Simulation of In�.��Ga�.�N solar cell

using AMPS-1D

Dennai Benmoussa*, Benslimane Hassane and Helmaoui Abderrachid

Laboratory in semiconductor devices

University of Bechar,

Bechar, Algeria

*benmoussade@gmail.com

Abstract— �������� alloys feature a bandgap ranging from

0.7eV to 3.4eV, covering almost the entire solar spectrum. To

optimize the efficiency and the best parameters of solar cells,

numerical simulations of �������� single junction. The

simulation modeling is important and indispensable for designing

and fabricating �������� single junction.

We changed the In doping and the thickness of the p- ��������

to determine the short circuit current density (Jsc), open circuit

voltage (Voc), fill factor (FF) and conversion efficiency (η).

For �������� single junction solar cell, the Jsc, Voc, and FF

have a strong dependence on the In composition. In composition

is a critical parameter to determine Jsc, Voc, FF, and η of InGaN

solar cells. ��.�����.��� Solar cell shows the maximum η ~ 23%.

The band gap of ��.�����.��� is 1.64 eV and is almost the same

with AlGaAs. When the total layer thickness is greater than 500

nm, the absorption becomes saturated and the η increases

smoothly. The simulation results are congruent with this trend.

Keywords-InGaN Solar Cell; p-n Junction; AMPS-1D;

simulation .

I. INTRODUCTION

In�Ga���N Alloys feature a bandgap ranging from the near infrared (0.7 eV) to the ultraviolet (3.4 eV) [1,2]. This range corresponds very closely to the solar spectrum making In�Ga���N alloys a promising candidate for radiation-resistant multi-junction solar cells [3]. Furthermore, it has been shown that In�Ga���N can be grown directly on Si substrates by a low temperature process, providing the potential for cheap multi-junction solar cells [4]. Previous simulations have shown that double-junction In�Ga���N /Si cells could have efficiencies as high as 31% [5]

In�Ga���N Solar cells have been fabricated by a number of groups although only for Indium compositions less than 30%[6–9]. One of the main challenges towards increasing the Indium content in these cells is p-type doping. P-type doping has only recently been established for InN and has been verified by electrochemical capacitance voltage measurements [10], variable magnetic field Hall effect [11] and thermo power [12]. The difficulty in doping InN p-type is believed to come from compensation by native defects. The Fermi-stabilization level [13] lies above the conduction band of In�Ga���N for Indium compositions greater than40%, causing native defects

in In�Ga���N to act as donors which pin the surface Fermi level in the conduction band [14]. In addition, in p-type In�Ga���N with x<60%, a surface inversion layer forms preventing direct contact to the p-type bulk [15,16]. These difficulties limit the range of Compositions available for fabricating solar cells.

This paper calculates the optimum efficiency of the In�Ga���N SJ cells and accordingly the best parameters of the PV structures, which include the doping density and thickness of each layer. InGaN SJ solar cells are the basic components of the high-efficiency InGaN tandem cells. So the work in this paper is important and indispensable for designing and fabricating InGaN tandem solar cells.

II. MODELLING AND SIMULATIONS

A. About AMPS-1D

AMPS-1D is the first-principles simulation tool developed by the Penn State/Electric Power Research Institute

(EPRI)[15]. AMPS software used in this study is based on the

first-principles, basic equations of semiconductors and solar

cells: Poisson’s equation, the continuity equation for free holes,

and the continuity equation for free electrons.

Determining transport characteristics then becomes a task of solving the three coupled non-linear differential equations, each of which has two associated boundary conditions. In AMPS, these three coupled equations are solved Simultaneously to obtain a set of three unknown state variables at each point in the device: the local vacuum level, the electron, and hole quasi-Fermi levels. From these three state variables, the free carrier concentrations, fields, currents, etc. can then be computed.

Besides the classical continuity equations, other semi-classical or quantum transport equations such as the Boltzmann equation, Quantum Hydrodynamic (QHD) model, Wigner function method, and non-equilibrium Green’s function method have also been applied to study the trans-port processes in photovoltaic devices so far [16–18].

