Mesoporous silica nanocomposite antireflective coating for Cu(In,Ga)Se2 thin film solar cells

Solar Energy Materials & Solar Cells 134 (2015) 359–363

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

Mesoporous silica nanocomposite antireflective coating for Cu(In,Ga) Se2 thin film solar cells Gareth Wakefield a, Megan Adair a, Martin Gardener a, Dieter Greiner b, Christian A Kaufmann b, Jonathan Moghal a,c,n a

Oxford Advanced Surfaces Group plc, Begbroke Science Park Oxford, OX5 1PF, United Kingdom Helmholtz-Zentrum Berlin für Materialien und Energie / PVcomB, Schwarzschildstrasse 3, 12489 Berlin, Germany c Department of Materials, University of Oxford, Parks Road, OX1 3PH Oxford, United Kingdom b

art ic l e i nf o

a b s t r a c t

Article history: Received 23 June 2014 Received in revised form 11 November 2014 Accepted 14 December 2014

A single layer anti-reflective (AR) mesoporous silica nanoparticles coating on Cu(In,Ga)Se2 (CIGS) thin film solar cells has been fabricated. Unlike traditional vacuum deposited AR coatings, this nanoparticle coating is applied using simple wet deposition techniques. In this study we investigate the structural properties and anti-reflection effects of the nanoparticle coating on the CIGS solar cell. The reduction in reflection and increase in the short circuit current shows the potential of this nanoparticle coating to compete with current technology. & 2014 Elsevier B.V. All rights reserved.

Keywords: Anti-reflection Mesoporous silica Short circuit current CIGS solar cells

1. Introduction Photovoltaic (PV) cells have been the topic of intense research over the past few years as they offer a low carbon alternative to fossil fuels. Traditionally silicon wafers have been used as the active component in PV cells [1], however thin film solar cells possess the advantage of low weight and a potentially lower cost of production, driving the $/W electricity cost down once mass production is achieved. Chalcopyrite Cu(In,Ga)Se2 (CIGS) thin films solar cells are one of the most promising thin film solar cell technologies due to their bandgap, high absorption coefficient and also due to the fact that they can be produced on flexible substrates, thus increasing the scope of possible applications [2,3]. Recently efficiencies of over 20% have been reported for small area CIGS cells [4–6]. Over the past decades significant research has been devoted to improving the efficiencies of CIGS cells. Most of this research has concentrated on absorber fabrication and various deposition techniques for the absorber layer [7–9]. All deposition methods other than co-evaporation need a post selenization and often an additional sulfurization process, which is an issue for high throughput manufacturing [10]. However, due to the good scalability of the deposition methods involved (mostly sputtering), the presently largest industrial n Corresponding author at: Department of Materials, University of Oxford, Parks Road, OX1 3PH Oxford, United Kingdom. Tel.: þ44 1865 854852; fax: þ44 1865 854808. E-mail address: [email protected] (J. Moghal).

http://dx.doi.org/10.1016/j.solmat.2014.12.022 0927-0248/& 2014 Elsevier B.V. All rights reserved.

fabrication facility relies on this technology and has recently demonstrated its high potential to reach world record efficiencies [11]. Direct sputtering of a CIGS thin film from a composite target without further selenization has also shown to be a viable option for a possible future one-step sputtering fabrication routine, further reducing production cost [12]. For the demonstration of world record efficiencies optical losses within the complete device have to be reduced. A standard approach to achieve this is to reduce the Fresnel reflection of the cell at the air/device interface. Commonly this is attained by application of an anti-reflection coating (ARC) [13–15]. Conventional ARC's for photovoltaic cells may consist of either multi or single layer structures [16]. Current ARC technology relies primarily upon vacuum deposition techniques such as sputtering or physical and chemical vapor deposition. However, although vacuum deposited coatings do suppress Fresnel reflection there are disadvantages associated with these coatings such as thermal expansion mismatch. Vacuum deposited coatings are expensive and may also suffer from material limitations, that is, they could be sensitive to humidity [17,18]. In terms of cost and from a processing perspective it is desirable to move away from vacuum deposited ARC's and have a single layer ARC. A number of examples have been discussed in the scientific literature [19–22]. One such approach is nanoscale architectures that completely suppress Fresnel refection due to their graded refractive index [23], however this requires multiple process steps and is not cost efficient. Sol-gel coatings are also common but their properties are not equivalent to multi-layer

