QUANTUM DOTS-SENSITIZED SOLAR CELL AND METHOD OF ENHANCING THE OPTOELECTRONIC PERFORMANCE OF A QUANTUM DOTS-SENSITIZED SOLAR CELL USING A CO-ADSORBENT

Abstract

The invention provides a quantum dots-sensitized solar cell and a method of enhancing the optoelectronic performance of a quantum dots-sensitized solar cell using a co-adsorbent, in which a bifunctional molecule is used as the co-adsorbent and is mixed with aqueous quantum dots to form a quantum dots sensitizer, thereby improving the photoelectric conversion efficiency of the solar cell.

Claims

1. A quantum dots-sensitized solar cell, comprising: a photoelectrode, formed on a first substrate and having a quantum dots sensitizer adsorbed thereon; a back electrode, formed on a second substrate; and a polysulfide electrolyte, injected between the photoelectrode and the back electrode; wherein the photoelectrode having the quantum dots sensitizer adsorbed is modified by a co-adsorbent, and the co-adsorbent has a structure of HS—R—COOH or HS—R—OH where R represents a substituted or unsubstituted organic carbon chain having 1 to 10 carbon atoms.

2. The quantum dots-sensitized solar cell according to claim 1, wherein the co-adsorbent having the structure of HS—R—COOH is selected from the group consisting of thioglycolic acid (TGA), L-Cystine, D-Cystine, DL-Cystine, L-cysteine (Cys), D-cysteine, DL-cysteine, L-homocysteine, N-isobutyryl-L-cysteine, N-carbamoyl-L-cysteine, glutathione (GSH), 2-mercaptopropionic acid (2-MPA)-3-mercaptopropionic acid (3-MPA), 4-mercaptobutyric acid, 6-mercaptohexanoic acid, 8-mercaptooctanoic acid, mercaptosuccinic acid, meso-2,3-dimercaptosuccinic acid, 2-methyl-3-sulfanylpropanoic acid, dihydrolipoic acid, thiolactic acid, methyl thioglycolate, ethyl thioglycolate, methyl 3-mercaptopropionate, and pentaerythritol tetrakis(2-mercaptoacetate).

3. The quantum dots-sensitized solar cell according to claim 1, wherein the co-adsorbent having the structure of HS—R—OH is selected from the group consisting of 1,4-dithiothreitol (DTT), L-(−)-dithiothreitol, trans-4,5-dihydroxy-1,2-dithiane, 1-mercapto-2-propanol, 2-mercaptoethanol (ME), 4-mercapto- 1-butanol, 3-mercapto-1-propanol, 6-mercapto-1-hexanol, and 8-mercapto-l-octanol

4. The quantum dots-sensitized solar cell according to claim 1, wherein the first substrate and the second substrate each are either of FTO transparent electrically conductive glass and ITO transparent electrically conductive glass.

5. The quantum dots-sensitized solar cell according to claim 1, wherein there is a layer of an oxide semiconductor selected from the group consisting of TiO.sub.2, SnO.sub.2, ZnO and SrTiO.sub.3 on the photoelectrode.

6. The quantum dots-sensitized solar cell according to claim 1, wherein the quantum dots sensitizer is a semiconductor material selected from the group consisting of CdS, CdSe, CdTe, PbS, PbSe, Ag.sub.2S, Ag.sub.2Se, AgS.sub.xSe.sub.1-x, CuS, Sb.sub.2S.sub.3, Sb.sub.2Se.sub.3, CdS.sub.xSe.sub.1-x, CdSe.sub.xTe.sub.1-x, InP, PbS.sub.xSe.sub.1-x, PbSe.sub.xTe.sub.1-x, AgInS.sub.xSe.sub.1-x, AgInS.sub.2, AgInSe.sub.2, AgInTe.sub.2, CuInS.sub.xSe.sub.1-x, CuInS.sub.xTe.sub.1-x, CuInS.sub.2, CuInSe.sub.2, CuInTe.sub.2, and CuIn.sub.2S.sub.3.

7. The quantum dots-sensitized solar cell according to claim 1, wherein a material of the back electrode is a metal sulfide selected from the group consisting of PbS, NiS, CoS, CuS, and Cu.sub.2S.

