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Optimization of indium tin oxide by pulsed DC power on single junction amorphous silicon solar cells
Thin Solid Films 519 (2011) 6053–6058
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Optimization of indium tin oxide by pulsed DC power on single junction amorphous silicon solar cells Rebecca K. Carlson, Yunsic Shim, William B. Ingler Jr. ⁎ Department of Physics & Astronomy, University of Toledo, Toledo, Ohio 43606, USA
a r t i c l e
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Article history: Received 19 February 2010 Received in revised form 11 March 2011 Accepted 15 March 2011 Available online 31 March 2011 Keywords: Amorphous materials Deposition process Indium tin oxide Optical properties Scanning electron microscopy Solar cells
a b s t r a c t We investigated the optimal deposition conditions of a thin indium tin oxide (ITO) film on an amorphous silicon (a-Si) single-junction solar cell using pulsed DC magnetron sputtering. Thin ITO films were deposited while power, deposition time, pressure, gas flow and temperature were varied to find such conditions. The efficiency of a-Si solar cells with ITO films was 6.65% at the optimal conditions — a pulsed DC power of 40 W, a deposition time of 460 s, a pressure of 0.53 Pa, gas flow of 16 sccm and 151 °C. On the other hand, an a-SiGe tandem solar cell with the ITO films made at the optimal conditions yields an efficiency of 7.20%. We have also examined the surface morphology of ITO coated a-Si solar cells, using atomic force microscopy. Interestingly, a change in power does not alter the surface morphology at small length scales, whereas at large scales, the lower power sample had a lower surface roughness than the samples made with higher powers. We also find that for the range of deposition conditions examined, the value of the roughness exponent does not change with α ≃ 2/3 and a thin layer of ITO does not modify the surface morphology significantly. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCO) have many applications in optoelectronic devices, such as flat panel displays and organic lightemitting diodes [1–3], and photovoltaic cells (see Ref. [4] for a recent review of high mobility TCO for thin-film solar cells). Indium tin oxide (ITO) is one of the most widely used transparent conducting oxides in such applications because it is a wide band-gap semiconductor (Egap = 3.75 to 4.0 eV), effective for transmitting visible and ultraviolet (UV) light, and has low resistivity in the low 10− 4 Ω cm range [5]. For photovoltaic cells, these characteristics are essential to produce an efficient solar cell. When light photons of a particular wavelength hit the cell, photons are absorbed in the intrinsic layer, producing a photoexcited electronhole pair. The electron (or hole) in the valence band is excited to the conduction band, these electrons or holes are moved towards the ntype layer or p-type layer respectively due to the built in potential of the depletion region between the layers, thus generating electricity when connected to a circuit. In particular, a hydrogenated amorphous silicon solar cell is a type of solar cell that has increased efficiency because Si―H bonds improve the electrical characteristics of the cell [6] (see Ref. [7] for a recent review of major thin-film solar cell technologies).
⁎ Corresponding author. E-mail address: [email protected] (W.B. Ingler). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.03.035
We note that the method in which the photodiode is constructed also affects the efficiency of solar cells. In a p-i-n photodiode, illumination through the p-layer is favorable because the holes have a shorter distance to travel to be collected, limiting recombination. If the drift carriers must travel a larger distance to be collected, charge can build up and the electric field will collapse, which can reduce the efficiency of the cell through recombination [6]. Thus, it is necessary to have the most transparent and conductive film possible on the p-layer. Otherwise, the efficiency will drop before the photons travel into the intrinsic layer (i-layer). ITO's wide band gap allows for maximized range of absorption of a-Si solar cells in the UV spectrum when doped. Conductivity of the ITO film increases as the indium doping increases because charges move effectively across the solar cell surface [8–10]. Even if the transparency and conductivity of the ITO are excellent, the interface between the ITO layer and the p-layer also plays a crucial role in determining the efficiency of solar cells. In particular, the interface is sensitive to the deposition conditions. A higher deposition temperature of ITO creates films with lower resistance, but it can damage the interface between the p-layer and the ITO if the temperature goes beyond the conditions at which the p-layer was deposited. In general, the interface losses should be less than 0.3 V. As a result, this naturally puts a limit on the deposition temperature of ITO on the top of p-layer of a-Si solar cells. In addition, the ITO film must have a high optical transmittance greater than 90%, a low sheet resistance below 100 Ω/sq and sheet resistivity below 6.5 × 10− 4 Ω cm [7,10,11]. The requirement of the low sheet resistance leads to an acceptable ITO thickness of about 100 nm [12].
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In this paper we report the optimal deposition conditions of ITO on an a-Si single-junction solar cell using pulsed DC magnetron sputtering. Although radio frequency (rf) deposition processes can lead to better and denser films due to an enhanced substrate bombardment by plasma ions (mainly Ar+) of moderate energy, one major problem with rf deposition process is the lower sputter rates [13]. Thus, the method of pulsed DC magnetron sputtering may be more desirable for the deposition of ITO thin films because they produce better quality films faster than rf magnetron sputtering [14]. Also, the electron pulse is beneficial for limiting arcing by reducing charges that gather through short bursts of positive voltage following a burst of negative voltage. This along with depositions done at a lower power results in less damage to the solar cell underneath compared to rf power and still produces a similar film in a shorter amount of time. The organization of this paper is as follows. In Section 2 we describe the details of our experiment. In Section 3, we present our results for the performance and optimization of deposition power, temperature, and gas pressure. These include I–V, transmittance, and quantum efficiency measurements. We also present results for the effects of deposition power and temperature on the surface roughness obtained from atomic force microscopy measurements. Finally, in Section 4 we present a brief conclusion of our results.
