- Email: [email protected]
Enhancement of GaAs solar cell performance by using a ZnO sol–gel anti-reflection coating
Solar Energy Materials & Solar Cells 123 (2014) 178–182
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Enhancement of GaAs solar cell performance by using a ZnO sol–gel anti-reflection coating Y.F. Makableh n, R. Vasan, J.C. Sarker, A.I. Nusir, S. Seal, M.O. Manasreh Department of Electrical Engineering, 3217 Bell Engineering Center, University of Arkansas, Fayetteville, AR 72701, United States
art ic l e i nf o
a b s t r a c t
Article history: Received 6 May 2013 Received in revised form 14 November 2013 Accepted 5 January 2014 Available online 1 February 2014
The performance of a GaAs p–n junction solar cell was investigated by coating the device with 110 nm thick ZnO sol–gel anti-reflection film. A post-furnace thermal annealing at 150 1C for 30 min was performed on the ZnO film after it was spin coated on the device with a speed of 8000 rpm. Ellipsometry was used to measure the reflectance, thickness, and the refractive index of the ZnO film. The solar cell performance was investigated by using the current–voltage technique from which the power conversion efficiency was extracted. The spectral response and quantum efficiency were also measured for the solar cell. An enhancement, after utilizing the ZnO anti-reflection coating, was observed on the order of 32%, 38, and 51% for the power conversion efficiency, spectral response, and quantum efficiency, respectively. & 2014 Elsevier B.V. All rights reserved.
Keywords: GaAs solar cells Anti-reflection coating ZnO sol–gel Performance enhancement
1. Introduction In recent years, fabricating high efficient solar cells is an essential part of the core research in the photovoltaic field [1,2]. Amongst different solar cell materials and structures, GaAs solar cells are receiving more attention worldwide [3] even though the multi-junction solar cells diluted the interest in GaAs single junction solar cells. The theoretical limit of the power conversion efficiency of GaAs solar cells is about 33%, which is due to their ability to generate high currents and voltages [4]. Despite this high theoretical power conversion efficiency limit, their performance did not reach its peak levels due to the relatively high refractive index [5–7] (n¼ 3.7), which reflects more than 35% of the incident light [8]. Several attempts were made to improve on the GaAs solar cell performance, such as by using the plasmonic effect, [8,9] surface texturing, [10] and the anti-reflection coating effect [10–12]. Among these attempts, the enhancement due to antireflection coatings is gaining more attention [12–14]. Several materials were used as anti-reflection coatings. Among them is ZnO, which appears to be an ideal anti-reflection film in case of GaAs based devices. This is due to the low value of the refractive index [15] and its wide band gap of 3.30 eV, which makes it transparent in the visible spectral region [16,17]. Zinc oxide films were reported to be synthesized by using different techniques, such as the sol–gel method [18], thermal evaporation [19], and chemical vapor deposition [20]. The sol–gel synthesis of
n
Corresponding author. E-mail address: [email protected] (Y.F. Makableh).
0927-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2014.01.007
ZnO has many advantages over other techniques as it is of low cost, less toxic and easy to implement to produce high quality films [18–21]. Furthermore, ZnO was used as a single layer antireflection coating for GaAs and Si solar cells with an enhancement in the device performance equivalent to or better than multi-layer coating schemes [5,11,22,23]. In this article, we report on the enhancement of the performance of GaAs p–n junction solar cells due to the addition of ZnO anti-reflection coating. The enhancement is observed in the characteristic properties of the solar cell, namely, in the power conversion efficiency (η), the spectral response, and the external quantum efficiency (EQE). The ZnO film was synthesized using the sol–gel method and was coated using a spin coating method. Low temperature furnace annealing was used to cure the ZnO films. The finished surfaces of the coated solar cells have bluish tint to them. The ZnO films were characterized by using the x-ray diffraction (XRD) and transmission and reflection spectroscopy. Ellipsometry was also used to measure the refractive index of the ZnO films in the entire visible spectral region. As expected, the anti-reflection coating reduces the reflection of the incident photons striking the surface of the GaAs solar cell leading to an increase in the photocarrier generation rate. The increase in the generation rate is the cause of the enhancement in the device performance. 2. Experimental details The GaAs solar cell structure was grown using the molecular beam epitaxy (MBE) technique. The structure was grown on a p-type substrate doped with [Zn] ¼1 1018 cm 3. The n-type side
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
3. Results and discussions
500
ZnO on GaAs ZnO
Intensity (a.u)
400 300
GaAs
ZnO
5.0
Refractive index
4.5 4.0 3.5 3.0 2.5
1.5 1.0
400
30 25
15 10 5 0 400
70
80
2θ (deg) Fig. 1. X-ray diffraction spectrum obtained for an annealed ZnO sol–gel film spin coated on a GaAs substrate.
450
500
550
600
650
700
750
800
Wavelength (nm) 65 60 55 50 45
800
60
Pristine GaAs GaAs with ZnO
20
30
50
800
35
100
40
700
40
35
30
600
45
200
20
500
Wavelength (nm)
40
0
GaAs substrate ZnO anti-reflection layer
2.0
Transmission %
Prior to coating with ZnO, the cell was etched using a 6% HCl solution for 20 s to remove any oxide layer from the solar cell surface. Then the solar cell was spin coated with the ZnO sol–gel solution with a speed of 8000 rpm for 30 s. Different spin coating speeds were tried, but 8000 rpm speed was found to be optimum for the device size used in this investigation. The ZnO film was annealed in ambient atmosphere at 150 1C for 30 min to evaporate the solvent (iso-propyl alcohol) and to cure the film. It has been noted that ZnO films were formed after annealing at 150 1C, as compared to other reported results [24,25] that used annealing temperatures more than 400 1C. The low annealing temperature is essential to avoid breaking the p–n junction. The presence of ZnO was confirmed by using XRD measurements as shown in Fig. 1. The
peaks observed at 381 and 441 are typical of ZnO peaks, and the broader peak at 661 is the GaAs peak. The GaAs peak is smaller and broader since the measurement was performed at an offset angle of 31 in order to avoid the lattice points of the GaAs crystal.
