Microcrystalline silicon–oxygen alloys for application in silicon solar cells and modules

Solar Energy Materials & Solar Cells ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Microcrystalline silicon–oxygen alloys for application in silicon solar cells and modules A. Lambertz a, V. Smirnov a, T. Merdzhanova a,n, K. Ding a, S. Haas a, G. Jost a, R.E.I. Schropp b,1, F. Finger a, U. Rau a a b

IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Gemany Utrecht University, Debye Institute for Nanomaterials Science, Physics of Devices, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands

art ic l e i nf o

Keywords: μc-SiOx:H Intermediate reflector Photon management Tandem cells Hetero junction cells Solar modules

a b s t r a c t Microcrystalline silicon oxide (mc-SiOx:H) alloys prepared by plasma enhanced chemical vapor deposition (PECVD) represent a versatile material class for opto-electronic applications especially for thin-film and wafer based silicon solar cells. The material is a phase mixture of microcrystalline silicon (mc-Si:H) and amorphous silicon oxide (a-SiOx:H). The possibility to enhance the optical band gap energy and to adjust the refractive index over a considerable range, together with the possibility to dope the material p-type as well as n-type, makes μc-SiOx:H an ideal material for the application as window layer, as intermediate reflector (IR), and as back reflector in thin-film silicon solar cells. Analogously, μc-SiOx:H is a suitable material for p- and n-type contact layers in silicon hetero junction (SHJ) solar cells. The present paper gives an overview on the range of physical parameters (refractive index, optical band gap, conductivity) which can be covered by this material by variation of the deposition conditions. The paper focuses on the interdependence between these material properties and optical improvements for amorphous silicon/ microcrystalline silicon (a-Si:H/mc-Si:H) tandem solar cells prepared on different substrates, such as Asahi (VU) and sputtered ZnO:Al. It gives a guideline on possible optical gains when using doped mc-SiOx:H in silicon based solar cells. As intermediate reflector in a-Si:H/mc-Si:H tandem cells mc-SiOx:H leads to an effective transfer of short circuit current generation from the bottom cell to the top cell resulting in a possible thickness reduction of the top cell by 40%. Within another series of solar cells shown in this paper a short circuit current density of 14.1 mA/cm² for an a-Si:H/mc-Si:H tandem solar cell with a mc-SiOx:H intermediate reflector is demonstrated. A SHJ solar cell on a flat (non-textured) wafer using p- and n-type doped mc-SiOx:H contact layers with an effective area efficiency of 19.0% is also presented. & 2013 Elsevier B.V. All rights reserved.

1. Introduction During recent years, microcrystalline silicon oxide (mc-SiOx:H) has established itself as a key material for functional layers in thin-film silicon solar cells and modules. It is a mixed phase material of an oxygen rich amorphous silicon oxide (a-SiOx:H) phase and a doped microcrystalline silicon (mc-Si:H) phase. The tunability of its refractive index and of its optical band gap combined with a suitable conductivity as p-type and n-type doped material enables its application as a replacement for the conventional doped layers for amorphous (a-Si:H) as well as microcrystalline (mc-Si:H) cells in thin-

n

Corresponding author. E-mail address: [email protected] (T. Merdzhanova). 1 Present address: Energy research Center of the Netherlands (ECN), Solar Energy, High Tech Campus Building 5 (WAY); p 057, 5656 AE Eindhoven; The Netherlands, and Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

film silicon tandem solar cells. The record of proven benefits starts with its application as an intermediate reflector (IR) between the two cells [1–6]. Due to its low refractive index n a portion of the light is reflected back into the a-Si:H top cell. Microcrystalline silicon oxide has also proven its capabilities to be beneficial for the use as n-layer for the microcrystalline silicon (mc-Si:H) single solar cell [7], not just improving the optical performance of the back reflector [6,8], but also improving the electrical properties of the mc-Si:H cell [9,10]. In addition doped mc-SiOx:H improves the performance when using it as window layer in amorphous silicon/microcrystalline silicon (a-Si:H/ mc-Si:H) tandem solar cells on ZnO coated substrates [11,12], especially due to its adaptable refractive index between Si and ZnO yielding an improved in-coupling of the light. An additional field of application are silicon heterojunction (SHJ) solar cells where mc-SiOx:H can serve as a beneficial contact layer material due to its favorable combination of electrical and optical properties. Partial implementation as emitter [13] or back contact material [14] and on both sides of the wafer as emitter and back contact [15] in SHJ solar cells has already been reported.

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For all these applications the required optical material properties are a high optical band bap (E04) (defined as the photon energy where the absorption coefficient α equals 104 cm-1) to avoid parasitic absorption and adaptable refractive index, preferably no 3. The required electrical conductivities are above 2
10−6 S/cm (for thicker IR 1
10-5 S/cm) to achieve a low series resistance (Rs) [3,16], and below 2
10−1 S/cm aiming a high shunt resistance (Rsh) (i) for shunt-quenching of cracks in the absorber layer [6] and (ii) for the module interconnection [17]. When interconnecting the cell stripes to a module a conductivity of the window layer above 2
10-1 S/cm would electrically connect the front TCO of neighboring cell stripes and thus lead to a shunt. The increase in series resistance Rs will be determined by the conductivity in growth direction of the mc-SiOx:H and the decrease in shunt resistance Rsh by its conductivity in lateral direction. An overview of the physical layer properties aimed for is given in Table 1. The conductivity limits are estimated as described in the device structure section. In addition, a function of mc-SiOx:H as a nucleation layer for microcrystalline silicon growth might be desired [18]. The doped mc-SiOx:H applications make use of the high electrical conductivity of the doped microcrystalline silicon mc-Si:H phase and the optical properties of the amorphous silicon oxide (a-SiOx:H) phase [3,16,19,20]. A small fraction of highly conductive doped mc-Si:H in the a-SiOx:H layer already provides a sufficient conductivity above 10−5 S/cm. This two-phase mixture allows decoupling partly the optimization task for the optical and electrical properties by adjusting the deposition parameters which deliver the appropriate mixture of the two phases and, unlike other material deposition processes for example sputtering; its insitu deposition is fully compatible to the industrial production of thin-film silicon and SHJ solar cells. The present study focuses on our recent developments of doped mc-SiOx:H material and its successful application in thin-film silicon single junction and tandem solar cells, modules, and silicon hetero junction solar cells. Relationships between the material properties and the performance of the solar cells are discussed.

a-Si:H i-layer thickness, carrier collection efficiency and the optical performance of the cell. The optical performance can be understood as a term for advantageous in-coupling, reduced parasitic absorption and improved trapping of the light. The mc-Si:H bottom cell absorbs the light with a longer wavelength of up to 1100 nm. Since

Fig. 1. A schematic drawing of the a-Si:H/mc-Si:H tandem solar cell. The tandem cells have a mc-SiOx:H window layer (pTop), intermediate reflector (nTop and/or pBot) and/or mc-SiOx:H back contact (nBot). The tandem solar cells presented in the paper have doped mc-SiOx:H layers within the n/p junction acting as intermediate layer with a thickness of 507 10 nm.

