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Fabrication and characterization of zinc oxide anti-reflective coating on flexible thin film microcrystalline silicon solar cell
Optik 124 (2013) 5397–5400
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Fabrication and characterization of zinc oxide anti-reflective coating on flexible thin film microcrystalline silicon solar cell M.Z. Pakhuruddin ∗ , Y. Yusof, K. Ibrahim, A. Abdul Aziz Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 26 October 2012 Accepted 22 March 2013
Keywords: Zinc oxide Anti-reflective coating Thin film microcrystalline silicon solar cell X-ray diffraction Transmittance
a b s t r a c t This paper studies the fabrication and characterization of 80 nm zinc oxide anti-reflective coating (ARC) on flexible 1.3 m thin film microcrystalline silicon (c-Si) solar cell. High resolution X-ray diffraction (HR-XRD) shows a c-axis oriented ZnO (0 0 2) peak (hexagonal crystal structure) at 34.3◦ with full width at half maximum (FWHM) of 0.3936◦ . Atomic force microscope (AFM) measures high surface roughness root-mean-square (RMS) of the layer (50.76 nm) which suggests scattering of the incident light at the front surface of the solar cell. UV–vis spectrophotometer illustrates that ZnO ARC has optical transmittance of more than 80% in the visible and infra-red (IR) regions and corresponds to band gap (Eg ) of 3.3 eV as derived from Tauc equation. Inclusion of ZnO ARC successfully suppresses surface reflectance from the cell to 2% (at 600 nm) due to refractive index grading between the Si and the ZnO besides quarterwavelength (/4) destructive interference effect. The reduced reflectance and effective scattering effect of the incident light at the front side of the cell are believed to be the reasons why short-circuit current (Isc ) and efficiency () of the cell improve. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a wide band gap II–VI semiconductor material. Recently, ZnO has shown high potential for applications in optoelectronic devices such as solar cells, light emitting diodes (LED), laser diodes and acoustic–optical devices [1]. In solar cells, ZnO thin films are used as an anti-reflective coating (ARC) and transparent conductive oxide (TCO) due to its high optical transmittance in the visible light region, high band gap energy (Eg ∼ 3.3 eV), optimum refractive index (n ∼ 2.0) and natural n-type electrical conductivity [2,3]. Flexible polymer substrate like polyimide (PI) has attracted interests of many parties in photovoltaic (PV) research and development activities due to its flexibility, light-weight, low-cost, high temperature resistance, low coefficient of thermal expansion, low moisture uptake and high moisture release characteristics, high voltage endurance and scalable roll-to-roll deposition process [4,5]. Due to its superior properties, PI has found wide applications as substrates in flexible thin film solar cells, printed circuits and high density interconnects [6]. In this paper, the fabrication and characterization of ZnO ARC on flexible (on PI) thin film microcrystalline silicon (c-Si) solar cell are investigated. 80 nm ZnO ARC is RF-sputtered on 1.3 m thin film
∗ Corresponding author. Tel.: +60125370721. E-mail address: [email protected] (M.Z. Pakhuruddin). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.03.117
c-Si solar cell at room temperature and structural properties, surface morphology, optical and electrical properties of the sputtered layer and solar cell are studied. 2. Materials and methods In this experiment, 80 nm of ZnO ARC is deposited by 13.56 MHz RF sputtering system (Model: Edwards Auto 500) on 1.3 m thin film microcrystalline Si (c-Si) solar cell stacks with cell area of 4 cm2 . The stacks comprise of n-type Si/intrinsic c-Si/p-type cSi/Al/PI in substrate-orientation. The solar cell (sample) is mounted on substrate holder, separated by 7 cm from the ZnO target and kept at room temperature (300 K). 20 sccm of argon (Ar) plasma (99.99% Ar gas purity), ZnO sputtering target (99.99999% purity), 200 W of RF power and 10 nm/min deposition rate are used for the deposition. The sputtering chamber is evacuated down to base pressure of 1.8 × 10−5 Torr, and the ZnO target is firstly sputtered with shutter in close position (i.e. target and target are isolated) for 15 min to decontaminate the target. After that, the shutter is opened to start the deposition of ZnO ARC and the substrate holder being constantly rotated to ensure uniform film coverage. During the deposition, the deposition rate and thickness are controlled and measured using quartz crystal monitor (FTM-7). After the deposition, the sample is sent for characterizations. Structural properties of the sample are characterized by HR-XRD (Model: PANalytical X’Pert PRO MRD PW3040). Surface morphology is inspected on AFM system (Model: ULTRA Objective). Optical
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Fig. 1. HR-XRD pattern of 80 nm ZnO ARC on thin film c-Si solar cell.