Reflectance (%)
of the GaAs was doped with [Si]¼2 1018 cm 3. A 100 nm n-type Al0.85Ga0.15As barrier was inserted in the cap layer and 100 nm ptype Al0.20Ga0.80As barrier was inserted in the buffer layer. These AlGaAs barriers are introduced as fence barriers to reduce the charge trapping effect. The high resolution optical profiler test revealed a high smooth surface, which was expected for the MBE grown structures. The sol–gel method [11] was followed to synthesize the ZnO film where zinc acetate dihydrate ( 499%) was used as a precursor and mono-ethanolamine was used as a stabilizer. In this method iso-propyl alcohol was used as a solvent medium. Throughout the synthesis, the solution mixture was kept on a hot plate at 50 1C and stirred constantly until a clear solution was obtained. The solution was then aged for 24 h before use. The solar cells were fabricated in a class 100 clean room using the standard photolithography procedure. The n-type and p-type ohmic contacts were formed using an e-beam deposition technique. The n-type metal contact consisted of AuGe/Ni/Au and the p-type metal contact consisted of Au/Zn/Au. The substrate temperature was held at 100 1C in the Angstrom Nexdrep e-beam chamber during the deposition of the metals. Each metal has a different calibration deposition rate. The power conversion efficiency was measured by using Newport, model 91291-100, 1.0 sun (1000 W/m2) AM 1.5G solar simulator in conjunction with a Keithley 4200 semiconductor characterization system. The spectral response measurements were recorded by using a Bruker FS 125HR Fourier-transform spectrometer and the EQE spectra were obtained by using an Oriel IQE 200 monochromator. All measurements were performed at room temperature.
179
Pristine GaAs GaAs with ZnO
1200
1600 2000 Wavelength (nm)
2400
2800
Fig. 2. The optical characteristics of ZnO film were obtained by using different experimental techniques. (a) Refractive index of ZnO layer grown on a GaAs substrate plotted along the refractive index of pristine GaAs substrate. (b) Reflectance spectra measured by using an ellipsometer and plotted for a GaAs substrate before and after spin-coated with ZnO film. (c) Transmission spectra of GaAs measured before and after spin-coated with ZnO film.
30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 0.0
Dark Current Without ZnO, FF = 0.69, η = 10.5% With ZnO, FF = 0.69, η = 14% (0.72, 51) Jsc= 57.4 mA/cm
Jsc= 76.9 mA/cm 0.2
2
(0.72, 68)
2
0.4
Voc= 0.93 V
2
Current Density (mA/cm )
0.6
0.8
1.0
Voltage (V) Fig. 3. The I–V characteristic of GaAs solar cell measured in the dark (black line) and under the illumination of 3.5 sun AM 1.5G solar simulator. The measurements we made before (blue line) and after (red line) spin-coating the solar cell with ZnO thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
30 20 GaAs pn junction solar cells coated with ZnO at different speeds 10 0 Dark Current -10 Pristine -20 Device A, coated at 6000 rpm -30 Device B, coated at 8000 rpm Device C, coated at 10000 rpm -40 Device D, coated at 12000 rpm -50 -60 -70 2 -80 Jsc= 76.8 mA/cm -90 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Voltage (V) Voc= 0.93 V
2
The refractive indices of both GaAs and ZnO film were measured using a J.A. Woolam VASE ellipsometer and plotted as a functional wavelength in Fig. 2(a). The optical reflectance spectra before and after depositing the ZnO thin film on top of GaAs substrate were also measured as shown in Fig. 2(b), while the transmission spectra were measured using a CARY 500 spectrometer as shown in Fig. 2(c). From the analysis of the ellipsometric measurements, the thickness of ZnO thin film was found to be about 110 nm and the average refractive index is about 1.65 in the spectral region between 400 and 900 nm. Both the reflectance and transmission spectra show the effect of anti-reflection coating after ZnO film was deposited on GaAs substrates. The reflectance is reduced from 33% to 3% at 650 nm as shown in Fig. 2(b), while the transmission is increased from 45% to 60% at 980 nm. Transmission above the band gap of GaAs could not be measured. The minimum reflectance achieved using ZnO sol–gel films (3% at 650 nm) is even smaller than the reflectance obtained by using ZnO nanostructures [26]. The current–voltage (I–V) characteristic of the GaAs solar cell is shown in Fig. 3 before and after coating the device with ZnO layer. The black curve represents the dark current, the blue and red curves represent the photocurrent before and after coating with ZnO, respectively. The short circuit current density increased from Jsc ¼57.4 mA/cm2 before coating to Jsc ¼76.9 mA/cm2 after coating with ZnO. On the other hand, the open circuit voltage is almost unchanged. The increase in the current density is thus the primary reason for the increase in η from η ¼10.5% before coating to η ¼14% after coating with ZnO, which is about 33% improvement. The present enhancement of 33% in the power conversion efficiency is higher than the reported enhancement using ITO nanostructures on GaAs solar cell [12] and equivalent to the enhancement of 32% obtained for GaAs solar cell by using a three-layer scheme antireflection coating [5]. However, the three-layer anti-reflection coating scheme for a Si solar cell produces a 39% enhancement in the power conversion efficiency [27]. The thickness of the ZnO sol–gel layer plays a major role in the power conversion efficiency of the solar cell since photon absorption and reflectivity depend on the thickness of the antireflection coating layer. Furthermore, the thickness of the antireflection layer using the spin coating technique depends on the device size [28]. Thus, the power conversion efficiency of the solar cell is affected by the thickness of the antireflection coating layer and the size of the device. The I–V characteristics of four devices coated with ZnO
Current Density (mA/cm )
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
Fig. 4. The I–V characteristics measured for four identical solar cells after spin coated with ZnO layer at different speeds.
30 27
Spectral response (arb. units)
180
24
Without ZnO With ZnO
21 18 15 12 9 6 3 0 400
500
600 700 800 Wavelength (nm)
900
1000
Fig. 5. The spectral response spectra were measured before (blue line) and after (red line) spin-coating the solar cell with ZnO thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
layers with different spin coating speeds, but a fixed speeding time (30 s) are plotted in Fig. 4. From this figure, the optimum power conversion efficiency is obtained for a speed of 8000 rpm as shown for device B. The spectral response measurements were performed for the solar cell before and after coating with ZnO. The measurements were made in the spectral region between 380 and 1000 nm as shown in Fig. 5. The response of the solar cell after coating with ZnO shows significant improvement in the entire spectral region with the maximum enhancement of 38% measured at 865 nm. The rapid decrease observed in the spectral response as the wavelength decreases is due to the beam-splitter response in the Fourier-transform spectrometer. This rapid decrease behavior was not observed in the EQE spectra as shown in Fig. 6 where the measurements were obtained by using a monochromator. The enhancement in EQE is quite significant after coating the device with ZnO layer, which is estimated to be about 51% at 830 nm. The step observed at 720 nm in the EQE spectra as shown in Fig. 6 is due to the change in the gratings. The quantum efficiency depends on the spectra response and therefore observing the behavior of the spectral response before and after coating the device with the ZnO antireflection layer provides a confirmation about the validity of the EQE spectral behavior.
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
degradation of the ohmic contacts. The ZnO layer was characterized by using XRD, reflectance, and transmission techniques. The thickness of the ZnO layer was optimized for the present device size by using different spin coating speeds. The enhancement of the solar cell was observed in all the I–V characteristics, power conversion efficiency, spectral response, and the external quantum efficiency. The most enhancement in the solar cell parameter was observed in the external quantum efficiency and found to be around 51%.
