GaAs solar cell

Accepted Manuscript Title: Numerical simulation of front graded and fully graded AlGaAs/GaAs solar cell Author: Nadia Messei M.S. Aida PII: DOI: Reference:

S0030-4026(15)00900-6 http://dx.doi.org/doi:10.1016/j.ijleo.2015.08.139 IJLEO 56063

To appear in: Received date: Accepted date:

19-8-2014 24-8-2015

Please cite this article as: N. Messei, M.S. Aida, Numerical simulation of front graded and fully graded AlGaAs/GaAs solar cell, Optik - International Journal for Light and Electron Optics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.08.139 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Numerical simulation of front graded and fully graded AlGaAs/GaAs solar cell Nadia Messei, M.S.Aida

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Thin films and interfaces laboratory, Faculty of Science, MentouriUniversity, Constantine. Algeria Abstract

One of the promising methods to enhance the performance of third generation solar cells is to use

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compositionally graded layers. The aim of this paper is to study the effect of fully graded AlGaAs solar cells using numerical simulation with SCAPS 1D . the gradient is simultaneously in the

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composition and in doping concentration . To attain this goal, we have optimized the standard p-i-n device in first step. Then, we have optimized the usual front graded device. simulation of the fully

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graded device was done and the most important observation is that the open circuit-voltage (Voc) is much higher than Voc of device with uniform band gap where the open circuit voltage density decrease inconsiderately.We showed that a grading strategy with fully graded device can indeed lead

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to a more favourable trade-of between Jsc and Voc than can be obtained with uniform AlGaAs device .

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Keywords: AlGaAs/GaAs; third generation; solar cell; fully graded cell; SCAPS1D.

Corresponding author:

E-mail address: [email protected] thin films and interface laboratory, Faculty of Science, Department of Physics, University Mentouri, Constantine. Algeria

1. Introduction The grading of the band-gap and other semiconductor properties, especially doping concentration, had been proposed a long time ago in order to improve solar cells efficiency both experimentally [1] 1 Page 1 of 19

and via computer simulation [2].Usually, the proposed profiles for improved solar cells with graded band-gap layers include grading in front, back or both front and back (double) of the solar cell [3] as it is shown in Fig.1(a), (b) and (c). Front grading leads to higher open-circuit voltage.Back grading in such a way the band gap increase toward the back contact (BSF) improves the electron collection at the junction due to the reduced bulk and surface recombination.Further more double grading can

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reduce Auger recombination and thermalisation in the cell [4]. Thus, band-gap engineering in the absorber layer can lead to enhanced overall performance of solar cells. An additional type of band-gap grading profile is also proposed by Dharmadasa et al [5] where the device is fully graded as it is

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illustrated in Fig.1(d).

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AlyGa1-yAs compounds are suitable to implement band-gap engineering in graded solar cells. This is for several reasons:(a) these compounds are characterised by direct band-gap for y values between y=0 (Eg =1.42 eV) and y=0.45(Eg =1.96 eV).Therefore those materials are efficient optical absorbers as the change in lattice constant is less than 1% (a

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required for graded solar cell; ( b) throughout the composition range from y =0(GaAs) to y=1(AlAs) AlyGa1-yAs

= aGaAs 0.0078y )[6,7], so graded regions

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can be grown without forming notable lattice mismatch or dislocations, (c) their technology is advanced . The greatest obstacle which faced the success of GaAs cells was the high cost of a singlecrystal GaAs substrate. Researchers are exploring several approaches to reduce the cost of GaAs

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devices. One of those approaches is growing of GaAs cells on cheaper, removable and re-usable GaAs substrates. This approach leads to obtaining GaAs thin films similar to those made of copper indium

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diselenide and cadmium telluride such as Alta devices [8].