Fig. 3. Optical transmittance of 80 nm ZnO ARC on thin film c-Si solar cell.
properties of the sample are studied on UV–vis spectrophotometer (Model: Hitachi U-2000). Film thickness and surface reflectance of the completed device (with and without the ZnO ARC layer) are measured by optical reflectometer system (Model: Filmetrics F20). I–V characteristics of illuminated thin film c-Si solar cells (fabricated with and without ZnO ARC) are measured by the solar simulator system (Model: Solar Simulator 1000 from Optical Radiation Corporation). For the illuminated I–V measurement, the 4 cm2 cell is placed under 220 W/m2 light intensity (Xenon Arc lamp) under AM1.5 (air mass) setting at 25 ◦ C and measured by the Leios IV Solar CT ONLINE (software) connected to Keithley source metre (Model: 2400).
deposition. The high roughness RMS suggests effective scattering of the incident light at the front surface of the solar cell which would help in increasing optical path length of the light inside the solar cell (i.e. optical thickness of the cell) [9]. A surface with a too high RMS roughness (typically more than 150 nm) is undesirable since it may cause the incident light, particularly of short wavelengths to be scattered at a very high angle into the ZnO ARC layer and get absorbed unnecessarily (within ZnO layer) instead of going into the cell absorber layer for photocurrent generation process [10]. Fig. 3 depicts the optical transmittance curve of 80 nm ZnO ARC on thin film c-Si solar cell, measured within spectral region of 200–1100 nm by the UV–vis spectrophotometer system. During the measurement, the solar cell on PI substrate is placed in the reference compartment in order to exclude its effects towards the transmittance value of the ZnO ARC. From this figure, it can be observed that the ZnO layer shows a cut-off wavelength at 380 nm and has very high optical transmittance, exceeding 80% in the visible and up to 85% in the IR region as published by other authors [11,12]. The high transmittance property until the end of the spectrum is highly desirable in thin film c-Si solar cells since the absorption profile of the absorber layer extends up to 1100 nm [13]. From the optical transmittance curve in Fig. 3, Fig. 4 is derived. For a direct band gap material like ZnO, relationship between the absorption coefficient (˛) and its band gap (Eg ) is given by Tauc equation [14] as the following:
3. Results and discussion The thickness of the sputtered ZnO is measured to be 80 nm by the optical reflectometer system. Fig. 1 illustrates the HR-XRD pattern of 80 nm ZnO ARC on thin film c-Si solar cell measured between 20◦ and 80◦ (2 theta). The diffraction pattern shows 6 peaks where 4 of them are attributed to the PI substrate: 22.1◦ , 25.9◦ , 44.6◦ and 64.9◦ . A low peak of Si (1 1 1) plane from the underlying c-Si thin film shows up at 28.8◦ . The peak at 34.3◦ belongs to c-axis oriented ZnO (0 0 2) crystal plane with hexagonal crystal structure. FWHM of the peak is 0.3936◦ , corresponding to crystallite size of 21.1 nm as calculated by Scherrer’s formula [7]. The small FWHM of the ZnO (0 0 2) peak indicates a good crystalline quality of the sample [8]. Fig. 2 shows the AFM image of 80 nm ZnO ARC on thin film c-Si solar cell measured with 30 m × 30 m spot size. From the figure, a high surface roughness RMS of the layer is evident (50.76 nm) due to texturing effects on the PI substrate before the cell
Fig. 2. AFM image of 80 nm ZnO ARC on thin film c-Si solar cell (30 m × 30 m spot size).
n
(˛hv) = B(hv − Eg )
(1)
where; ˛ is the absorption coefficient (cm−1 ); hv is the photon energy (eV); n = 2 (implies direct optical transition in ZnO); B is the constant; Eg is the optical band gap of the film (eV). From this relationship, (˛hv)2 is plotted as a function of photon energy (hv). The Eg of the ZnO ARC is then determined by
Fig. 4. Plot of (˛hv)2 as a function of photon energy (hv) of 80 nm ZnO ARC on thin film c-Si solar cell.
