3
Effect of the Geometrical Structure Of the Front Contact Probe on Series Resistance of CdTe/CdS Solar Cells
M. K. Al Turkestani
Department of Physics, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
E-mail: [email protected]
Abstract.The impact of front contact probe geometry in a CdTe/CdS solar cell on the photovoltaic performance is presented in this study.
A 5 × 5 mm2 cell was fabricated prior to revealing the front contact surrounding it by removing both CdTe and CdS layers. Gold was evaporated on the revealed front contact to create and test five different contact configurations. In addition, each configuration was investigated at four different distances from the edges of the cell itself so that the effect of distance between the probes could be studied. It was found that using metal configurations in the front contact reduced the series resistance of the device more than using a conventional point probe. The effect of such configurations on the series resistance measured under dark conditions is also presented in this paper.
Keywords: CdTe, solar cells, series resistance, transparent conductive oxide
Introduction
For CdTe-based solar cells, both the front contact and the metallurgical back contact play an important role in photovoltaic performance. The main goal in improving device performance is to reduce the resistance of these contacts, which in turn, reduces the
total resistance of the cell. The front contact normally consists of a stack of layers of conductive transparent oxides (TCO). Each layer plays a different role in the required optical and electrical properties of this contact. In addition, fabricating a low-resistance Ohmic back contact to the CdTe layer is advantageous, although difficult to achieve. This difficulty arises from the high value of the electron affinity of CdTe, higher than that of any existing metal. To overcome this issue, various methods have been used to create a tunneling junction between CdTe and the back contact. Such a junction may be formed by chemical etching of the back surface of the CdTe layer before applying the back contact [1], or using materials with certain chemical compositions for fabricating the back contact of the cell [2–4].
A typical procedure for testing the performance of CdTe solar cells is by connecting probes to the back contact and the front contact (TCO), revealed by removing the CdTe and CdS layers around the cell. In this paper, various gold (Au) configurations were designed to serve as a probe for the front contact. The effect of the configurations on the cell working parameters (open-circuit voltage Voc, short-circuit current Jsc, fill factor FF, and efficiency η) is demonstrated. The configuration with the minimum resistance was identified using measurements of the series resistance (Rs).
Such configurations may then be used for connecting solar cells in large-scale solar modules.
Experimental Details
The substrate used in this study was 5×5 cm2 Pilkington TEC10, with a 100 nm ZnO buffer layer deposited on top. The substrate was cleaned with a nylon brush and de-ionized water prior to rinsing thoroughly with de-ionized water, and dried with N2. An AJA International, Inc. ATC ORION sputtering system was used to deposit the CdS film. The sputtering target was of 5N purity. Deposition took place under a pressure of 5 mTorr Ar at 200°C with a power of 51 Watts over 16 minutes. Such conditions yielded a CdS film with a thickness of ~290 nm. The CdTe layer was grown on the CdS layer using a close space sublimation (CSS) kit built by EGC Ltd. Before CdTe film deposition, the source was preheated at 480°C for 20 minutes in a N2 static pressure of 200 mTorr. Due to this high pressure, CdTe deposition rate is very slow that it can be negligible in this experiment.
The CdTe film was then grown in two steps. First, the source temperature was increased to 605°C and the pressure reduced to 25 mTorr for 7 minutes. Second, the static pressure was broken by pumping the gas out while the heater was still on. In this stage, the actual pressure in the reactor reached ~5 mTorr after 10 seconds, the total duration of this step. The aim of the first step was to grow large CdTe grains [5], whereas the second step is designed to cover the voids among these grains. During growth, the substrate temperature changed from 440 to 490°C. The thickness of the final CdTe film was ~3.5 μm. CdCl2 treatment of the device was
performed by evaporating 200 nm of CdCl2 on the back surface of the device prior to annealing in air at 400°C for 20 minutes. The back surface of the cell was etched by nitric-phosphoric acid prior to the application of a 5×5 mm2 Au back contact. The Au contact thickness was ~100 nm. All thicknesses were determined using an Ambios XP 200 profiler.
