Enhancing Thermophotovoltaic Performance
Info: 3072 words (12 pages) Example Research Project
Published: 11th Oct 2021
Thermophotovoltaics have become very widely used and are on their way to becoming one of the most cost-efficient ways of generating energy from renewable and green sources. The Paper here proposes an idea to enhance the performance and cost-efficiency of the setups by combining a wide array of concepts and researches that have been done previously in order to improve their performance. The challenge is to integrate all these existing ideas in one setup to have benefits of all the technological researchers that have been done in the past.
As we know that a lot of the solar energy that we receive from the sun is still left untapped in the near and mid-infrared regions and the potential of the energy that we can get is also high, so our focus is to use graphene blackbody emitter to tap that energy , also the emitter system with silicon/silicon dioxide gives very fast and bright emissions which makes the energy generation faster to match the growing needs of the growing population.
The graphene emitter system is made very thin to make emission in the infrared range (however graphene layer is already taken very thin and its operation in this energy range is already excellent so much work shouldn't be needed in this case). The Photovoltaic cell is made from InSb which has a wavelength range of 1-5.5 micrometer Hongzhi Chen, King Wai Chiu La  for tapping energy which is like the back body graphene range of emissions but making the system thin would make the range more narrow and precise as done by Jonathan K. Tong, Wei-Chun Hsu so that we can target the wavelength regions where all the heat energy lies.
Even though the ranges for emission and absorption are same for the emitter and the semiconductor there still might be some mismatch that occurs during energy transfer , to prevent that the cell should be covered with graphene as done by Rongqian Wang, Jincheng Lu  .Same material coating improves the absorption of energy and makes it more resonant with the emitting material.
Finally, use of metal back reflectors can be made to enhance the efficiency of power that we get from this setup. Use of a perfect metal is recommended by Jonathan K. Tong, Wei-Chun Hsu  as it prevents emission and absorption after the energy has been harnessed which facilitates the energy conversion. However, Jonathan K. Tong, Wei-Chun Hsu used Ag with MgF2 spacers as it is very difficult to make and use perfect metal so use of Bragg mirror can also be made to achieve efficiencies as high as achieved via perfect metals.
Successful integration will allow us to tap most of the energy in the infrared spectrum which is currently being wasted. This coupled with the normal solar cells will allow us to harness the entire solar spectrum thereby raising the energy output from a green and renewable source of energy and the more energy-efficient solar cells become, the less will be the cost associated with the technology. That is less area would be needed for power generation and more and more tech companies would divert toward solar energy as the primary source of energy generation which could lead to a high rate of replacement of polluting non-renewable sources of energy like coal, petroleum, natural gas, etc.
Graphene thin-film high-speed emitter used in thermophotovoltaic cells for improved performance of the thermophotovoltaic cell.
Thermophotovoltaic materials emitter require high absorption and high emittance for improved performance of the cell. So here we try to target two most important properties that are - allowing most of the incoming light to be harnessed in a material and the ability to pass the photoelectric carriers quickly hence reducing the thermal losses that might happen in between by reducing the size of the system, that is the photons pass easily with less loss of energy , having a high-speed emitter which will decrease the photon transfer time . Graphene is a miracle material with lots of applications. Because of its high thermal and electrical properties, it has a wide range of applications. The use of graphene in solar cells is done in many ways like - Graphene with silicon solar cell, polymer solar cells, etc but all these applications focus on the inclusion of graphene at the junction of the solar cells to improve the electro flow rate, in this body of work, the focus is on use of graphene as an emitter on silicon or silicon dioxide surface to have a bright, planar and uniform light emission.
Graphene blackbody emitter which can absorb and emit the energy especially in that Near-infrared and mid-infrared region which is where most of the solar energy remains untapped. Research has been done previously by Yusuke Miyoshi, Yusuke Fukazawa  which states that graphene black body could be used for the high-speed emission in optical communication. The same principle could perhaps be applied in the thin film thermophotovoltaic cells. This type of emitter can be a good substitute for other rare earth materials that are relatively expensive to use because of their limited availability like platinum, yttrium oxide, etc. The near field region has a lot of potential for energy harnessing because in the far-field energy region where most of the current thermophotovoltaics operate the energy transfer is limited by propagation through the air. Near field, energy harnessing can overcome that limitation but the challenge is the low availability of materials for in photovoltaic cell that has a shorter bandgap as such energy could only be harnessed by semiconductors or materials with low bandgap values like indium antimony which has an approximate band gap of 0.17 electron volt V. B. Svetovoy and G. Palasantzas and the can tap 1-5.5 micrometer.
