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Title:
ANODIZED NIODIUM OXIDE PHOTOANODE FOR DYE SENSITIZED SOLAR CELL
Document Type and Number:
WIPO Patent Application WO/2013/052998
Kind Code:
A1
Abstract:
A dye sensitized solar cell in which the photoanode includes an anodized niobium oxide (Nb2O5) layer which has been annealed so that the surface exhibits orthorhombic Νb2O5. The anodisation conditions include the use of an electrolyte with low content of water and Fluorine ions at a voltage up to 20V and at a temperature of about 50°C. This results in a Nb2O5 crisscross nanoporous network. The efficiency of the solar cells is enhanced as a result of both generated current and voltage increase.

Inventors:
KALANTAR-ZADEH KOUROSH (AU)
OU JIAN ZHEN (AU)
Application Number:
PCT/AU2012/001223
Publication Date:
April 18, 2013
Filing Date:
October 11, 2012
Export Citation:
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Assignee:
UNIV RMIT (AU)
KALANTAR-ZADEH KOUROSH (AU)
OU JIAN ZHEN (AU)
International Classes:
H01G9/20
Foreign References:
EP2233614A12010-09-29
Other References:
FILHO ET AL.: "Photoelectrochemical Properties of Sol-Gel Nb205 Films", JOURNAL OF SOL-GEL SCIENCE AND TECHNOLOGY, vol. 8, 1997, pages 735 - 742
KOVENDHAN ET AL.: "Spray Deposited Nb205 Thin Film Electrodes for Fabrication of Dye Sensitized Solar Cells", TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, vol. 64, no. 1&2, February 2011 (2011-02-01), pages 185 - 188
EGUCHI ET AL.: "Nb205 Based Composite Electrodes for Dye-Sensitized Solar Cells", JOURNAL OF THE CERAMIC SOCIETY OF JAPAN, vol. 108, 2000, pages 1067 - 1071, XP001023503
Attorney, Agent or Firm:
MISCHLEWSKI, Darryl (P.O.Box 1254Camberwell, Victoria 3124, AU)
Download PDF:
Claims:
1. A dye sensitized solar cell in which the photoanode includes an anodized niobium oxide layer which has been annealed so that the. efficiency of the solar cells current and voltage is enhanced.

2. A dye sensitized solar cell as claimed in claim 1 in which the surface of the photoanode exhibits orthorhombic Nb20e.

3. A photoanode for use in a dye sensitized solar cell which includes a layer of niobium oxide formed by anodization followed by annealing.

A photoanode as claimed in claim 3 in which the surface of the photoanode exhibits nanostructured orthorhombic Nb20s.

5. A photo anode as claimed in claim 3 which is a ND2O5 crisscross

nanoporous network of up to thickness of about 16 μιτι. 6. A method of forming a photo anode as claimed in claim 4 in which the

anodisation conditions include the use of an polar organic solvent based electrolyte with content of water below 8% and fluoride ions below 3 % at a voltage of from 5 to 25V and at a temperature of about from 40C to 60C.

Description:
ANODIZED NIODIUM OXIDE PHOTOANODE FOR DYE SENSITIZED SOLAR

CELL

This invention relates to improved photoanodes for use in Dye Sensitized Solar Cells (DSSC) . Background to the invention

A DSSC consists of a photoanode immersed in an electrolyte containing a redox mediator and a cathode.

The photanode is generally a porous layer of a metal oxide. The most common metal oxides include materials such as TiC^, WO3, ZnO and Sn0 2 . These porous layers can be made of nanoparticles, nanorods, nanotubes, etc. The porous layer is covered with an organometalic dye. Ruthenium complexes are employed as dye sensitizer very early on and are still now the most commonly used sensitizer. The photoanode is usually placed on a transparent conducting layer to allow back illumination. The most common transparent conductive films are fluorine doped oxide (FTO) and indium tin oxide (ITO) layers. The electrolytes are usually iodide/triiodide and guanidine thiocyanate.

The cathode is usually a transparent conductive material covered with a catalyst of which the most common are platinum and palladium.

The DSSC operate by the sunlight passing through the transparent electrodes an incident on the dye layer. The dye is capable of absorbing photons from sunlight. The photons energy elevates an electron from the valence band to the conduction band of the dye. This electron is then scavenged by the porous metal oxide. The electrons flow toward the electrode where they are collected. Then they are sent to the load via a closed loop. After flowing through the external circuit, they are re- introduced into the cathode, flowing into the electrolyte and thence to the photoanode.