2. Device structures The device structures of the a-Si:H/mc-Si:H tandem solar cells and SHJ solar cell with integrated p- and n-type doped mc-SiOx:H material is shown in Figs. 1 and 2, respectively. The a-Si:H top cell's absorber layer which has typically a high optical band gap of E04 ¼1.9 eV is the first cell when describing the tandem cell from the light incident side. The top cell mostly absorbs the light with a short wavelength of up to 750 nm. The amount of light absorbed and consequently the generated current depends strongly on the

Fig. 2. Schematic illustration of the SHJ solar cell structure with PECVD grown emitter, back contact and buffer layers on flat p-type wafer covered with sputtered TCO and thermally evaporated silver.

Table 1 Targeted physical layer properties for the proposed applications in silicon solar cells. The applications of layers described in the table are sketched in Figs. 1 and 2. The limits of the conductivity were calculated as described later on. The n are indicating that the upper limit of the conductivity for this layer is not known. If the conductivity for these layers is higher as for doped ZnO [45] an additional laser scribing step would be necessary. s low limit [S/cm] growth s high limit [S/cm] lateral Further issues/benefits direction direction

Applied in

as…-layer/Ref.

E04 [eV]

mc-Si:H single junction a-Si:H/mc-Si:H tandem

Window [4,7]

42.2  2.5 2
10-6

2
10-1

-6

-1

SHJ

pTop [7,11,12] nTop [2,3,6] pBot [4] nBot [6,8–10] n window [13,15] p back contact [14,15]

42.2 42.2 42.2 42.2 42.2

n

 2.5 o 2.5 o 2.5 o 2.5  2.5

2
10 1
10-5 1
10-5 2
10-6 2
10-6

2
10 n n

TCO/window layer tunnel contact; growth compatible with TCO; nucleation layer for 〈i〉 mc-Si:H TCO/p tunnel contact; growth compatible with TCO n/p tunnel contact; nucleation layer for p-layer n/p tunnel contact; nucleation layer for i-layer n/TCO tunnel contact layer Tunnel contact TCO/n layer

42.2 o 2.5 2
10-6

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both cells are connected electrically in series, the cell with the lowest current limits the current of the device. This means to achieve high efficiencies, the current generated by top and bottom cell have to be well balanced. But to reduce the effect of lightinduced degradation on the cell performance, the amorphous top cell's thickness has to be kept as low as possible [21,22]. In order to obtain a sufficient top cell photocurrent the amount of light absorbed in the top cell can be increased by using an intermediate reflector layer (IR) which reflects the light with a photon energy above the band gap of the a-Si:H top cell. The effects of intermediate reflectors for such cell configurations have been demonstrated [1–5] and evaluated in model studies [23–25]. Fig. 1 indicates that either nTop or pBot or both of them can serve as IR. The other doped layers pTop and nBot are also important to improve the optical performance i.e. to reduce parasitic absorption. Beside the required optical properties the limits of the electrical conductivity are also listed in Table 1. These limits are important to avoid low values of fill factor (FF) and consequently efficiency (η) due to a high series resistant (Rs) or a low shunt resistant (Rsh) introduced by the doped layers. For example, to estimate the influence of the intermediate reflector on the FF we added numerically the Ohmic JV-characteristics of a series resistance Rs corresponding to the resistance of the intermediate reflector to a measured JV-characteristics of our standard tandem solar cell. For an additional series resistance of 1 Ω we calculated a decrease in FF of about 1%. When increasing this series resistance to 10 Ω we expect a drop in FF of about 7%. Based on these calculations we set the highest acceptable additional series resistance to be 1 Ω. This additional series resistance would be reached when an intermediate reflector with a thickness of 100 nm and a conductivity of 10-5 S/cm in growth direction is used. We define this conductivity value (10-5 S/cm) as acceptable lower conductivity for the intermediate reflector layer [s-limit]. For window layers or back contacts which are typically thinner (15–25 nm) the s-limit is accordingly lower. Correspondingly we estimated the minimum acceptable shunt resistance (10 kΩ) between two cell stripes when interconnecting in a module. This would be the result from a too high conductivity in lateral direction of 4 2
101 S/cm i.e. of the window layer (pTop). For this a scribe width between the TCO stripes in a module of 30 mm and a window layer (pTop) layer thickness of 15 nm were assumed. The SHJ solar cell combining c-Si wafer with a-Si:H thin-film technology benefits from both the high quality of the crystalline absorber and the formation of well passivated junctions at low temperature making it one of the most attractive concepts for high efficiency and low cost silicon photovoltaic devices. Motivated by the success of the Sanyo/Panasonic HIT concept using intrinsic a-Si:H buffer and doped a-Si:H contact layers [26], a growing number of research groups are searching for advanced alternative buffer and contact layer materials for SHJ solar cells. One possibility to improve the cell performance is to reduce the optical losses in the intrinsic and/or doped a-Si:H layers. We fabricated SHJ solar cells involving all buffer layers made from a-SiOx:H and all contact layers made from mc-SiOx:H materials as shown Fig. 2. Si-rich, intrinsic a-SiOx:H is a promising alternative to the intrinsic a-Si:H buffer layer. With a wafer passivation quality similar to that of a-Si:H, the a-SiOx:H might be able to improve the solar cell efficiency further due to its higher transparency giving rise to lower parasitic optical loss [27]. Further advantages are the suppression of epitaxial growth [28] and the higher thermal stability [29], both enabling more flexibility in the process development. Doped mc-SiOx:H is an attractive contact layer material due to its high conductivity and at the same time high optical band gap and favorable refractive index. Only the implementation as emitter [13] or back contact [14] material in SHJ solar cells has been reported so far.

