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Geometric light trapping for high efficiency thin film silicon solar cells
Solar Energy Materials & Solar Cells 98 (2012) 185–190
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Geometric light trapping for high efficiency thin film silicon solar cells ¨ ¨ Jordi Escarre´ n, Karin Soderstr om, Matthieu Despeisse, Sylvain Nicolay, Corsin Battaglia, Gregory Bugnon, Laura Ding, Fanny Meillaud, Franz-Josef Haug, Christophe Ballif Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, Rue A.-L. Breguet 2, 2000 Neuchˆ atel, Switzerland
a r t i c l e i n f o
abstract
Article history: Received 10 September 2011 Received in revised form 17 October 2011 Accepted 20 October 2011 Available online 17 November 2011
The imprinting of random square based pyramidal textures with micrometric scale at the air/glass interface of thin film silicon solar cells is presented as an efficient alternative to anti-reflective coatings to minimize reflection losses at the cell entrance. This novel processing step, which can be applied after cell or module manufacture, is found to simultaneously enhance light in-coupling and light-trapping in amorphous silicon/microcrystalline silicon tandem solar cells. A remarkable total current gain of more than 5% is demonstrated with the imprinting of such structures, resulting in a tandem cell with a high initial efficiency of 13% for a total absorber layer thickness below 1.5 mm. & 2011 Elsevier B.V. All rights reserved.
Keywords: Solar cells Thin film Light trapping Anti-reflective effect UV imprinting
1. Introduction All PV technologies can benefit from introducing anti-reflective (AR) schemes at the front surfaces. In this paper we focus on the case of thin film silicon (TF-Si) solar cells, which is particularly interesting for its specific optical properties. Indeed, the micromorph concept, which combines an amorphous silicon (a-Si:H) top cell with a microcrystalline silicon (mc-Si:H) bottom cell to form a tandem cell, has proven to be an ideal candidate to enhance the thin film silicon solar cells efficiency [1–3]. Moreover, the reduction of the active layer thickness was recently shown to be feasible while maintaining a high efficiency [3], allowing for costs reductions [4]. To further enhance efficiencies while keeping reducing layer thickness, the search for novel and more efficient ways to trap light inside the cells is therefore essential and can contribute to make TF-Si a very low cost technology [4]. A TF-Si solar cell behaves as a light waveguide, with the active silicon layers usually placed in between two transparent conductive oxide (TCO) layers. A textured TCO/Si front interface increases the light path inside the active layer thereby increasing the photo-generated current. Recently, a lot of efforts have been put in finding the optimal front TCO texture [5–9]. For maximum light absorption, it is necessary to enhance the light in-coupling
n
Corresponding author. Tel.: þ41 32 718 33 79; fax: þ 41 32 718 32 01. E-mail address: jordi.escarre@epfl.ch (J. Escarre´).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.10.031
by minimizing reflection losses at the air/glass substrate interface. There are mainly three ways to minimize these losses: via antireflective coatings [10,11], which diminish reflection by creating destructive interferences on the reflected light waves; via antireflecting structures [12], which lower the refractive index at the air/glass interface using either textured or porous glass on a subwavelength scale; or via geometric anti-reflective textures, which allow for multiple rebounds of the incoming light at the glass surface, enhancing transmission. The main advantage in using a geometrical approach is the spectral independence of the antireflective effect. This is the case as long as the features of the texture are much larger than the wavelengths of interest. In TF-Si solar cells, where the spectral range of interest comprises the near infra red up to 1.1 mm, textures with feature sizes of the order of 10 mm are therefore required. In crystalline silicon solar cells, the use of geometric square based pyramidal textures is a common approach to minimize primary reflection loss at the front of the device [13,14]. The pyramids are obtained by etching (100)-oriented monocrystalline wafers in alkaline solutions. The etching is anisotropic, being slower for the (111)oriented crystallographic planes. The intersection of four of these planes forms square based pyramids. The (111) facets are inclined by 54.71 from the horizontal plane, allowing for a double rebound of the incident light against the surface of the wafer (see Fig. 1 (a)). Thus, the expected reflectance is decreased to a value of R2F , where RF is the reflectance of the flat silicon. In TF-Si solar cells, the use of similar pyramidal textures on the glass substrate could therefore significantly reduce the air/glass
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Fig. 1. (a) SEM image of the glass sample with the UV imprinted pyramids at the front and (b) its ARS curve measured in transmittance mode.
interface reflection from RF ¼4% to R2F ¼0.16%. Given the potential for enhanced light in-coupling in TF-Si devices of such pyramidal textures, we propose a way of fabricating such structures on glass. The availability in the market of UV curable resins, which exhibit both high transparency and a refractive index close to the one of glass, makes UV imprinting a good candidate for this purpose. In this work, we show a total current gain above 5% when using such UV imprinted texture at the air/glass interface of high efficiency micromorph solar cells. The use of these pyramidal structures, in addition to reduce the primary reflectance, helps to improve light trapping, which leads to a total current gain higher than the one expected for a perfect planar anti-reflective coating in the air/glass interface ( 4%). Finally, we also compare the performance of this geometric pyramidal texture with other antireflective schemes.