2. Experimental details The ITO target used in this study was a 90/10 indium oxide–tin oxide mixture by weight and was 99.99% pure produced by Kurt J. Lesker. The deposition chamber had a circular, three inch diameter cathode gun, and the ITO was deposited onto a 4 in. × 4 in. a-Si singlejunction or SiGe tandem solar cell deposited on 430 stainless steel substrate. This a-Si single-junction solar cell substrate was placed in a rotating substrate holder. When depositing the ITO film, a mask from Microphoto Inc. was used to produce small dots of 0.25 cm2 area. The chamber contained two halogen heating lamps, controlled by an external transformer that was used for heating the substrate. During temperature calibration, the thermocouple was attached to the sample holder, but during deposition, and after calibration it was screwed onto the mantle underneath the lamp housing. This method gives a relative temperature to the applied voltage, but subsequent recalibrations show little to no drift when the chamber is recalibrated at a later time. The chamber was evacuated using a Pfeiffer vacuum roughing pump and turbo pump. A Pirani gage, barometer, and ion gage were used to measure chamber pressure. Argon gas was flowed into the chamber to form plasma in the vacuum chamber with a pressure of as low as 3.2 × 10− 5 Pa before adding argon. Compressed argon and argon/20% oxygen gases were injected into the deposition chamber using MKS mass flow controllers with a range of 0 to 10 sccm for each gas. The first part of the study was done at standard conditions determined for rf power in order to create baseline conditions used as the standard lab recipe for ITO coatings. Rf generator power (ENI, Model ACG-3B) with a frequency of 13.56 MHz for the In2O3 target was varied from 35 to 90 W, and substrate temperature was varied from 141 to 191 °C. Chamber pressure (Pc) was held at a constant 0.53 Pa, and the deposition time was varied to 7 to 12 min with 15 s intervals. For rf power, the sample was heated for 30 min at the lowest relative pressure, corresponding to a completely open turbo pump, and argon gas was run at 30 sccm to fill and heat the chamber with enough gas prior to the deposition. Then gas flow (fg) was decreased to 16 sccm at a pressure of 0.53 Pa for 2 min. The pre-sputtering period was done for 2 min at 60 W. The standard deposition time was 9 min and 15 s at a temperature of 151 °C. In the second part of the study the standard conditions developed using rf power were used as a baseline for pulsed DC power which was
generated with an ENI power supply (Model RPG-5G) with a pulse frequency of 201 kHz. For characterization of the as-deposited cells, standard dark current voltage (I–V) and AM1.5G light I–V measurements were made using a 1000 W Oriel solar simulator (Model 81192–1000). Standard quantum efficiency (QE) measurements were carried out in the wavelength range of 350 to 1000 nm using a 100 W Xe Oriel lamp (Model 66055) with an Oriel monochromator with a light chopper placed in front of the monochromator. These measurements were done for pulsed DC a-Si solar cell samples at the optimal conditions as well as on SiGe tandem solar cells and the corresponding rf solar cell in order to compare efficiencies at different wavelengths of light. In addition, ultraviolet–visible spectroscopy (UV–VIS) was used to find the transparency of the film. For solar cells, the transparency in the visible spectrum is particularly important as much of the light incident is from the visible spectrum. For example, lower transmittance in the visible spectrum indicates that the ITO is too thick, thus reducing the efficiency of a solar cell. The spectra were taken from ITO on Eagle 2000 glass deposited at 40 W for 15 min and 30 s. 3. Results 3.1. Optimization of deposition power 3.1.1. I–V results In order to determine the optimal power for the pulsed DC deposition, a sweep of every 10 W from 60 to 110 W was conducted. The goal of the sweep was to find at which power most of the working cells were closest to the center of the substrate and produced the most uniform film thickness. It was found that at 9 min and 15 s and at a power of 70 W had the most active cells with room open circuit voltages (rVoc), but the most even distribution was found at 60 W. Along with varying the power, the deposition time was changed by 15 s intervals to move the ideal working cells toward the center of the substrate and to try and coat the ITO evenly over the substrate. 60 W was determined to be more favorable than 70 W, with a time of 11 min. It was found that the ITO target was well aged that it was taking more time than 9 min and 15 s to form an acceptable layer. Four point probe tests show that the color corresponding to the active cells was a teal-blue (see Fig. 1). Eventually, the power was investigated at 25, 30, 33, 35, 38, 40, 43, 45, and 50 W. However, changing the power itself did not change the gradient distribution of colors. Other parameters such as pressure and gas flow were also considered. A change in the pressure from 0.53 Pa was tested with a range of 0.27 −1.07 Pa (see Table 1), but it had no positive effect on the result as the percent efficiency differed by 0.162%. We note that pressure of 0.27 Pa resulted in too slow growth. On the other hand, 1.07 Pa leads to too fast growth, resulting in a shunted solar cell. In addition, a change in the pressure from 0.53 Pa by a few Pa had no positive affect in improving uniformity. When the gas flow was changed from the standard flow rate, 16 to 6 sccm, it indeed lowered the values of the short circuit current Jsc (see Table 1) but did nothing to improve uniformity of the plasma past the center 36 dots, the same as the change in pressure did not improve uniformity during the deposition. Changing gas flow, pressure, and time had no effect on improving uniformity of the film, indicating that the nature of pulsed DC power for sputtering is to produce a denser and therefore a larger thickness gradient. The deposition done at 11 min, 16 sccm, and 0.53 Pa at 60 W were thicker, as can be seen by the observed color gradient in Fig. 1. We also varied deposition time, and found 9 min to be closer to earlier rf deposition results. 3.1.2. Thickness and UV–VIS measurements After results were not improving beyond Jsc values of 9–10 mA/ cm2, we speculated that if the pulsed DC was in fact causing damage to the solar cell by arcing during the deposition process, lower power
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Fig. 1. The transition from (a)–(d) going from 90, 80, 70, to 60 W, respectively, at 151 °C with chamber pressure Pc = 0.53 Pa, gas flow fg = 16 sccm and deposition time tdep = 9 min 15 s. The difference in power at the same temperature suggests that the color of the center where the best results were found was the light teal color as seen in (c) and (d).
