Effect of textured glass substrates coated with LPCVD-deposited SnO2:F on amorphous silicon solar cells

Solar Energy Materials & Solar Cells 140 (2015) 126–133

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Effect of textured glass substrates coated with LPCVD-deposited SnO2:F on amorphous silicon solar cells Amartya Chowdhury a,n, Dong-Won Kang a, Masanobu Isshiki b, Takuji Oyama b, Hidefumi Odaka b, Porponth Sichanugrist a, Makoto Konagai a,c a

Department of Physical Electronics, Tokyo Institute of Technology, Japan Research Center, Asahi Glass Co., Ltd., Japan c Photovoltaic Research Center (PVREC), Tokyo Institute of Technology, Japan b

art ic l e i nf o

a b s t r a c t

Article history: Received 25 July 2014 Received in revised form 31 March 2015 Accepted 2 April 2015

A new type of SnO2:F film has been deposited by low-pressure chemical vapor deposition (LPCVD) on textured glass substrates, and it is used for the fabrication of amorphous silicon (a-Si) thin film solar cells. These substrates are found to be highly effective in terms of their optical and electrical properties. They also facilitate the fabrication of efficient top cells for a future multi-junction solar cell. These substrates have a micron-order texture feature size, which is much larger than the a-Si absorber layer thickness. Still, they produce higher current in the 300–550 nm wavelength range with a higher open circuit voltage than cells on multi-scale textured ZnO:B substrates. This work examines the applicability of this new SnO2:F coated glass substrate with different texture profiles on a-Si solar cells in the wavelength range of 300–550 nm. & 2015 Elsevier B.V. All rights reserved.

Keywords: Solar cell Amorphous silicon Thin film Transparent conducting oxide SnO2:F Textured Substrate

1. Introduction Textured transparent conductive oxides (TCOs) are extensively utilized in thin-film photovoltaic devices as front contact electrodes. This facilitates significant reduction of material consumption with respect to wafer-based silicon solar cells [1]. Common TCO materials are tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), and impurity-doped (e.g., Al, Ga, B) zinc oxide. Nowadays doped zinc oxide (ZnO:Al or ZnO:B) films are becoming increasingly favorable owing to advantages such as low cost, nontoxicity, stability against a hydrogen plasma environment, and easy pre/post-deposition texturing possibilities [2–5]. Light confinement by proper substrate texturing becomes very important for a tandem micromorph solar cell because it consists of a 300-nm-thick amorphous silicon (a-Si) solar cell on the front and a 1.5- to 2-μm-thick microcrystalline silicon solar cell on the back [6]. For a single junction solar cell, the period and height of the texture pattern should be equal to or slightly greater than the absorber layer thickness [7,8]. However, for a tandem micromorph cell, the best suited feature size is difficult to estimate. One alternative way is to use a multi-textured substrate [2,9,10], as first reported by our group [11]. n

Corresponding author. E-mail address: [email protected] (A. Chowdhury).

http://dx.doi.org/10.1016/j.solmat.2015.04.003 0927-0248/& 2015 Elsevier B.V. All rights reserved.

Apart from texture pattern, the other requirements for TCO substrates are high carrier mobility or carrier density and high optical transparency. However, a higher carrier density leads to increased free-carrier absorption. This is detrimental to the nearinfrared (NIR) transparency. In this respect, commercially produced Asahi VU substrate, i.e. fluorine-doped tin oxide substrates grown by atmospheric pressure chemical vapor deposition (APCVD) have excellent properties. However, their small feature size is not suitable for a microcrystalline silicon solar cell part with a thickness of 1.5 μm in a tandem solar cell. Macro-order feature sizes are possible on doped ZnO substrates. There are many ways to control the feature size [9,10], including the use of metal organic chemical vapor deposition (MOCVD) [2] and post-deposition wet chemical etching in diluted HCl or HF acid [12,13]. However, these techniques fail to trap the crucial 300–380 nm wavelength range for the front a-Si cell because ZnO has a lower optical band gap, which is responsible for the total absorption of wavelengths in that range. Irrespective of the TCO material, another bottleneck for substrate texturization is that scattering is most pronounced in the short wavelength region. This also causes substantial absorption in the inactive layers such as the TCO and the p-layer, whereas absorption enhancement is most needed in the 550–1100 nm wavelength range of the spectrum. Recently, we have reported the superior properties of LPCVD-grown FTO films coating textured glass substrates and the