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3. Experimental details 3.1. mc-SiOx:H preparation Microcrystalline silicon oxide (mc-SiOx:H) films were deposited in a plasma enhanced chemical vapor deposition (PECVD) system using an excitation frequency of 13.56 MHz on glass substrates (Corning Eagle 2000). Deposition parameters were a substrate temperature of around 185 1C and a plasma excitation power of around 0.35 W/cm² with respect to the electrode area. The process gases used were silane (SiH4), carbon dioxide (CO2) and hydrogen (H2). As dopant source we used trimethylboron (TMB) diluted in helium (He) for the p-type material and phosphine (PH3) diluted in SiH4 for n-type material. More details on the preparation conditions are given in [4,16]. 3.2. Solar cells and modules preparation The substrate used in the thin-film solar cell developments was SnO2:F coated glass from the Asahi Glass Company (AGC) (both type U and type VU) or texture-etched ZnO:Al (Jülich ZnO:Al) [30–32]. In tandem solar cells (Fig. 1) and modules, the 〈p〉 and the 〈i〉 layer of the a-Si:H top cell are amorphous, the 〈n〉 layer is amorphous and/or microcrystalline. The 〈p〉 and 〈i〉 layers of the bottom cell are microcrystalline and the 〈n〉 layer is amorphous, using optimum phase mixture (OPM) material [33,34] for the mcSi:H absorber layer. The doped mc-SiOx:H layers were used as described (Fig. 1). To define the exact area of 1 cm² the tandem cells were laser patterned. For tandem solar cells and modules ZnO:Al/Ag back contacts were used. In some cases an antireflection foil from the company Solarexcel™ was used [35]. In addition mc-Si:H single junction solar cells were prepared in the sequence: glass/ZnO:Al/μc-SiOx:H〈n〉/μc-Si:H〈i〉/μc-Si:H〈p〉 and illuminated from the n-side. Additionally, n-side illuminated n-i-p solar cells, employing μc-Si:H n-layer and p-side illuminated p-i-n cells were prepared to enable a comparison of the different window layers. The thickness of the i-layers of solar cells was 1 mm, and the area of the individual devices was defined by the 1 cm2 Ag or ZnO:Al/Ag back contacts as described in the text. mc-Si:H single junction solar cells, tandem solar cells and SHJ solar cells were prepared in 10
10 cm² substrate area deposition system. The tandem solar modules were prepared in a 30
30 cm² substrate area deposition system and the interconnection was realized by a three-step patterning process selectively removing the TCO front contact (step 1), the thin-film silicon layer stack (step 2) and the thin-film silicon/back contact layer stack (step 3) of the solar module. The tandem solar modules consist of 26 cell stripes of 1
26 cm2 aperture area that are connected in series. The cell stripes are 1 cm wide, including a dead area width of 300 mm. In this study, optically pumped solid-state lasers with wavelengths of 355 nm (step 1) and 532 nm (step 2 and 3) were used. Details of the interconnections are given elsewhere [36]. Fig. 2 shows the schematic drawing of the silicon hetero junction (SHJ) solar cell structure. We used double side polished, 〈100〉 oriented, 250 mm thick p-type float zone silicon wafers with a doping concentration of (4.62 70.15)
1015 cm−3 as the cell absorber. Prior to deposition, the wafers were treated in a mixture of sulfuric acid and hydrogen peroxide to clean off organic residues, followed by dipping them for 30 s in diluted hydrofluoric acid (1%) to remove the oxide from the wafer surface. After the PECVD deposition of the silicon oxide layers, the cells were completed with sputtered indium tin oxide (ITO) on both sides, thermally evaporated silver grids through a shadow mask (active area ¼0.67 cm²) on the front side and full area silver on the rear side. The silver grid design consists of one centered busbar and

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nine grid fingers, which cover in total 33% of the cell area. The completed cells were then annealed on a heating plate at 200 1C for 2 min for the contact formation and the improvement of the passivation quality. 3.3. Characterization methods The optical properties of the microcrystalline silicon oxide (mc-SiOx:H) films were investigated by optical transmission and reflectance measurements and photo thermal deflection spectroscopy (PDS). The band gap E04 is used as a measurement for the transparency and is given by the photon energy at which the absorption spectra reaches α¼ 104 cm1. The refractive indices were calculated from the transmission and reflection spectra using the Fresnel equation. From conductivity measurements with coplanar Ag-electrodes we obtain the lateral conductivity (s) of the thinfilms. A double source (class A) AM 1.5 solar simulator was used to determine the cell efficiencies η at standard test conditions (AM 1.5G, 100 mW/cm², 25 1C), as well as fill factors FF, open circuit voltages Voc and short circuit current densities Jsc. To measure the EQE of the mc-Si:H single junction solar cells and silicon hetero junction (SHJ) solar cells an EQE setup using light from a monochromator was used. For the silicon hetero junction (SHJ) solar cells the light spot was focused between two silver grids during the EQE measurement. Considering that the integrated current density Jsc,int from EQE is more accurate for the SHJ solar cells than Jsc from the solar simulator due to possible Ag-grid area inaccuracies, we recalculated the efficiency ηact for the active area from FF, Voc and Jsc,int. The internal quantum efficiency IQE was determined from the equation IQE¼EQE/ (1-R), where R is the reflectance which was measured using a UV-VIS-NIR Perkin-Elmer photospectrometer with attached Ulbricht integrating sphere. To measure the EQE of tandem solar cells and solar modules an identical double source (class A) solar simulator with additional interference filters, chopper wheel and lock-in amplifier was used. The EQE of the top cell was measured under red bias light and of the bottom cell was measured under blue bias light. As a bias light source LED arrays were used. The IQE was determined as previously described. This setup was also used to measure the uniformity of the solar module's photovoltaic parameters and

the solar module's EQE. Therefore the module was separated by laser patterning perpendicular to the cell stripes in three 8.65 cm wide areas. The patterning allows JV and EQE measurements on 78 cells each with an area of 8.65
1 cm2. The (class A) xenon flasher from the company halm is used to perform current voltage (JV) measurements of solar modules with an aperture area of 676 cm2.

