Quantum Dot Solar Cells

Quantum Dot Solar Cells

2 INTRODUCTION: As industrialization speeds up in developing and under-developed countries with an alarming rise in population, global power consumption has become a big question mark as power resources are exhausting rapidly. Most of the global power is obtained through the burning of fossil fuels which produces carbon dioxide, a greenhouse gas. This has brought adverse effect on the atmosphere through global warming that has resulted in elevating temperatures across the world thus, creating an unbalance in the environment. Lescvhkies, K. S., (2009) quoted that atmospheric carbon dioxide concentrations have increased from 5 parts per million (ppm) to 385 ppm during the 20th century. As a consequence, the world is in a greater risk of facing drought and reduced crop yields if measures are not taken to control temperature (Lescvhkies, K. S., 2009). Scientists around the world have been working on to find alternatives of power sources that are environmental-friendly. Some of the few options available are nuclear power, hydroelectric power, geothermal power, wind power, tidal power and solar cell. Nuclear power is a complex technology usually used by developed countries. And in case of any mishap, the exposure to this may cause harmful effects on the people and surroundings. Hydroelectric power is limited to geographical conditions i.e. upon the amount of rainfall or location of rivers, lakes etc. A similar case is of tidal power that depends on climatic and weather conditions. In this regard, solar cell is safer, easier and abundant technology that can be utilized to generate power by all countries (Shang, X., 2012). Figure 1 shows solar light spectrum according to different atmospheric levels (Shang, X., 2012). Figure 1 Solar Light Spectrum (Shang, X., 2012).

3 SOLAR CELLS: Solar energy has been termed as the clean energy because it reduces the above mentioned dangerous effects on the environment and the people. However, even this technology suffers from some challenges. Firstly, it is very expensive to incorporate solar cell components making it less friendly in terms of cost effectiveness. Secondly, the power efficiency of solar technology is only 30% which is extremely low (MENSAH., n.d.). A solar cell works on the principle of photovoltaic effect. Figure 2 shows the structure, I-V curve and band diagram of a p-n junction solar cell. During equilibrium, the free electrons from the n region and the free holes from the p region recombine forming a depletion layer at the junction. The resulting positive and the negative ions i.e. the donor and the acceptor ions then create an electric field. Since there is no voltage output so the Fermi level stays flat. Once exposed to sunlight, excess electrons and holes are generated due to photon absorption which move in the opposite direction due to the effect of the electric field. The I-V curve of solar cell marks three important cases (Shang, X., 2012). Case 1 Open Circuit: If no load is present then the excess carriers generated due to photovoltaic effect will produce excess voltage called Vout. However, they will also reduce the electric field thus, minimizing the photocurrent value to zero. Case 2 Load: In case an external load is applied to the solar cell, then the excess carriers flow through it increasing the value of photocurrent and reducing the value of output voltage. As this continues, a point comes where the efficiency is maximum shown by the rectangular region in the curve. Case 3 Short Circuit: If the output connection is a short circuit then no voltage is observed at the output while the excess carriers produce maximum photocurrent. Figure 2 Structure, I-V curve and band diagram of a p-n junction solar cell (Shang, X., 2012).

As already discussed, solar cells produce an output efficiency of 30% which is too low. There are several reasons behind this value of efficiency. Following are the main factors that contribute to this low value: 4 Transmission losses Thermal losses Recombination of carriers Sunlight reflection at solar cell surface Resistance Temperature Out of the above mentioned factors, the first three play a vital role in the significantly decreasing the value of the solar cell s efficiency. In order to overcome this issue, the idea of quantum dots was proposed (MENSAH., n.d.). QUANTUM DOT SOLAR CELLS: In typical, bulk solar cells, photons with energy less than the bandgap are not utilized while photons exceeding the bandgap energy are accounted for which emit hot carriers. However, on cooling down the extra energy is wasted. Quantum dots have the advantage that the band gap is tuned in with the solar spectrum hence, reducing the problem faced with semi-conductors. One of the difference between semiconductors and quantum dots is that semiconductors can be easily classified as amorphous and crystalline while quantum dots have several types of twodimensional sheets and three-dimensional arrays. Their junctions can be prepared on costfriendly materials like plastic, glass and metal sheets (Jasim, K. E., n.d.). Since, electrons and holes are freely moving in a bulk semi-conductor so their energy is continuous and as the energy bands are close together so they form a continuous energy band. However, in case of lesser number of atoms the density of states become discrete. This gives rise to quantum wells, wires and dots. Bulk semi-conductors are three dimensional systems, quantum wells are two dimensional system, quantum wires are one dimensional system while quantum dots are zero dimensional system. Figure 3 shows the density of states of all these four systems schematically (Young, J., 2011). Figure 3 Schematic of density of states of Bulk semi-conductor with parabolic band diagram, Quantum well with step function band diagram, Quantum wire with spike like band diagram, Quantum dot with delta function band diagram (Young, J., 2011).

