Methods
Flame spray pyrolysis (FSP) was employed to synthesize pure ZnO and niobiumdoped ZnO nanoparticles containing 0.5-3 mol% Nb. Zinc naphthenate (Aldrich, 8
wt%Zn) and niobium (V) ethoxide (Aldrich, 99.999%) were used as precursors. The
precursors were dissolved in toluene/methanol (70/30 vol%) to obtain a 0.5 mol/l
precursor solution. In a typical run, the precursor was fed into a FSP reactor by a syringe
pump with a rate of 5 ml/min while 5 l/min O2 is being dispersed (5/5 flame). The gas
flow rates of methane and O2 supporting flamelets were 1.19, and 2.46 l/min,
respectively. The pressure drop at the capillary tip was kept constant at 1.5 bars by
adjusting the orifice gap area at the nozzle. Figure 1a shows the experimental setup for
the flame-made pure ZnO and Nb doped ZnO nanoparticles. The flame height was about
10-11 cm and showed an orange-yellowish flame appearance. After evaporation and
combustion of precursor droplets, particles are formed by nucleation, condensation,
coagulation, coalescence and Nb deposited on ZnO support1. Finally, the nanopowders
were collected on glass microfiber filters (Whatmann GF/D, 25.7 cm in diameter) with
the aid of a vacuum pump. X-ray diffraction (XRD) was employed to confirm the phase
and crystallinity of the nanopowders (Figure 1b). EDACs was used to confirm Nb
content in the resultant powders (Figure 2).
The nanopowders were highly crystalline, and the peak can be confirmed to be the
hexagonal structure of ZnO (JCPDS No.89-0510)2. Amorphous phase and Nb peak were
not found in these patterns due to low concentration of Nb. The BET-particles diameters
can be calculated from dBET = 6/SSABET x ρsample, where ρsample are calculated from the
density of ZnO (ρZnO = 5.61 x 103 kg/m3)3 and the density of NbO2(ρNbO2 = 5.9 x 103
kg/m3)4 taken into the account with the appropriate amount of both components. (Fig.
1c).
As the Nb concentrations increased, the specific surface areas (SSABET) increased
(from 80.36 to 136.2 m2/g) with decreasing dBET (from 13.31 to 7.85 nm). Figure 1d
shows the absorption spectra of pure ZnO and Nb/ZnO nanoparticles in 1-butanol. It was
found that the absorption spectra of ZnO doped with Nb at different ratio were quite
similar to that of pure ZnO. The absorption edges of all samples were about 375 nm
which were ascribed to the fundamental of pure ZnO, corresponding to the band-gap of
about 3.2 eV. It can be concluded that Nb did not affect the ZnO structure. The
absorption intensity of ZnO slightly increased with increasing Nb content.
The TEM image shows particles having clear spheroidal, hexagonal and rod-like
morphologies. The crystallite sizes of ZnO spheroidal and hexagonal particles were in the
range of 5-20 nm. ZnO nanorods were found to be ranging from 5-20 nm in width and
20-40 nm in length (Fig. 1e). Figure 1f shows the sizes of Nb nanoparticles are very tiny,
when compare with the size of ZnO nanoparticles.
Figure 1│ FSP experimental and the effect of Nb loading on ZnO nanoparticles. a, The experimental
setup for the synthesis Nb/ZnO nanoparticles. b-d, Powder XRD patterns (b), SSABET and dBET (c) and UVVis absorption spectra (d) of particles with Nb loading. e,f, TEM bright-field image of pure ZnO (e) and 3
mol% Nb/ZnO (f) nanoparticles.
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(a
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(c
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(b)
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Figure 2: EDACS results for ZnO powders with varying levels of niobium doping. 2a is no niobium
doping, 2b is .5% niobium doping, 2c is 1% niobium doping and 2d is 3% niobium doping.
Device fabrication.
Devices were prepared incorporating regioregular P3HT (Rieke Metals; Mw
48,000 g mol-1) and PCBM (99.5% purity) blends (both with and without added Nb/ZnO
solution) as a donor/acceptor bulk heterojunction active layer. The polymer solar cells
had the device configuration (Fig.1) using indium tin oxide (ITO) patterned glass
substrates (Delta Technologies Rs =10 Ω sq-1).
The substrates were cleaned in an ultrasonic bath with, deionized (DI) water,
acetone and isopropyl alcohol, and then dried under vacuum in an oven. The substrate
was treated with oxygen plasma for 90 min. Poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) (Baytron P) was deposited onto the ITO glass
substrate by spin coating at 4000 rpm and dried at 100 °C for 25 min. The ZnO
nanoparticles (pure ZnO, 0.5, 1 and 3 mol% Nb/ZnO) were prepared in 1-butanol to give
the concentration of 15 mg mL-1.
Four sets of P3HT:PCBM:Nb/ZnO blend solutions were prepared by adding
Nb/ZnO solution (31 vol%) to stirred solutions of P3HT:PCBM (1:0.7) in chlorobenzene
(1 ml). The P3HT:PCBM:Nb/ZnO solution blend was coated on top of the PEDOT:PSS
film. Finally, the substrates were transferred to a vacuum chamber to evaporate 1.5 nm of
lithium fluoride (LiF) (Aldrich) and 100 nm of aluminum (Al) (Sigma-Aldrich) as top
electrode. The active area of these devices is 0.38 +/- .04 cm2. After completion, the
devices were annealed for 3 min at 150 °C in the glove box. Devices with just 1-butanol
in the appropriate concentrations were also made in order to determine the effect of the
solvent on device performance (Figure 3). No substantial effect was observed.
Figure 3 – JV curves of devices containing varying amounts of 1-butanol and no ZnO nanoparticles.
The J-V characteristics of the solar cells were test in air at room temperature
(298 K) using a Keithley 2400 source measurement unit, and an Oriel filter was Xenon
lamp (150W) coupled with an AM1.5 filter was used as the light source. The light
intensity was 120 mW cm-2 on the sample surfaces measured by a photodetector. UV-vis
spectra of the spin-coated films were recorded on a Perking-Elmer Lambda Bio-40
spectrophotometer. External Quantum Efficiency (EQE) measurements were performed
using a Hitachi F-4500 fluorescence spectrophotometer.
References
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