Development of two-step etching approach for aluminum doped zinc. oxide using a combination of standard HCl and NH 4 Cl etch steps

Transcription

1 Development of two-step etching approach for aluminum doped zinc oxide using a combination of standard HCl and NH 4 Cl etch steps S. Fernández a), S.E. Pust b),j. Hüpkes b), F.B. Naranjo c) a) CIEMAT, Departamento de Energías Renovables, Unidad de Energía Solar Fotovoltaica, Avd. Complutense 22, Madrid. Spain. b) IEK5 Photovoltaik, Forschungszentrum Jülich GmbH, Jülich, Germany. c) Grupo de Ingeniería Fotónica, Departamento de Electrónica, Escuela Politécnica Superior, Universidad de Alcalá, Campus Universitario, Alcalá de Henares, Madrid, Spain Abstract A new etching method for ZnO:Al films deposited by radio-frequency sputtering is presented. The method is developed to achieve appropriate surface morphology for efficient light scattering. This etching method consists of a first step where the sample is dipped in standard diluted HCl (0.5 wt%) for 40 seconds (the standard Jülich etch process) and a subsequent step where a NH 4 Cl aqueous solution with concentrations ranging from 2 to 20 wt% is used. The introduction of the second step leads to a slight modification of the surface feature shape and an increase in the surface roughness of up to around 37% in relation with that obtained using only the first step. High haze values are also obtained, reaching up to 93% at 550 nm and strong light scattering into angles above 50º at 632 nm. On the other hand, the resistivity of the textured films remains low enough for cell application, being ranged from 6 to 13 Ω/sqr depending on the NH 4 Cl concentration used. Finally, in order to assess the role of the features obtained on the 1

2 surface as effective light trapping, the textured films are applied as front contact in silicon thin film solar cells. 1. INTRODUCTION Transparent conductive oxides (TCOs) play an important role in the thin film silicon solar cells, having a huge influence on the device efficiencies [1]. As integral part of these devices, TCOs can be used as a front electrode in p-i-n (superstrate) configuration or as part of back side reflector. In the first case, the TCO should fulfil two conditions: to show favourable physico-chemical properties for the posterior growth of the silicon and to provide the enhancement of the light scattering within the solar cell. In this sense, among the most common TCOs used, magnetron-sputtered aluminumdoped zinc oxide (ZnO:Al) presents several advantages such as high stability against hydrogen plasma and adequate post-deposition textured surface using wet-chemical etching [2]. Usually the single or mixed acids used for the post-deposition texturization purpose are the hydrochloric acid (HCl), hydrofluoric acid (HF), ammonium chloride (NH 4 Cl), nitric acid (HNO 3 ) and phosphoric acid (H 3 PO 4 ) [3-7]. The optimized textured etched films obtained using these acids normally provide an effective light trapping in silicon thin film solar cells [8]. It has to be realized that the light scattering properties depend on the shape and size of the features obtained after the etching process. Hence, to achieve appropriate features, it has to be considered on one hand, the strong influence of the film deposition parameters on the surface morphology [9-11] and on the other hand, the etching mechanism and the behaviour that the acids exert on the surface. As example, low deposition pressures lead to steep and sharp features, appropriate for light scattering. However granular features are obtained on the layer surface when increasing the sputtering deposition pressure, thus leading to bad light 2

