Process for the synthesis of air stable metal sulphide quantum dots

Abstract

The present invention discloses a process for the preparation of metal sulphide quantum dots by using a very low cost sulphur precursor as a sulphur source. The metal sulphide quantum dots finds application in optical devices selected from photovoltaic cells, photodetectors and light-emission devices.

Claims

1. A process for the preparation of metal sulphide QDs comprising the steps of: a) reacting a metal salt with a ligand in a solvent followed by heating at a temperature ranging from 90 to 95° C. under a vacuum for a period ranging from 1 to 2 h to afford a metal oleate or a metal amine solution; b) preparing a dithiocarbamic acid solution by mixing octyl dithiocarbamic acid with a ligand and a solvent to form a mixture followed by injecting said mixture to the metal oleate or metal amine solution of step (a) to obtain a dithiocarbamic solution; c) injecting acetone to the dithiocarbamic solution of step (b) as an anti-solvent to obtain a precipitate, followed by collecting particles of precipitate by centrifugation to obtain metal sulfide QDs; and d) dispersing said metal sulfide QDs in a non-polar solvent to obtain colloidal quantum dots.

2. The process as claimed in claim 1, wherein said metal is selected from the group consisting of Lead (Pb), Cadmium (Cd), Manganese (Mn), Zinc (Zn), Copper (Cu) and Tin (Sn).

3. The process as claimed in claim 1, wherein said salt of the metal is selected from the group consisting of an oxide salt, an acetate salt and a halide salt.

4. The process as claimed in claim 1, wherein said ligand is selected from the group consisting of oleic acid and oleyl amine.

5. The process as claimed in claim 1, wherein said solvent of step (a) and (b) is 1-octadecene.

6. The process as claimed in claim 1, wherein said non-polar solvent of step (d) is selected from the group consisting of toluene, chloroform, hexane or octane.

7. The process as claimed in claim 1, wherein said metal sulfide QDs have a particle size in the range of 2 nm to 10 nm.

8. The process as claimed in claim 1, wherein said metal sulfides QDs are stable and mono dispersed.

9. The process as claimed in claim 1, wherein said metal sulfides QDs absorb and emit in visible to NIR region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Synthesis and characterisation of the Sulfur containing ligand DTCA (Octyl Dithiocarbamic Acid); (1a) HRMS data of DTCA indicates mass of the DTCA; (1b) NMR spectra of DTCA

(2) FIG. 2: Characterization of Lead sulfide QDS (PbS QDs); (2a) NIR-UV Spectra of oleic acid capped PbS QDs at different temperatures (90, 100 and 110° C.) (2b) NIR-Photo luminescence spectra of PbS QDs at different temperatures (90, 100 and 110° C.); (2c) Tunable NIR-UV spectra of PbS QDs prepared at different condition; (2d) NIR-UV Spectra of oleic acid capped PbS QDs at different time interval after injection of DTCA; (2e) NIR-Photo luminescence spectra of PbS QDs at different time; (2f, 2g) Scalabality of PbS QDs from 2 mmol scale 50 mmol scale; (2f) NIR-UV Spectra of oleic acid capped PbS QDs; (2g) NIR-Photo luminescence spectra of PbS QDs; (2h, 2i) Stability of PbS QDs after 6 month; (2h) NIR-UV Spectra of oleic acid capped PbS QDs after 6 month; (2i) NIR-Photo luminescence spectra of PbS QDs after 6 month; (2j) PXRD of PbS QDs; (2k, 2l, 2m) TEM image of PbS QDs; (2n) HRTEM image of PbS QDS.

(3) FIG. 3: Characterization of Cadmium sulfide QDS (CdS QDs) (3a) PXRD of CdS QDs; (3b) UV-Vis Spectra and Fluorescence spectra of CdS QDs; (3c) TEM image of CdS QDs.

(4) FIG. 4: Characterization of Manganese sulfide QDS (MnS QDs) (4a) UV-Vis Spectra of MnS QDs; (4b) Fluorescence spectra of MnS QDs; (4c) TEM image of MnS QDs.

(5) FIG. 5: Characterization of Zinc sulfide QDS (ZnS QDs) (5a) PXRD of ZnS QDs; (5b) TEM image of ZnS QDs; (5c) Fluorescence spectra of ZnS QDs; (5d) UV-Vis Spectra of ZnS QDS.

