Efficiency enhancement of non-selenized Cu(In,Ga)Se2 solar cells employing scalable low-cost antireflective coating
© Jheng et al.; licensee Springer. 2014
Received: 16 April 2014
Accepted: 7 June 2014
Published: 4 July 2014
In this study, a non-selenized CuInGaSe2 (CIGS) solar device with textured zinc oxide (ZnO) antireflection coatings was studied. The ZnO nanostructure was fabricated by a low-temperature aqueous solution deposition method. With controlling the morphology of the solution-grown tapered ZnO nanorod coatings, the average reflectance of the CIGS solar device decreased from 8.6% to 2.1%, and the energy conversion efficiency increased from 9.1% to 11.1%. The performance improvement in the CuInGaSe2 thin-film solar cell was well explained due to the gradual increase of the refractive index between air and the top electrode of solar cell device by the insertion of the ZnO nanostructure. The results demonstrate a potential application of the ZnO nanostructure array for efficient solar device technology.
Antireflection coatings play a major role in enhancing the efficiency of photovoltaic devices by increasing light coupling into the region of the absorber layers. Presently, the standard antireflection coatings in thin-film solar cells are the transparent thin films with quarter-wavelength thickness. In addition, the quarter-wavelength thickness antireflection coating is typically designed to suppress optical reflection in a specific range of wavelengths [1, 2]. Also, it works only in a limited spectral range for a specific angle of incidence, typically for near-normal incidence. Recently, the availability of nanofabrication technology has enabled the engineering of materials with desired antireflection characteristics such as electron beam lithography and dry etching, which have been widely used to fabricate different antireflection nanostructures [3, 4]. However, they require expensive cost of equipment and technology for fabricating nanostructures on large-area solar cells. In addition, surface recombination defects induced by etch process will decrease the device performance. Consequently, the nanostructures fabricated by using bottom-up grown methods have been developed [5–7].
Recently, zinc oxide (ZnO) nanostructures have become regarded as suitable for forming efficient antireflection coatings, taking advantage of their good transparency, appropriate refractive index, and ability to be formed as textured coatings by anisotropic growth. Also, ZnO exhibits several favorable material characteristics, such as its abundance, wide direct band gap (3.3 eV), low manufacture cost, non-toxicity, large exciton binding energy, and chemical stability against hydrogen plasma [8, 9]. The synthesis of ZnO nanostructures is currently attracting considerable attentions because of their good physical properties. Various ZnO nanostructures have been demonstrated, including nanowires, nanotips, nanotubes, and nanocages [10–13]. This work proposes an effective non-selenized Cu(In,Ga)Se2 (CIGS) solar cell with ZnO nanorods on an aluminum (Al)-doped ZnO (AZO) seed layer. This is also of one-stage sputtering process, taking no toxic selenization procedure, low production cost, and no solvent pollution to the environment . It is thereby suitable for large area and mass production. In addition, a simple, low-cost, and environmentally friendly chemical solution-based deposition is developed for growing vertically oriented arrays of hexagonal ZnO nanorods at a low processing temperature. The improvements in the optical reflection properties, the current-voltage (I-V) characteristics and the external quantum efficiency (EQE) of non-selenized CIGS solar cell are demonstrated with the ZnO nanorod antireflection coatings.
CIGS-based photovoltaic devices were fabricated with the structure of soda-lime glass/Mo/CIGS/CdS/ZnO/AZO/Al contact. The p-type CIGS films were deposited by the process described previously , employing one-stage deposition cycle and a final heat treatment at 550°C. The cell is completed by a chemical bath deposited CdS buffer layer and a RF-sputtered ZnO/AZO transparent front contact (window layer). Finally, a grid of Al used as a top contact was deposited by sputtering with a contact mask. In order to fabricate the antireflection coating on the top surface of the non-selenized CIGS solar device, ZnO nanostructures were grown by the hydrothermal method. The reaction chemicals were prepared by mixing zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) and hexamethylene tetramine (C6H12N4, HMT) in aqueous solution. After the solution was stirred for 10 min, bare non-selenized CIGS solar cells were immersed vertically in this solution, and the sealed reaction bottle was heated up to 90°C. The pH value of the chemical solution was adjusted to the desired value from 6.5 to 8 by using 1,3-diaminopropane (DAP, Acros) solution . Field-emission scanning electron microscope (FESEM) images were taken using a JEOL JSM-7401 F instrument (Tokyo, Japan). In order to obtain cross-sectional images, samples were broken mechanically. The surface and cross-sectional microstructures of the films were investigated by FESEM operating at 10 kV. The crystalline structure of the ZnO films was observed by X-ray diffraction (XRD) with an automated Bruker D8 advance X-ray diffractometer (Madison, WI, USA) with CuKα radiation (40 kV and 30 mA) for 2θ values of over 20° to 60°. Energy dispersive spectroscopy (EDS) with standardless calibration, using an accelerating voltage of 10 kV, and a dead time of approximately 20%, was performed to determine the composition of deposited ZnO nanorods. Optical transmittance and reflectance were measured at normal incidence in the wavelength range of 400 to 1,200 nm with a Cary 500 UV-visible-near infrared spectrophotometer equipped with an integrated sphere. The current-voltage characteristics of solar cells were measured by a Keitheley 4200 semiconductor analyzer under the irradiation of simulated AM1.5 sunlight with the power density of 100 mW/cm2 at 25°C using a temperature controller.