Ordered Ti0 2 nanotube arrays in dye-sensitized solar cells, have been shown to enhance the efficiencies of both charge collection and light harvesting. However, their power conversion efficiency is still comparably lower than that of nanoparticle films due to their reduced available surface sites for dye loading and possibly the existence of exciton-like trap states. In order to seek a better solution for developing high efficiency DSSCs, Nb 2 0 5 may be a more suitable candidate compared to Ti0 2 in the view of its wider bandgap, higher conduction band edge and better chemical stability. Nb20 5 nanoparticles have been studied for the development of DSSCs, However, their photocurrents and photoconversion efficiencies have not reached those of T1O2 based DSSCs due to the reduction in their dye loading sites. It has been suggested that the large unit cell dimension of orthorhombic Nb20s, in comparison to anatase ΤΊΟ2, makes it a challenging task to obtain the optimum b205 morphologies for DCS applications.

WO2009/156321 uses Atomic Layer deposition techniques to form photoelectrodes which may be composed of metal oxides such as titanium and niobium.

WO2010/118375 which discloses titanium dioxide nanotubes formed by an anodization process. These are claimed to be superior to nanoparticulate titanium dioxide photoanodes by allowing direct free electron carrier pathways.

USA patent 7799989 to the discloses a mesoporous metal oxide in a DSSC. The metal oxide exemplified is titanium dioxide and is formed by a spl-gel method combined with heat treatment.

USA 2009/00904111 discloses a nano tube of titanium dioxide formed by anodizing.

It is an object of this invention to provide an improved photoanode for a DSSC. Brief description of the invention

To this end the present invention provides a dye sensitized solar cell in which the photoanode includes an anodized niobium oxide layer which has been annealed so that the efficiency of the solar cells current and voltage is enhanced.

The anodisation process results in a Nb20s crisscross nanoporous network.

In order to increase the efficiency of solar cells both the current and voltage of the cells should be enhanced.

The overall efficiency (ritotai) of the photovoltaic cell is calculated from the

photocurrent density (i P h), the open-circuit photovoltage (V oc ), the fill factor of the cell (ff) and the intensity of the incident light (l s = 1000 mW/cm 2 )

Htotal = iph X V 0C ff / Is

The ff of a solar cell is the ratio of the solar cells maximum power output (V pma x* ipmax) divided by its dummy power output (V oc x isc) for which i sc is the short-circuit photocurrent. In a DSSC the output voltage depends on the difference between the positions of Fermi level for the electrolyte and the conduction band of the metal oxide used in the photoanode. The most efficient and most common photanode currently used is based on titanium dioxide.

In this invention the voltage and photocurrent magnitude are increased to provide a significant increase in cell efficiency. Although b 2 05 is known as a possible photoanode material it has not been put forward as serious alternatives to titanium dioxide because reported efficiencies are low. It is the structure of the anodized niobium oxide that is responsible for the improved efficiency. The structure is composed of nanotubes formed by the method of anodizing and annealing the deposited Niobium layer. In this invention a Nb 2 0 5 crisscross nanoporous network is produced by a novel electrochemical anodization method at elevated

temperatures.

The foci of this invention are: (a) the implementation of the Nb 2 0s as the photoanode in a dye sensitized solar cell (b) a novel protocol for the formation of thick nanoporous Nb 2 05 using high temperature anodization. The most preferred anodisation conditions include the use of an electrolyte with low content of water and Fluorine ions at a voltage up to 20V and at a temperature of about 50C.

Preferably the temperature range for anodisation is 40C to 60 C more preferably 45C to 55C. The preferred voltage range used in the anodisation is 5 to 25 V.

During anodisation a preferred solvent is any polar electrolyte similar to ethylene glycol which is the most preferred solvent. The electrolyte preferably has a Fluoride content below 3 % most preferably below 1 %. The maximum water content in the electrolyte is preferably less than 8 %.

The anodised layer is annealed in the conventional way at a temperature in the range of 200C to 580C.

The outcome is a solar cell with -11.7% efficiency which is the largest ever reported for any dye-sensitized solar cells. An optimum thickness of the Nb 2 0 5 crisscross nanoporous network in the photochemical anode is about 16 micrometres. Detailed description of the invention

Preferred embodiments of the invention will be described.

In the drawings:

Figure 1 shows the schematic diagram of a dye sensitized solar cell (prior art).

