Modeling. Simulation and Comparative Study of New ...

Modeling. Simulation and Comparative Study of New Compound Alloy Based P-I-N Solar Cells an Efficient Way of Energy Management Rudrarup Sengupta

Vurikiti Prashant

Department of Electronics and Communication Engineering Heritage Institute of Technology, IEEE Student Member, Kolkata Chapter, Kolkata, India [email protected]

Department of Electronics and Communication Engineering Heritage Institute of Technology, Kolkata, India [email protected]

Tapas Chakrabarti

Department of Electronics and Telecommunication Engineering, Jadavpur University, Senior Member, IEEE, Kolkata, India su [email protected]

Dr. Subir Kumar Sarkar Department of Electronics and Communication Engineering Heritage Institute of Technology, Kolkata, India [email protected]

Abstract-This

_

2.03 eV is necessary for current matching in a two-tenninal stack. GaO.5AsO.5 has a very good lattice match with GaInP and has extremely good absorption coefficient which is generally used at the top layer or as substrate. In the proposed model a small percentage of Aluminium is mixed not only with GaInP but also GaAs, and it has given remarkable efficiency. A high Aluminium content will introduce an Al-O defect during the epitaxy process due to the high dissociation energy of the Al­ o bond [4]. Measures such as changing the V-III ratio, variation of the growth temperature, altering oxygen-free Aluminium precursors, etc., is generally taken to minimize the Al-O defect and enhance the performance of the cell. AlGaInP have been used previously in multi junction solar cells but in the proposed model AlGaAs have been used as the substrate and the top layer for the first time. As stated earlier about GaAs, absorption coefficient of this material increases even more when it is alloyed with AI. Focus is kept upon increasing efficiency so that more amount of solar energy can be trapped by comparatively lesser number of cells. A more energy efficient and cost effective proposal is made, which traps much more solar energy, finally converting renewable energy to electricity with high efficiency than the conventional ones. PIN solar cells of such high efficiency have not been modeled before using these specified equations as discussed later. The novelty of this work lies in adding a small percentage of Aluminium in both the materials mentioned above during epitaxy process. Also for the first time such high efficiency has been simulated with the use of only two compound alloys and a single junction. In this model, layers of p-type, n-type and intrinsic AlGaInP and used AlGaAs as top layer/substrate and simulation is done through ATLAS (SILVACO) which clocks an efficiency of 51.50%.

work is concentrated on developing a model of

PIN solar cell with some III-V compound material. Two new compound materials are used in this solar cell. One of them consists of Gallium Indium Phospide (GaInP) and Aluminium (AI), which reached the band-gap energy of 4.30eV with an I absorption coefficient of 1.6ge-4 cm- • Another one consists of Gallium Arsenide (GaAs) and Aluminium (AI), reaching a band gap energy (Eg) of 4.6 eV and absorption coefficient of 1.81e-4 I cm- • As the band gap energy of the material is increased, the material can absorb more photon energy from the optical source and can convert the optical energy into electrical energy more efficiently.

The

new

modeled

PIN

solar

cell

made

of

AIGaInP/AIGaAs has been developed in SILVACO TCAD which is a virtual fabrication and simulation lab. The cell has achieved the conversion efficiency of 51.50% with a Fill Factor (FF) of 91% under the AM1.5 illumination (1000 suns)

Keywords- PIN Solar Cell, TCAD, Efficiency (11), Absorption Coefficient (a), Band-gap Energy (Eg). 1.

INTRODUCTION

The advanced types of single junction solar cells are PIN cells. Cells with higher photogeneration rate have been made with layers of semiconducting alloys stacked upon one another matching their lattices. But simple PIN cells also have shown remarkable efficiencies of up to 20% using GaAs or GaInP [1][2]. Single junction solar cells of high efficiency can be of great use reducing the complexity of multi junction cells. Solar cells with layers of AlGaInP have given remarkable efficiency over the years. It is chosen due to its superior electron flow rate and wide bandgap. The material features a direct bandgap unlike silicon which make it favorable for photocurrent generation. The direct bandgap of this material system can be tuned to as high as 2.3 eV, where a band-gap of

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II.