Results and discussion
Conical region of ZnO nanorod must have a height (h) equal to at least 40% of the longest operational wavelength.
Center-to-center spacing of ZnO nanorod must be less than the shortest operational wavelength divided by the refractive index (n) of the material.
It was recognized that the size and the shape of nanorods grown on the non-selenized CIGS solar cell satisfy the theoretical requirements for the efficient antireflection coating fabrication.
Based on the results of flat-top and tapered ZnO antireflection coatings, we also observed that the light conversion efficiency was improved by over 10% and that photocurrent was increased by more than 11%. They can be attributed to the enhanced light absorption caused by the multiple photon scattering phenomena associated with the nanorod arrays. According to the weighted reflectance Rw with both the internal spectral response of the solar cell and the AM1.5 solar spectrum, we found that decreasing the nanorod tip diameter to 50 nm improved the Rw from 13.5% to 12.6% in the letter. According to the effective medium theory , the effective refractive index increases with the filling factor. The filling factors at the air-ZnO nanorod array interface are statistically estimated to be 17.21% and 12.47% for flat-top and tapered ZnO, respectively. Consequently, tapered ZnO nanorod arrays have the lowest effective refractive index at the interface.
Photovoltaic performance of non-selenized CIGS solar cells with different conditions of ZnO nanorod antireflection coating
Tapered ZnO nanorods
Improvement (η, %)
In summary, the effects of ZnO nanorods as a subwavelength-textured antireflection coating on non-selenized CIGS thin-film solar cell have been demonstrated in this work. Based on the moth-eye effect, the reflection on the surface of CIGS solar cell covered with nanostructured ZnO layer can be effectively eliminated. The surface morphology of ZnO nanostructures also played a critical role in the reduction of the reflection. With the coating of branched tapered ZnO nanorods, the average reflectance of the non-selenized CIGS solar cell decreased the magnitude by three times, and the energy conversion efficiency increased by 20%. The aqueous-grown ZnO nanostructures also can be fabricated with a large-area coating process at a temperature less than 90°C. It thereby would have a great potential for further application to flexible solar cell technology.
This work was supported by the Ministry of Science and Technology of Taiwan under contract nos.:103–2623–E–009–009–ET.
- Lee Y, Koh K, Na H, Kim K, Kang J-J, Kim J: Lithography-free fabrication of large area subwavelength antireflection structures using thermally dewetted Pt/Pd alloy etch mask. Nanoscale Res Lett 2009, 4: 364. 10.1007/s11671-009-9255-4View ArticleGoogle Scholar
- Jiang H, Yu K, Wang Y: Antireflective structures via spin casting of polymer latex. Opt Lett 2007, 32(5):575. 10.1364/OL.32.000575View ArticleGoogle Scholar
- Park SJ, Lee SW, Lee KJ, Lee JH, Kim KD, Jeong JH, Choi JH: An antireflective nanostructure array fabricated by nanosilver colloidal lithography on a silicon substrate. Nanoscale Res Lett 2010, 5: 1570. 10.1007/s11671-010-9678-yView ArticleGoogle Scholar
- Song YM, Park GC, Kang EK, Yeo CI, Lee YT: Antireflective grassy surface on glass substrates with self-masked dry etching. Nanoscale Res Lett 2013, 8: 505. 10.1186/1556-276X-8-505View ArticleGoogle Scholar
- Shin BK, Lee TI, Xiong J, Hwang C, Noh G, Choc JH, Myounga JM: Bottom-up grown ZnO nanorods for an antireflective moth-eye structure on CuInGaSe2solar cells. Sol Energy Mater Sol Cells 2011, 95: 2650. 10.1016/j.solmat.2011.05.033View ArticleGoogle Scholar
- Chao YC, Chen CY, Lin CA, Dai YA, He JH: Antireflection effect of ZnO nanorod arrays. J Mater Chem 2010, 20: 8134. 10.1039/c0jm00516aView ArticleGoogle Scholar
- Umar A, Lee S, Im YH, Hahn YB: Flower-shaped ZnO nanostructures obtained by cyclic feeding chemical vapour deposition: structural and optical properties. Nanotechnology 2005, 16: 2462. 10.1088/0957-4484/16/10/079View ArticleGoogle Scholar