with a longer deposition time could possibly reduce damage to the cell. In order to test this, a deposition was done on Eagle 2000 glass to determine the thickness of the ITO layer at the center of the substrate. Using the Dektak3ST, the thickness was found with the deposition done two times longer to ensure a detectable difference between the glass and the ITO layer. The thickness was found to be approximately 165 nm at the center of the cell. For 60 W, the ITO layer was approximately 95 nm thicker than an ITO film made with rf power. We have also examined the transmittance of ITO deposited a-Si solar cells. As shown in Fig. 2, the transmittance was highest at approximately 520 nm for the center of the solar cell as well as the outside edge. At this wavelength, the percent transmittance is approximately 85%. The thickness of the ITO on glass was double that of the typical ITO layer on a solar cell, decreasing the transparency slightly in the recorded spectra. The time of rf deposition considered to be standard was 9 min and 15 s with 60 W. On the other hand, we found that from the deposition rate of the solar cell, 43 W with 8 min produced the same film thickness using pulsed DC power. Other lower powers were also investigated at 25, 30, 33, 35, 38, 40, 45, and 50 W at times that corresponded to the same thickness of about 100 nm (see Table 2). The best results were obtained at a power of around 40 W and a time
of 7 min and 40 s. Corroborating data was found after carrying out depositions on four unique a-Si single junction solar cell substrates.
3.2. Optimization of temperature The second parameter that we have examined was the heating temperature of the substrate. The crossover observed in the I–V curves was thought to be linked to deposition temperatures that exceeded the temperature at which the p-layer was made, 191 °C. Temperature was changed in five degree increments from 141 to 156 °C, even up to 171 °C and 191 °C. These high temperatures adversely affected the performance of the solar cell, while the change
Table 1 Performance of ITO coated a-Si solar cells. Here, T, Pc, fg, tdep, FF and Pm denote growth temperature, chamber pressure, gas flow, deposition time, a filling factor and the maximum power, respectively. Power (W)
T (°C)
fg (sccm)
Pc (Pa)
tdep (s)
Voc (V)
Jsc (mA/cm2)
FF (%)
Pm (%)
60 60 60 60 30 30 30
151 151 141 141 146 151 156
16 16 16 6 16 16 16
0.53 0.80 0.53 0.53 0.53 0.53 0.53
630 600 555 555 465 450 465
0.805 0.736 0.689 0.824 0.794 0.727 0.778
8.51 8.92 12.30 7.46 11.56 9.39 11.63
58.46 58.55 52.45 53.08 54.55 51.66 53.53
4.005 3.843 4.443 3.262 5.01 3.53 4.84
Fig. 2. This figure contains spectra taken of ITO on Eagle 2000 glass that indicates high transparency in the visible spectrum. The ITO was deposited on the glass for 15 min and 20 s, rather than 7 min 40 s in order for the film to be thick enough for UV–VIS measurements. Here, Pc = 0.53 Pa and fg = 16 sccm were used.
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Table 2 Dektak thickness measurements of ITO made at double thickness on Eagle 2000 glass substrate at 151 °C with Pc = 0.53 Pa and fg = 16 sccm. The deposition times for 30, 40 and 45 W are 16 min, 15 min 20 s and 16 min, respectively. All thicknesses are in nm. Cell numbers 11 to 15 denote the center to edge of the substrate, respectively. Cell no.
30 W
40 W
45 W
11 12 13 14 15
160 170 130 140 175
110 110 125 140 150
100 100 145 100 125
of 5 to 10° did not seem to have a significant effect as the I–V values are similar (see Table 1). Similarly, as can be seen in Fig. 3, the results of the quantum efficiency indicate that the temperature was not affecting the interface between the p-layer and the ITO layer as the original scan of the rf sample (Fig. 3(a)) and the pulsed DC samples (Fig. 3(b)), as long as the deposition temperature of ITO is lower than the temperature at which the p-layer was made. Also note that both show similar trends near the 400 nm region (blue wavelengths). The solar cell has the best efficiency in the 500 −600 nm region (green wavelengths) as these are absorbed farther into the cell than the blue wavelengths (Fig. 3(b)). 3.3. a-SiGe tandem solar cell results Based on those results, we concluded that the optimal conditions for pulsed DC power deposition are 40 W, 7 min 40 s in total deposition time, 16 sccm, 0.53 Pa, and 151 °C. Using these standard conditions for pulsed DC power, we deposited ITO on a highly efficient a-SiGe tandem solar cells, and compared the results to those obtained using the current rf power standard. Detailed results are presented in
(a) RF power
(b) DC power
Fig. 3. QE of a-Si solar cells using (a) rf power and (b) pulsed DC on four substrates. Here Pc = 0.53 Pa and fg = 16 sccm were used. In (a) the deposition time tdep = 570 s and in (b), tdep = 450 (solid line), 450 (dashed line), 460 (filled circle), and 440 s (open circle).
