Open Access
Add to BibSonomyAbstract
We report on fabrication of plasmonic interfaces consisting of Ag nanoparticles on flat and textured Al:ZnO for use at the front surface of thin film solar cells to enhance light trapping and photo-conversion efficiencies. We show that outstandingly high transmittance haze is achieved from single step HCl surface textured Al:ZnO and demonstrate Ag dewetting on textured and flat Al:ZnO surfaces upon annealing at moderate temperatures. Optical response of these plasmonic interfaces clearly display plasmonic resonances in the visible and near infrared, which is crucial for enhancement of photovoltaic conversion efficiency in thin film solar cells.
© 2015 Optical Society of America
1. Introduction
Thin film architecture allows the advantage of using less material which is beneficial for lowering the fabrication budget of solar cells (SCs). However, as absorber layers become thinner, the corresponding absorption efficiency reduces. For thin film SCs, with bandgap of 1.7-1.8 eV, around 50% of the incident light is transmitted through the active layer [1]. Thus, it is crucial to increase the optical thickness of absorber layer to attain higher conversion efficiencies from thin film SCs. Unfortunately, the random pyramid texturing in KOH, which is commonly used in wafer-based bulk Si SCs, is incompatible with thin film SCs because the pyramids’ size exceeds the material thickness [2]. Several alternative light trapping schemes integrated in thin film SCs have been reported including photonic-plasmonic back reflector [3–5] and multiple-exciton generation to trap hot carriers [6]. Another approach is to stack absorber layers with different bandgaps in tandem cells to maximize absorption of solar radiation at wide range of energies [7]. However, fabrication and integration of complicated photon management interfaces increase the production cost of thin film SC. The concept of up-conversion to trap low energy photons using rare earth elements; such as Erbium (Er), have been investigated without a pronounced impact on thin film SC performance [8,9]. Alternatively, a great portion of recent research efforts is focused on optimum light trapping interfaces that maximize photons absorption on large modules without involving high fabrication cost and complexity.
Aluminum doped zinc oxide (Al:ZnO) can easily be modified by wet-chemical etching in a variety of acidic solutions such as HCl, HNO3, HF, HN4Cl [10–12]. Chemically fabricated randomly textured Al:ZnO front electrode has proven its effectiveness in increasing the short current density (Jsc) [13–15]. Incident light scattered at the interface of textured front Al:ZnO and absorber layer results in a prolonged absorption path in the active layer of a SC. In other words, the optical distance exceeds the active layer thickness, which results in an increased Jsc. With an optimum, lossless textured surface, Yablonovitch revealed a 4n2 (n being refractive index of the absorber layer) enhancement in the path length of light in absorber layer [16]. The enhancement is around 50 for materials with high refractive index such as Si where the density of optical modes is much higher as compared to air [16]. However, there are considerable amount of processing based losses and typical enhancement factors reached in the experimental studies are significantly smaller than the 4n2 limit. Consequently, decorating textured Al:ZnO with plasmonic metal nanoparticles (MNPs), such as silver (Ag), affords the possibility to utilize both the scattering potentials of textured Al:ZnO layers and plasmonic MNPs in a single light trapping design.
MNPs have already been successfully employed to enhance photon absorption efficiency in thin film solar modules thanks to their localized surface plasmons (LSPs) [17–21]. LSPs are collective oscillations of confined free electrons in the conduction band of MNPs under light excitation. The collective movement of electrons results in a buildup potential of polarization charges on the particle surface. The latter acts as a restoring force resonating at a characteristic resonance frequency, the LSP resonance frequency. Incident light with frequency matching the plasmon resonance frequency is strongly absorbed or scattered by the localized surface plasmons of MNP. At the resonance frequency of a low loss metal such as Ag with relatively moderate diameter of ~100 nm, a prominent fraction of incident light is scattered at a large cone angle and trapped in the optically denser neighboring medium [17,21–23]. The wavelength of maximum scattering efficiency by nanoparticles can be tuned towards the red and near infrared portion of the solar spectrum by careful choice of MNPs’ material, their location, size, distribution as well as the dielectric environment in their close vicinity [17–24]. Among different noble metals used to fabricate nanoparticles, silver has the maximum effective forward scattering cross-section in the visible range and thus the maximum path length enhancement for the SC [23]. In our previous publications, we proved that the nearby dielectric material has strong effect on the shape, size, distribution and thus on the plasmonic resonance of silver nanoparticles (AgNPs) prepared by dewetting technique [24–26]. In this technique, AgNP are formed by annealing a continuous thin metal film sputtered or evaporated on a given substrate [24,26]. Two main advantages of this technique are: i) it is possible to fabricate MNPs on larger scale substrates which is useful for mass production of SCs; ii) it is possible to control the average particle size and the size distribution by dewetting technique because they strongly depend on the thickness of metal thin film as well as on the annealing temperature. Thus by selecting adequate metal thickness and annealing temperature, the wavelength of maximum scattering can be easily shifted towards the red end of the solar spectrum.