2. Experimental Micromorph tandem cells are fabricated in the superstrate (p-i-n) configuration on a 0.5 mm thick glass (Schott AF45). Borondoped ZnO layers synthesized by low-pressure chemical vapor deposition (LPCVD) are used as transparent contacts. The cells are developed on a 1.4 mm thick doped ZnO layer and on a state-of-theart 2 mm thick ZnO bi-layer, optimum for micromorph cell application. The bi-layer combines a highly doped nucleation part with a non intentionally doped bulk, and is designed to minimize optical absorption and improve light scattering [15]. The silicon layers are deposited by plasma enhanced chemical vapor deposition (PECVD). The micromorph cells used in this study implement a 260 nm thick a-Si:H top cell and a 1.2 mm thick mc-Si:H bottom cell. Doped silicon rich silicon oxide layers are used in both junctions together with optimized intrinsic layer deposition processes, as detailed in [4]. In particular, a thin n-type silicon rich silicon oxide intermediate
reflector layer (20 nm) is used to maximize the response of the 260 nm thick top cell. For all cells, a low-doped 5 mm ZnO thick layer covered with a white dielectric reflector is used as back contact. The pyramidal features are transferred to the glass as follows: the texture of a (100) crystalline silicon wafer textured using a low concentration potassium hydroxide solution (KOH) is molded in polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning). The PDMS mold is prepared mixing the base and the curing agent with a ratio 10:1, degassing it in vacuum, dispensing it onto the wafer, and curing it at 46 1C for 12 h before careful separation from the wafer. Thus, the negative of the original pyramidal texture is transferred on the PDMS. This intermediate mold is used to UV imprint the positive pyramidal texture on the cells. A 25 mm thick UV curable layer (Ormocer, Micro Resist Technology GmbH) is spin coated on the front. This coating, with a refractive index of 1.518 at 635 nm, optically behaves as glass without increasing its absorption in the spectral range comprised between 400 and 1100 nm. UV stamping is carried under vacuum conditions, by applying a pressure of 1 bar and UV exposure ( 365 nm at 1.4 mW/cm2, 3 min) coming through the PDMS. After separation from the PDMS mold, the desired pyramidal texture is reproduced on the front of the cells. The quality of the replica is studied both morphologically and optically by means of scanning electronic microscopy (SEM) and angle resolved scattering (ARS) measurements, respectively. The ARS curve is measured using a green laser with a wavelength of 543 nm. The solar cells are characterized by measuring the current– voltage (I–V) characteristics and the external quantum efficiency (EQE) for both top and bottom cells. I–V measurements were carried out using a dual lamp solar simulator (Wacom WXS-140S-10) in standard test conditions (STC, 25 1C, AM 1.5 g spectrum, and 1000 W/m2). The short circuit current density (Jsc) of the devices is calculated integrating over the EQE curve after weighting with the AM 1.5 g spectrum. Finally, the cells were disposed for 200 h under 1 sun equivalent illumination, at 50 1C, for light induced degradation (LID) evaluation.
3. Results and discussion 3.1. Random square-based pyramids imprinted on glass The geometric pyramidal texture imprinted on bare glass is shown in Fig. 1(a). The image reveals the typical random square based pyramids of a (100) c-Si wafer etched in KOH. The tilt angle of the facets is determined by measuring the angle resolved scattering curve for the transmitted light. As the facets of different pyramids are parallel to each other, the ARS measurement depends on how the sample is oriented with respect to the incident beam. The transmitted light is projected in four intense spots, one for each facet, equispaced around a circle. The ARS curve shown in Fig. 1(b) is measured placing two antipodal spots on the way of the detector. Both maxima are detected at 311 measuring from the incident axis. The ray diagram of Fig. 2(a) shows how the incoming light interacts with the pyramidal texture. By applying Snell’s law, it is inferred that light propagates in the glass with an angle of 20.11 before being refracted out. This direction is defined at the entry when the light first strikes a facet. The incident angle is the facet tilting (af) while the refracted one is (af—20.11). Thus, the facet tilt angle can be estimated by numerically solving the following equation: 1:5 sinðaf 20:13 Þ ¼ sinðaf Þ
ð1Þ
As a result, the facets of the UV imprinted pyramids that we developed on glass are inclined by 51.51, slightly less than the
J. Escarre´ et al. / Solar Energy Materials & Solar Cells 98 (2012) 185–190
Exit
Entry
α1 ~ 51.5° α2 ~ 31.4° α3 ~ 20.1° α4 ~ 31° α4
Case α
α5 ~ 71.6° α6 ~ 71.6° α7 ~ 31°
Case β
α8 ~ 31.5° α9 ~ 51.6°
α7
α3
α6
α2
α5
α8 α9
α1
Fig. 2. Ray diagrams showing how light geometrically interacts with the pyramidal texture when (a) entering or (b) exiting the device.
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54.71 expected from the original texture. However, the condition to have a double rebound at the entry is still valid: the part of the incoming light, which is not transmitted in the glass propagates with an angle of 1031 regarding the incident direct direction and will therefore hit a second pyramid facet. The ray diagrams of Fig. 2(b) show how the outgoing light interacts with the pyramidal texture. The light, once being reflected on the back part of the glass, is directed against the interior of the pyramids either with an angle of 71.61 (case a) or 31.51 (case b). In the first case, the light continues its way inside the glass after being reflected twice against the pyramids, both rebounds at the same angle (71.61) and under total reflection conditions. The case a occurs when the exiting ray firstly strikes a facet parallel to the one, which previously had coupled the light in. For random pyramids, the probability that case a takes place has been estimated to be 44% [16]. For the case b, the total reflection condition is not achieved, and most of the light is refracted out. 3.2. Results on micromorph cells
Fig. 3. External quantum efficiency curves of exactly the same cell measured before and after the imprinting of the pyramids at the front glass. The results obtained for the cells grown on a 2 mm bi-layer and on a 1.4 mm ZnO front contacts are shown in (a) and (b), respectively.
