Method to enhance the efficiency of dye sensitized and perovskite solar cell

Abstract

The present disclosure relates to a method to enhance the efficiency and reduce interfacial charge transfer resistance in dye sensitized solar cell (DSSC) and a perovskite solar cell (PSC) by fabricating with Mg and La doped photoanodes. Mg and La co-doped into TiO.sub.2 has shown more than 20% efficiency than pristine TiO.sub.2 and more than 5% higher efficiency than the single doping of 1% La and Mg in TiO.sub.2 cells. Thus, the present disclosure relates to an improved photoanode material to be used in solar cells.

Claims

1. A method of synthesizing a solar cell, comprising: synthesizing a metal co-doped powder, wherein the synthesizing is done using following steps: creating a first mixture by stirring 3 gm Titanium (IV) isopropoxide and 100 ml propanol for 30 minutes at room temperature and then adding 0.0113 g Mg and 0.0167 g La; creating a second mixture by stirring 5 ml distilled water and 50 ml propanol for 30minutes at room temperature; creating a third mixture by adding 5 drops of HCL in the first mixture; creating a fourth mixture by adding the second mixture into the third mixture dropwise until a gel is formed; and creating fifth mixture by drying the fourth mixture at 100 degrees centigrade until it becomes a powder; synthesizing a photoanode with the metal co-doped powder; and assembling the solar cell with the photoanode, wherein assembling the solar cell with the photoanode enhances the efficacy of the solar cell, the assembling includes creating a sixth mixture by ball milling 0.5 gm the fifth mixture, 4 ml absolute ethanol, 24 drops of acetic acid, 2 drops of Triton X for 20 hours; creating a seventh mixture by sonicating the sixth mixture for 10 minutes; creating an eighth mixture by stirring the seventh mixture until the seventh mixture becomes more viscous; and creating photoanode by putting the eighth mixture on a conducting glass using doctor blading method and heating the conducting glass at 450 degrees centigrade.

2. The method of claim 1, wherein the method further comprises synthesizing a metal co-doped titanium oxide (TiO.sub.2) powder.

3. The method of claim 2, wherein the metal co-doped TiO.sub.2 powder is synthesized by a sol-gel method.

4. The method of claim 2, wherein the metal co-doped TiO.sub.2 powder is synthesized by a hydrothermal method.

5. The method of claim 1, wherein the method comprises Mg and La co-doped powder.

6. The method of claim 1, wherein the solar cell is a dye-sensitized solar cell.

7. The method of claim 5, wherein the method comprises 0.5% Mg and 0.5% La co-doped powder.

8. The method of claim 1, wherein the Mg and La salt vary from 0.01 to 3 mole percent concentration of the impurity with respect to Ti.sup.4+ions for synthesizing metal co-doped TiO.sub.2 powder.

9. The method of claim 1, wherein the method shows a 20% improvement over cells with pure TiO.sub.2 photoanode only.

10. A method of synthesizing a perovskite solar cell, comprising: synthesizing a metal co-doped powder; wherein the synthesizing is done using following steps: creating a first mixture by stirring 3 gm Titanium (IV) isopropoxide and 100 ml propanol for 30 minutes at room temperature and then adding 0.0113 g Mg and 0.0167 g La; creating a second mixture by stirring 5 ml distilled water and 50 ml propanol for 30 minutes at room temperature; creating a third mixture by adding 5 drops of HCL in the first mixture; creating a fourth mixture by adding the second mixture into the third mixture dropwise until a gel is formed; and creating fifth mixture by drying the fourth mixture at 100 degrees centigrade until it becomes a powder; synthesizing a photoanode with the metal co-doped powder; and assembling the perovskite solar cell with the photoanode, wherein assembling the perovskite solar cell with the photoanode enhances the efficacy of the solar cell.

11. The method of claim 10, wherein the method further comprises synthesizing a metal co-doped titanium dioxide (TiO.sub.2) powder.

12. The method of claim 11, wherein the metal co-doped TiO.sub.2 powder is synthesized by a sol-gel method.

13. The method of claim 11, wherein the metal co-doped TiO.sub.2 powder is synthesized by a hydrothermal method.

14. The method of claim 10, wherein the method comprises Mg and La co-doped powder.

15. The method of claim 14, wherein the method comprises 0.1-0.5% Mg and 0.1-0.5% La co-doped powder.

16. The method of claim 10, wherein the Mg and La salt vary from 0.01 to 3 mole percent concentration of the impurity with respect to Ti.sup.4+ions for synthesizing metal co-doped TiO.sub.2 powder.

