TW201517352A - Method for manufacturing photoanode of dye-sensitized solar cell - Google Patents

Abstract

The present invention provides a method for manufacturing photo-anode of dye-sensitized solar cell. When the photoanode absorbs dye, the ambient pressure in manufacturing is decreased to be negative pressure, so as to release the air or impurity of the photo-anode without absorbed dye, thereby increasing the area of the photo-anode for absorbing dye. Furthermore, the ambient pressure in manufacturing may be increased to be positive pressure, so as to raise the diffusivity of the dye and increase the contact between dye and the photo-anode. With the manufacturing method of the present invention, the dye adsorption capacity of photo-anode can be enhanced to thus increase the photoelectric conversion efficiency of the dye-sensitized solar cell prepared by the photo-anode manufactured by the manufacturing method of the present invention.

Description

Method for preparing photoanode of dye-sensitized solar cell

The present invention relates to a method of preparing a photoanode of a dye-sensitized solar cell.

The principle of power generation of dye-sensitized solar cells is to convert the absorbed solar energy into usable and stored electrical energy using colored dyes. The photoelectric conversion operation mode is that the dye is irradiated and excited by light; the excited state molecules transfer electrons to the conductive strip of the semiconductor nanoparticle by electron transfer process. This process of charging a semiconductor by dye absorption is called "dye sensitization", and it is possible to conduct the electrons in it and to be the electric energy we need.

The process of dye-sensitized solar cell is first coated on a transparent electrode substrate and sintered to form a metal oxide (eg, titanium dioxide) nanoporous electrode, and then adsorbed with a dye, and then coated with a platinum film between the opposite electrode substrates It is sealed with an electrolyte containing iodine. The most time-consuming two parts are electrode preparation and dye impregnation. Electrode preparation takes two days, which is mainly time consuming. The dye is impregnated for about 24 hours. It is generally impregnated with dye by wet impregnation. It is also the most common dye impregnation method. It is directly immersed in the dye solution under normal pressure. The dye solution diffuses into the pores of the electrode. Inside, the electrode is allowed to adsorb the dye.

Due to the porous material sintered by titanium dioxide, the diameter of the pores belongs to the range of the mesopores (the pore diameter is in the range of 2 to 50 nm). When the general dye is impregnated, the dye solution is diffused to the pores due to the viscosity and surface tension of the dye solution. The internal influence will be affected, which will take a long time. If the above problems can be overcome, the time required for dye impregnation can be shortened, and the higher photoelectric conversion efficiency can be achieved in a shorter time.

In view of the above problems, the present invention provides a method for preparing a photoanode of a dye-sensitized solar cell, which mainly combines negative pressure impregnation, can effectively shorten the time of dye impregnation, and can also have a high dye adsorption amount of the photoanode, thereby further The dye-sensitized solar cell made of the photoanode prepared by the invention has high photoelectric conversion efficiency.

It is an object of the present invention to provide a method for preparing a photoanode of a dye-sensitized solar cell, which shortens the time of dye impregnation to shorten the preparation time of the photoanode.

The object of the present invention is to provide a method for preparing a photoanode of a dye-sensitized solar cell, wherein the photoanode prepared has a high dye adsorption amount and is used for a dye-sensitized solar cell to achieve high photoelectricity of the dye-sensitized solar cell Conversion efficiency.

In order to achieve the above-mentioned various purposes and effects, the present invention discloses a method for preparing a photoanode of a dye-sensitized solar cell, comprising: providing a conductive substrate to a dip tank, wherein one of the conductive surfaces of the conductive substrate has a semiconductor layer (metal oxide or metal oxide composite); supplying a dye solution to the dip tank, the semiconductor layer is immersed in a dye solution, and the gas of the dip tank is extracted to reduce the pressure of the dip tank to a a first pressure, the first pressure is less than 1 atmospheric pressure; and introducing a shielding gas to the dip tank to increase the pressure of the dip tank to a second pressure, the second pressure being greater than 1 atmosphere to prepare a photoanode. The shielding gas system is selected from the group consisting of carbon dioxide, nitrogen, helium, argon, and mixtures thereof.