70 Without ZnO With ZnO
60 50
EQE (%)
181
40 30 20
Acknowledgments
10 0 400
500
600
700
800
900
1000
Wavelength (nm) Fig. 6. The external quantum efficiency spectra were measured before (blue line) and after (red line) spin-coating the solar cell with ZnO thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
This work was supported by the Air Force Office of Scientific Research (Grant no. FA9550-10-1-0136), the NSF-EPSCoR Program (Grant no. EPS-1003970), and NASA (Grant no. 2420261BBX11AQ36A). The authors would like to thank Scott Little for helping with the XRD measurements.
References It is apparent from the present results that ZnO sol–gel thin films are effective anti-reflection coating for GaAs solar cells as judged from the significant enhancement in the device performance, which includes the IV characteristic, the power conversion efficiency, the spectral response, and the external quantum efficiency. Zinc oxide films or nanostructures have been used as an antireflection coating as a single layer or part of anti-reflection schemes involve more than two dissimilar layers for different type of solar cells [29–36]. The achievement of enhancement in the device performance in the present results using ZnO sol–gel single layer is comparable to the best enhancement reported thus far. The significant enhancement in the present EQE of about 51% is quite remarkable for 110 nm thick ZnO layer knowing that the thin film was post-annealed at a low temperature (150 1C) for only 30 min. These annealing conditions are favorable of solar cell devices as compared to the high temperature annealing conditions, [12,23–39] which are typically performed at temperature higher than 300 1C. Additionally, the low annealing temperature condition is also important to avoid the degradation of the solar cell ohmic contact. For GaAs based solar cells, the refractive index for an optimum anti-reflective coating material should be in the range of (see for pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi example [40]) nARC ¼ no nGaAs , where nARC, no and nGaAs are the refractive indices of the anti-reflection coating film, air, and GaAs, respectively. The refractive index of ZnO of about 1.65 obtained from the ellipsometer lies near the optimum value of the best antireflection coating that can be used for GaAs. Thus, a simple scheme of a single layer of ZnO as the anti-reflection coating appears to provide enhancement equivalent to or better than the most complicated nanostructure schemes [28–39]. In addition to the significant enhancement of the solar cell performance, ZnO sol–gel layers do not require special equipment, highly toxic materials, or any complicated procedures to be deposited on top of solar cells. This will lead to simple and low cost anti-reflection coatings that can be implemented in the photovoltaic industry.
4. Conclusions In conclusion, a significant enhancement in the GaAs solar cell performance was achieved by utilizing a ZnO anti-reflection coating. The ZnO layer was grown by a sol–gel method and post-growth furnace annealed at a temperature low enough to void the breakdown of the p–n junction and prevent the
[1] M.A. Green, Third generation photovoltaics: ultra-high conversion efficiency at low cost, Prog. Photovolt.: Res. Appl. 9 (2001) 123–135, http://dx.doi.org/ 10.1002/pip.360. [2] A. Luque, A. Marti, A metallic intermediate band high efficiency solar cell, Prog. Photovolt.: Res. Appl. 9 (2001) 73–86, http://dx.doi.org/10.1002/pip.354. [3] K. Nakayama, K. Tanabe, H.A. Atwater, Plasmonic nanoparticle enhanced light absorption in GaAs solar cells, Appl. Phys. Lett. 93 (2008) 1904-1–1904-3, http://dx. doi.org/10.1063/1.2988288. [4] W. Shockley, H.H. Queisser, Detailed balance limit of efficiency of pn junction solar cells, J. Appl. Phys. 32 (1961) 510–519, http://dx.doi.org/10.1063/1.1736034. [5] L.K. Yeh, K.Y. Lai, G.J. Lin, P.H. Fu, H.H. Chang, C.A. Lin, J.H. He, Giant efficiency enhancement of GaAs solar cells with graded antireflection layers based on syringe like ZnO nanorods arrays, Adv. Energy Mater. 1 (2011) 506–510, http://dx.doi.org/ 10.1002/aenm.201100025. [6] J.B. Theeten, D.E. Aspens, R.P.H. Chang, A new resonant ellipsometric technique for characterizing the interface between GaAs and its plasma grown oxide, J. Appl. Phys. 49 (1978) 6097–6102, http://dx.doi.org/10.1063/1.324529. [7] D.T.F. Marple, Refractive index of GaAs, J. Appl. Phys. 35 (1964) 1241–1242, http://dx.doi.org/10.1063/1.1713601. [8] A.S. Hovhannisyan, Single-layer antireflection coating for GaAs solar cells, J. Contemp. Phys. 43 (2008) 136–138, http://dx.doi.org/10.3103/S1068337208030080. [9] J. Wu, Y.F.M. Makableh, R. Vasan, M.O. Manasreh, B. Liang, Strong interband transitions in InAs quantum dots solar cells, Appl. Phys. Lett. 100 (2012) 19071–1907-4, http://dx.doi.org/10.1063/1.3681360. [10] Y. Inomata, K. Fukui, K. Shirasawa, Surface texturing of large area multicrystalline silicon solar cells using reactive ion etching method, Sol. Energy Mater. Sol. Cells 48 (1997) 237–242, http://dx.doi.org/10.1016/S0927-0248(97)00106-2. [11] B.Y. Su, S.Y. Chu, Y.D. Juang, M.C. Lin, C.C. Chang, C.J. Wu, Efficiency enhancement of GaAs solar cells photovoltaic due to sol–gel derived anti-reflection AZO films, J. Electrochem. Soc. 159 (2012) H312–H316, http://dx.doi.org/10.1149/ 2.062203jes. [12] P. Yu, C.H. Chang, C.H. Chiu, C.S. Yang, J.C. Yu, H.C. Kuo, S.H. Hsu, Y.C. Chang, Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns, Adv. Mater. 21 (2009) 1618–1621, http://dx. doi.org/10.1002/adma.200802563. [13] Y.F. Makableh, R. Vasan, S. Lee, M.O. Manasreh, Enhancement of the performance of InAs quantum dots solar cell by using surface modification using poly-l-lysine homopolymers, Appl. Phys. Lett. 102 (2013) 1904-1–1904-3, http://dx.doi.org/10.1063/1.4789908. [14] S.L. Diedenhofen, G. Grzela, E. Haverkamp, G. Bauhuis, J. Schermer, J.G. Rivas, Broadband and omnidirectional anti-reflection layer for III/V multi-junction solar cells, Sol. Energy Mater. Sol. Cells 101 (2012) 308–314, http://dx.doi.org/ 10.1016/j.solmat.2012.02.022. [15] R.G. Heideman, P.B. Lambeck, J.G.E. Gardeniers, High quality ZnO layers with adjustable refractive indecies for integrated optics applications, Opt. Mater. 4 (1995) 741–755, http://dx.doi.org/10.1016/0925-3467(95)00028-3. [16] K.A. Salman, K. Omar, Z. Hassan, Effective conversion efficiency enhancement of solar cell using ZnO/PS antireflection coating layers, Sol. Energy 86 (2012) 541–547, http://dx.doi.org/10.1016/j.solener.2011.10.030. [17] Y.C. Chao, C.Y. Chen, C.A. Lin, J.H. He, Light scattering by nanostructured antireflection coatings, Energy Environ. Sci. 20 (2011) 3436–3441, http://dx.doi. org/10.1039/c0ee00636j. [18] S. Ilican, Y. Caglar, M. Caglar, Preparation and characterization of ZnO thin films deposited by sol–gel spin coating method, J. Optoelectr. Adv. Mater. 10 (2008) 2578–2583.
