Photonic nanostructures for advanced light trapping in thin crystalline silicon solar cells

Part of Topical Section onAdvanced Materials and Nanotechnology for Photovoltaics

Photonic nanostructures for advancedlight trapping in thin crystalline siliconsolar cells

Christos Trompoukis*,1,2, Islam Abdo1, Romain Cariou3, Ismael Cosme3, Wanghua Chen3, Olivier Deparis4,Alexandre Dmitriev5, Emmanuel Drouard6, Martin Foldyna3, Enric Garcia- Caurel3, Ivan Gordon1,Babak Heidari7, Aline Herman4, Loic Lalouat6, Ki-Dong Lee7, Jia Liu6, Kristof Lodewijks5, Fabien Mandorlo6,Inès Massiot5, Alexandre Mayer4, Vladimir Mijkovic5, Jerome Muller4, Regis Orobtchouk6, Gilles Poulain8,Patricia Prod’Homme8, Pere Roca i Cabarrocas3, Christian Seassal6, Jef Poortmans1,2, Robert Mertens1,Ounsi El Daif1, and Valérie Depauw1

1 Imec, Kapeldreef 75, 3001 Leuven, Belgium2KUL, Departement Elektrotechniek – ESAT, Kasteelpark Arenberg 10, 3001 Leuven, Belgium3 LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, France4 Solid-State Physics Laboratory, Research Centre in Physics of Matter and Radiation (PMR), University of Namur, 61 rue de Bruxelles,5000 Namur, Belgium

5Applied Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden6 Institut des Nanotechnologies de Lyon (INL) UMR 5720 CNRS-INSA-ECL-UCBL, Université de Lyon, Ecully 69134, France7Obducat Technologies AB, Scheelevägen 2, 223 63 Lund, Sweden8 Total M&S – New Energies, R&D Division, Tour Michelet, 24 Cours Michelet, La Défense 10, 92 069 Paris La Defense Cedex, France

Received 14 March 2014, revised 7 May 2014, accepted 21 May 2014Published online 30 June 2014

Keywords light trapping, photonic crystals, photonic nanostructures, silicon, solar cells, thin films

* Corresponding author: e-mail [email protected], Phone: þ32 16 28 15 25, Fax: þ32 16 28 80 60

We report on the fabrication, integration, and simulation, bothoptical and optoelectrical, of two-dimensional photonic nano-structures for advanced light trapping in thin crystalline silicon(c-Si) solar cells. The photonic nanostructures are fabricated bythe combination of various lithography (nanoimprint, laserinterference, and hole mask colloidal) and etching (dry plasmaand wet chemical) techniques. The nanopatterning possibilitiesthus range from periodic to random corrugations and frominverted nanopyramids to high aspect ratio profiles. Optically,the nanopatterning results in better performance than thestandard pyramid texturing, showing a more robust behaviorwith respect to light incidence angle. Electrically, wet etchingresults in higher minority carrier lifetimes compared to dryetching. From the integration of the photonic nanostructuresinto a micron-thin c-Si solar cell certain factors limiting theefficiencies are identified. More precisely: (a) the parasitic

absorption is limiting the short circuit current, (b) theconformality of thin-film coatings on the nanopatterned surfaceis limiting the fill factor, and (c) the material damage from dryetching is limiting the open circuit voltage. From opticalsimulations, the optimal pattern parameters are identified. Fromoptoelectrical simulations, cell design considerations arediscussed, suggesting to position the junction on the oppositeside of the nanopattern.

� 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Despite an annual growth rate of morethan 40% over the last decades and a large amount of globalinstallations over the last years (31–32 GWp per year), the

field of photovoltaics (PV) is still not competitive enoughwith respect to the cost of grid electricity supplied by fossilfuels, nuclear power, or even other renewable sources. At the

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same time, the cost-driven nature of the field results in anoverall conservative approach with respect to integratingnovel concepts and potential breakthroughs.