8. A method of enhancing the optoelectronic performance of a quantum dots-sensitized solar cell using a co-adsorbent, characterized in that a photoelectrode is dipped into a mixed solution of a co-adsorbent and a quantum dots sensitizer to increase the coverage of the quantum dots sensitizer on the photoelectrode and thereby improve the photoelectric conversion efficiency of the quantum dots-sensitized solar cell, wherein the co-adsorbent has a structure of HS—R—COOH or HS—R—OH where R represents a substituted or unsubstituted organic carbon chain having 1 to 10 carbon atoms.

9. The method according to claim 8, wherein the co-adsorbent having the structure of HS—R—COOH is selected from the group consisting of thioglycolic acid (TGA), L-Cystine, D-Cystine, DL-Cystine, L-cysteine (Cys), D-cysteine, DL-cysteine, L-homocysteine, N-isobutyryl-L-cysteine, N-carbamoyl-L-cysteine, glutathione (GSH), 2-mercaptopropionic acid (2-MPA) 3-mercaptopropionic acid (3-MPA), 4-mercaptobutyric acid, 6-mercaptohexanoic acid, 8-mercaptooctanoic acid, mercaptosuccinic acid, meso-2,3-dimercaptosuccinic acid, 2-methyl-3-sulfanylpropanoic acid, dihydrolipoic acid, thiolactic acid, methyl thioglycolate, ethyl thioglycolate, methyl 3-mercaptopropionate, and pentaerythritol tetrakis(2-mercaptoacetate).

10. The method according to claim 8, wherein the co-adsorbent having the structure of HS—R—OH is selected from the group consisting of 1,4-dithiothreitol (DTT) , L-(−)-dithiothreitol, trans-4,5-dihydroxy-1,2-dithiane, 1-mercapto-2-propanol, 2-mercaptoethanol (ME), 4-mercapto-1-butanol, 3-mercapto-1-propanol, 6-mercapto-1-hexanol, and 8-mercapto-1-octanol.

11. The method according to claim 8, wherein the quantum dots sensitizer is a semiconductor material selected from the group consisting of CdS, CdSe, CdTe, PbS, PbSe, Ag.sub.2S, Ag.sub.2Se, AgS.sub.xSe.sub.1-x, CuS, Sb.sub.2S.sub.3, Sb.sub.2Se.sub.3, CdS.sub.xSe.sub.1-x, CdSe.sub.xTe.sub.1-x, InP, PbS.sub.xSe.sub.1-x, PbSe.sub.xTe.sub.1-x, AgInS.sub.xSe.sub.1-x, AgInS.sub.2, AgInSe.sub.2, AgInTe.sub.2, CuInS.sub.xSe.sub.1-x, CuInS.sub.xTe.sub.1-x, CuInS.sub.2, CuInSe2, CuInTe.sub.2, and CUIn.sub.2S.sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1(a) is a TEM (Transmission Electron Microscope) diagram of the aqueous CuInS.sub.2 quantum dots, FIG. 1(b) is an XRD (X-ray Diffraction) diagram of the aqueous CuInS.sub.2 quantum dots, and FIG. 1(c) is an EDS (Energy Dispersive Spectrometer) diagram of the aqueous CuInS.sub.2 quantum dots.

[0024] FIG. 2 is schematic diagram showing assembly of the components of a solar cell.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0025] (Synthesis of the aqueous CuInS.sub.2 quantum dots)

[0026] A solution was prepared by adding 0.213 ml of CuCl.sub.2 solution, 0.553 ml of InCl.sub.3 solution, and 0.25 ml of sodium citrate (SC) solution (all prepared in advance) into a microwave reaction vial G30, being stirred until its color turned blue, adding 1000 ml of L-cysteine (Cys) precursor solution so that the color became transparent from blue, adding 17.48 ml of deionized water, and fast adding Na2S with stirring so that the color became yellow from transparent, wherein Cu:In:SC:Cys:S is 1:4:16:7.2:6.5.

