ZnS Quantum Dots Doped with Transition Metals for Photovoltaic Applications

Thilini Ekanayaka Author

03/27/2021 Added

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This study explores how the different dopants lead changes in band gap and discuss the characteristic of these doped quantum dots.

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[00:00:01.880]Hello, my name is Thilini Ekanayaka,
[00:00:04.320]I'm a graduate student
[00:00:05.690]at Department of Physics and Astronomy.
[00:00:08.360]And the topic of my talk is Zinc Sulfide Quantum Dots
[00:00:11.780]doped with transition metals for photovoltaic applications.
[00:00:16.480]Now, before I talk about quantum dots
[00:00:19.110]I would like to talk a little bit about solar cells.
[00:00:21.660]Solar cells are part of solution
[00:00:23.650]for existing energy crisis.
[00:00:25.720]There are many studies that have been done on solar cells
[00:00:28.880]for large scale production, with high efficiencies.
[00:00:32.540]The maximum efficiency that has been obtained so far
[00:00:35.900]is about 50% for multi-junction solar cells,
[00:00:41.227]and the efficiency of silicon solar cells are about 28%.
[00:00:46.200]Now, when it comes to a better solar cells
[00:00:49.330]it's not only the efficiency matters
[00:00:51.350]but also the material and cost.
[00:00:54.220]There are solar cells which has efficiencies close to 50%
[00:00:58.277]but the usage of this solar cells are low
[00:01:01.110]because they're expensive.
[00:01:02.870]Now, if you take Amorphous Silicon solar cells
[00:01:05.280]they are less expensive than Crystalline Silicon
[00:01:07.790]solar cells because the material is less expensive,
[00:01:11.270]therefore they have been used widely,
[00:01:14.490]even though the efficiency is lower than other solar cells.
[00:01:17.490]So it's important to use a cheap way to implement
[00:01:21.170]the solar cells.
[00:01:23.850]Okay. Now, among all these solar cells
[00:01:26.600]the solar cells that I am interested are
[00:01:29.860]the quantum dots sensitized solar cells.
[00:01:32.250]Quantum dot sensitized solar cells have drawn attention
[00:01:35.390]due to their low cost
[00:01:36.530]and extraordinary power conversion efficiency performance.
[00:01:39.900]Now, are which now, which is about 18% and are scalable
[00:01:43.650]to very large areas.
[00:01:45.890]Among all the different quantum dots
[00:01:48.100]the group of two-fours semiconductor quantum dot
[00:01:50.950]for example, cadmium sulfide- zinc sulfide.
[00:01:53.440]They have shown promise as solar cell materials
[00:01:57.270]for quantum dot sensitized solar cells.
[00:01:59.510]So the goal is to make such materials
[00:02:01.840]into efficient solar cells
[00:02:03.310]with fabrication techniques that are scalable
[00:02:06.280]to large area and inexpensive.
[00:02:10.150]Okay. Now, the group of two-four semiconductor quantum dots
[00:02:15.150]they have unique electronic and optical properties.
[00:02:18.890]Zinc sulfide have a band gap of 3.54eV
[00:02:22.070]and cadmium sulfide have a band gap of 2.42eV.
[00:02:26.390]Because the band gap are large,
[00:02:29.000]these materials could be used as window solar cells.
[00:02:33.290]You can still see out through the solar cell
[00:02:35.600]but the UV will be blocked
[00:02:37.110]and it will generate current.
[00:02:40.300]For photovoltaic applications if you want to absorb
[00:02:43.360]the light in the visible region,
[00:02:44.940]then you need to reduce these band gap.
[00:02:49.110]To reduce the band gap,
[00:02:50.830]there are different ways that we can do it.
[00:02:53.570]We can either change the size of the quantum dots,
[00:02:55.990]you can see by decreasing the size,
[00:02:58.580]the band gap increases.
[00:03:00.440]And also you can either add a shell to the core,
[00:03:05.530]which also changed the band gap.
[00:03:07.160]And third method is we can dope with transition metals.
[00:03:12.240]So in this talk, I'll be talking about this third method.
[00:03:16.150]Understanding how a dopant influence the band structure
[00:03:19.260]is important to better band gap engineering
[00:03:21.900]and thus achieving high efficiency photovoltaics.
[00:03:27.360]Okay, now let's see how we synthesize these quantum dots.
[00:03:31.140]So we use a wet chemical method to synthesize
[00:03:33.833]these quantum dot.
[00:03:35.380]First, we first the Zinc acetate dihydrate will dissolve
[00:03:40.040]in dimethyl sulfoxide and then 1-thioglycerol
[00:03:45.220]was added drop wise.
[00:03:47.370]The mixture was then heated to around 60 to 70 Celsius
[00:03:51.970]with constant stirring.
[00:03:53.750]And once it reached the temperature Sodium,
[00:03:57.450]aqueous Sodium Sulfide solution was injected to the mixture.
[00:04:00.470]And heated for nine hours with constant stirring.
[00:04:04.210]To prepare the quantum dots,
[00:04:06.350]the doped quantum dots with the desired
[00:04:09.770]transition metal,
[00:04:11.570]The transition metal was added to the Zinc acetate solution
[00:04:14.610]in the very first step.
[00:04:16.470]And in order to percipitate out the quantum dots
[00:04:18.730]a ratio one to six milliliter of unprecipitated solution
[00:04:22.010]to the acetone was used
[00:04:24.010]and the resulting powder were rinsed and centrifuge
[00:04:27.250]with methanol and isopropanol three times each.
[00:04:31.810]Okay. And once we get the powder
[00:04:35.180]we dissolve them in acetone.
[00:04:37.050]And here are the solutions that we obtain.
