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).

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.

2.6 2.4 2.2 polished ZnO:B

n 2.0

1.8 1.6

1.4 a ZnO:Al (unetched) ]

-1 105 polished ZnO:B 104 ZnO:Al (unetched) 103 102

abs. coeff. [cm coeff. abs. b 101

[%]

30 ZnO:Al (unetched, 0.8 m) A A polished ZnO:B (2.3 m)

10 c 0 absorptance 400 600 800 1000 wavelength [nm]

Figure 2: Refractive index n (a), absorption coefficient α (b) and calculated absorptance A (c) of sputter-etched and LPCVD-ZnO layers on glass. For the determination of the optical parameters, the initially rough ZnO:B layer was polished. The ZnO layer thicknesses are indicated in parenthesis.

2.2 Cell design and silicon layer depositions

A schematic cross-section of micromorph tandem solar cells deposited on LPCVD ZnO:B and sputter- etched ZnO:Al is illustrated in Fig. 3. The micromorph tandem cells consist of an a-Si:H top cell with an absorber layer thickness of 220 nm and a microcrystalline silicon (µc-Si:H) bottom cell with an absorber layer thickness of 1.1 µm. A buffer µc-Si:H p-layer is used as a first Si layer on top of the

TCO to provide a good contact at this interface. The cells incorporate silicon-rich silicon-oxide (SiOx) doped layers, which are known to improve (i) light management, by reducing optical losses, and (ii) electrical performance, by mitigating the influence of inhomogeneous, porous Si regions obtained on rough surfaces [21-21b]. An intermediate reflector based on n-type SiOx (denoted as SOIR) of 40 nm is introduced between the top and bottom cell to serve as a reflective layer that boosts the top-cell photocurrent (, enabling the use of thinner top cells and consequently reducing the associated light- induced degradation (LID). Like the other SiOx-based doped layers, the SOIR additionally increases the cell's resilience to the superstrate texture [21]. The top and bottom cells are deposited by plasma- enhanced chemical vapor deposition (PECVD) in an octopus cluster system [22] and in an industrial KAITM reactor [23], respectively. As a back electrode, we use 2.3 µm of lightly doped LPCVD-ZnO:B, applying the same recipe as for 2 the front electrode. Cell areas of 1 cm are defined by lift-off and SF6-plasma dry-etching (except for the cells with a Cr rear electrode, where the Si is not removed between the cells). A white quasi- Lambertian dielectric back reflector (BR) is placed behind the cells for characterization. An anti- reflective coating (ARC) consisting of a micrometric random square-based pyramidal texture embossed in a UV-curable resin at the air/glass interface is used to improve the light coupling into the

4 cell [24-25]. This coating also plays the role of a retro-reflective layer since it reflects part of the escaping diffuse scattered light back into the cell [24].

Figure 3: Schematic picture of the cross-section of micromorph cells deposited on a sputter-etched ZnO:Al (left) and an LPCVD ZnO:B (right) front electrode. The pyramids at the front of the cells depict the ARC.

2.3 Optical and electrical cell characterizations

The absorptance data presented in the following were deduced from the reflectance R (as A = 100 − R) measured in a spectrophotometer with an integrating sphere (Perkin Elmer, Lambda 900). External quantum efficiencies (EQEs) were measured with red and blue bias light to assess the EQE response of each sub-cell, denoted as EQEtop-cell for the a-Si:H top cell and EQEbot-cell for the µc-Si:H bottom cell. The probe beam used for the EQE measurements irradiates an area of approximately 1 mm×3 mm. Owing to the relatively small thickness of the glass superstrate (1.1 mm) and the reasonably large size of the cell (1 cm2), we assume only negligible optical losses in the glass sheet [26]. The current-voltage (J-V) curves were determined using a sun simulator combining halogen and xenon lamps in standard test conditions (25 °C, AM1.5g, 1000 W∙m-2).

