2011 IEEE First Conference on Clean Energy and Technology CET

Investigation of Buffer Layers, Front and Back Contacts for ZnxCd1_xS/CdTe Photovoltaic

Md. Sharafat Hossain\ Nowshad Amin1,2,3*, Nur Radhwa Harnzah\ M.M. Aliyu\ M.A. Matin\ T. Razykov1 and Kamaruzzaman Sopian2

IDepartment of Electrical, Electronic & Systems Engineering, Faculty of Engineering and Built Environment Universiti Kebangsaan Malaysia, UKM, Bangi 43600, Selangor, Malaysia.

2Solar Energy Research Institute

Universiti Kebangsaan Malaysia, UKM, Bangi 43600, Selangor, Malaysia. 3Center of Excellence for Research in Engineering Materials (CEREM), College of Engineering, King Saud University,

Riyadh 11421, Saudi Arabia. *Corresponding author: nowshad@eng.ukm.my

Abstract-A numerical analysis has been performed

utilizing Analysis of Microelectronic and Photonic

Structures (AMPS ID) simulator to explore the possibility

of higher efficiency and stable ZnxCd1_xS/CdTe cells.

Several cell structures with indium tin oxide (ITO) and cadmium stannate (Cd2Sn04) as front contact, zinc

stannate (Zn2Sn04) and zinc oxide (ZnO) as butTer layer

and antimony telluride (Sb2Te3) insertion with Nickle (Ni)

as back contact has been investigated in the conventional

(SnOz/CdS/CdTe/Ag) CdTe cell structures in which CdS is

replaced by zinc cadmium sulphide (ZnxCd1_xS) as window

layer. Efficiency as high as 18.0% has been found with 80

nm of ZnxCd1_xS window layer for x=O.05, 1 J1II1 of CdTe

layer and 100 nm Zn2Sn04 butTer layer without Sb2Te3

back contact. However, ZnO insertion shows low

conversion efficiency of 7.84% and 12.26%, respectively

with and without Sb2 Te3 back contact. It has been found that 1 J1II1 of CdTe absorber layer, 70 nm of ZnxCd1_xS

(x=O.05) window layer, 100 nm of Zn2Sn04 buffer layer

and 100 nm Sb2 Te3 back contact layer are sufficient for

high efficiency (>17.5%) ZnxCd1_xS/CdTe cells. Moreover,

it was found that the cell normalized efficiency linearly

decreases with the increasing operating temperature at the

temperature gradient of -0.25%I"C.

Keywords- ZnZSn04' buffer layer, ZnxCdl_xS, AMPS-ID.

1. INTRODUCTION

Thin film polycrystalline cadmium telluride (CdTe) based solar cell is one of the most potential candidates for photovoltaic energy conversion. It has shown long-term stable performance [1]-[2] and high efficiency [3]-[5] under AM1.5 illumination for terrestrial usage. Moreover, CdTe has a direct optical bandgap of 1.45 eV which is very close to the optimum bandgap for solar cells. Consequently, the thickness required for an absorption layer of CdTe cells makes the price of material relatively low [6]-[7]. However, homojunction CdTe solar cells have not shown encouraging efficiencies because of

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the short optical absorption length and the complexity of establishing a shallow junction with a high conductivity surface layer. Hence, these type of solar cells are usually heterojunction structures in where a transparent conducting semiconductor (window layer) is used as the hetero partner. Cadmium sulfide (CdS) has been found to be best matched for thin film CdTe solar cells. However, it is not evidence that it is the only best. The bandgap energy of CdS is comparatively small for CdS/CdTe solar cells, since 100nm CdS layer absorbs more than 35% of the incident photons with energy higher than 2.42 ev and there exists too much pinholes in CdS film of :'S100nm which causes significant amount of forward leakage current. Hence, the conversion efficiency of the cells decreases. Thus, wide and variable band-gap window layers are desirable. ZnxCd1-xS is attaining prominence as good candidate for wide band-gap material in the field of optoelectronic devices. Its bandgap can be tuned from 2.42 eV (CdS) to 3.6 eV (ZnS) and it provides a more transparent window layer without compromising the thickness which considerably decreases the absorption of incident photons although the thickness of ZnxCd1_xS is :'S100nm. The n-type ZnxCd1_xS compounds have been applied as a window layer to form heterojunction solar cell with different p-type materials such as Si [8], CuxS [9], CuInSe2 [10], CuGaSe2 [11], Cu(In,Ga)Se2 (CIGS) [12] and CdTe [13].

