The main research activity of this laboratory is based on the industrial and semi-industrial projects in the field of optoelectronic and semiconductor devices. These projects are done by the PhD and MSc students. Some of these works that will be explained in the section “Projects and Research Activities” are the design and simulation of photonic crystal devices, photonic crystal fibers, semiconductor lasers, photo detectors, solar cells and so on.
At present, the most researches in this laboratory are on the solar cells. One of the projects is being donein order to fabricate the laboratory sample of solar cells based on GaAs. Firstly, the design and simulation of the proper solar cell structures such as GaAs and AlGaAs/GaAs has been done. Then, the fabrication of the primitive sample is being donewith choosing the best structure and finally the final sample will be fabricated after the test and optimization of samples. Also, one of the main issues for improving the solar cell characteristics is utilizing the suitable antireflection coating and presenting the novel structures for it. In order to achieve these purposes, we are working on the antireflection coatings used in new generation solar cells such as thin film and quantum dot solar cells and cells based on the concentrators.
Projects and Research Activities
Design and Fabrication of GaAs Solar Cells :
As can be seen in the most papers, choosing the GaAs for achieving the best efficiency is the good choice because of its 1.424eV band gap energy. In the first step, we did a complete study on the types of solar cells. Then the design and simulation of the proper solar cell structures such as GaAs and AlGaAs/GaAs has been done. In the next step, the fabrication of the primitive sample is being donewith choosing the best structure and finally the final sample will be fabricated after the test and optimization of samples.
Investigation of the Electrical and Optical Properties of the Antireflection Coatings in the Third Generation Solar Cells :
One of the main issues for improving the solar cell characteristics is utilizing the suitable antireflection coating and presenting the novel structures for it. In order to achieve these purposes, we are working on the antireflection coatings used in new generation solar cells such as thin film and quantum dot solar cells and cells based on the concentrators. We are studying on the several types of materials utilized for antireflection coatings, optical properties of these materials and the different structures. There is an expectation which the novel proposals for one layer and multilayer antireflection coating can be presented.
Design and Modeling of WDM Photonic Crystal Integrated Devices Based on the Interaction of Cavity and Waveguide :
In summary, the resonance frequencies and the field distributions of 2D-PC were investigated. PC hybrid waveguides with quasi-flat and Lorentzian transmission spectrum were analyzed and modeled by using FDTD and CMT methods. The theoretical results derived by CMT were in good agreement with FDTD simulation results. A modified HW3 with extra rods in both ends of the CCW and Lorentzian transmission spectrum was proposed, which can be used in implementation of WDM filters. It was shown that in this case phase-shift is close to zero. Transmission of ultra-short pulses through the hybrid waveguide was also investigated. In this work, a low cross-talk and wideband waveguide intersection design based on two orthogonal hybrid waveguides in crossbar configuration was proposed and modeled by using the FDTD and CMT methods. In addition, it has been clearly proved that simultaneous crossing of ultra-short pulses through the intersection is possible with negligible interference. The proposed solution can be easily generalized to other 2D square as well as 3D cubic PCs. A modified HW2 with extra rods in both ends of the CCW and Lorentzian transmission spectrum was proposed, which can be used as the key element in implementation of WDM filters. It was shown that the phase-shift of the EM waves traveling between the modified HW2 cavities is close to zero. According to the theoretical theory using CMT in time, the performance of the proposed CDF was investigated and the conditions which lead to 100% drop efficiency were extracted.
Design of a Novel Structure for Photonic Crystal Fiber and Optimization of Its properties :
Photonics crystal fibers (PCFs) have attracted a great deal of attention in the optics community in recent years. In this work, new structures for PCFs are introduced. Also for the first time, a combination of intelligent methods is applied for optimization of these fibers.
PCFs have particular properties that are caused by their cladding structure and its ability of controlling the effective refractive index. The particular properties mentioned are ability to remain single mode at all wavelengths, great controllability in chromatic dispersion, low confinement loss, high values of birefringence, high nonlinearity, high numerical aperture and low bending loss.