3. Cell Results:

Fig. 4a shows the EQE and absorptance of micromorph cells, with an ARC at the air/glass interface, deposited on the two types of front electrode. Below 400 nm, the cell with ZnO:Al front electrode provides a higher EQE, due to its wider optical band gap. Above this wavelength, a higher EQEtop-cell is

5 measured for the cell grown on LPCVD ZnO:B; except below 675 nm, ZnO:B also provides the highest EQEbot-cell. Summing the EQE of the two sub-cells (see Fig. 4b), interesting features appear in the spectral response of the two types of tandem cell. In particular, a minimum is observed at around 675 nm for the cell deposited on ZnO:Al, which perfectly coincides with a minimum in cell absorptance at the same position. It has been shown that, for micromorph cells deposited on ZnO:Al, the presence of such a minimum is correlated to the SOIR thickness.

100

ZnO:Al [%]

80 ZnO:B A A 60

EQE 40

20 a 0 absorptance absorptance 80 ZnO:Al ZnO:B

[%] 60 sum

40 EQE

20 b 0

[%] 40

R R ZnO:Al / with Cr at the back ZnO:B / with Cr at the back 20 c

reflectance reflectance 0 450 600 750 900 1050 wavelength [nm]

Figure 4: EQE and cell absorptance with ARC (a) and summed EQE (b) of micromorph solar cells deposited on sputter-etched ZnO:Al and LPCVD ZnO:B. (c) Reflectance of cells with a highly absorbing Cr back electrode.

To understand the impact of the SOIR on the EQEsum response, a Cr layer was deposited directly on the Si layers at the rear of both types of tandem cells. With this strongly absorbing back contact, the reflected light mainly comes from the air/glass, glass/ZnO, ZnO/Si and Si/SOIR interfaces. For the cell deposited on LPCVD ZnO:B, the reflectance only slightly increases in the wavelength range from 300 nm to 1100 nm (see Fig. 4c, full triangles), suggesting that the reflection losses at the SOIR are negligible. In contrast, the reflectance of the cell on ZnO:Al shows interferences, clearly identified as oscillations over this spectral range (see Fig. 4c, full circles): A sharp maximum around 675 nm is observed, which perfectly coincides with the attenuation of the EQEsum at this wavelength; in addition, a second broader maximum at approximately 950 nm is observed in the reflectance spectrum. Both maxima indicate reflection losses, mostly at the SOIR interfaces, suggesting weaker light trapping in

6 the top cell for λ < 750 nm and, at the same time, indicating that less light is available for the bottom cell for λ > 850 nm.

Table 1 summarizes the electrical parameters of the two types of cells. The Voc of the cell on sputter- etched ZnO:Al surpasses that of the cell on LPCVD ZnO:B by 30 mV. The FF measured under AM1.5g is almost the same for both types of cells. As the FF of tandem cells depends on the current- matching conditions of their component cells, measuring the FF under various illumination spectra represents a powerful approach to assess their intrinsic electrical performance. A so-called current- matching setup can be used for this purpose [27-28]. This setup allows us to maintain the summed photocurrent of a tandem cell constant, while modifying the balance between the top-cell and bottom- cell photocurrents. Fig. 5a shows the FF under various irradiance spectra, varied from a blue-rich to a red-rich spectrum. Irrespective of the current-matching condition, the FF of the cell on ZnO:Al exceeds that obtained on ZnO:B. The excellent electrical properties with ZnO:Al, as evidenced by both the superior Voc value and higher FF curve, are attributed to the smoother topography of the ZnO:Al texture, which is more suitable for high-quality µc-Si:H growth. It is well documented that rough TCOs with sharp features used as superstrates lead to the formation of defective porous areas in

µc-Si:H that degrade both the FF and Voc of the corresponding cell [29-30]. The discrepancy in FF between ZnO:Al and ZnO:B increases with increasing Jsc- top − Jsc-bot. For positive mismatch values (Jsc-

top − Jsc-bot > 0), the tandem-cell response is dominated by the response of the µc-Si:H cell, which is more sensitive to surface roughness than the a-Si:H top cell.

Table 1: Key parameters of micromorph solar cells deposited on textured sputter-etched ZnO:Al and LPCVD ZnO:B under AM1.5g irradiation. The photocurrent values are derived from EQE measurements and the efficiencies are calculated with these values.

