Tungstate ion solution and hybrid photovoltaic device

Abstract

The invention concerns a solution of tungstate ions W.sup.6+ (VI) comprising as solvent at least one optionally partly etherified polyalcohol, a preparation method and uses thereof. The invention further concerns a layer comprising at least one tungsten oxide WO.sub.z comprising one or more polyoxotungstate complexes, methods for preparing the same and uses thereof, and in particular a photovoltaic device comprising said layer of material.

Claims

1. A solution of tungstate ions W.sup.6+ (VI) and counter-ions of the tungstate ions consisting of: as solvent at least one polyalcohol, optionally partly etherified, tungstate ions in the form of one or more polyoxotungstate complexes selected from the group consisting of [W.sub.6O.sub.19].sup.2−, [W.sub.10O.sub.32].sup.4− and [W.sub.7O.sub.24].sup.6−; and H.sup.+ protons, as counter ions of the tungstate ions.

2. The solution according to claim 1, wherein said at least one polyalcohol is a glycolic compound, optionally partly etherified.

3. The solution according to claim 1, wherein said at least one polyalcohol is ethylene glycol.

4. The solution according to claim 1, wherein said solution has a concentration of tungstate ions of 0.001 to 1 mol.Math.L.sup.−1.

5. The solution according to claim 1, wherein said solution is limpid.

6. The solution according to claim 1, wherein said solution has a dynamic viscosity of between 2 and 64 cP (mPa.Math.s) at 25° C. and at atmospheric pressure (101 325 Pa).

7. The solution according to claim 1, wherein said solution has a dynamic viscosity of between 4 and 54 cP (mPa.Math.s), at 25° C. and at atmospheric pressure (101325 Pa).

8. The solution according to claim 1, wherein said solution has a dynamic viscosity of between 4 and 25 cP (mPa.Math.s), at 25° C. and at atmospheric pressure (101325 Pa).

9. The solution according to claim 1, wherein said solution has a dynamic viscosity of between 8 and 19 cP (mPa.Math.s), at 25° C. and at atmospheric pressure (101325 Pa).

10. A method for preparing the solution according to claim 1, comprising increasing the temperature of a suspension or gel of H.sub.2WO.sub.4 as a polyoxotungstate complex precursor in a polyalcohol solvent.

11. The method according to claim 10, wherein the formation of the polyoxotungstate complex(es) is obtained by heating at a temperature higher than 120° C.

12. A method for preparing a layer of tungsten oxide WO.sub.z deposited on a solid substrate comprising deposition of the solution according to claim 1 on a solid substrate via a wet process and removal of the solvent from the solution in order to obtain a film of tungsten oxide WO.sub.z on the solid substrate, wherein z is greater than 2.7.

13. The method according to claim 11, wherein the removal of the solvent is conducted at a temperature higher than the boiling point of the solvent.

14. The method according to claim 11, wherein the deposition is performed by printing or coating selected from the group consisting of spin coating, ink jet printing, screen printing, heliogravure, slot die, roll to roll, or tape casting.

Description

(1) In the Figures:

(2) FIG. 1 illustrates titrimetric assay with sodium hydroxide (NaOH) of 20 ml of transparent solution synthesized in ethylene glycol according to Example 1.1 (dashed line: [H.sub.2WO.sub.4]=0.05 mol/L; solid line:[H.sub.2WO.sub.4]=0.1 mol/L).

(3) FIG. 2 illustrates the curves of standard architecture cells, glass/ITO/WO.sub.z or PEDOT:PSS/P3HT:PCBM/Ca/AI under AM 1.5 illumination at two different drying temperatures (150° C. and 225° C.) of the deposited film.

(4) FIG. 3 illustrates XPS 4f level spectra of tungsten, energy calibration on C1s pollution at 284.7 eV, layers deposited on ITO treated at 150° C. (right) and 225° C. (left), for one hour.

(5) FIG. 4 illustrates XPS 4f electron level spectra of tungsten for cells analysed post-mortem, dried at 250° C. for one hour. The amounts of tungsten in oxidation state 5 are limited to 6 and 3% respectively, the conversion yield is directly correlated with this reduction percentage.

(6) FIG. 5 provides scanning microscope images of the surface of films dried at 50° C., 10 minutes, obtained with the parent solution of tungsten oxide percursors in propylene glycol solvent.

(7) The term «according to the invention» refers to all the aspects and embodiments of the invention, including the examples and preferred embodiments thereof, including any of the combinations thereof with no limitation.

(8) Other objectives, characteristics and advantages of the invention will become clear to persons skilled in the art on reading the explanatory description referring to examples that are given solely for illustration purposes and in no manner limit the scope of the invention.