Both CdTe and CdS layers were removed from around the Au back contact using a scalpel and hydrochloric acid prior to rinsing the whole sample with de-ionized water. The aim of this step was to reveal the front contact (TCO) of the sample. Au was evaporated on the revealed TCO to create various configurations to which the probe of the front contact could be connected and, therefore, serve as front contact probes. These configurations are shown in Figure 1, referred to as the “front contact configuration”
hereafter. In front contact configuration a, the Au film surrounds the back contact from all sides. In configurations b, c, and d, the front contact progressively loses one side. In configuration e, no Au film is deposited on the front contact at all, i.e. the front probe was placed on the TCO, creating a conventional point contact. In all cases, the vertical distance between the edges of the back contact and those of the front contact configurations was kept at a value of z, where the value of z was 3.1, 8.4, 13.5, and 18.4 mm.
Note that for configuration e, z designates the distance between the edges of the back contact and the probe placed on the TCO directly.
Solar cell working parameters were determined by recording the current-voltage (J-V) characteristics for each configuration under AM1.5 illumination. J-V measurements were
performed using LabVIEW software, a Keithley 2400 source meter, and an Oriel 81160 solar simulator. In this procedure, a probe was connected to the back contact of the cell while the other probe was connected to the front contact configuration. In addition, J-V curves were recorded under dark conditions for each run in order to calculate the “dark” Rs. The entire experiment was conducted at the Stephenson Institute for Renewable Energy at the University of Liverpool.
Fig. 1. A schematic diagram of the Au film structures deposited on the front contact to create the front contact probe configurations. In configuration a, the Au film surrounded the back contact from all sides. Configurations b, c and d have 3, 2, and 1 sides of Au around the back contact, respectively. Configuration e has no Au at all, i.e. the probe was directly placed on the front contact. The symbol z represents the vertical distance between the edges of the back contact and the front contact configurations.
Results and Discussion
Figures 2 and 3 show a few selected J-V curves under illumination for the cell, from which the cell performance for all front contact
configurations are obtained for each value of z. As seen in these figures, both Voc and Jsc remained nearly unchanged. However, the FF changed as a function of z. Table 1 summarizes the values of all the performance parameters.
0.0 0.3 0.6
-0.018 -0.009 0.000
J (mA/cm2)
3.1 mm 8.4 mm 13.5 mm 18.4 mm
V (V)
Fig. 2. J-V curves measured under illumination for front contact configuration e (point contact) at various values of z.
0.0 0.3 0.6
-0.02 -0.01
0.00 V (V)
J (mA/cm2)
a b c d e
Fig. 3. J-V curves measured under illumination for all front contact configurations at z = 18.4 mm.
TABLE 1. The working parameters extracted from all J-V runs measured in this study.
Au
configuration z (mm) η (%) FF (%) Jsc (mA/cm2) Voc (V)
a
3.1 9.7 58.2 23.14 0.72 8.4 9.32 57.49 22.52 0.72 13.5 9.06 56.68 22.19 0.72 18.4 8.74 55.09 22.03 0.72
b
3.1 9.53 57.79 22.61 0.73
8.4 9.28 57.53 22.4 0.72
13.5 9.01 56.71 22.07 0.72 18.4 8.79 55.42 22.03 0.72
c
3.1 9.42 57.13 22.61 0.73 8.4 9.29 57.44 22.45 0.72 13.5 8.99 56.41 22.12 0.72 18.4 8.77 55.18 22.06 0.72
d
3.1 9.46 57.48 22.53 0.73 8.4 9.25 57.36 22.39 0.72 13.5 8.98 56.5 22.08 0.72 18.4 8.78 55.18 22.1 0.72
e
3.1 8.78 53.6 22.44 0.73 8.4 8.5 53.36 22.13 0.72
13.5 8.34 52.67 22 0.72
18.4 8.1 51.1 22.02 0.72
For all runs, the average value for Jsc was ~22.3 ± 0.3 mA/cm2 while Voc varied between 0.72 and 0.73 V. However, the variation in FF, and therefore in η, was more pronounced than that in Jsc and Voc. The reason for this variation in FF can be attributed to the change in Rs of the cell, which can be determined from the slope of the J-V curve in the forward bias region [6]. Rs was evaluated for each J-V curve recorded under illumination in this study (hereafter labeled “light Rs”). These results are shown in Table 2 and plotted in Figure 4. The deterioration in FF for all contact configurations is due to the increase in Rs is shown in Figure 5.
TABLE 2. Values for the light Rs obtained for all front contact configurations with various values of z. The unit of Rs is Ω cm2.