In this work, we prepare HCl textured, Al:ZnO films with high total and diffuse transmittance and low resistivity. We report, here, the effect of Al:ZnO surface texture on the formation of AgNPs and their scattering mechanisms. We further study the influence of AgNPs on the optical behavior of flat and textured Al:ZnO.
2. Experimental details
2.1 Al:ZnO deposition and texturing
Al:ZnO were deposited on 25 cm x 25 cm Schott glass using RF (13.56 MHz) magnetron sputtering process of 2% aluminum-doped zinc oxide ceramic target. Before sputtering, the glass was heated in the chamber for 30 minutes to obtain better crystallinity of sputtered film. To achieve the desired film thickness and sheet resistance, the heater temperature was maintained at 250 °C while the glass oscillated horizontally below the Al:ZnO cathode [27]. The chamber pressure was maintained at 5 mTorr under an Ar flow of constant 175 sccm flow rate, and a 500 watt RF power was applied to sputter the ceramic target. Texturing of Al:ZnO coated superstrates studied in this work were produced with one step wet chemical etching. After sputtering, the obtained Al:ZnO films were dipped in 5% diluted HCl agitated with a magnetic stirrer of 30 seconds. During texturing, HCl was kept at a moderate temperature. The texturing products on Al:ZnO surface were removed by carefully rinsing the superstrates in deionized water at room temperature and then dried by N2. A 4-point probe measurement device (JANDEL, RM3-AR) was used to measure the sheet resistances of flat and textured Al:ZnO films.
2.2 Ag deposition and AgNP fabrication
A uniform Ag film of 15 nm thickness was deposited by RF sputtering of a rectangular Ag target on flat and textured Al:ZnO substrates. We choose to deposit 15 nm thick Ag because the subsequent dewetting procedure ends up in a AgNP decorated surface with average size of ~100 nm, a size at which AgNPs have desirable plasmonic activity [28]. During sputtering, the substrates were kept at room temperature to prevent the pre-dewetting formation of Ag nanoislands at elevated sputtering temperatures [25]. The chamber pressure was kept at 5 mtorr by maintaining a fixed Ar flow rate of 175 sccm. AgNPs were fabricated by dewetting the substrates in N2 environment using a flow of 155 sccm for 60 minutes in a tube furnace. A total of 4 sample sets were prepared as follows:
- Sample Set A: Schott glass/1250 nm flat Al:ZnO
- Sample Set B: Schott glass/1250 nm flat Al:ZnO/15 nm Ag/Annealing at 200°C (300°C)
- Sample Set C: Schott glass/Textured Al:ZnO
- Sample Set D: Schott glass/Textured Al:ZnO/15 nm Ag/Annealing at 200°C (300°C)
All of the 4 sample sets from A through D are used as the front window interface of thin film SCs.
2.3 AFM and SEM imaging analysis
Average surface roughness and lateral feature size of flat and textured Al:ZnO were obtained using Atomic Force Microscope, AFM (Veeco Nanoscope5) in tapping mode. We imaged the Al:ZnO and AgNPs by a Scanning Electron Microscope, SEM (FEI, Model Quanta400 F) equipped by a field emission gun operating in ultra-high vacuum. For the thickness measurement of flat and deposited Al:ZnO layer, the cleaved facets of samples were mounted at 90° with respect to the SEM electron beam. In order to obtain high resolution SEM images, the samples were coated with 3 nm gold/palladium (Au/Pd) alloy.