The most common techniques for analyzing the PCFs are finite difference time/frequency (FDTD/FDFD) methods and the finite-element (FM) approach. In overall view, there is no complete analyzing method. In practice, the finite difference methods are very general and reliable (well established and tested) method. They are also accurate modal descriptors. But these conventional methods that simulate the optical characteristics of PCFs are computationally expensive and time consuming. In this thesis, an artificial intelligence method such as Neuro-Fuzzy system is used to establish a model that can predict the properties of PCFs. Simulation results show that this model is remarkably effective in predicting the properties of PCF such as dispersion, dispersion slope and confinement loss over the C communication band. The accuracy of this method is high about 98%.
Also this work proposes a combination of Differential Evolution (DE) and Estimation of Distribution Algorithm (EDA) (DE/EDA algorithm) to solve the optimization problem and to determine the PCF structure. This study is focused on the determination of the PCF structure that can lead to the minimum confinement loss and dispersion and nearly zero dispersion slope over C communication band. The optimized PCF exhibits a very low dispersion and ultra-low confinement loss at 1.55µm wavelength along with a nearly zero dispersion slope over the C communication band.
Equipments of Optoelectronics Lab.
Several Types of Gas and Semiconductor Lasers :
Several Types of Photodiodes and Photo detectors :
Optoelectronic Equipments Boxes :
Frequency Meter :
Other Equipments :
Recent Conference and Journal Papers
S. Mohammadnejad, N. Ehteshami, “Novel design to compensate dispersion for index-guiding photonic crystal fiber with defected core,” in proceedings of 2nd International Conference on Mechanical and Electronics Engineering (ICMEE), pp. 417-421, 2010.
In this paper we present a novel dispersion compensating photonic crystal fiber with defected core. The small central defect of air hole can flexibly control the chromatic dispersion properties of this kind of photonic crystal fiber. The 2-D finite difference frequency domain method (FDFD) with perfectly matched layers (PML) is used to investigate dispersion properties. By varying the size of defected core, it is possible to obtain high negative dispersion coefficient. The proposed photonic crystal fiber is suitable for broadband dispersion compensation in wavelength division multiplexing (DWDM) optical communication systems.
S. Mohammadnejad, N. Ehteshami, “A novel design to compensate dispersion for square-lattice photonic crystal fiber over E to L wavelength bands,” in proceedings of 7th International Symposium on Communication Systems Networks and Digital Signal Processing (CSNDSP) , pp. 654-658, 2010.
This paper reveals a novel square-lattice photonic crystal fiber for dispersion compensation in a wide range of wavelengths. The 2-D finite difference frequency domain method (FDFD) with perfectly matched layers (PML) is used to investigate dispersion properties. It has been shown theoretically that it is possible to obtain high negative dispersion coefficient and confinement losses less than 10-5 dB/m in the entire wavelengths band.
S. Mohammad Nejad, A. AlizadeVandchali, “ A Novel Structure Based on Nonlinear Photonic Crystal to Improve the Kerr Effect for all Optical Switch ,” in proceedings of The 3rd International Conference on Power Electronics and Intelligent Transportation System (PEITS), 2010.
M. Pourmahayabadi, S. Mohammad Nejad, “Design of Ultra-low and Ultra-Flattened Dispersion single mode Photonic Crystal Fiber by Differential Evolution Algorithm,” in proceedings of Applied Electronics , pp. 211 – 215, 2009.
In this paper, Differential Evolution (DE) algorithm is applied to design photonic crystal fibers structure with desired properties over the C communication hand. In order to determine the effective index of propagation of mode and then, the other properties of structure, Finite Difference Frequency Domain (FDFD) solver is used. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhihits a very low dispersion with a nearly zero dispersion slope over the C communication band.
K. Fasihi, S. Mohammadnejad, “ A Flexible Design of Waveguide Intersections with Low Cross-talk in Hexagonal Photonic Crystal Structures ,” in proceedings of Applied Electronics, pp. 1-3, 2009.
A new design for constructing adjustable waveguide intersections in rode-type photonic crystal of hexagonal lattice is proposed. The design utilizes the strong dependence of the defect coupling on the cavities field pattern and their alignments. By changing the radii of the coupled cavities, an adjustable low cross-talk intersection is obtained.
M. Pourmahayabadi, S. Mohammad Nejad, “Design of a Large Mode Area Photonic Crystal Fiber with Flattened Dispersion and Low Confinement Loss,” in proceedings of ICEE, Vol. 1, pp. 417 – 421, 2009.
