There are so many questions to consider when starting to use QDs for solar cell development. What are the best properties to choose? QDs vary in emission wavelength, surface chemistry, solubility, etc.
Hello Zoraida, if you are particularly looking for high efficiency solar cells, then my suggestion would be to go with quantum dot sensitized solar cells. In quantum dot sensitized solar cells, you require one electron transporter (preferably TiO2) sensitized with QDs. For QDs, you can always work with CdSe or CdTe. PbS and PbSe are better options than CdSe and CdTe but they have certain problems such as stability and toxicity. Dye sensitized solar cells, the achieved efficiency almost approach to 20 %. Therefore, experimentally, it is possible that replacing dyes with QDs can also give to this much efficiency. Since, the DSSCs have much progressed and QDSSCs are still under development so you have lot of scope. First thing factor which defines the efficiencies is the absorption of the incident light. For this purpose, you can have low gap QDs and there lots of chemistry is involved. You can work with size and shape of the QDs to optimize the absorption. Now second factor comes is how much photons are converted into excitons and then the charge separation. This factor will depend device engineering and here comes the measure challenge, which is the energy gap between TiO2 and QDs. Currently, this energy gap between CB and VB of TiO2 and CdSe QDs are quite high and if these gaps decreases the solar efficiency will increase drastically.
Here, if you use PbS and PbSe you will face very less barrier. But mostly around the globe, scientists are focusing on CdSe and CdTe because of their several advantages. Now third factor is the electron transport in TiO2. This factor is well optimized by changing the geometry of TiO2. So here you can use the optimized TiO2.
Now Scientist have adapted many approaches to increase the absorption by using core shell QDs. They increase absorption but they decrease conductivity and also alter energy barriers. Therefore, increase in one factor is compensated by decrease in other factors.
Now being physicist, I particularly look for the solutions by not changing chemical compositions and other materials. So my thinking is first what can we do outside the device to enhance the efficiency. This can be done in some ways:
1. use solar cell concentrators to concentrate the solar light inside the cell. This can be done by fabricating micolenses on backside of your cell from where light is coming. By this method you can concentrate the light inside the area you desire. This will give you some enhancement of efficiency.
2. We can increase the absorption by having plasmonic nanoparticles such as gold and silver mixed in your cell. Plasmonic nanoparticles have very high electromagnetic field around them that will increase your absorption and thereby will give you some more enhancement.
In the last, my suggestion would be to chose a system and start working on it because then in the mean while you will certainly see some ideas which will also help us.
Small energy gap QD family would be a nice choice because of the potentially high efficiency. For example, PbS, PbSe, PbTe. Would the polarity of ligand a matter in charge transfer and even the properties of solar cell?
Hi Ting,
Thank you for your response. I am not an expert in solar cells that is why I m looking for suggestions from the experts. regarding your question, I can only make a conjecture but I have not tried such a study. I hope the experts will responds and give us their insights.
Core shell structured QDs are the best to use in solar cell like (CdSe)ZnS...however we talk more in detail next time...
You can have to consider the band gap of QDs. Its extremely important. Please our paper in this regards. We have achieved highest efficiency using Cds QDs and CuInS2 as co-sensitizer layer. For reference you can read our paper.
Improved conversion efficiency of CdS quantum dot-sensitized TiO2
nanotube-arrays using CuInS2 as a co-sensitizer and an energy barrier layer, J. Mater. Chem., 2011, 21, 16430
Nanotechnology 22 (2011) 015202
ACS Applied Materials and Interface VOL. 2 • NO. 10 • 2910–2914 • 2010
Hi Ghafar,
Thank you for the suggestion. Can you send me an electronic copy?
Till date CdS/CdSe holds good with a decent stability and good conversion efficiency. Co-sensitized QDs are relatively better. Even though PbS sounds interesting, its stability and recombination makes it a poor candidate. Check for QDs in the bandgap range of ~1.3 eV, it should serve your purpose well. And do check out for Sb2S3 they too are hot selling cakes recently for heterojunction solar cells.
Hi Ghafar,
Thank you for the references. I appreciate it. Check your email as well. I sent you a response.
Is Sb2S3 stable under photon irradiation? I found in one of the papers that it is not.
Ghafar Ali, Can you please send me your papers to my email id as well? I'll appreciate it very much.
What can the experts in solar cells say about this: a combination of small and smaller QDs are better than a homogeneous one size QDs for the development of solar cells. Also, will the cadmium containing QDs be more efficient and cost effective than the other QDs such as those containing Sb?