(9) The examples form an integral part of the present invention, and any characteristic that appears to be novel compared with any prior art technique and arising from the description as a whole, including the examples, forms an integral part of the purpose of the invention and the invention in general.

(10) Therefore, the extent of the scope of each example is general.

(11) Also, in the examples all the percentages are weight percentages unless otherwise indicated, temperature is expressed in degrees Celsius unless otherwise indicated, and pressure is atmospheric pressure unless otherwise indicated.

EXAMPLES

Example 1: Preparation of Tungstate Ion Solutions According to the Invention

(12) Different tungsten reagents were tested: Na.sub.2WO.sub.4, (NH.sub.4).sub.2WO.sub.4 and H.sub.2WO.sub.4 and different glycols such as propylene glycol, ethylene glycol, propylene glycol propyl ether. The tested tungsten concentration ranged from 0.05 to 0.20 mol.Math.L.sup.−1. After agitation for 2 h, the suspension or gel of yellow colour was placed in an autoclave at a temperature of 120° C., for 24 h. The final solutions obtained were always yellow in colour but transparent and able to be used for at least 6 months. This solution is employed as such for the depositing of interface layers.

Example 1.1

(13) 0.374 g of H.sub.2WO.sub.4 and 30 mL of ethylene glycol are placed under magnetic stirring for 2 h; the mixture leads to a gel of yellow colour that is placed in an autoclave at 120° C. for 24 h.

Example 1.2

(14) The same protocol as in Example 1.1 is followed, using propylene glycol as solvent instead of ethylene glycol.

Example 1.3

(15) 0.494 g of Na.sub.2WO.sub.4 and 30 mL of ethylene glycol are placed under magnetic agitation for 2 h, the mixture leads to a gel of yellow colour that is placed in an autoclave at 120° C. for 24 h.

(16) The solutions obtained in Examples 1.1., 1.2. and 1.3. are transparent and stable as specified in the introduction to the above examples.

(17) Dynamic light scattering performed on the solutions obtained confirms the absence of any colloids having a hydrodynamic diameter greater than 1 nm.

(18) The polyoxotungstate complexes formed in solution are evidenced as follows:

(19) Titrimetric assay is performed on 20 mL of the transparent solution synthesized according to the above examples. Changes in the pH curve are monitored as a function of the volume of added base (sodium hydroxide). The curves have the same shape irrespective of the concentration of the precursor, in the range of 0.05 to 0.1 mol/L. Three points of equivalence are identified each of which corresponds to neutralisation of a different acidity. The number of moles of neutralised H.sup.+ ions relative to the number of moles of tungsten contained in the solution confirms the presence of different polyoxotungstate complexes in each pH domain (Table 1 below and FIG. 1).

(20) TABLE-US-00001 TABLE 1 polyoxotungstate complexes formed in solution (computed from points of equivalence on the black curve in FIG. 1) Polyanions in the solvent Equivalence X before each equivalence 1 ~0.6 W.sub.6O.sub.19.sup.2−, 2 ~1.2 W.sub.10O.sub.32.sup.4−, 3 ~2.3 W.sub.7O.sub.24.sup.6−, X represents the number of moles of H.sup.+ ions per mole of tungsten ion.

(21) The compositions inferred from XPS surface analyses differ from those of the prior art: WO.sub.2.985 and WO.sub.2.97 for 3 and 6% W.sup.5+ respectively, compared with WO.sub.2.7 for example for the sol-gel film of Guillain et al.

Example 2: Use of the Solution to Fabricate an Organic Photovoltaic Cell

(22) A conventional spin-coating is used to deposit the solution prepared in Example 1.1 to prepare a cell of standard architecture of glass/ITO/WO.sub.z/P3HT:PCBM/Ca/Al type. In parallel, following a similar operating mode, a cell is prepared having the architecture glass/ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al.

(23) The changes in short-circuit current density Jsc (mA/cm.sup.2) are then monitored as a function of changes in open-circuit voltage V.sub.oc(V).

(24) As illustrated in FIG. 2, the increase in drying temperature allows an increase in conversion yield up to a value higher than 3%. The improvement chiefly concerns open-circuit voltage and form factor. In comparison with a p-type conducting polymer conventionally used (PEDOT:PSS), irrespective of drying temperature, current density is greater on account of the transparency of the tungsten oxide layer prepared according to the present invention.

(25) Table 2 below summarises the characteristics arising from intensity/voltage curves performed on the aforementioned standard architectures.

(26) TABLE-US-00002 TABLE 2 p-layer Voc (mV) Jsc (mA/cm.sup.2) FF (%) PCE (%) PEDOT:PSS 579.2 8.78 63.5 3.30 WO.sub.z150° C. 444.4 9.39 53.4 2.23 WO.sub.z225° C. 547.2 9.53 60.1 3.13

(27) FIG. 3 illustrates an XPS spectrum of a tungsten oxide layer deposited on ITO, dried at 150° C. (right) and 225° C. (left), for one hour.

