Design Of Multijunction Solar Cell Using AMPS -1 D

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AMPS-1D it is aimed at designing multi junction solar cell with a high efficiency using efficient materials as CdTe, CIGS, and amorphous solids. This software ...
Design Of Multijunction Solar Cell Using AMPS -1 D Puja Verma

Tabassum Aara

Department of Electronics and Communication Engineering RVS College of Engineering & Technology Jamshedpur, India [email protected]

Department of Electronics and Communication Engineering RVS College of Engineering & Technology Jamshedpur, India [email protected]

Anshu Karna

Hirak Mazumdar

Department of Electronics and Communication Engineering RVS College of Engineering & Technology Jamshedpur, India [email protected]

Department of Electronics and Communication Engineering RVS College of Engineering & Technology Jamshedpur, India [email protected]

Abstract— As it is known that sunlight is the most abundant and pure source of resource hence it should be used for various purpose as the non-conventional source of energy so that cost of using other source of energy can be reduced. Many solar cell panels have been designed till date to store and use the solar energy for fulfilling the energy demands. By using the software AMPS-1D it is aimed at designing multi junction solar cell with a high efficiency using efficient materials as CdTe, CIGS, and amorphous solids. This software combines the physics of photovoltaic along with numerical methodology for designing and simulating solar cell. It is a phenomenal tool which helps in analyzing all kinds of data for solar cell and hence the time and cost for simulation is reduced and beforehand the properties like efficiency, fill factor etc. are determined. As more and more solar cells are designed and fabricated energy problems will be solved more easily and also the pollution and toxicity due to other materials can be solved especially in developing nations like India.

II. RELATED WORK Solar cells are being used in panel arrangement for years and these panels are arranged in a way to obtain more and more lights like on buildings, roof tops and vehicles. Many small solar cell are joined together to form panels and according to sun rays calculations these panels are placed at different places

Keywords— Solar cell, Multi Junction, AMPS-1D

I. INTRODUCTION Solar energy is the most abundant and clean source of energy available in nature that is why it is needed to utilize this energy more to solve the energy problems and for this reason solar cell which employs principle of photovoltaic to convert light energy to electrical energy is used . Electrons are freed once the energy exceeds bond energy[1]. Asymmetry is established in solar cells by contacting two semiconductors of opposite polarity which drives electron freed from incident light. Photons having energy greater than energy-band gap are able to create electron-hole pair needed for energy conversion[2,3].

978-1-4799-6908-1/15/$31.00 ©2015 IEEE

Fig. 1.

basic working of solar cell

A. SOLAR CELL STRUCTURE Here it is discussed about the basic simulation that we have performed. In this the basic solar structure has been used by using the material silicon. The first layer which is the acceptor layer is the p-type material of amorphous silicon hydrogenated carbon. The advantage of using the amorphous material is that in amorphous material there is no particular orientation and hence the material distribution is imperfect which helps in easy distribution of sun light ray. The second layer is the absorber layer which helps in absorbing photons for this we use intrinsic type amorphous hydrogenated silicon. The third layer is the donor layer that is n-type amorphous hydrogenated silicon.[7, 8] Fig. 2. Solar spectrum with respect to wavelength for space and terrestrial applications

The solar cell is fabricated and then searched for its spectral response. The spectral response of a solar cell is the short circuited current received for the wavelength of incident light. The fabrication of device is done by depositing p-type material and n- type material on silicon slice. The contact to ptype and n- type can be made by evaporating metal like aluminum and gold[4, 5]. The solar cell function is dependent on the short circuited current. This current depends on wavelength of light, absorption coefficient, diffusion constant and diffusion length in p and n regions. For high short circuit current there should be maximum number of carrier reaching the junction and long diffusion length[6].

1) Boundary condition Boundary conditions are very important as all the calculations are done by the simulating tool is as per the boundary condition. In these boundaries equation many parameters are being specified according to which other calculations are made. In which there are terms like PHIBO, SNO and PHIBL.[8, 9, 10, 11, 12] PHIBO = work function (front contact) - electron affinity (semiconductor in contact) PHIBL = work function (back contact)-electron affinity (semiconductor in contact) SNO = electrons at x=0 space SNL = electrons at x= L space SPO = holes at x=0 space SPL = holes at x=L space RF = reflection coefficient for light impinging at front contact surface RB = reflection coefficient for light impinging at back contact surface

FRONT CONTACT PHIBO = 1.54 Ev

Fig. 3. Graph showing V-I characteristics of a solar cell

The above graph is just an example of V-I characteristics of solar cell. The product of maximum current Imax and maximum voltage Vmax gives the power obtained. Efficiency of solar cell is also calculated using this graph.