But to avoid any mismatch between the Photo Voltic cell collection and the graphene blackbody emission we can tune the system by attaching a layer of graphene above the photovoltaic cell as is done by many researchers before Rongqian Wang, Jincheng Lu . This allows us to have improved coupling between the photovoltaic cell and our emitter and increases the performance by amplifying the near field thermal radiation.
This system could allow us to improve performance to a much greater extent by integrating past researches on the thin field emission, graphene blackbody emission for high speed emission allowing us to transfer more energy in a short time and reducing distance between emitter and cell to harness the full energy spectrum.
The graphene emitter is highly integrable that is it can be fabricated by using the chemical vapor deposition technique (to grow a single atom thick layer) and then it can be used in various positions in the silicon wafer. A two-dimensional array of graphene emitter has been made by Yusuke Miyoshi, Yusuke Fukazawa  which shows an array with a uniform intensity of light obtained from channels of graphene as shown in the Figure below.
Figure 1- Diagram of emission from 85-layer graphene device at a data rate of 1 Mbps under a random square voltage 
The research indicated that graphene emitter could be used as a high-density emitter on silicon chips/wafer as the array density of graphene was approx. a hundred times higher than a typical semiconductor light emitter maybe like hexagonal boron nitride etc.
Also, they used an alumina oxide insulation capping by atomic layer deposition.
Figure 2- The Figure shows emitter setup with multi-mode optical fiber. 
The Figure above shows their set up. The MMF here is multi-mode optical fiber which they used for optical communication application but instead, the proposition in this study is to use the emitter set coupled with an InSb photovoltaic cell with a graphene layer on its surface to improve emitter and cell coupling and enhance the Near infrared radiation tapping. A similar kind of setup has also been made by Rongqian Wang, Jincheng Lu  in which they used H- boron nitride emitter and graphene hybrid.
Figure 3-The Figure shows. A thermal emitter of temperature Temit made of h-BN-graphene heterostructure is placed with of a thermophotovoltaic cell of temperature Tcell made of an InSb p-n junction. The red arrows represent the heat flux radiated from the emitter to the cell. 
As shown in the Figure above but the system proposed with graphene emitter might be able to have much higher emission rates. Also, if we use thin-film setup for the emitter and the PV cell as used by Jonathan K. Tong, Wei-Chun Hsu we can improve the rates even further the concept here is to make a very thin emitter and cell to have a faster transfer of photons , electrons/holes and to have emission in the near-infrared and mid-infrared wavelength region.
In Jonathan K. Tong, Wei-Chun Hsu they reported that for Ge emitter systems with GaSb cell, the spectral heat flux that we get has a strong shape with longer wavelength slightly suppressed and shorter wavelengths (in sub near-infrared of 1-2 micrometer ) relatively enhanced as shown in the Figure below. However, the wavelength range from a blackbody would be in approx. 1-5 micrometer range in our case which could lead to better tapping in the solar spectrum. Also, the use of metal contacts as back reflectors on ends of emitter and cell could increase efficiency dramatically.
Figure 4- The Figure shows spectral heat flux that we get from a thin film Ge emitter in near and mid-infrared range th is the emitter thickness and tc is the thickness of cell. 
The combination of all these factors as mentioned above should lead to much greater performance than from changing one of the parameters or using one of the approaches. So, the system proposed in this paper would be like Figure 3 but with graphene with silicon emitter as shown in Figure 2. And both the cell(semiconductor ) and emitter are very thin with approx. 150 -1000 nm as Jonathan K. Tong, Wei-Chun Hsu did with a 100nm gap in between.
First Graphene black body high-speed emitter is synthesized by using chemical vapor deposition. As mentioned by Yusuke Miyoshi, Yusuke Fukazawa  the single layer of graphene sheet can be grown out and transferred onto a silicon oxide/silica substrate using polymethyl methacrylate. The patterning on the substrate is done by electron beam lithography, the oxygen plasma etching is done with Nickel mask followed by HCL removal.
Then Ti/Pd electrodes can be designed to be waveguides on an undoped silicon substrate for highspeed light emitters. The capped emitters as discussed above for better operation in air should be fabricated by deposition of 75-nm-Al2O3 insulator on the graphene emitter by an atomic layer deposition method. Yusuke Miyoshi, Yusuke Fukazawa .