2.4 Optical characterization
Reflection and transmission measurements are performed using an optical setup; as sketched in Fig. 1Reflection and transmission measurements are performed using an optical setup; as sketched in Fig. 1, built around an 8-inch, 5-port integrating sphere (Oriel 70679NS), the inside of which is coated with a diffuse reflective material. The light from a 250W QTH lamp is collimated, chopped and sent through the input (transmission) port and the output (reflection) ports of the sphere. The sample plane of the output port is tilted by 4°, such as all the specular light reflected from the sample surface, at 8° to the incident beam, strikes the dedicated specular reflection exit port.
A reference disk with known diffuse reflectance is used in the measurements. For total () transmission measurements, the sample is placed at the input port, and the output port is closed by the reference disk. For diffuse transmission (haze) measurements, the sample is placed at the input port, and the output port is left open. The haze (H), defined as , is computed by post-processing the diffuse and total transmission data. For total reflection measurements, the sample is placed at the output port. Each measurement results in a uniform intensity distribution inside the sphere surface, which is analyzed through a monochromator (Oriel 74100) attached to the detector port of the sphere. A UV-enhanced silicon photodetector (Oriel 70356) in combination with a lock-in amplifier is used to measure the unprocessed signal for the wavelength range of 350-1100 nm.
The measurements were taken from the glass side to mimic the superstrate configuration where the light strikes the SC at the front glass/Al:ZnO contact. The plasmonic resonances of AgNPs can be detected from the dip in total transmittance. This dip originates either from the trapping (and consequently absorption) of light in optically denser material or from the absorption of the AgNPs.
3. Results
Figure 2 shows the SEM cross-section images of flat (sample set A) and textured Al:ZnO (sample set C). A total of 1250 nm Al:ZnO were sputtered on the given 25 cm x 25 cm Schott glass. After texturing with 5% HCl, the thickness of Al:ZnO is reduced to 816 nm.
Fig. 2 Cross-sectional scanning electron microscope (SEM) images of flat Al:ZnO (left) and textured Al:ZnO (right).
Figure 3 displays SEM images of surface morphology and AFM layouts of flat and textured Al:ZnO. SEM images reveals smaller surface features present on the surface of flat Al:ZnO.
Texturing Al:ZnO with HCl results in crater-like valleys which appear clearly on the surface. The average surface roughness of as deposited (flat) Al:ZnO is 16 nm whereas it is 157 nm for HCl textured films. The AFM analyses were taken from 625 µm2 large square regions.
Texturing of flat Al:ZnO using 5% HCl for 30 seconds is successful in increasing the total and diffuse transmittance while maintaining superior electrical properties of the films. Using 4-point probe the sheet resistance of flat and textured Al:ZnO is found to be 1.3 Ω/□ and 8 Ω/□, respectively. The increase in the sheet resistance is expected since the textured Al:ZnO film is thinner and a lot more disruptive for the electron conduction due to its rougher surface morphology and higher concentration of trap charges than flat film. Figure 4 shows, in column from left to right the measured total and diffuse transmittance, and the calculated haze is transmittance, respectively. These data were obtained by illuminating the glass side as discussed in the experimental details (Fig. 1). Total transmittance of flat Al:ZnO shows oscillating crests and troughs coming from the interface induced by the thick Al:ZnO. It is clear that the flat Al:ZnO has very low measured diffuse transmittance, lower than 1%, and thus results in a low haze over the entire wavelength range. We attribute this low value to miniature average surface roughness of 16 nm. In line with the increase roughness, the diffuse transmittance of textured Al:ZnO shows a pronounced increase over the whole spectrum. In particular a diffuse transmittance maximum of about 70%, corresponding to an impressive 3 orders of magnitude increase, is attained at 550 nm wavelength. The calculated haze in transmittance of textured Al:ZnO covers a broad spectral range which is correlated to the vast increase in surface roughness.
Fig. 4 (a) Total transmittance, (b) diffuse transmittance, and (c) haze of bare flat (black) and textured Al:ZnO (red).
In order to examine the effect of AgNPs on the prepared Al:ZnO films, we used the construction in sample sets B and D. We prepared two samples from each of Sets B and D. Figure 5 shows the SEM images and the corresponding optical responses from two such samples bearing both flat Al:ZnO and AgNPs, one prepared by annealing at 200°C (see Fig. 5(a)) and the other at 300°C (see Fig. 5(b)). The optical response of bare sample is also included in the plots in order to serve as a reference for the effect of AgNPs on optical response. Annealing Ag thin film on flat Al:ZnO at 200°C results in AgNPs, yet the dewetting of the Ag on the Al:ZnO layer is observed to be rather incomplete. The sample annealed at 300°C, on the contrary, appears to have well-formed AgNPs with an increased average size with respect to 200°C.