The invention also discloses a method for preparing a photoanode of a dye-sensitized solar cell, comprising: providing a conductive substrate in a dip tank, wherein a conductive surface of the conductive substrate has a semiconductor layer (metal oxide or metal oxide) a dye solution is supplied to the dip tank, the semiconductor layer is immersed in a dye solution, and the gas of the dip tank is extracted to reduce the pressure of the dip tank to a negative pressure to prepare a photoanode.

1‧‧‧ system
10‧‧‧Dyeing tank (with agitator)
11‧‧‧Exhaust pump
12‧‧‧Protective gas storage tank
13‧‧‧Supercharged pump
14‧‧‧dye storage tank
15‧‧‧Dyner recovery unit
16‧‧‧Adjustment valve
17‧‧‧Precooler
18‧‧‧heater
19‧‧‧Filter
20‧‧‧ Jacket heater

First: a schematic diagram of a preparation system of a photoanode according to a first embodiment of the present invention;
Second drawing: it is a flow chart of the preparation method of the first embodiment of the present invention;
Third: it is a pressure change diagram of the dip tank of the first embodiment of the present invention;
Fourth drawing: a flow chart of a preparation method of a second embodiment of the present invention;
Figure 5 is a flow chart showing a preparation method of a third embodiment of the present invention;
Figure 6 is a pressure change diagram of the dip tank of the second embodiment of the present invention; and a seventh diagram: it is a flow chart of the preparation method of the fourth embodiment of the present invention.

In order to further understand and understand the features of the present invention and the effects achieved, the embodiments and the detailed descriptions are described as follows:

In the process of dye-sensitized solar cells, the photoanode is prepared by wet impregnation method. Due to the viscosity and surface tension of the dye solution, the diffusion of the dye solution into the pores will be affected by the above, which will take a long time. The dye is impregnated. In view of the above problems, the present invention provides a method for preparing a photoanode of a dye-sensitized solar cell, which mainly performs negative pressure impregnation to reduce the time of dye impregnation, shorten the preparation time of the photoanode, and increase the dye adsorption amount of the photoanode. Thereby, the photoelectric conversion efficiency of the dye-sensitized solar cell is improved.

Please refer to the first figure, which is a schematic diagram of a preparation system of a photoanode according to a first embodiment of the present invention; as shown in the figure, the embodiment provides a system 1 for preparing a photoanode of a dye-sensitized solar cell, which mainly The invention comprises a dip tank 10, an evacuation pump 11, a shielding gas storage tank 12, a booster pump 13, a dye storage tank 14, and a dye recovery device 15. The pumping pump 11 is connected to the dip tank 10 and is used for extracting the gas in the dip tank 10 to reduce the pressure of the dip tank 10, wherein the dip tank 10 further comprises a jacket heater 20, and the jacket heater 20 can be set to heat. The temperature is controlled to be between 0 degrees Celsius and 70 degrees Celsius; the shielding gas storage tank 12 is connected to the dip tank 10 through the booster pump 13, and the boosting pump 13 is extracted from the shielding gas storage tank. The protective gas in 12 is transferred to the dip tank 10 to increase the pressure dye storage tank 14 in the dip tank 10 to the dip tank 10 to provide a dye solution into the dip tank 10. The dye recovery device 15 is connected to the dip tank 10 to recover the dye solution in the dip tank 10.

The exhaust pump 11, the booster pump 13 and the dye recovery device 15 and the dip tank 10 may be provided with at least one regulating valve 16, such as a pressure reducing valve, a return valve or an expansion valve; or in the protective gas storage tank 12 A precooler 17 is disposed between the booster pump 13 and a heater 18 is disposed between the booster pump 13 and the dip tank 10; a filter 19 may be disposed between the dye recovery device 15 and the dip tank 10, the above adjustment The valve 16, the pre-cooler 17, the heater 18 and the filter 19 are common elements of the current photoanode preparation system, and will not be described herein.

