ES2350222A1 - Nitrogen-doped nanocrystalline tio2 for photovoltaic applications - Google Patents

Abstract

The present invention describes a method for preparing a nitrogen-doped nanocrystalline TiO2 by means of the pulsed laser technique, which confers on said nanocrystalline properties that make it suitable for use in photovoltaic devices, especially solar cells, be these of the Gratzel, hybrid or organic type.

Description

TiO 2 nitrogen-doped nanocrystalline for photovoltaic applications.

The present invention relates to a TiO2 doped with nitrogen, which has a 100% rutile structure, its procedure for obtaining and its use in the manufacture of a photovoltaic device

State of the art

Nitrogen oxide doped with nitrogen (N-TiO 2), is a material highly used in photocatalysis, antibacterial agent, etc.

In CN 101279250 A describes the application of a TiO_ {2} based photocatalyst doped with nitrogen whose doping allows the absorption of wavelengths in the visible spectrum and in the infrared. In another CN document 101026200 A describes a film preparation procedure of TiO2 doped with ammonia-controlled nitrogen and a high pressure reaction device. In document CN 101026024 A good TiO2 conductor doped with nitrogen is revealed in powder and its preparation procedure.

In the market is the use of N-TiO_ {2} in devices for photocatalysis (self-cleaning windows, Toyota vehicle deodorant, etc.) or related.

In addition, the application of N-TiO2 in solar cells is known ( T. Ma , et al . " High-efficiency dye-sensitized solar cell based on a nitrogen-doped nanostructured titania electrode ". Nano Letters, 2005 Dec; 5 (12), 2543-2547 ) and the application of N-TiO2 in solar cells obtained by a sol-gel process, obtaining a crystal of N-TiO2 whose structure presents a mixture of anatase phases , brookita and rutile ( T. Lopez-Luke , et al . " Nitrogen-doped and CdSe Quantum-Dot-Sensitized Nanocrystalline TiO_2 Films for Solar Energy Conversion Applications ". J. Phys. Chem. C, 2008, 112, 4, 1282-1292 ).

A hybrid photovoltaic device is characterized by the use of inorganic semiconductor oxides (TiO2, ZnO, Nb2O5, CeO2, SnO2, etc.) in direct contact with organic semiconductors ( dyes, polymers, electrolytes). The best examples of this type of solar cells are known solar type cells Gratzel or titanium oxide sensitivizado ( "dye-sensitized solar cell" or DSC), hybrid polymer solar cells ( "solar hybrid cells" or HSC) or Gratzel solar cells in solid state (" solid state dye-sensitized solar cell " or ss-DSC). Within the inorganic semiconductors used (TiO2, ZnO, Nb2O5, CeO2, SnO2, CeO2-TiO2, etc.) the most applied, Due to its high performance, it is the TiO_2 used in its anatase structure that has greater photoactivity. However, some variations using mixtures of TiO2 in its anatase and rutile structures can also be used. The most commonly used synthesis procedure for obtaining semiconductor oxides is sol-gel or hydrothermal synthesis techniques.

Recently, it was shown that the devices hybrid photovoltaic, formed by oxide / polymer interfaces, they have an oxygen exchange mechanism with the atmosphere necessary for its proper functioning as conveyors of charge (electrons). However, it is this same oxygen the responsible for the degradation of organic semiconductors (dyes, polymers, etc.) over time, thus limiting life Useful device. The ideal situation would then be the encapsulation of the device in an inert atmosphere to avoid degradation of the organic semiconductor, but the presence of inert atmospheres eliminates the oxygen required by the oxide semiconductor to function as a load conveyor. This mechanism of action is an important problem to maintain the stability of the organic material and to be able to use these photovoltaic devices in applications under inert atmospheres as in space equipment (satellites, space probes, etc.).

Description of the invention

In a first aspect, the present invention is refers to a procedure for obtaining nanocrystalline TiO2 doped with nitrogen in the 100% rutile phase comprising:

to)

Pulsed laser deposition ( pulse laser deposition or PLD) of TiO 2 on a transparent substrate electrode and fluorine-doped tin oxide conductor ( fluorine tin oxide or FTO), where said substrate is at a temperature between 400 ° C and 600 ° C,

b)

adding a gas mixture oxygen-nitrogen on the material obtained in the previous step, where the ratio of partial pressures of said gases is X: Y, with X + Y being a constant value between 1 and 15 Pa.