4. Results and discussion 4.1. mc-SiOx:H thin-films For the application of mc-SiOx:H in solar cells the electrical and optical properties have to be adjusted. In general a trade-off between high transparency and high conductivity, has to be dealt with. A low refractive index and a high band gap at a sufficient conductivity are desired for example if the material is used as intermediate reflector. A plot of conductivity versus optical band gap and refractive index like in Fig. 3 can serve as a figure of merit for the suitability of the mc-SiOx:H for the respective application. We defined a conductivity value of 10−5 S/cm as acceptable for the intermediate reflector layer as described in the device structure section. This s-limit is indicated by the dotted line in Fig. 3. mc-SiOx:H films with n- and p-type doping were developed on small size (10
10 cm²) and medium size (30
30 cm²) substrate areas. High conductivities (s) of about 10 S/cm were found for low band gaps of 1.8 eV (Fig. 3a) and high refractive indices of 3.7 (Fig. 3b). With increasing band gap E04 and decreasing refractive index n the conductivity is reduced. When a conductivity of s≈10-3 S/cm is reached, the s decrease is more pronounced for a further increase of E04 and decrease of n. For both n-type (circles) and p-type (squares) mc-SiOx:H material similar trends were observed. Also when preparing the n-type mc-SiOx:H material in the 30
30 cm² system (triangles) similar properties, compare to 10
10 cm² system, were achieved. Curves are guide to the eye and are taken as upper limit for the conductivity at a given E04 or n from different series using different gas flows. For n-type amorphous silicon oxide (a-SiOx:H) taken from literature [37] (full triangles) the absence of the doped mc-Si:H phase leads to smaller conductivities compared to mc-SiOx:H films at the same E04 and n.

Fig. 3. The electrical conductivity s for mc-SiOx:H n-type (circles, triangles) and p-type (squares) films as a function of the band gap energy E04 (a) and as a function of the refractive index n (b), respectively. Results of mc-SiOx:H n-type prepared in 30
30 cm² substrate area deposition system are also shown (triangles). The results of amorphous silicon oxide films (〈n〉 a-SiOx:H) from the literature [37] (full triangles) are shown for comparison. In both graphs the calculated lower conductivity limit s-limit for the use as intermediate reflector is indicated as a dotted line. Curves are guides to the eye and are taken as upper limit for the conductivity at a given E04 or n from different series using different gas flows.

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In addition to this, it is possible to select out of a broad refractive index range between 2 and 2.5 of highly transparent films (E04 4 2.2 eV) and achieve a conductivity above the s-limit. This flexibility makes μc-SiOx:H an attractive material for various applications in silicon solar cells. The E04 and n can be adjusted by the gas flow parameter which is the ratio between the CO2-flow and the SiH4-flow [4,16]. It was shown [4,16], that these mc-SiOx:H films with a s 41
10−5 S/cm have a considerable crystalline volume fraction, with the ability to act as nucleation layer for mc-Si:H [5].

4.2. Thin-film silicon solar cell and modules 4.2.1. Single junction solar cells The possibility to increase the optical band gap E04 and to adjust the refractive index n over a considerable range, together with appropriate electrical conductivities, makes μc-SiOx:H an attractive material for its application as a wide band gap window layer in thin-film silicon solar cells. This is demonstrated first in single junction mc-Si:H solar cells in nip configuration with illumination from the n-layer side. While high performance a-Si:H solar cells are usually illuminated through the p-side, solar cells utilizing μc-Si:H absorber layers may be illuminated from either p- or n-side, since the hole drift mobility in μc-Si:H films is much higher than in a-Si:H films [38,39]. It was shown that the performance of p-side and n-side illuminated solar cells utilizing microcrystalline silicon absorber layers is quite similar provided that the defect density of the absorber layer is sufficiently low [40]. For example, n-type microcrystalline silicon carbide (μc-SiC:H), having a wide optical gap of around 2.8 eV, can be used as a

Fig. 4. Reflectance spectra of μc-Si:H solar cells prepared with different window layers [7].

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window layer in single junction microcrystalline silicon n-i-p solar cells [41]. Fig. 4 compares the reflectance spectra for mc-Si:H solar cells employing three types of window layers: p- and n- type mc-Si:H (dashed and dotted curves) and n-type mc-SiOx:H [7] (solid curve). Here, mc-SiOx:H material with an optical band gap E04 of 2.2 eV and a refractive index n of 2.4 (Fig. 3) was chosen. Thus, anti-reflection conditions [7] are improved when a mc-SiOx:H layer is introduced between ZnO:Al (nZnO:Al 1.9) and the μc-Si:H absorber (nμc-Si:H  3.7). It can be seen that for the solar cells employing μc-SiOx:H window layers, the reflectance in the wavelength region between 400 nm and 650 nm is significantly reduced. Additionally, the mc-SiOx:H material selected here has a sufficiently high dark conductivity (around 10-2 S/ cm) and a high crystallinity (around 70%) to act both as an appropriate doped layer and as a nucleation layer for subsequent absorber layer growth. The EQE curve of n-side illuminated μc-Si:H n-i-p solar cell, prepared with ZnO:Al/Ag back contact, is shown in Fig. 5. The data are compared with the EQE of a standard optimized p-side illuminated μc-Si:H p-i-n solar cell. The observed longwavelenght response is consistent with the high transparency due to a wide optical band gap of 2.2 eV (Fig. 3) and low total reflectance (Fig. 4) for the wavelengths above 450 nm. This 1 μm thick single junction n-i-p solar cell shows a high JSC value of 24.8 mA/cm2 (1.4 mA/cm² higher than the reference cell), a FF of 65.1%, a Voc of 0.548 V resulting in an efficiency of 9.2% [5] and are summarized in Table 3. We have observed a slight increase in Voc in the case of the cell with n-type mc-SiOx:H window layer. This could be related to the larger amorphous fraction appeared during the initial growth of the absorber layer, because n-type mc-SiOx:H layer, serving as nucleation layers for subsequent absorber layer growth, has smaller crystallinity relative to the standard p-type mc-Si:H window layer [7]. The increased amorphous fraction can then contribute to slightly higher Voc value, and also to the reduction in FF [42]. To conclude, n-type μc-SiOx:H layers have been used as window layers in n-side illuminated microcrystalline silicon n-i-p solar cells. The results indicate that n-type μc-SiOx:H provides sufficient conductivity and crystallinity to function well as a doped layer and as a nucleation layer for microcrystalline i-layer growth. It also acts as a wide band gap window layer with a reduced light absorption (reduced parasitic absorption). The appropriate refractive index of this mc-SiOx:H window layer results in an improved anti-reflection effect, providing better light trapping and therefore higher currents in μc-Si:H single junction solar cells. The ability to improve light trapping in solar cells demonstrates the flexibility of mc-SiOx:H for usage as a doped layer in solar cells.