(6) FIG. 6: (a) Schematic device structure of the photovoltaic device, (b) J-V characteristics of the best performing solar cell under 1.5 AM illumination, (c) Comparison of photovoltaic figure of merits for different band gap PbS QDs. (d) Capacitance-voltage plot for best performing PbS QD (1.3 eV band gap) based solar. (e) and (f) The evolution of short circuit current (J.sub.sc) and open circuit voltage (V.sub.oc) respectively for best performing PbS QD based solar cell

(7) FIG. 7 shows scheme 1 which provides stepwise description of metal sulfide QDs synthesis using octyl dithiocarbamic acid

DETAILED DESCRIPTION OF THE INVENTION

(8) The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

(9) The present invention provides a process for the preparation of metal sulphide quantum dots by using a very low cost sulphur precursor as a sulphur source.

(10) In an embodiment, the present invention provides a process for the preparation of metal sulphide QDs comprising the steps of: a) reacting a metal salt with a ligand in a solvent followed by heating at temperature ranging from 90 to 95° C. under vacuum for a period ranging from 1 to 2 h to afford a metal oleate or a metal amine solution; b) preparing dithiocarbamic acid solution by mixing octyl dithiocarbamic acid with a ligand and a solvent followed by injecting to the metal oleate or metal amine solution of step (a) to obtain a solution; c) injecting acetone to the solution of step (b) as an anti-solvent to obtain a precipitate followed by collecting particles of precipitate by centrifugation to obtain metal sulfide QDs; and d) dispersing said metal sulfide QDs in a non-polar solvent to obtain colloidal quantum dots.

(11) In preferred embodiment, said metal is selected from the group consisting of Lead (Pb), Cadmium (Cd), Manganese (Mn), Zinc (Zn), Copper (Cu) and Tin (Sn).

(12) In another preferred embodiment, said salt of the metal is selected from the group consisting of oxide salt, acetate salt and halide salts.

(13) In still another preferred embodiment, said ligand is selected from oleic acid or oleyl amine.

(14) In yet another preferred embodiment, said solvent of step (a) and (b) is 1-octadecene.

(15) In still yet another preferred embodiment, said non-polar solvent of step (d) is selected from toluene, chloroform, hexane or octane.

(16) In yet still another preferred embodiment, said particles of precipitate of step (c) are dispersed by adding a non-polar solvent to obtain colloidal metal sulphide QDs.

(17) In yet still another embodiment, said metal sulfides QDs have particle size in the range of 2 nm to 10 nm.

(18) In yet still another embodiment, said metal sulfides QDs are stable and mono dispersed.

(19) In yet still another embodiment, said metal sulfides QDs absorb and emit in visible to NIR region.

(20) The present invention provides a process for the preparation of various metal sulphide QDs. More specifically the synthetic procedure involves two steps, wherein in the first step, Pb-oleate solution prepared by using Pb precursors [PbO, PbCl.sub.2 or Pb(ac).sub.2] and oleic acid acting as a ligand dissolved in oleyl amine (in room temperature) and injected to the Pb-oleate solution at a particular temperature (80-140° C.) to obtain metal sulphide QDs with controlled particle size. From octyl dithiocarbamic acid, sulfur liberates as H.sub.2S in presence of amine at a particular temperature and as a side product thiourea also formed. The liberated H.sub.2S reacts with metal oleate and oleic acid capped metal sulfide QDs are formed.

(21) The release rate of sulfur can be controlled by varying some reaction parameter like temperature and amine concentration. So, different sized QDs may be obtained by varying the release rate of sulfur. Octyl dithiocarbami acid based synthesis procedures have been used to produce tunable absorption spectra CQDs from 800-1300; with about 6 month air stability with narrow size distribution and comparable optical properties to the TMS based PbS QDS. The reaction scheme allowed in Scheme 1 as provided in FIG. 7.

(22) The metal sulfide QDs of the present invention finds application in optical devices selected from the group consisting of photovoltaic cells, photodetectors and light-emission devices.

(23) The colloidal metal sulfide QDs of the present invention finds application in optical devices selected from the group consisting of photovoltaic cells, photodetectors and light-emission devices.

(24) The as prepared metal sulfide QDs readily go into and form clear dispersions with non-polar organic solvents such as toluene. This dispersion could be used in thin film photovoltaic solar cells.