[0027] The resultant solution was placed in a microwave assisting device Microwave 300 at a standard mode, and a microwave reaction was carried out at 180° C. for 15 minutes. The cooling temperature was set to 55° C., and the pressure during the reaction was about 10.5-11 Bar. The color of the solution turned deep brown from yellow. After reaction, the solution was mixed with 2-propanol, and then centrifuged to collect a precipitate. The precipitate was placed in an oven at 40° C. for 16-18 hours, and the dried substance in deep brown color was the aqueous CuInS.sub.2 quantum dots as synthetized.

[0028] FIG. 1(a) shows the lattice structure of the aqueous CuInS.sub.2 quantum dots, obtained from an image analysis conducted with a high resolution transmission electron microscope. FIG. 1(b) shows the XRD signal of the aqueous CuInS.sub.2 quantum dots. In comparison with CuInS.sub.2 with tetragonal structure (JCPDS-15-0681), the XRD signal of the aqueous CuInS.sub.2 quantum dots quite correspond to that of CuInS.sub.2 with tetragonal structure which shows three main peaks (112), (220), and (312) at 2θ=28.2°, 46.8°, and 55.3°, respectively, and it was thus confirmed that the synthetized material was CuInS.sub.2 quantum dots. FIG. 1(C) is an EDS diagram of the aqueous CuInS.sub.2 quantum dots, which shows that there are element signals of Cu, In, S, and Au. Because the EDS element analysis was conducted by dropping the aqueous CuInS.sub.2 quantum dots on a gold specimen after dissolving, an Au signal appeared in the EDS result.

[0029] (Adsorption of the aqueous CuInS.sub.2 quantum dots)

[0030] The dried aqueous CuIn3.sub.2 quantum dots were dissolved in water, and then different kinds of co-absorbents were added in different concentrations so as to prepare aqueous CuInS.sub.2 quantum dots solutions containing the kinds and concentrations of co-absorbents listed in the Examples below. Subsequently, a photoelectrode in which a TiO.sub.2 film was formed on an FTO electrically conductive glass was immersed in each of the aforementioned aqueous CuInS.sub.2 quantum dots solutions at 40° C. for 24 hours, and then the photoelectrode was taken out, washed with methanol, and dried. Subsequently, a ZnS passivation layer was deposited thereon with the SILAR method. Finally, a TiO.sub.2 photoelectrode having CuInS.sub.2 quantum dots adsorbed was thus obtained.

[0031] The obtained TiO.sub.2 photoelectrode having CuInS.sub.2 quantum dots adsorbed was combined with the Cu.sub.2S back electrode, which was prepared with a spin coating method, and the polysulfide electrolyte, which was prepared in advance by the following steps of weighting 4.3232 g of Na.sub.2S, 0.1491 g of KCl, and 0.6401 g of S powder, dissolving these solid solutes in 7 ml of water, followed by adding 3 ml of methanol, and then adding 29.5 mg of GuSCN into 5 ml of the aforementioned mixed solution. The assembly of the solar cell is shown in FIG. 2.

[0032] Table 1 is the analysis result of the optoelectronic performance of the QDSSCs produced by using different concentrations of DTT as the co-adsorbent, in which J.sub.sc represents a short-circuit current density (where the short-circuit current is a current generated upon irradiation) and is defined as a current density measured when the applied voltage is zero, V.sub.oc represents an open-circuit voltage and is defined as a voltage applied when the measured current density is zero, FF (fill factor) is defined as an actual largest output power divided by a target output power (J.sub.sc×V.sub.oc), which is a dimensionless value, and can be used as an index for indicating the difference between the actual solar cell and the ideal solar cell, as a solar cell is closer to an ideal solar cell if the FF value is closer to 1, and η represents the photoelectric conversion efficiency of a solar cell and is defined as a ratio of the largest output power to the power of an incident light.