Still, there are major rooms to enhance the efficiency of CdTe solar cells in balancing the effects of ZnxCd1_xS with different buffer layers, front and back contacts on cell output parameters by improving open circuit voltage (Vac), short circuit current density (Jsc) and fill factor (FF). In this analysis, the conventional superstrate structure Sn02/CdS/CdTe/Ag [14] has been modified and investigation was performed for several modified structures by inserting different front and back buffer layers, front and back contacts by replacing window layer CdS with ZnxCd1-xS. Five layers were emphasized in the modified structures: a transparent and conducting oxide (TCO) which acts as a front contact, a n­ZnxCd1_xS film which is the so-called window layer, buffer layers in between TCO and window layer, a p-CdTe film which is the absorber layer made on top of window layer and

2011 IEEE First Conference on Clean Energy and Technology CET

the back contact on top of the CdTe layer. A little bit of pinholes may be formed in ZnxCd1-xS window layer but it can be fully removed by inserting a very thin resistive buffer layer in between TCO and window layer. The formation of pinholes is not as excessive as in CdS window layer in CdS/CdTe solar cells when the CdS layer thickness is reduced to <100 nm to enhance the blue response and the pinholes can't be fully eliminated by incorporating a thin resistive buffer layer. In addition, buffer layer improves window layer film morphology and ultimately increases conversion efficiency of the CdTe devices [15]. The stability of the cells can be improved by applying stable back surface reflectors (BSR) like Sb2Te3. We have designed and modeled several cell structures by implementing all of the above ideas utilizing AMPS 10 simulator aiming to achieve the best possible structure for ZnxCd1_xS/CdTe PV for optimum value of x which showed low resistivity of ZnxCd1_xS film.

II. MODELING AND SIMULATION

The typical superstrate structure of ZnxCd1_xS/CdTe solar cell and the modified structure to analyze of different buffer, front and back contact materials proposed in this analysis to explore the effect of electric field on the performance of the cells is shown in Fig. 1.

Light

Glass Substrate

p- CdTe(4 OOnm)

Back Co nta ct (Ag)

Li g ht

.: • Glass Substrate

F ront Contact (ITO/CTO)

Buffer layer ZnO;ZTO (1 0 nm)

n- ZnCdS (SO nm)

p- CdTe (1 000 nm)

Sb:Te,(1 Onm}

Back Contact (Ag/Nil

(a) (b) Fig. 1. Structures of the CdTe solar cells: (a) Conventional structure and (b)

Structure designed for variable absorber acceptor concentration.

It is evident from Fig. 1 that in the modified cell structure CdS window layer was replaced by ZnxCd1_xS to explore effect of Zn concentration on cell output parameters such as open-circuit voltage (Voe), short-circuit current (Jse) and fill factor (FF). Besides, the modified structure has TCO layer (ITOICd2Sn04) and buffer layer (ZnO/Zn2Sn04) in between glass substrate and ZnxCd1_xS window layer at the back contact Ni with Sb2 Te3 or Ag has been inserted. In this work, AMPS-ID was used to analyze the performance of the different cell structures possibilities of higher conversion efficiency. Four basic layers that have been focused in this modeling are the front contact layer (ITOICd2Sn04), buffer layer (ZnO/Zn2Sn04), n-ZnxCd1_xS layer, and p-CdTe layer. Table I shows the arrangement of all the material parameters used in this modeling which are extracted from [16]-[17]

In our previous work [17] we have found that 1 Ilm CdTe films are sufficient for fully absorption of potential photons and 80 nm ZnxCd1_xS window layer together with suitable buffer layer for high efficiency ZnxCd1_xS ICdTe solar cells. In

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this work, we have investigated seven different structures to inspect cell performances and effect of electric field on high efficiency ZnxCd1_xS/CdTe PV for optimum value of x.