M.Z. Pakhuruddin et al. / Optik 124 (2013) 5397–5400
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Table 1 Summary of I–V characteristics of thin film c-Si solar cells on PI substrate with and without 80 nm ZnO ARC. Cell
Cell designation
Isc (mA)
Voc (mV)
Imax (mA)
Vmax (mV)
F.F (%)
(%)
1 2
Without ZnO ARC With ZnO ARC
6.11 7.14
372 390
4.43 4.85
0.26 0.30
50.7 52.3
1.31 1.65
tested by Solar Simulator 1000 under 220 W/m2 light illumination at AM1.5 condition and temperature of 25 ◦ C. Fig. 6 plots the illuminated I–V curves of 1.3 m thin film cSi solar cells on PI substrate with and without 80 nm ZnO ARC. From the curves, it can be seen that without the ZnO ARC, the cell measures Isc of 6.11 mA and 1.31% efficiency. With the ARC, the Isc increases to 7.14 mA and this is believed to be due to reduced reflectance in the visible region (400–700 nm as previously depicted by Fig. 5) and effective scattering effect of the incident light at the surface of ZnO ARC. Voc shows a slight increase of 18 mV and cell efficiency increases to 1.65% due to the improvement in both Isc and Voc . 4. Conclusions Fig. 5. Surface reflectance of thin film c-Si solar cell on PI substrate with and without 80 nm ZnO ARC.
extrapolating the linear part of the curve to an interception at the x-axis where the (˛hv)2 = 0 (as shown by Fig. 4). From the linear extrapolation, the Eg of the ZnO window layer is determined to be ∼3.3 eV. This value shows a good corroboration with other published papers [2,11]. The high Eg of the 80 nm ZnO ARC is why it appears highly transparent to most of the incident photons. Fig. 5 shows the surface reflectance of the thin film c-Si solar cells on PI with and without 80 nm ZnO ARC. Without the ARC, the Si surface already shows a low reflectance (around 12% average) over the whole spectrum (400–1030 nm) since it is already textured. With the ZnO ARC on top of the cell, the surface reflectance is further suppressed to the minimum (down to only 2%) especially at the wavelength of 600 nm due to refractive index grading between the n-type Si and the ZnO ARC and quarter-wavelength coating thickness (/4) of the ZnO adopted in this study. With the quarter-wavelength thickness of the ZnO layer (at 80 nm), a completely destructive interference takes place at 600 nm (red light region) between the light reflected from the front and rear sides of ZnO. The maximum suppression is chosen at 600 nm since the solar spectrum peaks at this point [15]. Table 1 summarizes the short-circuit current (Isc ), maximum current (Imax ), open-circuit voltage (Voc ), maximum voltage (Vmax ), fill factor (F.F) and efficiency () of thin film c-Si solar cells on PI substrate with and without 80 nm ZnO ARC. Each cell (4 cm2 ) is
Fig. 6. I–V curves of illuminated (AM 1.5, 220 W/m2 , 25 ◦ C) thin film c-Si solar cells on PI substrate with and without 80 nm ZnO ARC.
In this paper, the fabrication and characterization of 80 nm ZnO ARC on flexible 1.3 m thin film microcrystalline silicon (c-Si) solar cell are studied. HR-XRD shows a high quality of c-axis oriented ZnO (0 0 2) peak (hexagonal crystal structure) at 34.3◦ with FWHM of 0.3936◦ . AFM measures 50.76 nm roughness RMS of the layer, suggesting scattering of the incident light at the front surface of the solar cell. UV–vis spectrophotometer illustrates that ZnO ARC has high optical transmittance, exceeding 80% in the visible and IR regions and corresponds to Eg ∼ 3.3 eV. Due to refractive index grading between the Si–ZnO and quarter-wavelength (/4) destructive interference effect, ZnO ARC reduces the cell surface reflectance to 2% (at 600 nm). Solar cell Isc and improve due to reduced reflectance and effective scattering effect of the incident light at the front side of the cell. Acknowledgements The support of Universiti Sains Malaysia and financial assistance from Incentive Grant 1001/PFIZIK/821061 is gratefully acknowledged. Thanks to DuPont Corporation (Malaysia and Singapore) for contributing the PI samples for this work. References [1] K. Ellmer, A. Klein, B. Rech, Transparent Conductive Zinc Oxide, Springer, Berlin, 2008. [2] J. Müller, G. Schöpe, O. Kluth, B. Rech, V. Sittinger, B. Szyszka, R. Geyer, P. Lechner, H. Schade, M. Ruske, G. Dittmar, H.