In order to further increase the competitiveness of PV, acombination of reduced fabrication costs and high solar cellefficiencies is required [1]. Since the cost of crystallinesilicon (c-Si) is still an important part of the module cost,using thinner c-Si substrates is a major trend. However, thistrend poses two main challenges. Firstly, the handling andprocessing issues of such thin substrates within the current c-Si solar cell technology fabrication methods. Secondly, thematerial’s incomplete light absorption due to the indirectband-gap of c-Si. Therefore, drastic innovations are requiredfor the fabrication of high efficiency thin-film c-Si solar cells.

Currently, methods for growing amorphous, nano- ormicro-crystalline silicon films on low-cost non-siliconsubstrates have reached a high level of maturity [2].However, these methods result in a very different materialthan standard mono- or poly-crystalline wafers, with a higherlight absorbance, but also a lower electronic quality, whicheventually limits their performances to lower maximumdevice efficiencies [2, 3].

Alternative approaches are to be used for achievingmono- or poly- c-Si layers, such as (1) the epitaxial growth ofsilicon on low-cost silicon [4, 5] and, (2) the transfer to acarrier substrate of a silicon film resulting from thereorganization of porous silicon, either with [6] or without [7]subsequent epitaxial thickening. In all cases, the thin-filmc-Si layers are held by a carrier substrate, thus eliminatingthe handling issues while enabling the processing on modulelevel.

At the device level, 19% efficient solar cells as thin as43mm have already been achieved [8]. Those cells utilize alight trapping scheme which consists of an antireflectioncoating (ARC), a front-side texture and a metallic backreflector. On one hand, the ARC and the front-side texture(micron-scale pyramids, randomly distributed at the frontside of the cell) result in low front side reflectance of theincoming light. On the other hand, the combination of thefront-side texture and the metallic back reflector enablesmultiple passes of light and therefore, increases a photon’soverall traveling distance (light trapping). However, suchlight trapping scheme reaches its limitations [9, 10] for c-Silayers thinner than 50mm, due to insufficient absorption.Moreover, the material waste (a few microns) during thefabrication of the random pyramid front texture isincompatible with such thin layers. Therefore, otherapproaches for highly efficient advanced light trapping areneeded.

Regarding advanced light trapping concepts, incorpo-rating a diffraction grating in a solar cell was first introducedin 1983 [11]. However, only recent developments in the fieldof nanophotonics [12] have brought the manipulation of lightusing periodic photonic nanostructures for PV into thespotlight. Recent analytical calculations and simulations [13,14] have shown that light trapping beyond the commonlyaccepted limit of Lambertian light scattering is possible

through the use of periodic photonic nanostructures. Theoptical properties of such nanostructures have been studiedand presented quite extensively during the last years. Moreprecisely, the use of rods [15, 16], one- [17, 18] and two-dimensional [19–27] periodic nanostructures, random, oramorphous patterning [28–30], Mie scatterers [31], super-cell approaches [32, 33], double side patterning [34–36] aswell as black silicon surfaces [37] have already beenproposed. However, the fabricated c-Si cells integrating suchnanophotonic concepts are significantly fewer and arerestricted so far to periodically nanopatterned thin-filmmonocrystalline silicon cells [20, 21], nanostructuredpolycrystalline silicon thin-film cells [38, 39] and to thickwafer-based black silicon [40–42] cells.

The lack of solar cells utilizing nanophotonic conceptscould be considered as an indication of how challenging theirfabrication is, mainly due to the trade-off between opticaland electrical properties. Since a solar cell is a device aimingat producing electrical power through charge carriercollection, any optical benefits from a light trapping schemeshould not be out-weighted by the material degradationdue to nanopatterning, limiting the efficiencies, as it oftenhappens [21, 43]. Besides, the nanopatterning should bestable enough to maintain its optical properties throughoutthe whole cell process [41]. For achieving minimal materialdegradation (i.e., good passivation and therefore, lowminority carrier recombination velocities) for the nano-patterned surfaces, four approaches have been proposed: (1)avoiding the texturing of silicon and applying the nano-pattern on dielectrics deposited on top of a passivated flatsilicon surface [43], (2) using a highly conformal passivationtechnique, such as atomic layer deposition of aluminumoxide, for highly demanding surface topographies, e.g.,black silicon [44], (3) adding a post patterning treatment ofthe plasma damaged surfaces [45] and (4) using a properetching so as to maintain high carrier lifetimes [46].