The nitrogen-doped nanocrystalline TiO2 (N-TiO2) is an inorganic semiconductor oxide whose crystalline structure has dimensions of the order of 10 nm and in which nitrogen ions have been introduced. TiO_ {2} It is found in nature in the form of minerals with structure rutile, anatase, brookita or mixture thereof. The TiO_ {2} in the Rutile type crystalline phase presents the best photovoltaic response when working under conditions of inert atmosphere (stability of the J_ {sc}). Therefore, the synthesis of the material in its rutile structure, because the anatase structure or anatase mixture and / or rutile and / or brookita does not have the desired properties. This behavior may be due to the greater rutile phase internal resistance compared to the anatase phase, which prevents that TiO_ {2} behaves like an almost metallic conductor to doping and during the extraction of oxygen that is carried out in inert atmospheres.

The use of deposition technique by Pulsed laser (PLD) has resulted in a TiO2 material with better performance (less loss of low photoactivity continued irradiation). The PLD technique achieves the Thin layer deposition of TiO2 on the substrate desired inside a camera.

Preferably, in the procedure described previously, the transparent and conductive substrate electrode of the step a) is at a temperature of 500 ° C.

Preferably, in the above procedure, Pulsed laser deposition is performed at an energy between 200 to 600 mJ and at a frequency between 1 and 20 Hz. More preferably, pulsed laser deposition is performed at a 400 mJ energy and at a frequency of 10 Hz.

Preferably, in the procedure described Previously, X + Y is 10 Pa.

Preferably, in the procedure described previously the ratio of partial pressures of the gas mixture oxygen-nitrogen is selected from 4: 6, 3: 7 or 2: 8 More preferably, the partial pressure ratio is 2: 8

In a second aspect, the present invention is refers to a nano-crystalline TiO2 doped with nitrogen obtainable by the procedure described above.

In a third aspect, the present invention is refers to a photovoltaic device comprising the TiO2 nitrogen-doped nanocrystalline described above, a conductive polymer and a metallic counter electrode.

In a preferred embodiment, in said photovoltaic device the conductive polymer is selected from P3HT (poly (3-hexylthiophene)), PCBM (ester acid methyl phenyl-C61-butyric), MEH-PPV (poly [2-methoxy-5- (2'-ethyl-hexyloxy) -1,4-phenylenevinylene]) or mixtures thereof. And more preferably, the polymer Driver is MEH-PPV.

In another preferred embodiment, in said photovoltaic device the metal counter electrode is selected between Ag, Au, Pt, Ca or Al. And more preferably, the counter electrode metallic is Ag.

The photovoltaic device object of the The present invention is composed of two thin layers, the N-TiO2 and a conductive polymer (preferably MEH-PPV), assembled between two electrodes current collectors, the transparent FTO and an Ag electrode evaporated. The thickness of the N-TiO 2 layer is approximately 400 nm, while that of the conductive polymer is 100 nm This configuration has been used for analysis of laboratory where time is required for practical purposes of solar cell life is short enough to carry out in the lab, but able to be compared to devices Similar photovoltaic Therefore, an optimal configuration would be one in which the N-TiO_ {2} was used Nanostructured (nanoparticles, nanowires, etc.) with a Greater surface area.

In a fourth aspect, the present invention is refers to the use of TiO2 doped with nitrogen previously described for the manufacture of a photovoltaic device.

As described above, the photovoltaic device preferably has a TOC / configuration.
N-TiO_2 / MEH-PPV / Ag, where TOC ( Transparent Conducting Oxide ) is a transparent and conductive substrate electrode, N-TiO2 is the nano-crystalline TiO2 nitrogen-doped, MEH-PPV is a Conductive polymer and Ag acts as a metallic counter electrode. The objective is to work with the photovoltaic device object of the present invention in inert atmospheres in order to improve the useful life of the solar cell where it is used.

And in a final aspect, the present invention is refers to the use of said photovoltaic device for manufacturing of a solar cell. Preferably, the solar cell is a cell solar type Gratzel, a hybrid solar cell or a solar cell organic

The N-TiO_2 obtainable by the procedure object of the present invention has a good stability over time of the solar cell where the photovoltaic device of which it is part, under conditions of inert atmosphere (N2, argon, etc.) and continuous irradiation of 1 sol (1000 W / m2, 1.5 A.M.). The dopings that are used vary with deposition conditions (partial pressure of oxygen and nitrogen).

Description of the figures

Fig. 1. Comparison of the first two hours of life of hybrid solar cells type FTO / TiO2 / MEH-PPV / Ag analyzed under conditions of irradiation continued at 1 sol (1,000 W / m2), A.M. 1.5 and in inert atmosphere The TiO2 used was synthesized by means of Two different techniques: (upper) PLD and (lower) sol-gel

Fig. 2. Comparison of hybrid solar cells of the TCO / TiO_2 / MEH-PPV / Ag type where the TiO2 has been used in its rutile (upper) and anatase phase (lower). Analysis carried out under irradiation conditions continued at 1 sol (1,000 W / m2), A.M. 1.5 and in atmosphere inert.