Au configuration
a b c d e
z (mm)
3.1 8.52 8.74 8.88 8.63 10.7 8.4 9.03 9.02 8.96 8.9 11.15 13.5 9.35 9.19 9.1 9.03 11.45 18.4 9.89 9.83 9.63 9.89 11.92
8 9 10 11 12
2 5
811141720 b a
d c
Front cone tact configuration Rs (Ω cm
2 )
z (m m)
Fig. 4. All values for light Rs for each front contact configuration as a function of z.
8 9 10 11 12
52 56 60
Rs light (Ω.cm2)
FF (%)
Fig. 5. The deterioration in FF due to the increase in Rs under illumination (light).
The most striking result is that Rs increases, on average, for configuration e (the point contact) in comparison to other configurations by ~2 Ω cm2. In contrast, the variation in Rs among configurations b, c, d, and e at each z value was insignificant. This observation implies that connecting devices with “point contacts”
in solar panels may reduce the FF due to an increase in Rs, which deteriorates the total conversion efficiency η. However, applying a metal strip next to one side of each cell may overcome this problem. From these results, surrounding the entire cell with a metal contact is not required to minimize the resistance caused by the front contact.
For each front contact configuration, Rs increased slightly with increasing z value. This behavior is expected due to the increase in the distance within the TCO in which current passes [7]. However, the increase in the absolute value of Rs as a function of z varied from one configuration to another, as shown in Table 2.
This behavior may be attributed to the uniformity and cleanliness of the TCO itself. This point will be discussed later.
The discussion above suggests that a metal strip should be deposited on the TCO as close as possible to the back contact.
Such a configuration may minimize the contribution of the front contact to the total value of Rs of the cell, and hence improve photovoltaic performance.
Rs was also measured for J-V curves under dark conditions (not shown here) in order to compare it with measurements under illumination. Table 3 and Figure 6 show these values.
TABLE 3. Rs measured under dark conditions for all front contact configurations with various values of z. The unit of Rs is Ω cm2.
Au configuration
a b c d e
z (mm)
3.1 47.96 45.66 43.2 43.98 43.1 8.4 39.65 41.7 43.44 42.64 43.42 13.5 39.82 42.55 42.03 38.3 43.74 18.4 40.67 47.66 47.57 46.73 47.57
36 38 40 42 44 46 48 50
25811141720 b a
d c
Front cone tact configuration
z (m m) Rs (Ω cm
2 )
Fig. 6. All values for Rs under dark conditions for each front contact configuration as a function of z.
There was no clear trend in Rs under dark conditions. Such a result may be explained by referring to the study of Proskuryakov et al. [8], which found that the contribution of the ZnO layer (in the front contact) to the total Rs vanishes under illumination. Thus, the behavior of Rs under dark conditions may be dominated by the ZnO layer in the TCO, which may not be uniform.
To investigate this hypothesis, two samples of the same substrate used in this study were cut into slides 3 cm wide (Sample A and Sample B). After cleaning, Au strip contacts were thermally deposited on the substrates to create a transfer length method (TLM) pattern; a technique used to measure thin film sheet resistance [9]. The distance between each two successive contacts was varied between 1 and 3.5 mm in steps of 0.5 mm. The resistance was measured across each of these distances and is supposed to vary linearly with distance [9]. However, as shown in Figure 7, the data did not follow this expectation. The resistance was obviously higher under dark conditions compared to that under illumination, which agrees with the work published Proskuryakov et al [8]. However, the change in the resistance under both dark and illuminated conditions was not linear with distance. Moreover, the general behavior of the resistance was different between samples A and B; even though the samples were identical. This implies that the TCO including the ZnO layer might be inhomogeneous in all substrates used in this study. This may explain the scattering of the Rs values shown in Figures 4 and 6, which was more severe for Rs
values measured under dark conditions (Figure 6) due to the ZnO layer effect as explained above. Therefore, if the front contact has a ZnO buffer layer, any improvement from the front contact probe configurations investigated in this study could be suppressed with insufficient levels of illumination.
90 120 150 180
1.0 1.5 2.0 2.5 3.0 3.5
90 120 150 180
A
B
Dark Light
Distance between contacts (mm)
Resistance (Ω)
Fig. 7. Resistance measured under dark and light conditions for the front contact (ZnO/TEC10 glass) as measured by the TLM. A and B denote two identical samples for which measurements were performed.