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coatings, the literature quotes a minimum reflection  0.8% [24] and there is a high temperature sintering (200–500 1C) step involved, which – due to the reduced thermal stability of thin film solar cells – makes direct application of this technology difficult. Nanoparticle systems such as SiO2 and TiO2 can be used to create single layer anti reflection coatings [25,26]. We have previously demonstrated the deposition of a robust single layer ARC consisting of mesoporous silica nanoparticles in a silica binder material on glass and polymeric substrates [27–29], which can suppress Fresnel reflections down to as low as 0.1%. The binder system is used with the mesoporous particles to fine tune the refractive index to match the substrate material to achieve optimum antireflection properties on any given material [27,28].

These coatings are applied using simple wet deposition techniques at ambient temperature. Even though the coating is a single layer, the reduction in Fresnel reflections is still significant at high angles of incidence, which is important for solar applications [27,28]. To our knowledge there are few studies that have directly deposited a single layer nanoparticle ARC using wet deposition techniques onto a CIGS photovoltaic cell. In this work, using a simple one step wet deposition technique, we have applied a single layer of mesoporous silica nanoparticles to the surface of a CIGS thin film photovoltaic cell and investigated its performance as an anti-refection coating.

2. Experimental details

Fig. 1. High resolution TEM images of mesoporous silica nanoparticles. The particles were deposited on a holey carbon grid.

Mesoporous silica nanoparticles were synthesized by the standard method reported by Bein et al. [30]. The coating was achieved by dispersing mesoporous silica nanoparticles in anhydrous isopropanol (3.4 wt%). The binder solution is made up of 100 μL of tetraethyl orthosilicate (TEOS), 2 mL of isopropanol and 50 μL of 0.1 M hydrochloric acid (HCL). The silica nanoparticles and the binder are mixed together in an appropriate ratio (for this study the ratio of particle to binder was 55:45). The change in refractive index is a function of how much binder is inserted into the gaps between the silica particles. The solution was then spin coated down onto a CIGS photovoltaic cell. By varying spin speed and dwell time (3000 rpm@20 s), the thickness of the film was altered to give a minimum reflection (maximum transmission) at a wavelength of approximately 750–800 nm. The type of CIGS thin film device used in this study consists of a Mo/CIGS/CdS/i ZnO/ZnO:Al layer stack on a glass substrate, with a Ni/Al front contact grid [31]. The CIGS thin film with a thickness of  2 mm and a surface roughness of o 200 nm is co-evaporated onto a Molybdenum back contact covered flat glass substrate.

ARC Layer ZnO:Al i-ZnO CdS

CIGS Layer

Fig. 2. (a) Plane view SEM of CIGS solar cell surface. (b) Plane view SEM image of nanoparticle ARC on top of CIGS cell. (c) Cross-section view SEM of CIGS solar cell with nanoparticle ARC.

G. Wakefield et al. / Solar Energy Materials & Solar Cells 134 (2015) 359–363

The top transparent layers, undoped and doped ZnO (radio frequency sputtered), CdS (chemical vapor deposition) have an intended thickness of 50 nm, 120 nm and 250 nm respectively. High resolution scanning electron microscopy (SEM) images were taken using a JEOL 6400 field emission Scanning Electron Microscope operated at 5 KV. Dark field transmission electron micrographs (TEM) of nanoparticle samples were taken using a 400 keV high resolution JEOL 4000HR transmission electron microscope. Particles were dispersed onto holey carbon grids supplied by Agar Scientific. Reflectance spectra were recorded using a Perkin Elmer Lambda 950 UV/VIS spectrometer. The reflectance spectra were recorded using the Universal Reflectance Accessory (URA) supplied by Perkin Elmer. Atomic force microscopy was performed using a digital instruments D3000 large sample AFM with a micro fabricated Si cantilever tip. The measurement was performed in “tapping mode” in air.