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performance of microcrystalline silicon cells fabricated on them [14]. This new substrate largely solves the previous problems inherent to the use of TCO substrates and has high carrier mobility as well as high transparency in the 400–1100 nm wavelength range. This substrate fractionally blocks the crucial 300–380 nm wavelength range. The multi-junction silicon solar cell developed by our group contains a microcrystalline silicon oxide n-layer in the back of the front cell to partially reflect photons [15]. The effective working wavelength range of individual solar cells also depends on the thickness, haze value of substrate and optical band gap of the absorber layer of the front cell. For present solar cells with 300 nm thick absorber layer, reflection as well as plasmonic losses from the back reflector comes into effect above 550 nm wavelength. Thus, for the present case, we calculate the cell efficiency of the front cell in the 300–550 nm wavelength range. Here we report for the first time the functional mechanism of a-Si solar cells on these substrates before they are applied to multi-junction type solar cells. The goal of this work is to maximize the current density in the 300–550 nm wavelength range while maintaining high open circuit voltage (Voc) and an acceptable fill factor (FF) on a micron-order textured substrate.

2. Experiment 2.1. FTO deposition Corning 7059 glass substrates of 0.7 mm thickness were macrotextured at different roughness levels using a radio frequency plasma reactive-ion etching (RIE) system. Carbon tetra-fluoride (CF4) was used as the etching gas. During RIE treatment, the power and CF4 flow rate were set at 200 W and 15 sccm, respectively. The glass etching time was varied from 20 to 80 min. The chamber pressure was varied from 7 to 15 Pa in order to achieve different levels of texturization. FTO films with fixed thickness of 3.6 mm were deposited on these substrates along with one (FTO flat) unetched flat glass substrate using LPCVD. SnCl4, H2O, and HF were used as source gases. The details of the LPCVD machine and the deposition conditions are described elsewhere [16,17]. The samples were passed through a 10-min argon plasma treatment after FTO layer deposition to reduce the occasional spikes and sharp edges on the sample surface. This is essential for both cell fabrication with reduced electrical shunting and atomic force microscopic (AFM) instrument tip protection during AFM imaging. AFM images of 20  20 mm2 were taken using the non-contact mode in the center of the substrates used for cell fabrication. It is found that the RMS roughness and maximum feature height are reproducible in most of the positions for FTO substrates with the same fabrication condition. So average value of RMS roughness and maximum height is given for each condition. However as these substrates have 2 different textures coming from the glass (feature size 10 mm) and FTO material, a scanning area of only 20  20 mm2 produces some fluctuations across different positions in FTO substrates when we try to measure height or angle distribution. The instrument accuracy is not very high for texture height from valley to peak (max. height) of over 1.5 μm. However, images in Fig. 1 show that the instrument has been able to achieve proper data collection as there are no flat white or dark regions. The AFM instrument failed to accurately measure the roughness for the highly textured ZnO:B substrate. For this reason, scanning electron microscopic (SEM) imaging was performed on selected ZnO:B and FTO substrates to allow a comparison of their texture patterns. Apart from roughness measurement, the carrier mobility and sheet resistance of the samples were also measured. The haze value and optical transmittance of the samples were measured using a photo-spectrometer. During optical property