Fig. 5 SEM, total reflection and total transmittance behavior of AgNP fabricated on flat Al:ZnO at annealing temperature of (a) 200°C and (b) 300°C. Total reflection and total transmittance (in red) of bare flat Al:ZnO is plotted to guide the reader.
Since the optical response measurements were taken from the glass superstrate side, it is not possible to extract the plasmonic resonance from the reflectance measurements. This is because at LSP, the incident light scattered by AgNPs, couples into Al:ZnO where the density of optical modes (or n) is higher than that of air. Thus in our optical measurements, the plasmonic resonance depicts itself in the form of a dip in transmittance curve. The sinusoidal fringes observed at reflection curves present the constructive and destructive interference between the light reflected at the Al:ZnO-AgNP interface and the glass-Al:ZnO interface. The plasmonic resonance dip on the transmission curve occurs at about 540 nm for the sample annealed at 200°C, which is observed to shift to about 580 nm for the sample annealed at 300°C. This red shift is due to the increase in the average AgNP size [24] by increasing the annealing temperature from 200°C to 300°C.
Figures 6 and 7 show the SEM images and the optical response of textured Al:ZnO samples with and without AgNPs (set D) at 200°C and 300°C, respectively. Dewetting of Ag at 200°C results in rather irregularly elongated (bean-like) AgNPs together with smaller spherical AgNPs as shown in Fig. 6(a) and magnified in Fig. 6(c). In our previous study, we proved that the FWHM of AgNP size distribution increases with increasing surface roughness for Ag films annealed at 200°C [24]. Those elongated AgNPs present in the sample annealed at 200°C are larger in size than that of the AgNPs formed at 300°C as shown in Figs. 7(a) and 7(c). The surface roughness hinders the complete coalescence of AgNPs at 200°C resulting in larger elongated AgNPs between smaller AgNPs. Increasing the annealing temperature to 300°C allows rapid dewetting of Ag thin film through larger surface features resulting in well-separated spherical AgNPs. The sample annealed at 300°C has higher measured total transmittance. This is in accordance with the reduced number of mostly absorbing smaller AgNPs as can be seen from SEM images with respect to 200°C samples, for wavelengths above 640 nm. Plasmonic resonance dip occurs at 530 nm for Ag annealed at 200°C and shifts to 520 nm for Ag annealed at 300°C. Blue shift in plasmonic resonance dips with increasing annealing temperature can be attributed to three possible mechanisms: 1- It reflects the size difference between larger AgNPs formed at 200°C and smaller AgNPs formed at 300°C; 2- It is a result of reduced interaction of AgNPs with increased interparticle space; 3- It is a result of increased symmetry in the shape of AgNPs. No plasmonic resonance appears in total reflectance measurement where the maximum scattering at LSP wavelength scatters preferentially towards Al:ZnO rather than air due to much higher index of refraction of Al:ZnO with respect to air [29].
Fig. 6 (a), (c) SEM; (b) total reflection; (d) total transmittance of AgNPs fabricated at 200°C on textured Al:ZnO. Total reflection and total transmittance (in red) of bare textured Al:ZnO is plotted to guide the reader.
Fig. 7 (a), (c) SEM; (b) total reflection; (d) total transmittance of AgNPs fabricated at 300°C on textured Al:ZnO. Total reflection and total transmittance (in red) of bare textured Al:ZnO is plotted to guide the reader.
Figure 8 presents the influence of AgNPs on the diffuse transmittance of flat (see Fig. 8(a)) and of textured Al:ZnO (see Fig. 8(b)). Scattering of AgNPs in all direction upon light excitation [29] and surface roughness induced by AgNPs on flat Al:ZnO slightly enhances diffuse transmittance as compared to bare samples which further increases for sample annealed at 300°C, for the whole spectrum range (see Fig. 8(a)), where dewetted AgNPs are farther separated. This enhancement in diffuse transmittance is also detected by the calculated haze in Fig. 8(c). Haze in transmittance increases up to 14% at 640 nm for sample set B annealed at 300°C compared to bare flat Al:ZnO.