Table 3 Direct comparison of rf vs. pulsed DC ITO coatings on (a) a-SiGe tandem solar cell along with (b) a-Si single-junction solar cell at the optimal conditions. Here Pc = 0.53 Pa and fg = 16 sccm were used. Deposition type
Power (W)
T (°C)
tdep (s)
Voc (V)
Jsc (mA/cm2)
FF (%)
Pm (%)
Yield
rf(a) Pulsed DC(a) Pulsed DC(b)
60 40 40
110 151 151
450 440 460
0.966 0.950 0.810
11.89 10.24 13.68
72.480 73.994 60.014
8.331 7.203 6.650
11/12 4/17 2/9
Table 3 along with the result for ITO coated a-Si single-junction solar cell made at the same optimal conditions. We note that these conditions were also applied to a-SiGe tandem solar cells in order to compare the efficiencies of these cells with those of an a-Si single junction solar cell. Typically, the a-SiGe cells would have Jsc of about 11 mA/cm2, Voc of 0.9 V and a fill factor (FF) of 70%. The optimal conditions showed an increase in the fill factor and approximately the same Voc. The Jsc values were approximately 9–10 mA/cm2 and there were three active dots. When the same test was done at 7 min and 20 s, the results were similar, with slightly higher fill factors and six out of twelve active dots. 3.4. Atomic force microscopy (AFM) measurements AFM measurements were conducted on a PicoSPMII system with silicon cantilevers having resonance frequencies at ~300 kHz. Scan resolution was 1024 × 1024 pixels which correspond to a scan area of 5000 nm × 5000 nm. Using height profiles obtained from AFM measurements, we have calculated the surface roughness W defined as root D E mean square (RMS) surface roughness [15], W = ½hðr Þ−〈h〉
2
1=2
, where h(r) is the height at the lateral position r and the mean height 〈h〉 = 1 = L2 ∑r hðr Þ with pixel size L. In order to investigate how power affected the surface morphology of the ITO films, AFM measurements were performed on samples made at pulsed DC powers of 35, 40, and 50 W (see Fig. 4 (a)–(c)). All the samples were made at 151 °C. Similarly, AFM scans were done on the ITO films grown with a pulsed DC power of 40 W at three different temperatures ranging from 140 to 191 °C (see Fig. 4(d)–(f)). Fig. 5 shows the RMS surface roughness W(l) measured over a square region of l × l as a function of the scan size l. For small l, the surface roughness exhibits power-law behavior with W ∼ lα, where α is the roughness exponent, and it saturates for large l when the scan length l is larger than the lateral correlation length ξ [15,16]. As can be seen in Fig. 5 (a), the saturated RMS surface roughness for 35 W samples indeed was lower, indicating a smoother surface at large length scales. However, the values for 40 and 50 W were similar (see inset of Fig. 5 (a)), suggesting that the change may not be linear. The rougher surface at a higher power can perhaps lead to the damage of the solar cell, decreasing the yield of active cells and the efficiencies of the cells. A power-law fit to the data for small l yields the roughness exponent α = 0.67 for all three cases as shown in Fig. 5 (a), indicating a change in power for the given range does not alter the surface morphology at small length scales. We note that the value of the roughness exponent obtained in our experiments (α = 0.67) is consistent with other experimental (α = 0.68) [17] and theoretical (α = 2/3) [18,19] results. We now discuss the effect of temperature on the surface roughness. The AFM scans in Fig. 4 (d)–(f) show that the surface roughness is visually similar at (c) 140 °C and (d) 156 °C. On the other hand, at 191 °C (f) there is a visible difference in the roughness. Accordingly, the results of the RMS surface roughness given in Fig. 5 (b) show that at large length scales, the roughness is similar at 140 and 156 °C and slightly higher at 191 °C. It is interesting to note that the result of the surface roughness for solar cell is almost the same as that for 191 °C, suggesting that a thin layer of ITO does not
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Fig. 4. AFM images of ITO films. Appended color bar in each image shows z-height in units of Å. (a)–(c) with three different values of pulsed DC power at 151 °C and (d)–(f) with 40 W but at three different temperatures. The sizes of images are all 5μm × 5μm. Here Pc = 0.53 Pa and fg = 16 sccm were used. The deposition time (a) tdep = 465 s, (b) 465 s, (c) 450 s, (c)–(e) 460 s.
modify the surface morphology significantly. We also note that at small length scales, the results of the surface roughness for 140, 156, and 191 °C exhibit the same power-law behavior with the roughness exponent α ≈ 2/3, as the results shown in Fig. 5 (a). In addition, as can be seen in Fig. 5 (b), this is also the case for stainless steel substrate at small scales. However, at large length scales the value of the roughness exponent shows a crossover to a higher value with α ∼ 0.9, indicating almost a linear increase with length scale. 4. Conclusions We have found that at the optimal conditions, total deposition time was ultimately 1 min and 35 s shorter for pulsed DC power than the conditions made with rf power. Although the time is shorter, comparing the results to the previous solar cells shows that Jsc, Voc, fill factor, etc. are close to the same values, but the pulsed DC values are lower than those done on the same sample with rf power. Although ITO layers were around 70 nm for the samples deposited by rf sputtering, the depositions done with pulsed DC needed to be thicker because pulsed DC power during the off times, allows the film particles to organize and fall in between the peaks of the solar cell underneath. From the UV–VIS spectra, the ITO was shown to have a high transparency and thus was not the cause of the lower Voc, Jsc, and Pm. Changing the pressure was thought to improve uniformity in the solar cell, but it was found to have little effect. The pulsed DC does not produce plasma that can be evenly distributed like rf. The color
gradient from DC means that only a few cells near the center, ideally, will produce proper results, but with an industrial process (i.e., a larger cathode and system), an even distribution is possible. The QEs of the a-Si solar cells show that the deposition temperature was not damaging the surface of the solar cell if the deposition is done below the temperature of the top-most layer of a-Si solar cell. Both the results for the QE of the rf and pulsed DC sample show the same trends, indicating that temperature was not damaging the cell and was not the cause of slightly lower I–V values for pulsed DC. Further studies are needed to understand the cause and ultimately to improve the efficiency of solar cells. As the a-SiGe tandem junction solar cells show, there is improvement in fill factor, but the other values are slightly lower than rf values. There are also less active cells as is expected. Pulsed DC will not produce active cells all over the substrate, but 4 to 6 semi active cells with rVoc ranging from 0.2 to 0.6 V indicating that pulsed DC power is affecting the solar cell. Future research could be done to examine a combination of rf sputtering followed by pulsed DC sputtering as one proposed in [13] to maximize the results coming from each type of powers characteristics. Acknowledgments This research was funded by the National Science Foundation for the Research Experience for Undergraduates and a U.S. Department of Energy grant DE-FG36-08GO18073. The authors would like to thank
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References 120 100 80 60 40 30
35
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45 50
55
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(a)
(b)
Fig. 5. The root mean square (RMS) surface roughness as a function of scan size l for the cases shown in Fig. 4. (a) For three different values of pulsed power at 151 °C and (b) at three different temperatures with a pulsed DC power of 40 W as well as the stainless steel substrate itself. The detailed experimental parameters are given in the caption of Fig. 4.