3 scattering properties [9,12,13]. Regarding the used etchant, and also as example, the features obtained with HF are much more homogeneous and in a higher number than etching with HCl [6], demonstrating the huge effect of the acid on the final surface morphology. In this work, a new etching process using a combination of two acids, HCl and NH 4 Cl, has been developed to etch approximately 800 nm-thick ZnO:Al films. Films are firstly etched with HCl (0.5 wt%) for 40 sec, parameters previously optimized for this material [15], and secondly etched with NH 4 Cl. The choice of the acids was based in the different acidy of both and the different possible behaviour found on the shape of the features. So, firstly the surface is etched in an aqueous solution of a strong acid (HCl), and hence, its effect on the surface is sharp. Secondly, when the main facets are defined, the sample is dipped into an aqueous solution of a weak acid (NH 4 Cl) which effect on the surface is to obtain a double feature within the main one. This new surface morphology could result an appealing option to improve the solar cell performance. The range of NH 4 Cl concentrations and the etching times studied are from 2 to 20 wt% and from 4 to 25 minutes, respectively. The effect that the second step based on NH 4 Cl has on the size and the shape of the surface features obtained after upon the first step based on HCl has been studied, analysing also the effect of the whole process on the electrical and optical properties of final textured films. In order to evaluate the surfaces obtained upon the new approach and trying to distinguish the effect on each chemical etchant used in it, they have been compared with those obtained using only NH 4 Cl. Finally, a comparison between the properties of the etched ZnO:Al films and those shown by commercially textured SnO 2 :F (Asahi-U type)/glass substrates has been established. a-si:h/µc-si:h tandem solar cells have been fabricated to evaluate the 3

4 suitability of the light scattering properties of the different textured glass/zno:al substrates. 2. EXPERIMENTAL The 800 nm-thick polycrystalline ZnO:Al films were deposited on a cleaned (10x10) cm 2 glass substrate (Corning Eagle XG) using RF magnetron sputtering in a vertical in-line system (VISS 300, VON ARDENNE Anlagentechnik GmbH, Dresden, Germany). A ceramic target consisting of ZnO with 1 w/w% Al 2 O 3 (Cerac Inc., Milwaukee, WI, USA) was used. The deposition was carried out at the substrate temperature of 300ºC, a discharge power density of 2 W cm -2, and an Argon working pressure of 0.1 Pa. Details about the sputtering deposition process and material properties can be found elsewhere [16]. 0.5 wt% HCl has been prepared by dilution of p.a. grade HCl (25 wt%, Merck, Darmstadt, Germany) with ultrapure deionised water (Millipore, Schwalbach, Germany). Regarding the NH 4 Cl aqueous solution, it was prepared by dissolving NH 4 Cl powder, with a purity of 99.5 %, in deionised water (ρ> 18 MΩ-cm) at RT. The etching mechanism of ZnO in diluted NH 4 Cl solution can be found in Ref. 7. The surface morphology was evaluated by an atomic force microscope (Multimode SPM, Veeco-Digital Instruments) operated in tapping mode and using antimony-doped silicon AFM tips (TESPSS tip from Veeco). The roughness of the films was quantified by the average value of the root mean square (RMS) deviation of the AFM measured height from the mean data plane in AFM 10x10 µm 2 images. Another statistical parameter A was also used to quantify the surface morphology in the XY-plane after the etching. It can be defined as the ratio between actual scanned 4

5 surface area (SA) and the area of its projection on the XY plane (PA) following the formula A = (SA - PA) / PA 100 (%) (1) The error bar in A calculation is about 10%. The specular and total optical transmittance spectra were performed by a Perkin- Elmer Lambda 1050 UV/Visible/NIR spectrophotometer with an integrating sphere of 6 mm, illuminating from the glass substrate side. From these measurements, the transmitted haze parameter H T (λ) was determined by the ratio of diffuse to total transmittance (T diffuse /T total ). The transmittance angular distribution function (ADF T ), defined as the intensity distribution of scattered light as function of the angle at which it is scattered, was also determined. For this measurement, a He-Ne laser operating at the wavelength of 635 nm was used as light source. The intensity of the scattered light from the sample was measured with a Silicon photodetector by changing the angle between the detector and the incident laser beam with a step of ϕ=2º. The intensities measured by the detector were normalized in order to obtain the relative angular dependency of the scattered light. The electrical sheet resistance R s was determined from the four-point probe method using a commercial Veeco Instrument system. The thickness of the etched films was measured using a surface profiler. The resulting textured ZnO:Al films were applied as substrates in tandem p-i-np-i-n amorphous/microcrystalline silicon (420 nm a-si:h/ 1.25 µm µc-si:h) solar cells. The Si films were deposited by plasma enhanced chemical vapor deposition (PECVD) in a (30 30) cm 2 reactor. Details of the PECVD Si deposition process have been described elsewhere [17,18]. The back contact consisted of sputter-deposited ZnO:Al from the same system as the front contacts and silver deposited by thermal evaporation 5