The following is a detailed description of the preparation method of the present invention. Please refer to the second drawing, which is a flow chart of preparing a photoanode according to a first embodiment of the present invention. As shown in the figure, the present embodiment provides a method for preparing a photoanode. And using the above preparation system for preparation, which first performs step S10 to provide a conductive substrate in the dip tank 10, one surface of the conductive substrate has a semiconductor layer, wherein the semiconductor layer has a plurality of holes, and the material of the conductive substrate can be One selected from the group consisting of indium tin oxide, fluorine-doped tin oxide, ZnOGa ₂ O ₃ , SnO ₂ -Sb ₂ O ₃ and the above mixture, the semiconductor layer of the present embodiment is a metal oxide: titanium dioxide. Next, in step S11, the dye storage tank 14 supplies the dye solution into the dip tank 10, and the semiconductor layer is immersed in the dye solution. The dye solution is mixed with the dye and the organic solvent, and the organic solvent must dissolve the dye. The protective gas (e.g., carbon dioxide, helium, nitrogen, argon, or a mixture of the above gases) is expanded, and the dye of this embodiment uses a photosensitizing dye. The organic solvent may be selected from the group consisting of an alcohol, a fatty alkane, acetonitrile, acrylonitrile, toluene, and a mixture of the above. The organic solvent of this embodiment is selected from the group consisting of ethanol. The shielding gas system of the present embodiment is selected from carbon dioxide. The dye solution of the present embodiment is supplied to the dyeing tank 10 for the dye storage tank 14. Of course, the dye solution can also be directly added to the dip tank 10, that is, the dye solution is not provided by the dye storage tank 14. The dip tank 10 will not be described here.

Next, in step S12, the pumping pump 11 extracts the gas in the dip tank 10 to reduce the pressure of the dip tank 10 to a first pressure, wherein the first pressure is a negative pressure, that is, less than 1 atmospheric pressure, and is between 0.001 psi. Between 14.6 psi, the air or impurities in the holes of the semiconductor layer are promoted to increase the area of the holes of the semiconductor layer which can adsorb the dye, thereby increasing the amount of dye adsorption of the semiconductor layer.

Then, in step S13, the booster pump 13 extracts the shielding gas in the shielding gas storage tank 12 to the dip tank 10, and increases the pressure in the dip tank 10 to a second pressure, wherein the second pressure is greater than 1 atmospheric pressure, and Between 100 psi and 3600 psi, the phenomenon of expanding fluid is generated, so the viscosity and surface tension of the dye solution are decreased, so that the dye solution has a reduced mass transfer resistance between the holes, that is, the dye solution is promoted in the semiconductor layer. The diffusion rate between the holes and the dye in the dye solution are adsorbed by the holes of the semiconductor layer to effectively increase the rate at which the semiconductor layer adsorbs the dye. However, the preparation method of this embodiment can continuously repeat steps S12 and S13 until the semiconductor layer can no longer adsorb the dye.

After step S13 is completed, step S17 is performed to release the shielding gas in the dip tank 10, and the dye recovery device 15 recovers the dye solution contained in the shielding gas. Next, in step S18, the conductive substrate coated with the dye is taken out from the dip tank 10, and the conductive substrate is the photoanode of the dye-sensitized solar cell. Finally, in step S19, the dye recovery device 15 recovers the dye solution in the dip tank 10.

The embodiment provides a method for preparing a photoanode, which mainly comprises a negative pressure impregnation and a pressure impregnation, and the negative pressure (pressure lower than 1 atmosphere) impregnation portion of the present invention uses the evacuation pump 11 to reduce the dip tank 10 The internal gas pressure mainly controls the pressure in the dip tank 10 to a negative pressure, and releases air or impurities in the pores of the semiconductor layer to which the dye is to be adsorbed, and increases the area of the semiconductor layer capable of adsorbing the dye, and the negative pressure The pressure is higher than the saturated vapor pressure of the dye solution in the dyeing tank to avoid rapid evaporation of the dye solvent during the impregnation process. In addition, the dye solution is depleted of the discharge dip tank 10 when the pressure is released or reduced to a negative pressure, so that it is preferable to impregnate the semiconductor layer with the dye solution during the entire impregnation process.