(25) FIG. 1 depicts synthesis and characterization of the Sulfur containing ligand DTCA (Octyl Dithiocarbamic Acid); (1a) HRMS data of DTCA indicates mass of the DTCA; (1b) NMR spectra of DTCA

(26) FIG. 2 depicts characterization of Lead sulfide QDS (PbS QDs); (2a) NIR-UV Spectra of oleic acid capped PbS QDs at different temperatures (90, 100 and 110° C.), with temperature particle size increases and absorption peak shifts towards red; (2b) NIR-Photo luminescence spectra of PbS QDs at different temperatures (90, 100 and 110° C.), with temperature particle size increases and emission peak shifts towards red; (2c) Tunable NIR-UV spectra of PbS QDs prepared at different condition; (2d) NIR-UV Spectra of oleic acid capped PbS QDs at different time interval after injection of DTCA, with time particle size increases due to long growth time and absorption spectra clearly indicates red shift of excitonic peak; (2e) NIR-Photo luminescence spectra of PbS QDs at different time; (2f, 2g) Scalabality of PbS QDs from 2 mmol scale 50 mmol scale; (2f) NIR-UV Spectra of oleic acid capped PbS QDs; (2g) NIR-Photo luminescence spectra of PbS QDs; (2h, 2i) Stability of PbS QDs after 6 month; (2h) NIR-UV Spectra of oleic acid capped PbS QDs after 6 month; (2i) NIR-Photo luminescence spectra of PbS QDs alter 6 month; (2j) PXRD of PbS QDs; (2k, 2l, 2m) TEM image of PbS QDs shows particles are highly monodispersed; (2n) HRTEM image of PbS QDS.

(27) FIG. 3 depicts characterization of Cadmium sulfide QDS (CdS QDs) (3a) PXRD of CdS QDs; (3b) UV-Vis Spectra and Fluorescence spectra of CdS QDs; (3c) TEM image of CdS QDs.

(28) FIG. 4 depicts characterization of Manganese sulfide QDS (MnS QDs) (4a) UV-Vis Spectra of MnS QDs; (4b) Fluorescence spectra of MnS QDs; (4c) TEM image of MnS QDs.

(29) FIG. 5 depicts characterization of Zinc sulfide QDS (ZnS QDs) (5a) PXRD of ZnS QDs; (5b) TEM image of ZnS QDs; (5c) Fluorescence spectra of ZnS QDs; (5d) UV-Vis Spectra of ZnS QDS.

(30) FIG. 6 depicts (a) Schematic device structure of the photovoltaic device, (b) J-V characteristics of the best performing solar cell under 1.5 AM illumination. (c) Comparison of photovoltaic figure of merits for different band gap PbS QDs. (d) Capacitance-voltage plot for best performing PbS QD (1.3 eV band gap) based solar. (e) and (f) The evolution of short circuit current (J.sub.sc) and open circuit voltage (V.sub.oc) respectively for best performing PbS QD based solar cell.

(31) FIG. 7 shows scheme 1 which provides stepwise description of metal sulfide QDs synthesis using octyl dithiocarbamic acid.

EXAMPLES

(32) Following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

Example 1: Synthesis of Octyl Dithiocarbamic Acid

(33) About 50 ml toluene was taken in 250 ml Rb flask and cooled, and then 20 mmol of CS.sub.2 (excess) added to it and stirred for some time. After 15 minute 10 mmol of Octyl amine added drop-wise into the CS.sub.2 solution. After 30 min very shiny crystalline organic compound formed. It was washed by hexane and dried by vacuum and recrystallized. The obtained product was analyzed by NMR and HRMS. The NMR spectrum results White shiney crystal. .sup.1H NMR (200 MHz, Chloroform-d) δ 5.44 (s, 2H, N—H, S—H), 3.60-3.48 (m, 1H), 3.02 (t, J=7.3 Hz, 1H), 1.70-1.59 (m, 2H), 1.27 (m, 10H), 0.88 (t, J=6.1 Hz, 3H). HRMS Spectra of Octyl dithiocarbamic acid. Two major peak observed at ˜130 and 206 due to Octyl Amine and Octyl dithiocarbamic acid respectively. In HRMS condition some of the compound (C8DTCA) decomposed to octyl amine.