In this paper, we present the achievements and progresstoward fulfilling the target of the project “PhotoNVol-taics” [47] which is to enable a new solar-cell generation,resulting from the combination of photovoltaics withphotonics. This generation could therefore be the alliancebetween the sustainability and efficiency of wafer-basedcells, with the simplicity and low cost of thin-film cells. Thepurpose of this manuscript is to highlight the limitations forreaching high device efficiencies with nanopatterning.Although high performances have been achieved usingnano- and micro-crystalline silicon solar cells, we arefocusing on mono-crystalline silicon thin films since theyoffer the potential to combine the advantages of wafer-basedmonocrystalline silicon cells (high Voc and FF among others)with thin-film technologies. The structure of the manuscriptreflects this approach.

More precisely, in the second section, the fabrication ofphotonic nanostructures and their optical performance, asachieved so far, are presented. A benchmarking with respectto the conventional random pyramid texturing is shown,highlighting the extra benefits of using wavelength-scale

2 C. Trompoukis et al.: Advanced light trapping in thin crystalline silicon solar cells

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nanopatterns. In the third section, the experimental integra-tion of the photonic nanostructures into thin-film c-Si solarcells is presented. Based on the characterization results of thefabricated cells, the factors limiting the short circuit current,the open-circuit voltage and the fill factor are identified.Moreover, suggestions for tackling the identified limitationsand eventually reaching high efficiency solar cells are given.In the fourth section, the simulation results for optimizingnot only the nanopattern parameters but also the deviceperformance with respect to the identified optical andelectrical limitations is shown. Finally, in the conclusions, asummary of the results is given.

2 Photonic nanostructures2.1 Fabrication Within a cost-driven PV field, inte-

grating photonic nanostructures into solar cell devicesshould be done using low cost techniques. Since thefabrication of photonic nanostructures includes both alithography and an etching step, control on both processes isnecessary. Some of the available lithography techniques andthe control on the etching are presented in this section.

2.1.1 Lithography techniques A variety of lithog-raphy techniques is available for the definition of either anordered (periodic) or a disordered (non-periodic or amor-phous) nanopattern.

Nanoimprint lithography (NIL) [48] is a moldingtechnique applied to the nanoscale patterning, wheresurface-relief patterns on a stamp are transferred to apolymer layer through a mechanical deformation [49]. It isconsidered to be the most cost-effective in producingnanopatterns over large area, with a proven resolution downto 10 nm. However, it still has limitations originating from itsinherent nature: a physical contact of stamps with polymers.High surface roughness and defects normally present on thecrystalline silicon substrates for PV, or particles in general,can therefore limit its applicability, in particular when theconventional NIL employing hard stamps is used.

This limitation is effectively avoided by using anintermediate polymer stamp (IPS) combined with soft presstechnology [50]. The IPS is a soft, intermediate stamp intowhich the master stamp is first replicated and in turn usedto transfer the structures onto the target substrates. The IPStechnology enables a contamination control and increasesthe master stamp lifetime by avoiding its direct use.Therefore, it greatly impacts the overall costs associated withNIL. The soft press technology employs a compressed gaswhich creates a homogeneous pressure distribution acrossthe entire imprint area. This allows the IPS and the substrateto conform to each other, eliminating negative effectscoming from surface roughness, thickness variation, anddefects on the substrate. The NIL process with IPS consistsof two imprint steps: the first step for the IPS fabrication andthe second for the imprint on resist-coated substrate, bothdescribed in Fig. 1a.

The first step can be either thermal or UV imprint,depending on the IPS material, which is normally UV-

transparent. The second imprint step can be done by eitherpure UV process or simultaneous thermal and UV (STU)process depending on the resist material. The resultingimprinted topography on the resist is shown in Fig. 1b.