Table 2 Photovoltaic parameters of our standard solar modules with and w/o AR-foil are shown. Additionally the results on tandem solar modules with various thicknesses of mc-SiOx:H n-layer implemented as intermediate reflector (nTop) are shown. Current density gain and loss calculated from the EQE for top cells (JQE,Top) and bottom cells (JQE,Bot), respectively. The aperture area of the modules is 676 cm2. η [%]

Fig. 5. EQE curve of best performance μc-Si:H n-i-p solar cell, illuminated through n-type μc-SiOx:H window layer, compared with a standard optimized pin solar cell.

Std. module +AR IR thk. series: 0 nm 60 nm 90 nm

ISC/JSC [mA]/ [mA/cm2]

JQE,Top, gain [mA/cm2]

JQE,Bot, loss [mA/cm2]

11.9 73.3 35.6/1.37

307/11.8

n/a

n/a

12.2 73.1 35.9/1.38

314/12.1

n/a

n/a

11.3 71.4 35.1/1.35 11.4 69.3 35.2/1.35 11.3 68.5 35.3/1.36

305/11.7 315/12.1 316/12.2

0 0.7 0.8

0 0.9 1.2

FF [%]

V/VOC per cell [V]

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Table 3 Initial photovoltaic parameters of best achieved device performances using doped mc-SiOx:H layers in different device structures. Device

η [%]

FF [%]

VOC [V]

JSC [mA/cm2]

mc-Si:H single junction solar cell [5] a-Si:H/mc-Si:H tandem solar cell with AR on Asahi (VU) a-Si:H/mc-Si:H tandem solar module SHJ solar cell [15]

9.2 13.5 11.4 19.0

65.1 71.7 69.3 79.6

0.548 1.33 1.36 0.677

24.8 14.1 12.1 35.8

Fig. 6. The current densities calculated from the quantum efficiency for a-Si:H top cells (open symbols) JQE,Top and bottom cells (full symbols) JQE,Bot versus the top cell thickness. The thickness of the mc-Si:H bottom cell is the same for all cells. The circles show the JQE of the cells with mc-SiOx:H nTop as intermediate reflector (IR), the triangles the JQE of the cells without IR. The dashed lines are a guide to the eye for the cells without IR and the solid lines are for the cells with mc-SiOx:H n-type IR (nTop). The arrow is indicating the possible thickness reduction when using a n-type mc-SiOx:H as n-layer for the top cell (nTop).

4.2.2. Tandem solar Cells 4.2.2.1. Possible thickness reduction of the a-si:H top cell. In the following the function of doped mc-SiOx:H in a-Si:H/mc-Si:H tandem solar cells is demonstrated. An n-type mc-SiOx:H (nTop) layer with a refractive index of 2.3 was implemented as intermediate reflector in tandem solar cells deposited on Asahi (U). The thickness of the absorber layer of the a-Si:H top cell was varied while keeping the mcSi:H bottom cell thickness constant at 1.8 mm. For comparison, the same was done for tandem cells without IR. The resulting current densities calculated from the external quantum efficiencies (JQE) of the a-Si:H top cell (JQE,Top) and of the mc-Si:H bottom cell (JQE,Bot) are shown as a function of the top cell thickness in Fig. 6. For both cell configurations, with and without IR, the JQE,top increases with the top cell's thickness, while the JQE,Bot decreases. For these tandem solar cells with a bottom cell thickness of 1.8 mm and without IR, a top cell thickness of 290 nm is needed to reach a JQE current matching between the individual cells. For a tandem solar cell with IR having the same bottom cell thickness, however, a top cell thickness of only 150 nm is sufficient to meet the current matching requirement. This demonstrates a way to achieve current matching for thinner top cells at a similar tandem cell short circuit current density of 11.5 mA/cm² by using an n-type mc-SiOx:H IR (nTop). The increase of JQE,Top is the consequence of the light reflected back into a-Si:H top cell by the IR. In conclusion, a mc-SiOx:H n-layer of the a-Si:H top cell (nTop) as intermediate reflector leads to an effective current transfer from the bottom cell to the top cell. For a typical bottom cell thickness the bottom cell thickness of 1.8 mm the top cell thickness can be reduced from 290 nm to 150 nm. For these exemplary devices the possible thickness reduction is around 40%. With improved transparency of the window and contact layers and enhanced optical functionality of the intermediate reflector also a higher stabilized efficiency can be expected. In particular the thickness of the a-Si:H top cell could be reduced leading to reduced degradation effects. Investigations on stabilized cells are currently under way.

Fig. 7. Current densities calculated from the quantum efficiency for top cells JQE,Top (squares), bottom cells JQE,Bot (circles) and sum JQE,Sum (triangles) are plotted for various cell structures. The application of the doped mc-SiOx:H layers and the usage of the antireflection foil (AR) is described along the x-axis of the graph. The positions where mc-SiOx:H layers are used are indicated by nTop, pBot and nBot as described in Fig. 1. The gray area represents cells deposited on Jülich ZnO:Al. The lines are guides to the eye.

