Microelectronic Engineering 111 (2013) 224–228

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Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

Silicon solar cells efficiency improvement with Nano ImprintLithography technology

S. Landis a,⇑, M. Pirot b, R. Monna b, Y. Lee a, P. Brianceau a, J. Jourdan c, S. Mialon c, P.J. Ribeyron b

a CEA-LETI, Minatec Campus, 17 rue des martyrs Grenoble, 38054 Cedex 9, Franceb CEA-INES, 50 avenue du Lac Léman, BP332, 73370 Le Bourget du Lac, Francec MPO Energy, Domaine de Lorgerie, 53700 Averton, France

a r t i c l e i n f o

Article history:Available online 8 April 2013

Keywords:Solar cellMulti crystalline SiNano Imprint LithographyAnti-Reflective surface

0167-9317/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.mee.2013.03.165

⇑ Corresponding author. Tel.: +33 0 4 38 78 44 03;E-mail address: slandis@cea.fr (S. Landis).

a b s t r a c t

A large scale patterning processes to produce patterned silicon surfaces with low reflectivity were devel-oped for silicon solar cells. Optical design, sample manufacturing, optical characterizations and cell effi-ciency measurements were conducted. Optical simulations were carried out to compute the reflectivity ofthe patterned surface to target the optimum shape to be manufactured in the silicon substrate. Patternedsurfaces were manufactured using thermal Nano Imprint Lithography over 125 � 125 mm2 Si-c and Si-mc wafer and proportional dry etching. A high aspect ratio inverted pyramid shapes were achieved inboth Si-c & Si-mc substrates. An effective reflectivity (Rw) of about 3% was achieved on multi-crystallinesilicon with the inverted pyramid pattern. The patterning process uniformity over the substrate was bet-ter than 97%. I(V) measurements of standard Si-mc KOH textured and Si-mc inverted pyramidal texturedusing Nano imprint and dry etching revealed that a drop of about 3% were induced in the open-circuitvoltage, a drop of about 3.4% for the fill factor, however with an increase of about 8.3% for the short-cir-cuit current. A gain of 0.33% absolute efficiency is obtained on the Si-mc Nano-imprinted cell compared tothe KOH textured Si-mc cell. The gain of the short-circuit current is directly connected to the gain ofreflectivity (8%) obtained on the finished solar cell.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction ing in a surface that is covered with inverted pyramids. Wet chemi-

Nanostructured surfaces are widely expected to play a significantrole in photonics, especially in wide-spread products like imagingsensors, solar cells, lightning devices and micro displays. Thoseexpectations are related to two kinds of benefits: improvement ofthe conversion efficiency from photon to electron (solar cells, photodetectors and image sensors) or from electron to photon (lightning,displays) and reduction of fabrication cost. The performance of asolar cell is critically dependent on the absorption of incidentphotons and their conversion to current. Several approaches havebeen proposed to improve the solar cell efficiency: light conversionapproaches [1], the light-mater interaction improvement using lightconcentration solutions [2], increasing the absorption [3] or lower-ing the surface reflectivity [4]. Inasmuch as more than 30% of inci-dent light is reflected from the silicon surface back to the air,surface treatments are required to manufacture high-efficiencysilicon solar cells. Anti-Reflective Coating (ARC) is usually performedon Si surface to reduce light reflection and to increase light absorp-tion. Acid [5] or alkaline [6] wet chemical etching approaches werealso proposed. Texturing of monocrystalline silicon (Si-c) is usuallydone in aqueous solutions of potassium or sodium hydroxide result-

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cal etchings are simple and low cost processes, however alkalineprocess suffers from the silicon crystalline orientations. For textur-ing of multicrystalline silicon (Si-mc) such crystal orientationdependent techniques are not efficient enough. An alternative tothin-film coatings and wet chemical etching is to pattern the surfacewith a periodically structured array [7,8]. However most of solutionsproposed today required high resolution and very expensive lithog-raphy techniques., Nano Imprint Lithography (NIL), considered ascost efficiency patterning technique, has already been used to createpatterns over organic solar cell [9,10], or manufacture photonic crys-tal over thin solar cells [11] to increase light coupling within thin wa-fer thickness. However few attends were made to combine NIL withdry plasma etching to offer a new attractive process flow to lower thesurface reflectivity of crystalline or multi crystalline solar cells [12]. Inthis paper we proposed a patterning process over 125� 125 Si-mccells using NIL with soft stamps and dry etching processes. We reportthe implementation of a micron scale antireflection pattern. The over-all anti-reflective effects were compared with a typical KOH wet etch-ing and I–V characteristics were carried out.