MATERIAL STUDY

Aluminium gallium indium phosphide (AIGaInP) is a semiconductor material which is often grown by heteroepitaxy on gallium arsenide (GaAs) or gallium phosphide (GaP) in order to form a quantum well structure. To achieve different electronic and optical properties, the semiconductor hetero­ structures exert quantum confinement and alloy composition. This is a remarkable progress in device field made during the past couple of decades. The increasing demand for optimal efficiency and greater power output has resulted in a steady growth in heterostructure complexity [5].

0.15f.!m of intrinsic layer is used as shown in fig 2. The doping profile of the given structure is shown on fig 3. Simulation of this structure is done through ATLAS (SILVACO). A virtual photocurrent is generated in the device which is generated by a beam of user defined wavelength. A wavelength of 300nm has been used for this study. ATLAS basically, mathematically solves the structure by iterative methods, to get the cathode current using different user defined mathematical models. In this study, the models used namely are, SRH, CONMOB, OPTR, AUGER, BGN AlGaAs

Using AIGaInP/AIGaAs, band gap energy and absorption coefficient of the PIN cell gets enhanced, which finally increases the mobility of free electrons and holes in the device. Thus, optimum power is utilized and maximum efficiency is achieved for the below mentioned structure. More than that, the efficiency of carrier confinement is strained in quantum well active layer; therefore, it becomes possible to minimize the reflectivity of the cathode-side. The threshold value also decreases with the increase of gain. With this, it is possible to attain further increase of the optical output. Also, the optical gain of the active layer is increased, which in tum makes it possible to attain an impeccable output power of improved efficiency. It should be carefully noted that these effects are achieved with the surface-emission of our PIN cell of the 300 nm band that uses the AIGaInP/AIGaAs system [3].

AIGaInP AIGaInP

(p) 0.02f.!m (p)

intrinsic

A1GaInP AIGaAs

(n) (n)

0.05 f.!m 0.15 f.!m 0.2 f.!m 1 f.!m

Fig 2 : Schematic dIagram of proposed PIN solar cell

Hence, a small percentage of Al is used in the alloy of GaInP and GaAs and a remarkable efficiency has been achieved. Absorption coefficient is calculated using the following equation, [6] a = a

E(l) -En (X)

o

(1)

ESJ (x) �

In equation (1), 0.0 is the value of absorption coefficient for GaInP and GaAs respectively, E(A) hclA, and Eg is the bandgap energy. Both of these are illustrated for the materials in table 1

Fig 3: Net Doping profile of proposed PIN solar A. SRH Recombination: it specifies Shockley Red Hall recombination using fixed lifetimes.

=

Table-I: Band-gap and absorption coefficient of concerned materials

Bandgap Energy (Eg) [7] Absorption Coefficient (a) III.

AIGalnP

AIGaAs

4.3 eV

4.6 eV

(2) In equation (2), ETF is the difference between trap energy and intrinsic Fermi level, TL: is the lattice temperature in Kelvin, TLTN O and TL T PO are electron and hole lifetimes, p and n are hole and electron densities and nje is intrinsic carrier density [8]. B.

1.69 e-4 cm-l

1.81 e-4 cm-

l

DEVICE STRUCTURE AND MODELS

Before schematic representation of the PIN solar cell is given in fig 2. The main absorption region of PIN homojunction is grown on a 1f.!m n type AIGaAs region with the same materials p type layer on top. The PIN layers consist of AIGaInP alloy. 0.05f.!m of p-type 0.2f.!m of n-type and

2015 International Conference on

Auger

recombination

(n (np-nt) + ( p (np -nt)

(3)

A UGER

Recombination:

technique, is given by the equation UAUGER

=

Here in equation (3), n and p are hole and electron densities and nj is the intrinsic carrier density [7].

C. Optical Recombination (OPTR): recombination model which can be stated as RAUGER

=

the

ACn(pn2 -nnts) + ACp(np2 + pnts)

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optical (4)

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In equation (4), p, n, nie are as described earlier. ACn and ACp are user definable whose default values related to Auger model which are taken as 8.225e-32 cm% and 1.732e-31 cm6/s respectively. Here, neither Klaassens temperature dependent model nor the narrow bandgap model is incorporated, as the BGN model is used [9].

indicates the high fill factor ratio, which is responsible for the high efficiency. One of the main objectives of this discussion is to compare the cell with most highly efficient single junction cells already fabricated.