The best quantum dot for solar cells is yet to be invented. Many researchers and scientist are trying hard for that. As of now the efficiency at lab level is reached upto 29 per. But still as of now CdSe holds good due to its bandgap of 1.7eV. But the bandgao can be varied accordingly . As per the cost too as Zoraida said Cadmium is best suitable.
Parikshit Sahatiya, Can you please provide us the reference of '29% efficiency Qn. dot sensitized solar cell'? It's a very high efficiency for any kind of solar cell. I am very eager to see the paper.
Ghafar Ali, thanks. You can give me the references of your papers on Sb2S3 based solar cell material in this forum. I'll download the papers.
Else, you can attach the full paper in your publication folder in RG. I'll download them.
Quantum dots are a form of nanocrystals that are made from semiconductor material such as silicon, germanium, cadmium sulfide, cadmium selenide, and indium phosphide. Quantum dots are only 1 nm to 12 nm in diameter (a nm is one billionth of a meter). Billions of dots could fit on the head of a pin! Because of their small size, quantum effects arise due to the confinement of electrons and holes; as a result, material properties are very different than the normal material. One important property is that the band gap is dependent on the size of the dots. When excited from an external source, dots formed from semiconductors emit light in the visible range as well as infrared and ultraviolet, depending on their size. The higher-frequency blue light is emitted by smaller dots suspended in solution (larger band gap); red light is emitted from solutions with larger dots (smaller band gap).
Thanks for comments of Bedir Yosif , A Kathalingam (Kathu).
I think CdTe and CdSe are good to be Quantum dots for solar cells.
We have a project started on CdTe and CdSe Quantum dots for solar cells.
I agree with you all, CdTe and CdSe seem to be very good for solar cells but we do need to improve their efficiency. Let me know if you all have had efficiencies better than what is currently published.
I'd like to add a different perspective to this question: all of you refer to colloidal QDs, that are QDs obtained by chemical processes. I think it is relevant to know that QDs can be obtained also by thin film deposition techniques such as MBE (Molecular Beam Epitaxy) or MOCVD (Metal-organic chemical vapour deposition): here QDs are made of a material (such as InAs) that is epitaxial to the substrate (in this case GaAs). In this case dimensions cannot be controlled as easily in colloidal QDs and there also material limitations. On the other hand these QDs are naturally included in a semiconductor materials; this makes the realization of solid state devices such as solar cells much easier.
I also think it is important to understand that there are some novel physical principles for which QDs are an attractive structure: for colloidal QDs I understand the advantage resides in the possibility of having MEG (Multiple Exciton Generation), a process that should allow to have more than one electron for one absorbed photon. For epitaxial QDs, the interest reside in the possibility of having an IB (Intermediate Band) within the gap of the host material.
I think people working with QDs should be more aware of what is going on with QDs produced with both systems: I reckon that from the synergy of these two different approaches some substantial improvements in these nanostructures might be obtained...
Luca,
Thank you for your response. It is interesting to know that thin films of QDs deposited or created by MBE or MOCVD can also be for solar cells. Can you enlighten us about the advantages of these QDs for solar cells? DO you have any publication that explains thier good points and bad points? This will be very helpful.
hello Zoraida, which types of solar cell you are interested in: DSSC, solution processed polymer, inorganic. According to that I can suggest you.
I agree with Luca Seravalli. Thanks for all above comments. Our QDs interest is to got a wide band gap, so, the efficiency will be very high in white lights.
Arunandan, whichever will be best to provide the highest efficiency.
Hello Zoraida, if you are particularly looking for high efficiency solar cells, then my suggestion would be to go with quantum dot sensitized solar cells. In quantum dot sensitized solar cells, you require one electron transporter (preferably TiO2) sensitized with QDs. For QDs, you can always work with CdSe or CdTe. PbS and PbSe are better options than CdSe and CdTe but they have certain problems such as stability and toxicity. Dye sensitized solar cells, the achieved efficiency almost approach to 20 %. Therefore, experimentally, it is possible that replacing dyes with QDs can also give to this much efficiency. Since, the DSSCs have much progressed and QDSSCs are still under development so you have lot of scope. First thing factor which defines the efficiencies is the absorption of the incident light. For this purpose, you can have low gap QDs and there lots of chemistry is involved. You can work with size and shape of the QDs to optimize the absorption. Now second factor comes is how much photons are converted into excitons and then the charge separation. This factor will depend device engineering and here comes the measure challenge, which is the energy gap between TiO2 and QDs. Currently, this energy gap between CB and VB of TiO2 and CdSe QDs are quite high and if these gaps decreases the solar efficiency will increase drastically.
Here, if you use PbS and PbSe you will face very less barrier. But mostly around the globe, scientists are focusing on CdSe and CdTe because of their several advantages. Now third factor is the electron transport in TiO2. This factor is well optimized by changing the geometry of TiO2. So here you can use the optimized TiO2.