In this paper, a novel structure of photonic crystal fiber (PCF) with large mode area, low confinement loss and flattened dispersion at C telecommunication band is presented. The numerical results revealed that these significant characteristics have been obtained by removing the first six surrounding air-holes in the transverse section. The perfectly matched layer (PML) for the boundary treatment and an efficient compact two dimensional finite-difference frequency domain (2-D FDFD) method were combined to model photonic crystal fibers. Macro-bending loss performance of the designed PCF is also studied and it is found that the fiber shows low bending losses for the smallest feasible bending radius of 5 mm.
K. Fasihi, S. Mohammad Nejad, “Design and Modeling of Hybrid Waveguides with Lorentzian Transmission Band,” in proceedings of ICEE, Vol. 1, pp. 301-305, 2009.
We present design and modeling of photonic crystal (PC) hybrid waveguides. The finite-difference time-domain (FDTD) and coupled-mode theory (CMT) methods are used to simulate the PC hybrid waveguide of square lattice. The bandwidth of the hybrid waveguide is investigated for different radius of the coupled cavities. A modified hybrid waveguide, with extra rods in both ends of the CCW and Lorentzian transmission spectrum was proposed, which can be used in implementation of WDM filters.
S. Mohammad Nejad, M. Aliramezani, M. Pourmahyabadi, “A Novel all-Solid Photonic Bandgap Fiber with Ultra-Low Confinement Loss,” in proceedings of ICEE, Vol. 1, pp. 268-272, 2009.
In this paper, a novel design of all-solid photonic bandgap fiber with ultra-low confinement loss is proposed. The confinement loss is reduced remarkably by managing the number of rods rings, up-doping level, pitch value, and rods diameters. Moreover, the designed PCF shows ultra-flattened dispersion in L and U-band. Furthermore, a new design, based on introducing of an extra ring of air holes on the outside of the all-solid bandgap structure, is then proposed and characterized. We demonstrate that it significantly reduces the fiber diameter to achieve negligible confinement loss. The validation of the proposed design is carried out by employing a tow dimensional finite difference frequency domain with perfectly matched layers.
A. Bahrami, S. Mohammadnejad, A. Rostami, “All-Optical multimode interference switch using nonlinear directional coupler as a passive phase shifter,” Fiber and Integrated Optics 30 (2011) 139-150. (Taylor & Francis)
This paper presents an all-optical multimode interference (MMI) switch in which the nonlinear directional coupler is utilized to realize a passive phase shifter. The proposed structure can be used either as a 1×2 or a 2×2 switch, with two inputs applied simultaneously in the latter case. The operation of the device is mainly based on the phase difference of the inputs of the MMI section. Beam propagation method is used for the design and simulation of the structure. The simulation results approve the low sensitivity on wavelength and fabrication tolerances. The crosstalk of the structure is equal to -31.6dB.
M. Pourmahayabadi, S. Mohammad Nejad, “Prediction of photonic crystal fiber characteristics by Neuro–Fuzzy system,” Optics Communications Vol. 282, pp. 4081–4086, 2009. (Elsevier)
The most common methods applied in the analysis of photonic crystal fibers (PCFs) are finite difference time/frequency domain (FDTD/FDFD) method and finite element method (FEM). These methods are very general and reliable (well tested). They describe arbitrary structure but are numerically intensive and require detailed treatment of boundaries and complex definition of calculation mesh. So these conventional models that simulate the photonic response of PCFs are computationally expensive and time consuming. Therefore, a practical design process with trial and error cannot be done in a reasonable amount of time. In this article, an artificial intelligence method such as Neuro–Fuzzy system is used to establish a model that can predict the properties of PCFs. Simulation results show that this model is remarkably effective in predicting the properties of PCF such as dispersion, dispersion slope and loss over the C communication band.
M. Pourmahayabadi, S. Mohammad Nejad, “Design of ultra-low and ultra-flattend dispersion single mode photonic crystal fiber by DE/EDA algorithm ,” Journal of Modern Optics, Vol. 56, pp. 1348-1357 , 2009. (Taylor & Francis)
This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the effective index of propagation of the mode and then, the other properties of structure, a finite difference frequency domain (FDFD) solver is applied. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits a dispersion of 0.22 psnm-1km-1 at 1.55 μm wavelength with a variance of ±0.4 psnm-1km-1 over the C communication band and a nearly zero dispersion slope.