17. The method of claim 10, wherein the method shows a 20% improvement over cells with pure TiO.sub.2 photoanode only.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments are illustrated by way of example and no limitation in the accompanying figures, like references indicate similar elements and in which:

(2) FIG. 1 shows X-ray diffraction (XRD) analysis of powders of 0.5 and 1 mol % Mg and La co-doped TiO.sub.2, 1 mol % La and Mg single doped TiO.sub.2.

(3) FIG. 2 shows current density Vs voltage (J-V) curves from a set of DSSC comprising cells with 0.5 and 1 mol % Mg and La co-doped TiO.sub.2 anodes, 1 mol % Mg and La single doped TiO.sub.2 anodes and pure TiO.sub.2 anodes.

(4) FIG. 3 shows recombination resistance plotted against bias voltages for the co-doped, single doped and pure TiO.sub.2 DSSC measured using EIS measurements.

DETAILED DESCRIPTION

(5) The present invention discloses a DSSC and a PSC with higher efficiency, enhanced charge transfer and reduced recombination using fabricated Mg and La photoanodes. The DSSC and PSC with Mg and La TiO.sub.2 anodes help in increasing dye absorption and solve electron injection problems faced by single Mg doped TiO.sub.2 anodes (Zhang et al., 2011); in reducing the recombination of injected free electrons with the electrolyte due to negative band edge shift caused by Mg ions and generate trapped states for longer lifetime of excited electrons caused by the presence of defects and the presence of interstitial La ions with empty f orbitals.

(6) As an experimental example to the disclosed method, the short circuit current (J.sub.sc) for the samples was significantly enhanced with a high open circuit voltage (V.sub.oc) thereby producing cells with 20% higher efficiencies than those with our TiO.sub.2 anodes and 5% higher than La doped TiO.sub.2 anodes. The transport properties of La and Mg co-doped TiO.sub.2 cells were observed with the help of Electrochemical Impedance Spectroscopy (EIS) measurements to have higher charge transfer resistances and longer electron lifetime than their doping counterparts, attributed to the negative band edge shifts and reduced trap defects concentration caused by the presence of the two metal ions within the lattice of the TiO.sub.2. Cells with 0.5% Mg and La co-doped TiO.sub.2 photoanodes gave 8.04% efficiency, 5% higher than La doped TiO.sub.2 cells (efficiency 7.7%), 15% higher than Mg doped TiO.sub.2 photoanodes (efficiency 6.9) and 20% higher than pristine TiO.sub.2 (efficiency 6.7%).

(7) Materials

(8) The following chemicals are used in this work: Titanium (IV) isopropoxide, TTIP (97%, Sigma-Aldrich), 2-propanol (97%, Sigma-Aldrich), Magnesium Acetate tetrahydrate (CH.sub.3COO).sub.2Mg.4H.sub.2O (>99.9%, Sigma Aldrich), Lanthanum (III) acetate hydrate, La(CH.sub.3CO.sub.2).sub.3.xH.sub.2O (>99.9%, Sigma-Aldrich), Absolute Ethanol (>99%, Sigma-Aldrich) and deionized water (DI).

(9) Synthesis of Mg and La Co-Doped, Single Doped and Pure TiO.sub.2 Powders:

(10) A sol gel method was used to synthesize pure TiO.sub.2 and the single and double Mg and La doped TiO.sub.2. To prepare pure TiO.sub.2, 3.0 g TTIP was added to 100 mL of IPA (to make 0.1M TTIP) and stirred for 30 mins. Next, 5 drops of HCl was added to the solution, and stirred for 10 to 15 mins. 15 mL of DI H.sub.2O/IPA mixture of 1:10 v:v prepared separately was added drop wise to the TTIP/IPA mixture allowed to stirred for another 3 to 4 hours, until the solution became cloudy. The resulting slightly cloudy solution was kept overnight for jellification. The final jell was dried in an oven, and a white powder was obtained and washed many times with absolute alcohol before drying. Next, the powder was crystallized by sintering in a furnace at 500 C. with a ramp time of 2 hrs and dwelling time of 5 hrs. For the single and co doped powders, required amounts were measured and heated in an oven at 100 C. for more than 2 hrs to remove all water of crystallization before adding to the TTIP/IPA mixture.