The pyramidal texture was imprinted at the air/glass interface of micromorph cells grown on two different front contacts. Fig. 3 compares the EQE curves for exactly the same cells measured before and after imprinting the pyramids, and Table 1 summarizes their parameters. In both cases, the solar cells with the pyramids exhibit higher top and bottom cell currents. For the cell with the bi-layer as a front contact, the currents increase from 12.41 to 13.32 mA/cm2, for the top cell, and from 12.33 to 12.76 mA/cm2, for the bottom one. The relative increase of the top and bottom cells are 7.3% and 3.5%. In a similar way, for the cell with 1.4 mm thick ZnO front contact, the current increases from 12.78 to 13.60 mA/cm2, for the top cell, and from 11.60 to 12.10 mA/cm2, for the bottom one. The relative increases for the top and bottom cells are 6.4% and 4.3%. For both front contacts, exactly the same total current gain (5.4%) is measured after imprinting the pyramids at the front. The proposed post-process imprinting therefore results in a substantial performance boost leading to an excellent 13% initial efficiency realized using the bilayer as a front contact and a total absorber layer thickness below 1.5 mm. The relative difference between EQE curves (top and bottom) for the cell deposited on the bi-layer front contact before and after imprinting the pyramids is shown in Fig. 4 as a function of the wavelength. In addition, two extra curves are added as EQE references measured on cells deposited on standard AR coated glass and glass front coated with submicrometric random structures. In the former case, an extra glass with coating consisting of a multilayer design: glass/A/B/A/B (A¼TiO2 and B¼SiO2) with a broadband anti-reflective spectrum is glued with UV lacquer at the front. In the latter case, a diffusive texture (based on 5 mm thick LPCVD grown ZnO), which also acts as a graded index antireflection layer, is UV imprinted at the front surface. It is seen in Fig. 4(a) that for wavelengths below 500 nm, where the EQE
Table 1 Parameters of the same cell measured before and after the imprinting of the pyramids at the front glass.
No pyramids Pyramids Pyramids after 200 h LID No pyramids Pyramids
Front contact
Voc (V)
FF
Jsc (Top) (mA/cm2)
Jsc (Bottom) (mA/cm2)
Efficiency (%)
Bi-layer (2 mm) Bi-layer (2 mm) Bi-layer (2mm) ZnO (1.4 mm) ZnO (1.4 mm)
1.38 1.38 1.37 1.35 1.35
0.725 0.730 0.660 0.770 0.770
12.41 13.32 12.8 12.78 13.60
12.65 12.94 12.8 11.60 12.10
12.4 13.0 11.5 12.0 12.6
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Fig. 5. Effect of the pyramids is significantly reduced by doubling the thickness of the front glass from 0.5 to 1 mm. The relative EQE gain is shown here for both thicknesses.
Fig. 4. Relative difference between the EQE curves of three equivalent micromorph cells grown on a 2 mm bi-layer before and after modification of their front. The EQE changes in the top and bottom cells are plotted in (a) and (b), respectively.
differences can only come from a better light in-coupling, the maximum gain ( 6%) is measured for the cell with geometric pyramids at the front. Therefore, the incoming light double rebound scheme, induced by the large pyramids is more efficient than the use of anti-reflective coatings or diffusive textures at the front, which leads to gain at 500 nm of 4 and 3%, respectively. This important gain is significantly above to the 4% expected when having an ideal light in-coupling at the front glass. Thus, the geometric pyramidal texture not only provides an optimum antireflective effect at the front but it also possibly reduces the amount of light exiting the device coming from the reflection at the glass-TCO and TCO-silicon interfaces. Before discussing the spectral range above 500 nm for the top cell, we first focus on the bottom cell shown in Fig. 4(b). In this case, the pyramid covered glass also performs better than the other references. For wavelengths above 600 nm, the gain of the cell with the AR coating linearly drops to 10%. These important losses are attributed to its double thick front glass. Indeed, because of the small size of the cells ( 0.7 1.5 cm2), the outgoing light, partially scattered after its path through the cell, can escape through the thick front glass sides after multiple reflections. In fact, the thicker the glass, the longer the light will travel inside the substrate, being more easily guided out from the small active area of the cell [17]. Note that this effect is negligible on modules, but it also has to be taken into account when using pyramids at the front of small cells. In this case, where the incoming light is already refracted into the glass with an angle of 20.11, important optical losses can be foreseen using thicker front glasses, even at the blue part of the spectra. To quantitatively estimate such EQE losses, the thickness of the front substrate was increased to 1 mm by gluing an extra glass with the imprinted pyramids. Fig. 5 compares the new total EQE gain
Fig. 6. Proportion of light absorbed in the top (or bottom) cell is calculated as the ratio between its EQE (top or bottom) and the total one (top þ bottom). Here, for different incident angles (a), it is shown how the absorption in each sub-cell is modified with respect to the measure at 01. The dashed and solid curves give information on top and bottom cells.
