesized by a rapid hot injection of TOP-Se in the presence of
CTAB. Although TOP free strategies are available for colloidal quantum dots, it
was barely applied for elongated nanocrystals. The preparation of these
´super-long tetrapods´is as follows: 1 mmol of CdO is added to 2 mL oleic acid
, and heated with stirring under inert conditions until a clear cadmium oleate
complex is formed. The solution is cooled to room temperature but 1 mL of
toluene should be added when the temperature lowers below 100 oC. This
helps the complex remain in liquid state. Similarly, equivalent amount of Se
(to Cd), is added to 20 mL of ODE and heated to 280 oC (or another
temperature depending on which the arm length and thickness is controlled).
Adding 0.1 mmol of CTAB to the prepared Cd-oleate, and injecting into the
ODE-Se adduct the synthesis is monitored at different growth times. The color
development corresponding to the particle size is found gradual which benefits
easy control of arm length and thickness.
Hollow nanoparticles
Hollow
nanoparticles/nanocrystals are spherical nano-objects formed as a direct
consequence of a classic mechanism in metallergy- the Kirkendall effect. The
phenomenon, induced from the difference in diffusivities of atoms, is a generic
route to diverse range of hollow nanostructures reported previously; such as
ZnO, Co3S4, CoO, CoSe2, CoTe, Co2P
and Ni2P nanoparticles, for example. Hollow nanocrystals and the
method of making is recently patented by Alivisatos and co-workers. []The
underlying mechanism of hollowing has been explained by the generation of small
Kirkendall voids near interfaces as a result of different interdiffusion rates
and surface diffusion of core material along the pore surface or a direct
elapse of the material in the core.
Very
recently, group of Wong at Rice University succeeded in developing a Kirkendell
process to synthesize CdSe hollow nanoparticles of less than 20 nm in size. The HNPs thereof were ligand
exchanged and applied in hybrid solar cell devices to study the charge transfer
reactions.
In
a typical synthesis, 2.47 g of cadmium nitrate tetrahydrate (Cd(NO3)2•4H2O, 8
mmol, purity = 99%, Sigma-Aldrich), 0.32 g of selenium powder (Se, 4 mmol,
purity = 99.999%, Sigma-Aldrich), 20 mL of 1-octadecene ("ODE",
purity = 90%, Sigma-Aldrich) and 0.15 g of hexadecyltrimethylammonium bromide
("CTAB", 0.4 mmol, purity >99%, Sigma-Aldrich) were added to a
four-neck flask, which was then heated slowly to 190 °C at a rate of 11 °C/min.
The Cd:Se:CTAB molar ratio was 2:1:0.1. The reaction flask was kept at this
temperature for 5 min and cooled down to room temperature, resulting is a
reddish-brown cloudy liquid. The resultant solution was centrifuged to remove
the solids, resulting in a clear reddish-brown liquid. Ethanol was then added
to the reddish brown liquid to crash out the HNPs. The HNP powder was washed
several times with ethanol before being dispersed in chloroform. The synthesis
method was also performed with no CTAB, and twice and triple the original
amount of CTAB (i.e., 0 mmol, 0.8 mmol, and 1.2 mmol).
III) Ligand Exchange Strategies
Pyridine Exchange
Presynthesized
and chloroform dispersed QDs (10 mg/mL) were added to a screw top vial with a
septum cap. The content was dried with a flow of Ar until the solvent
is completely evaporated or QDs are left a bit wet in some cases. The QDs were
redispersed with 10 mL of pyridine with a glide syringe through the septum in
the fume hood (under similar stirring and inert conditions). The ligand
exchange for quantum dots is exceptionally slow and requires controlled heating
and cooling steps. The pyridine exchange was comparatively more successful when
heating the content to not more than 100 oC for 12 hours in an oil bath while
redispersing to the same concentration so that all ligands are not evaporated
through the vent needle. The same concentration of NCs was left for additional
12 hours at room temperature and with stirring. Thereafter, few drops of
check-up sample were taken to assure the exchange has taken place and added to
5 mL hexane. A cloudy suspension means most of the pyridine exchange was
performed in which case the parent solution was filled with at least 40 mL of
hexane (until it turns cloudy) and centrifuged. The tendency of QDs capped in
pyridine ligands is to aggregate. The hexane washed precipitate is later
redissolved in chloroform to the desired concentration. This procedure was
repeated whenever the surface passivation strategy was unsuccessful (QDs remain
dissolved in the washing solvent).
