Fig 1: Quantum dot semiconductor containing concept CdSe a n-type semiconductor(Red) and PbS a p-type semiconductor (Grey)

Quantum dots (QD) are nanoparticles comprised of semiconductor materials, which exhibit enhanced electronic properties over bulk materials. These properties are defined by a highly adjustable structure of the quantum dot that yields enormous potential in many applications, including photovoltaic cells.[1] Photovoltaic cells (solar cells) convert light to electrical energy through the use of electron-hole pairs in a circuit. If combined with a quantum dot, a high quantum efficiency (percentage of photons transformed into current) is produced, generating a usable total efficiency.[2] The viability of dissolving these quantum dots into a suitable ink for 3D printing onto a universal solar cell and its processes are analyzed and discussed.


Overview

A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell (in that its electrical characteristics—e.g. current, voltage, or resistance—vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source. [1]

Quantum dot solar cells are often made by depositing quantum dots on thin film ceramics[2].

Solar cell concept [3]
  1. Make a basic diagram for an electronic device that could use this semiconductor and post diagram in your project page.

3D printed Device

Our photovoltaic cell would use CdSe and PbS quantum dots to convert photon's energy into current. They will be layered with a p-type (PbS) layer on top, a bulk nano-heterojunction (BNH), or mix of the two quantum dot types, in the middle, and an n-type (CdSe) layer on the bottom. Each ink will only be deposited in one layer by the 3D printer (the layer thickness of 0.3 mm in RepRaps[4] is already much thinner than the typical nanometer scale[5]), except for the CdSe layer, which will be deposited twice. This is because the increased n-type doping is suggested in BNH devices. Complete solidification between ink applications may not be necessary, as intermixing between layers may expand the BNH. There will also be carbon nanotubes dispersed throughout the layers to enhance conductivity.

The matrix that the quantum dots and single wall carbon nanotubes (SWNT's) would work in is ABS. ABS is already used in many 3D printing applications. It was selected because it has a low enough printing temperature (about 220° C) to work in a low temperature extruder, but a high enough temperature to retain its properties despite extended periods of time in direct sunlight. The ABS that would be used has a resistance of 104 ohm-cm, which would hopefully be improved by the addition of SWNT's.

The layers of the semiconductor portion are raised above a solar cell template: CdSe(crimson), mixed layer(dark red), PbS(Grey)

Materials and Processing

  1. A detailed BOM of the chemicals used to make it with links to sources and prices in a table - and total cost.

5

  1. Links to MSDSs for each of the chemicals.

6

Ink Synthesis

Sites have been found that would allow us to buy the QD's, ABS, and SWNT's. An advantage of purchasing the QD's from NOM corp is that they also have a CdS shell around the PbS, and a ZnS shell around the CdSe core. This higher band gap shell helps to prevent recombination. The Dia graphic describes how QD's could be synthesized at home.

The semiconductor will be made using 3 separate inks: a PbS ink, a CdSe ink, and a mixed ink. All of these inks should have 1x1016[6] QD's per cm3 and 100 mg/L of SWNT's[7]. Based on these concentrations and a print volume that was determined to be 1.95 cm3, the necessary quantities of each material is given in the 'Amount Needed' column of the bill of materials. This process will require a hopper, a heater, a screw extruder, and a mold. The synthesis process of each ink can be summarized with these steps:

  1. The ABS will be melted at 250° C.
  2. The melted ABS and appropriate concentrations of QD's and SWNT's will be mixed in a hopper.
  3. The liquid ink will be extruded in a mold with the same dimension (1.7 mm diameter) as before.


7

  1. Dia is a program that creates elaborate flowcharts and diagrams to simplify complicated processes. The figures shown to the right outline the basic steps taken to synthesize and 3-D print quantum dots, more specifically CdSe and PbS. The formation of these two quantum dots is also outlined.

8

  1. Outline purification methods and the methods needed to obtain acceptable purity for your material.

9

  1. A description of the testing procedures, equipment and specifications for the equipment used to determine if you obtained your target compound.

Testing

Ideal Semiconductor Ink Properties

This ink is mostly focused on extending the lifetime of charge carriers in the semiconductor. However, another factor that should be considered in the making of this ink, as in all photovoltaic cells, is the range and efficiency of the absorption of photons. These properties will help increase the efficiency of the ink.