3. Results and discussion Fig. 1 shows a high resolution dark field TEM image of the mesoporous nanoparticles, with the image defocused to highlight the pore structure. The nanoparticles have a size distribution from

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20–30 nm. The pore size of the particles is approximately 3 nm. It is the ratio of pore to particle volume which controls the refractive index of the particles – this should be as high as possible to allow the maximum loading of a binder system to be introduced between the particles to match the refractive index to the substrate [27–29]. Fig. 2(a) and 2(b) show the plane view scanning electron microscopy images of the CIGS cell with and without the nanoparticle. Fig. 2a shows the polycrystalline ZnO:Al transparent conductive oxide front contact with crystal sizes of  100– 300 nm in diameter. From Fig. 2b it can be seen that the nanoparticle layer conforms to the underlying structure of the ZnO:Al. Fig. 2c shows the cross-section image of the nanoparticle coating on the CIGS cell. The SEM cross-section image shows that the nanoparticles are densely packed and the film thickness is uniform with an average thickness of 220 75 nm. Atomic force micrographs of the CIGS cell with and without the anti-reflective coating are shown in Figs. 3(a) and (b). It can be seen that the coated particles are well packed and there is a noticeable difference in the surface roughness and structure once the ARC is deposited, which is in consistent with the SEM analysis. Fig. 3c shows the topography of the cell with and without ARC displayed as a line scan. Insight into the degree of coverage and change in surfaces roughness can be gained by examining how the height profile varies. There is a noteworthy reduction in surface

300 CIGS Cell CIGS Cell-ARC

250

Height / nm

200 150 100 50 0

0

2

4

6 Distance / µm

8

10

12

Fig. 3. (a) AFM image of CIGS solar cell. (b) AFM image of nanoparticle ARC on top of CIGS cell surface. (c) AFM topography of CIGS solar cell with and without nanoparticle ARC.

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roughness of the cell after the nanoparticle coating has been applied. The roughness has been reduced from 60 nm to 36 nm upon addition of the silica nanoparticle coating. It has been proposed that a high surface roughness between the ZnO:Al-air interface of a CIGS solar cell can result in increased light scattering and high haze (roughness is 41/10 of incident wavelength), hence a decrease in cell efficiency [32]. Therefore an additional benefit of the coating is a reduction in light scattering at the ARC/ZnO:Al-air interface. Fig. 4 shows the reflection curves for the CIGS solar cell with and without ARC at near normal (81) incidence. The main loss in the reflection spectrum of a CIGS solar cell at AM 1.5 occurs at 600–800 nm, where the AM1.5 photon flux is highest (see also Fig. 4) [33,34]. This is attributed to the high optical loss attributed to interference of thickness due to the transparent ZnO/ZnO:Al front contact[33]. The average reflection for the CIGS cell between 800–600 nm is 14%. It can be seen from Fig. 4 that the addition of the nanoparticle ARC does result in a significant reduction in the overall refection of the CIGS solar cell. The average reflection from 800-600 nm is o10%. As the coating is approximately 220 nm in thickness, the maximum suppression of reflection occurs at 750– 760 nm (15.3% no ARC, 9.2% with ARC). Other solar applications may require an ARC with minimum reflection shifted to a lower or higher energy part of the solar spectrum. By simply altering the concentration and deposition conditions the reflection minimum may easily be shifted from 400 nm–2000 mm with no loss of performance [27,28]. Table 1 shows best and average values for the performance of a number of CIGS photovoltaic devices before and after ARC application, measured under AM1.5 illumination at 100 mW/cm² and at 25 1C. Due to the fact that the relative error of the IV measurement is in the same order of magnitude as the current increase observed after the application of the ARC, 15 devices with nominal areas of 0.5 and 1.0 cm² on one substrate have been investigated in order to ensure statistical viability. As can be seen from the average values, 50 CIGS Cell CIGS Cell-ARC