182
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
[19] B.D. Yao, Y.F. Chan, N. Wang, Formation of ZnO nanostructures by a simple way of thermal evaporation, Appl. Phys. Lett. 81 (2002) 757–759, http://dx.doi.org/ 10.1063/1.1495878. [20] A.J. Cheng, Y. Tzeng, Y. Zhou, M. Park, T.S. Wu, C. Shannon, D. Wang, W. Lee, Thermal chemical vapor deposition growth of zinc oxide nanostructures for dye-sensitized cell fabrication, Appl. Phys. Lett. 92 (2008) 2113-1–2113-3, http://dx.doi.org/10.1063/1.2889502. [21] L. Znaidi, Sol–gel deposited ZnO thin films: a review, Mater. Sci. Eng. 174 (2010) 18–30, http://dx.doi.org/10.1016/j.mseb.2010.07.001. [22] T. Minemoto, T. Mizuta, H. Takakura, Y. Hamakawa, Antireflective coating fabricated by chemical deposition of ZnO for spherical Si solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 191–194, http://dx.doi.org/10.1016/j. solmat.2006.08.005. [23] J.Y. Checn, K.W. Sun, Growth of vertically aligned ZnO nanorod arrays as antireflection layer on silicon solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 930–934, http://dx.doi.org/10.1016/j.solmat.2010.01.005. [24] M. Ohyama, H. Kozuka, T. Yoko, Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution, Thin Solid Films 306 (1997) 78–85, http://dx.doi.org/10.1016/S0040-6090(97) 00231-9. [25] O. Tari, A. Aronne, M.L. Addonizio, S. Daliento, E. Fanelli, P. Pernice, Sol–gel synthesis of ZnO transparent and conductive films: a critical approach, Sol. Energy Mater. Sol. Cells 105 (2012) 179–186, http://dx.doi.org/10.1016/j. solmat.2012.06.016. [26] Y.J. Lee, D.S. Ruby, D.W. Peters, B.B. McKenzie, J.W.P. Hsu, ZnO nanostructures as efficient antireflection layers in solar cells, Nano Lett. 8 (2008) 1501–1505, http://dx.doi.org/10.1021/nl080659j. [27] S.Y. Lien, D.S. Wuu, W.C. Yeh, J.C. Liu, Tri-layer antireflection coatings (SiO2/ SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique, Sol. Energy Mater. Sol. Cells 90 (2006) 2710–2719, http://dx.doi.org/10.1016/j.solmat. 2006.04.001. [28] B.D. Washo, Rheology and modeling of the spin coating process, IBM J. Res. Dev. 21 (1977) 190. [29] I. Moreno, J.J. Araiza, Thin-film spatial filters, Opt. Lett. 30 (2005) 914–916. [30] J.Y. Chen, and K.W. Sun, Enhancement of the light conversion efficiency of silicon solar cells by using nanoimprint anti-reflection layer, Sol. Energy Mater. Sol. Cells 94 (2010) 629-633. 10.1016/j.solmat.2009.11.028.
[31] S.S. Lo, C.C. Chen, F. Garwe, T. Pertch, Broad-band anti-reflection coupler for a: Si thin-film solar cell, J. Phys. D: Appl. Phys. 40 (2007) 754–758, http://dx.doi. org/10.1088/0022-3727/40/3/010. [32] N.K. Park, Y.S. Kim, J.H. Chang, D.H. Kim, S.O. Ryu, T.J. Lee, H.D. Kim, W.J. Lee, Application of ZnO single crystal rods as anti-reflection material for PV cells, Mol. Cryst. Liq. Cryst. 565 (2012) 68–77, http://dx.doi.org/10.1080/15421406. 2012.692235. [33] K.A. Salman, K. Omar, Z. Hassan, Improved performance of a crystalline silicon solar cell based on ZnO/PS anti-reflection coating layers, Superlattice Microstr. 50 (2011) 517–528, http://dx.doi.org/10.1016/j.spmi.2011.08.014. [34] A. Verma, F. Khan, D. Kumar, M. Kar, B.C. Chakravarty, S.N. Singh, M. Husain, Sol–gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells, Thin Solid Films 518 (2010) 2649–2654, http://dx.doi.org/10.1016/j.tsf.2009.08.010. [35] Y.C. Chao, C.Y. Chen, C.A. Lin, Y.A. Dai, J.H. He, Antireflection effect of ZnO nanorod arrays, J. Mater. Chem. 20 (2010) 8134–8138, http://dx.doi.org/10. 1039/c0jm00516a. [36] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Kcanc, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Properties of 19.2% efficiency ZnO/CdS/CuInGaSe2 thin-film solar cell, Prog. Photovolt: Res. Appl. 11 (2011) 225–230, http://dx.doi.org/10.1002/pip.494. [37] S.J. An, J.H. Chae, G.C. Yi, G.H. Park, Enhanced light output of GaN-based lightemitting diodes with ZnO nanorods arrays, Appl. Phys. Lett. 92 (2008) 11081–1108-3, http://dx.doi.org/10.1063/1.2903153. [38] J.K. Kim, S. Chhajed, M.F. Schubert, E.F. Schubert, A.J. Fischer, M.H. Crawford, J. Cho, H. Kim, C. Sone, Light-extraction enhancement of GaInN light-emitting diodes by graded-refractive-index indium tin oxide anti-reflection contact, Adv. Mater. 20 (2008) 801–804, http://dx.doi.org/10.1002/adma.200701015. [39] Y.L. Chueh, Z. Fan, K. Takei, H. Ko, R. Kapadia, A.A. Rathore, N. Miller, K. Yu, M. Wu, E.E. Haller, A. Javey, Black Ge based on crystalline/amorphous core/ shell nanoneedle arrays, Nano Lett. 10 (2010) 520–523, http://dx.doi.org/10. 1021/nl903366z. [40] I. Moren, J.J. Araiza, M. Avendano-Alejo, Thin-film spatial filters, Opt. Lett. 30 (2005) 914–916.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Enhancement of GaAs solar cell performance by using a ZnO sol–gel anti-reflection coating Y.F. Makableh n, R. Vasan, J.C. Sarker, A.I. Nusir, S. Seal, M.O. Manasreh Department of Electrical Engineering, 3217 Bell Engineering Center, University of Arkansas, Fayetteville, AR 72701, United States
art ic l e i nf o
a b s t r a c t
Article history: Received 6 May 2013 Received in revised form 14 November 2013 Accepted 5 January 2014 Available online 1 February 2014
The performance of a GaAs p–n junction solar cell was investigated by coating the device with 110 nm thick ZnO sol–gel anti-reflection film. A post-furnace thermal annealing at 150 1C for 30 min was performed on the ZnO film after it was spin coated on the device with a speed of 8000 rpm. Ellipsometry was used to measure the reflectance, thickness, and the refractive index of the ZnO film. The solar cell performance was investigated by using the current–voltage technique from which the power conversion efficiency was extracted. The spectral response and quantum efficiency were also measured for the solar cell. An enhancement, after utilizing the ZnO anti-reflection coating, was observed on the order of 32%, 38, and 51% for the power conversion efficiency, spectral response, and quantum efficiency, respectively. & 2014 Elsevier B.V. All rights reserved.