TABLE-US-00001 TABLE 1 Efficiency of QDSSCs with different DTT concentrations J.sub.sc V.sub.oc FF η DTT Conc. (mA/cm.sup.2) (mV) (%) (%) Compar. .sup. 0M 0.282 290 46.8 0.038 Ex. 1 Compar. 0.1M 0.296 240 50.3 0.036 Ex. 2 Ex. 1 0.5M 2.046 478 54.5 0.533 Ex. 2 4.0M 7.207 592 60.6 2.587

[0033] It can he learned from the Comparative Examples 1 and 2 that the η values were merely 0.038% and 0.036%, respectively, for the aqueous CuInS.sub.2 quantum dots dissolved in pure water and low concentration 0.1 M of DTT. When the DTT concentration was increased to 0.5 M, all data of the device increased greatly, indicating that a high concentration disulfide bond reagent can stabilize the bifunctional molecule on the surfaces of the aqueous CuInS.sub.2 quantum dots so that the carboxylic group of the bifunctional molecule can successfully bond to TiO.sub.2, thereby greatly increasing the coverage and providing more excited electrons to increase J.sub.sc. V.sub.oc depends on the Fermi energy level of TiO.sub.2 and the potential difference of oxidation-reduction pairs in the electrolyte. The increase in the coverage of the aqueous CuInS.sub.2 quantum dots implies that more electrons are injected into TiO.sub.2 so that the Fermi energy level of TiO.sub.2 moves toward a negative potential, thereby increasing the potential difference with respect to the oxidation-reduction pairs of the electrolyte and increasing V.sub.oc. In the Example 2, the DTT concentration was further increased to 4.0 M, and therefore J.sub.sc increased greatly to 7.207 mA/cm.sup.2, V.sub.oc increased to 592 mV, FF increased to 60.6%, and η even increased from 0.533% to 2.587%, indicating that the reduction conducted in high concentration may provid the wide bandgap oxide semiconductor with a high load of aqueous CuInS.sub.2 quantum dots.

[0034] Table 2 is the analysis result of the optoelectronic performance of the QDSSCs produced by using different bifunctional molecules as the co-adsorbent.

TABLE-US-00002 TABLE 2 Efficiency of QDSSCs with 0.5M of different bifunctional molecules J.sub.sc V.sub.oc FF η co-adsorbent (mA/cm.sup.2) (mV) (%) (%) Ex. 1 DTT 2.046 478 54.5 0.533 Ex. 3 TGA 12.820 640 54.1 4.438 Ex. 4 Cys 4.462 544 56.8 1.379 Ex. 5 GSH 6.930 628 56.4 2.455

[0035] It can be learned from Table 2 that the photoelectric conversion efficiencies of the solar cells of the Examples 3 to 5, in which TGA, Cys, and GSH, respectively, were used as the co-adsorbent, all increased greatly, in comparison with the Example 1, in which DTT was used as the co-adsorbent. TGA, Cys, and GSH each have carboxylic groups in their molecular structures, which may be a main reason why the coverage increased greatly. Among them, TGA as the co-adsorbent has the best result for photoelectric conversion efficiency, followed by GSH, and the last is Cys, and this may be because TGA's molecular structure and thus steric effect are smaller so that the aqueous CuInS.sub.2 quantum dots could be successfully adsorbed to the surface of TiO.sub.2 and the photoelectric conversion efficiency greatly increased to 4.438%. GSH has two carboxylic groups in its molecular structure, which can provide a higher capability of bonding to TiO.sub.2. However, the said molecular structure and thus the steric effect are larger, so that its photoelectric conversion efficiency of 2.455% was worse than that of TGA. The steric effect of the molecular structure of Cys is between those of TGA and GSH, but its photoelectric conversion efficiency is worse than those of TGA and GSH. It is inferred that Cys is not higher in capability of reducing disulfide bonds than GSH, and therefore Cys is worse in efficiency than GSH even if its steric effect is smaller.

[0036] Table 3 is the analysis result of the optoelectronic performance of the QDSSCs produced by using different concentrations of TGA as the co-adsorbent.

TABLE-US-00003 TABLE 3 Efficiency of QDSSCs with different TGA concentrations J.sub.sc V.sub.oc FF η TGA Conc. (mA/cm.sup.2) (mV) (%) (%) Ex. 6 0.1M 9.230 616 56.7 3.225 Ex. 3 0.5M 12.820 640 54.1 4.438 Ex. 7 1.0M 14.015 642 51.8 4.661 Ex. 8 2.0M 14.415 642 52.1 4.821 Ex. 9 4.0M 14.837 630 52.6 4.920 Ex. 10 6.0M 13.705 650 50.9 4.534

[0037] According to the result of Table 2, TGA was the best co-absorbent for photoelectric conversion efficiency and thus was used in concentrations of 0.1 M to 6.0 M in the Examples 6 to 10, respectively. It can be learned from Table 3 that the photoelectric conversion efficiency increased as the TGA concentration was increased from 0.1 M to 4.0 M. The best result was the Example 9 with 4.0 M TGA used and the photoelectric conversion efficiency was 4.920%.

