Prospects of Novel CdZnTe Thin Film Solar Cells from Numerical Analysis

M. N. Imamzai1, M. J. Rashid1, N. A. Khan1, Q. Huda1, M. Akhtaruzzaman1, K. Sopian1

and N. Amin1,2,3*

1Solar Energy Research Institute, Universiti Kebangsaan Malaysia (UKM), 43600, Bangi, Selangor, Malaysia 2 Department of Electrical, Electronics and System Engineering, Universiti Kebangsaan Malaysia (UKM), 43600

Bangi, Selangor, Malaysia 3Advanced Materials Research Chair, Chemistry Department, College of Sciences, King Saud University, Riyadh 11421,

Saudi Arabia

*Corresponding Author: nowshad@eng.ukm.my

Abstract—The efficiency of CdTe based solar cell can be increased using ternary CdZnTe material as absorber layer. Cd1-xZnxTe has tunable bandgap depending on the composition. In this work the bandgap of CdZnTe layer (1.57 eV) which is in the optimum range, can be achieved with Zn composition of x=0.1. First the carrier concentration of absorber layer in the baseline case is increased then the thicknesses of absorber layer and window layer in the conventional baseline case are reduced and optimized. Finally an optimized cell structure is proposed. After optimization, the total thickness of the baseline case cell is reduced by factor four and results high efficiency. The cell structure in both baseline case and modified cell is: (SnO2/CdS/CdZnTe/Back Contact), however some material parameters are different. The performance parameters are found better in the optimized cell structure. We also investigated the effect of ZnO buffer layer and the operating temperature on the performance parameters. Keywords: Solar Cells, Thin Film Solar Cells, AMPS-1D, Bandgap, Efficiency

I. INTRODUCTION The free and infinite source of sun energy is one of the most significant types of the sustainable and renewable energy source. The direct conversion of the sunlight into electricity using solar cell is known as photovoltaic (PV). In PV technology, Si is the most widely used material, so far and it is still dominating the PV market [1,2]. However the manufacturing process of silicon based solar cells is expensive. Therefore thin film technology for solar cells becomes the subject of intense research in the PVs. One of the most promising thin film candidates is CdTe, because of their high conversion efficiency with reduced materials and stable cell operation [3,4]. Polycrystalline CdTe semiconductor has shown great potential as an absorber layer for thin film solar cells. The CdTe solar cells have some advantages. Firstly, the layer of CdTe solar cells can be deposited using different low cost techniques such as sputtering, closed space sublimation, chemical bath deposition and etc [5]. Secondly, CdTe has a direct optimum band gap (1.45 eV) with the high absorption coefficient over 5×105/cm [6,7], which means that the incidents photons with sufficient energy can be absorbed within a few

micrometers of CdTe absorber layer [8,9]. The requirement of less material reduces relatively the cost of CdTe based solar cells.

The highest reported conversion efficiency of CdTe solar cells is 20.4% [10] which is lower compared to the theoretical efficiency (near to 30%) [10]. In order to increase the efficiency of CdTe thin film solar cells ternary Cd1-xZnxTe absorber layer which is an extension of CdTe [11] can be used. CdZnTe has tunable bandgap between 1.45 eV and 2.2 eV and has high absorption coefficient, in which more than 90% photon can be absorbed within 1 μm thickness [12]. The maximum efficiency of 7.2% is recorded for CdZnTe solar cells by co-sputtering of CdTe and ZnTe targets [13]. The CdZnTe layer also can be used as back contact for CdTe solar Cells [14]. The practical efficiency of CdZnTe solar cell is usually very low which is likely due to the defects and chemical disorder of the CdZnTe layer [13]. Commercially available CdTe solar cells usually have 10 μm thick absorber layers.

In this work to reduce the cost of the CdZnTe solar cell the thicknesses of the absorber layer and window layer in a conventional baseline case of CdZnTe solar cell is optimized and a modified cell is achieved. The effect of buffer layer on modified cell structure also is investigated. Finally a modified cell structure with high efficiency and low cost is proposed.