2. Experimental

P-type 1–2 X.cm mc-Si substrates with a thickness of 220 lmwere used. First of all wafers were polished in an acidic bath in

S. Landis et al. / Microelectronic Engineering 111 (2013) 224–228 225

order to remove damages related to the saw. A 6.2 lm thick JSR re-sist where then spin coated on multi crystalline Si square wafers.The imprint process was performed in EVG520HE set-up with h-PDMS/PDMS composite stamp manufactured from a 200 mmdiameter Si crystalline wafer patterned with wet KOH etching(Fig. 1a). A silicon dioxide layer deposited on (100) silicon sub-strate was first patterned with DUV lithography and reactive ionetching processes. Then this patterned mask was used to carryout a selective wet alkaline etching of silicon. The master siliconstamp was then coated with anti-adhesive layer, Optool DSX fromDAIKIN [13] and a 32 lm thick h-PDMS layer were spin coated onthe top of inverted pyramid shape (Fig. 1b) and finally cover by3 mm thick PDMS layer (Fig. 1c,d). This soft composite stampwas then demolded from the Si master stamp and coated with Op-tool DSX anti-sticking layer lowering the surface energy down to

Fig. 1. (a) SEM cross section picture of inverted pyramid shapes wet etched in Si-c 200 mmaster. (c) Optical micrograph of the composite stamp and (d) optical image of the h-P

Fig. 2. SEM cross section pictures of pyramid shapes printed in the resist on Si-c (a) &etched patterns within the Si substrates for Si-c (c) & Si-mc substrates (d).

11 mJ/m2. The stamp was then printed at 135 �C under 2 bars for5 min into the resist to shape the inverted pyramid features(Fig. 2a & b). Thanks to the low stiffness of the composite moldand the micrometer scale of the pyramid shape, printings over Sigrain boundaries for Si-mc substrate, even for 4 lm to 5 lm stepsbetween two neighbouring grains, were possible without deform-ing the printed patterns. Dry etching was then performed in ICPmode using chlorine based chemistry in Centura AP from AppliedMaterial to remove the residual layer thickness and transfer theprinted shape within the Si-mc substrate (Fig. 2c & d). Low anduniform residual layer thickness (210 nm) compared to the featureheight (6.17 lm) was achieved over Si-c substrates thanks to theplanarity of surfaces. However, this was not possible for Si-mc sub-strates which presented from 4 lm to 5 lm mesa height at grainboundaries. The etching process was therefore developed to free

m wafer. (b) SEM cross section image of 32 lm thick h-PDMS layer coated on the SiDMS pyramid shapes over the PDMS back plane.

Si-mc (b) substrates. Corresponding SEM cross section pictures of pyramid shapes,

Fig. 3. Process flow for the patterned Si-mc solar cell manufacturing with NIL orKOH wet etching approaches.

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our self from the non-uniform residual layer thickness distributionobserved over the Si-mc substrate (Fig. 2b). It led also to an ampli-fication of the pyramid aspect ratio. Indeed, starting with an initialside wall angle of pyramid of about 54.74� (KOH wet etching of Si,Fig. 1a) with respect to the surface plane, the final side wall anglewas ranging between 71� up to 85�, leading to much more deeperinverted pyramid shape, up to 15 lm deep for a pyramid with asquare base of 9.5 lm (Fig. 2c and d). The remaining resist wasstripped away with oxygen plasma and HF treatment was thenperformed. The final process leads to excellent results in structur-ation with similar results for both flat single-crystal silicon andmulti crystalline substrates. The wafers were submitted to a POCl3diffusion in an open quartz tube furnace. The diffusion process re-sults in the formation of n++ surfaces on both sides with a sheetresistance value of 75 O/sq. Anti-reflection coating (72 nm thickSiN) and metallization by screen printing were carried out in orderto fit to the surface topography. The whole process flow is summa-rize in Fig. 3 and is also compared with a standard alkaline patter-ing process scheme. Finally illuminated and dark I (V)measurements were done. A reference group, with standard alka-line wet pattering process, was also manufactured to underlinethe impact of the patterning on the cell efficiency.