M. Pourmahayabadi, S. Mohammad Nejad, “ Advanced design and optimization of single mode photonic crystal fibers ,” Journal of Modern Optics, Vol. 56, pp. 1572-1581, 2009. (Taylor & Francis)
This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the properties of PCFs such as dispersion, dispersion slope and loss, an artificial intelligence method, the Nero-Fuzzy system, is applied. In addition, a special cost function which simultaneously includes the confinement loss, dispersion and its slope is used in the proposed design approach. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits an ultra low confinement loss and low dispersion at 1.55 μm wavelength with a nearly zero dispersion slope over the C communication band.
K. Fasihi, S. Mohammad Nejad, “Orthogonal Hybrid Waveguides: An Approach to Low Crosstalk and Wideband Photonic Crystal Intersections Design,” Journal of Lightwave Technology, Vol. 27, 2009. (IEEE)
A low crosstalk and wideband photonic crystal (PC) waveguide intersection design based on two orthogonal hybrid waveguides in a crossbar configuration is proposed. The finite-difference time-domain (FDTD) and coupled-mode theory (CMT) methods are used to simulate the hybrid waveguides of square lattice. The bandwidth (BW) and crosstalk of the intersection are investigated for various radii of the coupled cavities. It is shown that simultaneous crossing of the lightwave signals through the intersection with negligible interference is possible. The transmission of a 200-fs pulse at 1550 nm is simulated by using the FDTD method, and the transmitted pulse shows negligible crosstalk and very little distortion.
K. Fasihi, S.Mohammad Nejad, “Highly efficient channel-drop filter with a coupled cavity-based wavelength-selective reflection feedback, ” Optics Express, Vol. 17, pp. 8983-8997, 2009. (OSA)
We have proposed a three-port high efficient channel-drop filter (CDF) with a coupled cavity-based wavelength-selective reflector, which can be used in wavelength division multiplexing (WDM) optical communication systems. The coupling mode theory (CMT) is employed to drive the necessary conditions for achieving 100% drop efficiency. The finite-difference time-domain (FDTD) simulation results of proposed CDF which is implemented in two dimensional photonic crystals (2D-PC), show that the analysis is valid. In the designed CDF, the drop efficiency larger than 0.95% and the spectral line-width 0.78nm at the center wavelength 1550nm have been achieved.
Useful Scientific Information
Introduction to Solar Cells:
Nowadays, with the increasing energy consumption initiated mainly by scientific developments, aninevitable demand for new energy resources is sensed. This has made the human being to achieve the required energy by adopting different methods. Meanwhile environmental pollutions and limited natural resources are the dominant issue in front of energy production by conventional techniques. Hence the overall trend is to discover the renewable energies without the environmental pollution characteristics. The solar cell energy is one of the most important renewable energies which is in recent decades considered by human being. The sun born about 6000 million years ago and it is said that 4.2 million ton of its mass is converted to energy in each second. The core temperature of the sun is about 10 to 14 million centigrade degree while its surface energy with 5600 centigrade degree is propagated in electromagnetic wave form. The earth is located in 150 million kilometer distance from sun and the sun light takes about 8 minutes and 18 seconds to reach it. Although the amount of energy achieved by earth from sun is a small part of its radiation, but saved fossil energy in the earth, wind energy, and sea waves are the direct result of the sun energy. The amount of the sun energy which reaches the earth every day is sufficient for providing the one year necessities of all of the creatures on the earth.
Some of the advantages of photovoltaic cells can be expressed as follows:
• Sun energy resource is free and infinite.
• The cost of working with it is low.
• They have not any mobile parts.
• They have high reliabilities.
• High security
Solar energy can be converted directly or indirectly to the other types of energy such as heat and electricity. Photovoltaic effect can be used for converting the solar energy to electricity utilizing the solar cells. The basic structure of a solar cell is a p – n junction formed by semiconductor devices which produce the electrical energy with absorbing the light and migration of electrons between energy bands.
The principle of solar cells is the shining of light on a p – n junction and production of electron – hole pairs that makes the drift and diffusion currents. The condition of production of electron – hole pairs is that the photon energy be bigger than the band gap energy of the material used in solar cell. The band gap energy, Eg, is the energy level difference between the conduction and valence band of the semiconductor. Hence the band gap energy, Eg, is the necessary energy for transmitting the electron from the valence band to the conduction band and the additional energy is wasted in the heat form and the electron relaxation in the lowest point of the conduction bandtakes place. Therefore, the main condition for absorbing light by a material with the energy band gap Eg is as follows:
where h is the plank constant and ν is the frequency corresponding to the emitted light photon with wavelength λ = c/υwhich c is the light velocity.