127

measurement of glass/FTO structure, CH2I2 with refractive index 1.74 was used to reduce light scattering from the surface of FTO layer. However, it absorbs light below 400 nm wavelength and we lose some valuable information. 2.2. Application in a-SI:H solar cell Thin film p–i–n type a-Si:H single-junction solar cells were fabricated on the previously described FTO substrates. The total structure then became glass/FTO/p-a-SiC:H/i-a-Si:H/n-mc-SiO: H/Ag/Al. The silicon layers were deposited in a multi-chamber plasma enhanced chemical vapor deposition (PECVD) system using very high frequency (VHF) of 60 MHz as the only power source. SiH4, H2, CH3–SiH3, CO2, B2H6, and PH3 gases were used as source gases at a substrate temperature of approximately 180 °C. The thicknesses of the p-, i-, and n-layers were approximately 20, 300, and 40 nm, respectively. Ag and Al layers were deposited by simple thermal evaporation. The photocurrent density–voltage characteristics (J–V curves) of the fabricated cells were measured using the AM1.5G spectrum at 1 Sun intensity. The spectral response of these cells was analyzed using external quantum efficiency (EQE) measurements. The reflections of the cells were measured using a photo-spectrometer, and the internal quantum efficiency (IQE) of the cells was calculated using EQE and reflection data. Jsc values of the solar cells were calculated for the total wavelength range and for partial ranges such as 300–550 nm and 550–800 nm using the external quantum efficiency data and the AM1.5G spectrum. The initial efficiencies (η) of the solar cells before light-induced degradation were calculated using the short circuit current density (Jsc) obtained from EQE measurements and Voc and FF from the J–V curves. For comparison, cells were also deposited on flat un-etched FTO coated glass substrate and on double-textured ZnO:B (W-ZnO)-coated glass substrate [11].

3. Results and discussion 3.1. Substrate morphology The AFM images of FTO substrates prepared at different pressures and etching times are shown in Fig. 1. All substrates passed through a 10-min argon plasma treatment. This treatment reduced the sharp edges and occasional spikes on the FTO surface, which are thought to be responsible for cell shunting. Argon plasma treatment also found to be beneficial for the microcrystalline cell in the back side [18]. The variation of the root mean square (RMS) roughness with respect to RIE etching time and chamber pressure can be found in Fig. 2A. For FTO layer on flat glass substrate, the RMS roughness is 46.2 nm. In general, the RMS roughness of the substrates increases considerably as the chamber pressure increases. The etching time dependence of the RMS roughness is not very prominent; however, the RMS roughness tends to increase marginally with an increase in etching time. Overall, the RMS roughness varies in the range 147–335 nm and the peak-to-peak roughness varies in the range 996–1757 nm. Careful observations reveal that the FTO substrates always have some texturization with a feature size of approximately 1 μm. In addition, the substrates have a larger feature size of approximately 10 μm, which becomes more prominent at higher RIE chamber pressures. In other words, the smaller primary features arise due to roughness of FTO material itself and can be seen clearly in case of sample FTO-flat. The larger features were generated because of the textured glass substrate beneath the FTO layer. These larger features were not suppressed by the 3.6-μm-thick FTO layer. This is generally known as the double-textured (W-textured) substrate, and it has been used frequently by our group for MOCVD-deposited ZnO:B substrates [11,19–22].

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RMS roughness mainly indicates the height distribution of textured surface and in case of the double textured substrate, it can be dominated by the larger feature sizes (∼10 mm width) originating from the glass texturization. The properties of FTO material as well as the micro-order roughness (∼0.3 mm width) generated from it may depend on the feature size of the glass substrate beneath. The height and angle distribution from AFM image have been analyzed. As they do not provide any extra conclusive evidence, they are not

Time

discussed in this paper. On the other hand, optical haze value at infrared wavelength region can indicate more accurately the effect of plasma etching parameters on the glass texturization. Fig. 2B shows the variation of optical haze value at 800 nm wavelength of samples described in Fig. 2A. It can be seen, that Fig. 2A and B has a very similar trend with some minor differences. This indicates that in spite of having dual texturizations, the RMS roughness describes the sample surface property quite accurately. Both the figures indicate