Pyridine
exchange for TPs and HNPs was relatively facile since room temperature stirring
under inert conditions of 12 hours was effective with a final hexane wash and
dispersion in pyridine:chloroform (1:9).
Butylamine Exchange
The
short amine ligand exchange of quantum dots was done following a prior pyridine
exchange; in which case 10 mL of butylamine is added to 100 mg of nanocrystals.
The content was left stirring under argon for 24 hrs at 70 oC. Nanocrystals
obtained were precipitated with acetone. The final volume redissolved in 1:9
butylamine: chloroform solution. Similarly, the TPs and HNPs were treated for
coating exchanges overnight.
a)
Oleic acid
b)
Pyridine
c)
Butylamine
IV)
Photovoltaic
performances
HNP
|
Jsc
|
Voc
|
FF
|
Eff
|
Rs(Ωcm2)
|
Rsh
(MΩcm2)
|
Pyridine
|
0.74
|
500
|
40
|
0.15
|
193
|
1.61
|
Butylamine
|
1.83
|
649
|
48
|
0.6
|
71
|
1.15
|
IV)
Reproducing the syntheses
Few
syntheses of QDs and TPs were conducted to see the reproducibility of the
routes. For the experiments carried out it turns out to be far away from the
expectations in terms of homogeneity. The full width at half máximum was almost
twice the expected values. This is bad since there will be energy transfer in
between polydispersed nanocrystals.
Experimental results
The
fact that these experiments are susceptible to any changes in the syntheses
conditions imposed a bottle neck in preparing nanocrystals of typical high
quality characteristics. In the course of identifying the problem, the optical
properties of the newly produced quantum dots and tetrapods were compared to
common characteristic features.
Observation:
Changing the volume of the flask, although a slight difference in injection
temperatures, blue-shifted the absorption peak somehow. However, the synthesis
was expected to produce QDs (abs. 540 nm) and TPs (abs. 570 nm) with FWHM
values of less than 30 and 23 nm, respectively. It should be noted though a
proper heating mantle was not used which resulted in a huge drop in temperature
(more than 100 oC) upon room temperature injection of the TOP-Se. Unlike, previous
syntheses, the Cd-oleate was a yellowish solution at injection temperature
which showed some sort of impurities in the constituting reactants. As a
result, all new reagents were purchased from Sigma Aldrich to the same impurity
levels as in previous syntheses at Rice.
Experimental results
In
this experiment, all new reagents were utilized. Temperature control was done
in-situ using adjustable thermometers. The synthesis was better in that the
Cd-oleate turned almost clear after degasing the ODE for 30 min, adding OA and
degasing for 10 min and finally heating the reaction volume to 250 oC.
The experimental conditions were well controlled and the right absorption bands
were observed while homogeneity of the sample remains unresolved. This was evident
from the fewer number of band features and the broadness of the FWHM. However,
the PL peaks were so symmetric and no evidence of surface traps at higher
growth times.
VI) Conclusion
So
far, different shapes of particles were reported in literature, including,
dots, tetrapods, tripods, tear drops and even hollow nanoparticles. Further
optimization in getting highly crystalline nanocrystals is underway. Some of
the important nanostructures were such as dots and tetrapods are being
optimized to obtain the best results. However, in comparison to the the other
shapes, the tetrapods outperformed in solar cell applications owing to their
physical orientation that increases the electron mobility in the devices. No
solar cells of HNPs in DSCs are reported in literature so far. We have achieved
butylamine exchanged HNPs of higher efficiency compared to pyridine exchanged
counterparts.
Acknowledgement
Institute of Chemical Research of Catalonia (ICIQ)
Universitat Rovira i Virgili
Rice University
Prof. Emilio Palomares
Prof. Michael S Wong
Suravani Gullipalli