(topþbottom) with the previous one, i.e. with the pyramids imprinted directly on a 0.5 mm thick front glass. As expected, the performance of the pyramids is significantly lowered all over the whole spectra using thicker glass at the front. Interestingly, for wavelengths above 500 nm, the pyramids constantly increase the quantum efficiency of the top cell arriving to a remarkable gain of 12% at 700 nm wavelength (see Fig. 4(a)). This trend cannot be observed for the cells with no imprinted pyramids, where the gain in the top is kept nearly constant. Thus, the use of the pyramids at the front could also play an important role in order to keep the amorphous top cell as thin as possible, helping to achieve higher micromorph stable efficiencies. The pyramids enhance light absorption in the top cell, at the sacrifice of the bottom, possibly due to enhanced reflection at the thin silicon rich silicon oxide intermediate reflector layer. As it was discussed before, the geometry of the pyramids changes the direction of the incident light. In first approximation after refraction at the glass-TCO and TCO-Si, the light would propagate with an angle of 7.41 inside the silicon that could increase the reflectance at the intermediate reflector. In order to validate this hypothesis, the EQE of the cell without pyramids at the front is measured under different angles for the incident light. For each angle, the proportion of light absorbed in the top (or bottom) cell is calculated as the ratio between its EQE (top or bottom) and the total one (top þbottom). Fig. 6 shows how the absorption in each sub-cell is modified with respect to the measure at 01. Moreover, the same curves measured for the cell with the pyramids at the front is added for better comparison. It can be seen that an increase in the incident angle shifts the light absorption towards
J. Escarre´ et al. / Solar Energy Materials & Solar Cells 98 (2012) 185–190
the top cell. The maximum gain for the top cell is achieved at 600 nm. For lower wavelengths, the cell with pyramids behaves similar to the one measured with a tilting angle of 301. Indeed, the light impinging the glass at this angle should propagate through the silicon with an angle of 7.21, which is close to the 7.41 expected for the pyramids (again with a 201 angle impinging on the TCO/Si system). Therefore, in this spectral range, the gain in the top cell can be attributed to the increased ‘‘refraction’’ at the glass/TCO/Si interfaces produced by the pyramids at the entry. For wavelengths above 600 nm, the enhancement at the top cell for the tilted samples begins to diminish, while for the pyramids it remains higher (see Fig. 6). For these wavelengths the light, which has already been reflected in the intermediate or back reflector arrives back to the front glass, where the geometric pyramids should hinder its exit from the device. Thus, an enhanced reflection for the exiting light at the front could explain why the pyramids keep a higher top cell gain for wavelengths above 600 nm. To further investigate this ‘‘anti-escape’’ effect, we focus on how geometric pyramids behave at the back of the cell. As we pointed before, it is supposed that the pyramidal texture reflects better light than a simple glass. In order to test this point, the white dielectric back reflector was replaced by a glass with the imprinted pyramids with its flat side contacting the ZnO. Similarly, a back reflector consisting of bare glass was used as a reference. In Fig. 7(a), the EQE of the bottom cell with pyramids as back reflector produces 11.20 mA/cm2, whereas with bare glass on the back, the current is 10.78 mA/cm2. The relative difference between both curves is plotted in Fig. 7(b), together with the ones
189
Fig. 8. (a) Total cell currents (topþ bottom) measured by changing the incident angle for the incoming light.
measured using the previous anti-reflective coating and diffusive texture as back reflectors. This graph clearly proves that the geometric pyramids, besides to improve light in-coupling at the front also have the capabilities to enhance light trapping in the devices. Due to the geometric peculiarities of the pyramidal texture, it is important to ensure that the optical gain achieved at normal incidence does not vanish when light impinges to the device with a different angle. For this purpose, the EQE of the cell with pyramids at the front was measured under different angles of incoming light. Fig. 8 shows the total cell current (topþbottom) as a function of the angle. As a reference, the graph also includes measurements for equivalent cells deposited on bare glass and glass with the AR coating. For the pyramids, the current begins to slightly decrease at a lower angle ( 251) than the references. However, it remains higher than the others for all the measured angles. Finally, we want to stress the advantages in using UV imprinting to texture the air/glass interface of different photovoltaic devices. First, its high resolution allows transferring features with sizes below 100 nm, what makes this technique also suitable for copying other sub-wavelength anti-reflective textures on glass. High fidelity replicas by UV imprinting of the most common textures used in TF-Si were recently demonstrated [18,19]. Another advantage of using UV imprinting, a non-aggressive technique, is that the anti-reflective structures to be applied at the air/glass interface can be transferred on the glass after deposition of the whole cell. Lastly, UV imprinting appears to have the capabilities to texture high volume of large area substrates, which is required for the PV industry.
4. Conclusions
Fig. 7. (a) External quantum efficiency curves measured for the bottom cell with either bare glass or imprinted geometric pyramids acting as a back reflector. The relative difference between both curves is shown in (b). The performance as a back reflector of other approaches, which was used previously in this work at the front of the cells are also plotted in (b) as a reference.
We presented a novel geometric light trapping scheme for thin film silicon solar cells. It is based on imprinting micrometric size pyramids at the front of the glass after cell deposition. Applied to micromorph cells, this approach demonstrated to be effective in enhancing light in-coupling and in improving light trapping. A summed current gain (topþbottom) above 5% has been achieved using these pyramids at the front compared to a standard glass substrate, resulting in a noticeable high initial efficiency of 13% for a micromorph solar cell implementing a 1.2 mm thick bottom cell. This scheme has been shown to provide higher interplay with the intermediate reflector by increasing more top cell current (7.3%), at the expense of the bottom one (3.5%), while it possibly allows to enhance the reflection at the glass/air interface for light escaping the cell after getting reflected at the back reflector. These results open up the possibility to further reduce the thickness of the top cell to achieve more stable and efficient micromorph devices. In addition,
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the presented findings might be of high interest for other photovoltaic technologies.