3. Patterns characterizations

Optical reflectivity measurements of the patterned substrateswere carried out. The characterizations were performed withoutanti-reflective coating above the patterned sample and were con-ducted over samples prepared with different patterning processes(Fig. 4): KOH wet etching over multi crystalline (KOH mc-Si) andcrystalline silicon (KOH c-Si) substrates; acid etching over multicrystalline silicon (acid mc-Si); KOH wet etching over a periodicpatterned SiO2 mask on crystalline silicon substrate (pyr. inv. C-Si); and imprint & dry etching process over multi crystalline sam-ple (imprint). The weighted reflectivity, Rw (weighted by the solarspectrum on the range of 350 to 1000 nm), for all patterning pro-cess, are listed in Fig. 4a. The KOH mc-Si patterning configurationpresented the lowest performance, with a Rw of about 30.8%, sincefor some Si grain orientations the alkaline wet etching process isnot optimum and created some large flat mesas (see correspondingSEM picture on Fig. 4a). Acid wet patterning slightly decreases theRw down to 26.5% with some micro scale rounded shapes over thesurface. KOH wet etching over crystalline substrate, either ran-domly or periodically arranged, present much lower reflectivity,with a dark gray aspect and a Rw of about 12.2% to 13.1%. Finally,the imprint and dry etching process led to the lowest Rw, of about2.2%, 10% lower than the wet KOH etching process performed overSi-c samples. The textured mc-Si wafers present a deep black as-pect compared to un patterned substrate, confirmed by a 3%weighted reflectivity, with a homogeneity better than 97% on thewhole wafer (Fig. 4b and c). The exact shape of the inverted pyra-mid pattern appeared to be a key parameter to tune the opticalreflectivity of the sample.

We performed optical simulation in order to compute thereflectivity of the patterned surface to point out the impact ofthe pyramid angle (with respect to the surface plan) onto its opti-cal reflectivity. Ray-tracing method using Zemax software [14] hasbeen used. For the calculation, a bundle of rays is emitted from animaginary surface in front of the 3D textured model. Each ray has asingle wavelength. Using Snell-Descartes law and considering re-flected and multi reflected, transmitted and absorbed rays, thereflection coefficient is computed by an external code with abelesmatrix. The silicon material was described by its refractive spectralindex and its spectral absorption coefficient. The reflection prop-erty of the surface with respect to the design was then computed.

The course for one ray is considered to be finished when the ray istotally absorbed by the Si or if there is no new possible interception(ray goes too far from the 3D object). The reflected light is com-puted by a virtual detector (hemispherical in order to collect all re-flected light into the 2pi-steradiant volume) which lies in front ofthe textured surface. We found that an array of 9.6 lm periodmade of inverted pyramid shape showed an optical reflectivitydecreasing from 40% for a pyramid angle of 40� down 2% for a pyr-amid angle of 80�. The optical reflectivity is about 13% when thepyramid angle is 54.7� (obtained with. the KOH patterning). There-fore, sharpening the pyramid shape within the silicon substratewill drastically reduce the optical reflectivity thanks to light trap-ping and multiple reflections on the surface.

4. Cells characterization

The reflectivity of the finished cells after the anti-reflectivecoating and the screening printing was measured as low as 4.3%.This corresponds to 8.3% reduction of the weighted reflectivitycompared to the finished cell patterned with KOH process. This sig-nificant reduction of the overall reflectivity was mainly observed inthe 300–600 and 1000–1200 nm wavelength regimes. Due to thehigher aspect ratio of the etched pyramid, both screen printingand firing processes have been optimized to the new surface topol-ogy. As consequence, the width of the line, contact resistance(10 mOhm.cm2) and line resistance (0.35 Ohm.cm) are then equiv-alent to those obtained on the classic KOH textured references cells(12.5 mOhm.cm2 and 0.15 Ohm.cm respectively).

I(V) measurements under sun illumination were performed tocharacterize the short-circuit current, Jsc, and the open-circuitvoltage, VOC, the maximum voltage available from a solar cell(Fig. 5). The short-circuit current is due to the generation and col-lection of light-generated carriers. The open-circuit voltage corre-sponds to the amount of forward bias on the solar cell due to thebias of the solar cell junction with the light-generated current.From I(V) curves, the ‘‘fill factor’’, more commonly known by itsabbreviation ‘‘FF’’ with Voc and Jsc, determines the maximumpower from a solar cell can be determined. The FF is defined asthe ratio of the maximum power from the solar cell (Fig. 5) tothe product of Voc and Jsc. Graphically, the FF is a measure ofthe squareness of the solar cell and is also the area of the largestrectangle which will fit in the IV curve.