Reflectance / a.u

40

Fig. 5. EQE of a CIGS cell with and without ARC, in comparison to the photon flux for the AM1.5 spectrum.

the photocurrent jsc of the solar cell has increased by 4.9%, while the photovoltage Voc slightly dropped. Thus the gain in current density does not lead to the increase in efficiency. Nevertheless an overall increase in efficiency is achieved. A possible reason for the slightly lower Voc may be the slight change of the illuminating spectrum within the device due to the applied ARC, or possibly an adverse effect of the solvents used. The fill factor FF of the device has not been affected by the application of the ARC coating. Comparing these values to the traditional or alternative ARC coating technologies, that is, a MgF2 coating or a coating of ZnO nanorods, shows that the effectiveness of the ARC is in a comparable range. While ZnO nanorods have been demonstrated to increase the photocurrent of a CIGS solar cell to up to 5.9% [35], MgF2 is reported to show a current increase by 6.1% [36]. The ARC technology presented in this paper however is characterized by its ease of application, which is cheap, scalable and very fast. Fig. 5 shows the external quantum efficiency (EQE) of a CIGS solar cell with and without ARC applied. Similar to the reflectance shown in Fig. 4, EQE seems to be effective mainly for wavelengths o1000 nm. While the reflectance is measured on a complete device, the EQE is only measured on an area of  2–3 mm². This may account for slight differences observed for the effectiveness of the ARC when comparing reflectance and EQE.

30

4. Conclusions

20

10

0 400

600

800

1000

1200

1400

Wavelength / nm Fig. 4. Reflectance spectra of CIGS solar cell and CIGS solar cell with ARC.

Table 1 CIGS cell performance under AM1.5 illumination with and without nanoparticle ARC coating applied.

Average Best

ARC

Voc [mA[

FF [%]

jsc [mA/cm²]

No Yes No Yes

628 610 632 625

73.8 73.8 74.0 73.9

36.3 38.1 36.5 38.6

jsc up by

η [%]

4.9%

16.9 17.2 17.1 17.8

5.6%

In summary single layer anti-reflection coatings based on mesoporous silica nanoparticles have been shown to reduce Fresnel reflections from a CIGS thin film solar cell. The coating was applied using a simple wet deposition technique and optimized to reduce reflections from 600–800 nm. The coating conforms to the top ZnO:Al structure of the cell and the overall roughness of the cell/air interface is reduced. The short circuit current was also increased on average by 4.9% and the EQE below 1000 nm has been increased. The observed optical properties are comparable with that of currently available vacuum deposited AR coatings.

Acknowledgments This Partnership received financial support from the Knowledge Transfer Partnerships programme (KTP). KTP aims to help businesses to improve their competitiveness and productivity through the better use of knowledge, technology and skills that