Acknowledgments The authors gratefully acknowledge support by the Swiss Federal Office for Energy (OFEN) under contract 101191. References [1] J. Bailat, L. Fesquet, J.-B. Orhan, et al., Recent developments of high-efficiency micromorph tandem solar cells in KAI-M PECVD reactors, Proceedings of the Fifth World PVSEC, (2010), 2720–2723. [2] K. Yamamoto, A. Nakajima, M. Yoshimi, T. Sawada, S. Fukuda, T. Suezaki, M. Ichikawa, Y. Koi, M. Goto, T. Meguro, T. Matsuda, M. Kondo, T. Sasaki, Y. Tawada, A high efficiency thin film silicon solar cell and module, Solar Energy 77 (2004) 939–949. [3] M. Despeisse, C. Battaglia, M. Boccard, G. Bugnon, M. Charrie re, P. Cuony, ¨ ¨ ¨ ¨ S. Hanni, L. Lofgren, F. Meillaud, G. Parascandolo, T. Soderstr om, C. Ballif, Optimization of thin film silicon solar cells on highly textured substrates, Physica Status Solidi (a) 208 (8) (2011) 1863–1868. [4] T. Kratzla, A. Zindel, R. Benz, Oerlikon solar’s key performance drivers to grid parity, Proceedings of the Fifth World PVSEC, (2010), 2807–2810. [5] S. Nicolay, M. Despeisse, F.-J. Haug, C. Ballif, Control of LPCVD ZnO growth modes for improved light trapping in thin film silicon solar cells, Solar Energy Materials and Solar Cells 95 (2011) 1031–1034. ¨ [6] S.E. Pust, J. Worbs, J. Hupkes, S.O. Klemm, K.J.J. Mayrhofer, Electrochemical etching of zinc oxide for silicon thin film solar cell applications, ECS Transactions 33 (17) (2011) 41–55. [7] H. Saı¨, J. Haijun, M. Kondo, Impact of front and rear texture of thin-film microcrystalline silicon solar cells on their light trapping properties, Journal of Applied Physics 108 (2010) 044505. ¨ ¨ [8] C. Battaglia, J. Escarre´, K. Soderstr om, M. Charrie re, M. Despeisse, F.-J. Haug, C. Ballif, Nanomoulding of transparent zinc oxide electrodes for efficient light trapping in solar cells, Nature Photonics 5 (2011) 535–538.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Geometric light trapping for high efficiency thin film silicon solar cells ¨ ¨ Jordi Escarre´ n, Karin Soderstr om, Matthieu Despeisse, Sylvain Nicolay, Corsin Battaglia, Gregory Bugnon, Laura Ding, Fanny Meillaud, Franz-Josef Haug, Christophe Ballif Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory, Rue A.-L. Breguet 2, 2000 Neuchˆ atel, Switzerland
a r t i c l e i n f o
abstract
Article history: Received 10 September 2011 Received in revised form 17 October 2011 Accepted 20 October 2011 Available online 17 November 2011
The imprinting of random square based pyramidal textures with micrometric scale at the air/glass interface of thin film silicon solar cells is presented as an efficient alternative to anti-reflective coatings to minimize reflection losses at the cell entrance. This novel processing step, which can be applied after cell or module manufacture, is found to simultaneously enhance light in-coupling and light-trapping in amorphous silicon/microcrystalline silicon tandem solar cells. A remarkable total current gain of more than 5% is demonstrated with the imprinting of such structures, resulting in a tandem cell with a high initial efficiency of 13% for a total absorber layer thickness below 1.5 mm. & 2011 Elsevier B.V. All rights reserved.
Keywords: Solar cells Thin film Light trapping Anti-reflective effect UV imprinting
1. Introduction All PV technologies can benefit from introducing anti-reflective (AR) schemes at the front surfaces. In this paper we focus on the case of thin film silicon (TF-Si) solar cells, which is particularly interesting for its specific optical properties. Indeed, the micromorph concept, which combines an amorphous silicon (a-Si:H) top cell with a microcrystalline silicon (mc-Si:H) bottom cell to form a tandem cell, has proven to be an ideal candidate to enhance the thin film silicon solar cells efficiency [1–3]. Moreover, the reduction of the active layer thickness was recently shown to be feasible while maintaining a high efficiency [3], allowing for costs reductions [4]. To further enhance efficiencies while keeping reducing layer thickness, the search for novel and more efficient ways to trap light inside the cells is therefore essential and can contribute to make TF-Si a very low cost technology [4]. A TF-Si solar cell behaves as a light waveguide, with the active silicon layers usually placed in between two transparent conductive oxide (TCO) layers. A textured TCO/Si front interface increases the light path inside the active layer thereby increasing the photo-generated current. Recently, a lot of efforts have been put in finding the optimal front TCO texture [5–9]. For maximum light absorption, it is necessary to enhance the light in-coupling
n
Corresponding author. Tel.: þ41 32 718 33 79; fax: þ 41 32 718 32 01. E-mail address: jordi.escarre@epfl.ch (J. Escarre´).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.10.031
by minimizing reflection losses at the air/glass substrate interface. There are mainly three ways to minimize these losses: via antireflective coatings [10,11], which diminish reflection by creating destructive interferences on the reflected light waves; via antireflecting structures [12], which lower the refractive index at the air/glass interface using either textured or porous glass on a subwavelength scale; or via geometric anti-reflective textures, which allow for multiple rebounds of the incoming light at the glass surface, enhancing transmission. The main advantage in using a geometrical approach is the spectral independence of the antireflective effect. This is the case as long as the features of the texture are much larger than the wavelengths of interest. In TF-Si solar cells, where the spectral range of interest comprises the near infra red up to 1.1 mm, textures with feature sizes of the order of 10 mm are therefore required. In crystalline silicon solar cells, the use of geometric square based pyramidal textures is a common approach to minimize primary reflection loss at the front of the device [13,14]. The pyramids are obtained by etching (100)-oriented monocrystalline wafers in alkaline solutions. The etching is anisotropic, being slower for the (111)oriented crystallographic planes. The intersection of four of these planes forms square based pyramids. The (111) facets are inclined by 54.71 from the horizontal plane, allowing for a double rebound of the incident light against the surface of the wafer (see Fig. 1 (a)). Thus, the expected reflectance is decreased to a value of R2F , where RF is the reflectance of the flat silicon. In TF-Si solar cells, the use of similar pyramidal textures on the glass substrate could therefore significantly reduce the air/glass
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Fig. 1. (a) SEM image of the glass sample with the UV imprinted pyramids at the front and (b) its ARS curve measured in transmittance mode.
interface reflection from RF ¼4% to R2F ¼0.16%. Given the potential for enhanced light in-coupling in TF-Si devices of such pyramidal textures, we propose a way of fabricating such structures on glass. The availability in the market of UV curable resins, which exhibit both high transparency and a refractive index close to the one of glass, makes UV imprinting a good candidate for this purpose. In this work, we show a total current gain above 5% when using such UV imprinted texture at the air/glass interface of high efficiency micromorph solar cells. The use of these pyramidal structures, in addition to reduce the primary reflectance, helps to improve light trapping, which leads to a total current gain higher than the one expected for a perfect planar anti-reflective coating in the air/glass interface ( 4%). Finally, we also compare the performance of this geometric pyramidal texture with other antireflective schemes.
