Download Nanowire dye-sensitized solar cells

Nanowire dyesensitized solar cells
J. R. Edwards
Pierre Emelie
Mike Logue
Zhuang Wu
Nanowire DSCs
Intro and Background to solarcells,
particularly DSCs
 Why nanowires in DSC
 Fabrication and Characterization
 Mid-infrared and Summary

Solar Cells Introduction
Convert solar energy to electrical energy
 Excitonic photocells: Dye-sensitized cells
(DSCs)
 DSC are different than traditional
photovoltaic cells because of the electron
transport mechanism

Conventional Construction

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Two electrodes: one
transparent
Nanoparticle film (with
absorbing dye in 400-800nm
range)
Electrolyte
Incident photons create
excitons
Rapidly split with electron
transport by trap limited
diffusion.
Transport is slow but still
favorable over recombination
rates
Efficiencies

Power conversion
efficiency (η)
η
= (FF x |Jsc| x Voc)/Pin

Where

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

FF: fill factor
Jsc: short circuit
current
Voc: open circuit
voltage
Pin : incident light
power
Improving Jsc

Determined from how
well the absorption
spectrum of the dye
overlaps with the
solar spectrum.
Shortcoming of Conventional DSCs

Poor absorption of low-energy photons
 Tuning
absorption through dye mixtures have
be relatively unsuccessful
 Increase optical absorption through increasing
the nanoparticle film thickness but limited by
electron diffusion length
Problems with nanoparticle DSC’s
Poor absorption of red and infrared light
 Possible approach to overcome this is to
increase nanoparticle film thickness for
higher optical density
 Approach doesn’t work because film
thickness needed exceeds electron
diffusion length through the nanoparticle
film

Why use nanowires

Want to increase electron diffusion length
in anode
 By
replacing polycrystalline nanoparticle film
with an array of single crystalline nanowires

Electron transport in the wires is expected
to be several orders of magnitude faster
than percolation through a random
polycrystalline network
Why use nanowires
Using of sufficiently dense array of long, thin
nanowires as a dye scaffold, it should be
possible to increase dye loading while
maintaining efficient carrier collection
 Rapid transport provided by nanowire anode
would be favorable for designs using
nonstandard electrolytes

 Some
examples are polymer gels or solid
inorganic phases, in which the recombination
rates are high compared to liquid electrolyte cells
How was the cell made

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ZnO arrays were made in an
aqueous solution using a
seeded growth process,
modified to yield long wires.
A 10-15nm film of ZnO quantum
dots was deposited onto
F:SnO2 glass (FTO) substrates
by dip coating.
Wires were grown from the
nuclei through the thermal
decomposition of a zinc
complex.
Results from nanowire process



The two step process proved to
be a simple, low-temp. route to
making dense (35*109/cm2), on
arbitrary substrates of any size
The aspect ratio was boosted to
125 using polyethylenimine
(PEI), to hinder only lateral
growth of the nanowires in
solution.
The longest arrays (20-25μm)
have one-fifth the active surface
area of a nanoparticle anode
Electrical Characteristics of
Nanowires


The wire films are good electrical
conductors along the direction of
the wire axes. Two-point
electrical measurements of dry
arrays on FTO substrates gave
linear I-V traces.
This indicates barrier-free
contacts between the nanowire
and the substrate
Electrical Characteristics of
Nanowires
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Individual nanowires were
extracted from the arrays,
fashioned into FET’s and
analyzed to determine their
resistivity, carrier concentration,
and mobility.
Resistivity values ranged from .32.0 Ω-cm, with an electron
concentration of 1-5*1018 cm-3,
and a mobility of 1-5 cm2 V-1 s-1
From Einstein’s relation
D=kBTμ/e, the Dn was estimated
to be .05-.5 cm2 s-1 for single dry
nanowires
Electrical Characteristics of
Nanowires  The value of D = .05-.5 cm
s-1 is
several hundred times larger than the
highest reported values for TiO2 or ZnO
nanoparticle films in operating cells.
The conductivity of the arrays also
increased by 5-20% when the were
bathed in the standard DSC electrolyte
These tests show that facile transport
through the array is retained in devicelike environments, and should result in
faster carrier extraction in the nanowire
cell
n


2
I-V characteristics of the
Nanowires
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The wire films are good
electrical conductors along
the direction of the wire
axes.
barrier-free contacts
between nanowire and
substrate
Analyze a single Nanowire

Resistivity values ranged from
0.3 to 2 Ω cm

Electron concentration
1-5X1018cm-3

Mobility: 1-5cm2V-1s-1

Diffusion constant: 0.050.5cm2s-1 which is much
higher than the nanoparticle
case
Fill factors

FF=(VmpJmp) / (VocJsc)

FF reflects the power lost
inside the solar cell
I-V characteristics of the device