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reside within the UK Knowledge Base. KTP is funded by the Technology Strategy Board along with the other government funding organizations. The authors would like to thank the CEMMNT network and BegbrokeNano for structural characterization. References [1] D.E. Carlson, C.R. Wronski, Amorphous silicon solar cell, Appl. Phys. Lett. 28 (1976) 671–673. [2] T. Wada, N. Kohara, T. Negami, Characterization of the Cu(In,Ga)Se2/Mo interface in CIGS solar cells, Thin Solid Films 387 (2001) 118–122. [3] D.H. Choa, Y.D. Chunga, K.-S. Leea, K.H. Kima, J.H Kimc, S.J. Parka, J. Kimd, Photovoltaic performance of flexible Cu(In,Ga)Se2 thin-film solar cells with varying Cr impurity barrier thickness, Curr. Appl. Phys. 13 (2013) 2033–2037. [4] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, Solar cell efficiency tables (version 37), Prog. Photovolt. Res. Appl 19 (2001) 84–92. [5] P. Jackson, D. Hariskos, R. Wuerz, W. Wischmann, M. Powalla, Compositional investigation of potassium doped Cu(In,Ga)Se2 solar cells with efficiencies up to 20.8%, Phys. Status Solidi RRL 8 (2014) 219–222. [6] A. Chirila, P. Reinhard, F. Pianezzi, P. Blösch, A.R. Uhl, C. Fella, L. Kranz, C. Gretener, H. Hagendrofer, D. Jäger, R. Erni, S. Nishiwaki, S. Bücheler, A.N. Tiwari, Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells, Nat. Mater. 12 (2013) 1107–1111. [7] J.C. Chang, C.C. Chuang, J.W. Guo, S.C. Hsu, H.R. Hsu, C.S. Wu, T.P. Hsieh, An investigation of CuInGaSe2 thin film solar cells by using CuInGa precursor, Nanosci. Nanotechnol. Lett 3 (2011) 200–203. [8] V.K. Kapur, A. Bansal, P. Le, Asensio, O.I. Asensio, Non-vacuum processing of CuIn1  xGaxSe2 solar cells on rigid and flexible substrates using nanoparticle precursor inks, Thin Solid Films 431 (2003) 53–57. [9] M.E. Calixto, K.D. Dobson, B.E. McCandless, R.W. Birkmire, Controlling growth chemistry and morphology of single-bath electrodeposited Cu ( In , Ga ) Se2 thin films for photovoltaic application, J. Electrochem. Soc. 153 (2006) G521–G528. [10] Y.K. Liao, Y.C. Wang, Y.T. Yen, C.H. Chen, D.H. Hsieh, S.C. Chen, C.Y. Lee, C.C. Lai, W.C. Kuo, J.Y. Juang, K.H. Wu, S.J. Cheng, C.H. Lai, F.I. Lai, S.Y. Kuo, H.C. Kuo, Y.L. Chueh, Non-antireflective scheme for efficiency enhancement of Cu(In,Ga) Se2 nanotip array solar cells, ACS Nano 7 (2013) 7318–7329. [11] M.A. Green, K. Emery, Y. Hisikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (version 44), Prog. Photovolt.: Res. Appl 22 (2014) 701–710. [12] C.H. Chen, W.C. Shih, C.Y. Chien, C.H. Hsu, Y.H. Wu, C.H. Lai, A promising sputtering route for one-step fabrication of chalcopyrite phase Cu(In,Ga)Se2 absorbers without extra Se supply, Sol. Energy Mat. Sol. Cells 103 (2012) 25–29. [13] M.Y. Hsieh, S.Y. Kuo, H.V. Han, J.F. Yang, Y.K. Liao, F.I. Lai, H.C. Kuo, Enhanced broadband and omnidirectional performance of Cu(In,Ga)Se2 solar cells with ZnO functional nanotree arrays, Nanoscale 5 (2013) 3841–3846. [14] P.C. Tseng, M.A. Tsai, P. Yu, H.C. Kuo, Antireflection and light trapping of subwavelength surface structures formed by colloidal lithography on thin film solar cells, Prog. Photovol. Res. Appl 20 (2012) 135–142. [15] F.T. Chi, L.H. Yan, H.B. Lu, H.W. Yan, X.D Yuan, B. Jiang, Sol-Gel preparation of ultralow refractive index magnesium fluoride optical films for broadband antireflective coatings, Nanosci. Nanotechnol. Lett 4 (2012) 441–444. [16] L. Dumas, E. Quesnel, Optical properties of magnesium fluoride thin films produced by argon ion-beam assisted deposition, J. Vac. Sci. Technol 20 (2002) 102–106.