Now Scientist have adapted many approaches to increase the absorption by using core shell QDs. They increase absorption but they decrease conductivity and also alter energy barriers. Therefore, increase in one factor is compensated by decrease in other factors.
Now being physicist, I particularly look for the solutions by not changing chemical compositions and other materials. So my thinking is first what can we do outside the device to enhance the efficiency. This can be done in some ways:
1. use solar cell concentrators to concentrate the solar light inside the cell. This can be done by fabricating micolenses on backside of your cell from where light is coming. By this method you can concentrate the light inside the area you desire. This will give you some enhancement of efficiency.
2. We can increase the absorption by having plasmonic nanoparticles such as gold and silver mixed in your cell. Plasmonic nanoparticles have very high electromagnetic field around them that will increase your absorption and thereby will give you some more enhancement.
In the last, my suggestion would be to chose a system and start working on it because then in the mean while you will certainly see some ideas which will also help us.
Thank you Prof. Arunandan Kumar for discussing the factors separately which are critical in enhancing solar cell efficiency.
I am glad to see that you are in Universite de Bourgogne. It reminds me of my stay in Dijon for few months in 2002 as 'Professeur Invite' in the same University.
I have a question here. Why are Sulphides, Selenides and Tellurides mostly used as Qn. dot sensitizers? Only because of their low band gap? I know that another important point is band alignment with host matrix, like TiO2. But how to analyse this point?
Hello Dr. Suchitra, Nice to hear from you that you were in Dijon, it is a very nice place. I am just a postdoc in University de Bourgogne so can you please call me arunandan only because Prof. term sounds more respect than I deserve right now.
Your point is correct sulphides, selenides and tellurides are generally used because of their low band gaps.
And the second question is rather more challenging and is basically limiting the efficiency of QD SSCs. Currently, what people do is that they use the conduction and valence bands of individual TiO2 and QDs to analyze the interface. However, it would be more advantageous to analyze the band alignment for this interface. I have never worked with interface studies of QDs with other semiconductors. But according to my knowledge, It can be studied by UPS, however, UPS is not so frequently available, but UPS studies can give real picture of this interface.
Thank you so much, Arunandan for your valuable discussion. UPS is probably the right method for band structure/ band alignment studies. You are right. It is not that easily available.
Nice to read your mail about Dijon. I was in France several times and I love that country and the people. You will be surprised to know that I still have friends in Univ de Bourgogne and I am in regular touch with some of them.
Anyway.. best of luck for your career.
According to our researchs in this field, we had tuned the photocurrent response through controlling the size ( varying the band gap) of cadimium chalcogenides QDs . We concluded that CdSe QDs is the best candidate from CdX (X: S, Se, and Te) QDs to serve as a sinsetizer. The suitable CdSe QDs size is 4.54 nm corrosponding to 540 nm absorbtion edge.
For more details, see my publications.
I have a question in this context. Is there any data base for energy level values in eV (for VB and CB) of different semiconducting materials, relevant in solar cell application, like TiO2, SnO2, CdS, PbS, CdSe etc.? Can some one provide me the source of the data base, at least for the bulk material?
I do not know about any database but this book could be of use- Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices
By Roland Scheer, Hans-Werner Schock
Ali Badawi, what is the best energy conversion efficiency η you have achieved in CdSe sensitized cell? Did you use liquid electrolyte? Can you indicate the reference of the best efficiency work here?
We adsorbed different sizes of CdSe QDs to work as a sensitizer directly onto TiO2 NPs layer ( this technique is called Direct Adsorption DA ). The max. energy conversion effeciency was about 0.3 % corrosponding to 540 nm absorption edge of CdSe QDs.
More details, visit my publications.
Small gold or silver nanoparticles (10-50 nm) are extremely efficient in coupling light with electrons because of plasmonic effect, enhancing the efficiency. See for examples: Appl. Phys. Lett. 101, 053109 (2012) and Appl. Phys. Lett. 93, 191113 (2008)
Chromophors will have very good fluorescent efficiency
CdS/CdSe are also useful for the best solar cell applications.and in case of energy transfer those two are potential candidates .some of the applications of solar cell may be utilized by CuO windos .i think it is best of my knowledge
Some people are of opinion that PbS/PbSe are better candidates than CdS/CdSe. Can someone please shed light on this?
Regarding toxicity, which ones are comparatively better?
I have not tested the PbS/PbSe toxicity. However, there are publications from various groups about this.
Check this out: http://pubs.rsc.org/en/content/articlelanding/2013/cs/c2cs35392j#!divAbstract
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