TABLE I Material parameters for the designed ZnxCd1_xSfCdTe solar cell

Parameters n- n-ZnxCd,_ p-CdTe

Zn,Sn04 xS(x=0.05)

Thickness (�m) 0.10 0.1 1.00

Dielectric 9.0 9.3 9.4

Coeffi cient

Electron mobility 32 100 500

(cm'/Vs)

Hole mobility 3 40 60

(cm'/Vs)

Acceptor 0 0 10"

concentration (cm-')

Donor 10" 3.0xl0'6 0

concentration (cm-')

Bandgap (e V) 3.35 2.48 1.50

Density of states, 1.8xl019 2.1xl018 7.5xl0"

CB (cm-')

Density of states, 2.4xl018 1.7xl019 1.8xl018

VB (cm-')

Electron affinity 4.50 4.47 4.28

(eV)

1 erent ce II TABLE 11

d II structures an ce output parameters

Structure Voc Jsc(mA FF (V) fcm')

Structure A1 0.956 25.031 0.775

(ITO/Zno.osCdo.ssS/CdTe/Ag)

Structure A2 0.957 25.061 0.776

(CTOlZno.osCdo.ssS/CdTe/Ag)

Structure A3 1.17 19.75 0.565

(CTOlZnOlZnoosCdo.sS ICdTe/Ag)

Structure A4 0.957 25.5 0.777 (CTOIZTOlZnoosCdo95S ICdTe/Ag)

Structure A5 0.975 25.559 0.7

(CTOlZnODSCdOSSS ICdTelSb2Te3/Ni)

Structure A6

(CTOlZnOlZnoCSCd095S 0.884 18.945 0.55 ICdTe/Sb2Te3/Ni)

Structure A7

(CTOIZTOlZno.osCdo.ssS 0.975 25.592 0.71 ICdTe/Sb,Te,tNi)

III. RESULTS AND DISCUSSION

p-

Sb2Te3

0.10

55

1094

320

6.8xl019

0

0.30

1.0xl0"

1.0xl0'6

4.15

Efr (%)

17.2

17.92

12.26

17.97

16.4

7.84

16.45

The conventional Sn02/CdS/CdTe solar cell [14] was analyzed by AMPS-lO simulator initially by replacing CdS with ZnxCd1_xS and Sn02 with ITO as well as inserting different TCO, buffer layer and back contact. It was noticed that for low concentration of Zn (x :s 10%), the conversion

2011 IEEE First Conference on Clean Energy and Technology CET

efficiency (Eft), Voc, and FF are higher than for high content of Zn (> 10%) and Jsc was increased for x=O to x=O.l and almost same up to x=0.35 then started to decrease up to x=1. The electrical resistivity of ZnxCd1_xS layer increases from 1 O-cm (x=O) to 10

10 O-cm (x=l) [18]-[19]. In consideration to fabrication challenges, resistivity of ZnxCd1_xS film and simulation results in this work, x=0.05 was selected which exhibited low resistivity and high conversion efficiency in comparison to other values of x. In the following text, ZnxCd1_ xS window layer has been written to Zno.osCdo.9sS for our proposed cell structures.

The different cell structures and the cell output parameters (Voc, FF and Jsc) for each structure through AMPS 10 simulation with all the material parameters as in Table I, are shown in Table II.

The efficiency Voc, FF and Jsc of each structure are assimilated in Fig. 2. It is clear from Fig. 2 that the cell structures containing ZnO are showing deprived performance.

1.2

1.1

0.9

0 . 8

0.6

0.5

0 . 3

0.2

0.0

30

25

20

15

10

5

o

,....,

• Vo e (V)

• FF

• J s e ( I le i 2 )

Eff . ( %)

-� 1-

� l .1

Fig. 2. Performance of all cell structures.

To assist high efficiency Zno.osCdo.9sS/CdTe PV the Zno.osCdo.9sS window layer has been reduced to 80 nm but thin Zno.osCdo.9sS layer «100nm) may allow a small amount of forward leakage current to front contact through some pinholes of the thinner Zno.osCdo.9SS layer which is considerably less than the leakage current in CdS layer in comparison to CdS/CdTe solar cell [20]. In order to eliminate this leakage current a high resistive Buffer layer of suitable

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material must be inserted in between front contact TCO and Zno.osCdo.9sS window layer. It is possible to get benefit of the different properties of two TCOs by forming a buffer layer. High-efficiency CdTe and CIGS devices are usually fabricated with such buffer layer structures consisting of a highly conducting layer for low-resistance due to contact and lateral current collection and a much thinner high-resistivity buffer layer of an appropriate material to minimize leakage current through some possible pinholes in the Zno.osCdo.9sS window layer. By inserting a 100 nm thick resistive ITO, ZnO, or Zn2Sn04 layer, the Zno.osCdo.9sS layer thickness can be reduced <100 nm, which significantly improves the blue response of the Zno.osCdo.9sS/CdTe devices. The insertion of the smoother high-resistive buffer layer also improves the Zno.osCdo.9sS film morphology. Among many potential buffer layer materials ZnO and Zn2Sn04 has been implemented in this work. ZnO and Zn2S04 were included as a buffer layer with all the material parameters as in Table I. The Zn2Sn04 buffer layer insertion provides better result than ZnO insertion. It was also investigated that the spectral response has no effect on QE for Zn2Sn04 thickness variation from 50 nm to 500 nm . In consideration of fabrication limitation, the thickness of Zn2Sn04 buffer layer was selected as 100 nm .