363

[17] P. Lalanne, G.M. Morris, Design, fabrication, and characterization of subwavelength periodic structures for semiconductor antireflection coating in the visible domain, Proc. SPIE, 2776, 1996300. [18] Y.J. Chang, Y.T. Chen, Broadband omnidirectional antireflection coatings for metal-backed solar cells optimized using simulated annealing algorithm incorporated with solar spectrum, Opt. Express 19 (2011) A875–A887. [19] G. San Vicente, R. Bay´on, N. Germ´an, A. Morales, Long-term durability of sol– gel porous coatings for solar glass covers, Thin Solid Films 517 (2009) 3157–3160. [20] H.Y. Fan, A. Wright, J. Gabaldon, A. Rodriguez, C.J. Brinker, Y.B. Jiang, Threedimensionally ordered gold nanocrystal/silica superlattice thin films synthesized via sol–gel self-assembly, Adv. Funct. Mater. 16 (2006) 891–895. [21] Y. Hoshikawa, H. Yabe, A. Nomura, T. Yamaki, A. Shimojima, T. Okubo, Mesoporous silica nanoparticles with remarkable stability and dispersibility for antireflective coatings, Chem. Mater. 22 (2010) 12–14. [22] S.J. Wilson, M.C. Hutley, The optical properties of ‘moth eye' antireflection surfaces, Opt. Acta 29 (1982) 993–1009. [23] J.Q. Xi, M.F. Schubert, J.K. Kim, E.F. Schubert, M. Chen, S.Y. Lin, W. Liu, J.A. Smart, Nat. Photonics 1 (2007) 176–179. [24] R. Prado, G. Beobide, A. Marcaide, J. Goikoetxea, A. Aranzabe, Development of multi functional sol–gel coatings: anti-reflection coatings with enhanced selfcleaning capacity, Sol. Energy Mater. Sol. Cells 94 (2010) 1081–1088. [25] L. Xiangmei, H.J. Junhui, Superhydrophilic and antireflective properties of silica nanoparticle coatings fabricated via layer-by layer assembly and post calcinations, Phys. Chem. C 113 (2009) 148–152. [26] Y. Zhao, G. Mao, J. Wang, Colloidal subwavelength nanostructures for antireflection optical coatings, Opt. Lett. 30 (2005) 1885–1887. [27] J. Moghal, J. Kobler, J. Sauer, J. Best, M. Gardener, A.A.R. Watt, G. Wakefield, High-performance, single-layer antireflective optical coatings comprising mesoporous silica nanoparticles, Appl. Mater. Interfaces 4 (2012) 854–859. [28] J. Moghal, S. Reid, L. Hagerty, M. Gardener, G. Wakefield, Development of single layer nanoparticle anti-reflection coating for polymer substrates, Thin Solid Films 534 (2012) 541–545. [29] J. Moghal, A. Bird, A.H. Harris, B.D. Beake, M. Gardener, G. Wakefield, Nanomechanical study of thin film nanocomposite and PVD thin films on polymer substrates for optical applications, J. Phys. D: Appl. Phys 46 (2013) 485303. [30] K. Möller, J. Kobler, T. Bein, Colloidal suspensions of nanometer-sized mesoporous silica, Adv. Funct. Mater 17 (2007) 603–612. [31] R. Caballero, C.A. Kaufmann, V. Efimova, T. Rissom, V. Hoffmann, H.W. Schock, Investigation of Cu(In,Ga)Se2 thin-film formation during the multi-stage coevaporation process, Prog. Photovolt.: Res. Appl 21 (2013) 30–46. [32] A. Campa, J. Krc, J. Malmstrom, M. Edoff, F. Smole, M. Topic, The potential of textured front ZnO and flat TCO/metal back contact to improve optical absorption in thin Cu(In,Ga)Se2 solar cells, Thin Solid Films 515 (2007) 5968–5972. [33] R. Santbergen, J.M. Goud, M. Zeman, J.A.M. van Roosmalen, R.J.C. van Zolingen, The AM1.5 absorption factor of thin-film solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 715–723. [34] Y. Hagiwara, T. Nakada, A. Kunioka, mproved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer, Sol. Energy Mater. Sol. Cells 67 (2001) 267–271. [35] L. Aé, D. Kieven, J. Chen, R. Klenk, T. Rissom, Y. Tang, M.C. Lux-Steiner, ZnO nanorod arrays as an antireflective coating for Cu(In,Ga)Se2 thin film solar cells, Prog. Photovolt.: Res. Appl 18 (3) (2010) 209–213. [36] C.A. Kaufmann, A. Neisser, R. Klenk, R. Scheer, H.W. Schock, Design of a window layer for flexible Cu(In,Ga)Se2 thin film solar cell devices, MRS Proc 865 (2005) F7.5.