AMPS-1D supplies two different approaches to the process of recombination/generation. One is the density of states (DOS)/capture cross section model and the other is the carrier lifetime model. Zhang et al. [7] probably adopted the lifetime model in the simulation of In�.��Ga�.�N single junction solar

978-1-4673-6374-7/13/$31.00 ©2013 IEEE

cell, and set the hole lifetime as ��=6.5 ns,which is supposed to be equal to the hole lifetime of GaN [7]. In this study, we use the DOS model in the simulation of InGaN solar cells because the DOS model could provide much more information about recombination/generation in semiconductors than the lifetime model.

B. Structures of the simulated solar cell

The structure as designed is shown in figure1. For simple

simulations, we suppose that the reflection coefficients of front

and back surfaces are both 10% [6,7].

p-In�.��Ga�.�N

n-In�.��Ga�.�N

Figure 1.Structure of an In�.��Ga�.�N SJ solar cell.

C. Parameters for the simulation

Material parameter equations used for the simulation of the In�Ga���N SCs

• Band gap[2]

:�� !" # 0.7! & 3.4 1 * !" * 1.43 1 * !" • Electron affinity

[8,9]:+ # 4.1 & 0.7 3.4 * ��"

• Absorption coefficient [9]

:

, -" # 2.2 / 10�0 1.24/-" * ��

• Effective density of states in the conduction band [8]

:

23 # 40.9! & 1 * !"2.36 / 10��

• Effective density of states in the valence band[4]

:

27 # 45.3! & 1 * !"1.86 / 10�:

• Relative permittivity[8]

: ;< # 14.6! & 1 * !"10.4

• Carrier mobility [10]

:>? 2" # >@?A,? & CDEF,GHCDGI,G�HJK KL,GM N

OG ;

The above formulae with asterisk are obtained from the

linear fitting of the corresponding parameters of InN and GaN.

The carrier mobility of InGaN is assumed to be similar to GaN,

where i= n, p denotes electrons and holes, respectively, and N

the doping concentration, while the model parameters>@?A,? , >@PQ,? , 2�,? and R? depend on the type of semiconductor [10].

TABLE I MODEL PARAMETERS USED IN THE CALCULATIONS OF THE

CARRIER MOBILITIES.

Type of

carriers ST�U,V WT�X��Y�� ST�U,V WT�X��Y��" �Z,V WT�[" R?

Electrons 100 55 2 / 10�\ 1 Holes 170 3 3 / 10�\ 2

TABLE II THE PARAMETERS FOR THE SIMULATION OF IN �.��GA �.�N

SOLAR CELL

Layers

Parameter p - ��.�����.��� n- ��.�����.���

Thickness(µm) 0.1--2 0.230

Relative permittivity 12.58 12.58

Electron mobility S�

(cm²/Vs)

498 498

Hole mobility S^

(cm²/Vs)

130 130

Carrier density, � _` ^

WT�[" P:1e+15-1e+19 n: 1.00e+017

Optical band gap,aZ

(eV)

1.64 1.64

Effective density,�W WT�[" 1.57e+018 1.57e+018

Effective density,�b

WT�[" 3.62e+019 3.62e+019

Electron affinity ,χ(eV) 5.33 5.33

Figure 2.AMPS simulation front panel contains the device and layer grid

parameters, and general layer parameters

III. RESULTS AND DISCUSSIONS

These simulations have been made for choosing the best

structure parameters for the optimal performance of In�.��Ga�.�N SJ cells. The proper total thickness was calculated first, followed by the optimum doping concentration and the best thickness of the front layer.

A. Optimum thickness of the front layer

With 1×1017

cm-3

carrier concentrations and 4.230 µm total thicknesses, the corresponding efficiency and Jsc of the solar cell were calculated by changing the p-layer thickness from 0.05 to 2 ųm. In relation to the collection efficiency, the p-layer thickness played a certain role in Jsc. By simulations, it was found that the efficiency for variations of the p-layer thickness had a peak value as well as Jsc (shown in figure3,4)

0,0 0,5 1,0 1,5 2,0

22,05

22,10

22,15

22,20

22,25

22,30

22,35

Cu

rren

t d

en

sity (

mA

)

p-layer thicknesses(µm)

Figure 3. Current density versus the p-layer thickness

0,0 0,5 1,0 1,5 2,0

22,4

22,5

22,6

22,7

22,8

22,9

De

vic

e to

tal effic

ien

cy (

%)

p- thicknesses(µm)

Figure 4. efficiency versus the p-layer thickness

On the one hand, when the p-layer thickness was decreased, Jsc increased due to the enhancement of the effective collection efficiency resulting from the decreasing distance between the surface and the space charge region.