D. Band Gap Narrowing (BGN): specifies band gap narrowing models which is expressed as an analytical model relating to variation in bandgap to doping concentration given by,

(5) In equation (5) bandgap.E, bandgap.N, bandgap.C parameters can be specified according to Klaassens model. The default values for bandgap.E, bandgap.N, bandgap.C are 9.158e-3 V, 1.297e7 cm-3 and 0.5 respectively. Variation of bandgap models are not introduced here as our material band­ gap is not variable [10]. E.

Concentration

Dependent

Mobility

(CONMOB):

specifies that a concentration dependent mobility model be used for silicon and gallium arsenide. This model is a doping versus mobility table valid for 300K only. This model is used to solve the top layer and substrate. In addition to all these equations, Fermi level and models for fixed Fermi are solved for the final current calculation. The simulator solves basic Poisson equation and continuity equation for holes and electrons separately which fall under the drift diffusion model. All important generation and recombination mechanisms are taken into account. The spontaneous recombination and optical absorption can be calculated with quantum mechanics using Fermi's golden rule, which may be important for novel solar cells using quantum well and quantum dot materials. For optical simulation relating to electron and hole generation due to incident light, simulator takes into account Fresnel's reflection, refraction and transmission. The optical and electrical properties of the materials are taken from the sopra database. For thin film solar cell, ID simulation is done keeping in mind direction and intensity of solar spectrum. The average solar spectrum energy incident on the earth is taken to be 1000W/m2. Area of the cell exposed to incident spectrum on is given by 111m by 111m. IV.

RESULTS AND DISCUSSIONS

Fig 4 : I-V Characteristics Curve B. Comparative Analysis

Use of PIN cells with amorphous silicon or GaAs [9] [10] is quite common. Here a comparative analysis is made between the proposed structure and two more structures. In one of them the PIN cell is made of AIGaInP only but the substrate and top layer is made of GaAs [4] (model 1). (Schematic diagram is given in fig 6.) Another comparison is made with a PIN cell made of GalnP with substrate of GaAs [9] [10] (model 2). (Schematic diagram is given in fig 7) All the results found in the previous section are calculated for these cells and compared. Efficiency comparison of the proposed model with other models yields the fact that the proposed model is a more energy efficient alternative. For the sake of simplicity width of each region is taken same for all three cells. Table 2 illustrates the different results obtained for different solar cells. The symbols Jse is for current density, Voe is for open circuit voltage, Pm is for maximum power, V m is for maximum operational voltage, 1m is for maximum operational current, FF is Fill Factor and Eff is efficiency. A comparative analysis is done in the I-V characteristics curve by plotting I-V characteristics of all the cells on the same graph (fig 8). The red line represents I-V curve of proposed model. GaAs (p) 0.02 11m AIGalnP AIGalnP

A. Results Obtained

AIGalnP

For the cell given in fig 2, there is no lattice mismatch or defect traps as GalnP is very well matched with GaAs. On solving the models the current equation is derived and from that, graph is plotted. With the known formulae open circuit voltage (Voc), short circuit current density (Jsc), maximum power (Pm) and its corresponding maximum voltage (Vm) and current (1m) is calculated. With the corresponding formulae Fill Factor and Efficiency are also calculated. All these values are given in table 2. The I-V curve is given in fig 4. An air mass ratio of 1.5 is used. The area under the curve

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GaAs

(p)

0.05 11m

(intrinsic) (n) (n)

0.15 11m 0.2 11m 1 11m

FIg 6 : Schematic DIagram of Model-l GaAs (p) 0.02 11m 0.05 11m GaInP (P) (intrinsic) 0.15 11m GaInP 0.2 11m GaInP (n) 1 11m GaAs (n) FIg 7 : Schematic DIagram of Model-2

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It is compared with model 1 (AIGaInP model 2 (GaInP & GaAs) in fig 8.