[0038] Table 4 is the analysis result of the optoelectronic performance of the QDSSCs produced by using 4.0 M TGA as the co-adsorbent and immersing the photoelectrode for different periods of time.

TABLE-US-00004 TABLE 4 Efficiency of QDSSCs with immersion in 0.4M TGA for different periods of time. Immersion J.sub.sc V.sub.oc FF η Time (mA/cm.sup.2) (mV) (%) (%) Ex. 11 0.5 hr 7.634 626 61.2 2.926 Ex. 12 1 hr 9.225 630 57.7 3.352 Ex. 13 3 hr 12.133 630 57.0 4.360 Ex. 14 6 hr 12.998 630 54.5 4.461 Ex. 15 12 hr 13.691 624 54.0 4.615 Ex. 9 24 hr 14.837 630 52.6 4.920

[0039] According to the result of Table 3, 0.4 M TGA exhibited the best result for photoelectric conversion efficiency among different concentrations of TGA, and thus was used as the co-adsorbent in the Examples 11 to 15 to determine the influence of immersion time on the photoelectric conversion efficiency of the solar cell. It can be learned from Table 4 that J.sub.sc increased from 7.634 mA/cm.sup.2 to 12.133 mA/cm.sup.2 and the photoelectric conversion efficiency increased from 2.926% to 4.360% when the immersion time was increased from 0.5 hours to 3 hours, while J.sub.sc increased from 12.133 mA/cm.sup.2 to 14.837 mA/cm.sup.2 and the photoelectric conversion efficiency increased merely from 4.360% to 4.920% when the immersion time was increased from 3 hours to 24 hours. In other words, taking the immersion time of 3 hours as a cut-off point, the photoelectric conversion efficiency rose rapidly before 3 hours and tended to rise gently after 3 hours. Also, it can be learned from the results of all immersion time conditions that as the immersion time was increased, FF decreased from 61.2% at 0.5 hours to 52.6% at 24 hours, indicating that the coverage of the quantum dots increases as the immersion time increases, but too many quantum dots will cause a continuous increase in internal impedance and thus a reduction in FF, thereby slowing down the rising of the photoelectric conversion efficiency.

[0040] The TiO.sub.2 photoelectrode has to be immersed in a solution composed of both TGA co-adsorbent and aqueous CuInS.sub.2 quantum dots so as to have a better photoelectric conversion efficiency. In the Comparative Examples 3 to 5, the optoelectronic performance of the QDSSCs produced from aqueous CuInS.sub.2 quantum dots with the TGA co-adsorbent added in different sequences was analyzed.

[0041] In Table 5, the Comparative Example 3 was to immerse the TiO.sub.2 photoelectrode in the TGA co-adsorbent for 24 hours and then in the aqueous CuInS.sub.2 quantum dots for 24 hours; the Comparative Example 4 was to immerse the TiO.sub.2 photoelectrode in the aqueous CuInS.sub.2 quantum dots for 24 hours and then in the TGA co-adsorbent for 24 hours; the Comparative Example 5 was to immerse the TiO.sub.2 photoelectrode only in the TGA co-adsorbent for 24 hours without being immersed in any aqueous quantum dots.

TABLE-US-00005 TABLE 5 Efficiency of QDSSCs with different immersion sequences J.sub.sc V.sub.oc FF η (mA/cm.sup.2) (mV) (%) (%) Compar. 0.597 354 53.8 0.114 Ex. 3 Compar. 1.411 460 55.2 0.358 Ex. 4 Compar. 0.495 410 38.1 0.077 Ex. 5 Ex. 3 12.820 640 54.1 4.438

[0042] It can be learned from Table 5 that the photoelectric conversion efficiencies of the Comparative Examples 3 and 4 were far lower than that of the Example 3. On the other hand, the photoelectric conversion efficiency of the Comparative Example 5, in which the TiO.sub.2 photoelectrode was immersed only in the TGA co-adsorbent for 24 hours without being immersed in any aqueous quantum dots, is even merely 0.077%. It is thus demonstrated that a mixed solution composed of both TGA co-adsorbent and aqueous CuInS.sub.2 quantum dots results in a better photoelectric conversion efficiency.