7 Pa Name: FTO-20-7 Max. height: 996 nm RMS roughness: 147.2 nm

Chamber Pressure 10 Pa Name: FTO-20-10 Max. height: 1443 nm RMS roughness: 224.2 nm

15 Pa Name: FTO-20-15 Max. height: 1682 nm RMS roughness: 321.4 nm

Name: FTO-40-7 Max. height: 1228 nm RMS roughness: 184.2 nm

Name: FTO-40-10 Max. height: 1400 nm RMS roughness: 223.4 nm

Name: FTO-40-15 Max. height: 1720 nm RMS roughness: 315.9 nm

Name: FTO-80-7 Max. height: 1446 nm RMS roughness: 174.8 nm

Name: FTO-80-10 Max. height: 1619 nm RMS roughness: 242.1 nm

Name: FTO-80-15 Max. height: 1757 nm RMS roughness: 335.5 nm

20 min

40 min

80 min

Name: FTO Flat Max. height: 415 nm RMS roughness: 46.2 nm FTO on flat glass substrate

Fig. 1. 3D AFM images with constant color scale (0–1.76 μm) of FTO substrates prepared at different RIE chamber pressures and processing times. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A. Chowdhury et al. / Solar Energy Materials & Solar Cells 140 (2015) 126–133

that FTO substrate properties depend heavily on chamber pressure and marginally on plasma etching time. Maximum haze value of around 94% is obtained for samples processed at 15 Pa chamber

360 340 320

Chamber pressure (Pa) 7 10 15

RMS roughness (nm)

300 280 260 240 220 200 180 160 140

20

30

40

50

60

70

80

Etching Time (min)

100 95

Haze Value at 800 nm

90 85 80 75

Chamber pressure (Pa) 7 10 15

70 65 60

20

30

40

50

60

70

80

Etching Time (min) Fig. 2. Variation of (A) RMS roughness and (B) Haze value of substrates prepared at different etching times and chamber pressures.

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pressure. The haze values of FTO substrates will be discussed in more detail later. Fig. 3 shows the SEM images of FTO and W-ZnO substrates. Although both undergo the same RIE treatment for 40 min at a pressure of 15 Pa, their appearances are remarkably different. The nano-order feature size dominates the W-ZnO substrate surface with the presence of a circular macrotexture of 5–6 μm diameter. For the FTO substrate, the macro-order feature pattern is similar, whereas the nano-order feature pattern is more than 300 nm in width. Table 1 shows the electrical properties of these FTO materials. The carrier mobility of these FTO substrates reduces from 87.4 to 75.3 cm2/V s as the RMS roughness increases in the aforementioned range. This reduction of carrier mobility may be related to increase in defect densities in the grain boundary etc. of the polycrystalline FTO on highly textured substrates. Indeed Sommer et al. reported similar trends in their experiments [23]. As seen in Table 1, these FTO substrates still have a mobility that is at least four times higher than their W-ZnO counterpart [16]. These mobility values are quite appreciable as the depositions were made on highly textured substrates. The advantage of FTO materials over W-ZnO counterpart is not only the carrier mobility but also the transparency in blue and infrared wavelength region. Fig. 4A and B shows haze ratio and transparency of these FTO substrates in 300–1000 nm wavelength range. CH2I2 was used during transmittance measurement and it blocks the wavelengths below 400 nm. So graphs are shown from 400 nm wavelength onward. As expected, it can be observed that the haze factor increases quite considerably with increase in roughness. In 500–1000 nm wavelength range, the FTO samples on textured glass substrate have almost twice haze value compared to the sample FTO-flat. In other words, it can be argued that at 500– 700 nm wavelength range the haze value depends equally on the texturization of FTO material and texturization of the glass substrate beneath. At higher wavelength, haze value is totally dominated by the high level of texturization of glass substrate. This is the reason for using these double texture FTO substrate in double junction micro-morph solar cell. The high level of glass texturization is essential to scatter light in the microcrystalline silicon back cell of a double junction solar cell. Lower micro-order texturization and slightly higher refractive index of FTO substrates compared to W-ZnO substrate make the later one slightly less reflective. So FTO substrates have slightly lower transmission with respect to W-ZnO substrate when the transparency of air/glass/FTO/CH2I2/air stack is measured. As FTO substrates have higher optical band gap, they are more transparent compared to ZnO as a material. However, the measurements of optical band gap or refractive index of these textured materials on flat glass substrate using ellipsometer are not very accurate. From Fig. 4A it can be seen that transparency does not change much with change in fabrication condition for the FTO substrates. However, for FTO-flat sample, the transparency is quite high. This may be due to

