Thin Solid Films 520 (2011) 680–683

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A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Moback contact

J.L. Peña a,b,⁎, O. Arés a, V. Rejón a, A. Rios-Flores a, Juan M. Camacho a, N. Romeo b, A. Bosio b

a Applied Physics Department, CINVESTAV-IPN Mérida, C.P. 97310 Mérida, Yucatán, Mexicob Dipartimento di Fisica, Università di Parma, Campus Universitario, Parco Area delle Scienza, 43100 Parma, Italy

⁎ Corresponding author at: Applied Physics Departme97310 Mérida, Yucatán, Mexico.

E-mail address: jlpena@mda.cinvestav.mx (J.L. Peña

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.04.193

a b s t r a c t

a r t i c l e i n f o

Available online 4 May 2011

Keywords:Solar cellCdS/CdTeBack contactSeries resistance

CdS/CdTe thin film solar cells with an area of 1 cm2were obtained and studied in detail. A ZnO buffer layer wasdeposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also byusing RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of thesolar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched byusing Br–Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Motargets. The procedure developed in this work led us to make systematically solar cells with good efficiency.However, the series resistance has a high value for an area of 1 cm2 (22 Ωcm2). In order to make moredetailed study, the solar cell with an area of 1 cm2 was divided in a 3×3 matrix. A good homogeneity in cellproperties is observed and the efficiency increases tomore than 11%, fundamentally through decreasing seriesresistance.

nt, CINVESTAV-IPNMérida, C.P.

).

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Series resistance (Rs) is a very important parameter in solar cells,since a high value of it limits the efficiency of the cell through the fillfactor (FF) [1,2]. In the case of CdTe cells, it is a very important aspect,as the back contact can have a strong contribution at Rs due to the highwork function of the CdTe semiconductor [3,4]. Wu [5] has pointedout that the Rs in CdTe cells is closely associatedwith the quality of theCdS/CdTe junction and its record cell, with efficiency of 16.5% has aRs=1 Ωcm2. In this work, wemake a detailed study of Rs in CdTe cellscontacted with Cu/Mo [6,7]. This contact is poorly studied in the caseof cells grown on glass substrate [8] in comparison to others such asCu/Au [9] and with buffer layers [10]. In the case of cells grown onMofoils a thin layer of Au is used between Cu and Mo to improve thiscontact [11]. Back contacts of Cu/Mo have been reported without rollover and metallization with this element, yield long-term stable CdTesolar cells [12]. An ohmic Cu free back contact with MoOx as thecontact buffer has been found recently with a minimum Rs of4.7 Ωcm2 [13].

2. Experimental details

The CdS/CdTe cell developed in this work is prepared on glasssubstrate coated with indium tin oxide (ITO) as a transparentconducting oxide (TCO). We used a buffer layer of ZnO (200 nm)

film deposited by RF magnetron sputtering starting from a metal Zntarget with 4 N purity. The used ZnO film has a resistivity of about103Ω cm measured by four probe method and helps to reduce shuntscaused by holes from CdS thin film and enhance the open-circuitvoltage (Voc) and the FF [5]. Zinc oxide has been identified by Romeoet al. [14] and Matin et al. [15] as a potential candidate for buffer layerinto CdTe cells, some researches like Mazzamuto et al. [16] andPerrenoud et al. [17] have used successfully ZnO as a buffer layer. Ingeneral the use of this material semi-insulating has allowed us toimprove the reproducibility and performance of our solar cells. Ourinvestigation on the benefits of ZnO as buffer layer is underway andwill soon be published. CdS films (160 nm) were grown by RFsputtering also with a target of CdS with 4 N purity. CdTe films (8 to10 μm thick) are grown by CSS using two rectangular graphite blocks(9×9×0.5 cm), heated by special halogen lamps (they do notproduce contamination). The separation between graphite blocks is3–5 mm and the atmosphere during the growth was of 0.1 mbar of O2

and 0.4 mbar of Ar. The source and substrate temperature were 590 °Cand 520 °C, respectively. The rate of deposition of CdTe film is about1 μm/min. The chlorination is performed using Freon gas that containschlorine in its composition, those one decomposes at 400 °C, it allowschlorine releasing. This method was developed by Romeo [16,18]. Thetreatment is done at 400 °C in a quartz ampoule in amixture of Freon–argon with to a total pressure in the range of 400–800 mbar. Freonpressure is varied between 20 and 50 mbar. After 5 min treatment theFreon–argonmixture is removed and high vacuum is restored in orderto remove surface residues of Cl, which can influence the next step ofthe back contact. The ohmic contact on CdTe films was performed byetching the CdTe surface with Br2:CH3OH solution followed by the

Table 1Parameters of cells with different areas and for a cell with a small area (0.034 cm2)optimized by light soaking. Rsh is the shunt (parallel) resistance.