Prof. Xianbo Liao for producing a-Si single-junction solar cells used in this study as well as Dr. Qi Hua Fan for the use of germanium silicon solar cells. The authors would also like to thank Xianbi Xiang for running QE scans.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Optimization of indium tin oxide by pulsed DC power on single junction amorphous silicon solar cells Rebecca K. Carlson, Yunsic Shim, William B. Ingler Jr. ⁎ Department of Physics & Astronomy, University of Toledo, Toledo, Ohio 43606, USA
a r t i c l e
i n f o
Article history: Received 19 February 2010 Received in revised form 11 March 2011 Accepted 15 March 2011 Available online 31 March 2011 Keywords: Amorphous materials Deposition process Indium tin oxide Optical properties Scanning electron microscopy Solar cells
a b s t r a c t We investigated the optimal deposition conditions of a thin indium tin oxide (ITO) film on an amorphous silicon (a-Si) single-junction solar cell using pulsed DC magnetron sputtering. Thin ITO films were deposited while power, deposition time, pressure, gas flow and temperature were varied to find such conditions. The efficiency of a-Si solar cells with ITO films was 6.65% at the optimal conditions — a pulsed DC power of 40 W, a deposition time of 460 s, a pressure of 0.53 Pa, gas flow of 16 sccm and 151 °C. On the other hand, an a-SiGe tandem solar cell with the ITO films made at the optimal conditions yields an efficiency of 7.20%. We have also examined the surface morphology of ITO coated a-Si solar cells, using atomic force microscopy. Interestingly, a change in power does not alter the surface morphology at small length scales, whereas at large scales, the lower power sample had a lower surface roughness than the samples made with higher powers. We also find that for the range of deposition conditions examined, the value of the roughness exponent does not change with α ≃ 2/3 and a thin layer of ITO does not modify the surface morphology significantly. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxides (TCO) have many applications in optoelectronic devices, such as flat panel displays and organic lightemitting diodes [1–3], and photovoltaic cells (see Ref. [4] for a recent review of high mobility TCO for thin-film solar cells). Indium tin oxide (ITO) is one of the most widely used transparent conducting oxides in such applications because it is a wide band-gap semiconductor (Egap = 3.75 to 4.0 eV), effective for transmitting visible and ultraviolet (UV) light, and has low resistivity in the low 10− 4 Ω cm range [5]. For photovoltaic cells, these characteristics are essential to produce an efficient solar cell. When light photons of a particular wavelength hit the cell, photons are absorbed in the intrinsic layer, producing a photoexcited electronhole pair. The electron (or hole) in the valence band is excited to the conduction band, these electrons or holes are moved towards the ntype layer or p-type layer respectively due to the built in potential of the depletion region between the layers, thus generating electricity when connected to a circuit. In particular, a hydrogenated amorphous silicon solar cell is a type of solar cell that has increased efficiency because Si―H bonds improve the electrical characteristics of the cell [6] (see Ref. [7] for a recent review of major thin-film solar cell technologies).
⁎ Corresponding author. E-mail address: [email protected] (W.B. Ingler). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.03.035
We note that the method in which the photodiode is constructed also affects the efficiency of solar cells. In a p-i-n photodiode, illumination through the p-layer is favorable because the holes have a shorter distance to travel to be collected, limiting recombination. If the drift carriers must travel a larger distance to be collected, charge can build up and the electric field will collapse, which can reduce the efficiency of the cell through recombination [6]. Thus, it is necessary to have the most transparent and conductive film possible on the p-layer. Otherwise, the efficiency will drop before the photons travel into the intrinsic layer (i-layer). ITO's wide band gap allows for maximized range of absorption of a-Si solar cells in the UV spectrum when doped. Conductivity of the ITO film increases as the indium doping increases because charges move effectively across the solar cell surface [8–10]. Even if the transparency and conductivity of the ITO are excellent, the interface between the ITO layer and the p-layer also plays a crucial role in determining the efficiency of solar cells. In particular, the interface is sensitive to the deposition conditions. A higher deposition temperature of ITO creates films with lower resistance, but it can damage the interface between the p-layer and the ITO if the temperature goes beyond the conditions at which the p-layer was deposited. In general, the interface losses should be less than 0.3 V. As a result, this naturally puts a limit on the deposition temperature of ITO on the top of p-layer of a-Si solar cells. In addition, the ITO film must have a high optical transmittance greater than 90%, a low sheet resistance below 100 Ω/sq and sheet resistivity below 6.5 × 10− 4 Ω cm [7,10,11]. The requirement of the low sheet resistance leads to an acceptable ITO thickness of about 100 nm [12].
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In this paper we report the optimal deposition conditions of ITO on an a-Si single-junction solar cell using pulsed DC magnetron sputtering. Although radio frequency (rf) deposition processes can lead to better and denser films due to an enhanced substrate bombardment by plasma ions (mainly Ar+) of moderate energy, one major problem with rf deposition process is the lower sputter rates [13]. Thus, the method of pulsed DC magnetron sputtering may be more desirable for the deposition of ITO thin films because they produce better quality films faster than rf magnetron sputtering [14]. Also, the electron pulse is beneficial for limiting arcing by reducing charges that gather through short bursts of positive voltage following a burst of negative voltage. This along with depositions done at a lower power results in less damage to the solar cell underneath compared to rf power and still produces a similar film in a shorter amount of time. The organization of this paper is as follows. In Section 2 we describe the details of our experiment. In Section 3, we present our results for the performance and optimization of deposition power, temperature, and gas pressure. These include I–V, transmittance, and quantum efficiency measurements. We also present results for the effects of deposition power and temperature on the surface roughness obtained from atomic force microscopy measurements. Finally, in Section 4 we present a brief conclusion of our results.