4.2.2.2. Optical improvements when using doped mc-SiOx:H. The results in Fig. 6 were shown to demonstrate the possible thickness reduction due to the use of mc-SiOx:H n-layer (nTop) as intermediate reflector. For these cells the efficiencies are below 12%. Our standard a-Si:H/mc-Si:H tandem solar cell (w/o IR) has a FF of 72% and Voc of 1380 mV; to reach an initial efficiency above 14% a JSC of more than 14 mA/cm² is necessary. This would require thicker absorber layers in the cells, reduced parasitic absorption in the doped layers and current matching between the a-Si:H top cell and mc-Si:H bottom cell. Therefore we implemented doped mc-SiOx:H in tandem cells with a thicker a-Si:H top cell thickness of 400 nm and a mc-Si:H bottom cell thickness of 3.2 mm. We show results of the tandem cells with focus on the JQE,Top, JQE,Bot and the sum of both cells (JQE,Sum) in Fig. 7. In Figs. 8 and 9 the EQE, IQE and R spectra are shown. For clarity the spectra were distributed over two figures, Figs. 8 and 9. These figures show the EQE of the a-Si:H top cell in the wavelength range between 350 nm and 750 nm, of the mc-Si:H bottom cell in the wavelength range between 500 nm and 1100 nm, and the sum of both EQESum. The internal quantum efficiency IQE was determined from the equation IQE¼EQESum/(1-R), where the reflectance R is shown in Fig. 8(a) and Fig. 9(a) versus the wavelength (λ). Using these results we discuss the optical gains and losses versus the spectra due to the substitution of doped Si layers by doped mc-SiOx:H layers on different substrates as Asahi (VU) and Jülich ZnO:Al. The doped mc-SiOx:H is used as n-layer for the top cell (nTop), p-layer for the bottom cell (pBot) and n-layer for the bottom cell (nBot) (Fig. 1). The layers of the tandem cell which are substituted by doped mc-SiOx:H layers are indicated at the x-axis of Fig. 7 and the substrates used for the cells are indicated at the Fig. 7.

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Fig. 8. Reflectance R in (a), external quantum efficiency EQE (solid line) for the a-Si: H top cell, the mc-Si:H bottom cell, the sum of both cell EQE and the internal quantum efficiency IQE (dotted line) in (b) for the tandem cells versus the wavelength. The results are shown for tandem cells without mc-SiOx:H layers and for tandem cells with mc-SiOx:H layers implemented as nTop layer and pBot layer which is indicated at the graph. For all cells Asahi (VU) substrate was used as substrate.

Fig. 9. (a) Reflectance R, (b) external quantum efficiency EQE (solid line) for the a-Si:H top cell, the mc-Si:H bottom cell, the sum of both cell EQE and the internal quantum efficiency IQE (dotted line) for the tandem cells versus the wavelength. The results are shown for tandem cells with mc-SiOx:H layers implemented as nTop, pBot and nBot layer grown on Asahi (VU) substrates and on Jülich ZnO:Al substrates.

Fig. 7 shows for the standard tandem cell “w/o mc-SiOx:H” layers a JQE,Top of 13 mA/cm² and a JQE,Bot of 14.2 mA/cm². The top cell is therefore the current limiting cell in the device. For the tandem cell which has “mc-SiOx:H as nTop” with a refractive index of 2.3 as the intermediate reflector, JQE,Top is increased to 14.2 mA/cm² and JQE,Bot decreased to 12.1 mA/cm² (Fig. 7). The increase of JQE,Top by 1.2 mA/cm² due to the nTop as intermediate reflector demonstrates that it transfers current from the bottom cell to the top cell even in a thick tandem device, although only a smaller part of the light reaches the intermediate reflector compare to the structure with thinner top cells as described in Fig. 6. Unfortunately, also the JQE,Sum is reduced by 1 mA/ cm². This decrease is mainly caused by the increase in parasitic absorption visible from the difference between the IQE spectra of the cell w/o IR and with nTop mc-SiOx:H IR in the wavelength range

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between 650 nm and 900 nm (Fig. 8b). From the difference between the IQE spectra it is not possible to distinguish where the additional parasitic absorption takes place, in the intermediate reflector or in the TCO. Just the part of the reduction in the bottom cell JQE,Bot can be assigned to the increase in reflectance between 800 nm and 900 nm (Fig. 8a). This light does not reach the mc-Si:H bottom cell because it is reflected by the intermediate reflector which is in front of it. This light is not absorbed by the a-Si:H top cell either because it has an energy below the band gap of a-Si:H. To decrease the total reflection of the cell we selected a material as intermediate reflector with a lower reflectivity. Thus we selected mc-SiOx:H material which has a higher refractive index and therefore has a lower refractive index contrast with the a-Si:H top cell. We used “mc-SiOx:H as pBot” (Fig. 7) with a refractive index of 2.5. For this cell the JQE,Top decreases to 13.7 mA/cm² and the JQE, Bot increases to 13 mA/cm² with respect to the afore mentioned cell “mc-SiOx:H as nTop”. The lower JQE,Top is most likely the result of the higher refractive index n¼ 2.5 of the used IR. But, on the other hand the JQE,Bot is increased by 1 mA/cm² which can be attributed to the reduced parasitic absorption visible from the gain in to the IQE between λ ¼600 nm and λ ¼900 nm (Fig. 8 b) compare to the afore mentioned cell using n-type mc-SiOx:H with a refractive index of 2.3 as nTop layer. From Fig. 8 we can conclude that by adjusting the refractive index of the intermediate reflector one can vary the transferred current to the top cell [4,16]. This was shown by the differences between tandem cell using the nTop IR with a refractive index of 2.3 and the pBot IR with a refractive index of 2.5. A lower refractive index can lead to an increased reflection out of the cell with a photon energy below the band gap of the a-Si:H top cell (Fig. 8a). Next we combined n-type and p-type mc-SiOx:H as intermediate reflector in the n/p tunnel recombination junction (nTop+pBot) of the tandem cell and also used n-type mc-SiOx:H as the n-layer of the bottom cell (nBot) with a thickness of around 30 nm. To reduce the reflection of the light out of the device which has an energy below the band gap of the a-Si:H top cell we implemented 〈n〉 mcSiOx:H as nTop with a higher refractive index of 2.4 as discussed for the results of Fig. 8. These cells were deposited on Asahi (VU) and on Jülich ZnO:Al to demonstrate the influence of the substrate on the optical performance for the tandem cells using doped mc-SiOx:H layers. The results of these cells are shown in Figs. 7 and 9. Note, for the “nTop+pBot+nBot” solar cell on Asahi (VU), this combination of doped mc-SiOx:H layers contributes to almost fulfilled current matching between the a-Si:H top cell and the mc-Si:H bottom cell yielding in a JQE,Top of 13.8 mA/cm² and a JQE,Bot of 14.0 mA/cm². This is due to the gain of 1 mA/cm² in JQE,Bot for the bottom cell (Fig. 7) compared to the cell using a mc-SiOx:H pBot. This gain in JQE,Bot and the resulting current matching can therefore be attributed to the implemented mcSiOx:H as nTop, pBot and nBot layer on Asahi (VU). This cell reached an efficiency of 12.9%, FF of 71.4%, Voc of 1.31 V and Jsc of 13.8 mA/cm². The highest initial efficiency achieved in the series is shown in Table 3.