Keywords: GaAs solar cells Anti-reflection coating ZnO sol–gel Performance enhancement
1. Introduction In recent years, fabricating high efficient solar cells is an essential part of the core research in the photovoltaic field [1,2]. Amongst different solar cell materials and structures, GaAs solar cells are receiving more attention worldwide [3] even though the multi-junction solar cells diluted the interest in GaAs single junction solar cells. The theoretical limit of the power conversion efficiency of GaAs solar cells is about 33%, which is due to their ability to generate high currents and voltages [4]. Despite this high theoretical power conversion efficiency limit, their performance did not reach its peak levels due to the relatively high refractive index [5–7] (n¼ 3.7), which reflects more than 35% of the incident light [8]. Several attempts were made to improve on the GaAs solar cell performance, such as by using the plasmonic effect, [8,9] surface texturing, [10] and the anti-reflection coating effect [10–12]. Among these attempts, the enhancement due to antireflection coatings is gaining more attention [12–14]. Several materials were used as anti-reflection coatings. Among them is ZnO, which appears to be an ideal anti-reflection film in case of GaAs based devices. This is due to the low value of the refractive index [15] and its wide band gap of 3.30 eV, which makes it transparent in the visible spectral region [16,17]. Zinc oxide films were reported to be synthesized by using different techniques, such as the sol–gel method [18], thermal evaporation [19], and chemical vapor deposition [20]. The sol–gel synthesis of
n
Corresponding author. E-mail address: [email protected] (Y.F. Makableh).
0927-0248/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2014.01.007
ZnO has many advantages over other techniques as it is of low cost, less toxic and easy to implement to produce high quality films [18–21]. Furthermore, ZnO was used as a single layer antireflection coating for GaAs and Si solar cells with an enhancement in the device performance equivalent to or better than multi-layer coating schemes [5,11,22,23]. In this article, we report on the enhancement of the performance of GaAs p–n junction solar cells due to the addition of ZnO anti-reflection coating. The enhancement is observed in the characteristic properties of the solar cell, namely, in the power conversion efficiency (η), the spectral response, and the external quantum efficiency (EQE). The ZnO film was synthesized using the sol–gel method and was coated using a spin coating method. Low temperature furnace annealing was used to cure the ZnO films. The finished surfaces of the coated solar cells have bluish tint to them. The ZnO films were characterized by using the x-ray diffraction (XRD) and transmission and reflection spectroscopy. Ellipsometry was also used to measure the refractive index of the ZnO films in the entire visible spectral region. As expected, the anti-reflection coating reduces the reflection of the incident photons striking the surface of the GaAs solar cell leading to an increase in the photocarrier generation rate. The increase in the generation rate is the cause of the enhancement in the device performance. 2. Experimental details The GaAs solar cell structure was grown using the molecular beam epitaxy (MBE) technique. The structure was grown on a p-type substrate doped with [Zn] ¼1 1018 cm 3. The n-type side
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
3. Results and discussions
500
ZnO on GaAs ZnO
Intensity (a.u)
400 300
GaAs
ZnO
5.0
Refractive index
4.5 4.0 3.5 3.0 2.5
1.5 1.0
400
30 25
15 10 5 0 400
70
80
2θ (deg) Fig. 1. X-ray diffraction spectrum obtained for an annealed ZnO sol–gel film spin coated on a GaAs substrate.
450
500
550
600
650
700
750
800
Wavelength (nm) 65 60 55 50 45
800
60
Pristine GaAs GaAs with ZnO
20
30
50
800
35
100
40
700
40
35
30
600
45
200
20
500
Wavelength (nm)
40
0
GaAs substrate ZnO anti-reflection layer
2.0
Transmission %
Prior to coating with ZnO, the cell was etched using a 6% HCl solution for 20 s to remove any oxide layer from the solar cell surface. Then the solar cell was spin coated with the ZnO sol–gel solution with a speed of 8000 rpm for 30 s. Different spin coating speeds were tried, but 8000 rpm speed was found to be optimum for the device size used in this investigation. The ZnO film was annealed in ambient atmosphere at 150 1C for 30 min to evaporate the solvent (iso-propyl alcohol) and to cure the film. It has been noted that ZnO films were formed after annealing at 150 1C, as compared to other reported results [24,25] that used annealing temperatures more than 400 1C. The low annealing temperature is essential to avoid breaking the p–n junction. The presence of ZnO was confirmed by using XRD measurements as shown in Fig. 1. The
peaks observed at 381 and 441 are typical of ZnO peaks, and the broader peak at 661 is the GaAs peak. The GaAs peak is smaller and broader since the measurement was performed at an offset angle of 31 in order to avoid the lattice points of the GaAs crystal.
Reflectance (%)
of the GaAs was doped with [Si]¼2 1018 cm 3. A 100 nm n-type Al0.85Ga0.15As barrier was inserted in the cap layer and 100 nm ptype Al0.20Ga0.80As barrier was inserted in the buffer layer. These AlGaAs barriers are introduced as fence barriers to reduce the charge trapping effect. The high resolution optical profiler test revealed a high smooth surface, which was expected for the MBE grown structures. The sol–gel method [11] was followed to synthesize the ZnO film where zinc acetate dihydrate ( 499%) was used as a precursor and mono-ethanolamine was used as a stabilizer. In this method iso-propyl alcohol was used as a solvent medium. Throughout the synthesis, the solution mixture was kept on a hot plate at 50 1C and stirred constantly until a clear solution was obtained. The solution was then aged for 24 h before use. The solar cells were fabricated in a class 100 clean room using the standard photolithography procedure. The n-type and p-type ohmic contacts were formed using an e-beam deposition technique. The n-type metal contact consisted of AuGe/Ni/Au and the p-type metal contact consisted of Au/Zn/Au. The substrate temperature was held at 100 1C in the Angstrom Nexdrep e-beam chamber during the deposition of the metals. Each metal has a different calibration deposition rate. The power conversion efficiency was measured by using Newport, model 91291-100, 1.0 sun (1000 W/m2) AM 1.5G solar simulator in conjunction with a Keithley 4200 semiconductor characterization system. The spectral response measurements were recorded by using a Bruker FS 125HR Fourier-transform spectrometer and the EQE spectra were obtained by using an Oriel IQE 200 monochromator. All measurements were performed at room temperature.