On the other hand, the collection efficiency of the space charge region would be weakened as the region would be too close to the surface if the surface recombination was considered. And Jsc would decrease because of the reduction in the effective collection efficiency.

Accordingly, as the p-layer thickness was increased, both the efficiency et Jsc increased first and then decreased. The best efficiency was obtained with about 600 nm p-layer thickness curves in figure (3, 4) show that.

Optimum doping of the front layer

For that presented optimization, we have chosen a range of hole doping concentration between 1015 to1019 (cm−3) for p- layer. The figure 4 schematizes the illuminated characteristics J-V of In�.��Ga�.�N solar cell.

According to these results, we notice a maximal density of current (22.45mA/cm

2) with devise total efficiency (22.99%)

for a value of the doping of the P- layer Na = 1015

(cm−3

)

10^15 5*10^15 10^16 5*10^16 10^17 5*10^17 10^18 5*10^18 10^19 --

17

18

19

20

21

22

23

Cu

rre

nt d

en

sity (

mA

)

doping concentrations NA(cm

-3)

Figure 4. Current density versus the p-layer doping

10^15 5*10^15 10^16 5*10^16 10^17 5*10^17 10^18 5*10^18 10^19 --

18

19

20

21

22

23

Devic

e t

ota

l e

ffic

ien

cy (

%)

doping concentrations NA(cm

-3)

Figure 4. efficiency versus the p-layer doping

Optimum performance of an cd�.��ef�.�2 SJ solar cell

Both with 1015

cm−3 carrier concentrations, the 600 nm thick p-layer and the 230 nm thick n-layer, the optimum performance of the In�.��Ga�.�N SJ solar cell is shown in figure7. The corresponding PV parameters (open-circuit voltage Voc, short-circuit current Isc, fill-factor FF and efficiency (η) are all summarized in Table 2.

TABLE III. PV PARAMETERS OF THE OPTIMIZED INP /GE DEVICE.

0,0 0,2 0,4 0,6 0,8 1,0 1,2

0

5

10

15

20

25

Cu

rre

nt

de

nsity (

mA

)

Voltage(V)

I(V)

TheJ–Vcharacteristic of the In0.65Ga0.35N SJ solar cell with the optimum

efficiency calculated.

IV. CONCLUSION

The optimum performance of the In�.��Ga�.�N solar cell is simulated by calculating the efficiencies with different parameters of the structure, including the doping concentration and the thickness of each layer. When the carrier concentrations of the front and basic regions are 10

15cm

−3,the

thickness of the p-layer and the n-layer are 600 nm and 270 nm, respectively, the optimum efficiency calculated is 22.98% (AM1.5G, 100 mW cm

−2, 0.32–1.32µm), and the simulation

contributes to designing and fabricating InGaN ultra-high efficiency solar cells.

ACKNOWLEDGMENT

We would like to acknowledge the use of AMPS-1D

program that was developed by Dr. Fonash’s group at

Pennsylvania State University (PSU).

REFERENCES

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[3] S. Jacobs and C. P. Bean, “Fine particles, thin films and exchange anisotropy,” in Magnetism, vol. III, G. T. Rado and H. Suhl, Eds. New York: Academic, 1963, pp. 271–350.

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[5] R. Nicole, “Title of paper with only first word capitalized,” J. Name Stand. Abbrev., in press.

[6] Zhang, X. B. et al., “Simulation of In0.65Ga0.35N single-junction solar cells,” J. of Phys. D: Appl. Phys. 40, 7335-7338 (2007)..

[7] Hamzaoui, H., Bouazzi, A. S. and Rezig, B., “Theoretical possibilities of InxGa1-xN tandem PV structures,” Sol. Energy Mater. Sol. Cells 87, 595-603(2005).

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[9] Li, N., “Simulation and Analysis of GaN-based Photoelectronic Devices,” Dissertation (in Chinese), Institute of Semiconductors, Chinese Academy ofSciences, Beijing, 3(2005).

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PV parameters ghW Xij ff η(%)

22.46 1.14 0.894 22.99