6 through a mask to determine a cell area of (1 1) cm 2. To ensure comparability, all cells have been prepared in the same deposition process. Solar cells were characterized with a Wacom WXS 140 S solar simulator (Wacom Electric Co., Saitama, Japan) under standard test conditions (AM1.5, 100 mw cm -2, 25 C). The external quantum efficiency (EQE) was measured by differential spectral response (DSR) at zero bias. 3 RESULTS AND DISCUSSION 3.1 As-deposited and reference samples The as-deposited 800 nm-thick ZnO:Al films showed a surface roughness of around 13.4 nm, a high transmittance in the visible range (~ 80%), a sheet resistance of 3.33 ± 0.04 Ω/sq and a resistivity as low as 2.6 x 10-4 Ω cm. These films after upon the optimized etch process labelled in this work as standard Jülich etch (HCl (0.5 wt% for 40 seconds), which corresponds to the first step of the double etching method developed, presented RMS values of nm, a deterioration of the sheet resistance up to 6.2 ± 0.1 Ω/sq and haze values at the wavelength of 550 nm and 800 nm of 70 % and 35 %, respectively. On the other hand, the commercially textured SnO 2 :F (Asahi-U type)/glass substrate also used here for comparison showed a RMS of around 43 nm and a sheet resistance of 10.5 ± NH 4 Cl based etching process In order to establish the effect of each chemical etchant on the surface morphology, the as-deposited samples were initially dipped in NH 4 Cl-based aqueous solution with concentrations ranging from 5 to 20 wt%. Table 1 summarizes the shape 6

7 of the surface features obtained, defined basically by the RMS value and the ΔA parameter from AFM measurements, together with the electrical properties of the etched films. The properties of the as-deposited film and the reference samples (ASAHI-U and the one labelled as standard Jülich sample) are also included. As the data show in Table 1, the RMS and ΔA values with the etchant concentration and the etching time increase, while maintaining the sheet resistance values close to that shown by the standard Jülich reference sample. On the other hand, RMS values close to this reference sample were only obtained using concentrations higher than 15 wt% (sample G). In spite of this, the features obtained on the NH 4 Cl etched surfaces were clearly different from the ~ nm size craters typically achieved using the standard HCl etch [15]. On the contrary, they are more similar to the pyramid-like morphology presented by ASAHI-U, as it can be appreciated in Figure 1. This figure summarizes the AFM images in areas of 10x10 μm 2 of the reference samples (Fig. 1a) and samples A, C, E and G, which show the higher RMS values reached with different NH 4 Cl concentrations from 5 to 20 wt%, respectively (Fig. 1b). However, regardless the NH 4 Cl concentration used, the RMS values reached always remained lower than those showed by the reference samples. In order to achieve higher RMS values, and taking into account previous results [19], a two-step etching process using firstly high NH 4 Cl concentrations for a short time and secondly a low NH 4 Cl concentrations for a long time was applied. The data of this NH 4 Cl based two step process are included in Table 1. In Figure 2, the AFM images in areas of 10x10 μm 2 of the surface of the samples H and I performed with this method are shown. Data from Table 1 indicate a slight increase of the RMS values with a similar ΔA parameter only for sample I; however, pyramidal facets much more distinguished than in the case of sample G were observed, which could result in a more efficient light scattering by the surface. To evaluate the 7