179
Pristine GaAs GaAs with ZnO
1200
1600 2000 Wavelength (nm)
2400
2800
Fig. 2. The optical characteristics of ZnO film were obtained by using different experimental techniques. (a) Refractive index of ZnO layer grown on a GaAs substrate plotted along the refractive index of pristine GaAs substrate. (b) Reflectance spectra measured by using an ellipsometer and plotted for a GaAs substrate before and after spin-coated with ZnO film. (c) Transmission spectra of GaAs measured before and after spin-coated with ZnO film.
30 20 10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 0.0
Dark Current Without ZnO, FF = 0.69, η = 10.5% With ZnO, FF = 0.69, η = 14% (0.72, 51) Jsc= 57.4 mA/cm
Jsc= 76.9 mA/cm 0.2
2
(0.72, 68)
2
0.4
Voc= 0.93 V
2
Current Density (mA/cm )
0.6
0.8
1.0
Voltage (V) Fig. 3. The I–V characteristic of GaAs solar cell measured in the dark (black line) and under the illumination of 3.5 sun AM 1.5G solar simulator. The measurements we made before (blue line) and after (red line) spin-coating the solar cell with ZnO thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
30 20 GaAs pn junction solar cells coated with ZnO at different speeds 10 0 Dark Current -10 Pristine -20 Device A, coated at 6000 rpm -30 Device B, coated at 8000 rpm Device C, coated at 10000 rpm -40 Device D, coated at 12000 rpm -50 -60 -70 2 -80 Jsc= 76.8 mA/cm -90 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Voltage (V) Voc= 0.93 V
2
The refractive indices of both GaAs and ZnO film were measured using a J.A. Woolam VASE ellipsometer and plotted as a functional wavelength in Fig. 2(a). The optical reflectance spectra before and after depositing the ZnO thin film on top of GaAs substrate were also measured as shown in Fig. 2(b), while the transmission spectra were measured using a CARY 500 spectrometer as shown in Fig. 2(c). From the analysis of the ellipsometric measurements, the thickness of ZnO thin film was found to be about 110 nm and the average refractive index is about 1.65 in the spectral region between 400 and 900 nm. Both the reflectance and transmission spectra show the effect of anti-reflection coating after ZnO film was deposited on GaAs substrates. The reflectance is reduced from 33% to 3% at 650 nm as shown in Fig. 2(b), while the transmission is increased from 45% to 60% at 980 nm. Transmission above the band gap of GaAs could not be measured. The minimum reflectance achieved using ZnO sol–gel films (3% at 650 nm) is even smaller than the reflectance obtained by using ZnO nanostructures [26]. The current–voltage (I–V) characteristic of the GaAs solar cell is shown in Fig. 3 before and after coating the device with ZnO layer. The black curve represents the dark current, the blue and red curves represent the photocurrent before and after coating with ZnO, respectively. The short circuit current density increased from Jsc ¼57.4 mA/cm2 before coating to Jsc ¼76.9 mA/cm2 after coating with ZnO. On the other hand, the open circuit voltage is almost unchanged. The increase in the current density is thus the primary reason for the increase in η from η ¼10.5% before coating to η ¼14% after coating with ZnO, which is about 33% improvement. The present enhancement of 33% in the power conversion efficiency is higher than the reported enhancement using ITO nanostructures on GaAs solar cell [12] and equivalent to the enhancement of 32% obtained for GaAs solar cell by using a three-layer scheme antireflection coating [5]. However, the three-layer anti-reflection coating scheme for a Si solar cell produces a 39% enhancement in the power conversion efficiency [27]. The thickness of the ZnO sol–gel layer plays a major role in the power conversion efficiency of the solar cell since photon absorption and reflectivity depend on the thickness of the antireflection coating layer. Furthermore, the thickness of the antireflection layer using the spin coating technique depends on the device size [28]. Thus, the power conversion efficiency of the solar cell is affected by the thickness of the antireflection coating layer and the size of the device. The I–V characteristics of four devices coated with ZnO
Current Density (mA/cm )
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
Fig. 4. The I–V characteristics measured for four identical solar cells after spin coated with ZnO layer at different speeds.
30 27
Spectral response (arb. units)
180
24
Without ZnO With ZnO
21 18 15 12 9 6 3 0 400
500
600 700 800 Wavelength (nm)
900
1000
Fig. 5. The spectral response spectra were measured before (blue line) and after (red line) spin-coating the solar cell with ZnO thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
layers with different spin coating speeds, but a fixed speeding time (30 s) are plotted in Fig. 4. From this figure, the optimum power conversion efficiency is obtained for a speed of 8000 rpm as shown for device B. The spectral response measurements were performed for the solar cell before and after coating with ZnO. The measurements were made in the spectral region between 380 and 1000 nm as shown in Fig. 5. The response of the solar cell after coating with ZnO shows significant improvement in the entire spectral region with the maximum enhancement of 38% measured at 865 nm. The rapid decrease observed in the spectral response as the wavelength decreases is due to the beam-splitter response in the Fourier-transform spectrometer. This rapid decrease behavior was not observed in the EQE spectra as shown in Fig. 6 where the measurements were obtained by using a monochromator. The enhancement in EQE is quite significant after coating the device with ZnO layer, which is estimated to be about 51% at 830 nm. The step observed at 720 nm in the EQE spectra as shown in Fig. 6 is due to the change in the gratings. The quantum efficiency depends on the spectra response and therefore observing the behavior of the spectral response before and after coating the device with the ZnO antireflection layer provides a confirmation about the validity of the EQE spectral behavior.
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
degradation of the ohmic contacts. The ZnO layer was characterized by using XRD, reflectance, and transmission techniques. The thickness of the ZnO layer was optimized for the present device size by using different spin coating speeds. The enhancement of the solar cell was observed in all the I–V characteristics, power conversion efficiency, spectral response, and the external quantum efficiency. The most enhancement in the solar cell parameter was observed in the external quantum efficiency and found to be around 51%.