Figure 2 shows an Energy state diagram of Nb2os and TiC>2-based DSSCs featuring their operation principle, in which CS, CE, CB, VB, VAC, and NHE stand for conductive substrate, counter electrode, conduction band, valence band, vacuum, and normal hydrogen electrode, respectively;

Figure 3 illustrate the photon to current conversion efficiency of photoanode surfaces of varying thicknesses ;

Figure 4 illustrates the current voltage characteristics of the invention versus

Titanium dioxide;

Figure 6 shows SEM images of a niobium oxide nanoporous network ;

Figure 7 shows the crystal structure of a niobium oxide nanoporous networkbefore and after annealing;

Figure 8 shows characteristics of the DSSCs fabricated using crystalline Nb 2 0 5 nanoporous networks and anatase T1O2 nanotube arrays;

Figure 9 shows performances of the DSSCs fabricated using crystalline NbaOs nanoporous networks and anatase Ti0 2 nanotube arrays;

Figure 10 shows Electrochemical impedance spectra of the DSSCs fabricated using crystalline b20s nanoporous networks and anatase T1O2 nanotube arrays.

Method of production of the photoanode

Anodization

1) Clean the niobium foil or film

2) The electrolyte for anodisation in the order of 1 % NH 4 F (0.5g) and 2% H 2 0 (1mL) in 50 mL ethylene glycol

3) Anodization conditions - 10 V at 50C

Annealing

The furnace is set for 440°C for 90 min rate is 1 °C /min

A low ramp up temperature is required to assure that the film does not crack. A layer sample of 1.5 μηι is shown in figure 5. After annealing the surface exhibits orthorhombic Nb 2 Os . The structures have nano dimensions (the walls of our structure are less than 15 nm, which are placed in nano domain).

By applying a voltage against a reference electrode the surface of the Nb foil is selectively etched.

In the anodization process of Nb foil, high chemical dissolution of the oxide in the fluorine containing electrolytes, and formation of thick barrier layer on the bottom of the pores prohibit further oxidation induced by ion diffusion. These are suggested to be the main two factors, preventing the formation of porous oxide layer thicker than 500 nm. In order to increase the layer thickness to several micrometers, ethylene glycol is used as the solvent instead of water, which helps to reduce the chemical dissolution effect on the porous oxide layer. The anodization electrolyte temperature is increased to 50°C to enhance the diffusion rate of ions; hence the growing rate of the porous oxide layer. The selection of electrolyte temperature was a process that required optimization. Temperatures higher than 50°C greatly increased the chemical dissolution effect, even in the presence of ethylene glycol and at lower than 50°C, the growth rate was not efficient.

In the anodization process of niobium (Nb), two factors are suggested to be the main causes for preventing the formation of porous layer thicker than 500 nm . One is high chemical dissolution rate of the porous layer caused by fluorine ions in electrolytes; other is the formation of thick barrier layer on the bottom of the pores, which prohibits further ions diffusion and growth of porous layer. In order to increase the layer thickness to several micrometers, we firstly utilize ethylene glycol instead of water as the solvent, which helps to reduce the chemical dissolution effect on the porous layer. Secondly, we increase the electrolyte temperature to 50°C to enhance the diffusion rate of ions; hence the growth rate of the porous layer. The selection of electrolyte temperature is a process that requires optimization. We find out that the chemical dissolution effect on porous layer is greatly increased even in the presence of ethylene glycol for temperatures above 50°C, while the growth rate of porous layer drops rapidly at temperatures below 50°C.

Fabrication Example

The starting material was niobium foil (99.95% purity, Sigma Aldrich) with a thickness of 0.25 mm. The surface of each specimen was cleaned ultrasonically with acetone, then washed with isopropyl alcohol and distilled water, and finally dried in a stream of compressed dry nitrogen. Electrolytes consisting of 50 ml_ of ethylene glycol (98% anhydrous, Sigma Aldrich) with 0.15-0.50 g of NH 4 F (98% purity, Sigma Aldrich) and 0-8 vol % water were initially conditioned by applying 5- 20 V between a platinum cathode and a niobium anode. The electrolytes were kept at a constant temperature of 20, 50, or 80 °C during each experiment. Anodization durations of 0.5-4.0 h resulted in films comprising 1.5-6.0 micrometre thick nanoporous structures. - about 20 hours of anodization results in 16 μιη thickness that may be slightly fragile. After anodization, the samples were carefully washed with distilled water and dried in a nitrogen stream. After anodization, the sample was annealed in air at a temperature of 440 °C for 20 min, with a slow ramp up and down rate of 1 °C/min.