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2. The numerical simulation package SCAPS 1D The simulation has been carried out using the simulation package, SCAPS 1D, this is a onedimensional solar cell device simulator, developed at Electronics and Information Systems (ELIS), University of Gent [9]. SCAPS is freely available to the PV research community. The user can describe a solar cell as a stack of up to seven layers with different properties, such as thickness, optical absorption, doping, defect densities and defect distribution. It is then possible to simulate a number of common measurements: I-V, QE, C-V, C-f. From version 2.8 onward, SCAPS also implements graded solar cells [10]. A variety of interpolation laws are available to set the position dependent composition y of each layer: y(x). These interpolation laws can also be applied to set the composition dependence of all relevant semiconductor properties in a layer, the most relevant properties are the band-gap Eg(y) and the electron affinity (y). Both of them combined with the composition profile y(x) give the ‘grading’ of these parameters, e.g. Eg(x) = Eg[y(x)]. In fact, the grading of up to eighteen properties can be set. A special interpolation scheme for the optical absorption (, y) has been implemented and tested for the Ga-Al-As material system [10]. 2 Page 2 of 19

3. Input parameters 3.1. Front and Back Contacts and Surfaces Contacts can be assumed ohmic or, depending on the focus of the modeling, The metal work function Φm (for majority carriers) in SCAPS can be input by the user . However, we can also choose the

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option “flat bands”. At the contacts a (wavelength dependent) reflection/transmission can be set . The reflection at the back surface has only minor influence on the achievable short-circuit current density

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(Jsc), and these influences only become noticeable if the absorber is chosen to be fairly thin. 3. 2.Materials parameters for the Al-Ga-As system

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The most obvious effect of the partial substitution of Ga with Al in GaAs is an increase of band-gap of the AlyGa1-yAs alloy and a decrease in the electron affinity. In AlyGa1-yAs alloys the band-gap varies from 1.42 to 2.16 eV as y goes from 0 to 1 [11].In this system, however, the band-gap is direct in the

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case of GaAs, but indirect in the case of AlAs. There is a transition from direct to indirect band-gap at y=0.45 [12]. In alloy semiconductors the variation of band gaps with composition (y) can generally be [14] in AlyGa1-yAs alloy the band gap is given by

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expressed as the form

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(eV)……………..when y<0,45 [13,14,15 ] …….. when y>0,45[13,14,15]

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and

The electron affinity is given in [13,14,15 ] by

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(y)= 4.07 - 1.1y (eV) when y<0,45 [13,14,15 ]

And (y)=3.64 - 0.14x (eV) when y>0,45 [13,14,15 ] However, most physical properties are influenced by Al alloying, and thus become composition dependent: the effective density of the band states NC(y) and NV(y), the electron and hole mobilities n(y) and p(y), the dielectric constant (y), electron and hole thermal velocity, electron and hole effective mass mn and mp the optical absorption (,y) etc. We used the composition dependent materials properties given in [13,14,15 ] and gathered in table.1.in this work we assume that the grading in the structure is always between y=0 and y=0.4 that means we use direct band gap material. However we gave the parameters of indirect band gap material because we need them to calculate parameters of the window. We assume that the doping densities ND and NA are controlled by the introduction of dopants during the production process. Consequently, any reasonable doping profile ND(x) and NA(x) can be obtained.

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For doping and defect densities, SCAPS can handle both: a direct grading profile such as ND(x) and an indirect profile (through composition grading) ND[y(x)]. From the accumulated knowledge to date, it seems that the dominating defect at room temperature is known as the EL2 defect [14]. It was proved that this defect remains nearly constant while varying the

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Al content in AlGaAs over a wide range [16]. This defect is donor-like in character and is located at the middle of the energy gap. A wide range of capture cross-sections and densities have been reported in literature depending on the deposition method [16, 17] . the Characteristics of EL2 Trap used in

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this work are illustrated in table.1.

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4. Optimization of the uniform cell

The schematic energy-band diagram under equilibrium condition for a typical AlGaAs p-i-n solar cell with a uniform band-gap profile is illustrated in Fig.2 .The basic solar cell structure under study is an

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GaAs p-i-n structure grown on a highly doped (1×1018 cm-3) n-type GaAs substrate. The structure is completed with a thin, 2×1018 cm-3 highly doped p type Al0.8Ga0.2As (Eg 2.09 eV) as a window layer.