Conclusion
In this work, various front contact probe configurations for a CdTe/CdS solar cell were investigated. The goal of this study was to identify the best configuration in terms of the reduction in total series resistance of the device. It has been found that applying a single strip of Au on the TCO next to the edge of the back contact significantly reduced Rs compared to placing the probe directly on the TCO. However, applying more than one Au strip around the back contact did not improve Rs more than a single strip. The reduction of the spacing between the Au strip and the edge of the back contact seems to reduce Rs slightly. Therefore, a metal strip should be deposited on the TCO as close as possible to the edge of the back contact in order to obtain the minimum value of Rs. In contrast, Rs measured under dark conditions seemed to be dominated by the contribution from the ZnO layer in the TCO more than that of the front probe configurations tested in this work.
Acknowledgements The author is grateful to Prof. Ken Durose and his research group at the Stephenson Institute for Renewable Energy, University of Liverpool, where all experimental work was undertaken. This work was financially supported by King Abdulaziz City for Science and Technology (KACST).
References
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[2] N. Romeo, A. Bosio and A. Romeo, An innovative process suitable to produce high-efficiency CdTe/CdS thin-film modules, Sol. Energy Mater. and Sol. Cells, 94, 2 (2010).
[3] S. H. Demtsu, D. S. Albin, J. W. Pankow and A. Davies, Stability study of CdS/CdTe solar cells made with Ag and Ni back-contacts, Sol. Energy Mater. and Sol. Cells 90, 2934 (2006).
[4] V. Barrioz, Y. Y. Proskuryakov, E. W. Jones, J. D. Major, S. J. C. Irvine, K.
Durose and D. A. Lamb, Highly arsenic doped CdTe layers for the back contacts of CdTe solar cells, in: Materials Research Society, Spring meeting, Symposium Y.San Francisco, CA, USA, (Cambridge University Press, San Francisco, CA, USA, 2007), pp. 367–372(2007).
[5] J. D. Major, Y. Y. Proskuryakov and K. Durose, Nucleation and Grain Boundaries in CdTe/CdS Solar Cells, in: Material Research Society, Spring meeting, Symposium M.San Francisco, USA, (Cambridge University Press, San Francisco, USA, 2009), pp. 223–233 (2009).
[6] J. Nelson, The Physics of Solar Cells (Imperial College Press, London) , p. 34 (2003).
[7] G. T. Koishiyev and J. R. Sites, Impact of sheet resistance on 2-D modeling of thin-film solar cells, Sol. Energy Mater. and Sol. Cells 93, 350 (2009).
[8] Y. Y. Proskuryakov, K. Durose, M. K. Al Turkestani, I. Mora-Sero, G. Garcia- Belmonte, F. Fabregat-Santiago, J. Bisquert, V. Barrioz, D. Lamb, S. J. C.
Irvine and E. W. Jones, Impedance spectroscopy of thin-film CdTe/CdS solar cells under varied illumination, J. Appl. Phys. 106, 44507 (2009).
[9] D. Schroder, Semiconductor Material and Device Characterization (John Wiley &
Sons, Hoboken, , p. 146 (2006).
ﻲﻓ ﺔﻳﻣﺎﻣﻻا ﻝﻳﺻوﺗﻟا طﺎﻘﻧﻟ ﻲﺳدﻧﻬﻟا ﻝﻛﺷﻟا رﻳﺛﺄﺗ
ﺔﻳﻠﺧ
CdTe/CdS
ﺔﻳﺳﻣﺷﻟا ﺔﻳﻠﺧﻠﻟ ﺔﻳﺑرﻬﻛﻟا ﺔﻣوﺎﻘﻣﻟا ﻰﻠﻋ ﺔﻳﺳﻣﺷﻟا
ﻲﻧﺎﺗﺳﻛرﺗﻟا ﻝﻳﻠﺧ دﻣﺣﻣ
ءﺎﻳزﻳﻔﻟا مﺳﻗ -
ﺔﻳﻘﻳﺑطﺗﻟا موﻠﻌﻟا ﻪﻳﻠﻛ -
ىرﻘﻟا ما ﺔﻌﻣﺎﺟ
ﺔﻛﻣ - ﺔﻳدوﻌﺳﻟا ﺔﻳﺑرﻌﻟا ﺔﻛﻠﻣﻣﻟا .