V FF J (top) J (bot.) J (sum) Eff. Superstrate oc sc sc sc (V) (%) (mA/cm2) (mA/cm2) (mA/cm2) (%)

LPCVD ZnO:B 1.365 72.2 13.25 12.5 25.75 12.3

after 300 h of 1.350 67.6 12.9 12.4 25.3 11.3 light soaking

sputter-etched 23.3 1.395 72.0 11.5 11.8 11.55 ZnO:Al

after 300 h LID 1.385 67.2 11.3 11.7 23.0 10.5

The higher optical performance obtained for the cell deposited on LPCVD ZnO:B compensates for its weaker electrical properties; irrespective of the irradiance spectrum – also under AM1.5g – the efficiency in the initial state is higher for the cell grown on ZnO:B than the cell deposited on ZnO:Al (see solid lines in Fig. 5b).

Fig. 5b shows the influence of the mismatch on the cell efficiency. With respect to the initial state (solid lines), 300 h of light soaking shifts the curves downward (dashed lines). The LPCVD ZnO:B provides higher efficiency in both the initial and light-soaked cells. A closer analysis of Fig. 5b shows that, after LID, the peak of the efficiency curves shifts towards higher photocurrent mismatch values (vertical bars), while the efficiencies under AM1.5g (see squares) move slightly in the other direction. This result suggests that the highest stabilized efficiencies under AM1.5g should be obtained for cells

7 that are strongly bottom limited in the initial state, with a positive mismatch of about 2 mA/cm2, as already observed in previous work [27, 6]. In practice, this can be achieved by increasing the SOIR thickness [27] or by increasing the a-Si:H i-layer thickness (at the cost of more severe LID).

According to Fig. 5b (see arrows), a particularly high Jsc-top boost is required with ZnO:Al as compared to ZnO:B to reach maximum efficiency. This requirement may in fact be critical for the following reasons: first, Jsc-top is less sensitive to the SOIR thickness for cells grown on ZnO:Al compared to cells grown on ZnO:B [35-36]. Thus, for the specific a-Si:H and µc-Si:H layers considered here, a significantly thicker SOIR may not be sufficient to provide the required photocurrent mismatch of 2 mA/cm2 in the initial state. Second, thickening the SOIR leads to increasing parasitic reflection losses – especially in the case of ZnO:Al – which impair the summed photocurrent and consequently the cell performance. Therefore, reaching the maximum efficiency by simply increasing SOIR thickness is not necessarily an option for cells deposited on single-texture sputter-etched ZnO:Al. In contrast, note that multi-texture sputter-etched ZnO layers obtained by chemical etching in various acidic solutions would fulfill this requirement more efficiently since such textures provide a higher Jsc-top and also a higher sensitivity to SOIR thickness.

We have seen in the previous section that the main advantage of cells deposited on the investigated LPCVD ZnO:B resides in the better light management. In the next section, we aim to understand the origin of the differences in optical performance found on both types of front TCO.

80.0 a ZnO:Al

77.5

[%] 75.0 ZnO:B FF 72.5

12.8 initial 12.4 b ZnO:B 12.0 11.6 ZnO:Al 11.2 10.8

efficiency [%] efficiency 10.4 10.0 -2 -1 0 1 2 3 4 2 Jsc-top- Jsc-bot [mA/cm ]

Figure 5: FF (a) and efficiency (b) as a function of the photocurrent mismatch Jsc-top - Jsc-bot. The squares indicate the values obtained under AM1.5g derived from the photocurrent values obtained from the EQEs. The dashed lines illustrate the efficiency curves after LID. The blue dotted line represents the efficiency curve expected when applying a two-step annealing treatment to a ready- made ZnO:Al, as proposed in [31, 32] (see details in the text). The arrows denote the gain in Jsc- top − Jsc-bot required to reach maximum efficiency after LID.

3.2 Origin of the difference in the optical behavior of cells grown on sputter-etched ZnO:Al and LPCVD ZnO:B

From the cell absorptance depicted in Fig. 4a, we see that LPCVD ZnO:B provides better light coupling into the cell than sputter-etched ZnO:Al, via roughness-induced refractive index grading. Both component cells of the tandem device benefit from this greater light coupling.

