Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

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Nano-imprinted rear-side diffration gratings for absorption enhanement in solar ells Alexander Mellor, Hubert Hauser, Aron Guttowski, Christine Wellens, Benedikt Blási, Ignaio Tobías, Antonio Marti,

1 Nano-imprinted rear-side diffration gratings for absorption enhanement in solar ells Alexander Mellor, Hubert Hauser, Aron Guttowski, Christine Wellens, Benedikt Blási, Ignaio Tobías, Antonio Marti, Antonio Luque ABSTRACT As wafer-based solar ells beome thinner, light-trapping textures for absorption enhanement will gain in importane. In this work, rystalline silion wafers were textured with wavelength-sale diffration grating surfae textures by nanoimprint lithography using interferene lithography as a mastering tehnology. This tehnique allows fine-tailored nanostrutures to be realized on large areas with high throughput. Solar ell preursors were fabriated, with the surfae textures on the rear side, for optial absorption measurements. Large absorption enhanements are observed in the wavelength range in whih the silion wafer absorbs weakly. It is shown experimentally that bi-periodi rossed gratings perform better than uni-periodi linear gratings. Optial simulations have been made of the fabriated strutures, allowing the total absorption to be deomposed into useful absorption in the silion and parasiti absorption in the rear refletor. Using the alulated silion absorption, promising absorbed photourrent density enhanements have been alulated for solar ells employing the nano-textures. Finally, first results are presented of a passivation layer deposition tehnique that planarizes the rear refletor for the purpose of reduing the parasiti absorption. Keywords: solar ell, light trapping, nano-imprint lithography, diffration grating, rystalline silion, photoni rystal, absorption enhanement. 1. INTRODUCTION Most ommerially produed solar ells use rystalline silion (-Si) wafers as light absorbers. Due to the indiret band gap of -Si, photons with energies lose to the band edge are absorbed weakly. By texturing the rear surfae of the -Si wafer, photons are sattered inreasing their optial path length in the absorber [1]; a proess known as light trapping. This an lead to higher absorption, higher urrent output, and ultimately higher solar ell effiienies. Light trapping is expeted to gain in importane as the industry moves to thinner wafers in an effort to redue osts assoiated to the volume of silion feedstok used. One option for a rear-side surfae texture is a periodi wavelength-sale diffration grating ethed into the -Si wafer. This was first proposed for thin-film solar ells by Sheng et al. [2], and was later applied to wafer-based ells by Heine and Morf [3]. Diffration gratings used for this purpose are typially a few hundred nanometres deep and onsume far less of the silion wafer than onventional surfae textures, whih have feature sizes of some tens of mirons [4, 5]. What's more, for normal inidene, they offer the possibility to provide greater absorption enhanement than unordered rough surfaes [6]. To be industrially relevant, the diffration grating must be produed heaply and quikly. A proess hain ombining nano-imprint lithography (NIL) with interferene lithography as a mastering tehnology has been developed at the Fraunhofer ISE for this purpose [7,8]. Using this tehnique, fine-tailored nano-strutures an be realized on large areas with high throughput. In this work, the above mentioned proess hain has been used to fabriate wavelength sale linear and rossed diffration gratings on -Si wafers. The wafers have been proessed into solar ell preursors for optial measurements. Absorption spetra are presented, showing high absorption enhanements in the 1 urn urn wavelength range in whih the -Si wafer absorbs weakly, and onfirming that bi-periodi gratings offer better absorption enhanement than uni-periodi gratings. Optial simulations have been made of the fabriated solar ell preursors. These show exellent

agreement with the measured spetra, and allow the total absorption to be deomposed into useful absorption in the silion and parasiti absorption in the refletor.