Fig. 3. Comparison of hybrid solar cells of type FTO / TiO_2 / MEH-PPV / Ag using the TiO2 and TiO2 doped with nitrogen (O: N ratio of 2: 8). Analysis carried out under conditions of continuous irradiation at 1 sol (1,000 W / m2), A.M. 1.5 and in an inert atmosphere.

Fig. 4. Curves IV obtained at different times of irradiation under atmosphere of N 2 obtained during the analysis of hybrid solar cells applying TiO2 (upper) and N-TiO2 (lower). The biggest difference between both devices is the type of curve IV that in the TiO_ {2} case is almost flat after only 5 minutes of analysis, while the N-TiO_2 maintains an FF more than 30%. Analysis carried out under conditions of continued irradiation at 1 sol (1,000 W / m2), A.M. 1.5 and in inert atmosphere

Fig. 5. First 50 hours of irradiation continued in an atmosphere of N2 and 1,000 W / m2, A.M. 1.5 for the solar cell using the N-TiO2. It does special emphasis on IV curves at different times of analysis where a filling factor (FF) greater than 25% is shown.

Fig. 6. Comparison of the photovoltaic response of hybrid solar cells using TiO2 and the N-TiO2 as electron transporters in hybrid solar cells. Analysis carried out under conditions of irradiation continued at 1 sol (1,000 W / m2) A.M. 1.5 and in inert atmosphere

Fig. 7. Photovoltaic response of the first 50 hours of hybrid solar cells using the N-TiO2 as electron transporters. Analysis carried out under conditions of continuous irradiation at 1 sol (1,000 W / m2) A.M. 1.5 and in an inert atmosphere.

Fig. 8. Schematic diagram of a solar cell hybrid where the bilayer is shown N-TiO2 / Polymer between the two FTO electrodes and Ag.

Examples

Example

1. Procedure for obtaining nanocrystalline TiO_2 doped with nitrogen in the 100% rutile phase

Obtaining the rutile phase depends completely of the transparent substrate that is used. He material growth on doped tin oxide substrates with indium (Indium Tin Oxide, ITO) will result in a material with anatase structure, or anatase-rutile mixture, while synthesized on a transparent and conductive substrate type fluorine-tinned tin oxide (Fluor Tin Oxide, FTO) gives as result obtaining the material in its rutile form.

TiO_ {2} layers are deposited by Pulsed laser deposition technique (PLD, press laser deposition). The white material used was TiO2 sintered in the form of tablet at 1,100 ° C for 4 hours. The distance was set 50mm white substrate and laser deposition pulsed was performed at an energy of 400 mJ, which corresponds to a 2 J cm -2 creep on the surface of the targets. The repetition frequency of the laser pulses was 10 Hz. The sum of the partial pressures of the gas mixture oxygen-nitrogen inside the chamber was 10 Pa. A substrate temperature of 500 ° C was used.

For the growth of doped layers with nitrogen, the sum of the partial pressures of the gas mixture oxygen-nitrogen remained constant at 10 Pa. partial pressure of nitrogen varied between 0 and 8 Pa while the partial oxygen pressure was the complementary value until reach the total pressure of 10 Pa.

2. Solar cell manufacturing

The most basic configuration for manufacturing of the solar cell is in a bilayer structure where a layer fine of an organic semiconductor, (e.g. conductive polymer), which select between P3HT, PCBM, MEH-PPV, etc., or mixtures thereof, is deposited by means of the spin technique coating on the N-TiO2 (see Figure 8.). To this Type of solar cells are known as hybrid solar cells. The N-TiO_ {2} in this case is in the form of a layer fine but nanostructured layers of it can be used material that allow a greater surface area such as nanoparticles, nanowires, etc. In this way the N-TiO_ {2} acts as a carrier material for electrons (ETM).

Therefore, like ETM, the N-TiO_ {2} can be used not only in one hybrid solar cell, but can also be used in a cell solar type Gratzel and in an organic solar cell. As a cape intermediate in a tandem type organic solar cell or as a layer buffer in an exciton solar cell, in which the photon absorption of light produces a pair electron-hole called exciton. The separation Efficient exciton in the electron and the gap is necessary for the energy generation. This separation is carried out in the interface between the polymer and the oxide.