70 Without ZnO With ZnO
60 50
EQE (%)
181
40 30 20
Acknowledgments
10 0 400
500
600
700
800
900
1000
Wavelength (nm) Fig. 6. The external quantum efficiency spectra were measured before (blue line) and after (red line) spin-coating the solar cell with ZnO thin film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
This work was supported by the Air Force Office of Scientific Research (Grant no. FA9550-10-1-0136), the NSF-EPSCoR Program (Grant no. EPS-1003970), and NASA (Grant no. 2420261BBX11AQ36A). The authors would like to thank Scott Little for helping with the XRD measurements.
References It is apparent from the present results that ZnO sol–gel thin films are effective anti-reflection coating for GaAs solar cells as judged from the significant enhancement in the device performance, which includes the IV characteristic, the power conversion efficiency, the spectral response, and the external quantum efficiency. Zinc oxide films or nanostructures have been used as an antireflection coating as a single layer or part of anti-reflection schemes involve more than two dissimilar layers for different type of solar cells [29–36]. The achievement of enhancement in the device performance in the present results using ZnO sol–gel single layer is comparable to the best enhancement reported thus far. The significant enhancement in the present EQE of about 51% is quite remarkable for 110 nm thick ZnO layer knowing that the thin film was post-annealed at a low temperature (150 1C) for only 30 min. These annealing conditions are favorable of solar cell devices as compared to the high temperature annealing conditions, [12,23–39] which are typically performed at temperature higher than 300 1C. Additionally, the low annealing temperature condition is also important to avoid the degradation of the solar cell ohmic contact. For GaAs based solar cells, the refractive index for an optimum anti-reflective coating material should be in the range of (see for pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi example [40]) nARC ¼ no nGaAs , where nARC, no and nGaAs are the refractive indices of the anti-reflection coating film, air, and GaAs, respectively. The refractive index of ZnO of about 1.65 obtained from the ellipsometer lies near the optimum value of the best antireflection coating that can be used for GaAs. Thus, a simple scheme of a single layer of ZnO as the anti-reflection coating appears to provide enhancement equivalent to or better than the most complicated nanostructure schemes [28–39]. In addition to the significant enhancement of the solar cell performance, ZnO sol–gel layers do not require special equipment, highly toxic materials, or any complicated procedures to be deposited on top of solar cells. This will lead to simple and low cost anti-reflection coatings that can be implemented in the photovoltaic industry.
4. Conclusions In conclusion, a significant enhancement in the GaAs solar cell performance was achieved by utilizing a ZnO anti-reflection coating. The ZnO layer was grown by a sol–gel method and post-growth furnace annealed at a temperature low enough to void the breakdown of the p–n junction and prevent the
[1] M.A. Green, Third generation photovoltaics: ultra-high conversion efficiency at low cost, Prog. Photovolt.: Res. Appl. 9 (2001) 123–135, http://dx.doi.org/ 10.1002/pip.360. [2] A. Luque, A. Marti, A metallic intermediate band high efficiency solar cell, Prog. Photovolt.: Res. Appl. 9 (2001) 73–86, http://dx.doi.org/10.1002/pip.354. [3] K. Nakayama, K. Tanabe, H.A. Atwater, Plasmonic nanoparticle enhanced light absorption in GaAs solar cells, Appl. Phys. Lett. 93 (2008) 1904-1–1904-3, http://dx. doi.org/10.1063/1.2988288. [4] W. Shockley, H.H. Queisser, Detailed balance limit of efficiency of pn junction solar cells, J. Appl. Phys. 32 (1961) 510–519, http://dx.doi.org/10.1063/1.1736034. [5] L.K. Yeh, K.Y. Lai, G.J. Lin, P.H. Fu, H.H. Chang, C.A. Lin, J.H. He, Giant efficiency enhancement of GaAs solar cells with graded antireflection layers based on syringe like ZnO nanorods arrays, Adv. Energy Mater. 1 (2011) 506–510, http://dx.doi.org/ 10.1002/aenm.201100025. [6] J.B. Theeten, D.E. Aspens, R.P.H. Chang, A new resonant ellipsometric technique for characterizing the interface between GaAs and its plasma grown oxide, J. Appl. Phys. 49 (1978) 6097–6102, http://dx.doi.org/10.1063/1.324529. [7] D.T.F. Marple, Refractive index of GaAs, J. Appl. Phys. 35 (1964) 1241–1242, http://dx.doi.org/10.1063/1.1713601. [8] A.S. Hovhannisyan, Single-layer antireflection coating for GaAs solar cells, J. Contemp. Phys. 43 (2008) 136–138, http://dx.doi.org/10.3103/S1068337208030080. [9] J. Wu, Y.F.M. Makableh, R. Vasan, M.O. Manasreh, B. Liang, Strong interband transitions in InAs quantum dots solar cells, Appl. Phys. Lett. 100 (2012) 19071–1907-4, http://dx.doi.org/10.1063/1.3681360. [10] Y. Inomata, K. Fukui, K. Shirasawa, Surface texturing of large area multicrystalline silicon solar cells using reactive ion etching method, Sol. Energy Mater. Sol. Cells 48 (1997) 237–242, http://dx.doi.org/10.1016/S0927-0248(97)00106-2. [11] B.Y. Su, S.Y. Chu, Y.D. Juang, M.C. Lin, C.C. Chang, C.J. Wu, Efficiency enhancement of GaAs solar cells photovoltaic due to sol–gel derived anti-reflection AZO films, J. Electrochem. Soc. 159 (2012) H312–H316, http://dx.doi.org/10.1149/ 2.062203jes. [12] P. Yu, C.H. Chang, C.H. Chiu, C.S. Yang, J.C. Yu, H.C. Kuo, S.H. Hsu, Y.C. Chang, Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns, Adv. Mater. 21 (2009) 1618–1621, http://dx. doi.org/10.1002/adma.200802563. [13] Y.F. Makableh, R. Vasan, S. Lee, M.O. Manasreh, Enhancement of the performance of InAs quantum dots solar cell by using surface modification using poly-l-lysine homopolymers, Appl. Phys. Lett. 102 (2013) 1904-1–1904-3, http://dx.doi.org/10.1063/1.4789908. [14] S.L. Diedenhofen, G. Grzela, E. Haverkamp, G. Bauhuis, J. Schermer, J.G. Rivas, Broadband and omnidirectional anti-reflection layer for III/V multi-junction solar cells, Sol. Energy Mater. Sol. Cells 101 (2012) 308–314, http://dx.doi.org/ 10.1016/j.solmat.2012.02.022. [15] R.G. Heideman, P.B. Lambeck, J.G.E. Gardeniers, High quality ZnO layers with adjustable refractive indecies for integrated optics applications, Opt. Mater. 4 (1995) 741–755, http://dx.doi.org/10.1016/0925-3467(95)00028-3. [16] K.A. Salman, K. Omar, Z. Hassan, Effective conversion efficiency enhancement of solar cell using ZnO/PS antireflection coating layers, Sol. Energy 86 (2012) 541–547, http://dx.doi.org/10.1016/j.solener.2011.10.030. [17] Y.C. Chao, C.Y. Chen, C.A. Lin, J.H. He, Light scattering by nanostructured antireflection coatings, Energy Environ. Sci. 20 (2011) 3436–3441, http://dx.doi. org/10.1039/c0ee00636j. [18] S. Ilican, Y. Caglar, M. Caglar, Preparation and characterization of ZnO thin films deposited by sol–gel spin coating method, J. Optoelectr. Adv. Mater. 10 (2008) 2578–2583.