(11) Preparation of Photoanodes:

(12) 4 g of the crystalized metal doped anatase powders were mixed with 24 drops of acetic acid [>99%, Sigma-Aldrich], a drop of Triton X-100 [laboratory grade, Sigma-Aldrich] and 5 mL of absolute ethanol was added to the paste. The mixture was then sealed tight and ball milled overnight. The resulting paste was removed, and stirred in an agate mortar until an appropriate viscosity was achieved before pasting onto the FTO conducting glass substrate.

(13) Assembly of Photoanodes into a Cell:

(14) Fluorine doped Tin oxide coated glass [Nippon sheet glass 10-12 sq.sup.1] was carefully cut in to flakes of 2 cm0.5 cm. The glass flakes were washed in isopropanol and in ethanol for 10 mins consecutively using an ultrasound bath. After washing, the cleaned and dried flakes were heated on a heating tub at 300 C. for another 10 mins and kept ready for pasting on. The paste with required viscosity was doctor bladed on the clean FTO conducting glass substrates and left to dry before scrapping to a required active area (0.50.5) cm.sup.2. All prepared anodes were ramped to 500 C. in the furnace in air for 30 mins and a dwelling time of 10 mins to remove all carbon compounds. The additive-free electrodes were dipped in ruthenium N719 dye [((cis-diisothiocyanato-bis (2,2-bipyridyl-4,4-dicarboxylato) ruthenium(II) bis (tetrabutylammonium), DYSOL, Switzerland, Australia] overnight at room temperature after which they were removed, washed with excess ethanol and dried. The dry dyed films were used directly as photo-anodes for DSSC, where each was combined with a Pt-coated glass counter electrode, and sandwiched with electrolyte [EL-HPE, High performance Electrolyte, DYSOL]. The electrolyte was introduced by placing a drop on the active area of the photo-anode, and the counter electrode placed carefully on it, after which both electrodes were held firm together by crocodile clips. Current-voltage and EIS measurements were later performed after a stabilization time of 20 mins in normal room exposure.

(15) Characterization of the Assembled Cell:

(16) To confirm that the impurities do not affect the phase composition of the TiO.sub.2 and to ensure (but not quantified) that some of the metal impurities actually got substituted into the TiO.sub.2 lattice structure, XRD measurements were carried out with SHIMADZU Maxima-X, 7000 series at a voltage and current of 40 kv and 30 mA, respectively. The XRD is set to scan at a step size of 0.02 deg, and the scan speed of 2 deg/min over 202theta80 using Cuk radiation of 1.54060 A. The Voc, FF and Jsc of the assembled cells are estimated from current-voltage using a Kiethley Source meter (2400, KIETHLEY source meter) after exposing the cells to approximately 0.9 sunlight (100 mW/cm.sup.2) under white light condition.

(17) The electronic processes taking place within the DSSC; charge transport resistance and recombination kinetics, are analyzed by Electrochemical Impedance spectroscopy (EIS) using Solartron SI 1260 Impedance/Gain-phase Analyzer in standalone mode. The EIS spectra were collected in the dark over the frequency range 0.01 Hz to 1 MHz at room temperature. A series of Nyquist plots for the Imaginary vs real impedance were recorded at different negative bias voltages of 0.25V to 0.5 V for an interval of 50 mV and at amplitude of 10 mV.

(18) Results and Analysis

(19) FIG. 1 shows that distinct diffraction peaks corresponding to 101, 004, 200, 105, 211, 204, 116, 220, 215 diffraction peaks of anatase. However, these peaks are quite shifted to higher degrees, due to lattice distortion caused by the incorporation of the dopant ions into the TiO.sub.2 lattice. To further confirm this incorporation, no extra peaks can be seen for these concentrations of doping. The insert of FIG. 1 represents the 101 diffraction peak for the stated samples. It can now be observed that, the shifts of 1 mol % concentrations are slightly greater than that of 0.5 mol % Mg and La TiO.sub.2 powders. This indicates lesser crystal distortions for 0.5 mol % Mg and La co-doped TiO.sub.2 than the other samples. It should be clarified that 0.5 mol % Mg and La powders has a total of 1 mol % impurity. Thus, it shows an approximate amount of impurity into TiO.sub.2, while minimizing the defect concentration for effective charge transport. This is confirmed further by increased resistance to charge recombination for DSSC with 0.5 mol % Mg and La co-doped TiO.sub.2 anodes compared to other cells with single doped and pure photoanodes.