BACK CONTACT PHIBL = 0.20 Ev

SNO = 1.00e + 007cm/sec

SNL = 1.00e + 007cm/sec

SPO = 1.00e + 007cm/sec

SPL = 1.00e + 007cm/sec

RF = 0.00

RB = 0.60

B. SOLAR CELL PARAMATERS The general modelling was done on the basis of standard AM 1.5 (air mass) used for global illumination condition at

300 K. The illumination conditions are taken for light conditions to obtain I-V characteristics in light conditions. The below window shows the different layer information and what are the different values which are kept and maintained for the modeling. There are different values which need to be specified like donor concentration (ND) and acceptor concentration (NA) and different density of the carrier and their mobility. The spectral parameters were specified as per the AM 1.5 standard which are used for terrestrial operation and the gap states parameters are specified. III. PRELIMINARY CONCEPTS A. MULTIJUNCTION LAYER Multi-junction (MJ) solar cells are solar cells with multiple p–n junctions made of different semiconductor material. Each material's p-n junction will produce electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell's sunlight to electrical energy conversion efficiency. . Multi junction solar cell are being developed and used for achieving higher efficiency than single device solar cell. Mostly material of IIIV group of the periodic table. In the fabrication process of multi junction one layer is placed beneath another with a lower band gap than the previous one this is because the longer wavelength of sun has lower photon energy which needs to be passed below to be absorbed by the lower sub cells. Efficient use of solar spectrum is very important so that sunlight is absorbed efficiently by sub cells with an accurate band gap match so that less of the photonic energy is lost during electron hole combination pair. Multi junction is made with upper and lower layers and varieties of middle layer with tunnel junctions to access more charge carrier. Absorber layers are used which may be organic or inorganic which are used to absorb more and more carriers. Every layer in a multi junction is a n-type (thin) layer on a p type substrate (thick) [12, 13]. The layers are grown by Metal Oxide Chemical vapour Deposition (MOCVD) on epitaxial ready substrate. A buffer layer is also added to compensate for any dislocations or defects. A tunnel junction is used to pass the current between top and bottom layers for conductivity of current. Each of the front and back surface has a window layer which is used to reduce surface recombination losses. These are generally layers which are in alloy form. The fabrication is completed with the ohmic contacts which are very necessary for proper flow of current [14]. The design basis is monolithic formation which is possible mainly due to tunnel junctions which connect all the parts necessary. The quantum mechanics theory is followed for tunnel junction who works on probabilistic logistics rather than being definite. The cell size, structure and metallization should be maintained to go with the varying current densities [15].

Fig. 4. Multijunction operation

The top layer emitter is highly doped to reduce semiconductor resistance losses. The carrier transport proceeds normal growth layers and then to top layer hence as carrier in bulk will go there so doping is maintained relatively high. It will reduce the voltage drop due to high lateral resistance loss however doping should not be too high as it can hinder absorption [15, 16]. With increase in temperature the band gap of the layers decreases leading to a decrease in open circuited voltage and increase in short circuit current but this ultimately decreases efficiency hence the temperature should not be increased randomly in multi junction solar cells. This effect can be compensated with increasing concentration. There has been a need for multi junction solar cell. It has been seen that solar cell has always suffered several loss mechanism may it be recombination losses, black body losses. The most dominant loss is due to the inability of cell to absorb all power in photon and hence inability to give output as much as the input. Electrons should also have enough energy to overcome the band gap of material. Conventional solar cell also had problems that some of the materials used were insensitive to certain wavelength of light. By making multi junctions many different layers are used having different band gaps which are suitable for absorbing different wavelengths of light some are suitable for infrared some for whole visible spectrum and hence like this the whole wavelength is utilised. Conveniently solar cell is fabricated as such to have largest band gap at the top most layer and subsequently decreasing the band gap hence the shortest wavelength received at top and with proper adjustment of reflector and absorber layers the junctions are fabricated to absorb whole of light[17]. Many researchers have developed different types of multi junction to achieve efficiency greater than single cell. They can be either homo junction (same material with different mole fractions to vary the band gaps) or hetero junction (different materials are used for different layers). In fact in

different layers unique combinations like that of organic and inorganic materials can also be tried. These give an idea of how a solar cell can work efficiently utilizing all the solar energy with materials best suited based on its availability and economic proportions [18].

