Nanowire dye- sensitized 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 Two electrodes: one transparent Nanoparticle film (with absorbing dye in nm 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 |J sc | x V oc )/P in Where  FF: fill factor  J sc : short circuit current  V oc : open circuit voltage  P in : incident light power

Improving J sc 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 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*10 9 /cm 2 ), 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 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 Ω-cm, with an electron concentration of 1-5*10 18 cm -3, and a mobility of 1-5 cm 2 V -1 s -1 From Einstein’s relation D=k B Tμ/e, the D n was estimated to be cm 2 s -1 for single dry nanowires

Electrical Characteristics of Nanowires The value of D n = cm 2 s -1 is several hundred times larger than the highest reported values for TiO 2 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

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

Analyze a single Nanowire Resistivity values ranged from 0.3 to 2 Ω cm Electron concentration 1-5X10 18 cm -3 Mobility: 1-5cm 2 V -1 s -1 Diffusion constant: cm 2 s -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 J sc and V oc 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

V oc &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 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 H 2 /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.

J sc 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 Excitation pulses: 400 nm, 510 nm, 570 nm Probe pulse: Mid-infrared Experimental setup for transient absorption measurements

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 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 Ultrafast step at <250 fs appears to be independent of pump energy N719 dye ZnO nanowire films 2.2 ± 1.1 ps 4.4 ± 1.4 ps 5.3 ± 1.3 ps  Long time constant varies with pump wavelength Bi-exponential kinetics:

Mid-Infrared Transient Absorption Measurements Results Dye N719 Films pumped at 400 nm Tri-exponential kinetics <250 fs, 20 ps, 200 ps 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 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 Advantages Limitations Available area for dye adsorption limits the efficiency of the nanowire cell