Smaller device shows a
higher Jsc and Voc

The fill factor and
efficiency for the smaller
one are 0.37, 1.51% and
0.38, 1.36% for the larger
one

The inset shows the
external quantum
efficiency against
wavelength of the larger
one
Voc &FF against light intensity
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The open-circuit voltage
depends logarithmically on light
flux.
The FFs are lower than the
nanoparticle devices, and fall
off with increasing light
intensity.
FFs don’t change with cell size
Short cut current & efficiency
against light intensity

The short cut current depends
linearly on light flux

Low FF results in the low
efficiency

The efficiency curve is pretty
flat about 5mW cm-2
The effect of annealing treatments

350°C for 30 min in H2/Ar increases the emission at 400nm

No treatment can increase the FF significantly.
Surface roughness factor

Surface roughness factor
describes how rough a
surface is.

A roughness factor
shows the ration
between the real
electrode surface
area and the
geometrical area
Here, roughness
factor is defined as
the total film area per
unit substrate area.

Jsc vs. roughness factors


the rapid saturation and
subsequent decline of the
current from cells built with
12-nm TiO2 articles, 30-nm
ZnO particles or 200-nm ZnO
particles confirms that the
transport efficiency of particle
films falls off above a certain
film thickness
the nanowire films show a
nearly linear increase in Jsc
that maps almost directly
onto the TiO2 data. Because
transport in the thin TiO2
particle films is very efficient,
this is strong evidence of an
equally high collection
efficiency for nanowire films
as thick as ~25 μm

Better electron transport within the nanowire photoanode is a
product of both its higher crystallinity and an internal electric field
that can assist carrier collection by separating injected electrons
from the surrounding electrolyte and sweeping them towards the
collecting electrode.

The Debye–Hückel screening length of ZnO is about 4 nm for a
carrier concentration of 10X18 cm-3, which is much smaller than the
thickness of the nanowire film, so that the nanowires can support the
sort of radial electric field.

The existence of a an axial field along each nanowire encourages
.
carrier motion towards the external circuit
Debye–Hückel screening length

The ions in an electrolyte have a screening effect on the electric field
from individual ions. The screening length is called the Debye length
and varies as the inverse square root of the ionic strength.
Mid-Infrared Transient Absorption Measurements
Basic Principles
Experimental setup for transient absorption measurements
• Excitation pulses: 400 nm, 510 nm, 570 nm
• Probe pulse: Mid-infrared
Mid-Infrared Transient Absorption Measurements
Applications
• Electron injection dynamics from the dye molecules into the semiconductor surface
h
dye  dye*
ZnO
dye  dye   e 
*
• These excess electrons have a broad and featureless absorption spectrum in the range
400-800 nm
 Study of the electron injection rate
• Comparison between the nanoparticles and the nanowires
• Particle and wire films have dissimilar surfaces onto which the sensitizing dyes adsorbs
- ZnO particles present an ensemble of surfaces having various bonding
interactions with the dye
- ZnO wire arrays are dominated by a single crystal plane (100) that accounts
for over 95% of their total area
Mid-Infrared Transient Absorption Measurements
Results
Bi-exponential kinetics:
a exp(  1 x)  b exp(  2 x)
 1 : Ultrafast time constant
 2 : Long time constant
2.2 ± 1.1 ps
4.4 ± 1.4 ps
5.3 ± 1.3 ps
N719 dye
ZnO nanowire films
Ultrafast step at <250 fs appears
to be independent of pump energy
 Long time constant varies
with pump wavelength
Mid-Infrared Transient Absorption Measurements
Results
Dye N719
Films pumped at 400 nm
a exp(  1 x)  b exp(  2 x)  c exp(  3 x)
Tri-exponential kinetics
<250 fs, 20 ps, 200 ps
a exp( 1 x)  b exp(  2 x)
Bi-exponential kinetics
<250 fs, 3 ps
 Faster electron injection in nanowires
The difference in the injection amplitudes is due to the
larger surface area of the nanoparticle film
Mid-Infrared Transient Absorption Measurements
Conclusion
• Electron injection dynamics from the dye molecules into the
semiconductor surface have been monitored by femtosecond transient
absorption spectroscopy
• It has been observed that the transient responses for wires and particles
are considerably different
• The electron injection in nanowires is faster than in nanoparticles which is
in agreement with previous results
• The ultrafast step for nanowires show a weak dependence on pump
wavelength
• The long time constant for nanowires depends on the pump wavelength
Summary
Advantages
• The nanowire dye-sensitized solar cell shows promising results comparing
with the nanoparticle version which is the most successful excitonic solar
cell
• Using ZnO wire array, the ordered topology improves the electron
transport to the electrode
• It may improve the quantum efficiency of DSCs in the red region
• More comparative studies of wire and particle devices are needed
Limitations
• Available area for dye adsorption limits the efficiency of the nanowire cell