Fig. 3. SEM images of (A) FTO substrate and (B) W-ZnO substrate with glass substrate prepared using RIE with 15 Pa chamber pressure and 40 min etching time.

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Table 1 Dependence of solar cell performance on substrate roughness. Sample

RIE time (min)

RIE pressure (Pa)

RMS roughness (nm)

Mobility (cm2/ V s)

Sheet resistance (Ω/□)

Voc (V) FF

Jsc (mA/ cm2)

η (%)

Jsc at EQE 300–550 nm (mA/cm2)

FTO-20-7 FTO-20-10 FTO-20-15 FTO-40-7 FTO-40-10 FTO-40-15 FTO-80-7 W-ZnO FTO-flat

20 20 20 40 40 40 80 40 0

7 10 15 7 10 15 7 15 0

147.2 224.2 321.4 184.2 223.4 315.9 174.9 – 46.2

87.4 82.5 72.5 85.6 83.0 75.1 82.7 18.0 79.3

10.2 9.6 13.5 11.1 10.2 11.8 9.8 8.5 8.8

0.934 0.933 0.927 0.934 0.926 0.922 0.929 0.905 0.933

14.30 14.81 15.04 14.40 15.27 15.01 14.86 16.12 14.19

9.62 9.87 9.81 9.61 9.92 9.57 9.85 10.58 9.45

7.21 7.23 7.24 7.30 7.29 7.24 7.11 7.19 6.66

0.720 0.714 0.704 0.714 0.701 0.692 0.714 0.725 0.721

substrate is the stability of the former one. These LPCVD deposited FTO substrates do not degrade in open air condition due to atmospheric moisture.

100 90

3.2. Cell performance

80

Haze value (%)

70

RIE

60 50 40 30 20 10 0 400

Pressure (Pa) 7 7 7 10 10 10 15 15 15

Duration (Min) 20 40 80 20 40 80 20 40 80

FTO Flat W-ZnO

500

600

700

800

900

1000

Wavelength (nm) 90 85

Optical transmittance (%)

80

RIE

75 70 65 60 55 50 45 40 400

Pressure (Pa) 7 7 7 10 10 10 15 15 15

Duration (Min) 20 40 80 20 40 80 20 40 80

Flat FTO W-ZnO

500

600

700

800

900

1000

Wavelength (nm) Fig. 4. Variation of (A) Haze value and (B) optical transmittance (using CH2I2) of FTO and ZnO substrates prepared at different etching times and chamber pressures.

low light travel path inside FTO material as the glass substrate is flat which reduces scattering. Apart from these electrical and optical properties, other advantages of using FTO substrate over MOCVD deposited W-ZnO