Fig. 8 (a) Diffuse transmittance of bare flat Al:ZnO and AgNPs fabricated on flat Al:ZnO at 200°C and 300°C; (b) Diffuse transmittance of bare textured Al:ZnO and AgNP fabricated on textured Al:ZnO at 200°C and 300°C; (c) haze of flat and textured Al:ZnO and of AgNPs fabricated on flat and textured Al:ZnO at 200°C and 300°C.
Decorating textured Al:ZnO with AgNPs reduces diffuse transmittance as seen in Fig. 8(b). Diffuse transmittance of sample set D annealed at 200°C and 300°C exhibit broad dip around plasmonic resonance wavelength. The haze of sample set D is very similar to that of sample set C as can be seen from comparing red, green and blue data points in Fig. 8(c). Thus we conclude that the formation of AgNPs on textured Al:ZnO reduces both diffuse (see Fig. 8(b)) and total (see Figs. 6(d) and 7(d)) transmittance of Al:ZnO but their ratio (haze) is conserved throughout the entire wavelength range.
The reduction in measured total transmittance has three origins. First contribution arrives from the fact that the light is scattered by AgNPs in all directions, yet mostly preferentially into Al:ZnO due to the asymmetry in the index of refraction of Al:ZnO and air [29]. The second contribution is coming from the intrinsic absorption of light in AgNPs; and the final contribution is absorption of light by Al:ZnO and glass substrate itself.
4. Discussion and conclusion
We have investigated the plasmonic scattering behavior of AgNPs fabricated on flat and textured Al:ZnO that can be easily integrated on large scale thin film SCs. We have experimentally demonstrated that high diffuse transmittance; as well as high transmittance haze, of textured Al:ZnO can be achieved, without reducing the total transmittance, by a single step chemical texturing using HCl only. High diffuse transmittance can be achieved using a combination of HCl and HF [10,12]. However, using HF assisted etching of glass coated Al:ZnO will affect the surface of glass itself and the resultant high diffuse transmittance is coming from the contributions of textured Al:ZnO as well as textured glass. The advantage of using HCl; but not HF, lies in the fact that the obtained high diffuse transmittance is associated with surface textured Al:ZnO only. Textured Al:ZnO has an average surface roughness of 157 nm which corresponds to the desired thin absorber layer thickness of the SC. We attribute this drastic increase in diffuse transmittance (haze) over the entire spectral range to the vast increase of average surface roughness of textured Al:ZnO.
The AgNPs are prepared by the simple dewetting technique. Dewetting of AgNP on flat AZO at 200°C forms rather interconnected smaller AgNPs where Al:ZnO is sputtered at high temperature resulting in strong surface adherence with consecutive Ag thin film. Increasing the annealing temperature to 300°C is enough to break the strong adherent between flat Al:ZnO and Ag thin film leading to a complete coalescence of larger AgNP atop the flat Al:ZnO surface. Still, increasing the annealing temperature to 400°C or 500°C do result in larger spherical AgNP; ≥ 500 nm in diameter, which does not support LSP upon light excitation [25]. Dewetting of AgNPs on textured Al:ZnO at 200°C and 300°C results in well-shaped AgNPs distributed in conformal manner within the Al:ZnO craters as shown clearly in Fig. 6(c) and Fig. 7(c); respectively. In this case, the Al:ZnO surface is treated with HCl so the consecutive Ag thin film does not adhere on textured Al:ZnO surface. LSP of AgNPs are detected from total transmittance measurements and not from total reflection since the measurements were obtained by illuminating the glass side so the maximum scattering at AgNPs resonance wavelength couples into higher refractive index material; the Al:ZnO [29].Obtained AgNPs resonate closer to the red portion of the solar spectrum which is crucial to enhance the photon path length in this spectral region where the photons are weakly absorbed [23]. The total and diffuse transmittance of AgNPs on flat and textured Al:ZnO are lower than that of bare flat and textured Al:ZnO because most of the light scattered by AgNPs is coupled into optically (n~2) and geometrically thick (1.25 µm and 0.81 µm for flat and textured, repsectively) Al:ZnO. Thus once those Al:ZnO/AgNPs interfaces are integrated into thin film SCs such as those made of hydrogenated amorphous silicon (n~4), a pronounced fraction of light scattered by AgNPs are expected to preferentially couple into the absorber layer where the density of optical modes are double that of Al:ZnO [29].