(28) FIG. 4 illustrates an XPS spectrum of two cells analysed post-mortem after a drying time of one hour at 250° C. (on the left 6% W.sup.5+ and on the right the spectrum with less than 3% W.sup.5+).

Example 3: Methods for Forming the p-Interface Layer in a Standard OPV Architecture of PIN Type

(29) A tungstate ion solution according to the invention (in particular Example 1.1) is deposited on a transparent conductor or semiconductor substrate. The deposit methods used can be selected from among spin coating, coating/blading (Doctor Blade, slot die, tape casting), printing methods (ink jet, screen printing, heliogravure . . . ), etc. It is also possible to apply patterning to a substrate.

(30) From a previously described tungstate ion solution according to the invention, the tungsten oxide layer is formed by ink jet printing, in particular by spin coating or coating, using a print platform integrating print modules generating droplets of 1 pL to 100 pL (pL: pico-litre). Depending on the type of underlying layer, print strategies are adapted to obtain homogeneous layers of desired thickness and the tungsten oxide concentration of the formulation used. The thermal annealing applied is similar to that used for the layers formed by coating. The layer according to the invention is used on a semiconductor surface of organic or hybrid solar cells.

Example 4: Preparation of Organic Photovoltaic Cells

(31) The prepared photovoltaic cells comprise a substrate in glass or plastic (PET, PEN), coated with an ITO layer itself coated with an n-semiconductor oxide such as TiO.sub.2. The latter is coated with an active film composed of a P3HT/PCBM mixture. The active layer is successively coated with a tungsten oxide layer prepared according to the present invention to form an interface layer, then with a silver anode. The configuration of the cell is therefore the following: substrate/ITO/TiO.sub.2/polymer+PCBM/WO.sub.z/anode.

(32) The conditions for spin coating deposition are the following:

(33) Step 1: A layer of TiO.sub.2 is prepared from a precursor solution by spin coating (see method in international application WO2013/050222). Coating time is 60 s at 1000 rpm.sup.−1 then 30 s at 2000 rpm.sup.−1. The thickness of the layer obtained is about 50 nm, deposition being performed in air, followed by drying over a hot plate at 150° C. for 1 h.

(34) Step 2: Deposition of the active layer is performed by spin coating a composition of P3HT/PCBM, on the n layer, at about 1500 rpm.sup.−1 for 40 s then 2000 rpm.sup.−1 for 35 s.

(35) Step 3: A tungsten oxide layer (about 50 nm) is deposited by spin coating at 2000 rpm.sup.−1 for 25 s then 3000 rpm.sup.−1 for 25 s.

(36) The cell thus prepared is annealed in a glove box for 15 mn at 150° C.

(37) Step 4: A silver electrode (100 nm) is then deposited by vacuum evaporation.

(38) The cells are characterized in a glove box under a controlled atmosphere. The current-voltage characteristics (I(V)) are recorded on a Keithley® SMU 2400 instrument under AM1.5 illumination at a power of 1000 W.Math.m.sup.−2.

Example 5

(39) The inventors have sought to improve the method for depositing the tungsten oxide layer to obtain optimal open-circuit voltage and optimal photovoltaic conversion efficiency.

(40) Different drying temperatures were tested.

(41) It appears that, on drying, a portion of the tungsten in oxidation state 6 is reduced to oxidation state 5. This mixed valence of the transition metal modifies the electronic properties of the semiconductor, which is undesirable. It is sought to prevent the presence of tungsten in oxidation state 5. The increase in temperature allows a decrease in the amount of reduced tungsten. This observation allows correlation between the best results of the photovoltaic cells and a lesser quantity of W.sup.5+. XPS studies on photovoltaic cells analysed post-mortem confirmed this interpretation (FIG. 4).

Example 6

(42) The inventors also used different concentrations of polyoxotungstate solution to optimise the homogeneity of the deposited films to prepare a photovoltaic cell. The tungstate ion solutions were concentrated before deposition on the substrate.

(43) It was observed that the concentrated solutions always maintain their stability and transparency after evaporation of one half of the amount of solvent present. Solutions synthesized before and after «pre-concentration» were also deposited in thin films by spin coating onto ITO glass. FIG. 5 gives scanning microscope images of the surface of films dried at 50° C., 10 minutes, obtained with the parent solution of tungsten oxide precursors in propylene glycol solvent in an autoclave at 120° C. for 24 hours (left image), and with this pre-concentrated solution (right image) before deposition. The results obtained show that the films are more homogeneous when the solutions are more concentrated.