Since quantum dots have fewer number of atoms so excitations are only confined to a small space within the dot that produces discrete energy levels. Another difference is that in bulk semiconductors when a photon with energy greater than the band gap is absorbed then electron-hole pair generation takes place in which electron from conduction band migrates to the valence band. However, in case of quantum dots when a photon is absorbed in the above mentioned condition then the excited electron which is also called a hot carrier generates several electron-hole pair. Figure 4 explains this effect in the energy band diagram (Jasim, K. E., n.d.). 5 Figure 4 Thermalization process in a) Bulk semi-conductor b) Quantum Dot (Jasim, K. E., n.d.) STATE OF THE ART IN QUANTUM DOTS: There are there main types of quantum cells that are used to utilize solar energy. They are: Quantum Dot Sensitized Solar Cells: The cell structure of QDSC is shown in figure 5. The structure consists of a mesoporous oxide film, QDs, electrolyte and a counter-electrode. When the photons are absorbed then electron-hole pairs are released rapidly which are separated into electrons and holes at the oxide interface. The electrons pass through the oxide layer while the holes are discharged through the electrolyte (Tian, J., & Cao, G., 2013). Figure 5 Schematic illustration of Quantum Dot Sensitized Solar Cell (Tian, J., & Cao, G., 2013)

6 Quantum Dot Solar Cells based on Bi-layer Heterojunctions: In order to improve reliability of quantum dots, a bi-layer heterojunction was introduced between a layer of p-type quantum dots and n-type quantum dots of different material. Figure 6 shows its labelled structure (Lescvhkies, K. S., 2009). Figure 6 Structure of Bi-layer Heterojunction Quantum Dot Solar Cells (Lescvhkies, K. S., 2009). The photons are absorbed by both the donors and the acceptors. However, the thermal energy is insufficient to disintegrate the electron-hole pairs which remain intact. This break-up is caused by the heterojunction. If the excitation occurs inside the acceptor, the hole is transmitted to the donor while the electron remains behind in the acceptor. On the other hand, if the excitation takes place in the donor then the electron travels to the acceptor while the hole remains in the donor (Lescvhkies, K. S., 2009). Quantum Dot Solar Cells based on Schottkey junctions: Instead of making use of separate electrons and holes transport layer, thin layers of semiconductor quantum dots are applied between electrodes of different functionality in this case. Figure 7 shows its structure (Lescvhkies, K. S., 2009). Figure 7 Structure of Schottkey Junction Quantum Dot Solar Cells (Lescvhkies, K. S., 2009).