Structural Characterization. The instruments used were a FEI Nova NanoSE for SEM; a JEM-3010 300 kV for cross-sectional TEM; and a Bruker D8

DISCOVER microdiffractometer fitted with a general area detector diffraction system (GADDS) for X-ray diffraction. HRTEM samples were prepared by using a cleaned razor blade to scrape the nanostructures onto a holey carbon (Formvar) grid, allowing the surface tension of the grid to hold the nanostructures in place. The HRTEM images were taken using a JEOL2100F HRTEM operating at 200 kV. EDX measurements were carried out on the FEI Nova NanoSEM. Raman

measurements were performed using a system incorporating an Ocean Optics QE 6500 spectrometer, a 532 nm 40 mW laser as the excitation source and a notch filter used to prevent measurements below 100 cm "1 . XPS measurements were performed on a VG-3 OF instrument using Al non-monochromated X-rays (20 kV, 15 mA) with the hemispherical energy analyzer set at a pass energy of 100 eV for the survey spectrum and 20 eV for the peak scans.

Solar Cell Fabrication.

The NbaOs electrode was first immersed in a 0.2 mM N3 dye (Solaronix) in a mixture of acetonitrile and tert-butyl alcohol (volume ratio 1 :1) and kept at room temperature for 24 h. The counter electrode was 30 nm thick platinum, sputtered on a FTO substrate (Delta Technologies). The electrolyte was a solution of 0.6 M 1- butyl-3-methylimidazolium iodide (Sigma Aldrich), 0.03 M 12 (Sigma Aldrich), 0.10 M guanidinium thiocyanate (Sigma Aldrich), and 0.5 M 4-tert-butyl pyridine (Sigma Aldrich) in a mixture of acetonitrile and valeronitrile (volume ratio, 85:15). The dye- adsorbed b205 photoanodes and platinum counter electrodes were assembled into a sandwich-type cell and sealed with a hot-melt sealant with a thickness of 25micrometre (Solaronix). The NbaOs electrodes had dimensions of 5 mm x 5 mm (i.e., 0.25 cm 2 ).

Solar Cell Characterization.

Photovoltaic measurements were performed using an AM 1.5 solar simulator (Photo Emission Tech.). The power of the simulated light was calibrated to 100 mW cm '2 by using a reference silicon photodiode with a power meter (1835-C, Newport) and a reference silicon solar cell to reduce the mismatch between the simulated light and AM 1.5.

Current Voltage curves were obtained by applying an external bias to the cell and measuring the generated photo- current with a Keithley model 2400 digital source meter. The voltage step and delay time of photocurrent were 10 mV and 40 ms, respectively. DSSCs fabricated using commercially available Ti0 2 nanoparticle- coated glass plates (Dyesol, Australia) were tested and compared with their factory specifications in order to standardize the cell fabrication and testing conditions. The IPCE values for the cells were determined using a system comprising a

monochromator (Cornerstone 330), a 300 W Xenon arc lamp, a calibrated silicon photodetector, and a power meter. To establish the dye loading of the sensitized Nb20 5 nanoporous networks and Ti0 2 nanotube arrays, samples sensitized with N3 were placed into a 10 mM solution of KOH to desorb the dye. Absorbance spectra of the desorbed dye were examined using a spectrophotometric system consisting of a Micropack DH-2000 UV— is— NIR light source and an Oceanoptics HR4000 spectrophotometer. EIS spectra were measured in the dark under different bias voltages using a CHI 700 electrochemical workstation with

impedance analyzer in a two-electrode configuration. A 10 mV AC perturbation was applied ranging between 00 kHz and 10 mHz.

In addition to anodization duration, the surface morphology and thickness of the nanoporous network may also be controlled by varying fluoride content, applied voltage, water content and temperature.

Fluoride content in the electrolyte plays an important role on the surface morphology of the porous network. Insufficient fluorine ions in the electrolyte results in failure to form organized pores from the initial compact oxide layer. On the other hand, excessive fluorine ions in the electrolyte may cause the partial destruction on organized appearance of porous network due to chemical dissolution.

Increasing the applied voltage up to 20 V may enlarge the pore size due to the enhancement of the chemical dissolution effect on the porous network. However, the voltage breakdown effect dominates when applied voltage exceeds 20 V, which only results in the formation of a compact oxide layer.