After this the emitter system should be coupled with the photovoltaic cell made of InSb which as stated above has a low bandgap making it easy to tap the energy in the near-infrared and midinfrared wavelength range. Also, in the near-infrared and the mid-infrared range this cell should have a graphene layer to prevent mismatch of energy transfer from emitter to the cell.
The emitter if made thin leads to a thermal well effect thus producing the radiation in a specific infrared wavelength analogous to what is shown in Figure 4 for Ge emitters. Also decreasing the PV cell thickness has been reported to increase the efficiency of heat transfer. As given by Jonathan K. Tong, Wei-Chun Hsu the efficiency of 20.7 percent is achieved for a GaSb cell for emitter temperature of 2000 K and a collector temperature of 300 K. The metal reflectors (they used silver with MgF2 spacers) at the ends of both emitter and the PV cell are proposed to increase the efficiency even further to approximately 38.7 percent for the Ge emitter Jonathan K. Tong, Wei-Chun Hsu but this is if we use pure metal back reflectors . Similar kind of results could also, be expected for this integrated system.
The distance between the emitter and the photovoltaic cell should be kept as low as possible in 100 nm range. Ognjen Ilic, Marinko Jablan states that the Separation gap is an important parameter for enhancement of black body transfer rate, for separations in the order of tens of nanometers high theoretical efficiencies (≈ 40%) compared to conventional thermoelectric, and high power densities (120 W cm2 at T =1200K for the hot-side) compared to state-of-art farfield TPV(thermophotovoltaic ) systems can be obtained.
The emitter is kept at a temperature which is higher than the temperature of the photovoltaic cell so that energy flows from high to low temperature. The plasmons that are generated in graphene interact strongly with the infrared photons that enhances the thermal radiation due to the strong light-matter interactions. As stated in Rongqian Wang, Jincheng Lu  the fact that the emitter and the absorber (that is the layer of graphene on the photovoltaic cell) makes the heat transfer more efficient due to resonant exchange of energy between the emitter and the absorber.
The energy transferred is then finally converted into electricity by the photoelectric conversation of infrared and near-infrared wavelengths.
Benefits of the integration and challenges
If the integration of all the above elements is successful, we can have many benefits like:
High-speed emission from graphene with Silicon/Silicon Dioxide that is also very bright and uniform this could not even increase the efficiency but also the speed at which the energy is transferred / harnessed from the sun rays. The Response time for the optical communication setup was reported as 100 Ps (picoseconds) by Yusuke Miyoshi, Yusuke Fukazawa .
The graphene layer on the InSb photovoltaic cell enhances the near field thermal radiation tapping, minimizes any mismatch between radiation emitted and absorbed as the emitter and layer on top of Photovoltaic cell are made from the same material there is a resonant energy transfer.
The use of InSb can facilitate the near and mid-infrared energy tapping as the bandgap for this is very low around (0.17 ev).
The distance reduction between the emitter and the photovoltaic cell can let us harness most of the energy in the near field range and we get a good spectral heat flux curve with thin-film emitters as shown in Figure 4 which will allow us to tap the near-infrared thermal energy, as most of the energy from sun rays exist in that region .
The metal back reflectors are also used as mentioned earlier by Jonathan K. Tong, Wei-Chun Hsu which are known to increase the efficiency of the system even further.
As graphene is relatively cheaper than other semi-conductor emitters such as Hexagonal boron nitride as used by in Rongqian Wang, Jincheng Lu  which is also shown in Figure 3 and it can improve the cost feasibility of the configuration.
The integration of all these parameters could potentially improve the performance of the solar cell on a completely different scale. As most of the researches have already been done for each of the components, there is a high possibility that the integration would lead to a further improvement in the efficiency, performance, and decrease in the overall cost of the setup.
The Challenge could be unforeseen errors that may arise when we try and integrate all the components. However, steps to prevent mismatch have been taken like stated by in Rongqian Wang, Jincheng Lu  by using the same kind of materials at the interface of photovoltaic cell and the emitter being the same material, but actual experimentation might reveal some other problems. Also making everything in nanoscale could increase the cost of the setup however a general micro scale system like in Figure 4 thickness system could also give great performance and efficiency if cost becomes too high for making precise nano scale setup.
2. DOI: 10.1038/srep10661
3. DOI: 10.1103/PhysRevApplied.12.044038
4. DOI: 10.1038/s41467-018-03695-x
5. DOI: 10.1103/PhysRevApplied.2.034006 ·
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