Subsequently, the pressurized impregnation is carried out, and the pressurized impregnation portion is passed through the shielding gas into the dip tank 10 to increase the gas pressure in the dip tank 10, so that the diffusion coefficient of the dye in the dip tank 10 rises as the pressure increases. Increasing the pressure in the dip tank 10 to a positive pressure to generate an expanding fluid, so that the dye solution rapidly diffuses between the holes of the semiconductor layer, so that the semiconductor layer rapidly adsorbs the dye in the dye solution, as can be seen from the above, The preparation method of the present embodiment not only effectively increases the rate at which the semiconductor layer adsorbs the dye, but also makes the semiconductor layer have a high dye adsorption amount, and the dye-sensitized solar cell prepared through the photoanode prepared above has good photoelectric conversion efficiency.

There is provided a photoanode according to the preparation method of the first embodiment, wherein the dye solution dye uses D719 (photosensitive dye), the dye concentration is, and the organic solvent of the dye solution uses absolute alcohol. Please refer to the third figure, which is a pressure change diagram of the dip tank according to the first embodiment of the present invention; the temperature of the jacket heater is set to 40 ° C, and then the pressure drop in the dip tank is first shown as shown in the figure. To the first pressure (negative pressure, 5.8 psi) and maintained for 30 minutes (step S12); then increase the pressure in the dip tank to a second pressure (positive pressure, 900 psi) (step S13), and then repeatedly control the dip tank The pressure is switched to a first pressure or a second pressure, and the above pressure maintenance time includes a time of boosting or depressurizing. During the whole operation, the temperature in the dip tank is controlled at the same time, and the temperature in the dip tank is changed between 10 ° C and 55 ° C. The photoanode can be prepared by preparing the above parameters in only 1.5 hours. The thickness of the photoanode is about 15 μm, and the area of the photoanode measured by a single measurement is 16 mm 2 (4 mm x 4 mm), and the photoelectric conversion efficiency value is calculated on average using a plurality of sets of sample data (excluding poor packaging, liquid leakage, etc.). The dye-sensitized solar cell was irradiated with an incident light intensity of 100 mW/cm 2 using a solar light source simulator, and photoelectric conversion efficiency values were calculated after Isc, Voc, and FF were measured. The formula for the photoelectric conversion efficiency value is as follows:
Efficiency value = Isc * Voc * FF / Pin, where Isc is the short circuit current, Voc is the open circuit voltage, FF is the fill factor, and Pin is the incident light power.

The photoanode prepared by the above parameters was prepared into a dye-sensitized solar cell, and the dye-sensitized solar cell was tested to have a photoelectric conversion efficiency value of 8.15%. The photoanode is prepared by a general impregnation method, and the semiconductor layer is mainly impregnated into the dye solution at atmospheric pressure and room temperature. This process takes 10 hours to complete the preparation of the photoanode, and the photoanode is prepared into a dye-sensitized solar cell. After the above test, the photoelectric conversion efficiency of the dye-sensitized solar cell was 7.83%. Obviously, the photoelectric conversion efficiency of the dye-sensitized solar cell prepared by the photoanode prepared in the first embodiment was prepared by the general impregnation method. The dye-sensitized solar cell made of the photoanode has good photoelectric conversion efficiency, and the preparation time of the photoanode of the first embodiment is also less than that of the photoanode prepared by the general impregnation method, so the present embodiment is used. The preparation method provided not only improves the photoelectric conversion efficiency of the dye-sensitized solar cell, but also greatly reduces the preparation time, thereby saving the preparation cost.

Please refer to the fourth figure, which is a flow chart of the preparation method of the second embodiment of the present invention; as shown in the figure, the preparation method of the embodiment differs from the preparation method of the first embodiment in the pressure adjustment of the dip tank, and The pressure adjustment of the dip tank of the embodiment first performs step S12 and step S13. After step S13 is performed, step S14 is performed to adjust the pressure in the dip tank to maintain the pressure in the dip tank at a predetermined positive pressure (ie, greater than 1 atmospheric pressure, between 100 psi and 3600 psi), wherein the pressure in the dip tank can be adjusted to increase the pressure of the dip tank by introducing a shielding gas to the dip tank, or by discharging the dip tank through a regulating valve (pressure reducing valve) The gas is used to reduce the pressure of the dip tank. Steps S12 to S14 may then be repeatedly performed or step S12 and step S14 (see FIG. 5) may be repeatedly performed until the semiconductor layer is unable to adsorb the dye.