صﻠﺧﺗﺳﻣ :
ﻟا ﻝﻛﺷﻟا رﻳﺛﺄﺗ ﺔﺳارد مﺗ ثﺣﺑﻟا اذﻫ ﻲﻓ طﺎﻘﻧﻟ ﻲﺳدﻧﻬ
ﻝﻳﺻوﺗﻟا -
ﺔﻳﻠﺧ ﻲﻓ دﻟوﺗﻣﻟا ﻲﺋﺎﺑرﻬﻛﻟا رﺎﻳﺗﻟا ﻝﻳﺻوﺗﻟ مدﺧﺗﺳﺗ ﻲﺗﻟا
CdTe/CdS ﺔﻳﺟرﺎﺧﻟا ةرﺋادﻠﻟ ﺔﻳﺳﻣﺷﻟا
- ﺔﻳﻠﺧﻟا ءادأ ﻰﻠﻋ
ﺔﻳﺳﻣﺷﻟا . مﺟﺣﺑ ﺔﻳﻠﺧ ﻊﻳﻧﺻﺗ مﺗ
×5 مﻠﻣ 5 ﻝﻛ ﺔﻟازا مﺛ نﻣو 2
ﺔﻳﺳدﻧﻬﻟا ﻝﺎﻛﺷﻻا ﻊﻳﻧﺻﺗﻟ كﻟذو ﺔﻳﻠﺧﻟا ﻩذﻫ ﻝوﺣ ﺔﺑﺳرﻣﻟا ﺔﻳﺷﻏﻻا ﻣﻟا ﻝﻳﺻوﺗﻟا طﺎﻘﻧﻟ ﺎﻬﺗﺳارد دار
. دادﻋا مﺗ ﺔﻔﻠﺗﺧﻣ ﺔﻳﺳدﻧﻫ ﻝﺎﻛﺷا 5
دﻧﻋ ﻝﻛﺷ ﻝﻛ رﺎﺑﺗﺧا مﺗ ﺎﻣﻛ ﺔﻳﻠﺧﻟا ﺔﻓﺎﺣ نﻣ ﺔﻔﻠﺗﺧﻣ تﺎﻓﺎﺳﻣ 4
ﺔﻳﺳﻣﺷﻟا . نﻣ دﺣاو طﻳرﺷ ﺔﻓﺎﺿا نا ﻰﻟا ﺔﺳاردﻟا ﻩذﻫ تﺻﻠﺧ
ﻣﻟا ﻝﺻوﻣﻟا مدﺧﺗﺳ
) بﻫذﻟا ( ﺔﻳﻠﺧﻟا نﻣ بﻳرﻗو يزاوﻣ ﻝﻛﺷﺑ
نا نﻳﺣ ﻲﻓ ،ظوﺣﻠﻣ ﻝﻛﺷﺑ ءﺎﺑرﻬﻛﻠﻟ ﺔﻳﻠﺧﻟا ﺔﻣوﺎﻘﻣ ضﻔﺧ ﺔﻳﺳﻣﺷﻟا ﻝﻛﺷﺑ ﺔﻣوﺎﻘﻣﻟا ضﺎﻔﺧﻧا ﻰﻟا يدؤﻳ مﻟ دﺣاو طﻳرﺷ نﻣ رﺛﻛا ﺔﻓﺎﺿا رﺑﻛا . اذﻫ ﻝﻳﺻوﺗ قرط رﺎﻳﺗﺧا ﻲﻓ رﻳﺑﻛ ﻝﻛﺷﺑ مﻬﺳﺗ دﻗ ﺔﺟﻳﺗﻧﻟا ﻩذﻫ
ا حاوﻟﻷا نﻳوﻛﺗﻟ ﺔﻳﺳﻣﺷﻟا ﺎﻳﻼﺧﻟا نﻣ عوﻧﻟا مﺟﺣﻟا ةرﻳﺑﻛ ﺔﻳﺳﻣﺷﻟ
.
ﺔﺳاردﻟ كﻟذو مﻼظﻟا ﻲﻓ كﻟذﻛو ةءﺎﺿﻹا تﺣﺗ ﺔﺳاردﻟا ﻩذﻫ تﻣﺗ ﺔﻳﺋﺎﺑرﻬﻛﻟا ﺔﻣوﺎﻘﻣﻟا ضﺎﻔﺧﻧا رادﻘﻣ ﻰﻠﻋ ةءﺎﺿﻻا ىوﺗﺳﻣ رﻳﺛﺄﺗ ﺔﻳﺳﻣﺷﻟا ﺔﻳﻠﺧﻠﻟ .