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Such structure is fairly common in the literature [18]. In most devices the window layer is followed by a p-GaAs contactable capping layer. The major reason for this capping layer is to avoid oxidation of Al in AlyGa1-yAs layers. Our simulation with SCAPS shows that the elimination of this layer increases

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the efficiency from 24 to 30 %. It is clear that the effect of this layer on the front of the structure is

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destructive to solar cell efficiency . GaAs layer acts as a filter, especially in the blue. We assume that optimization of layer thicknesses can be done independently in two steps using

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SCAPS : first, optimization of Al0.8Ga0.2 As window. This shows that most favorable thickness is ~4 nm. Increasing thickness more than 4nm has a detrimental effect on Jsc and efficiency. The wellknown role of larger band-gap window is the reduction of surface recombination and window absorption losses. Second, optimization of layer thicknesses was done one by one, one can use 1.1m as an optimum thickness for the p-type layer, 0.3 m for the i-type layer, and 1.5 m for the n-type layer.

5.effect of grading

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In uniform layers, the driving forces for electrical current are the electrostatic potential gradient ∇Φ

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(drift current) and the concentration gradients ∇n and ∇p (diffusion current). When grading is present,

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additional driving terms should be added: the gradient of the electron affinity ∇χ, the gradient of the

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band gap ∇Eg, and the gradients of the effective density of states in the conduction and valence bands:

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∇(log NC) and ∇(log NV). Also, the electron and hole continuity equations are modified by the presence

of a mobility gradient ∇μn or ∇μp (eq.(1)), and the Poisson equation is modified by a gradient ∇ε in

dielectric constant (eq.(2)). These modified equations have been described in the literature [19] [20] and are implemented and solved in SCAPS [20].

(1)

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(2)

n

and

p

to the electrostatic potential Φ[20]

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the ‘band potentials’

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When gradients in electron affinity χ or band gap Eg are present, this formalism is extended by adding

The graded material can be viewed as a series of sub-layers or sheets .each layer is divided in SCAPS n

in a number (typically 100 or larger) of sub-layers or sheets, The effective electrostatic potentials Φ p

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and Φ evaluated (in SCAPS) at each sub-layer or sheet are given by equations 1 and 2 (n for electron, p for holes ) [10] :

with

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(3)

(4)

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with

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(the factor kT/q or KT is needed, all 3 terms must have the same dimension of voltage, Eg is an energy, Eg/q is a voltage) .Here NC0 and NV0 are arbitrarily chosen reference values for the density of

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states NC in the conduction band and NV in the valence band. 6 .Common front graded cell

The grading is supposed as starting from Al0.4Ga0.6As at the beginning of the p-layer ( direct band gap). We have done an optimization of the depth of the grading toward the junction designed by Wpg in Fig.3. Better results of short-circuit current density and efficiency are obtained from 0.2 m depth of front grading. If grading goes deeper than 0.2m the efficiency and Jsc decrease while FF and Voc remain constant. As a result of our simulation , front grading is beneficial when applied to the first 200nm of the p-type layer as it is illustrated in Fig.4.The band-gap variation at the front represented by ΔEc in Fig.3 was optimized as well. Since AlyGa1-yAs is a direct band-gap material when y<0.45 and indirect band gap material when y >0.45,the maximum value of ΔEc that can be used is 0.56 eV (y=0.45).This indicates that the front grading starts from Al0.45Ga0.55As (y=0.45) and decreases uniformly until reaching the pure GaAs (y=0) substrate at the end of Wpg (already optimized). Fig.5. illustrates how variation in ΔEc can affect device parameters. The front grading using maximum ΔEc 6 Page 6 of 19

improves Jsc from 31.12to 31.58 mA/cm2 and efficiency from 30.09 to 30.59 %.FF and Voc remain insensitive. Our optimum values will be Wpg =0.2 m and ΔEc =0.56 eV. This increase in efficiency in the case of front grading is attributed principally to an improved collection efficiency caused by the built-in electric field that results from the band gap grading.