[00:04:40.980]Here, this a shows the
[00:04:44.200]pure zinc sulfide quantum dot
[00:04:45.920]and b,c,d show single doped zinc sulfide,
[00:04:49.120]and e,f,g shows the double doped zinc sulfide,
[00:04:52.540]and h shows the tri doped zinc sulfide.
[00:04:56.110]X-ray diffraction measurement done on these doped
[00:04:59.290]and undoped quantum dot powders
[00:05:01.890]to investigate the crystal structure
[00:05:03.920]and the size of the quantum dots.
[00:05:06.334]XRD of both undoped and doped shows a zinc blende structure
[00:05:12.150]with broad peaks. And the doped quantum dot crystal structure,
[00:05:16.180]size and lattice remain mostly unchanged.
[00:05:19.150]And average size that was obtained was around
[00:05:23.050]two to three nanometer.
[00:05:26.210]Okay. And optical absorption measurements were done
[00:05:30.190]on doped and undoped quantum dot solutions.
[00:05:32.740]And here figure a and b shows
[00:05:34.460]these optical absorption spectrums.
[00:05:37.380]The cobalt doped zinc sulfide
[00:05:38.940]and cobalt co-doped zinc sulfide with nickel and manganese
[00:05:43.840]and tri-doped zinc sulfide with nickel and manganese,
[00:05:48.020]they show a high peak around 500 nanometers.
[00:05:52.290]This indicate that doping
[00:05:54.040]with cobalt allow electronic transition
[00:05:57.250]at energies corresponding to the visible light wavelength.
[00:06:00.860]And the cobalt- nickel doped zinc sulfide
[00:06:03.630]has the highest absorbance in the visible range.
[00:06:07.980]Now the tauc plots were obtained for both doped
[00:06:10.780]and undoped quantum dots using these absorption spectrums
[00:06:13.960]and figure c and d shows these tauc plots
[00:06:17.700]that we obtained.
[00:06:20.020]Using tauc plots we can obtain the optical band gap
[00:06:23.130]of the materials and this band gap represent
[00:06:26.000]the minimum energy that required to excite an electron
[00:06:28.950]from the valence band to the conduction band.
[00:06:32.490]And here are the band gap values
[00:06:35.010]that we obtained using the tauc plot.
[00:06:37.860]For the pure zinc sulfide the band gap was
[00:06:40.560]it's around 3.9eV.
[00:06:43.550]And the band gap of single doped quantum dots
[00:06:46.540]has been reduced compared to zinc sulfide
[00:06:50.670]except for the cobalt doped zinc sulfide
[00:06:52.820]which has a higher band gap
[00:06:54.980]in which the band gap has been increased.
[00:06:58.240]And it is also important to notice that
[00:07:00.670]the band gap of doubled doped quantum dots,
[00:07:04.262]nickel-manganese and cobalt- nickel,
[00:07:06.100]they, the band gap seems to be wider
[00:07:09.660]in comparison to undoped and single doped
[00:07:13.900]quantum dots.
[00:07:15.610]Now these transition metals
[00:07:17.630]they have different oxidation states
[00:07:19.650]which affect the change in the band gap.
[00:07:22.078]Zinc sulfide has vacancies in it's lattice
[00:07:24.930]and by doping with the transition metals
[00:07:27.920]can interact with these vacancies
[00:07:29.710]and could change the band gap.
[00:07:31.500]So this is something that we need to study
[00:07:33.420]in depth. To do that, we need to do
[00:07:35.890]Photoemission Spectroscope measurement
[00:07:37.980]and Scanning Tunneling Spectroscope measurement on these
[00:07:40.770]doped quantum dots.
[00:07:42.200]By doing so we can get an idea
[00:07:46.130]how these co-doped and tri-doped systems,
[00:07:49.470]which transition metals has been interacting
[00:07:51.930]with the vacancy state in zinc sulfide lattice
[00:07:54.270]and which oxidation states.
[00:07:57.130]Okay. Now,
[00:07:59.460]as summary we did, we synthesize a single doped quantum dots
[00:08:04.830]and co-doped, tri-doped single dope quantum dots.
[00:08:08.560]And we did optical characterization
[00:08:11.080]and X-ray diffraction measurements.
[00:08:13.060]X-ray diffraction measurement tells us that by doping
[00:08:16.220]the crystal structure, lattice constant
[00:08:18.250]they have not been changed.
[00:08:20.250]And from the optical characterization.
[00:08:26.210]we obtained the tauc plots and using tauc plots
[00:08:28.790]we saw that the cobalt-doped quantum dots
[00:08:31.120]seems to have the co-doped quantum dots seems to have
[00:08:34.600]wider band gap in comparison to undoped,
[00:08:37.900]undoped and single doped zinc sulfide
[00:08:40.040]and absorption data shows that cobalt-nickel doped
[00:08:43.870]zinc sulfide has the highest absorbance
[00:08:46.720]in the visible range.
[00:08:48.630]which make the, these quantum dot
[00:08:51.930]the best candidate for optoelectronic device fabrication.
[00:08:55.590]And as for the future work as I mentioned before
[00:08:58.540]we are planning to do X-ray Photoelectron spectroscopy
[00:09:01.690]and Scanning Tunneling Microscope measurements
[00:09:05.730]and also we are planning to make a solar cell
[00:09:07.730]using this cobalt-nickel doped zinc sulfide
[00:09:10.960]and using a printing method in order to make this solar cell.
[00:09:18.450]I would love to thank Dr. Takashi Komesu
[00:09:21.140]and Dr. Andrew Yost for supporting this work.
[00:09:25.090]This project was supported
[00:09:26.560]by the Nebraska Public Power District
[00:09:29.030]through the Nebraska Center for Energy Sciences Research.
[00:09:33.730]Thank you.

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ZnS Quantum Dots Doped with Transition Metals for Photovoltaic Applications

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