LETTERS 0.20 thick enough to support the sort of radial electric field (depletion layer)that is impossible in smaller TiO2 or ZnO nanoparticles with fewer carriers. This upward band bending at the nanowire surface should suppress recombination by corralling injected electrons within the wire cores. At the same time, an axial field along each 油订水 nanowire encourages carrier motion towards the external circuit. These macroscopic fields should act synergistically to increase 0.10 electron transport relative to nanoparticle cells, which lack such fields Ambipolar diffusion is consequently a less dominant mechanism in the nanowire devices A switch from particles to wires also affects the kinetics of Time delay (ps) charge transfer at the dye-semiconductor interface, as particle and wire films have dissimilar surfaces onto which the sensitizing dye adsorbs. Whereas Zno particles present an ensemble of surfaces having various bonding interactions with the dye, our wire arrays are dominated by a single crystal plane(the [100))that accounts for over 95% of their total area. We used femtosecond transient absorption spectroscopy to measure the rate of electron injection from photoexcited ruthenium dyes into nanowire and nanoparticle films. Dye-sensitized samples were excited with 400-nm, 510-nn or 570-nm pulses and the free carrier concentration of the oxide Figure 4 Transient mid-infrared absorption traces of dye-sensitized Zno nanowire was monitored with a mid-infrared probe(see Figs S8 and S9).The NW)and zno nanoparticle(NP) films pumped at 400 nm. The large difference in transient responses for wires and particles(Fig 4)were considerably injection amplitudes is due to the larger surface area of the particle film Injection in different. Injection in wires was characterized by bi-exponential wires is complete after-5 ps but continues for -100 ps in the particle case. A high- resolution trace (inset shows the ultrafast step (250 fs)and -3ps rise tme for a whereas the particle response was tri-exponential and significantly slower(time constants: <250 fs, 20 ps, 200 ps). Our data on particle wavelength(see Figs S8 and S9), Partides were synthesized and films were prepared injection are in excellent agreement with published results using dye N719)as described elsewhere. Films were deposited on A,O, substrates. validating our evidence for faster electron injection in nanowires. petra are offset by -0.05 absorbance units for clarit The nanowire dye-sensitized solar cell is an exciting variant of the most successful of the excitonic photovoltaic devices. As an ordered topology that increases the rate of electron transport, a nanowire electrode may provide a means to improve the quantum indium tin oxide). The efficiency of our devices is fairly flat above efficiency of DSCs in the red region of the spectrum, where their a power density of-5 mW" performance is currently limited. Important differences in transport, To assess the relative efficiency with which carriers are extracted interna ution a d light scattering should make m the nanowire devices, we compare in Fig 3 the short- comparative studies of wire and particle devices fruitful. Raising ircuit current densities of the wire cells to those of Tio2 and the efficiency of the nanowire cell to a competitive level depends on ZnO nanoparticle cells as a function of the internal surface area achieving higher dye loadings through an increase in surface area. (roughness factor). A hypothetical photoanode that maintained a We are now extending our synthetic strategy to design nanowire factor would trace out a line in this plot that gradually tapered off advantages of the nanowire geometry are even more onon.The at high surface areas to a large Js value(>25 mA cm-2). In contrast, for other types of excitonic photocells, such as inorganic- the rapid saturation and subsequent decline of the current from cells hybrid devices, in which an oriented, continuous and built with 12-nm TiO2 particles, 30-nm Zno particles or 200-nm inorganic phase of the proper dimensions could greatly improve the Zno particles confirms that the transport efficiency of particle collection of both electrons and holes. films falls off above a certain film thickness, as we argued above. Crucially, the nanowire films show a nearly linear increase in s that maps almost directly onto the TiO2 data. Because transport in the METHODS hin TiO2 particle films is very efficient(with Js =7.8-8.7 mA cm-2 at a roughness factor of 250), this is strong evidence of an equally array of SYNTHESIS OF NANOWIRE ARRAYS addition, the nanowire cells generate considerably higher currents hlm of Zno