Near zero water content in the electrolyte results in the formation of randomly distributed surface pores and stratified layers in the cross section. On the other hand, water acts as a passivator when its content exceeds 4 vol%, as the surface pore size becomes smaller and the thickness of the porous network is reduced. Increasing the electrolyte temperature up to 80 °C greatly enhances the growing rate of the porous network up to four times larger for the pore size formed and four times thicker for the thickness at 50 °C compared to those at 20 °C. However for the temperature above 80 °C, the chemical dissolution effect on the porous network becomes and the organised pore structure deteriorates.

Figure 1 illustrates the structure of a standard prior art DSSC.

Figure 2 illustrates the efficiency of an Nb 2 0 5 based DSSC of this invention compared with a titanium dioxide DSSC. The difference between the ΤΊΟ2 conduction band and electrolyte Fermi level is 0.7 eV (4.9-4.1 eV) while this value is ~1.1 eV (4.9-3.8) for Nb 2 Os that obviously shows the excellent advantages of this invention.

Photocurrent magnitude depends on the efficiency of photon to electron conversion by the photoanode. The photon to electron conversion in our Nb 2 0s system is superior to Ti0 2 as can be seen in Figure 3. The incident photon-to-current conversion efficiency (IPCE) spectra of 4 DSSC samples are presented in Figure 3 as can be seen for the same thickness of the cells the photocurrent conversion is greater for Nb20 5 compared to Ti0 2 .

Figure 4 shows that the voltage and photocurrent of tis invention results in larger efficiencies compared to the T1O2 photoanodes. Table 1 illustrates the efficiencies of this invention where 1 1.7% is achieved compared to 1 1.2 % which is the highest ever reported for T1O2 based

dyesensitized solar cells prior to this invention. Table 1- properties of standard dye sensitized solar cells vs standard Nb20 5 based based cells.

Figure 6 shows . a, top view of the nanoporous network (pore size: 30-50 nm, side wall thickness: 10-20 nm). The anodization was performed at 10 V in the electrolyte of 50 ml ethylene glycol with 0.25 g NH 4 F and 4 vol% H 2 0 at 50°C for 2 hr.

b, bottom view of the nanoporous network (grain size: 40-50 nm).

c, cross-sectional view of the top of the nanoporous network.

d, cross-sectional view of the whole nanoporous network (thickness: ~4.3 μιτι). The porous structure is continuous from top all the way down to the bottom.

e, f, higher magnification SEM images of cross-sectional view of nanoporous network with structure A (e) and B (f) appearances.

g, 3D schematic of the top and cross-sectional views of the nanoporous network. The scanning electron microscopy (SEM) image in Figure 6a represents the surface morphology of as-deposited niobium oxide layer obtained via anodization. A highly- organized pore distribution is observed with nano-sized pores, ranging from 30 to 50 nm. The side walls are around 10 to 20 nm thick. The bottom of the anodized layer, showed in Figure 6b, consists of uniform and interconnected grains with the diameter of approximately 50 nm t> which is close to that of the pore dimensions. Similar to the anodization of Ti and Al, the formation of these grains could be resulted from the mechanical stress induced by niobium oxide volume expansion effect. Figure 1d depicts a ~4 pm thick porous layer which is formed after 2 hr anodization with a stratified structure consists of several different regions. By zooming in the cross-sectional images, a -100— 200 nm thick porous layer can be seen on top (Figure 6c). This layer, with similar morphology and thickness to previously-reported anodized niobium oxide structures, could be the initial porous layer formed at the beginning of the anodization process. The absence of a barrier layer underneath this initial layer further evidences the enhancement of ions diffusion rates in our unique anodization method at an elevated temperature. From the SEM images, this in-between layer seems to be made of compressed vein-like nanostructured networks, which are connected to each other via lateral openings. The top view schematic of the in-between layer is shown in Figure 6g. If the layer is cut along the dot line or any lines in parallel, 'structure A\ which demonstrates the 'nano-veins' morphology, can be observed (Figure 6f). These 'nano-veins' have the internal diameters ranging from 30 to 60 nm, while the 'nano-vein valves' diameters typically reduce in diameters to ~40 to 80% of the veins internal diameters. The valves are the residuals grown from the internal walls, possibly due to the chemical dissolution caused by fluorine ions in electrolytes. Another type of structure is also observed in the cross-sectional images (Figure 6e) that assigned to 'structure B', which can be mathematically obtained by cutting the layer along the dash line. It consists of compressed pore networks with sparse distribution. This crisscross nanoporous network can provide excellent directional paths for electron transfer in addition to enhanced surface area, which fulfils the prerequisites for developing highly-efficient DSCs. The growth of this nanoporous network is very efficient with approximately ~2 pm/hr growth rate for the first 3 hr. However, the organized porous surface is partially destroyed due to excessive chemical dissolution when the anodization duration exceeds 2 hr. In addition, saturation occurs and the thickness does not increase further after 3 hr anodization. The surface morphology and thickness of the nanoporous network can also be controlled by varying fluoride content, applied voltage, water content and temperature. In Figure 7 (a) Dashed blue line is the XRD pattern of an as-anodized niobium oxide nanoporous network underthe same fabrication conditions as Figure 2, and the red line is the XRD pattern of the network after annealing at 440 °C in air for 20 min. Orthorhombic N b205 (ICDD 27-1003) is indicated with * , while Nb metal (ICDD 35-0789) is indicated with 2. (b) Corresponding Raman spectra of the as-anodized (dashed blue line) and post-annealed network (red line), (c) HRTEM image of one of the nanoporous areas for Nb205 nanoporous networks after annealing. The 0.39 nm spacings with no cross-hatches indicate that the orthorhombic phase is present, (d) Corresponding selected area diffraction pattern indexed to the Nb205 orthorhombic phase.