Fig. 4. (a) Reflectance measurements of patterned Si, either Si-c or Si-mc substrates without anti-reflective coating. Acid (acid mc-Si) or alkaline (mc-Si, c-Si, c-Si poly)patterning approaches are compared to imprint/etching processes. Optical photograph of 125 � 125 square Si-mc substrate, without pattering (b), and covered by invertedpyramid shapes (c) shown in Fig. 1d.

Fig. 5. Graph of cell output current (solid line for imprint, dotted line for KOHetching) as function of voltage. Also shown are the cell short-circuit current (Jsc @V = 0 mV) and open-circuit voltage (Voc @ I = 0 mA) points, as well as the powercurves (solid line with filled rounds for imprint, dotted line with crosses for KOHetching).

Fig. 6. (a) Short-circuit current (Jsc), (b) open-circuit voltage (Voc), (c) fill factor and(d) cell efficiency measurements of a patterned Si-mc solar cells either with KOHwet etching or imprint & dry etching processes. The increase or decreases of cell’scharacteristics between the two patterning processes are underline.

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We observe a higher recombination current on the Si-mc in-verted pyramidal texturized using nano-imprint and dry etching,and a lower open circuit voltage (Fig. 6). In our case, we found that

228 S. Landis et al. / Microelectronic Engineering 111 (2013) 224–228

the gain in short circuit current (Jsc) gain was correlated to reflec-tivity decrease. However, lower values are observed for both theopened circuit voltage (�15 mV, i.e. �3%) and the Fill Factor(�2.5, i.e. �3.4%) in the case of the imprinted and dry etched pat-tern compared to the alkaline one. The lower Voc is probably re-lated to a degradation of the front surface due to the plasmaetching process. Dark current–voltage curve (not shown) indicateda twice higher recombination current on the imprint textured cells,what highlights a strong recombination in the space charge region,leading to the observed Voc drop. We also performed measure-ments of the minority carrier diffusion length by LBIC measure-ments to check the influence of the dry patterning process on theelectrical quality of the bulk. No significant impact of the pattern-ing process was observed as since the minority carrier’s diffusionlength was of about 530 lm for the KOH patterning process and590 lm for the imprinting process respectively. The small increaseof the minority carrier diffusion length on the imprinted cells maycome from a better optical confinement, which induce a generationof minority carriers closer to the surface. Regarding the FF thesmall decrease is probably related to a lower shunt resistance forimprinted substrate. As a consequence an absolute efficiency gainof 0.33% was obtained applying the imprinting process on Si-mcsolar cells compared to KOH patterning (Fig. 6). This gain is quitelow compared to the theoretical gain which may be obtained. Asmentioned previously, the current under illumination is directlyproportional to the reflectivity. Therefore a 1% gain in reflectivityresults in a 1% gain in short circuit current. As a consequence, keep-ing both VOC and FF parameters constants, a decrease of 8% of thereflectivity should lead to a gain of absolute efficiency over 1%.

5. Conclusions

We developed a patterning process, based on Nano ImprintLithography with soft stamp and dry etching, over non flat andfragile multi-crystalline silicon substrate, to create inverted pyra-mid shapes. The transfer of the inverted pyramid shape in the Si-mc provided by plasma etching showed an amplified aspect ratiocompared to the stamp manufactured with standard KOH wetetching process. A very low optical reflectivity property, lower than3%, was achieved whatever the crystalline orientation of the silicongrain. This low reflectance is associated to light trapping and multi-

ple reflections in the very sharp pyramid shape manufacturedwithin the silicon. Compared to other wet patterning approaches,our process presents the lowest weighted reflectivity over the solarspectrum with a gain at least of 8%. We also showed that the shortcircuit current gain, 8.3%, was directly associated to the reflectivitygain. An absolute 0.33% efficiency gain is demonstrated on Si-mcsolar cells (16.8% compared to 16.5% with our reference alkalineprocess). These promising results are however below those ex-pected in terms of efficiency due to losses in open circuit voltage(�3%) and fill factor (�3.4%). This open circuit voltage degradationmight be generated by the plasma etching and/or by the aggressivepatterned profile. Further works are ongoing for dry etching opti-mization and also both the screen printing and firing steps. Lower-ing the Voc and FF drops, an efficiency gain of 1% may be targeted,if the reflectivity gain is completely transformed in efficiency gain.

Acknowledgments

The partial support from the PV20 project supported by theOSEO funding is gratefully acknowledged.

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