2. Experimental details The ITO target used in this study was a 90/10 indium oxide–tin oxide mixture by weight and was 99.99% pure produced by Kurt J. Lesker. The deposition chamber had a circular, three inch diameter cathode gun, and the ITO was deposited onto a 4 in. × 4 in. a-Si singlejunction or SiGe tandem solar cell deposited on 430 stainless steel substrate. This a-Si single-junction solar cell substrate was placed in a rotating substrate holder. When depositing the ITO film, a mask from Microphoto Inc. was used to produce small dots of 0.25 cm2 area. The chamber contained two halogen heating lamps, controlled by an external transformer that was used for heating the substrate. During temperature calibration, the thermocouple was attached to the sample holder, but during deposition, and after calibration it was screwed onto the mantle underneath the lamp housing. This method gives a relative temperature to the applied voltage, but subsequent recalibrations show little to no drift when the chamber is recalibrated at a later time. The chamber was evacuated using a Pfeiffer vacuum roughing pump and turbo pump. A Pirani gage, barometer, and ion gage were used to measure chamber pressure. Argon gas was flowed into the chamber to form plasma in the vacuum chamber with a pressure of as low as 3.2 × 10− 5 Pa before adding argon. Compressed argon and argon/20% oxygen gases were injected into the deposition chamber using MKS mass flow controllers with a range of 0 to 10 sccm for each gas. The first part of the study was done at standard conditions determined for rf power in order to create baseline conditions used as the standard lab recipe for ITO coatings. Rf generator power (ENI, Model ACG-3B) with a frequency of 13.56 MHz for the In2O3 target was varied from 35 to 90 W, and substrate temperature was varied from 141 to 191 °C. Chamber pressure (Pc) was held at a constant 0.53 Pa, and the deposition time was varied to 7 to 12 min with 15 s intervals. For rf power, the sample was heated for 30 min at the lowest relative pressure, corresponding to a completely open turbo pump, and argon gas was run at 30 sccm to fill and heat the chamber with enough gas prior to the deposition. Then gas flow (fg) was decreased to 16 sccm at a pressure of 0.53 Pa for 2 min. The pre-sputtering period was done for 2 min at 60 W. The standard deposition time was 9 min and 15 s at a temperature of 151 °C. In the second part of the study the standard conditions developed using rf power were used as a baseline for pulsed DC power which was
generated with an ENI power supply (Model RPG-5G) with a pulse frequency of 201 kHz. For characterization of the as-deposited cells, standard dark current voltage (I–V) and AM1.5G light I–V measurements were made using a 1000 W Oriel solar simulator (Model 81192–1000). Standard quantum efficiency (QE) measurements were carried out in the wavelength range of 350 to 1000 nm using a 100 W Xe Oriel lamp (Model 66055) with an Oriel monochromator with a light chopper placed in front of the monochromator. These measurements were done for pulsed DC a-Si solar cell samples at the optimal conditions as well as on SiGe tandem solar cells and the corresponding rf solar cell in order to compare efficiencies at different wavelengths of light. In addition, ultraviolet–visible spectroscopy (UV–VIS) was used to find the transparency of the film. For solar cells, the transparency in the visible spectrum is particularly important as much of the light incident is from the visible spectrum. For example, lower transmittance in the visible spectrum indicates that the ITO is too thick, thus reducing the efficiency of a solar cell. The spectra were taken from ITO on Eagle 2000 glass deposited at 40 W for 15 min and 30 s. 3. Results 3.1. Optimization of deposition power 3.1.1. I–V results In order to determine the optimal power for the pulsed DC deposition, a sweep of every 10 W from 60 to 110 W was conducted. The goal of the sweep was to find at which power most of the working cells were closest to the center of the substrate and produced the most uniform film thickness. It was found that at 9 min and 15 s and at a power of 70 W had the most active cells with room open circuit voltages (rVoc), but the most even distribution was found at 60 W. Along with varying the power, the deposition time was changed by 15 s intervals to move the ideal working cells toward the center of the substrate and to try and coat the ITO evenly over the substrate. 60 W was determined to be more favorable than 70 W, with a time of 11 min. It was found that the ITO target was well aged that it was taking more time than 9 min and 15 s to form an acceptable layer. Four point probe tests show that the color corresponding to the active cells was a teal-blue (see Fig. 1). Eventually, the power was investigated at 25, 30, 33, 35, 38, 40, 43, 45, and 50 W. However, changing the power itself did not change the gradient distribution of colors. Other parameters such as pressure and gas flow were also considered. A change in the pressure from 0.53 Pa was tested with a range of 0.27 −1.07 Pa (see Table 1), but it had no positive effect on the result as the percent efficiency differed by 0.162%. We note that pressure of 0.27 Pa resulted in too slow growth. On the other hand, 1.07 Pa leads to too fast growth, resulting in a shunted solar cell. In addition, a change in the pressure from 0.53 Pa by a few Pa had no positive affect in improving uniformity. When the gas flow was changed from the standard flow rate, 16 to 6 sccm, it indeed lowered the values of the short circuit current Jsc (see Table 1) but did nothing to improve uniformity of the plasma past the center 36 dots, the same as the change in pressure did not improve uniformity during the deposition. Changing gas flow, pressure, and time had no effect on improving uniformity of the film, indicating that the nature of pulsed DC power for sputtering is to produce a denser and therefore a larger thickness gradient. The deposition done at 11 min, 16 sccm, and 0.53 Pa at 60 W were thicker, as can be seen by the observed color gradient in Fig. 1. We also varied deposition time, and found 9 min to be closer to earlier rf deposition results. 3.1.2. Thickness and UV–VIS measurements After results were not improving beyond Jsc values of 9–10 mA/ cm2, we speculated that if the pulsed DC was in fact causing damage to the solar cell by arcing during the deposition process, lower power
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Fig. 1. The transition from (a)–(d) going from 90, 80, 70, to 60 W, respectively, at 151 °C with chamber pressure Pc = 0.53 Pa, gas flow fg = 16 sccm and deposition time tdep = 9 min 15 s. The difference in power at the same temperature suggests that the color of the center where the best results were found was the light teal color as seen in (c) and (d).