The following is a description of the preparation of the photoanode according to the preparation method of the second embodiment, wherein the dye solution dye is D719, the dye concentration is, and the organic solvent of the dye solution is absolute alcohol. Please refer to the sixth figure, which is a pressure change diagram of the dip tank according to the second embodiment of the present invention; the temperature of the jacket heater is set to 40 ° C, and then the pressure drop in the dip tank is first shown as shown in the figure. To the first pressure (negative pressure, 5.8 psi) and maintained for 30 minutes (step S12); then increase the pressure in the dip tank to a second pressure (positive pressure, 900 psi) and maintain for 15 minutes (step S13), then lower The pressure in the dip tank is to a third pressure (positive pressure, 400 psi), the third pressure is less than the second pressure, and is greater than 1 atmosphere, and maintained for 30 minutes (step S14); then the pressure in the dip tank is lowered to the first Pressure (negative pressure, 5.8 psi) (step S12), and maintained for 30 minutes; finally increase the pressure of the dip tank to a fourth pressure (positive pressure, 200 psi) (step S14), the above pressure maintenance time includes boost or depressurization time. It can be seen from the above that the present embodiment mainly controls the pressure in the dip tank to be switched to positive pressure or negative pressure, and the pressure in the dip tank can be adjusted before the step S12 is repeatedly performed, and the pressure in the dip tank is predetermined. Positive pressure (such as the third pressure and the fourth pressure), the temperature in the dip tank is controlled at the same time during the whole operation, so that the temperature change in the dip tank is between 20 ° C and 50 ° C, using the above parameters It takes only 2.5 hours to prepare a photoanode.

However, a dye-sensitized solar cell is prepared by the photoanode prepared by the preparation method of the second embodiment. After the dye-sensitized solar cell is tested, the photoelectric conversion efficiency value of the dye-sensitized solar cell is 8.011%; The dye-sensitized solar cell produced by the photoanode prepared by the method has a high photoelectric conversion efficiency value, and the preparation method using the second embodiment can be short compared to the use of the general impregnation method using the preparation method of the second embodiment. The time for preparing the photoanode effectively improves the preparation efficiency and reduces the preparation cost.

It can be seen from the above that the preparation method of the present invention can effectively shorten the impregnation time, that is, shorten the preparation time and make the prepared photoanode high, as long as the pressure of the dip tank is controlled during the preparation process to switch between the negative pressure and the positive pressure. A dye-sensitized solar cell with photoelectric conversion efficiency.

In addition, please refer to the seventh figure, which is a flowchart of the preparation method of the fourth embodiment of the present invention; as shown in the figure, the preparation method of the present embodiment is different from the preparation method of the first embodiment in that the embodiment is The preparation method omits the pressurized impregnation portion, that is, the step S13 is omitted, and the preparation method of the present embodiment only performs the negative pressure impregnation, but the time required to prepare the photoanode by the preparation method of the present embodiment is 2 hours (the pressure maintenance time includes the pressure increase). With the time of depressurization, it takes less time to prepare the photoanode than the general impregnation method. At the same time, a dye-sensitized solar cell was prepared by using the photoanode prepared by the preparation method of the present embodiment, and the dye-sensitized solar cell was tested, and the photoelectric conversion efficiency value was 7.921%, which was also compared with the photoanode prepared by the general impregnation method. The dye-sensitized solar cell produced has a high photoelectric conversion efficiency value.

In summary, the present invention provides a method for preparing a photoanode of a dye-sensitized solar cell, which mainly performs negative pressure impregnation, that is, controls the pressure of the preparation environment to perform dye impregnation under a negative pressure state, so that air or impurities in the semiconductor layer are made. Released to increase the dye adsorption area of the semiconductor layer, thereby increasing the amount of dye adsorption of the photoanode. It can also be combined with pressurized impregnation to control the pressure of the preparation environment in a positive pressure state to reduce the mass transfer resistance of the dye solution between the photoanodes, thereby increasing the diffusion rate of the dye solution in the photoanode and shortening the preparation time of the photoanode.