As mentioned earlier, a-Si solar cells with identical layer structures have been fabricated on these FTO substrates, and the results are given in Table 1. Samples are named FTO-t-p, where t and p are numbers representing the RIE time (min) and pressure (Pa), respectively. The cells FTO-80-10 and FTO-80-15 are absent in the table as they are shunted completely. This is likely due to a very high degree of texturization in comparison to the absorber layer thickness and/or the presence of occasional spikes on the FTO substrate. A maximum initial efficiency of 9.92% is obtained for cells on a moderately textured FTO-40-10 substrate. This efficiency is quite low compared to the efficiency of cells on a W-ZnO substrate. However, our goal is to produce a solar cell with the highest performance as the front cell of a multi-junction cell, i.e., in the 300–550 nm wavelength range. Indeed, in this wavelength range, Jsc values in most of the cells on FTO substrates are higher than those in W-ZnO substrates. The cause of this improvement can be found in Fig. 5, where the external quantum efficiencies of some selected solar cells are given together with the absorption of the solar cells. Although the 3600-nm-thick FTO layers are much thicker than the 1500-nm-thick ZnO:B layers, the lower band gap and much higher optical absorption coefficient of the latter films completely block the 300–380 nm wavelength range. The SEM images in Fig. 3 show that the W-ZnO substrate contains more nano-order textures than the FTO substrate, and this reduces front-surface reflection. As a result, the cell on the W-ZnO substrate has a higher absorption of light and subsequently, a higher EQE in the 400–550 nm wavelength range. The EQE curves of cells on FTO substrates dominate over those on W-ZnO substrates in the 300–390 nm wavelength range. Calculations show that the Jsc value generated for this wavelength range is 0.385 mA/cm2 for the FTO-40-10 cell, whereas the same for a cell on the W-ZnO substrate is 0.131 mA/cm2. This 0.254 mA/cm2 gain in current density is small but not negligible. Ultimately, it helps to surpass the current density of a cell on the W-ZnO substrate in the 300–550 nm wavelength range. This happens despite the fact that a cell on the W-ZnO substrate has a lower front-surface reflection, as seen in Table 1 and Fig. 5. The cells on FTO substrates do not show much variation in their EQE curves in the 300–550 nm wavelength range. However, all of them have some kind of interference pattern in the 550–800 nm wavelength range. This is likely due to some of the light reflecting back to the air after reaching the back silver–aluminum electrode, which causes the ripple in the EQE curves. Thus, the FTO substrates with feature sizes in the micron-range are not suitable for proper light trapping for a 300-nm-thick single junction a-Si solar cell. However, in the ultimate multi-junction solar cell, a microcrystalline

A. Chowdhury et al. / Solar Energy Materials & Solar Cells 140 (2015) 126–133

40 50 0.936

1.0

150

131

200

250

300

350

Voc (V)

0.8

0.928 0.924 0.920 0.72

0.6 FF

0.71

Substrate FTO-20-7 FTO-40-10 W-ZnO:B FTO-Flat

400

500

600

2

700

Wavelength (nm) Fig. 5. External quantum efficiency of selected solar cells on FTO and W-ZnO substrates.

bottom cell will cover the 550–1100 nm wavelength range; therefore, this lack of absorption in the 550–800 nm range is not a matter of concern in the present work. The top cell in a multi-junction solar cell should have higher Jsc for the lower wavelength region of the AM1.5G spectrum. The top cell should also have a lower front reflection in the total wavelength region of the AM1.5G spectrum. In other words, the goal of the present study is limited to improving the current density of solar cells in the 300–550 nm wavelength range while maintaining good Voc and modest FF values. The external quantum efficiency and optical absorption of the solar cells on FTO substrates change completely when flat glass substrate is used in place of textured glass substrate. The macrotextured glass substrate reduces the front reflection by around 10% which can be seen in Fig. 5. The interference pattern in light absorption is more prominent for cell on FTO-flat substrate compared to textured FTO substrate for wavelengths over 550 nm. The threshold of interference pattern with respect to photon wavelength starts quite early around 500 nm for flat glass substrate. This suggests that the macro-texture of the glass substrate has some role to play regarding light confinement in case of single textured 300 nm thick a-Si solar cell. This finding matches quite well with the haze value difference of FTO-flat and other textured FTO substrates in 400–700 nm wavelength range. For near infrared wavelength range, use of double textured FTO substrate is must for multi-junction solar cell as the haze value is quite poor for FTO-flat substrate. Fig. 6 shows the variations of Voc, FF, Jsc, and Jsc in the 300– 550 nm wavelength range with respect to the RMS roughness of the substrate prepared at different RIE etching times and chamber pressures. Fig. 6C gives the values of the short circuit current density for the total 300–800 nm wavelength range with respect to the RMS roughness of different substrates. A maximum Jsc of 15.27 mA/cm2 is obtained for a cell on the FTO-40-10 substrate. The general tendency of increasing Jsc with increasing RMS roughness can be seen in most cases. This directly indicates the general positive effect of a higher substrate roughness on light confinement. However, there are some cases, such as the cell on FTO-40-15 substrate, where Jsc does not increase with an increase in substrate roughness. The cause of the reduction of Jsc is not clear. Fig. 6C and D illustrates that the change in the total Jsc is dominated by Jsc in the 550–800 nm range and the contribution of Jsc in the 300–550 nm range is small. Table 1 shows that the optimum value of Jsc in the 300–550 nm wavelength range is obtained for an RMS roughness of approximately 200 nm. This is

