Nanowire dye-sensitized solar cells

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Nanowire dye-sensitized solar cells

• J. R. Edwards
• Pierre Emelie
• Mike Logue
• Zhuang Wu

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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

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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Fill factors

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

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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

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The effect of annealing treatments

• 350C for 30 min in H2/Ar increases the emission
  at 400nm
• No treatment can increase the FF significantly.

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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.

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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

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• 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.

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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.

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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

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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

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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
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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
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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

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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