7 Once light is absorbed, excitations are produced which cause either recombination or disintegration of electrons and holes in the film or dissociation in the Schottkey junction. CHALLENGES AND SOLUTIONS: Semi-conductor quantum dots have been a break-through in solar energy since, it supports high absorption coefficient and has a tunable bandgap (Tian, J., & Cao, G., 2013). However, there were some initial challenges faced during the emergence of this technology. Initially, dyesensitized cells (DSC) were used that gave an output efficiency of 11%. But as the quantum dot solar cell technology emerged with its high absorption power, dye-sensitized cells became obsolete. And even quantum dot solar cells were not free of problems. Sensitizers are used for the conversion of photonic energy to electrical energy. These sensitizers need hole-transporting materials electrolytes to regenerate sensitizers and to collect electrons. The iodide electrolyte utilized in DCS proved to be corrosive for QDSC semi-conductor therefore, researchers tested many materials that were non-corrosive to QDSC semi-conductors and found polysulphide as the most suitable one (Photovoltaic news and py jobs., n.d.). Another challenge was the loss in output due to impurities in quantum dots. Because quantum dots have small size therefore, they are prone to disorders and impurities. This leads to reduction in the overall efficiency of this technology. Another contributing factor is the inclusion of impurities at the time of doping. To avoid the population of impurities in quantum dots, they are manufactured in extremely high vacuum chambers in rivaling space. Epitaxy, a technique that utilizes molecular beam to control the production of quantum dots at microscopic level is used for this purpose (Kidd, T., n.d.). TECHNICAL CHALLENGE AND SOLUTION: One of the most important technical challenge was the increment in the value of photocurrent. Even though quantum dots improved the working principle of solar cells but still this improvement in the efficiency was not up to the mark. Therefore, efforts had to be made to optimize the structure of quantum dots in order to maximize the value of photocurrent. For this, one such suggestion implores a quantum-dot-in-a-fence structure which comprises of InAs QDs enclosed by thin AlxGa1 xas fence layers. This has a higher band gap and thus, these fences facilitate high photo-carrier generation instead of recombination which improves the overall efficiency of solar cells to up to 45%. Another method to improve photocurrent is light trapping. Study have shown that metal nanoparticles have the capability to improve the absorption of light by a factor of 300 but their use within the same proximity of quantum dot can degrade the material quality of quantum dot. Therefore, surface nanoparticles can be used instead, as they provide good light scattering property that improves absorption power (Zheng, Z., Ji, H., Yu, P., & Wang, Z., 2016). FUTURE WORK: Structural design and material used for the generation of p-n junction is a domain that still needs more research work. Since, excitations cause low binding energy that results in increased free electrons and holes so further study on this issue is to be done for concrete decision. Another

8 area of study in quantum dots is its preservation from oxygen and moisture by experimenting with the properties of surface materials. This effort will pave way in improving the mobility of carriers and lifetime of semi-conductors. Metal oxides have been used for the production of p-n junction. Moreover, many metal sulphides have also been utilized for this purpose. Different materials should be tested for the making of p-n junctions so that absorption and extraction of carriers can be improved (Cheng, C., n.d). Designing of semi-conductor quantum dots with larger wavelength of optical absorption should be carried out. Study should also focus on enhancing photoelectrodes to adjust more quantum dots and decrease the rate of charge recombination (Tian, J., & Cao, G., 2013). SUMMARY: The idea of green energy that reduced the harmful effects faced globally gave boost to solar cells. However, the low value of efficiency of solar cells accelerated the process of research in quantum dot solar cells; a technology that provides tunable band gap. Quantum dot solar cells have several types, a few of which have been discussed above in this report. However, even this technology have some drawbacks like that of lower efficiency and needs improvement. Future work on this field includes finding ways of maximizing efficiency and absorption power using optimum resources and exploring the structural depths of quantum dots in order to address challenges mentioned above. Quantum dot solar cell is a promising technology with a wide scope of providing energy solutions. REFERNCES: Tian, J., & Cao, G. (2013). Semiconductor quantum dot sensitized solar cells (Publication) Photovoltaic news and py jobs. (n.d.). Retrieved July 19, 2016, from http://www.pv- magazine.com/archive/articles/beitrag/quantum-dots--the-pros-and-cons-in-pv- _100010173/572/#axzz4Eyph0DQX. Jasim, K. E. (n.d.) Quantum Dot Solar Cells. In Solar Cells New Approaches and Reviews. Cheng, C. (n.d). Semiconductor Colloidal Quantum Dots For Photovltaic Applications. The University of Oxford. Shang, X. (2012). Study of Quantum Dots on Solar Energy Applications. Royal Institute of Technology Stockholm. Lescvhkies, K. S. (2009). Assembly and Characterization of Quantum Dot Solar Cells. The University of Minnesota. Young, J. (2011). Patterned Quantum Dots for Solar Cell Applications. University of Illinois at Urbana-Champaign MENSAH. (n.d.). SOLAR CELL AND ITS APPLICATION (Issue brief). Retrieved July 21, 2016, from

https://indico.cern.ch/event/145296/contributions/1381075/attachments/136874/194161/presenta tion_at_knust.pdf Zheng, Z., Ji, H., Yu, P., & Wang, Z. (2016). Recent Progress Towards Quantum Dot Solar Cells with Enhanced Optical Absorption. Retrieved July 22, 2016, from http://www.ncbi.nlm.nih.gov/pmc/articles/pmc4877335/ Kidd, T. (n.d.). Harnessing Quantum Dots for High Efficiency Solar Cells. Retrieved July 23, 2016, from http://www.iowaenergycenter.org/harnessing-quantum-dots-for-high-efficiencysolar-cells/ 9