II. METHODOLOGY AND MODELING OF STRUCTURES In this work AMPS-1D (Analysis of Microelectronic and Photonic Structures) software is used for simulation and to analyze the CdZnTe solar cells [15]. The conventional baseline case of CdZnTe solar cells structure consists of three main layers (SnO2/CdS/CdZnTe) with front and back contact layer. In the conventional baseline case of CdZnTe solar cells, SnO2 with 500 nm thickness is used as transparent conducting oxide (TCO), CdS with 100 nm thickness is used as front contact (or window layer) and CdZnTe with 4 μm thickness is used as absorber layer. The bandgap Eg=1.57 eV is selected for CdZnTe (absorber layer) thin film solar cells because it is in the range of optimum bandgap. Moreover this bandgap of CdZnTe can

2nd International Conference on Green Energy and Technology, September 2014 126

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be obtained practically using the compositiCd1-xZnxTe [16]. Then the conventional baseoptimized and modified. The cells are antemperature T=300 K. The main layers contacts properties are shown in table 1 andFigure 1 shows the structures of conventionaand modified cell structures respectively. Ithe total thickness of the baseline case strucfrom 4600 nm to 1150 nm in modified structoxicity, cost, time and energy required for facell is expected to be reduced.

Table 1: Main layers properties used in simulat

Table 2: Front contact and back contact properties us

III. ANALYSIS AND RESUL

A. Analysis of Conventional BaselinThe structure of conventional baseline case and shown earlier in part 2. The output paconventional baseline case 1 cell are: Jsc= Voc= 0.99 V, FF= 0.693, Eff= 18.12 %. Thethick (4600 nm). To reduce the cost the thickis reduced and optimized. By reducing the thcontact from 500 nm to 100 nm the effperformance parameters remain constant.

Table 2: front contact and back contact properties

General layer properties Parameters Sn CdS CdZ

°

9 10 10

μ [

100 100 25

μ [

25 25 7

n,p [ ]

n: n: P:5×

[ev]

3.6 2.42 1.

[ ]

2.20 × 2.20 × 1.5×

[ ]

1.80× 1.80× 1.80×

[ev] 4.5 4.5 4.

General device properties parameters Front contact Bac

[ev] 0.10

[cm/s] 1× 1

[cm/s] 1× 1 0.01

ion of x=0.1 in eline structure is nalyzed at room

properties and d 2, respectively. al baseline case1 It is shown that cture is reduced

cture. Hence, the fabrication of the

tion [17,18]

sed in simulation

LTS

ne Case 1 is introduced

arameters of the 28.82 mA/ ,

e baseline case is kness of this cell hickness of front ficiency and all

s in simulation

Figure1: Baseline case 1 and modif(not to scale)

Hence the thickness of 100contact to analyze the absothicknesses. For achieving tconcentration of baseline ca

to 5× achieof absorber layer and winincreasing the carrier concebaseline case 1) to 5×efficiency increases from increase in efficiency is due By increasing the carrier conbuilt-in potential increases wthat will be collected at conthe Jsc increases (equation 1overall increase of photogensubsequently by increasingincreases. The structures abaseline case 1 and 2 arrespectively.

) (1)

) (2)

B. Effect of Absorber Layer aPerforma

In this part, the carrier co5×1014 cm-3 to 5×1015 cm-3 awindow layer thicknesses are

ZnTe ZnO

.20 9

50 100

70 25

× n:

57 3.3

2×

× 1.5×

70 4.35

ck contact 1.25

1×

1×

0.99

fied cell structures of CdZnTe solar cell

0 nm is selected for the front orber layer and window layer to an ultra-thin cell the doping ase1 is increased from 5× vable value. Then the thickness

ndow layer are optimized. By entration from 5× (in (in baseline case 2) the 18.12 % to 22.935%. This to the increase of Jsc and Voc.

ncentration of absorber layer the which the number of electrons

ntact increases and subsequently ). The increase of Voc is due to

neration current (equation 2) and g Jsc and Voc, the efficiency and I-V characteristics of the re shown in figure 2 and 3

)

)

and Window Layer Thickness on ance Parameters

oncentration is increased from and then the absorber layer and e optimized. Figure 4 shows the

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effect of absorber layer and window layer thicknesses on performance parameters.

Figure 2: The structures of baseline case 1 and 2 (not to scale)

Figure 3: I-V charateristics of baseline case 1 and 2

The thickness of absorber layer is varied from 1 μm to 4 μm. It can be seen that by decreasing the absorber layer thickness the short circuit current exhibit small decrease in their magnitude. This is because of the absorption of less photon by the reduced layer thickness of the absorber layer. The increase of absorber layer thickness cause big area of p-n junction as the main part of p-n junction is formed in CdZnTe layer. However, by changing the thickness of absorber layer the open circuit voltage remains constant. The increase in FF with the lessening of the thickness of

absorber layers reduces the series resistance. On the other hand by decreasing the thickness of absorber layer the efficiency decreases and this is due to the reduction of Jsc. It shows that efficiency almost follows the short circuit current trend. By decreasing the thickness of absorber layer from 4 μm to 1 μm the efficiency decreases from 22.935% to 21.717%. This means that the thickness is decreased by factor four, whereas the efficiency decreases only around 1.2%. The optimized thickness of absorber layer is 1000 nm. The absorber layer with 1μm is selected for next step (optimization of window layer).