4.2.2.3. Optical performance of tandem cell on ZnO. Light trapping, in-coupling of the light, parasitic absorption and therefore, the generated current depends strongly on the substrate, especially if the surface texture is different as it is for the Jülich ZnO:Al [30–32] compared to the Asahi (VU). To evaluate the optical performance of the mc-SiOx:H as “nTop+pBot+nBot” layers in this solar cell configuration on Jülich ZnO:Al we prepared the same layer stack as for the former described solar cell on Asahi (VU) substrate. It also means we used 〈p〉 a SiC:H as p-layer for the a–Si:H top cell (pTop) which is known to result in a poor electrical contact between the p-layer and the ZnO:Al [43] resulting in a low Voc of 1.28 V and FF of 62.7%, but gives us the highest grade of optical comparability. Solving the ZnO/p contact

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issue for example by using the 〈p〉 mc-SiOx:H as contact layer is shown in [11,12]. For this tandem cell using mc-SiOx:H as “nTop+pBot+nBot” layers grown on ZnO:Al the JQE,Top is slightly lower (Fig. 7) compare to this cell grown an Asahi (VU) substrate, which is also visible in the lower EQE value for λo500 nm (Fig. 9 b). From the reflectance and the IQE spectra (Fig. 9a/b) for λo500 nm we can conclude that the reduction in EQE for the top cell is due to a higher parasitic absorption. Most likely this increase in parasitic absorption is related to the lower band gap of the ZnO:Al compared to the SnO2 used for the Asahi (VU) substrate [44]. The JQE,Bot is 15.2 mA/cm² for the cell grown on Jülich ZnO:Al substrate (Fig. 7) which is 1.2 mA/cm² higher than on the Asahi (VU) substrate. The IQE is higher and the reflectance similar for λ4500 nm for the cell on ZnO:Al substrate compared to the cell on Asahi (VU) substrate (Fig. 9) which can be attributed to a lower absorption of the ZnO:Al. An IQE above 98% in the wavelength between 550 nm and 700 nm, and a JQE,Sum ¼ 28.6 mA/cm² demonstrates the low parasitic absorption of the doped mc-SiOx:H layers and the Jülich ZnO:Al substrate. By using AR-foil on this device from the company Solarexcel™ the JQE,Sum was further increased to 29.5 mA/cm² with JQE,Top ¼13.9 mA/cm² and JQE, Bot ¼ 15.6 mA/cm² (Fig. 7).

4.2.3. Tandem solar modules Our standard tandem solar module technology process without IR yields efficiencies of 11.9% and with AR-foil 12.2% (Table 2) with an aperture area of 676 cm2. Recently we developed n-type mc-SiOx:H intermediate reflectors (nTop) for this technology in order to increase the Jsc which gives perspective for further improvement of the module efficiency. There are additional requirements for the application of mc-SiOx:H n-layer as IR in modules. First, the deposition parameters have to be scaled up to large areas i.e. deposition gas flow and plasma power have to be adapted. Fig. 3 in the mc-SiOx:H film section shows similar electrical and optical properties of thin-film mc-SiOx:H n-doped material deposited in the 10
10 cm2 and 30
30 cm2 substrate size deposition systems. This demonstrates that all deposition parameters can be scaled up to larger areas. Second, a good uniformity of photovoltaic parameters over the aperture area of 676 cm2 is required. Fig. 10 shows the fill factor FF, open circuit voltage Voc, and short circuit current density Jsc uniformity data for a module with 60 nm thick IR. The standard deviation of the factor values FF is 7 0.8%, for the Voc 7 0.01 V and the Jsc 7 0.09 mA/cm2 which is in the same range as compare to modules without mc-SiOx:H IR (not shown). The third requirement for mc-SiOx:H n-layer integrated as IR in module is the sufficient low conductivity to be able to use the same module interconnection as for modules without IR. When using a highly conductive intermediate reflector like a doped ZnO IR an additional laser scribing step is necessary [45]. As expected

from the conductivity data on doped mc-SiOx:H films (Fig. 3) we demonstrate that this additional laser scribing step is not necessary when using an mc-SiOx:H IR and the three steps laser pattering process [36] is sufficient. The photovoltaic parameters of tandem solar modules with various thicknesses of n-type mc-SiOx:H layer implemented as intermediate reflector (nTop) are summarized in Table 2. The tandem solar modules have a top cell thickness of 400 nm and a bottom cell thickness of 2.5 mm. The applied intermediate reflector has an E04 gap of 2.3 eV, a refractive index of 2.5, and a dark conductivity of 2.7
10−3 S/cm. The electrical and optical material properties are shown in Fig. 3. The three tandem solar modules show similar initial conversion efficiency in the range of 11.3–11.4%. The solar module without mc-SiOx:H nTop layer as IR shows efficiency of 11.3% and FF of 71%. The open-circuit voltage is 35.1 V (corresponding to 1.35 V per cell) and the short circuit current (Isc) is 305 mA. The aperture area is used for calculation of short circuit current density (Jsc) of 11.7 mA/cm2. Integration of nTop mc-SiOx:H layer with thickness of 60 nm between the top and bottom cell leads to an increase of the short circuit current up to 315 mA (corresponding to 12.1 mA/cm2) and the efficiency was slightly improved. Further increase of the IR layer thickness to 90 nm results in an improvement of the JSC, while FF decreases. The increase of top cell current by 0.8 mA/cm2 calculated from the EQE spectra (data not shown) is observed. However, stronger loses in the JQE0 Bot (  1.2 mA/cm2) result in a current-limited bottom cell (see Table 2). In summary, a successful implementation of n-type mc-SiOx:H (nTop) layer as intermediate reflector (IR) to a tandem solar module with aperture area of 676 cm2 is achieved. A good uniformity for all photovoltaic parameters is shown for modules with mc-SiOx:H n-layer as intermediate reflector compared to module without IR (not shown). An increase in the top cell current by 0.8 mA/cm2 after integration of the IR layer is observed and the resulting photovoltaic parameters are summarized in Table 3. Our results demonstrated that the standard three steps laser pattering process [36] works also for modules using an n-type mc-SiOx:H as IR.