with a longer deposition time could possibly reduce damage to the cell. In order to test this, a deposition was done on Eagle 2000 glass to determine the thickness of the ITO layer at the center of the substrate. Using the Dektak3ST, the thickness was found with the deposition done two times longer to ensure a detectable difference between the glass and the ITO layer. The thickness was found to be approximately 165 nm at the center of the cell. For 60 W, the ITO layer was approximately 95 nm thicker than an ITO film made with rf power. We have also examined the transmittance of ITO deposited a-Si solar cells. As shown in Fig. 2, the transmittance was highest at approximately 520 nm for the center of the solar cell as well as the outside edge. At this wavelength, the percent transmittance is approximately 85%. The thickness of the ITO on glass was double that of the typical ITO layer on a solar cell, decreasing the transparency slightly in the recorded spectra. The time of rf deposition considered to be standard was 9 min and 15 s with 60 W. On the other hand, we found that from the deposition rate of the solar cell, 43 W with 8 min produced the same film thickness using pulsed DC power. Other lower powers were also investigated at 25, 30, 33, 35, 38, 40, 45, and 50 W at times that corresponded to the same thickness of about 100 nm (see Table 2). The best results were obtained at a power of around 40 W and a time
of 7 min and 40 s. Corroborating data was found after carrying out depositions on four unique a-Si single junction solar cell substrates.
3.2. Optimization of temperature The second parameter that we have examined was the heating temperature of the substrate. The crossover observed in the I–V curves was thought to be linked to deposition temperatures that exceeded the temperature at which the p-layer was made, 191 °C. Temperature was changed in five degree increments from 141 to 156 °C, even up to 171 °C and 191 °C. These high temperatures adversely affected the performance of the solar cell, while the change
Table 1 Performance of ITO coated a-Si solar cells. Here, T, Pc, fg, tdep, FF and Pm denote growth temperature, chamber pressure, gas flow, deposition time, a filling factor and the maximum power, respectively. Power (W)
T (°C)
fg (sccm)
Pc (Pa)
tdep (s)
Voc (V)
Jsc (mA/cm2)
FF (%)
Pm (%)
60 60 60 60 30 30 30
151 151 141 141 146 151 156
16 16 16 6 16 16 16
0.53 0.80 0.53 0.53 0.53 0.53 0.53
630 600 555 555 465 450 465
0.805 0.736 0.689 0.824 0.794 0.727 0.778
8.51 8.92 12.30 7.46 11.56 9.39 11.63
58.46 58.55 52.45 53.08 54.55 51.66 53.53
4.005 3.843 4.443 3.262 5.01 3.53 4.84
Fig. 2. This figure contains spectra taken of ITO on Eagle 2000 glass that indicates high transparency in the visible spectrum. The ITO was deposited on the glass for 15 min and 20 s, rather than 7 min 40 s in order for the film to be thick enough for UV–VIS measurements. Here, Pc = 0.53 Pa and fg = 16 sccm were used.
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Table 2 Dektak thickness measurements of ITO made at double thickness on Eagle 2000 glass substrate at 151 °C with Pc = 0.53 Pa and fg = 16 sccm. The deposition times for 30, 40 and 45 W are 16 min, 15 min 20 s and 16 min, respectively. All thicknesses are in nm. Cell numbers 11 to 15 denote the center to edge of the substrate, respectively. Cell no.
30 W
40 W
45 W
11 12 13 14 15
160 170 130 140 175
110 110 125 140 150
100 100 145 100 125
of 5 to 10° did not seem to have a significant effect as the I–V values are similar (see Table 1). Similarly, as can be seen in Fig. 3, the results of the quantum efficiency indicate that the temperature was not affecting the interface between the p-layer and the ITO layer as the original scan of the rf sample (Fig. 3(a)) and the pulsed DC samples (Fig. 3(b)), as long as the deposition temperature of ITO is lower than the temperature at which the p-layer was made. Also note that both show similar trends near the 400 nm region (blue wavelengths). The solar cell has the best efficiency in the 500 −600 nm region (green wavelengths) as these are absorbed farther into the cell than the blue wavelengths (Fig. 3(b)). 3.3. a-SiGe tandem solar cell results Based on those results, we concluded that the optimal conditions for pulsed DC power deposition are 40 W, 7 min 40 s in total deposition time, 16 sccm, 0.53 Pa, and 151 °C. Using these standard conditions for pulsed DC power, we deposited ITO on a highly efficient a-SiGe tandem solar cells, and compared the results to those obtained using the current rf power standard. Detailed results are presented in
(a) RF power
(b) DC power
Fig. 3. QE of a-Si solar cells using (a) rf power and (b) pulsed DC on four substrates. Here Pc = 0.53 Pa and fg = 16 sccm were used. In (a) the deposition time tdep = 570 s and in (b), tdep = 450 (solid line), 450 (dashed line), 460 (filled circle), and 440 s (open circle).