Example 2: Synthesis of PbS QDS

(34) A mixture of lead oxide (2 mmol, 0.45 g) or lead acetate (2 mmol), oleic acid (4 mmol, 1.26 ml) and 1-octadecene (8 mmol, 2.56 ml) in a flask was heated and degassed under vacuum at 95° C. for 1-2 h, followed by adding 15 ml of 1-octadecene. The temperature was set to 90-140° C. About 75 mg octyl dithiocarbamic acid (0.75 mmol) was dissolved in 0.5 ml oleyl amine and another 10 ml of 1-ODE added to it. The reaction was initiated by rapid injection of the dithiocarbamic acid solution. Immediately after injection, the heating mentle was removed and the flask was allowed to cool down gradually to room temperature under stirring. QDs were purified in air by adding acetone, followed by centrifugation. The extracted QDs were dispersed in toluene for solar cell fabrication. The purified PbS QDs were characterized by PXRD, UV-VIS spectroscopy, NIR-PL spectra and TEM. The PXRD pattern of the PbS nanocrystals prepared by this method is shown in FIG. 2j, which shows a high degree of crystallinity with all the peaks matching with the Bragg reflections of the standard cubic rock-salt structure of PbS (JCPDS #05-0592). The absorption and emission of these PbS nanocrystals could be tuned by varying the reaction parameter like temperature, time and octyldithiocarbamic acid concentration. It can be clearly seen that each of the sample displays well defined excitonic peak (absorption peak) and emission peak and the absorption peak position varied gradually from 915 to 1300 nm (1.35-0.95 eV) (FIG. 2c). The TEM image of this sample unveils the presence of monodispersed particles (particle sizes ranges 2.4-5 nm) which is also exemplified by their self assembly into two dimensional hexagonally close packed structures. High resolution TEM images showed in FIG. 2n indicates that the particles are highly crystalline with well-resolved lattice planes corresponding to an interplanar spacing of 0.29±0.02 nm, consistent with the (200) d-spacing of the PbS bulk rock salt structure.

Example 3: Synthesis of CdS QDs

(35) Similarly a mixture of cadmium acetate (2 mmol) [not CdO, as it required very high temperatures about 300° C.], oleic acid (1 ml), oleyl amine (1 ml) and 1-octadecene (8 mmol, 2.56 ml) in a flask was healed and degassed under vacuum at 105° C. for 2-4 h, followed by adding 15 ml of 1-octadecene. The temperature was set to 160-200° C. About 75 mg octyl dithiocarbamic acid (0.75 mmol) was dissolved in 0.5 ml oleyl amine and another 10 ml of 1-ODE added to it. The reaction was initiated by rapid injection of the dithiocarbamic acid solution. Immediately after injection, the heating mentle was removed and the flask was allowed to cool down gradually to room temperature under stirring. QDs were purified in air by adding acetone and methanol, followed by centrifugation. The extracted QDs were dispersed in toluene. The purified CdS QDs were characterized by PXRD, UV-VIS spectroscopy, PL spectra and TEM. The PXRD patterns of these samples (FIG. 3a) confirm the formation of cubic phase (JCPDS-75-0581) of CdS. The UV-Vis and PL spectra (solid black line is the UV-Vis and dotted black line is photoluminescence spectra) of the CdS QDs prepared at 200° C. are displayed in FIG. 3b. The dotted black line (FIG. 3b) indicates pure band gap emission. High resolution TEM images showed in FIG. 3c indicate that the particles are highly crystalline and the average size calculated from TEM is ˜3.3±0.5 nm.

Example 4: Synthesis of MnS QDs

(36) Similarly a mixture of Manganese acetate (2 mmol), oleic acid (1 ml), oleyl amine (1 ml) and 1-octadecene (8 mmol, 2.56 ml) in a flask was heated and degassed under vacuum at 105° C. for 2-4 h, followed by adding 15 ml of 1-octadecene. The temperature was set to 160-200° C. About 75 mg octyl dithiocarbamic acid (0.75 mmol) was dissolved in 0.5 ml oleyl amine and another 10 ml of 1-ODE added to it. The reaction was initiated by rapid injection of the dithiocarbamic acid solution. Immediately after injection, the heating mentle was removed and the flask was allowed to cool down gradually to room temperature under stirring. QDs were purified in air by adding acetone and methanol, followed by centrifugation. The extracted QDs were dispersed in toluene. The purified MnS QDs were characterized by PXRD, UV-VIS spectroscopy, PL spectra and TEM. The UV-Vis and PL spectra (FIG. 4a is the UV-Vis and FIG. 4b is photoluminescence spectra) of the MnS QDs prepared at 200° C. are displayed in FIG. 4. The dotted black line (FIG. 4b) indicates pure band gap emission. High resolution TEM images showed in FIG. 4c indicate that the particles are highly crystalline and the average size calculated from TEM is ˜6.9±1.6 nm.