The J-V curve for selected cell structures as in Table II, when utilizing all the material parameters with 80 nm ZnxCd1_ xS (x=O.05) and CdTe layer thickness of 1 !-lm is shown in Fig. 3. Structures AI, A2, A4 without BSF showed higher Jsc and Voc with good FF which in tum showed higher efficiency in comparison to other structures. The lowest Voc of structure A6 is due to combined effect of ZnO and Shz Te3 in comparison to structure A3 which showed highest Voc. The existence of roll over was found in some of the structures with Sb2Te3.

30

25 ---- 20 1"1

- A I -A2 E '" 15 - A4 - A7 --

-<I': E 10 '-'

..., 5

0

0.0 0.1 0.2 0.3 O.y O(� 0.6 0.7 0.8 0.9 1.0

Fig. 3. The J-V curves for selected structures.

The band diagram of some structures (A2, A4, A 7) is plotted in FigA which exhibited high efficiency and Jsc as indicated in TABLE II. The band discontinuity of 0.23 eV at Zno.osCdo.9sS/CdTe junction is found. There is a rectifYing back contact with Ag material in structure A2 which may be responsible for rollover of the cell. The electron rectifYing potential caused by Sb2 Te3 BSF in the structure A 7 can also be seen from Fig. 4.

2011 IEEE First Conference on Clean Energy and Technology CET

2

c- O ..... � � -1 � � -2

0.0 0.2 0.4

B

VB

0.6 0.8 Position (Jlln)

StJ'Ucture .U

Structure .. u

Stl'Ucture A

1.0 1.2

Fig. 4. Band diagram of some selected structures.

1.4

Temperature plays a very significant role for affecting the performance of the solar cells in practical operation. At higher operating temperature, parameters such as the electron and hole mobility, carrier concentrations and band gaps of the materials are affected, which was investigated in this analysis An exploration has also been performed utilizing AMPS with operating temperature ranged from 27°C to 100°C to understand the effects on the cell performance for the cell structures of CTO/ZTO/ZnoosCdo9sS/CdTe/Sbz TeiNi and CTO/ZTO/ZnoosCdo9sS/CdTe/Ag, as shown in Fig. 5.

1.2

t(1.0 = � ;g 0.8

... '"" "0 0.6 � .� � 0.4 ... � 0.2

0.0

25 35

_Structure A1 with ITO

-'-Structure A7 with CTO, ZTO and

Sb2Te3/Ni

45 55 65 75 85 Temperature eq

95

Fig. 5. Effect of temperature on structures Al and A7.

105

It is obvious from the Fig. 4 that the conversion efficiency linearly decreased with the increase of operating temperature at a temperature coefficient (TC) of -0.25o/oI°C which also indicated the degree of stability of the cell with BSF at higher operating temperature. This result is in good agreement with the related work [17], [21].

IV. CONCLUSIONS

Finally, we would conclude that the best structure is structure A4 (CTO/ZTO/Znooscdo9sS/CdTe/Ag) which showed highest efficiency and good cell output parameters and if we consider the stability of the cell at higher operating temperature then the structure A6 (CTO/ZTO/ZnoosCdo9SS

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ICdTe/Sbz TeiNi) is the best although it exhibited the lowest efficiency that have been achieved from this numerical study. The computed operating temperature gradient provides some valuable hints to fabricate higher efficiency and stable Zno.osCdo.9sS/CdTe solar cells with standard fabrication techniques which are comparable to any other reported cells .

ACKNOWLEDGMENT

This work is supported by the department of Electrical, Electronic & System Engineering and Solar Energy Research Institute (SERI), UKM, Malaysia through the research grant UKM-GUP- TK-08-16-057 and by NPST program by King Saud University, Saudi Arabia, Project Number: 10-ENE 1039-02.

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2011 IEEE First Conference on Clean Energy and Technology CET

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