quantum dots, 3-4 nm in diameter, by dip-coating in a concentrated ethanol solution. than either of the Zno particle cells over the accessible range of Nanowires were grown by immersing seeded roughness factors(55-75% higher at a roughness of 200). This is molecular weight, Aldrich) at 92"C for 25 hours. Because nanowire growth slowed after this period. direct confirmation of the superiority of the nanowire photoanode substrates were repeatedlly introduced to fresh solutio reaction time of up to 50 hours). The arrays were then rinsed with deionized water and baked in air at Better electron transport within the nanowire photoanode is a 00"C for 30 minutes to remove any residual organics and to optimize cell performance. product of both its higher crystallinity and an internal electric field SOLAR CELL FABRICATION AND CHARACTERIZATION Nanowire arrays were first sensitized in a solution(0.5 mmolF)of(Bu, N), Ru(dcbpyH)(NCS)(N71 rom the surrounding electrolyte and sweeping them towards the如知由抛 Dupont). The internal space of the cell ( roughly one-third of the thickness of the space-charge layer in the)b5AML须mM05M4由如3m学由 collecting electrode. The Debye-Huckel screening length of Zno was illed witha liqui semiconductor at the semiconductor-electrolyte junction) is about dificiency (eoe) values t 4 nm for a carrier concentration of 108 cm3, making our nanowires semon Lmp coupled to a monochromator, and calibrated with a silicon photodiode. naturematerialsVol.4IJuNe2005iwww.nature.com/naturematerials @2005 Nature Publishing GroupLETTERS 458 nature materials | VOL 4 | JUNE 2005 | www.nature.com/naturematerials indium tin oxide). The effi ciency of our devices is fairly fl at above a power density of ~5 mW cm–2. To assess the relative effi ciency with which carriers are extracted from the nanowire devices, we compare in Fig. 3 the short￾circuit current densities of the wire cells to those of TiO2 and ZnO nanoparticle cells as a function of the internal surface area (roughness factor). A hypothetical photoanode that maintained a near-unity carrier collection effi ciency independent of roughness factor would trace out a line in this plot that gradually tapered off at high surface areas to a large Jsc value (>25 mA cm–2). In contrast, the rapid saturation and subsequent decline of the current from cells built with 12-nm TiO2 particles, 30-nm ZnO particles or 200-nm ZnO particles confi rms that the transport effi ciency of particle fi lms falls off above a certain fi lm thickness, as we argued above. Crucially, the nanowire fi lms show a nearly linear increase in Jsc that maps almost directly onto the TiO2 data. Because transport in the thin TiO2 particle fi lms is very effi cient (with Jsc = 7.8–8.7 mA cm–2 at a roughness factor of 250), this is strong evidence of an equally high collection effi ciency for nanowire fi lms as thick as ~25 μm. In addition, the nanowire cells generate considerably higher currents than either of the ZnO particle cells over the accessible range of roughness factors (55–75% higher at a roughness of 200). This is direct confi rmation of the superiority of the nanowire photoanode as a charge collector. Better electron transport within the nanowire photoanode is a product of both its higher crystallinity and an internal electric fi eld 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 (roughly one-third of the thickness of the space-charge layer in the semiconductor at the semiconductor–electrolyte junction) is about 4 nm for a carrier concentration of 1018 cm-3, making our nanowires thick enough to support the sort of radial electric fi eld (depletion layer) that is impossible in smaller TiO2 or ZnO nanoparticles with fewer carriers. This upward band bending at the nanowire surface should suppress recombination by corralling injected electrons within the wire cores. At the same time, an axial fi eld along each nanowire encourages carrier motion towards the external circuit. These macroscopic fi elds should act synergistically to increase electron transport relative to nanoparticle cells, which lack such fi elds. Ambipolar diffusion is consequently a less dominant mechanism in the nanowire devices. A switch from particles to wires also affects the kinetics of charge transfer at the dye–semiconductor interface, as particle and wire fi lms have dissimilar surfaces onto which the sensitizing dye adsorbs. Whereas ZnO particles present an ensemble of surfaces having various bonding interactions with the dye, our wire arrays are dominated by a single crystal plane (the {100}) that accounts for over 95% of their total area. We used femtosecond transient absorption spectroscopy