8 scattering capability of the features obtained, haze parameter in transmission in the wavelength range from 350 to 1200 nm was calculated. The results are displayed in Figure 3. The haze parameter at the wavelength of 550 nm increased from 26 % to 61 % when increasing the NH 4 Cl concentration from 5 to 20 wt%, values still significantly lower than the haze showed by the reference sample, standard Jülich of 70 %. However, the haze factor achieved for sample I was slightly higher than that value, 81 %, which confirms the assumption of superior scattering capability of samples etched under the two-step process. The same tendency was observed for the haze value at the wavelength of 800 nm, varying from 5 % to 41 %. The last value also corresponds to sample I, in comparison with 35% shown by the standard Jülich sample. 3.3 Two step etching approach based on HCl and NH 4 Cl Taking into account the superior properties for light scattering achieved using the NH 4 Cl-based two-step process, we evaluated the double etching process but combining the two different acids, HCl and NH 4 Cl. For this approach, in the first step, the sample is dipped in 0.5 wt% HCl for 40 s (the standard Jülich etch process); and in the second step a lower ph solution based on NH 4 Cl concentrations ranging from 2 wt% to 15 wt% is held for a long time with the aim of enhancing the roughness as much as possible and introducing new facets. Table 2 summarizes the morphological and electrical results of the samples etched with this approach for different NH 4 Cl concentrations and times and Figure 4 displays the AFM images in areas of 10x10 µm 2. Results presented in Table 2 show a slight electrical worsening after the second step, what could lead to a detrimental of the final device characteristics. Nevertheless, the sheet resistance values are quite similar to that of ASAHI U, so adverse effects on the 8

9 solar cell were not expected from an electrical point of view. On the other hand, an increase of the RMS values and a slight increase of ΔA were achieved, while a double feature in the crater shape was observed. The different features obtained with both acids can be explained by the superior acidity of HCl compared to NH 4 Cl, giving rise to a double facet within the crater formed after the HCl etch. Those facts could be beneficial to increase the scattering capability in comparison with the samples etched only with the first optimized step. Figure 5 shows the calculated haze as function of the wavelength for this set of samples. For all cases the haze values at 550 and 800 nm were higher than the standard Jülich sample. Hence, the second step introduced and its associated morphology led to superior scattering properties. Figure 6a displays the ADF T, plotted in logarithmical scale, of the reference ASAHI-U and standard Jülich sample, and sample I, the best one obtained using the NH 4 Cl-based two-step process. Additionally, Figure 6b shows the ADF T of the reference samples together with samples 2 and 3. From both Figures it becomes obvious that two different curve shapes are measured depending on the surface features obtained after the etching. The broadest ADF T functions were reached by the sample I being close to that shown by ASAHI-U, as it was expected by the similarity between the features obtained (see Figures 1a and 2 and the parameters obtained from AFM measurements summarized in Tables 1 and 2 for these samples). The ADF T s shown by the samples 2 and 3 were closer to the standard Jülich sample. This is attributed to the dominated facet is that formed after the etching in HCl, which is defining the main shape of the ADF T. The ADF T of ASAHI-U can be linearly interpolated in Cartesian plot, while the ZnO:Al textured films did not follow the same tendency, being able to obtain a good polynomial fit in the logarithmic representation, and hence, a Gaussian fit in the Cartesian plot [20,21]. The width at half maximum of the obtained Gaussian fit depended stronger on the RMS 9

10 and ΔA values, demonstrating the close relationship between ADF T and the surface features. 3.4 Textured ZnO:Al/glass substrates in tandem solar cells To evaluate the quality of the new substrate morphologies generated by the presented two step etching method for light trapping purposes, a-si:h/µc-si:h tandem solar cells have been deposited on samples 2, 3, being those ones showing the double features in Fig. 4, and on the standard Jülich sample as a reference. The light trapping performance of these devices has been checked by means of EQE measurements, differentiating between the top a-si:h component cell that absorbs the light of wavelengths roughly between 300 and 800 nm, and the bottom µc-si:h component cell absorbing mainly in the (near) infrared range. The resulting EQE data is depicted in Fig. 7. In this specific cell configuration. the reference cell deposited on the "standard Jülich" reference delivered short circuit current densities for the top (J sc, top ) and bottom (J sc, bottom ) component cell of 11.5 and 11.7 ma cm 2, respectively. These values are derived from the EQE data plotted in Fig. 7 as grey curves. After the additional etch step in NH 4 Cl solutions of different concentrations the generated double features (cf. Fig. 4) led to an improved light trapping in the wavelength region above 600 nm solely in the bottom component cell. For sample 3 (5 min in 10 wt% NH 4 Cl as second etch step, Fig. 7a), a current increase of 0.4 ma cm 2 in the bottom µc-si:h component cell (J sc, bottom = 12.1 ma cm 2 ) was observed. This is in accordance with the observation by Owen et al. [22] with HF etching attempts that the introduction of smaller features into the HCl-generated craters mainly lead to improvements in the long wavelength region. 10