- Zamfirescu M, Kavokin A, Gil B, Malpuech G, Kaliteevski M: ZnO as a material mostly adapted for the realization of room-temperature polariton lasers. Phys Rev B 2002, 65: 161205.View ArticleGoogle Scholar
- Klingshirn C, Hauschild R, Priller H, Decker M, Zeller J, Kalt H: ZnO rediscovered—once again!? Superlattices Microstruct 2005, 38: 209. 10.1016/j.spmi.2005.07.003View ArticleGoogle Scholar
- Kim K, Debnath PC, Lee DH, Kim S, Lee SY: Effects of silver impurity on the structural, electrical, and optical properties of ZnO nanowires. Nanoscale Res Lett 2011, 6: 552. 10.1186/1556-276X-6-552View ArticleGoogle Scholar
- Chen H, Wu X, Gong L, Ye C, Qu F, Shen G: Hydrothermally grown ZnOmicro/nanotube arrays and their properties. Nanoscale Res Lett 2010, 5: 570. 10.1007/s11671-009-9506-4View ArticleGoogle Scholar
- Ko YH, Kim MS, Park W, Yu JS: Well-integrated ZnO nanorod arrays on conductive textiles by electrochemical synthesis and their physical properties. Nanoscale Res Lett 2013, 8: 28. 10.1186/1556-276X-8-28View ArticleGoogle Scholar
- Chong SK, Dee CF, Rahman SA: Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires. Nanoscale Res Lett 2013, 8: 174. 10.1186/1556-276X-8-174View ArticleGoogle Scholar
- Jheng BT, Liu PT, Wu MC, Shieh HP: A non-selenization technology by co-sputtering deposition for solar cell applications. Opt Lett 2012, 37(13):2760. 10.1364/OL.37.002760View ArticleGoogle Scholar
- Lee YJ, Sounart TL, Liu J, Spoerke ED, McKenzie BB, Hsu JWP, Voigt JA: Tunable arrays of ZnO nanorods and nanoneedles via seed layer and solution chemistry. Cryst Growth Des 2008, 8(6):2036. 10.1021/cg800052pView ArticleGoogle Scholar
- Kuo ML, Poxson DJ, Kim YS, Mont FW, Kim JK, Schubert EF, Lin SY: Realization of a near-perfect antireflection coating for silicon solar energy utilization. Opt Lett 2008, 33: 2527. 10.1364/OL.33.002527View ArticleGoogle Scholar
- Wilson SJ, Hutley MC: The optical properties of moth-eye antireflection surfaces. Opt Acta 1982, 29: 993. 10.1080/713820946View ArticleGoogle Scholar
- Southwell WH: Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces. J Opt Soc Am A 1991, 8: 549. 10.1364/JOSAA.8.000549View ArticleGoogle Scholar
- Raguin DH, Morris GM: Antireflection structured surfaces for the infrared spectral region. Appl Opt 1993, 32: 1154. 10.1364/AO.32.001154View ArticleGoogle Scholar
- Wei SH, Zhang SB, Zunger A: Effects of Ga addition to CuInSe2on its electronic, structural, and defect properties. Appl Phys Lett 1998, 72: 3199. 10.1063/1.121548View ArticleGoogle Scholar
- Chao YC, Chen CY, Lin CA, He JH: Light scattering by nanostructured anti-reflection coatings. Energy Environ Sci 2011, 4: 3436. 10.1039/c0ee00636jView ArticleGoogle Scholar
- Jung SM, Kim YH, Kim SI, Yoo SI: Characteristics of transparent conducting Al-doped ZnO films prepared by dc magnetron sputtering. Curr Appl Phys 2011, 11: S191. 10.1016/j.cap.2010.11.101View ArticleGoogle Scholar
- Mahdjoub A, Zighed L: New designs for graded refractive index antireflection coatings. Thin Solid Films 2005, 478: 299. 10.1016/j.tsf.2004.11.119View ArticleGoogle Scholar
- Baek SH, Jang HS, Kim JH: Characterization of optical absorption and photovoltaic properties of silicon wire solar cells with different aspect ratio. Curr Appl Phys 2011, 11: S30.View ArticleGoogle Scholar
- Baek SH, Noh BY, Park IK, Kim JH: Fabrication and characterization of silicon wire solar cells having ZnO nanorod antireflection coating on Al-doped ZnO seed layer. Nanoscale Res Lett 2012, 7: 29. 10.1186/1556-276X-7-29View ArticleGoogle Scholar
- Ashour ES, Sulaiman MYB, Ruslan MH, Sopian K: a-Si:H/SiNW shell/core for SiNW solar cell applications. Nanoscale Res Lett 2013, 8: 466. 10.1186/1556-276X-8-466View ArticleGoogle Scholar
- Jheng BT, Liu PT, Wang MC, Wu MC: Effects of ZnO-nanostructure antireflection coatings on sulfurization-free Cu2ZnSnS4absorber deposited by single-step co-sputtering process. Appl Phys Lett 2013, 103: 052904. 10.1063/1.4817253View ArticleGoogle Scholar
- Chang CH, Caballero JAD, Choi HJ, Barbastathis G: Nanostructured gradient-index antireflection diffractive optics. Opt Lett 2011, 36(12):2354. 10.1364/OL.36.002354View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.