to measure the rate of electron injection from photoexcited ruthenium dyes into nanowire and nanoparticle fi lms. Dye-sensitized samples were excited with 400-nm, 510-nm or 570-nm pulses and the free carrier concentration of the oxide was monitored with a mid-infrared probe (see Figs S8 and S9). The transient responses for wires and particles (Fig. 4) were considerably different. Injection in wires was characterized by bi-exponential kinetics with time constants of less than 250 fs and around 3 ps, whereas the particle response was tri-exponential and signifi cantly slower (time constants: <250 fs, 20 ps, 200 ps). Our data on particle injection are in excellent agreement with published results25, validating our evidence for faster electron injection in nanowires. The nanowire dye-sensitized solar cell is an exciting variant of the most successful of the excitonic photovoltaic devices. As an ordered topology that increases the rate of electron transport, a nanowire electrode may provide a means to improve the quantum effi ciency of DSCs in the red region of the spectrum, where their performance is currently limited. Important differences in transport, internal electric fi eld distribution and light scattering should make comparative studies of wire and particle devices fruitful. Raising the effi ciency of the nanowire cell to a competitive level depends on achieving higher dye loadings through an increase in surface area. We are now extending our synthetic strategy to design nanowire electrodes with much larger areas available for dye adsorption. The advantages of the nanowire geometry are even more compelling for other types of excitonic photocells, such as inorganic–polymer hybrid devices26, in which an oriented, continuous and crystalline inorganic phase of the proper dimensions could greatly improve the collection of both electrons and holes. METHODS SYNTHESIS OF NANOWIRE ARRAYS Arrays of ZnO nanowires were synthesized on FTO substrates (TEC-7, 7 Ω per square), Hartford Glass Co.) that were fi rst cleaned thoroughly by acetone/ethanol sonication and then coated with a thin fi lm of ZnO quantum dots, 3–4 nm in diameter, by dip-coating in a concentrated ethanol solution. Nanowires were grown by immersing seeded substrates in aqueous solutions containing 25 mM zinc nitrate hydrate, 25 mM hexamethylenetetramine and 5–7 mM polyethylenimine (branched, low molecular weight, Aldrich) at 92 °C for 2.5 hours. Because nanowire growth slowed after this period, substrates were repeatedly introduced to fresh solution baths in order to obtain long wire arrays (total reaction times of up to 50 hours). The arrays were then rinsed with deionized water and baked in air at 400 °C for 30 minutes to remove any residual organics and to optimize cell performance. SOLAR CELL FABRICATION AND CHARACTERIZATION Nanowire arrays were fi rst sensitized in a solution (0.5 mmol l–1)of (Bu4N)2Ru(dcbpyH)2(NCS)2 (N719 dye) in dry ethanol for one hour and then sandwiched together and bonded with thermally platinized FTO counter electrodes separated by 40-μm-thick hot-melt spacers (Bynel, Dupont). The internal space of the cell was fi lled with a liquid electrolyte (0.5 M LiI, 50 mM I2, 0.5 M 4-tertbutylpyridine in 3-methoxypropionitrile (Fluka)) by capillary action. Cells were immediately tested under AM 1.5G simulated sunlight (300 W Model 91160, Oriel). Intensity measurements were made with a set of neutral density fi lters. External quantum effi ciency (EQE) values (uncorrected for transmission and refl ection losses) were obtained with a 150-W xenon lamp coupled to a monochromator, and calibrated with a silicon photodiode. 0 5 10 15 Time delay (ps) 0.04 0.03 0.02 0.01 0 Absorbance 0 20 40 60 80 100 Time delay (ps) 0.20 0.15 0.10 0.05 0 Absorbance NP NW Figure 4 Transient mid-infrared absorption traces of dye-sensitized ZnO nanowire (NW) and ZnO nanoparticle (NP) fi lms pumped at 400 nm. The large difference in injection amplitudes is due to the larger surface area of the particle fi lm. Injection in wires is complete after ~5 ps but continues for ~100 ps in the particle case. A high￾resolution trace (inset) shows the ultrafast step (<250 fs) and ~3 ps rise time for a nanowire sample. The slower time constant showed a weak dependence on pump wavelength (see Figs S8 and S9). Particles were synthesized6 and fi lms were prepared25 (using dye N719) as described elsewhere. Films were deposited on Al2O3 substrates. Spectra are offset by ~0.05 absorbance units for clarity. nmat1387-print.indd 458 mat1387-print.indd 458 10/5/05 3:46:24 pm 0/5/05 3:46:24 pm ©2005 NaturePublishingGroup © 2005 Nature Publishing Group