Figure 4 also shows the effect of window layer thickness on performance parameters. The thickness of window layer is changed from 50 nm to 300 nm. It shows that by decreasing the thickness of window layer the efficiency and short current increase. The increase in efficiency is due to the increase of short circuit current. The increase in short circuit current is likely due to the reduced current losses for the wavelength below 510 nm. However, by decreasing the thickness of window layer the open circuit voltage is constant and the fill factor increases a bit. The fill factor is increased because by decreasing the thickness of window layers the series resistance of the bulk decreases. The thickness of window layer is reduced from 300 nm to 50 nm and thus the efficiency increases from 20.607% to 21.8%. To reduce the cost significantly it is reasonable to decrease the thickness of window layer to 50 nm. The optimized thickness of window layer is then set to 50 nm. The modified cell (with 1150 nm total thickness) has 21.8% efficiency which is higher than the 18.12% efficiency of baseline case 1 (with 4600 nm thickness). Table 3 and 4 show the material parameters and performance parameters of the baseline case 1 and modified structure respectively.

C. Effect of Buffer Layer on Performance Parameters

Figure 5 shows the I-V characteristics of Modified cell with and without front buffer layers. In this work ZnO is investigated as front buffer layer in the structure of modified cell. By inserting front buffer layer short circuit current increases very little and thus the subsequent effect in the increase of efficiency is insignificant. However, by inserting buffer layer the open circuit voltage and fill factor remain constant. In this figure 5 the I-V characteristic graphs of modified structure with front buffer layer and without front buffer layer are almost identical, thus they overlap. It means the effect of buffer layer on modified structure is insignificant. Since the effect of mentioned buffer layers on the modified cell is insignificant, the final modified cell structure is proposed without buffer layer. Performance parameters of the modified cell with ZnO buffer layer are shown in table 5.

The Structure and band diagram profile of the final modified cell are shown in figure 6. The high Voc and Jsc of the modified cell are in line with the small value of conduction band offset Ec and valence band discontinuity. High values of Ec and Ev are known to reduce Voc and Jsc, respectively.

128

Figure 4: Effect of abso

Table3: Material parameters of the baseline case 1 anstructure

Table 4: Performance parameters of the Baseline case 1

D. Effect of Temperature on Performance PFinal Modified Cell

In this work the temperature is changed 127°C. It is evident from the figure 7 that effdecreases with operating temperature. Whetemperature increases the electrons in the extensive energy, but these electrons insteadin the electricity become unstable and recomholes, hence the short circuit current decrincreasing the temperature the bandgap of se

Material Baseline case 1 MoFront Contact

(SnO2) Thickness =500 nm

Eg = 3.6 eV Thickn

EgWindow layer

(CdS) Thickness = 100 nm

Eg = 2.42 eV Thick

Eg

Absorber layer (CdZnTe)

Thickness = 4000 nm Eg = 1.57 eV

Hole density = 5×1014

ThicknEg

Hol5×

Performance parameters Baseline case1 M

( mA/ ) 28.82 (Volt) 0.99 FF 0.693

Eff (%) 18.12

orber and window layers thicknesses on performance parameters of

nd Modified cell

1 and Modified cell

Parameters of

from 27°C to fficiency linearly en the operating solar cells gain

d of contributing mbine with other reases. Also by

emiconductor

decreases, thus Voc also decthe efficiency decreases. The°C is 21.806%, and when the°C the efficiency becomes 2obtained at T=127 °C. Modibuffer layer has TC = -0.048has good stability at higher te

Figure 5: I-V characteristics ofront bu

Table 5: Performance parameters

l

odified Cell ness = 100 nm g = 3.6 eV kness = 50 nm g = 2.42 eV ness = 1000 nm g = 1.57 eV le density =

×1015

Modified Cell

28.893 1.06 0.781 21.8

Jsc (mA/cm

Modified Cell 28.893 Modified Cell

with ZnO buffer layer

28.899

the cell

reases and subsequently Jsc and e efficiency of the cell at T= 27 e temperature is increased to 37