Example 5: Synthesis of ZnS QDs

(37) Similarly a mixture of Zinc acetate (2 mmol), oleic acid (1 ml), oleyl amine (1 ml) and 1-octadecene (8 mmol, 2.56 ml) in a flask was heated and degassed under vacuum at 105° C. for 2-4 h, followed by adding 15 ml of 1-octadecene. The temperature was set to 160-200° C. About 75 mg octyl dithiocarbamic acid (0.75 mmol) was dissolved in 0.5 ml oleyl amine and another 10 ml of 1-ODE added to it. The reaction was initiated by rapid injection of the dithiocarbamic acid solution. Immediately after injection, the healing mentle was removed and the flask was allowed to cool down gradually to room temperature under stirring. QDs were purified in air by adding acetone, followed by centrifugation. The extracted QDs were dispersed in toluene. The purified ZnS QDs were characterized by PXRD, UV-VIS spectroscopy, PL spectra and TEM. The PXRD patterns of these samples (FIG. 5a) confirm the formation of cubic phase (JCPDS-05-566) of ZnS. The UV-Vis and PL spectra (FIG. 5d is the UV-Vis and FIG. 5c is photoluminescence spectra) of the ZnS QDs prepared al 200° C. are displayed in FIG. 5. The dotted black line (FIG. 5c) indicates pure band gap emission. High resolution TEM images showed in FIG. 5b indicate that the particles are highly crystalline and the average size calculated from TEM is ˜3.5±0.5 nm.

Example 6: Perovskite Ligand Exchange and Film Fabrication

(38) The oleic acid capped PbS CQDs were synthesized by using C.sub.8DTCA as a sulphur source. The perovskite solution-phase ligand exchange was carried out in Argon atmosphere. The perovskite ligand exchange was carried out and purified by slightly modified to previously reported method. The starting concentration of CQD solution was set at ˜10 mg/ml in octane. For solution-phase ligand exchange, 5 mL of dimethyl formamide (DMF) solvent containing 0.1 M of PbI.sub.2 and 0.02 M of PbBr.sub.2 and 0.1 M of MAI were added to the vial and mixed vigorously at 45-50° C. for about 20-30 minutes. A 5 ml of PbS CQD octane solution (10 mgml.sup.−1) was added to 5 ml of precursor solution in Argon atmosphere. These were mixed vigorously for 1-2 min until the CQDs completely transferred to the DMF phase. The DMF solution was washed three times with octane to remove the residual OA ligands. After ligand exchange, CQDs were precipitated via die addition of toluene, and were separated by centrifugation. After 20 min of drying, the CQDs were then redispersed in butylamine (200 mgml.sup.−1) to facilitate the film deposition. The exchanged ink was deposited by single-step spin-coating at 2,500 r.p.m. for 30 s to achieve˜200 nm thickness.

Example 7: PbS CQDs Solar Cell Fabrication

(39) The solar cells were prepared on a pre-patterned ITO substrate (2.5 cm×2.5 cm). Two layers of ZnO nanoparticles were deposited on the substrate by spin coating at 3500 rpm. The perovskite-capped CQD film was further annealed at 70° C. for 10 min under nitrogen, atmosphere. Two layers of EDT ligand exchanged CQDs were deposited on top of perovskite-capped CQD film by spin-casting following reported method. Top electrodes were deposited by thermal evaporator from Hind high vacuum, model BC-300 at a base pressure of 3×10.sup.−6 mBar. 10 nm MoO.sub.3 was deposited at 0.1 Ås.sup.−1, followed by 50 nm of Au deposition at 0.5 Ås.sup.−1 and finally 100 nm Ag was deposited at 1 Ås.sup.−1 to complete the film formation.

Advantages of the Invention

(40) 1. The present invention gives access to large quantities of monodispersed metal sulfide QDs with good optical properties. 2. The as prepared metal sulfide QDs readily go into and form clear dispersions with non-polar organic solvents such as toluene. This dispersion could be used in thin film photovoltaic solar cells. 3. The key advantage that process can be done in continuous flow method for industrial scale synthesis. 4. The synthesized PbS QDs are air-stable for several months (more than 3 months) and they readily self-assemble into ordered lattices and present simple low-cost method resulted in a record solar power conversion efficiency of 4.64%.