2. Experimental Micromorph tandem cells are fabricated in the superstrate (p-i-n) configuration on a 0.5 mm thick glass (Schott AF45). Borondoped ZnO layers synthesized by low-pressure chemical vapor deposition (LPCVD) are used as transparent contacts. The cells are developed on a 1.4 mm thick doped ZnO layer and on a state-of-theart 2 mm thick ZnO bi-layer, optimum for micromorph cell application. The bi-layer combines a highly doped nucleation part with a non intentionally doped bulk, and is designed to minimize optical absorption and improve light scattering [15]. The silicon layers are deposited by plasma enhanced chemical vapor deposition (PECVD). The micromorph cells used in this study implement a 260 nm thick a-Si:H top cell and a 1.2 mm thick mc-Si:H bottom cell. Doped silicon rich silicon oxide layers are used in both junctions together with optimized intrinsic layer deposition processes, as detailed in [4]. In particular, a thin n-type silicon rich silicon oxide intermediate
reflector layer (20 nm) is used to maximize the response of the 260 nm thick top cell. For all cells, a low-doped 5 mm ZnO thick layer covered with a white dielectric reflector is used as back contact. The pyramidal features are transferred to the glass as follows: the texture of a (100) crystalline silicon wafer textured using a low concentration potassium hydroxide solution (KOH) is molded in polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning). The PDMS mold is prepared mixing the base and the curing agent with a ratio 10:1, degassing it in vacuum, dispensing it onto the wafer, and curing it at 46 1C for 12 h before careful separation from the wafer. Thus, the negative of the original pyramidal texture is transferred on the PDMS. This intermediate mold is used to UV imprint the positive pyramidal texture on the cells. A 25 mm thick UV curable layer (Ormocer, Micro Resist Technology GmbH) is spin coated on the front. This coating, with a refractive index of 1.518 at 635 nm, optically behaves as glass without increasing its absorption in the spectral range comprised between 400 and 1100 nm. UV stamping is carried under vacuum conditions, by applying a pressure of 1 bar and UV exposure ( 365 nm at 1.4 mW/cm2, 3 min) coming through the PDMS. After separation from the PDMS mold, the desired pyramidal texture is reproduced on the front of the cells. The quality of the replica is studied both morphologically and optically by means of scanning electronic microscopy (SEM) and angle resolved scattering (ARS) measurements, respectively. The ARS curve is measured using a green laser with a wavelength of 543 nm. The solar cells are characterized by measuring the current– voltage (I–V) characteristics and the external quantum efficiency (EQE) for both top and bottom cells. I–V measurements were carried out using a dual lamp solar simulator (Wacom WXS-140S-10) in standard test conditions (STC, 25 1C, AM 1.5 g spectrum, and 1000 W/m2). The short circuit current density (Jsc) of the devices is calculated integrating over the EQE curve after weighting with the AM 1.5 g spectrum. Finally, the cells were disposed for 200 h under 1 sun equivalent illumination, at 50 1C, for light induced degradation (LID) evaluation.
3. Results and discussion 3.1. Random square-based pyramids imprinted on glass The geometric pyramidal texture imprinted on bare glass is shown in Fig. 1(a). The image reveals the typical random square based pyramids of a (100) c-Si wafer etched in KOH. The tilt angle of the facets is determined by measuring the angle resolved scattering curve for the transmitted light. As the facets of different pyramids are parallel to each other, the ARS measurement depends on how the sample is oriented with respect to the incident beam. The transmitted light is projected in four intense spots, one for each facet, equispaced around a circle. The ARS curve shown in Fig. 1(b) is measured placing two antipodal spots on the way of the detector. Both maxima are detected at 311 measuring from the incident axis. The ray diagram of Fig. 2(a) shows how the incoming light interacts with the pyramidal texture. By applying Snell’s law, it is inferred that light propagates in the glass with an angle of 20.11 before being refracted out. This direction is defined at the entry when the light first strikes a facet. The incident angle is the facet tilting (af) while the refracted one is (af—20.11). Thus, the facet tilt angle can be estimated by numerically solving the following equation: 1:5 sinðaf 20:13 Þ ¼ sinðaf Þ
ð1Þ
As a result, the facets of the UV imprinted pyramids that we developed on glass are inclined by 51.51, slightly less than the
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Exit
Entry
α1 ~ 51.5° α2 ~ 31.4° α3 ~ 20.1° α4 ~ 31° α4
Case α
α5 ~ 71.6° α6 ~ 71.6° α7 ~ 31°
Case β
α8 ~ 31.5° α9 ~ 51.6°
α7
α3
α6
α2
α5
α8 α9
α1
Fig. 2. Ray diagrams showing how light geometrically interacts with the pyramidal texture when (a) entering or (b) exiting the device.