FTO-40-15

14.8 14.4 14.0 7.3

800 Jsc in 300-550 nm

0.0 300

FTO-40-10

15.2

Jsc

0.2

0.70 0.69

(mA/cm )

0.4

7.2

(mA/cm2)

EQE (%) and Cell absorption

0.932

7.1

Etching time 20 min 40 min 80 min

7.0 6.7 6.6 40 50

150

200

250

300

350

RMS roughness (nm) Fig. 6. Effect of substrate roughness variation by changing RIE chamber pressure on solar cell parameters: (A) Voc, (B) FF, (C) Jsc, and (D) Jsc in the 300–550 nm wavelength range. Red circles indicate the Y-axis values for flat FTO substrates with RMS roughness of 46.2 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

expected as higher roughness not only reduces front-surface reflection but also increases the optical loss in the TCO as well as in the p-layer. Furthermore, while deposition time for the p-layer is kept constant, the effective thickness of the p-layer may decrease below a certain threshold value as the substrate surface roughness increases. Fig. 6A shows that this thinning of the p-layer and the increase in the RMS roughness in turn reduce the effective electric field and carrier collection in the p- and n-layers. The variations in Voc and FFs of the a-Si solar cells are given in Fig. 6A and B. A maximum Voc of 0.934 V is obtained from solar cells on smoother FTO substrates. However, Voc of cells reduces monotonically with an increase in surface roughness. As mentioned earlier, higher surface roughness may adversely affect the electric field inside the cell as well as Voc. However, it is difficult to calculate the actual thickness of the p-layer in each case and compensate for the thickness loss during deposition. It can be seen that the Voc and FF are very similar for cells on FTO-flat and FTO20-7 substrate. This loosely indicates that the Voc and FF start to drop when RMS roughness of FTO substrate become larger than 200 nm. Sakai et al. [24] reported the reduction of open circuit voltage due to defective zones in absorber layer for highly textured substrate. In the present study, this is also a possibility. A high Voc is always desired in solar cells as it creates a higher electric field inside the cell and improves the carrier collection. In addition, the FFs of the solar cells monotonically reduce as the RMS roughness increases. This may be explained by a longer charge carrier travel path and higher recombination centers, which are evolved due to a non-perpendicular electric field with a lower magnitude. Fig. 7A and B shows the variations of the external and internal quantum efficiencies with respect to the RMS roughness of the substrates at a wavelength of 400 nm. The EQE and IQE have optimum high values around an RMS roughness of 225 nm. For higher RMS roughness values, the IQE drops slightly, but the EQE remains

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EQE at 400 nm

132

40 50 0.68

150

200

250

300

350

0.66 0.64 0.62

IQE at 400 nm

0.60 0.74 0.72 0.70

Adv. reflection in 650-750 nm (%)