The radial spectrum of sun in the surface of the earth is almost different from that in the space. Finally, the absorbed photons are in the wavelength range of :
But the photons out of this range couldn’t be absorbed and the crystal is transparent for them. The sun light that receives the earth is composed of these wavelengths: 47% infrared, 46% visible light and 7% ultraviolet. Hence the solar cells should have the higher absorption in the range of infrared and visible light. The optical path for light from a sky resource to the earth is called as the Air Mass (AM). The light is attenuated in passing from the aerospace in effect of scattering and absorption. The air mass in the out of aerospace (i. e. satellites) and the zenith at the sea surface is AM0 and AM1, respectively.In AM1, the sun light is normal to the surface of the cell. AM1.5 means that the sun is in the place that the radiation path length on the earth is 1.5 times of that in AM1 which is equivalent with the solar radiation spectrum with angle 48 degrees. The light intensity for AM0 and AM1 is almost equal to 1366W/m2 and 925W/m2, respectively.
The solar cells can be divided to some categories which is presented in follows in the time sequence:
• Single crystal silicon solar cells (c-Si)
• Polycrystalline (poly-Si) or multicrystalline (mc-Si) silicon solar cells
• Thin film amorphous silicon solar cells
• Organic solar cells
Dye-Sensitized solar cells
Polymeric solar cells
Liquid crystal solar cells
Some of the last types of solar cells which the researches are done about them just in the primitive states are called the third generation solar cells.
History of Solar Cells:
The photovoltaic effect was first recognized in 1839 by French physicist Alexandre Edmond Becquerel which observed the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution when he saw a voltage develop when light fell upon the electrode. Becquerel whoseresearches was began from his childhood as a student, and then as an assistant, to his scientist father, observed this phenomenon in the age of 19. His father Antoine César Becquerel (1788-1878) was the discoverer of piezoelectricity. This effect firstly was studied by Heinrich Hertz on the solid materials such as selenium in 1870. But the first report of the PV effect in a solid substance appeared in 1877 when two Cambridge scientists, W. G. Adams and R. E. Day, described in a paper to the Royal Society the variations they observed in the electrical properties of selenium when exposed to light. However, in 1883 Charles Edgar Fritts, a New York electrician constructed a selenium solar cell that was in some respects similar to the silicon solar cells of today. It consisted of a thin wafer of selenium covered with a grid of very thin gold wires and a protective sheet of glass. But his cell was very inefficient. The efficiency of a solar cell is defined as the percentage of the solar energy falling on its surface that is converted into electrical energy. Less than 1% of the solar energy falling on these early cells was converted to electricity. Nevertheless, selenium cells eventually came into widespread use in photographic exposure meters. In 1887, Heinrich Hertz discovered that ultraviolet light altered the lowest voltage capable of causing a spark to jump between two metal electrodes.In 1888 Russian physicist AleksandrStoletov built the first photoelectric cell (based on the outer photoelectric effect discovered by Heinrich Hertz earlier in 1887). In 1904, Wilhelm Hallwachs discovered that a combination of copper and cuprous oxide is photosensitive. In the next year Albert Einstein published his paper on the photoelectric effect. The lower efficiency impasse was finally overcome with the development of the silicon solar cell by Russell Ohl in 1941. The modern photovoltaic cell was developed in 1954 at Bell Laboratories. The highly efficient solar cell was first developed by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson in 1954 using a diffused silicon p-n junction.
The next researches after the paper of Einstein about the photoelectric effect can be followed as:
Polish scientist Jan Czochralski developed a way to grow single-crystal silicon
Albert Einstein wins the Nobel Prize for his theories (1904 research and technical paper) explaining the photoelectric effect.
1954 Photovoltaic technology is born in the United States when Daryl Chapin, Calvin Fuller, and Gerald Pearson develop the silicon photovoltaic (PV) cell at Bell Labs—the first solar cell capable of converting enough of the sun’s energy into power to run every day electrical equipment.
Hoffman Electronics achieved 8% efficient photovoltaic cells.
