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Comparison of LPCVD and Sputter-Etched Zno Layers Applied As Front Electrodes in Tandem Thin-Film Silicon Solar Cells

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Comparison of LPCVD and Sputter-Etched Zno Layers Applied As Front Electrodes in Tandem Thin-Film Silicon Solar Cells Comparison of LPCVD and sputter-etched ZnO layers applied as front electrodes in tandem thin-film silicon solar cells Etienne Moulin1, Karsten Bittkau2, Michael Ghosh2, Grégory Bugnon1, Michael Stuckelberger1, Matthias Meier2, Franz-Josef Haug1, Jürgen Hüpkes2, Christophe Ballif1 1Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin-Film Electronics Laboratory, rue de la Maladière 71b, 2002 Neuchâtel, Switzerland 2IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany Abstract: Aluminum-doped zinc oxide (ZnO:Al) layers deposited by sputtering and boron-doped zinc oxide (ZnO:B) layers deposited by low-pressure chemical vapor deposition (LPCVD) are well-established materials for front electrodes in thin-film silicon solar cells. In this study, both types of front electrodes are evaluated with respect to their inherent properties and their surface textures in micromorph tandem solar cells in the superstrate configuration. The silicon layer stack investigated here consists of a 220-nm-thick amorphous silicon top cell, a 40-nm-thick intermediate reflector and a 1.1-µm-thick microcrystalline silicon bottom cell; for this specific silicon layer stack, the LPCVD ZnO:B provides higher power conversion efficiency than its sputtered ZnO:Al counterpart. The growth-friendly surface topography of ZnO:Al yields better electrical performance. However, the optical performance is the limiting factor. Detailed analysis of the experimental results allows us to clearly distinguish the origin of the observed differences. Several possible upgrades are discussed to further improve performance of cells grown on sputter-etched ZnO:Al. 1. Introduction: The thin-film silicon (TF-Si) solar technology offers many advantages, including low production costs (<0.5 $/Wp [1]), the abundance and non-toxicity of the base materials and low material usage (thicknesses in the µm range). In addition, TF-Si solar modules can be deposited on low-cost substrates, including glass, ceramic and lightweight flexible plastic or metal foils [2], and they can be semi-transparent, which offers a large degree of freedom for building integration and utilization in electronic devices. The most common transparent conductive oxides (TCOs) used as front electrodes in TF-Si solar modules that allow for large-scale production and high power conversion efficiency are (i) self- textured boron-doped zinc oxide (ZnO:B) deposited by low-pressure chemical vapor deposition (LPCVD) [1, 3], (ii) self-textured fluorine-doped tin oxide (SnO2:F) prepared by atmospheric-pressure CVD (APCVD) [2] and (iii) sputter-etched aluminum-doped zinc oxide (ZnO:Al) [4]. We often find one of these TCOs at the front of TF-Si cells or modules reported to have the highest efficiencies. For instance, TEL Solar recently reported an impressive stabilized efficiency of 12.24% for gen-5 modules (i.e. 1.1×1.3 m2) in the micromorph tandem configuration grown on LPCVD ZnO:B [5]. At the cell level (1 cm2), a certified stabilized efficiency of 13.4% for triple-junction solar cells grown on sputter-etched ZnO:Al was obtained by LG-Electronics [6]. The PV-Lab in Neuchâtel 1 communicated a certified stabilized efficiency of 12.6% for a micromorph cell deposited on LPCVD ZnO:B [7]. Excellent results were also found by HyET Solar for flexible tandem modules grown on SnO2:F [2]. The record certified efficiency for a single-junction amorphous silicon (a-Si:H) cell was obtained with SnO2:F used as front electrode [8]. This non-exhaustive list stresses the high quality of these three types of TCO materials for TF-Si solar cell applications. Multi-scale front-electrode textures have been proposed to achieve the highest solar module/cell efficiencies [9-17]. In these electrodes, a micrometer-scale texture provides high-quality Si growth and light trapping, while light in-coupling and other optical properties are promoted by sub-micron features. Although excellent results were obtained on these multi-scale textures, the present study focuses on single textures, based on both sputter-etched ZnO:Al and LPCVD ZnO:B. IEK-5 in Jülich (Germany) and PV-Lab in Neuchâtel (Switzerland), collaborating in the framework of the European project Fast-Track, have gained valuable experience over the last decades regarding these respective TCO materials. Here, we evaluate the potential of these two types of front electrodes for micromorph tandem solar cells. Sensitive to the fact that PV-Lab (Neuchâtel) has focused its activities mainly on the development of cells deposited on LPCVD ZnO:B, we are well aware that the results presented here for cells grown on ZnO:Al are not optimized results. However, we believe that the present comparison can provide