Table 3 Direct comparison of rf vs. pulsed DC ITO coatings on (a) a-SiGe tandem solar cell along with (b) a-Si single-junction solar cell at the optimal conditions. Here Pc = 0.53 Pa and fg = 16 sccm were used. Deposition type
Power (W)
T (°C)
tdep (s)
Voc (V)
Jsc (mA/cm2)
FF (%)
Pm (%)
Yield
rf(a) Pulsed DC(a) Pulsed DC(b)
60 40 40
110 151 151
450 440 460
0.966 0.950 0.810
11.89 10.24 13.68
72.480 73.994 60.014
8.331 7.203 6.650
11/12 4/17 2/9
Table 3 along with the result for ITO coated a-Si single-junction solar cell made at the same optimal conditions. We note that these conditions were also applied to a-SiGe tandem solar cells in order to compare the efficiencies of these cells with those of an a-Si single junction solar cell. Typically, the a-SiGe cells would have Jsc of about 11 mA/cm2, Voc of 0.9 V and a fill factor (FF) of 70%. The optimal conditions showed an increase in the fill factor and approximately the same Voc. The Jsc values were approximately 9–10 mA/cm2 and there were three active dots. When the same test was done at 7 min and 20 s, the results were similar, with slightly higher fill factors and six out of twelve active dots. 3.4. Atomic force microscopy (AFM) measurements AFM measurements were conducted on a PicoSPMII system with silicon cantilevers having resonance frequencies at ~300 kHz. Scan resolution was 1024 × 1024 pixels which correspond to a scan area of 5000 nm × 5000 nm. Using height profiles obtained from AFM measurements, we have calculated the surface roughness W defined as root D E mean square (RMS) surface roughness [15], W = ½hðr Þ−〈h〉
2
1=2
, where h(r) is the height at the lateral position r and the mean height 〈h〉 = 1 = L2 ∑r hðr Þ with pixel size L. In order to investigate how power affected the surface morphology of the ITO films, AFM measurements were performed on samples made at pulsed DC powers of 35, 40, and 50 W (see Fig. 4 (a)–(c)). All the samples were made at 151 °C. Similarly, AFM scans were done on the ITO films grown with a pulsed DC power of 40 W at three different temperatures ranging from 140 to 191 °C (see Fig. 4(d)–(f)). Fig. 5 shows the RMS surface roughness W(l) measured over a square region of l × l as a function of the scan size l. For small l, the surface roughness exhibits power-law behavior with W ∼ lα, where α is the roughness exponent, and it saturates for large l when the scan length l is larger than the lateral correlation length ξ [15,16]. As can be seen in Fig. 5 (a), the saturated RMS surface roughness for 35 W samples indeed was lower, indicating a smoother surface at large length scales. However, the values for 40 and 50 W were similar (see inset of Fig. 5 (a)), suggesting that the change may not be linear. The rougher surface at a higher power can perhaps lead to the damage of the solar cell, decreasing the yield of active cells and the efficiencies of the cells. A power-law fit to the data for small l yields the roughness exponent α = 0.67 for all three cases as shown in Fig. 5 (a), indicating a change in power for the given range does not alter the surface morphology at small length scales. We note that the value of the roughness exponent obtained in our experiments (α = 0.67) is consistent with other experimental (α = 0.68) [17] and theoretical (α = 2/3) [18,19] results. We now discuss the effect of temperature on the surface roughness. The AFM scans in Fig. 4 (d)–(f) show that the surface roughness is visually similar at (c) 140 °C and (d) 156 °C. On the other hand, at 191 °C (f) there is a visible difference in the roughness. Accordingly, the results of the RMS surface roughness given in Fig. 5 (b) show that at large length scales, the roughness is similar at 140 and 156 °C and slightly higher at 191 °C. It is interesting to note that the result of the surface roughness for solar cell is almost the same as that for 191 °C, suggesting that a thin layer of ITO does not
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Fig. 4. AFM images of ITO films. Appended color bar in each image shows z-height in units of Å. (a)–(c) with three different values of pulsed DC power at 151 °C and (d)–(f) with 40 W but at three different temperatures. The sizes of images are all 5μm × 5μm. Here Pc = 0.53 Pa and fg = 16 sccm were used. The deposition time (a) tdep = 465 s, (b) 465 s, (c) 450 s, (c)–(e) 460 s.
modify the surface morphology significantly. We also note that at small length scales, the results of the surface roughness for 140, 156, and 191 °C exhibit the same power-law behavior with the roughness exponent α ≈ 2/3, as the results shown in Fig. 5 (a). In addition, as can be seen in Fig. 5 (b), this is also the case for stainless steel substrate at small scales. However, at large length scales the value of the roughness exponent shows a crossover to a higher value with α ∼ 0.9, indicating almost a linear increase with length scale. 4. Conclusions We have found that at the optimal conditions, total deposition time was ultimately 1 min and 35 s shorter for pulsed DC power than the conditions made with rf power. Although the time is shorter, comparing the results to the previous solar cells shows that Jsc, Voc, fill factor, etc. are close to the same values, but the pulsed DC values are lower than those done on the same sample with rf power. Although ITO layers were around 70 nm for the samples deposited by rf sputtering, the depositions done with pulsed DC needed to be thicker because pulsed DC power during the off times, allows the film particles to organize and fall in between the peaks of the solar cell underneath. From the UV–VIS spectra, the ITO was shown to have a high transparency and thus was not the cause of the lower Voc, Jsc, and Pm. Changing the pressure was thought to improve uniformity in the solar cell, but it was found to have little effect. The pulsed DC does not produce plasma that can be evenly distributed like rf. The color
gradient from DC means that only a few cells near the center, ideally, will produce proper results, but with an industrial process (i.e., a larger cathode and system), an even distribution is possible. The QEs of the a-Si solar cells show that the deposition temperature was not damaging the surface of the solar cell if the deposition is done below the temperature of the top-most layer of a-Si solar cell. Both the results for the QE of the rf and pulsed DC sample show the same trends, indicating that temperature was not damaging the cell and was not the cause of slightly lower I–V values for pulsed DC. Further studies are needed to understand the cause and ultimately to improve the efficiency of solar cells. As the a-SiGe tandem junction solar cells show, there is improvement in fill factor, but the other values are slightly lower than rf values. There are also less active cells as is expected. Pulsed DC will not produce active cells all over the substrate, but 4 to 6 semi active cells with rVoc ranging from 0.2 to 0.6 V indicating that pulsed DC power is affecting the solar cell. Future research could be done to examine a combination of rf sputtering followed by pulsed DC sputtering as one proposed in [13] to maximize the results coming from each type of powers characteristics. Acknowledgments This research was funded by the National Science Foundation for the Research Experience for Undergraduates and a U.S. Department of Energy grant DE-FG36-08GO18073. The authors would like to thank
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References 120 100 80 60 40 30
35
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Watt
(a)
(b)
Fig. 5. The root mean square (RMS) surface roughness as a function of scan size l for the cases shown in Fig. 4. (a) For three different values of pulsed power at 151 °C and (b) at three different temperatures with a pulsed DC power of 40 W as well as the stainless steel substrate itself. The detailed experimental parameters are given in the caption of Fig. 4.
Prof. Xianbo Liao for producing a-Si single-junction solar cells used in this study as well as Dr. Qi Hua Fan for the use of germanium silicon solar cells. The authors would also like to thank Xianbi Xiang for running QE scans.
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