[0043] Table 6 is the analysis result of the optoelectronic performance of the QDSSCs produced by using aqueous CdSe, CdSe.sub.xTe.sub.1-x, AgInSe.sub.2, and AgInS.sub.2 quantum dots.

TABLE-US-00006 TABLE 6 Efficiency of aqueous CdSe, CdSe.sub.xTe.sub.1−x, AqInSe.sub.2, and AqInS.sub.2 QDSSCs with the TGA co-adsorbent co- J.sub.sc V.sub.oc FF QD adsorbent (mA/cm.sup.2) (mV) (%) η (%) Compar. CuInS.sub.2 — 0.282 282 46.8 0.038 Ex. 6 Ex. 16 CuInS.sub.2 4M TGA 14.478 630 53.1 4.864 Compar. AgInS.sub.2 — 0.178 212 47.5 0.018 Ex. 7 Ex. 17 AqInS.sub.2 4M TGA 6.518 382 64.0 1.594 Compar. CdSe — 0.385 264 45.7 0.046 Ex. 8 Ex. 18 CdSe 4M TGA 4.759 552 53.8 1.413 Compar. CdSe.sub.xTe.sub.1−x — 0.640 384 46.0 0.113 Ex. 9 Ex. 19 CdSe.sub.xTe.sub.1−x 4M TGA 14.38 630 50.5 4.578 Compar. AgInSe.sub.2 — 1.538 492 50.4 0.381 Ex. 10 Ex. 20 AgInSe.sub.2 4M TGA 16.39 610 54.1 5.411

[0044] It can be learned from Table 6 that the photoelectric conversion efficiencies of the solar cells using the aqueous CdSe.sub.xTe.sub.1-x and AgInSe.sub.2 quantum dots and the TGA co-adsorbent also increased notably. Also, the photoelectric conversion efficiencies of the AgInS.sub.2 and CdSe QDSSCs increased from 0.018% and 0.046% to 1.594% and 1.413%, respectively. Although their improvements are not as obvious as that of CuInS.sub.2, yet it can still be demonstrated that the TGA co-adsorbent can be used with various aqueous quantum dots to improve the coverage.

[0045] Table 7 further provides the analysis result of the optoelectronic performance of the QDSSCs produced by immersing the TiO.sub.2 photoelectrode in different concentrations of GSH, 3-MPA, and Cys co-adsorbents for 24 hours, which shows that different kinds of co-adsorbents all can improve the efficiencies of the QDSSCs by increasing their concentrations.

TABLE-US-00007 TABLE 7 Efficiency of the QDSSCs with different concentrations of GSH, 3-MPA, and Cys J.sub.sc V.sub.oc FF η co-adsorbent (mA/cm.sup.2) (mV) (%) (%) Compar. 0.1M GSH 0.108 490 41.2 0.136 Ex. 11 Ex. 5 0.5M GSH 6.930 628 56.4 2.455 Ex. 21 1.0M GSH 12.234 676 51.7 4.273 Ex. 22 2.0M GSH 11.862 660 53.5 4.185 Compar. 0.1M 3-MPA 0.878 496 58.8 0.256 Ex. 12 Ex. 23 0.5M 3-MPA 2.620 574 56.0 0.842 Ex. 24 1.0M 3-MPA 5.648 600 58.2 1.973 Ex. 25 2.0M 3-MPA 10.596 624 53.5 3.536 Ex. 26 4.0M 3-MPA 11.693 632 51.2 3.787 Compar. 0.1M Cys 0.707 412 48.0 0.140 Ex. 13 Ex. 4 0.5M Cys 4.462 544 56.8 1.379 Ex. 27 1.0M Cys 8.174 616 57.4 2.888 Ex. 28 2.0M Cys 9.403 618 53.1 3.084 Ex. 29 4.0M Cys 9.539 626 52.8 3.154