The above is only the embodiment of the present invention, and is not intended to limit the scope of the present invention, and the variations, modifications, and modifications of the shapes, structures, features, and spirits described in the claims of the present invention are It should be included in the scope of the patent application of the present invention.

Claims (24)

A method for preparing a photoanode of a dye-sensitized solar cell, comprising:
Providing a conductive substrate on a dip tank having a semiconductor layer on one surface thereof;
Supplying a dye solution to the dip tank, the semiconductor layer being immersed in the dye solution;
Extracting the gas of the dip tank to reduce the pressure in the dip tank to a first pressure, the first pressure is less than 1 atmosphere; and introducing a shielding gas to the dip tank to increase the pressure in the dip tank to a first The second pressure is greater than 1 atmospheric pressure. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the dye of the dye solution is a photosensitizing dye. The method for producing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the shielding gas system is selected from the group consisting of carbon dioxide, nitrogen, helium, argon, and a mixture thereof. The method for producing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the semiconductor layer is a metal oxide. The method for producing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the semiconductor layer is a metal oxide composite. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the temperature in the dip tank is between 0 ° C and 70 ° C. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the step of reducing the pressure in the dip tank to the first pressure and increasing the pressure in the dip tank to the first The second pressure step. A method of producing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the first pressure is between 0.001 psi and 14.6 psi. The method of preparing a photoanode of a dye-sensitized solar cell according to claim 1, wherein the second pressure system is between 100 psi and 3600 psi. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 1, further comprising:
The pressure in the dip tank is adjusted and the pressure in the dip tank is greater than 1 atmosphere. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 10, wherein the pressure of the dip tank is between 100 psi and 3600 psi. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 10, wherein the step of reducing the pressure in the dip tank to the first pressure is repeated, and the pressure in the dip tank is increased to the first The second pressure step and the step of adjusting the pressure in the dip tank. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 10, wherein the step of reducing the pressure in the dip tank to the first pressure and the step of adjusting the pressure in the dip tank are repeated. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 10, wherein the step of adjusting the pressure in the dip tank comprises:
The shielding gas is introduced into the dip tank to increase the pressure in the dip tank. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 10, wherein the step of adjusting the pressure of the dip tank further comprises:
The gas in the dip tank is discharged to reduce the pressure in the dip tank. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 1, further comprising:
Preparing a photoanode in the dip tank;
Release the shielding gas in the dip tank, and recover the dye solution contained in the shielding gas;
Removing the photoanode from the dip tank; and recovering the dye solution in the dip tank. A method for preparing a photoanode of a dye-sensitized solar cell, comprising:
Providing a conductive substrate on a dip tank having a semiconductor layer on one surface thereof;
Supplying a dye solution to the dip tank, the semiconductor layer is immersed in the dye solution; and extracting the gas of the dip tank to reduce the pressure in the dip tank to a negative pressure. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 17, further comprising:
Preparing a photoanode in the dip tank;
Release the shielding gas in the dip tank, and recover the dye solution contained in the shielding gas;
Removing the photoanode from the dip tank; and recovering the dye solution in the dip tank. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 17, wherein the dye in the dye solution is a photosensitizing dye. The method for producing a photoanode of a dye-sensitized solar cell according to claim 17, wherein the shielding gas system is selected from the group consisting of carbon dioxide, nitrogen, helium, argon, and a mixture thereof. The method for producing a photoanode of a dye-sensitized solar cell according to claim 17, wherein the semiconductor layer is a metal oxide. The method for producing a photoanode of a dye-sensitized solar cell according to claim 17, wherein the semiconductor layer is a metal oxide composite. The method for preparing a photoanode of a dye-sensitized solar cell according to claim 17, wherein the temperature in the dip tank is between 0 ° C and 70 ° C. The method of preparing a photoanode of a dye-sensitized solar cell according to claim 17, wherein the pressure of the dip tank is a negative pressure of between 0.001 psi and 14.6 psi.