3. Comparative examples

Figure 3 shows the advantage of using the TiO2 doped with nitrogen compared to TiO2 conventional. In both cases the materials were synthesized. using the PLD technique. In the first 13 hours of life of the solar cell you get a retention of properties photovoltaic of up to 80% when the TiO_2 is doped with nitrogen, while the undoped material maintains only 20% of its initial photovoltaic properties in the same period of time (in both cases using a UV filter that cuts the length UV wavelength less than 400 nm).

The metallic behavior of the doped TiO_2 in the rutile type crystalline phase it is reflected in curves IV, typically used to analyze the properties of a photovoltaic device The more "square" the curve is the greater the filling factor (FF) and the greater the efficiency of the solar cell where the photovoltaic device is used. A curve IV-straight indicates an ohmic behavior, or that the photovoltaic device is capable of transforming the photons into current and voltage but the power generation is zero or has unacceptable efficiency. Figure 4 reflects the description above, showing the IV curves of solar cells that they use the TiO_2 doped with nitrogen and without doping (both in their rutile phase). It can be seen that in the case of TiO_2 without doping the curve is practically flat (FF <25%) after analyze the solar cell in an inert atmosphere for a few hours, while in the case of N-TiO_ {2} the curve is maintained for many more hours. This behavior is attributed to the extraction of oxygen (reduction) from the structure TiO 2 crystalline during irradiation processes continued in inert atmospheres.

Figure 5 shows the behavior of the solar cells that use the low N-TiO2 conditions of continuous irradiation and nitrogen atmosphere. Be shows in each stage the corresponding curve IV. IV curves obtained at different stages of the analysis show a factor of filling greater than 30%. Even at long analysis times the answer of the doped solar cell does not exhibit ohmic behavior.

Figure 6 shows the comparison of parameters of the solar cells that use the TiO2 and the N-TiO2 observed during the first 7 hours in functioning. TiO_2 without doping has properties Very low photovoltaic (ohmic behavior) in just one pair of hours of analysis while the solar cell material using N-TiO_ {2} is clearly superior to its TiO2 homolog without doping.

Figure 7 shows the first 50 hours of life of the solar cell using N-TiO2. We observe that the parameters such as filling factor (FF), voltage (Voc) or current density (Jsc) remain reasonably stable with time. In some cases they increase, as in the case of voltage, and in another decreases a bit, as with the density of stream.

Figure 8 schematically shows the configuration of the photovoltaic device of the invention used In a solar cell.

Claims (16)

1. Procedure for obtaining TiO_ {2} nanocrystalline doped with nitrogen in the 100% rutile phase that understands:

to)

pulsed laser deposition of TiO2 on a transparent substrate electrode and type conductor fluorine doped tin oxide, where said substrate is found at a temperature between 400ºC and 600ºC,

b)

adding a gas mixture oxygen-nitrogen on the material obtained in the stage a), where the ratio of partial pressures of said gases is X: Y, with X + Y being a constant value between 1 and 15 Pa.

2. Method according to claim 1, where the substrate of stage a) is at a temperature of 500 ° C 3. Procedure according to any of the claims 1 or 2, wherein the pulsed laser deposition is performs at an energy of between 200 to 600 mJ and at a frequency of between 1 and 20 Hz. 4. Method according to claim 3, where the pulsed laser deposition is performed at an energy of 400 mJ and at a frequency of 10 Hz. 5. Method according to claim 1, where X + Y is 10 Pa. 6. Procedure according to any of the claims 1 to 5, wherein the partial pressure ratio is select between 4: 6, 3: 7 or 2: 8. 7. Method according to claim 6, where the partial pressure ratio is 2: 8. 8. TiO2 nitrogen-doped nanocrystalline obtainable by the process according to claims 1 to 7. 9. Photovoltaic device comprising the TiO2 nitrogen-doped nanocrystalline according to claim 8, a conductive polymer and a counter electrode metal. 10. Photovoltaic device according to the claim 9, wherein the conductive polymer is selected from P3HT, PCBM, MEH-PPV or mixtures thereof. 11. Photovoltaic device according to the claim 10, wherein the conductive polymer is MEH-PPV. 12. Photovoltaic device according to claims 9 to 11, wherein the metal counter electrode is select between Ag, Au, Pt, Ca or Al. 13. Photovoltaic device according to the claim 12, wherein the metal counter electrode is Ag. 14. Use of TiO2 doped with nitrogen according to claim 8 for the manufacture of a device photovoltaic 15. Use of the photovoltaic device according to the claims 9 to 13 for the manufacture of a solar cell. 16. Use according to claim 15, wherein the solar cell is selected from a Gratzel type solar cell, a hybrid solar cell or an organic solar cell.