4.3. Silicon hetero junction solar cells Finally a-SiOx:H and doped mc-SiOx:H layers can also be very successfully used for the fabrication of SHJ solar cells, where all contact and buffer layers were replaced by SiOx. With optimized intrinsic a-SiOx:H buffer [15], phosphorous doped mc-SiOx:H emitter and boron doped mc-SiOx:H back contact layers, a high conversion efficiency η of 19.0% (active area¼ 0.67 cm²) was achieved with open circuit voltage Voc ¼667 mV, short circuit current density Jsc ¼35.8 mA/cm² and FF¼79.6% on a flat, 〈100〉-oriented, 250 mm thick p-type wafer (Table 3). This is, to the best of our knowledge, the highest efficiency

Fig. 10. Photovoltaic parameters for the sub cell stripes of a module with 60 nm thick intermediate reflector. To determine the homogeneity of the photovoltaic parameters over the module area, 26 cell stripes are additional separated by laser patterning perpendicular to the cell stripes in three areas indicated with 1, 2 and 3. The standard deviations of FF, Voc and Jsc are shown on the top of the graph. The colors indicate the deviation of the individual values as indicated in the legend.

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Fig. 11. Solar cell parameters (a)–(c) versus the oxygen content of the n-type mc-SiOx:H emitter layer as well as the spectrally resolved cell (e) absorptance, (f) external quantum efficiency and (g) internal quantum efficiency for the respective samples.

ever shown for SHJ solar cells using mc-SiOx:H contact layers. As a demonstration of the beneficial optoelectronic properties of mc-SiOx:H for SHJ solar cell application, we present an oxygen content series of the n-type mc-SiOx:H emitter layer. Fig. 11 shows solar cell parameters Voc, Jsc, FF, and η plotted versus the oxygen content cO as well as the spectrally resolved cell absorptance Acell, external quantum efficiency EQE, and internal quantum efficiency IQE for the respective samples. The variation of the composition was obtained by changing the CO2 gas flow ratio during deposition. The series reveals decreasing Voc with increasing cO (Fig. 11a), which is attributed to the reduction of doping efficiency as indicated by the decrease of the free carrier absorption α0.6 (absorption coefficient at a photon energy of 0.6 eV), giving rise to deteriorated field-effect passivation. The evolution of Jsc in correlation with cO (Fig. 11b) composes two overlapping effects. On the one hand, the optical loss in the emitter layer is noticeably reduced with increasing cO due to increased optical band gap E04 as reflected in the gain of the IQE in the short wavelength range (Fig. 11g). On the other hand, the interference patterns, caused by the usage of flat wafers, were different for varying cO, since the refractive index reduces with rising cO. A shift of the interference maxima can be clearly observed on the cell absorptance (Fig. 11e). Thus, the first increase of Jsc with increasing cO is predominantly due to the optical gain from the emitter. The subsequent drop from cO ¼24% to 34% is attributed to less beneficial interference patterns with higher reflectance loss. Up to a cO of 24%, the FF remained at a high level, which suggests that the series resistance of the emitter layer is not limiting the FF yet (Fig. 11c). The pronounced decrease of FF from cO ¼24% to 34% is explained by the decrease of the corresponding dark conductivity from 10-3–10-8 S/cm. We found the optimal oxygen content for emitter layers at 24% as a compromise between the aforementioned competing effects (Fig. 11d). The excellent Jsc value was found to mainly arise from the low optical losses in the mc-SiOx:H emitter layer. Optical improvement by mc-SiOx:H back contact was not observed due to the usage of rather thick wafers (thickness 250 mm) and the absence of any lighttrapping scheme. However, for bifacial SHJ solar cells on thin, textured wafers, the optical gain with a-SiOx:H and mc-SiOx:H on both sides of the wafer instead of a-Si:H is expected to be significant. The reason for the lower Voc as compared to those of state-of-the-art

SHJ solar cells [26] was identified as Si epitaxial growth from high resolution TEM images, which suggests that the epitaxial growth is not always completely suppressed during a-SiOx:H deposition [46]. This problem can be solved by adjusting the deposition conditions and/or using 〈111〉-orientated wafers instead [47]. Moreover, a higher Voc is expected when transferring the concept from p-type onto ntype wafer [48]. This work delivers promising results and encourages further development of SHJ solar cell using a-SiOx:H buffer and mc-SiOx:H contact layers.

5. Conclusions Doped microcrystalline silicon oxide (mc-SiOx:H) material has demonstrated its flexibility and favorable properties for the application in thin-film as well as in wafer based silicon solar cells. The possibility to increase optical gap E04 and to adjust the refractive index n over a considerable range, together with appropriate electrical conductivity, has been demonstrated. In mc-Si:H single junction solar cells, doped μc-SiOx:H reduces parasitic light absorption and reflection as wide band gap material with an appropriated refractive index. When using the 〈n〉 mc-SiOx:H as intermediate reflector in a-Si:H/mc-Si:H tandem solar cells, the amount of current transfer allows a decrease of the top cell thickness by about 40%. On Asahi (VU) substrates, when using mc-SiOx:H as n/p contact between the cells and as n-layer of the bottom cell, we achieved a short circuit current density of 14.1 mA/cm². Solar cells on Jülich ZnO:Al as a substrate yielded a total current density JQE,Sum of 29.5 mA/cm². We demonstrate a successful implementation of n-type mc-SiOx:H as intermediate reflector (IR) to a tandem solar module. Finally the application to wafer based solar cells has demonstrated an effective area conversion efficiency η¼19.0% combined with an open circuit voltage Voc ¼667 mV.

Acknowledgments The authors thank their colleagues at the IEK5-Photovoltaik who contributed to this work. A part of this work was carried out in the framework of the FP7 project “Fast Track”, funded by the EC under Grant agreement no. 283501. We acknowledge also the

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Please cite this article as: A. Lambertz, et al., Microcrystalline silicon–oxygen alloys for application in silicon solar cells and modules, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.05.053i