182
Y.F. Makableh et al. / Solar Energy Materials & Solar Cells 123 (2014) 178–182
[19] B.D. Yao, Y.F. Chan, N. Wang, Formation of ZnO nanostructures by a simple way of thermal evaporation, Appl. Phys. Lett. 81 (2002) 757–759, http://dx.doi.org/ 10.1063/1.1495878. [20] A.J. Cheng, Y. Tzeng, Y. Zhou, M. Park, T.S. Wu, C. Shannon, D. Wang, W. Lee, Thermal chemical vapor deposition growth of zinc oxide nanostructures for dye-sensitized cell fabrication, Appl. Phys. Lett. 92 (2008) 2113-1–2113-3, http://dx.doi.org/10.1063/1.2889502. [21] L. Znaidi, Sol–gel deposited ZnO thin films: a review, Mater. Sci. Eng. 174 (2010) 18–30, http://dx.doi.org/10.1016/j.mseb.2010.07.001. [22] T. Minemoto, T. Mizuta, H. Takakura, Y. Hamakawa, Antireflective coating fabricated by chemical deposition of ZnO for spherical Si solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 191–194, http://dx.doi.org/10.1016/j. solmat.2006.08.005. [23] J.Y. Checn, K.W. Sun, Growth of vertically aligned ZnO nanorod arrays as antireflection layer on silicon solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 930–934, http://dx.doi.org/10.1016/j.solmat.2010.01.005. [24] M. Ohyama, H. Kozuka, T. Yoko, Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution, Thin Solid Films 306 (1997) 78–85, http://dx.doi.org/10.1016/S0040-6090(97) 00231-9. [25] O. Tari, A. Aronne, M.L. Addonizio, S. Daliento, E. Fanelli, P. Pernice, Sol–gel synthesis of ZnO transparent and conductive films: a critical approach, Sol. Energy Mater. Sol. Cells 105 (2012) 179–186, http://dx.doi.org/10.1016/j. solmat.2012.06.016. [26] Y.J. Lee, D.S. Ruby, D.W. Peters, B.B. McKenzie, J.W.P. Hsu, ZnO nanostructures as efficient antireflection layers in solar cells, Nano Lett. 8 (2008) 1501–1505, http://dx.doi.org/10.1021/nl080659j. [27] S.Y. Lien, D.S. Wuu, W.C. Yeh, J.C. Liu, Tri-layer antireflection coatings (SiO2/ SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique, Sol. Energy Mater. Sol. Cells 90 (2006) 2710–2719, http://dx.doi.org/10.1016/j.solmat. 2006.04.001. [28] B.D. Washo, Rheology and modeling of the spin coating process, IBM J. Res. Dev. 21 (1977) 190. [29] I. Moreno, J.J. Araiza, Thin-film spatial filters, Opt. Lett. 30 (2005) 914–916. [30] J.Y. Chen, and K.W. Sun, Enhancement of the light conversion efficiency of silicon solar cells by using nanoimprint anti-reflection layer, Sol. Energy Mater. Sol. Cells 94 (2010) 629-633. 10.1016/j.solmat.2009.11.028.
[31] S.S. Lo, C.C. Chen, F. Garwe, T. Pertch, Broad-band anti-reflection coupler for a: Si thin-film solar cell, J. Phys. D: Appl. Phys. 40 (2007) 754–758, http://dx.doi. org/10.1088/0022-3727/40/3/010. [32] N.K. Park, Y.S. Kim, J.H. Chang, D.H. Kim, S.O. Ryu, T.J. Lee, H.D. Kim, W.J. Lee, Application of ZnO single crystal rods as anti-reflection material for PV cells, Mol. Cryst. Liq. Cryst. 565 (2012) 68–77, http://dx.doi.org/10.1080/15421406. 2012.692235. [33] K.A. Salman, K. Omar, Z. Hassan, Improved performance of a crystalline silicon solar cell based on ZnO/PS anti-reflection coating layers, Superlattice Microstr. 50 (2011) 517–528, http://dx.doi.org/10.1016/j.spmi.2011.08.014. [34] A. Verma, F. Khan, D. Kumar, M. Kar, B.C. Chakravarty, S.N. Singh, M. Husain, Sol–gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells, Thin Solid Films 518 (2010) 2649–2654, http://dx.doi.org/10.1016/j.tsf.2009.08.010. [35] Y.C. Chao, C.Y. Chen, C.A. Lin, Y.A. Dai, J.H. He, Antireflection effect of ZnO nanorod arrays, J. Mater. Chem. 20 (2010) 8134–8138, http://dx.doi.org/10. 1039/c0jm00516a. [36] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Kcanc, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Properties of 19.2% efficiency ZnO/CdS/CuInGaSe2 thin-film solar cell, Prog. Photovolt: Res. Appl. 11 (2011) 225–230, http://dx.doi.org/10.1002/pip.494. [37] S.J. An, J.H. Chae, G.C. Yi, G.H. Park, Enhanced light output of GaN-based lightemitting diodes with ZnO nanorods arrays, Appl. Phys. Lett. 92 (2008) 11081–1108-3, http://dx.doi.org/10.1063/1.2903153. [38] J.K. Kim, S. Chhajed, M.F. Schubert, E.F. Schubert, A.J. Fischer, M.H. Crawford, J. Cho, H. Kim, C. Sone, Light-extraction enhancement of GaInN light-emitting diodes by graded-refractive-index indium tin oxide anti-reflection contact, Adv. Mater. 20 (2008) 801–804, http://dx.doi.org/10.1002/adma.200701015. [39] Y.L. Chueh, Z. Fan, K. Takei, H. Ko, R. Kapadia, A.A. Rathore, N. Miller, K. Yu, M. Wu, E.E. Haller, A. Javey, Black Ge based on crystalline/amorphous core/ shell nanoneedle arrays, Nano Lett. 10 (2010) 520–523, http://dx.doi.org/10. 1021/nl903366z. [40] I. Moren, J.J. Araiza, M. Avendano-Alejo, Thin-film spatial filters, Opt. Lett. 30 (2005) 914–916.