2 agreement with the measured spetra, and allow the total absorption to be deomposed into useful absorption in the silion and parasiti absorption in the refletor. Expeted absorbed photourrent density enhanements are alulated using the results of these simulations. Finally, first results of a method for reduing the parasiti absorption in the rear refletor are presented. This onsists in liquid phase deposition of the rear side passivation layer, whih leaves a planar surfae onto whih the refletor an be deposited. 2. FABRICATION OF REAR TEXTURED SOLAR CELL PRECURSORS BY NANO- IMPRINT LITHOGRAPHY Both linear and rossed gratings were fabriated on rystalline silion substrates using nano-imprint lithography (NIL), with interferene lithography as a mastering tehnology, followed by reative ion ething (RIE). This tehnique allows fine-tailored nano-strutures to be realized on large areas with high throughput: a neessity for industrialisation. A detailed desription of the proess hain an be found in Refs. [8, 9]. The master strutures were realised on photoresist oated glass substrates by two-beam interferene lithography followed by a development step. The line grating masters were realised by a single exposure. The rossed grating masters were realised by two exposures with a 90 rotation of the substrate in between. The inverse pattern of the master struture was repliated on an addition-uring polydimethylsiloxane (PDMS) stamp by ast moulding. The silion wafer was then oated on the rear side with a low visosity free-radial uring NIL resist. The stamp was pressed onto the resist and pressure was maintained while the resist was exposed to UV light through the transparent stamp. This left a replia of the master struture on the photoresist. The pattern was then transferred into the underlying silion wafer by RIE. The RIE was strongly anisotropi, favouring ething normal to the wafer plane. The grating depth is determined by the ething time and an be ontrolled aurately. The residual photoresist was removed by plasma ashing. SEM mirographs of linear and rossed gratings ethed into the silion substrates are shown in Figure 1 (left) and (right) respetively. It an be seen that the desired binary profile and lose to 50% duty yle has been ahieved in the silion. A period of 1 urn was hosen for the diffration gratings, this having been found to be optimum for 40 urn thik -Si solar ells in previous numerial studies [10, 11]. The grating depth was 300 nm for the line grating and 200 nm for the rossed grating. These depths had been found to provide the greatest absorption enhanement in preliminary experiments. Figure 1. SEM mirographs of Linear (left) and rossed (right) diffration grating textures in -Si wafers produed by NIL Solar ell preursors were fabriated for optial haraterisation. Although this work is ultimately aimed at appliation to wafers with thiknesses around 40 urn, we have used 200 urn thik monorystalline silion wafers in this study, due to availability and ease of handling. These were oated with a 63 nm thik SiN anti refletion oating (ARC) on the front side. The wafers were textured on the rear side by NIL as desribed above. A Si0 2 passivation layer was deposited on the textured surfae by plasma enhaned hemial vapour deposition (PECVD). Finally, an Al refletor was deposited on the passivation layer by evaporation. An SEM mirograph of the rear-surfae ross-setion of the linear grating sample is shown in Figure 2. The Si0 2 passivation layer has been oloured red for ease of visualization. A planar referene was also fabriated. This onsisted of a 200 urn thik rystalline silion wafer with a 63 nm SiN ARC on the front side and a 500 nm thik PECVD Si0 2 passivation layer followed by an Al refletor on the rear side.

Figure 2. SEM mirograph of the rear-surfae ross-setion of the linear grating solar ell preursor. The Si0 2 passivation layer has been digitally oloured in red for ease of visualization. 3.

3 Figure 2. SEM mirograph of the rear-surfae ross-setion of the linear grating solar ell preursor. The Si0 2 passivation layer has been digitally oloured in red for ease of visualization. 3. MEASURED AND SIMULATED ABSORPTION SPECTRA The wavelength dependent absorption of all solar ell preursors was measured by refletion spetrosopy using a Fourier transform spetrometer and an integrating sphere. The absorption was alulated by Absorption = 1 - Refletion, under the assumption that there is zero transmission through the Al refletor. The absolute absorption enhanement ompared to the planar referene is shown for both linear and rossed grating samples in Figure 3. This is the absorption in the textured samples minus the absorption in the planar sample. It an be observed that, in all ases, there is a signifiant absorption enhanement in the wavelength range lose to the silion band edge in whih silion absorbs weakly (1 urn -1.2 pi). The absorption enhanement is muh stronger for rossed gratings than for linear gratings. This onfirms experimentally the previous theoretial preditions that bi-periodi gratings lead to stronger absorption enhanement than uni-periodi gratings [6, 12]. E CD O CD.C 111 o Linear grating Crossed grating i Silion Band Edge O to < CD + _3 O to < Wavelength (urn) Figure 3. Absolute absorption enhanements for solar ell preursors employing linear and rossed gratings ompared to a planar referene.