constructive information about the potential and limits of these two types of TCO. The present study is organized as follows: first, we introduce the opto-electronic and structural properties of sputter-etched ZnO:Al and LPCVD ZnO:B. Second, the electrical performance under AM1.5 illumination of micromorph cells co-deposited on these two types of TCOs is compared. The cells are then characterized under various illumination spectra, which is equivalent to modifying the photocurrent-matching conditions of the two component cells. This provides useful information about the intrinsic electrical performance and quality of the deposited cells. Finally, we discuss the origin of the differences observed for the two types of cells grown on these TCOs and address solutions from the literature to further increase cell performances. 2. Experimental details and TCO characterization: 2.1 Single-texture ZnO:Al- and ZnO:B-based superstrates 1.1-mm-thick glass sheets (Eagle XG, Corning) were used as superstrates. The ZnO:Al layers were deposited at a substrate temperature of 300 °C using RF-sputtering from a ceramic target with 1 wt.% Al2O3 doping [17b]. Their initial thickness was approximately 800 nm. High short-circuit photocurrents (Jsc) in TF-Si solar cells require efficient light trapping, which is usually obtained by texturing of the interfaces. As sputtered ZnO grows almost perfectly flat – with a root mean square (rms) roughness typically well below 15 nm for the deposition regimes commonly applied for high- quality films – a chemical treatment has to be performed to texture its surface. Here, this treatment consisted of a chemical etching in a solution of 0.5%-diluted hydrochloric acid for 30 s; the anisotropic etching resulted in the formation of randomly distributed crater-like textures at the ZnO surface with a correlation length in the micrometer range (see Fig. 1a). An rms roughness of around 130 nm was evaluated by atomic force microscopy (AFM). The final average thickness was about 600 nm. Owing to its relatively high charge-carrier mobility (µ ≈ 40 cm2·V-1·s-1) and high carrier 20 -3 density (n ≈ 3.7x10 cm ), this TCO layer provided a low sheet resistance Rsq of around 5 Ω/sq (≈ 8 Ω/sq after etching). 2 Figure 1: 8×8 µm2 AFM scans showing the surface morphology of a sputter-etched ZnO:Al (a) and a LPCVD ZnO:B (b) layer, used as front electrodes in micromorph tandem solar cells. The LPCVD-ZnO:B layers deposited in this study represent the state-of-the-art front electrodes at IMT (Neuchâtel) for micromorph tandem solar cells. They are deposited at 180 °C, have a thickness of approximately 2.3 µm and their rms roughness, determined by AFM, is approximately 100 nm. In contrast to sputter-etched ZnO:Al, LPCVD ZnO:B develops an as-grown surface texture made of pyramidal features (see Fig. 1b); these features are much steeper than those observed at the ZnO surface. The ZnO:B layers considered in this work exhibit a carrier mobility µ of about 30 cm2·V-1·s-1, 19 -3 and a carrier density n of approximately 3×10 cm , which gives a sheet resistance Rsq in the order of 20 Ω/sq. The ZnO films were optically characterized by means of photo-thermal deflection spectroscopy (PDS) and ellipsometry. For this purpose, the as-grown rough LPCVD-ZnO layers were polished to allow for a straightforward determination of the optical parameters and to avoid roughness-induced light- trapping effects, which would result in an overestimation of the absorption; the optical properties of ZnO:Al were obtained on flat films prior to etching. The refractive index n of ZnO:B varies from approximately 2.5 to 1.9 over the relevant wavelength region for solar cells, i.e. between 300 nm and 1100 nm (see Fig. 2a). Owing to its larger free-carrier density, the refractive index of ZnO:Al exhibits a stronger drop, from 2.5 to 1.5 over this spectral range. The absorption coefficient derived by PDS is significantly higher for ZnO:Al than for ZnO:B – by around one order of magnitude over the full spectrum – except in the ultraviolet (UV) region (see Fig. 2b). Fig. 2c depicts the calculated absorptance A of the ZnO films deduced from the absorption coefficient α, as 퐴 = 1 − 푒−훼(휆)푑, with d the film thickness. Only for wavelengths λ below 400 nm is the absorptance of LPCVD ZnO:B larger than that of ZnO:Al: the higher carrier density of ZnO:Al leads to a wider optical bandgap, which results in a shift towards shorter wavelengths of the fundamental absorption edge as compared to ZnO:B. This effect is referred to as the Burstein-Moss effect [18]. At the other end of the spectrum, in the near-infrared (NIR) wavelength region, the higher free-carrier density of ZnO:Al leads to a higher absorptance, which is mostly dictated by the position of the free- electron plasma resonance [19]. In the intermediate wavelength region, the higher absorptance of ZnO:Al is related to defect-induced sub-bandgap absorption [20] and to the tale of the free-electron plasma resonance.
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