From the XRD patterns (Figure 7a), the as-anodized nanoporous network appears to be amorphous and only niobium peaks are present in the

diffractogram. Annealing of the porous network results in crystallization into a Nb205 orthorhombic structure (ICDD 27- 1003, a = 6.168, b = 29.312, and c = 3.936 A) with diffraction peaks at 28.3,49.7,55.1 ,58, and 63.1° (more detailed analysis of XRD patterns are provided in the Supporting Information). The Raman spectra (Figure 7b) confirm the results obtained from the XRD measurements. For the as-anodized layer, a broad peak centered at 650 cm -1 represents the symmetric stretching mode v (O— Nb— O) of amorphous niobium oxide. 30

Another weak peak at 242 cm -1 can be assigned to bending modes of N — O— b linkages. The Raman peaks become more prominent after annealing. In addition, there is a peak shift from 650 to 690 cm -1 , and a new peak appears at 303 cm -1 , both indicative of the orthorhombic nature of the annealed nanoporous network. According to theory, the orthorhombic and hexagonal (some literature states it as "pseudo- hexagonal") Nb 2 0s structures both have very similar atomic structures. High-resolution transmission electron microscopy (HRTEM) was utilized to further identify the crystal structure of this nanoporous network. Figure 7c shows the HRTEM image of the

Nb205 nanoporous network in which parallel lattice fringes with spacings of 0.39 nm have been observed. This spacing has been seen previously and observed this spacing to be indicative of the orthorhombic phase. Figure 7d shows a selected area diffraction (SAD) pattern of the b 2 0 5 structure, which has been indexed to the orthorhombic phase.

In Figure 8 (a) Configuration of the DSSC fabricated incorporating the Nb205 nanoporous network, (b) IPCE spectra of DSSCs fabricated using b2os nanoporous networks and Ti02 nanotube arrays of various thicknesses. IPCE was calculated using the relation IPCE % = 1 00hcj(A)/eAP(A), where h denotes the Planck constant, c the velocity of light, e the electron charge, A the wavelength (A) the photocurrent density at A, and P(A) the power density of light at A. (c) Table of dye coverage detail of the Nb 2 0 5 rianoporous networks and T1O2 nanotube arrays. N3 dye attached on Nb 2 0s nanoporous networks and T1O2 nanotube arrays was desorbed by immersing the samples into a 10m KOH solution. The concentration of the desorbed dye determined by UV— vis spectroscopy was used to calculate the dye coverage in the samples, (d) UV— vis absorption spectra of N3 dye desorbed from b20s nanoporous networks and Ti0 2 nanotube arrays of various thicknesses.

The DSSCs based on ~2 and ~4pm thick crystalline Nb 2 0 5 nanoporous networks were assembled using a conventional configuration (Figure 8a). The incident photon-to-electron conversion efficiency (IPCE) spectra of these DSSCs are shown in Figure 8b in comparison to anatase Ti0 2 nanotube arrays with similar thicknesses as a benchmark