-P. Bochem, State-of-the-art midfrequency sputtered ZnO films for thin film silicon solar cells and modules, Thin Solid Films 442 (2003) 158–162. [3] J. Müller, O. Kluth, S. Wieder, H. Siekmann, G. Schöpe, W. Reetz, O. Vetterl, D. Lundszien, A. Lambertz, F. Finger, B. Rech, H. Wagner, Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates, Sol. Energy Mater. Sol. Cells 66 (2001) 275–281. [4] H. Cai, D. Zhang, Y. Xue, K. Tao, Study on diffusion barrier layer of silicon-based thin-film solar cells on polyimide substrate, Sol. Energy Mater. Sol. Cells 93 (2009) 1959–1962. [5] P.K. Shetty, N.D. Theodore, J. Ren, J. Menendez, H.C. Kim, E. Misra, J.W. Mayer, T.L. Alford, Formation and characterization of silicon films on flexible polymer substrates, Mater. Lett. 59 (2005) 872–875. [6] A. Pecora, L. Maiolo, M. Cuscunà, D. Simeone, A. Minotti, L. Mariucci, G. Fortunato, Low-temperature polysilicon thin film transistors on polyimide substrates for electronics on plastic, Solid-State Electron. 52 (2008) 348–352. [7] J.I. Langford, A.J.C. Wilson, Scherrer after sixty years: a survey and some new results in the determination of crystallite size, J. Appl. Crystallogr. 11 (1978) 102–113. [8] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour, Thin Solid Films 442 (2003) 80–85.
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[9] M.A. Green, Silicon Solar Cells: Advanced Principles and Practice, Centre for Photovoltaic Devices and Systems, University of New South Wales, Sydney, 1995. [10] J. Nelson, The Physics of Solar Cells, Imperial College Press, London, 2003. [11] A. Limmanee, P. Krudtad, S. Songtrai, C. Piromjit, J. Sritharathikhun, K. Sriprapha, ZnO back reflector prepared by MOCVD technique for flexible solar cell applications, Curr. Appl. Phys. 11 (2011) S206–S209.
[12] J. Müller, B. Rech, J. Springer, M. Vanecek, TCO and light trapping in silicon thin film solar cells, Solar Energy 77 (2004) 917–930. [13] A. Dussan, R.H. Buitrago, R.R. Koropecki, Microcrystalline silicon thin films: a review of physical properties, Microelectron. J. 39 (2008) 1292–1295. [14] G.P. Smestad, Optoelectronics of Solar Cells, SPIE Press, Washington, 2002. [15] S.R. Wenham, M.A. Green, M.E. Watt, R. Corkish, Applied Photovoltaics, second ed., Earthscan, London, 2007.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Fabrication and characterization of zinc oxide anti-reflective coating on flexible thin film microcrystalline silicon solar cell M.Z. Pakhuruddin ∗ , Y. Yusof, K. Ibrahim, A. Abdul Aziz Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 26 October 2012 Accepted 22 March 2013
Keywords: Zinc oxide Anti-reflective coating Thin film microcrystalline silicon solar cell X-ray diffraction Transmittance
a b s t r a c t This paper studies the fabrication and characterization of 80 nm zinc oxide anti-reflective coating (ARC) on flexible 1.3 m thin film microcrystalline silicon (c-Si) solar cell. High resolution X-ray diffraction (HR-XRD) shows a c-axis oriented ZnO (0 0 2) peak (hexagonal crystal structure) at 34.3◦ with full width at half maximum (FWHM) of 0.3936◦ . Atomic force microscope (AFM) measures high surface roughness root-mean-square (RMS) of the layer (50.76 nm) which suggests scattering of the incident light at the front surface of the solar cell. UV–vis spectrophotometer illustrates that ZnO ARC has optical transmittance of more than 80% in the visible and infra-red (IR) regions and corresponds to band gap (Eg ) of 3.3 eV as derived from Tauc equation. Inclusion of ZnO ARC successfully suppresses surface reflectance from the cell to 2% (at 600 nm) due to refractive index grading between the Si and the ZnO besides quarterwavelength (/4) destructive interference effect. The reduced reflectance and effective scattering effect of the incident light at the front side of the cell are believed to be the reasons why short-circuit current (Isc ) and efficiency () of the cell improve. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a wide band gap II–VI semiconductor material. Recently, ZnO has shown high potential for applications in optoelectronic devices such as solar cells, light emitting diodes (LED), laser diodes and acoustic–optical devices [1]. In solar cells, ZnO thin films are used as an anti-reflective coating (ARC) and transparent conductive oxide (TCO) due to its high optical transmittance in the visible light region, high band gap energy (Eg ∼ 3.3 eV), optimum refractive index (n ∼ 2.0) and natural n-type electrical conductivity [2,3]. Flexible polymer substrate like polyimide (PI) has attracted interests of many parties in photovoltaic (PV) research and development activities due to its flexibility, light-weight, low-cost, high temperature resistance, low coefficient of thermal expansion, low moisture uptake and high moisture release characteristics, high voltage endurance and scalable roll-to-roll deposition process [4,5]. Due to its superior properties, PI has found wide applications as substrates in flexible thin film solar cells, printed circuits and high density interconnects [6]. In this paper, the fabrication and characterization of ZnO ARC on flexible (on PI) thin film microcrystalline silicon (c-Si) solar cell are investigated. 80 nm ZnO ARC is RF-sputtered on 1.3 m thin film
∗ Corresponding author. Tel.: +60125370721. E-mail address: [email protected] (M.Z. Pakhuruddin). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.03.117
c-Si solar cell at room temperature and structural properties, surface morphology, optical and electrical properties of the sputtered layer and solar cell are studied. 2. Materials and methods In this experiment, 80 nm of ZnO ARC is deposited by 13.56 MHz RF sputtering system (Model: Edwards Auto 500) on 1.3 m thin film microcrystalline Si (c-Si) solar cell stacks with cell area of 4 cm2 . The stacks comprise of n-type Si/intrinsic c-Si/p-type cSi/Al/PI in substrate-orientation. The solar cell (sample) is mounted on substrate holder, separated by 7 cm from the ZnO target and kept at room temperature (300 K). 20 sccm of argon (Ar) plasma (99.99% Ar gas purity), ZnO sputtering target (99.99999% purity), 200 W of RF power and 10 nm/min deposition rate are used for the deposition. The sputtering chamber is evacuated down to base pressure of 1.8 × 10−5 Torr, and the ZnO target is firstly sputtered with shutter in close position (i.e. target and target are isolated) for 15 min to decontaminate the target. After that, the shutter is opened to start the deposition of ZnO ARC and the substrate holder being constantly rotated to ensure uniform film coverage. During the deposition, the deposition rate and thickness are controlled and measured using quartz crystal monitor (FTM-7). After the deposition, the sample is sent for characterizations. Structural properties of the sample are characterized by HR-XRD (Model: PANalytical X’Pert PRO MRD PW3040). Surface morphology is inspected on AFM system (Model: ULTRA Objective). Optical
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Fig. 1. HR-XRD pattern of 80 nm ZnO ARC on thin film c-Si solar cell.
Fig. 3. Optical transmittance of 80 nm ZnO ARC on thin film c-Si solar cell.
properties of the sample are studied on UV–vis spectrophotometer (Model: Hitachi U-2000). Film thickness and surface reflectance of the completed device (with and without the ZnO ARC layer) are measured by optical reflectometer system (Model: Filmetrics F20). I–V characteristics of illuminated thin film c-Si solar cells (fabricated with and without ZnO ARC) are measured by the solar simulator system (Model: Solar Simulator 1000 from Optical Radiation Corporation). For the illuminated I–V measurement, the 4 cm2 cell is placed under 220 W/m2 light intensity (Xenon Arc lamp) under AM1.5 (air mass) setting at 25 ◦ C and measured by the Leios IV Solar CT ONLINE (software) connected to Keithley source metre (Model: 2400).