Also, Jsc- top and Jsc- bot obviously depend on the light trapping in the i-layers, which is given by the angular intensity distribution (AID) of the light within these layers. To assess the AID of the light transmitted to the top cell, we performed calculations based on the scalar scattering theory, using the Dominé model [39]. Fig. 6 presents the simulated AID for normal incidence at λ = 650 nm of the light refracted at the TCO/a-Si:H interface. We see that the AID in a-Si:H peaks at larger angles and extends to larger angles for LPCVD ZnO:B compared to ZnO:Al. The AID of the scattered light during its first path through the top cell is thus more appropriate for efficient light trapping and consequently for high light absorption. Furthermore, the absence of interference fringes in the absorptance spectrum of the cell on ZnO:B below 750 nm underlines the low level of optical coherence in the top cell. This effect may be associated with faster randomization of the scattered light within the top cell, due to the broader AID of the light during its first path in Si and a more suitable interaction with the textured a-Si:H/SOIR interface.

0.08

0.06 LPCVD ZnO:B sputter-etched ZnO:Al

0.04

0.02 counts [arb. unit] [arb. counts

0.00 0 20 40 60 80 angle  [°]

Figure 6: Angular intensity distribution at λ = 650 nm of the scattered light transmitted from the front TCO layer, made of LPCVD ZnO:B or sputter-etched ZnO:Al, to the a-Si:H top cell, through the textured interface, at normal incidence. The distribution is normalized to unit area. The AFM topographies of the LPCVD-ZnO:B and the ZnO:Al layers are used as inputs in the simulations.

Fig. 7a shows the internal quantum efficiency (IQE), defined as IQE = EQE / (100−R), of the cells. Since the IQE represents the EQE normalized to the cell absorptance, it highlights the relative amount of light that is absorbed in the active i-layers and is parasitically absorbed in the non-active layers. Fig. 7a demonstrates that the relative parasitic absorption losses are significantly higher for the cell grown on ZnO:Al since its IQE is lower than that of the cell grown on ZnO:B. These losses result mostly from the stronger losses in the ZnO:Al layer (cf. Fig. 2c).

By equitably splitting the parasitic losses between the reflectance and the EQE, as described in Ref [40], we can assess the light-trapping ability of the two types of ZnO over the full spectral range. The relevance of this approach was already validated in former publications, where a good match could be obtained between experimental devices comprising highly transparent layers with adequate refractive index and calculations based on this assumption [40]. In the case of ideal carrier collection, we can write: 퐸푄퐸 + 푅 + 퐴푝 = 100, where 퐴푝 represents the parasitic absorptance. This is equivalent to

퐸푄퐸 + 푅 = 100 − 퐴푝. No parasitic absorption in the solar cells would imply that 퐸푄퐸푐 + 푅푐 =

100%. This normalization is obtained by defining 퐸푄퐸푐 = 100 × [퐸푄퐸⁄(100 − 퐴푝)] and 푅푐 =

100 × [푅⁄(100 − 퐴푝)], which means splitting equitably the parasitic losses between EQE and reflectance. The EQEc values of the cells deposited on sputter-etched ZnO:Al and LPCVD ZnO:B are shown in Fig. 7b; assuming an identical light coupling for both cells – by multiplying the EQEc over the full spectral range by a constant factor in order to obtain the same EQEc in the short wavelength region – we see that both EQEc are almost identical in the NIR region. A minimum at 670 nm is found on ZnO:Al, which is ascribed to the reflection losses at the SOIR interfaces. The good correlation between both curves above 750 nm suggests that the light-trapping ability of both types of textures is equivalent in this spectral range. This result can be explained by the fast randomization of the AID of the scattered light within the Si layers after multiple reflections at the front, rear and SOIR interfaces.

The lower EQEbot-cell obtained for the cell grown on ZnO:Al as compared to the cell grown on ZnO:B is then mainly explained by higher parasitic losses and not by a weaker light-trapping ability in this spectral range.

100 LPCVD ZnO:B 80

60 [%]

sputter-etched ZnO:Al

40 IQE

20 a 0

60 [%]

40 EQEc 20 b 0 450 600 750 900 1050 wavelength [nm]

Figure 7: IQE (a) and EQEc (b) of micromorph tandem cells deposited on sputter-etched ZnO:Al and LPCVD ZnO:B. These EQEc are calculated by splitting equitably the absorption losses between the reflectance and the EQE. The dashed line indicates the expected EQEc with ZnO:Al considering similar light-coupling properties to ZnO:B.