The IPCE spectrum of the dye-loaded 4 im thick Nb 205 nanoporous network indicates two broad peaks in a wide wavelength range (400— 750 nm). The center of the first peak is at 450 nm with a peak magnitude of 52.4%, while the other is at 575 nm with a 61.1% conversion efficiency. Significant conversion efficiencies can also be observed for wavelengths up to 750 nm. These impressive results are far superior to any other reported Nb20 5 -based DSSCs that we are aware of and approximately 20% higher than that of T1O2 nanotube arrays with a similar thickness. This enhancement becomes even more obvious for the 2 micrometre thick Nb 2 0 5 nanoporous network, which surprisingly shows an almost 100% improvement when compared with that of a T1O2 nanotube array. This noteworthy improvement of the conversion efficiency could be primarily due to the enhanced surface area of the Nb 2 0 5 nanoporous networks. The 2 and 4 micrometre thick Nb 2 0 5 nanoporous networks have a greater surface area by factors of 100 and 45%, respectively, in comparison to the equivalentTi0 2 nanotube arrays according to the dye coverage characterization presented in Figure 8c,d.

In figure 9 (a) Current— voltage characteristics of DSSCs fabricated using N b2os nanoporous networks and T1O2 nanotube arrays of various thicknesses under the testing condition of Sun AM 1 ;5 (100 mW cm -2 ), (b) Table of power conversion efficiencies of the Nb 2 0s nanoporous networks and T1O2 nanotube arrays. The efficiency was calculated using the relation 17 = JSCVOCFF/Pt, where Jsc is the short circuit current density, Voc the open circuit voltage, FF the fill factor, and Pt the incident power density (100 mW cm -2 ), (c) Photovoltage decay measurements of DSSCs fabricated using N 2os nanoporous networks and Ti0 2 nanotube arrays of various thicknesses, (d) Effective electron lifetime determined by open circuit voltage decay for DSSCs fabricated using Nb205 nanoporous networks and T1O2 nanotube arrays of various thicknesses. The effective electron lifetime is determined using the relation v n =— keTidVoc/dt]- Ve, where k B T is the thermal energy, dVoc/dt is the derivative of open circuit voltage transient, and e is the positive elementary charge.

The current— oltage characteristics of these DSSCs are shown in Figure 9a,b. The DSSC with a ~4micrometre thick Nb 2 0 5 nanoporous network yields a power conversion efficiency of 4.1%. In comparison, a Ti0 2 nanotube array based DSSC with similar thickness has an efficiency of only 2.7%, which is 51% below that of Nb 2 Os. Similarly, there is a 47% improvement in power conversion efficiency for a b 2 05 nanoporous network when compared to a Ti0 2 nanotube array when both thicknesses are reduced to ~2 im. Note that the presence of a higher open circuit voltage voc for Nb 2 Os when compared toTi0 2 is a commonly reported characteristic. This is mainly due to wider band gap (~0.29 eV wider) and higher conduction band edge energy (as shown in Figure 2) of Nb 2 Os, as voc is

proportional to the difference between the Fermi level of the Nb 2 05 (or Ti0 2 ) electrode and the electrochemical potential of the redox couple.

In addition to the increase in open circuit voltage, the significant improvement in the short circuit photo- current density JSC may be due to a larger amount of loaded dye and also possibly less dye agglomerates formed on the oxide surfaces due to the better chemical stability of Nb 2 0 5 . However, it is widely known that the major drawbacks for utilizing nano- architectures, especially those that are based on nanoparticles, are the increase in the random diffusion of electrons (which leads to a higher likelihood of electron entrapment) and the increase in grain boundary density (which gives rise to defect states in the band gap that perform as trap centers for the free electrons). Hence, these effects can largely degrade the electron transport and recombination kinetics. Although ΤΊΟ2 nanotube arrays have been suggested to offset these

drawbacks, recent research reveals that the possible fluoride and nitrogen impurities in T1O2 nanotubes, mainly embedded during anodization, can create additional trap states, which is implied to be the cause for lower electron mobility and shorter electron- lifetime. Here our Nb 2 0s nano- porous networks can provide continuous and directional electron transfer pathways, which to a high degree, perform similar to those of Ti0 2 nanotube arrays. More importantly, they have much better chemical stabilities, hence minimizing the fluoride and nitrogen doping effect. No impurities were found within these nanoporous networks by utilizing various surface characterization techniques, including XRD, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS)

Open circuit voltage decay (OCVD) measurements were performed to estimate effective electron lifetime and verify the above assumptions.

