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Nanowire dye-sensitized solar cells

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Nanowire dye-sensitized solar cells J. R. Edwards Pierre Emelie Mike Logue Zhuang Wu – PowerPoint PPT presentation

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Title: Nanowire dye-sensitized solar cells


1
Nanowire dye-sensitized solar cells
  • J. R. Edwards
  • Pierre Emelie
  • Mike Logue
  • Zhuang Wu

2
Nanowire DSCs
  • Intro and Background to solarcells, particularly
    DSCs
  • Why nanowires in DSC
  • Fabrication and Characterization
  • Mid-infrared and Summary

3
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

4
Conventional Construction
  • 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

5
Efficiencies
  • Power conversion efficiency (?)
  • ? (FF x Jsc x Voc)/Pin
  • Where
  • FF fill factor
  • Jsc short circuit current
  • Voc open circuit voltage
  • Pin incident light power

6
Improving Jsc
  • Determined from how well the absorption spectrum
    of the dye overlaps with the solar spectrum.

7
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

8
Problems with nanoparticle DSCs
  • Poor absorption of red and infrared light
  • Possible approach to overcome this is to increase
    nanoparticle film thickness for higher optical
    density
  • Approach doesnt work because film thickness
    needed exceeds electron diffusion length through
    the nanoparticle film

9
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

10
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

11
How was the cell made
  • 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 FSnO2 glass (FTO) substrates by dip
    coating.
  • Wires were grown from the nuclei through the
    thermal decomposition of a zinc complex.

12
Results from nanowire process
  • The two step process proved to be a simple,
    low-temp. route to making dense (35109/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

13
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

14
Electrical Characteristics of Nanowires
  • Individual nanowires were extracted from the
    arrays, fashioned into FETs and analyzed to
    determine their resistivity, carrier
    concentration, and mobility.
  • Resistivity values ranged from .3-2.0 O-cm, with
    an electron concentration of 1-51018 cm-3, and a
    mobility of 1-5 cm2 V-1 s-1
  • From Einsteins relation DkBTµ/e, the Dn was
    estimated to be .05-.5 cm2 s-1 for single dry
    nanowires

15
Electrical Characteristics of Nanowires
  • The value of Dn .05-.5 cm2 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 device-like
    environments, and should result in faster carrier
    extraction in the nanowire cell

16
I-V characteristics of the Nanowires
  • The wire films are good electrical conductors
    along the direction of the wire axes.
  • barrier-free contacts between nanowire and
    substrate

17
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18
Analyze a single Nanowire
  • Resistivity values ranged from 0.3 to 2 O cm
  • Electron concentration 1-5X1018cm-3
  • Mobility 1-5cm2V-1s-1
  • Diffusion constant 0.05-0.5cm2s-1 which is much
    higher than the nanoparticle case

19
Fill factors
  • FF(VmpJmp) / (VocJsc)
  • FF reflects the power lost inside the solar cell

20
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

21
Voc FF against light intensity
  • 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 dont change with cell size

22
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

23
The effect of annealing treatments
  • 350C for 30 min in H2/Ar increases the emission
    at 400nm
  • No treatment can increase the FF significantly.

24
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.

25
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

26
  • 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 DebyeHückel 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.

27
DebyeHückel 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.

28
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

29
Mid-Infrared Transient Absorption Measurements
Applications
  • Electron injection dynamics from the dye
    molecules into the semiconductor surface
  • 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

30
Mid-Infrared Transient Absorption Measurements
Results
Bi-exponential kinetics
2.2 1.1 ps
4.4 1.4 ps
5.3 1.3 ps
  • Long time constant varies with pump wavelength

N719 dye ZnO nanowire films
Ultrafast step at lt250 fs appears to be
independent of pump energy
31
Mid-Infrared Transient Absorption Measurements
Results
Dye N719 Films pumped at 400 nm
Tri-exponential kinetics lt250 fs, 20 ps, 200 ps
  • Bi-exponential kinetics
  • lt250 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
32
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

33
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
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