Adv. reflection in 400-500 nm (%)

0.68 16 14 10 9 8 7 32 30 28 26 24 22 20 18 16

Etching time 20 min 40 min 80 min

40 50

150

200

250

300

be similar to or slightly larger than the thickness of the absorber layer for a single junction cell. Conversely, the present work indicates that for a top cell, the feature size is only important to reduce the front-surface reflection. Although feature size in the micronorder is not very important for light confinement in an a-Si top cell of a multi-junction solar cell, it plays an important role during the light confinement in the bottom cell. To decouple the effect of glass texturization and FTO material texturization, the samples on FTO Flat substrate gave some important clues. It indicates that the mobility and sheet resistance did not get too much deteriorated with glass texturization. The same is also true for Voc and FF of the solar cells. From Fig. 5 it is also proved that glass texturization plays a vital role in reducing the front reflection. The performance of an a-Si top cell in the 300–550 nm wavelength range is linked with a higher substrate roughness in terms of reduced front reflection and increased shunting. The optimum RMS substrate roughness value is found to be approximately 200 nm. It is also found that this FTO substrate in its present state is better in 300–550 nm wavelength range than the W-ZnO substrate previously used by our group.

4. Conclusions

350

RMS roughness (nm) Fig. 7. Effect of substrate roughness variation by changing RIE chamber pressure on different parameters: (A) EQE, (B) IQE, (C) average reflection in the 400–500 nm wavelength range, and (D) average reflection in the 650–750 nm wavelength range. Red circles indicate the Y-axis values for flat FTO substrates with RMS roughness of 46.2 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

constant. Analyzing both the EQE and IQE, it can be seen that the effects of the p-layer and the FTO layer absorption for solar cells on a highly textured substrate are not negligible. The cause of the lower IQE for substrates with RMS roughness values below 180 nm cannot be definitively explained, although the absorption in the textured FTO layer likely plays a vital role. Fig. 7C and D shows the variations of the average reflectance of completed solar cells in the 400–500 nm and 650–750 nm wavelength ranges. As expected, it can be seen that the reflections monotonically reduce as substrate roughness increases and maximum reflection is observed for solar cell on FTO-flat substrate. Overall, the justification of describing cell performance in terms of the RMS roughness of the substrate is important but complex, because it is a multi-scale textured substrate and AFM only gives an average roughness value. In principle, RMS roughness is determined by the difference in height and does not include any information about the lateral size. However, as seen from the images, most data points follow a trend and more importantly, Fig. 2A and B shows very similar trend. This indicates the effectiveness and justification of correlating the cell performance with the RMS roughness of the substrates. At higher chamber pressure, the effect of the RIE etching time on substrate morphology is not clear in terms of AFM-measured RMS roughness values or optical haze values, which do not produce clear trends. FFs of the cells seem to be low (o0.72) in comparison to the ideal value (40.74). However, it may still be satisfactory as the substrate texturization height is quite high. The presence of a very thin ZnO layer over the FTO substrate may prevent some damage to the FTO layer during p-layer deposition by VHF power. However, it will come at the expense of further light absorption in the 300–380 nm wavelength range and reduced Voc. There are reports [7,8] that the substrate texture feature size should

The new LPCVD-grown multi-scale textured FTO substrate has proved to be excellent in terms of optical transparency as well as carrier mobility. In comparison with its W-ZnO counterpart, it has an unmatched transparency for wavelengths below 400 nm and four times higher mobility. The glass substrate texturization not only reduces the front reflection, but also provides a way to texturize the FTO substrate. This substrate facilitates a higher current density in the 300–550 nm wavelength range and a higher Voc. The height of the texturization can reach four to five times the thickness of the a-Si absorber layer. The FFs of the cells (o0.72) are encouraging. Overall, this FTO/a-Si cell configuration is found to be suitable as a front cell of a multi-junction solar cell.

Acknowledgment This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI), Japan.

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