The measured absorption is the sum of the useful absorption in the silion and the parasiti absorption in the refletor. For wavelengths above the silion band edge (-1.

4 The measured absorption is the sum of the useful absorption in the silion and the parasiti absorption in the refletor. For wavelengths above the silion band edge (-1.2 urn), the silion is transparent and all absorption takes plae in the refletor (it is assumed that the Si0 2 is transparent at all wavelengths). The measured absorption enhanement in this range demonstrates that the presene of the grating inreases the refletor absorption, as has been predited in Ref [11]. To disern how muh of the measured absorption ours in the silion, and how muh ours in the aluminium refletor, optial simulations have been made of the fabriated strutures using the simulation tehnique presented in Ref. [6]. In the simulation tehnique, the diffration-grating region (the textured rear surfae of the silion, the passivation layer and the refletor) is treated separately from the rest of the struture (the -Si wafer and the ARC). The diffration-grating region is simulated wave-optially using rigorous oupled wave analysis (RCWA) to yield a far-field intensity sattering matrix. This is ombined with a simple steady state matrix formalism, whih desribes the propagation of the orders within the silion bulk, their absorption, their partial loss at the front surfae, and their re-interation with the grating via the sattering matrix. The angle dependent refletivity of the ARC is alulated using a text-book transfer matrix method. Thus the diffration grating and the ARC are treated oherently, but the bulk absorber is treated inoherently. This is appropriate for strutures whose absorber thikness is great than the oherene length of the inident light (~1 urn for solar illumination), as is the ase for these strutures. Beause the diffration grating region and the bulk silion are simulated separately, it is easy to separate the silion absorption and the refletor absorption without time onsuming near-field spatial interpolation. Figure 4 shows the measured (red triangles) and simulated (solid blak line) total absorption for the solar ell preursors with rossed (a) and linear (b) gratings. Good agreement an be seen between the measured and simulated results, partiularly in the ase of the linear grating. The simulated absorption has been deomposed into absorption in then silion wafer and absorption in the aluminium refletor; these are the green and blue urves respetively. It an be seen that, in the light-trapping wavelength range (1 urn urn), the parasiti refletor absorption is omparable in magnitude to the useful silion absorption. A D Total Absorption (measured) Planar Referene (measured) -Total Absorption (simulated) -Silion Absorption (simulated) -Aluminium Absorption (simulated) o </> < Wavelength (jim) Wavelength (urn) Figure 4. Measured and simulated absorption spetra for the linear (a) and rossed (b) grating solar ell preursors. The simulated total absorption (blak urve) is deomposed into the useful silion absorption (green urve) and the parasiti aluminium absorption (blue urve). Red triangles and blue squares show the measured absorption spetra for the textured sample and the planar referene respetively.