(20) The current density-voltage curves in FIG. 2 compares the performance of the DSSC with the different photoanodes. The DSSC all had an active area of 0.25 cm.sup.2 and calibrated under same conditions and exposure time as explained above in the detained description. The cells parameters are compared based on the single and co-doping and the mol percentages of the different dopant ions present in the lattice of the TiO.sub.2 anode. The cell with 0.5 mol % Mg and La photoanode showed the highest short circuit current, J.sub.sc of 14.2 mA/cm.sup.2 and power to current conversion efficiency (PCE) of 8.1%. This is a 20% calculated improvement over cells with pure TiO.sub.2 anode which had a J.sub.sc of 12.4 and A PCE of 6.7%. On the other hand, cells with 1 mol % La and Mg single doped TiO.sub.2 anodes recorded J.sub.sc of 13.3 and 12.2 respectively and PCEs of 7.7% and 6.9% respectively. Therefore, the invention presents a cell with 0.5 mol % Mg and La co-doped TiO.sub.2 anode with an approximate PCE increase of 5% and 17% over cells and 1 mol % La and Mg single doped TiO.sub.2 anodes respectively. This can first be attributed to the slightly less distortion as observed in the XRD analysis of FIG. 1. Cells with higher percentage of dopant concentration higher and lower than those mentioned above show lower DSSC photovoltaic performances, attributed to either higher trap concentrations (for percentage higher than 1 mol % for single doping and higher than 0.5 mol % for double doping) that lead to recombination at trap sites. Cells with lower concentrations of dopant than 0.5 mol % for double doping and 1 mol % for single doping, showed lower DSSC photovoltaic performances as well attributed to less concentration of traps and detrapping sites to curb recombination and increase electron life time. The table below (Table 1) shows the DSSC parameters.

(21) TABLE-US-00001 TABLE 1 DSSC performance parameters from current-voltage measurements. Batch Eff Voc Isc Pmax Number FF (%) (V) (mA) (mW) Vmp Imp Area pure TiO.sub.2 0.639 6.703 0.763 3.094 1.508 0.593 2.542 0.25 0.5% La 0.657 7.607 0.719 3.628 1.712 0.523 3.273 0.25 0.5% Mg 0.703 7.046 0.781 2.886 1.585 0.601 2.639 0.25 0.5% MgLa 0.687 8.044 0.743 3.546 1.810 0.553 3.272 0.25 1% La 0.690 7.676 0.751 3.329 1.727 0.567 3.044 0.25 1% Mg 0.686 6.845 0.744 3.016 1.540 0.560 2.749 0.25 1% MgLa 0.677 6.765 0.733 3.067 1.522 0.564 2.698 0.25 2% La 0.733 5.654 0.767 2.262 1.272 0.612 2.077 0.25 2% Mg 0.712 5.499 0.782 2.223 1.237 0.608 2.035 0.25 2% MgLa 0.686 6.845 0.744 3.016 1.540 0.560 2.749 0.25

(22) FIG. 3 presents a charge transfer study in the DSSC with anodes of the different metal doped TiO.sub.2 samples. It is observed that cells with 1 mol % La doped TiO.sub.2 show higher resistance to recombination compared to cells with 1 mol % Mg doped TiO.sub.2 and those with pure TiO.sub.2 photoanodes. The doped TiO.sub.2 photoanodes all show higher resistance to recombination than the pure TiO.sub.2, due to the slight increase in concentration of trap sites hence their higher cell efficiency. The higher charge transfer resistance (or low rate of recombination) for cells with La doped TiO.sub.2 photoanode can be attributed to the filling of the empty f suborbitals of the La ion, the smaller particle size and higher dye adsorption or the increase in oxygen vacancy reported (Zhang., 2010). Interestingly, cells with 0.5 and 1% Mg and La TiO.sub.2 photoanodes show enhanced charge transport properties due to the additional/combine effect of the presence of La and Mg ions in the TiO.sub.2 lattice that increase both injection at the oxide-dye as well as reduces the rate of electron recombination at the oxide-electrolyte interfaces. Cells with 0.5 mol % Mg and La cells show higher resistance to recombination and hence higher cell efficiency due to appropriate concentration of trap sites and energy level distribution relative to their 1 mol % single doping counterparts. Therefore, 1 mol % Mg and La doped TiO.sub.2 cells have a higher defect compared to those of 2 mol % single doping cells.

(23) Although the present embodiments have been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the various embodiments. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.