Laser interference lithography: Laser interferencelithography (LIL) becomes a popular method for fabricatingperiodic patterns over large areas, at high throughput and lowcost. A review of the basic principles of LIL and its use inphotonic crystal devices, optical telecommunications, datastorage, and the integrated circuit industry are described byLu and Lipson [51]. Applications in the photovoltaic area areillustrated in Refs. [52, 53].

As shown in Fig. 2a, a UV laser (l¼ 266 nm) beam isexpanded and spatially filtered through a pinhole to generatea coherent beam that illuminates both the mirror and thesample of the Lloyd’s mirror interferometer. Part of the lightis reflected on the mirror surface and interferes with the

Figure 1 (a) Schematic process flow for the nanoimprintlithography (NIL) using intermediate polymer stamp (IPS) and(b) cross sectional SEM image of a resulting imprint on the resist.

Figure 2 (a) Schematic optical setup used for the fabrication of thenanostructures using LIL and (b) top view of the 2D array patternon the photoresist.

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portion of the beam that is directly illuminating the sample.This interference gives a line pattern with a continuousvarying periodicity (P) given by

P ¼ l2sin u

; ð1Þ

where l is the wavelength of the laser beam and u is the anglebetween the incidence light and the sample normal. Bychanging the incidence angle u, the periodicity can easilybe adjusted. Two-dimensional arrays are made by doubleexposure with a sample rotated by 908 on a photoresist.Shapes of the pattern in the photoresist layer are shown inFig. 2b. Limitations of the patterned size are related to thespace of the LIL setup. The diameter of the beam can beenlarged by increasing the distance between the spatial filterand the Llyod’s mirror interferometer reaching areas up to1.2m� 1.2m [54].

Hole-mask colloidal lithography (HCL) [55] is a bottom-up nanopatterning technique based on the self-assembly ofsurface-charged polystyrene (PS) beads onto a sacrificialresist layer of poly(methyl methacrylate) (PMMA) pre-coatedby a thin polymer film of poly(diallyldimethylammoniumchloride) (PDDA). The negatively charged PS beads areadsorbed on the positively charged PMMA surface byelectrostatic interactions. The coexistence of repulsive inter-particle interactions and attractive bead-substrate interactionsleads to a short-range ordering of the particles into anamorphous array. The combination of the sacrificial layerwith a thin-film mask with nanoholes, also named hole-mask,allows fabricating a wide variety of nanostructures that areapplied in a broad range of contexts, from photovoltaics andoptical nanoantennas to spintronics [30, 56–58].

In the case of silicon nanopatterning, as outlined inFig. 3a, we use a further developed process where the beadsare assembled directly on the sample’s surface, reminiscentto the standard colloidal lithography technique [59].

However, the nanopatterning process still relies on thefabrication of a hole-mask film used to etch the siliconsurface. First, the PS beads are deposited on a triple-layerprecursor film of PDDA, poly(sodium 4-styrenesulfonate)(PSS) and aluminium chloride hydroxide (ACH) thatprovides the surface the positive charge needed for thebeads adsorption. A hole-mask is then obtained throughthe evaporation of a thin-film mask and tape-stripping of thebeads. The size of the holes and the surface area fill factor(ratio between the area covered by the holes and the totalarea), depend on both the initial bead size and the etchtechnique.

A strong advantage of this process is its tolerance withrespect to surface roughness. We are able to process a widerange of surfaces, from smooth monocrystalline silicon(c-Si) surfaces (Fig. 3b) to rough surfaces like polycrystal-line silicon films (Fig. 4). The comparison between NILand HCL on a rough polysilicon surface with respect to thepatterned surface coverage is shown in Fig. 4, highlightingeach technique’s tolerance to the surface roughness.

2.1.2 Etching techniques The etching of silicon hasbeen well documented throughout the years [60–62]. We arepresenting here two main possibilities with certain degreesof etch selectivity: dry plasma etching and wet chemicaletching. In the following section, we briefly present thetechniques used within the frame of this project and thepossibilities of tuning the profiles of the corrugations (holes)by using different etch chemistries and etch parameters.