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54.71 expected from the original texture. However, the condition to have a double rebound at the entry is still valid: the part of the incoming light, which is not transmitted in the glass propagates with an angle of 1031 regarding the incident direct direction and will therefore hit a second pyramid facet. The ray diagrams of Fig. 2(b) show how the outgoing light interacts with the pyramidal texture. The light, once being reflected on the back part of the glass, is directed against the interior of the pyramids either with an angle of 71.61 (case a) or 31.51 (case b). In the first case, the light continues its way inside the glass after being reflected twice against the pyramids, both rebounds at the same angle (71.61) and under total reflection conditions. The case a occurs when the exiting ray firstly strikes a facet parallel to the one, which previously had coupled the light in. For random pyramids, the probability that case a takes place has been estimated to be 44% [16]. For the case b, the total reflection condition is not achieved, and most of the light is refracted out. 3.2. Results on micromorph cells
Fig. 3. External quantum efficiency curves of exactly the same cell measured before and after the imprinting of the pyramids at the front glass. The results obtained for the cells grown on a 2 mm bi-layer and on a 1.4 mm ZnO front contacts are shown in (a) and (b), respectively.
The pyramidal texture was imprinted at the air/glass interface of micromorph cells grown on two different front contacts. Fig. 3 compares the EQE curves for exactly the same cells measured before and after imprinting the pyramids, and Table 1 summarizes their parameters. In both cases, the solar cells with the pyramids exhibit higher top and bottom cell currents. For the cell with the bi-layer as a front contact, the currents increase from 12.41 to 13.32 mA/cm2, for the top cell, and from 12.33 to 12.76 mA/cm2, for the bottom one. The relative increase of the top and bottom cells are 7.3% and 3.5%. In a similar way, for the cell with 1.4 mm thick ZnO front contact, the current increases from 12.78 to 13.60 mA/cm2, for the top cell, and from 11.60 to 12.10 mA/cm2, for the bottom one. The relative increases for the top and bottom cells are 6.4% and 4.3%. For both front contacts, exactly the same total current gain (5.4%) is measured after imprinting the pyramids at the front. The proposed post-process imprinting therefore results in a substantial performance boost leading to an excellent 13% initial efficiency realized using the bilayer as a front contact and a total absorber layer thickness below 1.5 mm. The relative difference between EQE curves (top and bottom) for the cell deposited on the bi-layer front contact before and after imprinting the pyramids is shown in Fig. 4 as a function of the wavelength. In addition, two extra curves are added as EQE references measured on cells deposited on standard AR coated glass and glass front coated with submicrometric random structures. In the former case, an extra glass with coating consisting of a multilayer design: glass/A/B/A/B (A¼TiO2 and B¼SiO2) with a broadband anti-reflective spectrum is glued with UV lacquer at the front. In the latter case, a diffusive texture (based on 5 mm thick LPCVD grown ZnO), which also acts as a graded index antireflection layer, is UV imprinted at the front surface. It is seen in Fig. 4(a) that for wavelengths below 500 nm, where the EQE
Table 1 Parameters of the same cell measured before and after the imprinting of the pyramids at the front glass.
No pyramids Pyramids Pyramids after 200 h LID No pyramids Pyramids
Front contact
Voc (V)
FF
Jsc (Top) (mA/cm2)
Jsc (Bottom) (mA/cm2)
Efficiency (%)
Bi-layer (2 mm) Bi-layer (2 mm) Bi-layer (2mm) ZnO (1.4 mm) ZnO (1.4 mm)
1.38 1.38 1.37 1.35 1.35
0.725 0.730 0.660 0.770 0.770
12.41 13.32 12.8 12.78 13.60
12.65 12.94 12.8 11.60 12.10
12.4 13.0 11.5 12.0 12.6
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Fig. 5. Effect of the pyramids is significantly reduced by doubling the thickness of the front glass from 0.5 to 1 mm. The relative EQE gain is shown here for both thicknesses.
Fig. 4. Relative difference between the EQE curves of three equivalent micromorph cells grown on a 2 mm bi-layer before and after modification of their front. The EQE changes in the top and bottom cells are plotted in (a) and (b), respectively.
differences can only come from a better light in-coupling, the maximum gain ( 6%) is measured for the cell with geometric pyramids at the front. Therefore, the incoming light double rebound scheme, induced by the large pyramids is more efficient than the use of anti-reflective coatings or diffusive textures at the front, which leads to gain at 500 nm of 4 and 3%, respectively. This important gain is significantly above to the 4% expected when having an ideal light in-coupling at the front glass. Thus, the geometric pyramidal texture not only provides an optimum antireflective effect at the front but it also possibly reduces the amount of light exiting the device coming from the reflection at the glass-TCO and TCO-silicon interfaces. Before discussing the spectral range above 500 nm for the top cell, we first focus on the bottom cell shown in Fig. 4(b). In this case, the pyramid covered glass also performs better than the other references. For wavelengths above 600 nm, the gain of the cell with the AR coating linearly drops to 10%. These important losses are attributed to its double thick front glass. Indeed, because of the small size of the cells ( 0.7 1.5 cm2), the outgoing light, partially scattered after its path through the cell, can escape through the thick front glass sides after multiple reflections. In fact, the thicker the glass, the longer the light will travel inside the substrate, being more easily guided out from the small active area of the cell [17]. Note that this effect is negligible on modules, but it also has to be taken into account when using pyramids at the front of small cells. In this case, where the incoming light is already refracted into the glass with an angle of 20.11, important optical losses can be foreseen using thicker front glasses, even at the blue part of the spectra. To quantitatively estimate such EQE losses, the thickness of the front substrate was increased to 1 mm by gluing an extra glass with the imprinted pyramids. Fig. 5 compares the new total EQE gain
Fig. 6. Proportion of light absorbed in the top (or bottom) cell is calculated as the ratio between its EQE (top or bottom) and the total one (top þ bottom). Here, for different incident angles (a), it is shown how the absorption in each sub-cell is modified with respect to the measure at 01. The dashed and solid curves give information on top and bottom cells.