L(mm)

Jsc(mA/cm2)

Voc

(V)FF(%)

Efficiency(%)

Rs

(Ω cm2)Rsh

(Ω cm2)Area(cm2)

10 14.4 0.751 48 5.2 22.6 472.6 18 13.8 0.742 57 5.9 16.2 702.9 0.645 11.9 0.769 69 6.3 8.0 631 0.252.8 13.5 0.751 72 7.3 5.1 1023 0.0781.85 22.4 0.73 69.7 11.4 3.7 554 0.034

681J.L. Peña et al. / Thin Solid Films 520 (2011) 680–683

deposition of two films, Cu (5 nm) and Mo (0.75 μm), grown by RFsputtering in Ar atmosphere. After, the device is annealed at 200 °C for20 min in an Ar atmosphere. Once completed, the cells were delimitedby mechanical scribing by using a stainless steel tip in a square form,with area=L2 (L is the side length). In our fabrication method a lightsoaking of the cells is included. This is performed for 20 min with thecell put under a halogen lamp. The light intensity on the cell is about900 mW/cm2 and its temperature rises to 100 °C. With this lightsoaking we can optimize the cell performance, achieving increases inboth Voc and short-circuit current density (Jsc). We will discuss aboutthis in more detail below. The thickness of different deposited filmsfor cell fabrication was measured using a Vecco Dektak-8 StylusProfiler. Finally, the current density–voltage (J–V) measurementswere determined using a SourceMeter from Keithley 2420. SpecificLabVIEW software was used to collect the J–V data. The Rs and shuntresistance (Rsh) were determinate by the slope of J–V characteristicsat I≥40 mA in forward and −0.2 V in reverse, respectively [19].

3. Results and discussion

Fig. 1 shows the current density–voltage J–V characteristics of cellson the same substrate with areas from 1 cm2 to 0.08 cm2. Table 1shows the parameters of these four cells and the ones for a cell with asmall area (0.034 cm2) optimized by light soaking. Note that in all J–Vcharacteristics, there is no presence of rollover [12]. From Fig. 1 we seethat these cells have close values of Jsc and Voc, but different values ofRs. We correlated fluctuations in Jsc and Voc to local variations in theproperties of the cell. Fig. 2 shows the J–V characteristics of an array of3×3, cut on the same substrate, from a 1 cm2 cell, this figure shows afairly good homogeneity, with little fluctuations in Jsc and Voc asobserved in Fig. 1. This result supports the idea that fluctuations in Jscand Voc of Fig. 1 are determined by spatial inhomogeneities in the cell.Fig. 3 shows simulated J–V characteristics with a diode simple model,where Rs was varied, considering Rsh sufficiently high to not influencethe FF (our Rsh experimental values are sufficiently high to have animportant influence in this parameter). This classical figure shows theeffect of Rs in solar cells [2] and makes clear the fact that Jsc is notaffected by Rs up to 25 Ωcm2, but not the FF. As it is seen from Fig. 1,the experimental curve of the cell with 1 cm2 is qualitatively similar tothe theoretical curve with 10 Ωcm2. From the above considerations,we can see that these cells are limited primarily by the value of Rs. Inorder to know the quality of the diode in our cells, we estimated

0.0 0.2 0.4 0.6 0.8 1.0

-15

-10

-5

0

5

10

J (m

A/c

m2 )

V (V)

1 cm2

0.64 cm2

0.25 cm2

0.08 cm2

Fig. 1. J–V characteristics of cells with different areas.

reverse saturation diode current (J0), and diode ideality factor (n) forthe smallest cell (to reduce the Rs effect in forward dark J–Vcharacteristic). This estimation was performed through ln (J) versusV curves in darkness [1,20]. A J0≈10−11A/cm2 and n=1.7 areobtained in forward [19], and J0≈10−10A/cm2 in reverse. The resultsconfirm a good quality of the cell diode. In addition, these cells do notshow the J–V curve crossing between dark and light, another featurefor well behaved cells [16]. There are reports in literature of cells withVoc and Jsc with good values but low FF, determined by a high value ofRs, due to a non-optimized back contact [2].

Fig. 4 shows the experimental dependence of Rs with the Ldimension of the cells. This is clearly a parabolic dependence of thetype a+b L2, according to models developed for thin film solar cells,which take into account the distributed resistance of the TCO [21]. Wearrive at this type of relationship, in which the first term correspondsto the contribution of the absorber and the second to the contacts.According to this model, the result in the figure would indicate acontribution of CdTe on the order of 104Ω cm which corresponds wellwith what we measured by four-point lighting conditions and withthose reported in literature [8,22]. Fig. 4 shows that FF versus Rs depen-dence is linear, which corresponds to the Formula FFs=FF0 (1−Rs/Rch)where Rch=Voc/Jsc [23]. FF0 is the fill factor that provides the quality ofthe cell junction and it is interesting to note that extrapolation of the linein Fig. 4, gives 78%, which corresponds to the value reported for cells ofthe state of the art [5]. This result proves that the optimized Rs value inour cells, leads to an achievable efficiency over 11% in 1 cm2 area.