  1. Homogenous distribution of carbon nanotubes among the quantum dots will help conduct improve its ability to act as a charge acceptor.
  2. Homogenous distribution of the quantum dots allows for more efficient use of the surface area.
  3. Intermixing of the p-type and n-type QD's in the middle layer to allow a greater interfacial area for diffusion to occur to the junction before recombination occurs within the p- or n-section[8].
  4. Range of QD diameters so that a wide range of photon energies can be absorbed[9].
  5. Thin layers.

In-Situ Analysis

This process takes the trouble of melting and re-solidifying the ABS in a form compatible with the low temperature extruder before 3d printing the ink. This allows us to perform in-situ analysis on the ink before it is printed.

To test the homogeneity of the quantum dots within the ABS, it would be useful to section off some of the semiconductor ink and take an SEM (scanning electron microscope) image of the section. A coating of chromium or graphite may need to be applied if the ink is not conductive enough[10]. This in-situ analysis would allow the mixing of the QD's to be tested before they are printed out as a solar cell. Also, if a range of QD sizes are observable within the solidified ink, it follows that a range of photon energies will be able to be observed.

If the ink is not suitably homogenous, the ABS ink can be remelted and remixed, possibly at a higher temperature to increase solubility. This in-situ analysis can be repeated until the ink is suitably homogenous.

Post-fabrication Testing

  1. A description of the testing procedures, equipment and specifications for the equipment used to determine if you obtained your target compound.

Discussion

3D Printing Applications

Everyone has already seen the old calculators that use solar cells to help power them. It's quite probable that in the future many smart phones, tablets, and other user devices will run on solar power as well. Our application uses a material that many believe will greatly improve solar cell efficiency, quantum dots, as the solar cell semiconductor for these consumer devices. 3D printing is the superior way to deposit quantum dots onto these devices. Several other techniques are used to deposit quantum dot semiconductors onto surfaces. These include templated assembly and spin casting. However, 3D printing has an advantage for printing solar cells over both of these deposition techniques.

The templated assembly requires that a pattern produced by a resist be placed on the substrate where the QD's are supposed to go. This allows for very precise placement of QD's, but is usually only used if one type of QD is to be deposited[11]. For solar cells using a p-type and an n-type layer, putting a resist on top of the bottom layer may disrupt the properties of the interface between the layers. Spin casting, on the other hand, allows for multiple layers, such as QD's and graphene, to be deposited on substrate, but it lacks precision [12]. Spin casting uses a flat wheel spinning rapidly to distribute the solution uniformly across the surface of the substrate. It does not allow for depositing around designs or across surfaces that are not extremely flat.

The watch design displayed in the OpenScad shows the advantages of 3D printing a small photovoltaic cell using QD's. It allows for different design choices to be made, such as the holes for separate dials in the watch, while still depositing layers evenly across the surface. It also allows for multiple layers, which a photovoltaic cell requires, to be deposited. Some limits are still in place before user devices will be a reasonable place to use this application. PbS a uses a heavy metal, which is frowned upon in our society for consumer products. Also, these cells could improve in efficiency if they are thinner. However, quantum dot materials and 3D printing layer thickness are sure to improve in the future, making this a very powerful application.


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  1. Design the semiconductor portion of the device in OpenSCAD, paste the code directly into your project page
The layers of the semiconductor portion CdSe(crimson), mixed layer(dark red), PbS(Grey) are shown as they would be printed
Colored rendition of the quantum dot semiconductor on the face of a wrist watch
The PbS(grey) is raised with a conceptual view of the ideal mixed layer being a homogeneous mixture
The layers of the semiconductor portion are shown to display thickness CdSe(crimson), mixed layer(dark red), PbS(Grey)