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7. Fully graded cell

In order to obtain the entirely graded p-i-n-type device structure, we started from the common device structure previously optimized in this work. After, the Al amount in AlyGa1-yAs was linearly reduced

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from y=0.4at the front end to y=0 at the back end of the device. This means that most material parameters are affected as it is shown in Table.1. At the same time, the electrical conductivity was

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gradually converted from p-type to n-type. In the front end, shallow acceptor density(NA) starts from 1x1018 cm-3 and gradually reduces. At the back end, shallow donor density ND starts from 1x1018 cm-3 and gradually reduces towards the junction. From SCAPS band diagram, the depletion region is about

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0.6μm and starts from 1.2μm from the front surface. If one reduces ND and NA to the range 1x1015cm-3 the depletion region will cover the full width of the device as it is illustrated in fig.6.As a result, the

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photo-generated charge carriers are separated by the built-in electric field in the whole thickness of the active solar cell structure and, hence, the probability of R&G process is further reduced. Fig.7. shows the quantum efficiency of this device and Table.2.illustrates comparison between PV parameters of the

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Conclusion

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fully graded structure solar cell and uniform composition cell .

In this work , we have used SCAPS 1D to study the effect of front grading and fully grading in p-i-n

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AlGaAs solar cell . The front graded device shows an enhancement in PV parameters over that of uniform solar cell. In the case of fully graded solar cell with depletion region equal to the thickness of the cell ,the most important observation is that the open circuit-voltage (Voc) is much higher than Voc of device with uniform band gap where the open circuit voltage density decrease inconsiderately as table.2 shows .We showed that a grading strategy with fully graded device can indeed lead to a more favourable trade-of between Jsc and Voc than can be obtained with uniform AlGaAs device . Acknowledgements

The author would like to acknowledge gratefully Marc Burgelman from ELIS, University of Gent

for his precious instructions. Figure Captions Fig.1. Characteristic graded structure profiles. a-front, b-double, c-back, d-fully graded , Fig.2. Uniform p-i-n AlGaAs/GaAs solar cell under equilibrium .a) energy-band diagram b) schematic diagram of the material structure 7 Page 7 of 19

Fig.3. Band gap diagram of p-i-n solar cell with front band gap grading. Wpg is the depth of the grading toward the junction P means that the layer is p-type, n means that the layer is n-type. ΔEc is the band-gap variation at the front.

Fig.5. Variation of Jsc and efficiency as a function of ΔEc.. Fig.6. Energy band diagram of the totally graded structure.

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Fig.7. Quantum efficiency of the fully graded device as function of wavelength.

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Fig.4. Variation of Jsc and efficiency as a function of depth of the front grading Wpg.

Table Captions Table1. AlGaAs solar cell baseline. p/n refers to electron/hole properties. Φm metal work function, S surface recombination velocity, ε dielectric constant, μ mobility, νth thermal velocity, Eg band gap energy, NC and NV effective density of states.

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Table2.Comparison between device parameter of uniform and fully graded device

References [1] J. A. Hutchby, Appl. Phys. Lett 26, (1975) 457. [2] J. E. Sutherland and J. R. Hauser, IEEE transaction on electron devices, vol.ed-24, no. 4,april (1977). 9 Page 9 of 19

[3] A. Morales-Acevedo, Solar Energy 83 (2009) 1466–1471 [4] N. H. Rafat, S. E.-D. Habib, Sol Energy Materials and Solar cells55 (1998) 341-361. [5] I. M. Dharmadasa, J. S. Roberts and G. Hill, Sol. Energy Materials and Solar cells. 88, (2005) 413-422. [6] john H.DAVIES, the physics of low dimensional semiconductor , Cambridge university press

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,1998

[7] Jasprit Singh, Electronic and Optoelectronic Properties of Semiconductor Structures,

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Cambridge University ,Press 2003

[8] M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovolt: Res.

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Appl. (2013) 1–11

[9] M. Burgelman, P. Nollet and S. Degrave, Thin Solid Films, 527 (2000) 361-362. [10] M. Burgelman, J. Marlein, Proceedings of the 23rd European Photovoltaic Conference,

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Valencia, Spain ( 2008 ) 2151-2155.

[11] S.Z.Sze, Physics of semiconductor devices, second edition, wiley, 1981.