&

GaAs) and

Table 2 : Comparison Chart

Jsc (Aim') Voc(V) Pm (W) Vm(V) Im(mA) FF (ratio) EfI(%)

GalnP&

AIGalnP

AIGalnP&

&AIGaAs

GaAs

GaAs

(proposed

(Existing

(Existing

model)

model-I)

model-2)

0.24e+006

0.0I77ge+006

0.017e+006

1.65103

1.09038

1.06639

4.344e-003

2.25575e-003

2.12178e-003

1.55

0.950001

0.95

2.80288

2.37447

2.23345

0.912105

0.818616

0.823003

51.5037

21.5529

20.2729

Fig 8 : I-V Characteristics Curves - Comparative Study V.

REFERENCES [I]

Y. G. Xiao, z. Q. Lt, M. Lestrade, and Z. M. Simon, "Modelling of InGaN PIN solar cells with defect traps and polarization interface charges" Li Crosslight Software Inc. , 121-3989 Henning Drive, Burnaby, BC, V5C 6P8, Canada 978-1-4244-5892-9/101$26.00 ©2010 IEEE.

[2]

Md. Abu ShahabMollah, Md. Liton Hossain, Md. Imtiaz Islam, Abu FarzanMitul , "High Efficiency InGaN Based Quantum Well & Quantum Dot Solar Cell" ©Academia 2014.

[3]

http://www.google.comipatentsIEPI780849BI?c1=en.

[4]

Lu Hongbo, Li Xinyi Zhang Wei., Zhou Dayong, Shi Mengqi, "A 2.05 eV AIGainP sub-cell used in next generation solar cells" Lu Hongbo, Li Xinyi Zhang Wei., Zhou Dayong, Shi Mengqi Sun Lijieand Chen Kaijian, Research Center for Photovoltaics, Shanghai Institute of Space Power-Source, Shanghai 200245, China, Journal of semiconductors, September 2014 Vol. 35, No. 9.

[5]

T. M. Ritter (a), B. A. Weinstein (b), R. E. Viturro (c), and D. P. Bour (d), "Energy Level Alignments in Strained-Layer GaInP/AIGaInP Laser Diodes : Model Solid Theory Analysis of Pressure- Photoluminescence experiments" T. M. Ritter (a), B. A. Weinstein (b), R. E. Viturro (c), and D. P. Bour (d) phys. stat. sol. (b) 211, 869 (1999)in revised form September 28, 1998.

[6]

Aniruddha Singh Kushwaha, Pramila Mahala and Chenna Dhanavantri, "Optimization of p-GaN/inGaN/n-GaN Double Heterojunction p-i-n Solar Cell for High Efficiency : Simulation Approach"; International Journal of Photoenergy, Vol-2014, Art.ID819637.

[7]

http://www.nextnano.comlnextnano3/tutoriallIDtutoria,-AIGaInP_onGa As.htm.

[8]

W. Shockley and W. T. Read, "Statistics of the Recombinations of Holes and Electrons", Jr Phys. Rev. 87, 835 The American Physical Society Published I September 1952© 1952.

[9]

Chih-Tang Sah, "Carrier Generation and Recombination in P-N Junctions and P-N Junction Characteristics", Shockley Semiconductor Lab., Mountain View, Calif. Noyce, R.N. ; Shockley, WProceedings of the IRE (Volume:45 , Issue: 9) Sept. 1957 IEEE Journals and Magazines.

CONCLUSION

In this paper one PIN solar cell structure with the III-V compound material of AIGaInP and AIGaAs is developed in the environment of VWF of Silvaco TCAD. The materials used in this solar cell are also developed in this software. The solar cell has achieved an efficiency of 51.5037% and Fill Factor of 92%. Short circuit current density (Jsc) 0.24e+006 Alm2 and the open-circuit voltage (Vo c) 1.65 V have been observed. It can be noted that, high cost compound material =

=

2015

used in the solar cell is made cost effective by adding Aluminium which in tum shoots the efficiency. Instead of fabricating multi junction costly highly efficient solar cells, which are mainly used for satellites, comparable efficiency is observed by optimizing material characteristics of only two optical compound alloys, in a single junction. With a simple and energy efficient structure, it is a proposal for more efficient way of energy management. More investigation and research work is required in the physical environment along with real-time fabrication to achieve optimum better result of this solar cell.

[10] S. C. Jain, J. M. McGregor and D. J. Roulston, "Band-gap narrowing in novel III-V semiconductors" 68, 3747 (I990)© 1990 American Institute of Physics

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