In the case of the CdS/CdTe cells there are reports of the effect oflight soaking in aging [24], but in our knowledge, not on the beneficialeffect of soaking for short times of minutes. In the case of amorphousSi cells in Ref. [25] reported that Voc increases with soaking, but it isunclear whether the mechanism is the reduction of recombination inthe junction or in the bulk of the absorber. Anyway, our results show

-0.2 0.0 0.2 0.4 0.6 0.8-20

-15

-10

-5

0

5

J(m

A/c

m2 )

V(V)

Fig. 2. J–V characteristics of an array of 3×3, cut on the same substrate, from a cell withan area of 1 cm2.

0.0 0.2 0.4 0.6 0.8

-25

-20

-15

-10

-5

0

5

10

10 Ohm cm2

1 Ohm cm2

100 Ohm cm2

J(m

A/c

m2 )

V(V)

25 Ohm cm2

Fig. 3. Simulated J–V characteristics with a diode simple model, where Rs value wasvaried.

0.0 0.2 0.4 0.6 0.8 1.0-25

-20

-15

-10

-5

0

5

10

J(m

A/c

m2 )

V(V)

Fig. 5. J–V characteristics of an optimized small cell by using light soaking.

682 J.L. Peña et al. / Thin Solid Films 520 (2011) 680–683

simultaneous increases of Jsc and Voc, which has to be related to adecrease in the rate of recombination in the cell. This presumablycould be determined by an ion-assisted redistribution in internalelectric fields produced by lighting and a temperature of about 100 °C.Fig. 5 shows the J–V characteristic of a cell of small area where thelight soaking treatment has been optimized. As seen in Table 1, thiscell has very good properties (more than 11% efficiency) but still theRs value is higher than that reported by Wu [5]. In larger areas, Rs

limits the efficiency due to the fact that it tends to a value close to25 Ωcm2. In Ref. [12] it is demonstrated that the carrier concentration,in the case of Cu/Mo contacts, is lower than that of the Sb2Te3/Mo,however the barrier height is low enough to create a quasi-ohmiccontact. However, this low carrier density in the back contact zone,can be explained through relatively high Rs observed in our work withCu/Mo contacts. XRD studies performed to our cells after the Br2:CH3OH treatment, followed by Cu deposition by RF sputtering, showthat the Cu1.4Te alloy is formed in the surface of CdTe film, when theCu film is very thin (≈5 nm). This is the best alloy for back contactcontaining Cu [26]. It is important to comment that the Mo/Cu hasprovided to our cells more stability and robustness Cu/Au contacts

2 4 6 8 10

5

10

15

20

25

0 5 10 15 200

10

20

30

40

50

60

70

80

90

100

Rs = 3.62 + 0.19 L2

L (mm)

FF

(%

)

Rs (

Ohm

cm

2 )

Rs (Ohm cm2)25

a b

Fig. 4. a) Series resistance dependence with the square cell side length dimensions andb) fill factor versus series resistance values measured in the cells.

used in Ref. [8] allowed to obtain values of Rs of about 3 Ωcm2 in1 cm2 area, but with bad reproducibility and fast degradation of cellproperties.

4. Conclusion

Square solar cells (of side L) obtained in this work show aquadratic behavior in L for the series resistance. This behavior wasreported previously for distributed resistance models, which considerthe contribution to Rs of transparent front contact. Extrapolating thelinear dependence of FF as a function of Rs we got a FF of 78% as for thecells of the state of the art. This corresponds to the good properties ofour diode cells with a J0 of the order of 10−10A/cm2 in dark. The highvalue of series resistance of our cells with Cu/Mo back contact(23 Ωcm2 for cell with 1 cm2 and 4 Ωcm2 for cell with 0.03 cm2)could be related to a low carrier density characteristic of this contact.However this carrier concentration is sufficient to have a barrierheight low enough to create a quasi-ohmic contact without thepresence of rollover in J–V characteristics. Through a procedure oflight soaking for several minutes and low temperatures (100 °C) wereach 11.4% efficiency, which may be related with ionic redistributionin the junction region that improves the quality of the cell.

Acknowledgment

This work has been supported by CONACYT-México under contractFORDECYT-116157. J. L. Peña acknowledges CONACYT-México byfinancial sabbatical support received under contract of EstanciasSabaticas No-128592. A. Rios-Flores acknowledges CONACYT-Méxicoby scholarship to study in the Applied Physics Department ofCINVESTAV-IPN Unidad Mérida. The authors acknowledge OswaldoGómez, Willian Cahuich and Roberto Sanchez by technical supportand Lourdes Pinelo by secretarial assistance.

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