deposition. The high roughness RMS suggests effective scattering of the incident light at the front surface of the solar cell which would help in increasing optical path length of the light inside the solar cell (i.e. optical thickness of the cell) [9]. A surface with a too high RMS roughness (typically more than 150 nm) is undesirable since it may cause the incident light, particularly of short wavelengths to be scattered at a very high angle into the ZnO ARC layer and get absorbed unnecessarily (within ZnO layer) instead of going into the cell absorber layer for photocurrent generation process [10]. Fig. 3 depicts the optical transmittance curve of 80 nm ZnO ARC on thin film c-Si solar cell, measured within spectral region of 200–1100 nm by the UV–vis spectrophotometer system. During the measurement, the solar cell on PI substrate is placed in the reference compartment in order to exclude its effects towards the transmittance value of the ZnO ARC. From this figure, it can be observed that the ZnO layer shows a cut-off wavelength at 380 nm and has very high optical transmittance, exceeding 80% in the visible and up to 85% in the IR region as published by other authors [11,12]. The high transmittance property until the end of the spectrum is highly desirable in thin film c-Si solar cells since the absorption profile of the absorber layer extends up to 1100 nm [13]. From the optical transmittance curve in Fig. 3, Fig. 4 is derived. For a direct band gap material like ZnO, relationship between the absorption coefficient (˛) and its band gap (Eg ) is given by Tauc equation [14] as the following:
3. Results and discussion The thickness of the sputtered ZnO is measured to be 80 nm by the optical reflectometer system. Fig. 1 illustrates the HR-XRD pattern of 80 nm ZnO ARC on thin film c-Si solar cell measured between 20◦ and 80◦ (2 theta). The diffraction pattern shows 6 peaks where 4 of them are attributed to the PI substrate: 22.1◦ , 25.9◦ , 44.6◦ and 64.9◦ . A low peak of Si (1 1 1) plane from the underlying c-Si thin film shows up at 28.8◦ . The peak at 34.3◦ belongs to c-axis oriented ZnO (0 0 2) crystal plane with hexagonal crystal structure. FWHM of the peak is 0.3936◦ , corresponding to crystallite size of 21.1 nm as calculated by Scherrer’s formula [7]. The small FWHM of the ZnO (0 0 2) peak indicates a good crystalline quality of the sample [8]. Fig. 2 shows the AFM image of 80 nm ZnO ARC on thin film c-Si solar cell measured with 30 m × 30 m spot size. From the figure, a high surface roughness RMS of the layer is evident (50.76 nm) due to texturing effects on the PI substrate before the cell
Fig. 2. AFM image of 80 nm ZnO ARC on thin film c-Si solar cell (30 m × 30 m spot size).
n
(˛hv) = B(hv − Eg )
(1)
where; ˛ is the absorption coefficient (cm−1 ); hv is the photon energy (eV); n = 2 (implies direct optical transition in ZnO); B is the constant; Eg is the optical band gap of the film (eV). From this relationship, (˛hv)2 is plotted as a function of photon energy (hv). The Eg of the ZnO ARC is then determined by
Fig. 4. Plot of (˛hv)2 as a function of photon energy (hv) of 80 nm ZnO ARC on thin film c-Si solar cell.
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5399
Table 1 Summary of I–V characteristics of thin film c-Si solar cells on PI substrate with and without 80 nm ZnO ARC. Cell
Cell designation
Isc (mA)
Voc (mV)
Imax (mA)
Vmax (mV)
F.F (%)
(%)
1 2
Without ZnO ARC With ZnO ARC
6.11 7.14
372 390
4.43 4.85
0.26 0.30
50.7 52.3
1.31 1.65
tested by Solar Simulator 1000 under 220 W/m2 light illumination at AM1.5 condition and temperature of 25 ◦ C. Fig. 6 plots the illuminated I–V curves of 1.3 m thin film cSi solar cells on PI substrate with and without 80 nm ZnO ARC. From the curves, it can be seen that without the ZnO ARC, the cell measures Isc of 6.11 mA and 1.31% efficiency. With the ARC, the Isc increases to 7.14 mA and this is believed to be due to reduced reflectance in the visible region (400–700 nm as previously depicted by Fig. 5) and effective scattering effect of the incident light at the surface of ZnO ARC. Voc shows a slight increase of 18 mV and cell efficiency increases to 1.65% due to the improvement in both Isc and Voc . 4. Conclusions Fig. 5. Surface reflectance of thin film c-Si solar cell on PI substrate with and without 80 nm ZnO ARC.