According to Figure 5c,d, electrons in these Nb 2 C>5 nanoporous networks exhibit much longer effective lifetimes in comparison to those in " Π02 nanotube arrays, indicating a combined effect of higher energy level of conduction band of b 2 05 and fewer recombination centers in the nanoporous networks. We further analyzed the electron transport and recombination properties in the photoanodes of Nb 2 05 nanoporous networks and T1O2 nanotube arrays by utilizing

electrochemical impedance spectroscopy (EIS). EIS is regarded as a powerful technique to characterize the transport and recombination in DSSCs, and appropriate physical models have been developed to interpret the results. EIS spectra of Nb 2 05 and Ti0 2 were measured in the dark under the bias voltage of — 0.67 and— 0.60 V, respectively. A well- developed equivalent circuit by Juan et al. was employed to fit the experimental data of impedance spectra for extracting parameters of DSSCs related to electron transport and

recombination, using the electrochemical workstation in-built simulation software. Among several key parameters involved in this equivalent circuit, three fitted parameters, which are electron transport resistance (R w ) in the oxide layer, charge transfer resistance of the charge recombination between electrons in the oxide and I 3" ions in the electrolyte (R k ), and chemical capacitance of the oxide layer (C g ), could be used in order to determine the parameters for the evaluation of cell performance, such as the electron transport time (T d ), effective electron lifetime (T eff ), and effective diffusion length (U^

In figure 10 (a) Nyquist plots of the measured and fitted impedance spectra of Nb 2 0 5 nanoporous networks and ΤΊΟ2 nanotube arrays with thicknesses of ~2 and ~4pm. They were measured in the dark under the bias voltage of— 0.67 and— 0.60 V for NbaOs and Ti0 2 , respectively, using a CHI 700 electrochemical workstation with impedance analyzer in a two- electrode configuration. A 10 mVAC perturbation was applied ranging between 100 kHz and 10 mHz. (b) Detailed impedance spectra of the marked region shown in panel a. (c) Their corresponding Bode phase plots of measured and fitted impedance spectra, (d) Summary of parametric analysis of the impedance spectra.

According to the Nyquist plots of the impedance spectra in Figure 10 a,b, it is seen that the radiuses of the semicircles of the Nb205 nanoporous networks are almost 0-fold larger than those of T1O2 nanotube arrays of similar

thicknesses, which implies that Nb 2 C>5 films have much larger charge transfer resistances and, therefore, greatly enhanced electron lifetimes. This conclusion is also supported by both the Bode phase plot displayed in Figure 10c, in which the left frequency peaks of b20 5 films (centered at around— 0.8 Hz) are more left- shifted as compared to those of Ti0 2 , as well as the OCVD results shown in Figure 9d. The fitted parameters shown in Figure 10d indicate that the T eff values of 2 and 4 micrometre b20s nanoporous networks are 8.31 and 5.71 s, respectively, which are approximately 13- and 11 -fold longer than those of T1O2 nanotube arrays with similar thicknesses. However, their T d values are almost 4 times longer than their T1O2 counterparts, hence leading to smaller electron diffusion rates. This could be possibly due to the lower natural conductivity of ND2O5 itself/Fortunately, their superiorly enhanced effective electron lifetimes compensate the shortfall of smaller electron diffusion rates in the photoanodes, which results in enhanced L e ff in comparison to those of Ti0 2 nanotube arrays. Additionally, we have conducted complementary transient absorption spectroscopy for electron recombination investigations of our b 2 0s nanoporous network in comparison to T1O2 nanotube arrays of similar thicknesses.

This invention teaches fabrication of Nb 2 0 5 crisscross nanoporous networks with a thickness up to— 4 micrometre by using a unique electrochemical anodization method at elevated temperatures that provides an efficient process to obtain porous morphologies. This nanoporous network offers superior dye-loading sites, excellent continuous and directional pathways for electron transfer, as well as enhanced effective electron lifetimes. These advantages, together with the relatively wider band gap and high conduction band edge of b20 5 , result in an ideal material for the creation of photoanodes for highly efficient DSSC

applications. Demonstrated conversion efficiency of 4.1 % from a—4

micrometre thick Nb20 5 photoanode is >50% higher than that of a Ti0 2 nanotube array with a similar thickness. The 4.1% efficiency achieved may be increased by synthesizing thicker porous structures as well as utilizing front-side illumination by realizing these structures on transparent conductive substrates. The descriptions are for the solar cell with 6 μηι metal oxide layers which has the efficiency of 4.5% for Nb 2 0 5 . The 12.3% efficiency is obtained for the solar cells for Nb 2 0 5 layers with of 16 μιη thickness. From the above it can be seen that this invention provides a unique and significant improvement in DSSC efficiency. Those skilled in the art will realise that this invention may be implemented in embodiments other than those illustrated without departing from the core teachings of this invention.




 
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