5 The absorbed photourrent density in the silion (J phsi )and in the aluminium refletor (J p h,ai) have been alulated by J ph,si = 1e] J ph,al=q e A Si f (!) \ A Al Aml5G d^ A<l.2jUm where ^Ami.so is photon flux of the AM1.5G spetrum, q e is the elementary harge, is the useful silion absorption (green urves in Figure 4) and is the absorption in the aluminium refletor (blue urves in Figure 4). The J ph,si is the urrent density that would be extrated from the ell if all the eletron-hole pairs generated in the silion reahed the external ontats; it represents an upper bound for the short iruit urrent density (J s ). The J ph,ai is simply the number of photons absorbed in the aluminium refletor represented as a urrent density for easy omparison. An upper integration limit of 1.2 urn has been hosen so as to only inlude useful photons that are wastefully absorbed in the refletor. Table 1 shows the alulated J phjsi and J ph4 i for solar ells employing the linear and rossed grating textures as well as the planar referene struture, as well as the absolute J phsi and J ph4 i enhanement of the textured ells ompared to the planar referene, denoted AJ phsi and AJ ph4 respetively and shown in bold. Table 1. Calulated J phsi and J ph/u for 200 urn thik solar ells with a planar rear side and with rear-side linear and rossed grating surfae textures. A denotes the absolute differene between the textured solar ell and the planar referene. all units are mam 2 Jph.Si djpk,si JphAl djpk,al Planar referene Linear grating Crossed grating These results show that signifiant J ph>si enhanements an be expeted for solar ells employing rear side diffration gratings, even for ells as thik as 200 urn. The expeted J ph>si enhanement for the rossed grating is almost twie that of the linear grating. It an also be seen from Table 1 that the diffration grating auses a parasiti absorption enhanement that is roughly equal to the useful absorption enhanement. Photons absorbed in this way might otherwise have been absorbed in the silion and onverted to external urrent. It is therefore expeted that by reduing the refletor absorption in this range, greater silion absorption and therefore greater J phsi enhanements than those presented in Table 1 ould be ahieved. A prospetive means of reduing this parasiti absorption is presented in the following setion. All experimental and numerial results presented so far are for 200 urn thik wafers. To give an idea of what might be ahieved for thinner solar ells, simulations have been made of 40 urn thik solar ells employing the grating strutures desribed in Setion 2. The J ph,si and tsj phsi have been alulated as before and are presented in Table 2. The J ph,si enhanements are more pronouned for a 40 urn ell than for a 200 urn ell, as would be expeted. The expeted J P P,SÍ for a 40 urn solar ell with rear-side rossed grating is similar to that of a 200 urn ell with no grating. Table 1. Calulated J phsi for 40 urn thik solar ells with a planar rear side and with rear-side linear and rossed grating surfae textures. all units are mam" 2 JphSi AJ p h,si Planar Linear Crossed referene grating grating

4. PLANARIZATION OF THE REAR REFLECTOR Numerial studies of similar strutures have shown that the parasiti refletor absorption an be dereased by around 50% if the aluminium refletor is planar, as

6 4. PLANARIZATION OF THE REAR REFLECTOR Numerial studies of similar strutures have shown that the parasiti refletor absorption an be dereased by around 50% if the aluminium refletor is planar, as opposed to onformal with the Sigrating[ll]. The refletors in the strutures studied here are lear onformal (Figure 2). This is due to the vapour phase PECVD Si0 2 deposition proess, whih deposits a onformal layer of Si0 2 onto the textured silion surfae. In order to deposit a planar refletor, the PECVD proess has been replaed by a liquid phase Si0 2 deposition. This onsists in deposition of a dispersion of olloidal Si0 2 nano-partiles onto the textured surfae by spin oating. This liquid phase proess leaves a planar Si0 2 surfae even when the underlying substrate is textured. This an be seen in Figure 2 (top), whih shows a spin-oated Si0 2 layer on top of a textured photoresist-on-glass substrate after drying in ambient onditions. Figure 2 (bottom) shows a solar ell preursor with a linear grating, spin-oated passivation layer and Al refletor. It an be seen that the addition of the Al refletor auses the previously flat Si0 2 surfae to modulate slightly, ausing a slight modulation of the Al refletor. We expet that this effet ours beause of the tempering during the evaporation proess. Nonetheless, the Al surfae is learly more planar for the spin-oating method than for the PECVD method. Perfetion of this method and investigation into the effet the refletor planarization has on the parasiti refletor absorption and the useful silion absorption is a subjet of on-going researh. SÍO2 surfae Figure 5. Top: a planar Si0 2 layer deposited on a textured photo-resist-on-glass substrate via spin oating. Bottom: ross setion of a linear-grating-textured -Si wafer oated with Si0 2 via spin-oating followed by evaporation of an aluminium refletor. Note that the bottom image is upside down with respet to the top image.

5. CONCLUSIONS -Si wafers have been textured with wavelength-sale diffration gratings by NIL using interferene lithography as a mastering tehnology.