extrapolating the linear part of the curve to an interception at the x-axis where the (˛hv)2 = 0 (as shown by Fig. 4). From the linear extrapolation, the Eg of the ZnO window layer is determined to be ∼3.3 eV. This value shows a good corroboration with other published papers [2,11]. The high Eg of the 80 nm ZnO ARC is why it appears highly transparent to most of the incident photons. Fig. 5 shows the surface reflectance of the thin film c-Si solar cells on PI with and without 80 nm ZnO ARC. Without the ARC, the Si surface already shows a low reflectance (around 12% average) over the whole spectrum (400–1030 nm) since it is already textured. With the ZnO ARC on top of the cell, the surface reflectance is further suppressed to the minimum (down to only 2%) especially at the wavelength of 600 nm due to refractive index grading between the n-type Si and the ZnO ARC and quarter-wavelength coating thickness (/4) of the ZnO adopted in this study. With the quarter-wavelength thickness of the ZnO layer (at 80 nm), a completely destructive interference takes place at 600 nm (red light region) between the light reflected from the front and rear sides of ZnO. The maximum suppression is chosen at 600 nm since the solar spectrum peaks at this point [15]. Table 1 summarizes the short-circuit current (Isc ), maximum current (Imax ), open-circuit voltage (Voc ), maximum voltage (Vmax ), fill factor (F.F) and efficiency () of thin film c-Si solar cells on PI substrate with and without 80 nm ZnO ARC. Each cell (4 cm2 ) is
Fig. 6. I–V curves of illuminated (AM 1.5, 220 W/m2 , 25 ◦ C) thin film c-Si solar cells on PI substrate with and without 80 nm ZnO ARC.
In this paper, the fabrication and characterization of 80 nm ZnO ARC on flexible 1.3 m thin film microcrystalline silicon (c-Si) solar cell are studied. HR-XRD shows a high quality of c-axis oriented ZnO (0 0 2) peak (hexagonal crystal structure) at 34.3◦ with FWHM of 0.3936◦ . AFM measures 50.76 nm roughness RMS of the layer, suggesting scattering of the incident light at the front surface of the solar cell. UV–vis spectrophotometer illustrates that ZnO ARC has high optical transmittance, exceeding 80% in the visible and IR regions and corresponds to Eg ∼ 3.3 eV. Due to refractive index grading between the Si–ZnO and quarter-wavelength (/4) destructive interference effect, ZnO ARC reduces the cell surface reflectance to 2% (at 600 nm). Solar cell Isc and improve due to reduced reflectance and effective scattering effect of the incident light at the front side of the cell. Acknowledgements The support of Universiti Sains Malaysia and financial assistance from Incentive Grant 1001/PFIZIK/821061 is gratefully acknowledged. Thanks to DuPont Corporation (Malaysia and Singapore) for contributing the PI samples for this work. References [1] K. Ellmer, A. Klein, B. Rech, Transparent Conductive Zinc Oxide, Springer, Berlin, 2008. [2] J. Müller, G. Schöpe, O. Kluth, B. Rech, V. Sittinger, B. Szyszka, R. Geyer, P. Lechner, H. Schade, M. Ruske, G. Dittmar, H.-P. Bochem, State-of-the-art midfrequency sputtered ZnO films for thin film silicon solar cells and modules, Thin Solid Films 442 (2003) 158–162. [3] J. Müller, O. Kluth, S. Wieder, H. Siekmann, G. Schöpe, W. Reetz, O. Vetterl, D. Lundszien, A. Lambertz, F. Finger, B. Rech, H. Wagner, Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates, Sol. Energy Mater. Sol. Cells 66 (2001) 275–281. [4] H. Cai, D. Zhang, Y. Xue, K. Tao, Study on diffusion barrier layer of silicon-based thin-film solar cells on polyimide substrate, Sol. Energy Mater. Sol. Cells 93 (2009) 1959–1962. [5] P.K. Shetty, N.D. Theodore, J. Ren, J. Menendez, H.C. Kim, E. Misra, J.W. Mayer, T.L. Alford, Formation and characterization of silicon films on flexible polymer substrates, Mater. Lett. 59 (2005) 872–875. [6] A. Pecora, L. Maiolo, M. Cuscunà, D. Simeone, A. Minotti, L. Mariucci, G. Fortunato, Low-temperature polysilicon thin film transistors on polyimide substrates for electronics on plastic, Solid-State Electron. 52 (2008) 348–352. [7] J.I. Langford, A.J.C. Wilson, Scherrer after sixty years: a survey and some new results in the determination of crystallite size, J. Appl. Crystallogr. 11 (1978) 102–113. [8] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour, Thin Solid Films 442 (2003) 80–85.
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