Code for wrist watch:

translate ([0,0,-3.005]) {color ("Black",1) cylinder (h = 3.5, r=25, $fn=100);}

// cutting into the first 2 layers (CdSe)

difference() {

translate ([0,0,.5]) {color ("Crimson",1) cylinder (h = 1.5, r=25, $fn=100);} translate ([0,7,1]) {color ("Crimson",1) circle (6.5, $fn=50); }

translate ([7.25,-5.5,1]) {color ("Crimson",1) circle (6.5, $fn=50); }

translate ([-7.25,-5.5,1]) {color ("Crimson",1) circle (6.5, $fn=50) ; }

}

// cutting into the mixed layer (CdSe + PbS)

difference() {

translate ([0,0,2]) {color ("DarkRed",1) cylinder (h = 1, r=25, $fn=100);} translate ([0,7,1.995]) {color ("DarkRed",1) cylinder (h = 1.5, r=6.7, $fn=50);}

translate ([7.25,-5.5,1.995]) {color ("DarkRed",1) cylinder (h = 1.5, r=6.7, $fn=50);}

translate ([-7.25,-5.5,1.995]) {color ("DarkRed",1) cylinder (h = 1.5, r=6.7, $fn=50);}

}

// cutting into the final layer (PbS)

difference() {

translate ([0,0,3]) {color ("Grey",1) cylinder (h = 1, r=25, $fn=100);} translate ([0,7,2.995]) {color ("Grey",1) cylinder (h = 1.5, r=6.9, $fn=50);}

translate ([7.25,-5.5,2.995]) {color ("Grey",1) cylinder (h = 1.5, r=6.9, $fn=50);}

translate ([-7.25,-5.5,2.995]) {color ("Grey",1) cylinder (h = 1.5, r=6.9, $fn=50);}

}

translate ([1,17,1]) {color ("Gold",1) cube([1,6,4]); }

translate ([-1,17,1]) {color ("Gold",1) cube([1,6,4]); }

translate ([-23,0,1]) {color ("Gold",1) cube([6,1,4]); }

translate ([0,-23,1]) {color ("Gold",1) cube([1,6,4]); }

translate ([17,0,1]) {color ("Gold",1) cube([6,1,4]); }

rotate([0,0,60]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,30]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,120]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,150]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,210]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,240]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,300]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

rotate([0,0,330]) translate ([17,0,1]) {color ("Black",1) cube([6,1,4]); }

translate ([0,-1.5,0]) {color ("White",1) cylinder (h = 6, r=1, $fn=50);}

rotate([0,0,35]) translate ([-1,-2,6]) {color ("White",1) cube([14,1.5,1]); }

rotate([0,0,270]) translate ([1,-.5,6]) {color ("White",1) cube([18,1.5,1]); }

rotate([0,0,120]) translate ([-1,.5,6]) {color ("White",1) cube([22,.5,1]); }

// small circles

rotate([0,0,0]) translate ([0,4,1]) {color ("White",1) cube([.5,6,.5]); }

rotate([0,0,0]) translate ([-2.75,6.75,1]) {color ("White",1) cube([6,.5,.5]); }

rotate([0,0,235]) translate ([-.25,6,1]) {color ("White",1) cube([.5,6,.5]); }

rotate([0,0,235]) translate ([-2.75,8.75,1]) {color ("White",1) cube([6,.5,.5]); }

rotate([0,0,135]) translate ([.5,6,1]) {color ("White",1) cube([.5,6,.5]); }

rotate([0,0,135]) translate ([-2.25,8.75,1]) {color ("White",1) cube([6,.5,.5]); }


15

http://www.appropedia.org/File:Watch_Face_STL.stl


Compiled and rendered version of the quantum dot layers on the face of a watch

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Important Facts about quantum dots

  • to generate more than one electron-hole pair for every photon absorbed.[13]

Project goals

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header 1 header 2 header 3
row 1, cell 1 row 1, cell 2 row 1, cell 3
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Discussion

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Conclusions

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References

  1. [Solar Cell - Wikipedia] October 11, 2013.
  2. QD solar cell company October 15, 2013
  3. Solarcellcentral October 11, 2013.
  4. RepRap layer thickness October 10, 2013
  5. BNH thickness October 10, 2013
  6. QD concentration October 10, 2013
  7. SWNT concentration October 14, 2013
  8. BNH interface October 14, 2013
  9. Diameter and absorption October 6, 2013
  10. SEM info October 14, 2013
  11. Templated assembly of CdSe October 6, 2013
  12. Spin casting with QD's and graphene October 6, 2013
  13. Solarcellcentral October 11, 2013.

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