Transmittance haze of textured Al:ZnO and that of AgNPs on textured Al:ZnO are the same because the fraction of light reduced from total transmittance is also reduced from diffuse transmittance, hence the ratio is the same where the reduction due to extinction by AgNPs is two-fold: scattering based coupling into Al:ZnO and parasitic absorption by AgNPs. It is important to obtain the optical response of the textured and plasmonic interface by cloning the same geometry of such an interface in the desired SC application. The application can be performed in two ways: in the case the interface is to be placed on the front surface, then it should be illuminated from glass side; and in the case the interface is to be placed on the back surface, then it is better to send the light directly to the interface. This is because the optical response is highly sensitive to the surrounding refractive index and hence the density of optical modes as it can be seen in reflection measurements.
Acknowledgments
H. Nasser greatly acknowledges the Scientific and Technological Research Council of Turkey (TÜBİTAK BİDEB-2215) for financial support. A. Bek acknowledges the financial support from Bilim Akademisi - The Science Academy, Turkey under the BAGEP 2013 program, European Union FP7 Grant nr. 270483, the Scientific and Technological Research Council of Turkey (TÜBİTAK) Grant Nrs. 113F239, 113M931 and 113F375. R. Turan acknowledges European Union FP7 grant CHEETAH with nr. 609788. Authors wish to thank Mr. Mete Günöven for his support with optical measurements.
References and links
1. A. C. Atrel, A. García-Etxarri, H. Alaeian, and J. A. Dionne, “Toward high-efficiency solar upconversion with plasmonic nanostructures,” J. Opt. 14(2), 024008 (2012). [CrossRef]
2. P. Campbell and M. A. Green, “Light trapping properties of pyramidally textured surfaces,” J. Appl. Phys. 62(1), 243–249 (1987). [CrossRef]
3. F.-J. Haug, T. Söderström, D. Dominé, and C. Ballif, “Light trapping effects in thin film silicon solar cells,” in Proceedings of Material Research Society Symposium A, Vol 1153, A. Flewitt, Q. Wang, J. Hou, S. Uchikoga, A. Nathan, eds. (Cambridge University Press, 2009), pp. A13–01.
4. R. Biswas, J. Bhattacharya, B. Lewis, N. Chakravarty, and V. Dalal, “Enhanced nanocrystalline silicon solar cell with a photonic crystal back reflector,” Sol. Energy Mater. Sol. Cells 94(12), 2337–2342 (2009). [CrossRef]
5. V. E. Ferry, M. A. Verschuuren, H. B. T. Li, E. Verhagen, R. J. Walters, R. E. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express 18(Suppl 2), A237–A245 (2010). [PubMed]
6. M. C. Beard, K. P. Knutsen, P. Yu, J. M. Luther, Q. Song, W. K. Metzger, R. J. Ellingson, and A. J. Nozik, “Multiple exciton generation in colloidal silicon nanocrystals,” Nano Lett. 7(8), 2506–2512 (2007). [CrossRef] [PubMed]
7. M. Kupich, D. Grunsky, P. Kumar, and B. Schröder, “Preparation of microcrystalline single junction and amorphous–microcrystalline tandem silicon solar cells entirely by hot-wire CVD,” Sol. Energy Mater. Sol. Cells 81(1), 141–146 (2004). [CrossRef]
8. F. Lahoz, C. Pérez-Rodríguez, S. E. Hernández, I. R. Martín, V. Lavín, and U. R. Rodríguez-Mendoza, “Upconversion mechanisms in rare-earth doped glasses to improve the efficiency of silicon solar cells,” Sol. Energy Mater. Sol. Cells 95(7), 1671–1677 (2011). [CrossRef]
9. T. Trupke, M. A. Green, and P. Würfel, “Improving solar cell efficiencies by up-conversion of sub-band-gap light,” J. Appl. Phys. 92(7), 4117 (2002). [CrossRef]
10. J. I. Owen, J. Hüpkes, H. Zhu, E. Bunte, and S. E. Pust, “Novel etch process to tune crater size on magnetron sputtered ZnO:Al,” Phys. Status Solidi A 208(1), 109–113 (2011). [CrossRef]