d/dx(−ε(x)dϕ/dx)=q*[p(x)−n(x)+ND+(x)−NA−(x)+p(x)−n(x) t t

(1)

1/ q(dJ n / dx) = −Gop (x) + R(x)

(2)

1/ q(dJ p / dx) = Gop (x) + R(x)

(3)

To analyze the transport behavior of semiconductor electronic and optoelectronic device structures. These three coupled non-linear differential equations with associated boundary conditions i.e

Fig. 5. Simple structure showing multi junction arrangement

ψ (0) = ψ 0 − v

(4)

ψ (L) = 0

(5)

J P (0) = −qS po ( p0 (0) − p(0))

(6)

J P (L) = qS pL ( p(L) − p0 (L))

(7)

J n (0) = qSno (n(0) − n0 (0))

(8)

J n (L) = −qSnL (n0 (L) − n(L))

(9)

The above figure is just a simple arrangement to give an overview of multi junction construction. IV. METHOD AND MATERIAL A. SIMULATION TOOL In this paper AMPS 1D (Analysis of Microelectronic & Photonic Structures) simulation tool is used. This software is developed by Stephen J Fonash at The Pennsylvania State University, USA.[20] AMPS is a numerical simulator used for analyzing and designing homo and hetero junctions based semiconductor devices. This software can operate in two modes: DOS (Density of States) modes and lifetime mode. The DOS model is used for material having significant defect densities like amorphous silicon. The user here has to specify the energy gap distribution and its spatial distribution[21]. According to these the recombination traffics is determined and its effect on electric field is observed. In lifetime approach it is assumed that recombination model can be formed out of linear models. Generally DOS models are employed in cases where there is much dislocation and less surety . More than one absorber layer with different bandgaps (i.e. multi junction layers) is applied to effectively convert the photo energy into electricity due to use of additional region from solar spectrum. This approach mainly consist of Poisson’s equation, Continuity equation for free electrons and holes,

are solved to obtain set of 3 unknown state variables. These are The electrostatic potential, The hole quasi-Fermi level & The electron quasi-Fermi level at each point of device. From these variables carrier concentration, fields, currents can be computed. The method of finite differences and the Newton-Raphson techniques (find roots to get guess for values) are incorporated by the computer to determine the state variables. Device being analyzed is divided into segments by a mesh of grid points. (Solved for each grid point).After interring these three state variables as a function of x, the band edges, electric field, trapped charge, carrier populations, current densities, recombination profiles, and any other transport information may be obtained. For solar cell structure, collection efficiencies as a function of voltage, light bias & temperature can be obtained. From these easy methods of computerized simulations an overview can be made about how exactly the solar cell will behave and will it be feasible to design such solar cell hence

unnecessary wastage of time and money is saved with use of this software. The above window appears where the users have to specify about what they want to see in their graphs. There are different types of graphs available say for electric field distribution, efficiency, Dark and light characteristics etc. and these are available with respect to position, width, thickness etc. and every graph can be viewed under thermodynamic equilibrium or normally under different conditions. B. DIFFERENT MATERIAL IN SOLAR CELL There are different materials employed in solar cells among which are light absorber, schottky barrier, ohmic contact and transparent contact [22]. Light absorber- In any solar cell the key factor is to absorb the excited photons of solar cell and to convert these in a pair of electron and holes which can work externally in producing current. For this correct absorber is chosen which can be thick metal or just a semi-conductor with high absorption coefficient. These materials should be flexible according to the ray of light and should have capacity to absorb maximum of sun rays. Schottky barrier – In addition to materials for absorption there are materials used for blocking. For proper flow of carriers and ultimately current a proper sequence is maintained. There are some materials which are used to block one type of carrier whereas allowing the other type to pass through. In this manner the proper flow of electrons and holes is maintained so that the generation and flow of current is maintained. Ohmic contact – These are used to provide contacts between PV cells and cell electrodes and grid for a normal flow of current to load. In this normally metals are used as they are good conductors. These materials should have low electrical and optical loss. Transparent conductive oxides can also be used for this purpose.