11 The trend of an improvement for long wavelengths is also observed for the solar cell on sample 2 (20 min in 5 wt% NH 4 Cl as second etch step, Fig. 7b), expressed as a J sc, bottom of 11.8 ma cm 2. In this case, however, the top component cell suffers from limited blue response that might be explained by non-optimized growth conditions for such morphologies and worse material properties especially close to the TCO/p/i interfaces. Thus the top cell delivers 11.1 ma cm 2 only, resulting in a deterioration of the overall device performance. However, a significant shift of the intercept point of top and bottom cell EQE indicates a transfer of light from bottom to top cell. This is either related to a thicker top cell absorber or to light trapping in the short wavelength range. As the cells were co-deposited, we assume the small features on the surface to induce this short wavelength light trapping effect. 4. CONCLUSIONS The surface morphology of ZnO:Al layers and its close relationship with the scattering properties depending on the treatment its surface with different chemical etchants such HCl and NH 4 Cl was studied. The morphological analysis of the features reached after the etching in these different acids varied from being close to pyramidallike shape using only NH 4 Cl at different concentrations to crater-like shape when the HCl was part of the etching. Taking into account this last background, a new double step etching approach combining both acids, firstly HCl and secondly NH 4 Cl, was developed. This method led to obtain double features in the crater due to the effect of the NH 4 Cl. Using this combination, textured films with RMS values up to 154 nm, haze values up to 93% at 550 nm and 53% at 800 nm, and light scattering into angles above 50 º were obtained. The deposition of tandem a-si:h/µc-si:h solar cells has shown an improved light trapping in the long wavelength region for samples with such double 11

12 features. This renders this novel etching approach as a promising candidate for a further optimization of the ZnO:Al front contact morphology in terms of light trapping issues for specific cell designs. Acknowledgements Partial financial support was provided by the Spanish Ministry of Science and Innovation under the projects AMIC (ENE c04-01) and INNDISOL (IPT ) and by the German Federal Environment Ministry (BMU, grant A). F.B. Naranjo thanks the Spanish government project TEC C02-02 for partial financial support. The authors would also like to thank A. Soubrie from Centro de Microscopía Electrónica Luis Bru for her advice and AFM measurements.the assistance in TCO and solar cell deposition and characterization by Janine Worbs, Joachim Kirchhoff and Simone Bugdol (all Forschungszentrum Jülich GmbH) is gratefully acknowledged. References [1] A. Gordijn, J.K. Rath, R.E.I. Schropp, Prog. Photovolt.: Res. Appl. 14 (2006) 305. [2] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schöpe, C. Beneking, H. Wagner, A. Löffl, H.W. Schock, Thin Solid Films 351 (1999) 247. [3] J. W. Kwon, E. S. Kim, J. Microelectromech. Systems 14 (2005) 603. [4] G. X.-Yong, L. Q.-Geng, L. Y.-Fen, L. J.-Xiao, Brazilian J. Phys. 38 (2008) 336. [5] S.-H. Nam, M.-H. Kim, D. G. Yoo, S. H. Jeong, D. Y. Kim, N.-E. Lee, J.-H. Boo, Surf. Rev. Let. 17 (2010) 121. [6] J. I. Owen, J. Hüpkes, H. Zhu, E. Bunte, S. E. Pust, Phys. Status Solidi A 208 (2011)