(topþbottom) with the previous one, i.e. with the pyramids imprinted directly on a 0.5 mm thick front glass. As expected, the performance of the pyramids is significantly lowered all over the whole spectra using thicker glass at the front. Interestingly, for wavelengths above 500 nm, the pyramids constantly increase the quantum efficiency of the top cell arriving to a remarkable gain of 12% at 700 nm wavelength (see Fig. 4(a)). This trend cannot be observed for the cells with no imprinted pyramids, where the gain in the top is kept nearly constant. Thus, the use of the pyramids at the front could also play an important role in order to keep the amorphous top cell as thin as possible, helping to achieve higher micromorph stable efficiencies. The pyramids enhance light absorption in the top cell, at the sacrifice of the bottom, possibly due to enhanced reflection at the thin silicon rich silicon oxide intermediate reflector layer. As it was discussed before, the geometry of the pyramids changes the direction of the incident light. In first approximation after refraction at the glass-TCO and TCO-Si, the light would propagate with an angle of 7.41 inside the silicon that could increase the reflectance at the intermediate reflector. In order to validate this hypothesis, the EQE of the cell without pyramids at the front is measured under different angles for the incident light. For each angle, the proportion of light absorbed in the top (or bottom) cell is calculated as the ratio between its EQE (top or bottom) and the total one (top þbottom). Fig. 6 shows how the absorption in each sub-cell is modified with respect to the measure at 01. Moreover, the same curves measured for the cell with the pyramids at the front is added for better comparison. It can be seen that an increase in the incident angle shifts the light absorption towards
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the top cell. The maximum gain for the top cell is achieved at 600 nm. For lower wavelengths, the cell with pyramids behaves similar to the one measured with a tilting angle of 301. Indeed, the light impinging the glass at this angle should propagate through the silicon with an angle of 7.21, which is close to the 7.41 expected for the pyramids (again with a 201 angle impinging on the TCO/Si system). Therefore, in this spectral range, the gain in the top cell can be attributed to the increased ‘‘refraction’’ at the glass/TCO/Si interfaces produced by the pyramids at the entry. For wavelengths above 600 nm, the enhancement at the top cell for the tilted samples begins to diminish, while for the pyramids it remains higher (see Fig. 6). For these wavelengths the light, which has already been reflected in the intermediate or back reflector arrives back to the front glass, where the geometric pyramids should hinder its exit from the device. Thus, an enhanced reflection for the exiting light at the front could explain why the pyramids keep a higher top cell gain for wavelengths above 600 nm. To further investigate this ‘‘anti-escape’’ effect, we focus on how geometric pyramids behave at the back of the cell. As we pointed before, it is supposed that the pyramidal texture reflects better light than a simple glass. In order to test this point, the white dielectric back reflector was replaced by a glass with the imprinted pyramids with its flat side contacting the ZnO. Similarly, a back reflector consisting of bare glass was used as a reference. In Fig. 7(a), the EQE of the bottom cell with pyramids as back reflector produces 11.20 mA/cm2, whereas with bare glass on the back, the current is 10.78 mA/cm2. The relative difference between both curves is plotted in Fig. 7(b), together with the ones
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Fig. 8. (a) Total cell currents (topþ bottom) measured by changing the incident angle for the incoming light.
measured using the previous anti-reflective coating and diffusive texture as back reflectors. This graph clearly proves that the geometric pyramids, besides to improve light in-coupling at the front also have the capabilities to enhance light trapping in the devices. Due to the geometric peculiarities of the pyramidal texture, it is important to ensure that the optical gain achieved at normal incidence does not vanish when light impinges to the device with a different angle. For this purpose, the EQE of the cell with pyramids at the front was measured under different angles of incoming light. Fig. 8 shows the total cell current (topþbottom) as a function of the angle. As a reference, the graph also includes measurements for equivalent cells deposited on bare glass and glass with the AR coating. For the pyramids, the current begins to slightly decrease at a lower angle ( 251) than the references. However, it remains higher than the others for all the measured angles. Finally, we want to stress the advantages in using UV imprinting to texture the air/glass interface of different photovoltaic devices. First, its high resolution allows transferring features with sizes below 100 nm, what makes this technique also suitable for copying other sub-wavelength anti-reflective textures on glass. High fidelity replicas by UV imprinting of the most common textures used in TF-Si were recently demonstrated [18,19]. Another advantage of using UV imprinting, a non-aggressive technique, is that the anti-reflective structures to be applied at the air/glass interface can be transferred on the glass after deposition of the whole cell. Lastly, UV imprinting appears to have the capabilities to texture high volume of large area substrates, which is required for the PV industry.
4. Conclusions
Fig. 7. (a) External quantum efficiency curves measured for the bottom cell with either bare glass or imprinted geometric pyramids acting as a back reflector. The relative difference between both curves is shown in (b). The performance as a back reflector of other approaches, which was used previously in this work at the front of the cells are also plotted in (b) as a reference.
We presented a novel geometric light trapping scheme for thin film silicon solar cells. It is based on imprinting micrometric size pyramids at the front of the glass after cell deposition. Applied to micromorph cells, this approach demonstrated to be effective in enhancing light in-coupling and in improving light trapping. A summed current gain (topþbottom) above 5% has been achieved using these pyramids at the front compared to a standard glass substrate, resulting in a noticeable high initial efficiency of 13% for a micromorph solar cell implementing a 1.2 mm thick bottom cell. This scheme has been shown to provide higher interplay with the intermediate reflector by increasing more top cell current (7.3%), at the expense of the bottom one (3.5%), while it possibly allows to enhance the reflection at the glass/air interface for light escaping the cell after getting reflected at the back reflector. These results open up the possibility to further reduce the thickness of the top cell to achieve more stable and efficient micromorph devices. In addition,
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the presented findings might be of high interest for other photovoltaic technologies.
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