Hoffman Electronics achieves 14% efficient photovoltaic cells.
The first Nimbus spacecraft – asatellite powered by a 470-watt photovoltaic array was launched.
A family in Morrison, Colorado, installs a 12-kilowatt solar electric system on its Home.
The best research cell efficiencies can be seen in this graph.
Types of Solar Cells:
In the different sources, the various categorization saboutthe solar cell shave been made. Here, one of these divisions is presented.
1. Crystalline Silicon Solar Cells
The most prevalent bulk solar cells are crystalline silicon (c-Si) solar cells. According to the type and size of the crystal, the bulk silicon crystal can be divided.
• Single-crystalline silicon (c-Si)
• Poly-crystalline (poly-Si) or multi-crystalline (mc-Si) silicon
2. Non-crystalline (amorphous) silicon thin film solar cells
The low cost is one of the advantages of solar cells based on the amorphous silicon (a-Si). Silicon and hydrogen are two components of the a-Si alloy. Moreover the characteristic of an a-Si alloy is that have a high absorption coefficient. Only a thin layer is required to absorb light and so the cost will be reduced.
3. Thin film GaAs Solar cells
The first condition for materials that should be used in a photovoltaic solar energy conversion device is matching the band gap with solar spectrum and having high mobility and high carrier lifetime.These conditions are met by many of compounds of II-VI, III-V and Si. The group of III-V despite the high cost of extraction and production of semiconductors, in space applications that cost is not important factor in them are used with great success. In 1961, Shockley and Queisser by considering a hybrid solar cellas a black body with thetemperature of 300 K showed that the highest efficiency solar cell,regardless of the typeof technologyused, is 30% and can be achieved for a cell with a band gap equal to 1.39eV.By considering that the band gap of GaAs is equal to 1.42eV, this material is suitable for solar cells. Thin film solar cells made of GaAs is named the second generation of solar cells.
4. Solar cells based on organic material
Solar cells made from organic materials are less efficient than their other counterparts. But due to low cost manufacturing capability and flexibility are suitable for nonindustrial usage. The solar cells based on organic materials include Dye-sensitized solar cells, polymer solar cells and solar cells based on liquid crystals.
• Dye Sensitized Solar Cells (DSSC)
The basic structure of a DSSC is importing of an optimized n-type transparent semiconductor (with wide band gap) in a lattice of nanoscale columns in contact with nanoparticles or coral-shaped blugings.
Schematic of DSSC
(S. J. Fonash, Solar Cell Device Physics (Second Edition) Elsevier, 2010.)
The surface of lattice is covered by a single layer of dye or quantum-dots that act as dye. Then electrolyte is used to achieve a channel between dye and anode. Dyes absorb the light and generate exciton, whichwill be separated in the interface of semiconductor and dye and cause to create electrons with photons for semiconductor and oxide dye molecules by electrolyte.
• Polymer solar cells
Polymer solar cells have special properties. Because of solubility of the active materials in this type of cells in most of organic solvents, polymer solar cells have the flexibility potential and ability to be fabricated in a printing process similar printing newspapers.
The ability to build of polymer solar cells as continuous manufacturing process
(Nat. Photonics, vol. 2, pp. 287–289, 2008.)
Recently power conversion efficiency about 6% is reported but this value is very less than the efficiency that is needed in typical applications.
• Solar cells based on liquid crystals
In some types of these solar cells, pillar liquid crystals are used for the fabrication. A group of liquid crystals can be in the pillar state. The pillar state is the case that the constitutive molecules of liquid crystal are placed like a disc and make some pillars. Previously, this group of liquid crystal called the disc liquid crystals.
5. Solar cells based on quantum-dots
A limiting factor for energy conversion efficiency of solar cells with a unit energy gap is that the energy of absorbed photon is higher than the band gap of semiconductor is wasted due to the interaction of electron-phonon until the carriers reach to the edge of band gap called relaxation.
Solar cell based on quantum-dots
(IEEE Transactions on electron devices, vol. 49, pp. 1632-1639, 2002.)
In recent years, new methods for reducing these losses are presented by using quantum structures like quantum dotes and quantum wells.
Improvingthe efficiency in photovoltaic solar cells by using quantum dot impact ionization
(Phys. Rev. B, vol. 60, pp. R2181-R2184, 1999.)
In these structures when carriers in semiconductor limited to certain ar