11. W. L. Lu, K. C. Huang, C. H. Yeh, C. I. Hung, and M. P. Houng, “Investigation of textured Al-doped ZnO thin films using chemical wet-etching methods,” Mater. Chem. Phys. 127(1-2), 358–363 (2011). [CrossRef]
12. E. Bunte, H. Zhu, J. Hüpkes, and J. Owen, “Novel texturing method for sputtered zinc oxide films prepared at high deposition rate from ceramic tube targets,” EPJ Photovo. 2, 20602 (2011). [CrossRef]
13. M. Berginski, J. Hüpkes, W. Reetz, B. Rech, and M. Wuttig, “Recent development on surface-textured ZnO:Al films prepared by sputtering for thin-film solar cell application,” Thin Solid Films 516(17), 5836–5841 (2008). [CrossRef]
14. O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schöpe, C. Beneking, H. Wagner, A. Löffl, and H. W. Schock, “Texture etched ZnO:Al coated glass substrates for silicon based thin film solar cells,” Thin Solid Films 351(1-2), 247–253 (1999). [CrossRef]
15. J. Müller, G. Schöpe, O. Kluth, B. Rech, V. Sittinger, B. Szyszka, R. Geyer, P. Lechner, H. Schade, M. Ruske, G. Dittmar, and H.-P. Bochem, “State-of-the-art mid-frequency sputtered ZnO films for thin film silicon solar cells and modules,” Thin Solid Films 442(1-2), 158–162 (2003). [CrossRef]
16. E. Yablonovitch and G. D. Cody, “Intensity enhancement in textured optical sheets for solar cells,” IEEE Trans. Electron. Dev. 29(2), 300–305 (1982). [CrossRef]
17. K. R. Catchpole and A. Polman, “Plasmonic solar cells,” Opt. Express 16(26), 21793–21800 (2008). [CrossRef] [PubMed]
18. Z. Ouyang, X. Zhao, S. Varlamov, Y. Tao, J. Wong, and S. Pillai, “Nanoparticle-enhanced light trapping in thin-film silicon solar cells,” Prog. Photovolt. Res. Appl. 19(8), 917–926 (2011). [CrossRef]
19. F. J. Beck, A. Polman, and K. R. Catchpole, “Tunable light trapping for solar cells using localized surface plasmons,” J. Appl. Phys. 105(11), 114310 (2009). [CrossRef]
20. C. Eminian, F.-J. Haug, O. Cubero, X. Niquille, and C. Ballif, “Photocurrent enhancement in thin film amorphous silicon solar cells with silver nanoparticles,” Prog. Photovolt. Res. Appl. 19(3), 260–265 (2011). [CrossRef]
21. U. Güler and R. Turan, “Effect of particle properties and light polarization on the plasmonic resonances in metallic nanoparticles,” Opt. Express 18(16), 17322–17338 (2010). [CrossRef] [PubMed]
22. K. R. Catchpole, S. Mokkapati, F. Beck, W.-C. Wang, A. McKinley, A. Basch, and J. Lee, “Plasmonics and nanophotonics for photovoltaics,” MRS Bull. 36(06), 461–467 (2011). [CrossRef]
23. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]
24. I. Tanyeli, H. Nasser, F. Es, A. Bek, and R. Turan, “Effect of surface type on structural and optical properties of Ag nanoparticles formed by dewetting,” Opt. Express 21(S5Suppl 5), A798–A807 (2013). [CrossRef] [PubMed]
25. H. Nasser, Z. M. Saleh, E. Özkol, M. Günoven, A. Bek, and R. Turan, “Fabrication of Ag nanoparticles embedded in Al:ZnO as potential light-trapping plasmonic interface for thin film solar cells,” Plasmonics 8(3), 1485–1492 (2013). [CrossRef]
26. M. Z. Borra, S. K. Güllü, F. Es, O. Demircioğlu, M. Günöven, R. Turan, and A. Bek, “A feasibility study for controlling self-organized production of plasmonic enhancement interfaces for solar cells,” Appl. Surf. Sci. 318, 43–50 (2014). [CrossRef]
27. V. Sittinger, F. Ruske, W. Werner, B. Szyszka, B. Rech, J. Hüpkes, G. Schöpe, and H. Stiebig, “Al:ZnO films deposited by in-line reactive AC magnetron sputtering for a-Si:H thin film solar cells,” Thin Solid Films 496(1), 16–25 (2006). [CrossRef]
28. S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109(7), 073105 (2011). [CrossRef] [PubMed]
29. M. Schmid, R. Klenk, M. Ch. Lux-Steiner, M. Topič, and J. Krč, “Modeling plasmonic scattering combined with thin-film optics,” Nanotechnology 22(2), 025204 (2011).