13 [7] M. Metha, C. Meier, J. Electromech. Soc. 158 (2011) H119. [8] J. Müller, B. Rech, J. Springer, M. Vanecek, Solar Energy 77 (2004) 917. [9] J. Hüpkes, B. Rech, O. Kluth, T. Repmann, B. Zwaygardt, J. Müller, R. Drese, M. Wuttig, Sol. Energy Mater. Sol. Cells 90 (2006) [10] S. J. Tark, M. G. Kang, S. Park, J. H. Jang, J. C. Lee, W. M. Kim, J. S. Lee, D. Kim, Curr. Appl. Phys. 9 (2009) [11] D.-W. Kang, S.-H. Kuk, K.-S. Ji, S.-W Ahn. M.-K. Han, Phys. Status Solidi C 3-4 (2010) 925. [12] O. Kluth, G. Schöpe, J. Hüpkes, C. Agashe, J. Müller, B. Rech, Thin Solid Films 442 (2003) 80. [13] S. Faÿ, L. Feitknecht, R. Schlüchter, U. Kroll, E. Vallat-Sauvain, A. Shah, Sol. Energy Mater. Sol. Cells 90 (2006) [14] J. Hüpkes, J. I. Owen, E. Bunte, H. Zhu, S. E. Pust, J. Worbs, G. Jost, Proceedings of 25 th European Photovoltaic Solar Energy Conference and Exhibition, Valencia, Spain (2010) p [15] H. Zhu, J. Hüpkes, E. Bunte, J. Owen, S. M. Huang, Sol. Energy Mater. Sol. Cells 95 (2011) 964. [16] M. Berginski, J. Hüpkes, M. Schulte, G. Schöpe, H. Stiebig, B. Rech, M. Wuttig, J. Appl. Phys. 101 (2007) [17] T. Roschek, T. Repmann, J. Müller, B. Rech, H. Wagner, J. Vac. Sci. Technol. 20 (2002) 492. [18] B. Rech, T. Roschek, T. Repmann, J. Müller, R. Schmitz, W. Appenzeller, Thin Solid Films 427 (2003)

14 [19] S. Fernández, O. de Abril, F. B. Naranjo, J. J. Gandía, Proceedings of the 8 th International Conference on Coatings on Glass and Plastics, Braunschweig, Germany, (2010) pp [20] O. Isabella, A. Campa, M.C.R. Heijna, W.J. Soppe, A.J.M. Van Erven, R.H. Franken, H. Borg, M. Zeman, Proceedings of the 23 rd EUPVSEC, Valencia, Spain (2008) pp [21] J. Krč, M. Zeman, O. Kluth, F. Smole, M. Topič, Thin Solid Films 426 (2003) 296. [22] J. I. Owen, J. Hüpkes, E. Bunte, S. E. Pust, A. Gordijn, Proceedings of the 25 th EUPVSEC, Valencia, Spain (2010) pp

15 Figure Captions. Fig 1. AFM images in areas of 10x10 µm 2 of the (a) reference samples and (b) textured surfaces achieved upon etching in NH 4 Cl with concentrations ranging from 5 to 20 wt%. Fig. 2. AFM images in areas of 10x10 µm 2 of the surfaces of the samples H and I achieved using the two-step method based on different NH 4 Cl concentrations. Fig. 3 Haze as a function of wavelength of the reference samples and some samples described in Table 1. The arrow indicates the shift from increasing the relative NH 4 Cl concentration. Fig. 4 AFM images in areas of 10x10 µm 2 of the sample surfaces etched with the double step method based on the combination HCl and NH 4 Cl, described in Table 2. Fig. 5 Spectral haze of samples 1, 2, 3 and 4 etched with the double step method. Fig. 6 The measured angular distribution function of diffusely transmitted light (ADF T ) plotted in logarithmical scale for the reference samples and (a) sample I, (b) samples 2 and 3. Fig. 7 Individual EQE the two component cells in a-si:h/µc-si:h solar cells on (a) sample 3, (b) sample 2. The EQE of a solar cell from the same absorber deposition 15

16 process on a standard Jülich reference substrate is plotted in grey for comparison. The sums of both component cells, representing the overall EQE, are shown as dashed lines. The component cell currents are included in the plots. 16