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[12] A. KITAI, Principles of Solar Cells, LEDs and Diodes, Wiley ,2011 [13] http://www.ioffe.ru/SVA/NSM/Semicond/AlGaAs/index.html, Jun, 2013

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[14] S.Adachi , properties of aluminium gallium arsenide , INSPEC , 1993.

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[15] Handbook series on Semiconductor Parameters, VOLUME 2: Ternary and Quaternary A3B5 Semiconductors, edited by M. Levinshtein, S. Rumyantsev and M. Shur , 1999 by World Scientific

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Publishing Co. Pte. Ltd.

[16] P. K. Bhattacharya, T. Matsumoto, Cryst.Growth 68(1984) 301-304. [17] S. Markram-Ebied, “Nature of EL2: The Main Native Midgap Electron Trap in VPE and Bulk GaAs,” in Semi-insulating III-V Materials, D. Look, Editor, Shiva Publishing Ltd., England, 1984

[18] M. Begotti & all , Cryst. Res. Technol. 40, No. 10–11, (2005)1033 – 1038 . [19] Stephen J. Fonash ; Solar Cell Device ,Physics, ACADEMIC PRESS ,1981 [20] C. Snowden, Introduction to semiconductor device modelling, World Scientific, 1986.

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Table .1.

Back

Flat band

Flat band

Sn [cm/s]

7

10

107

Sp [cm/s]

107

107

Layer Properties at 300°[13,14,15] Al0.8Ga0.2As 10.6

μn (cm2/Vs)

212

μp [cm2/Vs]

126

mn(in unit of m0)

0.19

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ε/ε0

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AlyGa1-yAs (y<0 .45)

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Φm [eV]

Front

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General Device properties

mp(in unit of m0)

0.71

2.3×105

νthn (m/s)

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νthp (m/ s)

1.4×105

2.5 ×1019×(0.063+0.083y)3/2

1.6×1019

Nv(cm-3)

2.5×1019×(0.51-0.25y)3/2

4×1018

d

Nc(cm-3)

Caracteristics of EL2 trap[14,16,17] 1015

Energy level Et(eV)

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Concentration (cm-3)

0.75 below Ec

Capture cross section σ (cm ) 4x10-16

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2

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Table .2. Voc(V)

Jsc(mA/cm2)

FF(%)

Efficiency (%)

Uniform

1.100

31.12

87.55

30.09

Fully graded

1.350

30 .00

89.00

34.01

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an

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cr

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device

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Figure

Eg

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Figure.1. N. Messei et al.

Ec

a-

cr

Ev b-

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Ev

c-

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d-

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Figure

Front contact

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Figure.2. N. Messei et al.

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p+-AlGaAs (4.5 nm) p-GaAs (1.5 μm) EFp

Eg p

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p

i-GaAs (0.5 μm)

Ec EFn

i

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n-GaAs (1.5 μm)

n

Back contact

n+-GaAs substrate

Ev

b-

Ac

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a-

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Figure

ΔEc Eg

Wpg

an

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Ec

cr

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Figure.3. N. Messei et al.

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Ev

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Figure

cr

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Figure.4. N. Messei et al.

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2

J sc(mA/cm )

31,6 31,4 31,2

0,0

0,2

0,4

0,0

0,2

0,4

0,6

an

31,0 0,8

1,0

M

30,4

30,2

ed

30,0 0,6

0,8

1,0

1,2

ce pt

Wpg(um)

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efficiency %

30,6

1,2

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Figure

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Figure.5. N. Messei et al.

cr

2

Jsc(mA/cm )

31,6

31,2 31,0 0,0

0,1

0,2

0,0

0,1

0,2

0,3

0,4

0,3

0,4

0,5

an

30,2

30,0

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30,4

0,5

0,6

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ed

EC (eV)

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efficiency %

30,6

0,6

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31,4

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Figure

cr

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front contact

Back contact

hν

Eg min

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hν

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Eg max

Solar radiation

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Fig.6. N.Messei et al

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Figure

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Figure.7. N. Messei et al.

100

cr

80

QE%

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60

an

40

0 300

400

500

M

20

600

700

800

900

Ac

ce pt

ed

wavelength(nm)

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