3.3 Further optical improvement of tandem cells deposited on single-texture sputter- etched ZnO:Al

The highest performance revealed in this study, for the specific Si layer stack studied here, was found on LPCVD ZnO:B. We would like to stress that LPCVD ZnO:B is the standard superstrate used at IMT (Neuchâtel). Hence, all processes used in this work are optimized on this specific TCO. Improvements could certainly be obtained on sputter-etched ZnO:Al by fine-tuning the deposition conditions of the Si layer and by adjusting the Si layer thicknesses.

Also, several upgrades might be envisaged to improve the optical performance of cells on ZnO:Al, such as (i) the implementation of a metallic BR adjacent to the Si layers to improve light trapping [33- 34], (ii) the insertion of a refractive-index matching layer at the TCO/Si interface to improve light coupling into the cell [37-38] and (iii) the use of front TCO layers with better opto-electrical properties. With respect to (iii), two-step annealing treatments, in which TCOs are subjected to a conventional annealing treatment followed by an annealing treatment under a capping layer, can be used to improve the transparency of ready-made ZnO:Al layers while maintaining their sheet resistance [31-32]. Adopting this approach, micromorph solar cells and modules with remarkable efficiencies as high as

12.1% at the cell level (and 11.6% for a mini-module [4, 41]) were obtained, thanks to a Jsc-sum improvement of 4% and to an unexpected increase in Voc (by 1.5%) [41]. Note that the reference cells in Ref. [4] provide comparable results to those found in the present study. Therefore, considering similar relative gains in Jsc-sum and Voc, we find that efficiencies almost similar to those obtained with ZnO:B are achievable with annealed ZnO:Al layers (dotted curve in Fig. 5b); even higher efficiencies are probably within reach when considering the envisaged upgrades. For the specific Si layer stack considered here, the main limiting factor to reach the highest stabilized efficiency under the AM1.5g spectrum with ZnO:Al resides in the non-optimal photocurrent mismatch: under AM1.5g, cells on ZnO:Al are strongly top limited, yet their highest performance is expected for bottom-cell limitation. This result has motivated the development and implementation of double-texture sputter-etched

ZnO:Al front electrodes, which have been proven to efficiently enhance Jsc-top and provide a more appropriate photocurrent balance between the component cells [ ]. Although the cell design presented here is optimized for LPCVD ZnO:B and the obtained results cannot be generalized to any arbitrary Si layer stack, we believe that this study illuminates the potential of both TCOs and their required intrinsic properties and surface textures for further improved cell efficiencies.

4. Conclusions:

We have compared the performance of two widely applied single-texture TCOs, namely sputter-etched ZnO:Al and LPCVD ZnO:B, as front-electrode materials for tandem TF-Si solar cells. The superstrate cell based on ZnO:Al showed superior electrical performance, mostly due to its growth-friendly surface morphology, and the high conductivity of ZnO:Al as compared to the applied ZnO:B. A ~2% higher Voc and a ~3% (rel.) higher “intrinsic” FF for positive photocurrent mismatches were measured on ZnO:Al as compared to LPCVD ZnO:B. The main drawback of ZnO:Al, for the particular Si layer stack investigated here, is its optical performance. The ZnO:Al employed here exhibits a low sheet resistance, and thus a relatively high absorptance, which leads to stronger parasitic losses in the bottom cell. Sputter-etched ZnO:Al has light-trapping properties as good as LPCVD ZnO:B in the NIR. However, it provides a weaker light coupling into the cell and less efficient light trapping in the top cell, which results in a non-optimized photocurrent balance between the two component cells. Despite the several optical upgrades discussed, this last issue is designated as the main limiting factor that prevents us from reaching the highest stabilized efficiency under AM1.5g with sputter-etched ZnO:Al.

Acknowledgements:

We would like to acknowledge Jordi Escarré, Karin Söderström, and Jan-Willem Schüttauf for constructive discussions, Mustapha Benkhaira and Ulrike Gerhards for ZnO depositions, and Loïc Garcia for technical assistance. This work was carried out in the framework of the FP7 project “Fast Track”, funded by the EC under grand agreement no 283501. Project Cheetah + OFEN

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