electrolytes. The solids can be metal, non-metal or semiconductor. Most cases use semi-conductors. The crystalline material possesses well defined crystal structure having lattice. These can be body centered, face centered, edge centered etc. the combination of single crystals make poly crystal which have long range order. Amorphous material are disordered that is it does not have any definite structure or lattice like in this paper amorphous silicon hydrogen was used where the material will be disordered but the variations will be there according to the bonds between the silicon and hydrogen and also the transfer of light rays will also depend on the combinations. These are the general material properties, for the selection of materials especially in multi junction is done based on the correct lattice matching, current matching and high performance optoelectronic properties. Crystal constant α is matched generally for lattice matching nowadays some flexibility has been provided yet to a degree it is better to match the constant to avoid crystal imperfection or dislocation of atoms [24, 25, 26]. The material chosen must have high absorption coefficients, high carrier lifetime and high motilities.

V. RESULT AND DISCUSSION By giving various parameters and specifying all the values the graph for I-V characteristics was obtained in which all the values like efficiency, fill factor (FF), open circuit voltage (Voc), short circuit current density (Jsc). The layer thickness of the acceptor p-type was set to be 8 nm, the intrinsic layer was kept at 500 nm, and the donor layer was kept 15 nm thick. The ratio was maintained and a graph was obtained which had efficiency of 7.169%, Jsc = 14.749 mA\cm2, fill factor of 0.552, V0c = 0.880 v.

Transparent contact – These materials are used to facilitate maximum light entry. Their properties are that they are transparent, conductive and have suitable work functions. Materials like tin oxide, indium tin oxide etc. are used for this purpose. C. MATERIAL CHOICE Since solar cells are being developed researchers have been implementing it with various materials to increase the efficiency of cells and to use the solar energy to its maximum, hence there is a list of varieties of materials tried and tested till date. Firstly basic Si semiconductor was used but then development took place and many more were added to the list [23]. Solids as well as liquids materials are used in solar cells. Homo junction, hetero junction, metal semiconductor and some dye sensitized solar cell use all solid material and there are others which use both solid liquid structures. Materials can be organic and inorganic and solids can be crystalline, noncrystalline and amorphous. The liquids used are generally

Fig. 6.

Graph between current and voltage in light conditions

[5]

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Fig. 7. Result graph for bands

[11] ANALYSIS OF MICROELECTRONIC AND PHOTONIC STRUCTURE

IN THREE DIMENSIONS by Nghia Dai Nguyen 101 Innovation Blvd., Suite 102 , University Park, PA 16802

The above graph is showing different bands as conduction band, valence band, Fermi level, hole and electron bands.

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CONCLUSION

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In this paper a solar cell with amorphous material was designed. A series of simulations were carried out for the results. Than an efficiency of 7.186% was obtained. Hence it can be concluded that by proper understanding of the software and materials properties the designing of solar cell is possible and in near future with increase in efficiency solar energy for mankind can be utilized . In all the papers different materials ranging from amorphous, crystalline, nano crystalline and tandem cells can be studied. Till now efficiency of 28% has been achieved by different researchers and everyone is utilizing the software platform to design in best possible way. Through this experiment we learnt new methods and ways of designing and it is intended to apply our knowledge and focus for developing much better solar cells which are more efficient so that more of solar energy can be utilized.

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[19] R. H. Franken, R. L. Stolk, H. Li, C. H. M. Van der Werf, J. K. Rath, R. E. ACKNOWLEDGEMENT

It is a great privilege to express our deep sense of gratitude and indebtedness to our Director Dr. M.P. Singh who gave us the opportunity to carry out this project. Our thanks are also reserved for Prof. N. Roy Chowdhury, Head or Department Electronics & Communication Engineering, R.V.S.C.E.T. Jamshedpur and other member of Electronics & Communication Department. We also thanks to our project guide Prof. Hirak Mazumdar. We have done this project in a group of five members and with everyone’s individual cooperation we have done this work. REFRENCES [1]

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