21.321% and finally 16.963% is ified cell with ZnO or 43% /�C, it means that this cell emperature.

of modified cell with and without uffer layers

s of the modified cell with ZnO buffer layer

2) Voc (V) FF Eff (%)

1.06 0.781 21.8

1.06 0.781 21.806

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Figure 6: Structure and band diagram profile of the final modified cell Figure 7: Effect of temperature on performance parameters of the final modified cell

IV. CONCLUSION In this work to reduce the cost and save the time and energy required for fabrication of the solar cell the thickness of the conventional baseline case of CdZnTe solar cell is reduced and optimized. To achieve an ultra-thin solar cell first, the carrier concentration of the baseline case is increased then the thicknesses of absorber layer and window layer are reduced and optimized, finally a modified cell is proposed (figure 6). It is shown that by reducing the total thickness of the cell from 4600 nm (in baseline case) to 1150 nm (in modified cell) the efficiency is increased. It can be seen that by only 1 μm absorber layer (CdZnTe) more than 20% efficiency can be obtained. Also it is shown that the effect of ZnO buffer layer on modified cell is insignificant. The modified cell has good stability at higher operating temperature (TC= -0.04843% /�C). The obtained results clearly reveal a good prospect of the CdZnTe solar cells. References [1] A. Lique and S. Hegedus, of Photovoltaic Energy Conversion and Engineering: John Wiley & Sons LTD, Chichester, West Sussex, England, 2003. [2] Kanevce, A. Anticipated Performance of Cu(In,Ga)Se2 solar Cells in the thin-film limit 2007. [3] K. Zweibel, Thin film PV manufacturing: Materials costs and their optimization, Solar Energy materials and solar cells, vol. 63, pp. 375-386, 2000.

[4] W. Diehl, V. Sittinger, B. Szyszka, Thin film Technology in Germany Volume 193, Issues 1-3, Pages 329-334, April 2005. [5] J. Pantoja, Enriquez, X. Mathew, G. Hernandez, U. Pal, C. Magana, D. Acosta, R. Guardian, J.Toledo, G. Contreras Puente, and J. Chave Carvayar, CdTe/CdS Solar cells on flexible Molybdenum substrates, Solar energy materials and solar cells, vol. 82, pp. 307-314, 2004. [6] A. Morales-Acevedo, Can we improve the record efficiency of CdS/Cd Te solar cells? Solar energy materials and solar cells, vol. 90, pp. 2213-2220, 2006. [7] Dieter Bonnet, Manufacturing of CSS CdTe solar cells, ANTEC Solar GmbH, ArnstaÈdterStrasse 22, 99334 Rudisleben Germany 2000. [8] N. Amin, K. Sopian, and M. Konagai,"Numerical modeling of CdS/Cd Te and CdS/CdTe/ZnTe Solar cells as a function of CdTe Thickness, Solar energy materials and solar cells, vol. 91, pp. 1202-1208, 2007. [9] M.Matin, N. Amin, A. Zaharim, K. Sopian, L. Perlovsky, D. Dionysiou, L. Zadeh, M. Kostic, C. Gonzalez-Concepcion, and H. Jaberg, Ultra -thin high efficiency CdS/CdTe thin film solar Cells From numerical analysis, in wseas International Conference. Proceedings.Mathematics and Computers Science and Engineering, 2009. [10] Steve Krum, first solar sets world record for CdTe solar cells Efficiency, http://www.firstsolar.com accessed on 25 February 2014 [11] A. Morales-Acevedo, "Analytical model for the photocurrent of Solar Cells based on graded Band-gap CdZnTe thin films, Solar Energy materials and solar cells, vol. 95, pp. 2837-2841, 2011. [12] M. Hossain, M. Aliyu, M. Matin, M. Islam, K. Sopian, M. Karim, and N. Amin, Prospects of Zn x Cd 1-x S window layer in CdTe Thin film solar cells from numerical analysis, International Conference on the Developments in Renewable Energy Technology (ICDRET), pp. 1-4, 2012. [13] S. H. Lee, A. Gupta, S. Wang, A. D. Compaan, B. E. McCandless, SputteredCdZnTe FilmsFor top Junctions in tandem Solar Cells, 2004. [14] N. Amin, A. Yamada, and M. Konagai, "Effect of ZnTe and CdZnTe Alloys at the Back Contact of 1- μm-Thick CdTe thin Film Solar Cells, Japanese Journal of Applied Physics, Vol.41, pp. 2834-2841, 2002.

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