17 Table Caption. Table 1. Description of the wet-chemical etching process used to texture approximately 800 nm-thick ZnO:Al films deposited on Corning glass. The final thicknesses after the etching together with the RMS and A values, calculated from 10x10 µm 2 AFM images, and the sheet resistances are shown. For comparison, the parameters from standard Jülich textured ZnO:Al films and the commercial substrate, labelled as ASAHI-U, are also included. Sample Wet-chemical etching Process Time Thickness (nm) RMS (nm) A (%) R sheet (Ω/sq) ASAHI-U ± 0.2 Asdeposited ZnO:Al Standard Jülich ± 0.04 HCl (0.5%) 40 sec ± 0.1 A NH 4 Cl (5%) 20 min ± 0.03 B NH 4 Cl (10%) 10 min ± 0.1 C NH 4 Cl (10%) 15 min ± 0.2 D NH 4 Cl (15%) 4 min ± 0.05 E NH 4 Cl (15%) 8 min ± 0.2 F NH 4 Cl (20%) 4 min ± 0.2 G NH 4 Cl (20%) 6 min ± 0.2 H I NH 4 Cl (15%) + NH 4 Cl (5%) NH 4 Cl (20%) + NH 4 Cl (5%) (4 + 20) min ± 0.5 (4 + 20) min ± 0.5 Table 1. S. Fernández et al.

18 Table 2. Description of the double step etching approach based on a combination of HCl (0.5 wt%) 40 sec and different NH 4 Cl concentrations (from 2 to 15 wt%) to texture the ZnO:Al films deposited on Corning glass. The final thicknesses, the RMS and A values, calculated from 10x10 µm 2 AFM images, and the sheet resistances of the etched films have been included. Sample Wet-chemical etching Thickness Process t (min) (nm) RMS (nm) A (%) R sheet (Ω/sq) HCl (0.5), 40 sec ± 0.4 NH 4 Cl (2%), t HCl (0.5), 40 sec ± 0.6 NH 4 Cl (5%), t HCl (0.5), 40 sec ±0.1 NH 4 Cl (10%), t HCl (0.5), 40 sec ± 0.6 NH 4 Cl (15%), t Table 2. S. Fernández et al.

19 a) ASAHI-U Standard Jülich textured ZnO:Al 0 nm 200 nm 0 nm 800 nm 2.0µm 2.0µm HCl (0.5 wt%) 40 sec

20 b) NH 4 Cl (5 wt%) 20 min NH 4 Cl (10 wt%)15 min 800 nm 2.0µm 2.0µm NH 4 Cl (15wt%) 8 min NH 4 Cl (20wt%) 6 min 2.0µm 2.0µm 0 nm

21 1 st step: NH 4 Cl (15 wt%), 4 min 1 st step: NH 4 Cl (20 wt%), 4 min 800 nm 2.0µm 2.0µm nd step: NH 4 Cl (5 wt%), 20 min 2 nd step: NH 4 Cl (5 wt%), 20 min 2.0µm 2.0µm 0 nm

22

23 HCl +NH 4 Cl (2 wt%), 25 min HCl +NH 4 Cl (5 wt%), 20 min 800 nm 2.0µm 2.0µm HCl +NH 4 Cl (10 wt%), 5 min HCl +NH 4 Cl (15 wt%), 4 min 2.0µm 2.0µm 0 nm

24

25

26

27 e x te rn a l q u a n tu m e ffic ie n c y E Q E " S t a n d a r d J ü l i c h " " S t a n d a r d J ü l i c h " % N H 4 C l (5 m in ) m A c m m A c m m A c m m A c m w a v e le n g th [n m ] -2-2

28 e x te rn a l q u a n tu m e ffic ie n c y E Q E " S t a n d a r d J ü l i c h " " S t a n d a r d J ü l i c h " + 5 % N H 4 C l (2 0 m in ) m A c m m A c m m A c m m A c m w a v e le n g th [n m ] -2-2