7 5. CONCLUSIONS -Si wafers have been textured with wavelength-sale diffration gratings by NIL using interferene lithography as a mastering tehnology. The wafers have been proessed into solar ell preursors for optial haraterisation. Measured absorption spetra show a signifiant absorption enhanement in the wavelength range in whih the -Si wafers absorb weakly (1 urn pi). Bi-periodi rossed gratings are shown to produe greater absorption enhanements than uniperiodi linear gratings. Optial simulations have been made of the solar ell preursors allowing the total absorption to be deomposed into useful absorption in the silion and parasiti absorption in the aluminium refletor. The total absorption alulated by the simulations shows good agreement with the measured absorption. Based on the alulated silion absorption, absorbed photourrent enhanements have been alulated for solar ells employing the nanotextures. These are 0.86 (1.28) and 1.55 (2.63) mam" 2 for 200 urn (40 urn) thik -Si solar ells employing linear and rossed gratings respetively. It has also been shown that the introdution of the grating auses the parasiti refletor absorption to inrease by as muh as the usefiil silion absorption. First results of a method for planarization of the rear refletor to prevent this have been presented. ACKNOWLEDGEMENTS Parts of this work were funded by the Comunidad de Madrid under ontrat number S2009/ENE-1477 ( Numnaia-II), the Spanish Siene Ministry under ontrat number ENE C02-01(NANOGEFFES), the German Federal Ministry of Environment, Nature Conservation and Nulear Safety under ontrat numbers (NanoTex) and A (ThETA), and the German Federal Ministry of Eduation and Researh under ontrat number 03SF0401 (InfraVolt). Alexander Mellor gratefully aknowledges the Comunidad de Madrid for finanial support through the sholarship Personal Investigador de Apoyo. REFERENCES [1] Yablonovith, E. and Cody, G. D., "Intensity Enhanement in Textured Optial Sheets for Solar Cells," IEEE Trans. Elet. Dev., Ed-29(2), 300 (1982). [2] Sheng, P., Bloh, A. N. and Stepleman R. S., "Wavelength-seletive absorption enhanement in thin-film solar ells," Applied Physis Letters, 43(6), (1983). [3] Heine, C, and Morf, R. H., "Submirometer gratings for solar energy appliations," Appl. Opt., 34(14), (1995). [4] Zhao, J., Wang, A., Green M. A. et al, "19.8% effiient "honeyomb" textured multirystalline and 24.4% monorystalline silion solar ells," Applied Physis Letters, 73(14), (1998). [5] Zhao, J., Wang, A., and Green, M. A., "24-5% Effiieny silion PERT ells on MCZ substrates and 24-7% effiieny PERL ells on FZ substrates," Progress in Photovoltais: Researh and Appliations, 7(6), (1999). [6] Mellor, A., Tobias, I., Marti, A. et al, "Upper limits to absorption enhanement in thik solar ells using diffration gratings," Progress in Photovoltais: Researh and Appliations, 19(6), (2011). [7] Hauser, H., Mihl, B., Kübler, V. et al, "Nanoimprint Lithography for Honeyomb Texturing of Multirystalline Silion," Energy Proedia 8, (2011). [8] Blasi, B., Hauser, H., Walk, C. et al, [Photon Management Strutures Based on Interferene Lithography and Nanoimprint Proesses], Hamburg(2011). [9] Blasi, B., Hauser, H., Hohn O. et al, "Photon Management Strutures Originated by Interferene Lithography," Energy Proedia, 8, (2011). [10] Mellor, A., Tobias, I., Marti A. et al, "A numerial study of Bi-periodi binary diffration gratings for solar ell appliations," Solar Energy Materials and Solar Cells, 95(12), (2011). [11]Peters, M., Rüdiger, M., Hauser H. et al, "Diffrative gratings for rystalline silion solar ells optimum parameters and loss mehanisms," Progress in Photovoltais: Researh and Appliations, n/a-n/a (2011). [12] Yu, Z., Raman, A. and Fan, S. "Fundamental limit of nanophotoni light trapping in solar ells," Proeedings of the National Aademy of Sienes USA, 107, (2010).

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