Dry plasma etching: reactive ion etching (RIE) andinductively coupled plasma etching (ICP) are used. Theetched profile is independent of the crystal orientation due tothe significant part of physical etching originating fromthe ion bombardment included in the process. Therefore, theprofiles show a parabolic U-shape whose sidewall slope canbe tuned depending on the etching parameters.

For RIE, different combinations of gases (SF6, CHF3,and O2) are used at low pressure. Lateral etching underneathof the etch-mask leads to under-etch. Longer etch timesresult in an overall expansion of the corrugation shape with aprogressive change on the sidewall slope (Figs. 5a and 7a).However, this can lead to the presence of negative slopesresulting in problems with the conformality of subsequently

Figure 3 (a) Schematic process flow for the HCL nanopatterningand (b) HCL-nanopatterned c-Si surface, with 150 nm wide holes:top right – top view of the patterned c-Si wafer; bottom right –cross-sectional profile of the nanopatterned surface.

Figure 4 (a) HCL-nanopatterned and (b) NIL-nanopatternedpolycrystalline silicon surface showing the effect of roughness onthe resulting topography.



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deposited films on top of the nanostructured surfaces, as itwill be discussed in a next section.

For ICP, the pattern is transferred into a SiO2 hard maskby RIE using CHF3. The c-Si is etched through the SiO2

hard mask by ICP using Cl2. The SiO2 hard mask is thenremoved by buffered HF (BOE). The process is done at alow pressure in order to keep a high verticality of sidewall(Fig. 5b). Since such processes do not present over- orunder-etching, the lateral size of the final pattern is directlydetermined by the pattern imprint by lithography while itsdepth can be tuned by controlling the duration of ICP–Cl2process.

Wet chemical etching: Alkaline solutions based ontetramethyl ammonium hydroxide (TMAH), sodium hy-droxide (NaOH), or potassium hydroxide (KOH) are used.Those solutions are extensively used for wafer-based solarcells, for the formation of micron-scale random pyra-mids [63]. The etching mechanism is based on the factthat the etch rates depend on the crystallographic planeorientation of the substrate. For a (100) oriented siliconwafer, the TMAH etching reveals the (111) crystallographicplanes, thus leading to square-based inverted triangularpyramids (Fig. 6).

For both dry plasma etching and wet chemical etching,the etched profile can be tuned by changing the etchingparameters (power of the plasma, the etching time, theoxygen concentration for the dry process or the TMAHconcentration, the temperature and the etching time for thewet chemical process). The evolution of the shape withrespect to the etch time (with the other process parametersfixed) is shown in Fig. 7.

The various combinations of the presented lithographytechniques and etching processes offer certain degrees offreedom (e.g., on period, area fill factor, shape and positionof corrugations on the surface) for defining the topographies

that will result in the most efficient light trapping. Moreprecisely, for lithography, each features a different andspecific strong point, namely up-scaling for nanoimprintlithography, quick adjustability for laser interferencelithography, and high tolerance to roughness for holecolloidal lithography. As for etching, each features its ownresulting profile. Therefore, the nanopatterning possibilitiesrange from periodic to random surface topographies andfrom nanopyramids to high aspect ratio profiles, each with itsown optical and electrical properties.

2.1.3 Optical characterization In order to check therelevance of photonic nanostructures for application in PV,we first study their anti-reflective properties on thick(700mm) wafers and a benchmark is done with respect tothe state-of-the-art random pyramid texturing (Fig. 8). Forcomparing the overall performance, the integrated values ofreflectance are used as the figure of merit. They are the ratio

Figure 5 Etched shape after (a) RIE and (b) ICP.

Figure 7 Evolution of the etching profile with time for (a) the dryplasma etching process and (b) for the wet chemical etchingprocess.

Figure 6 Etched shape for c-Si (100) TMAH wet etching,highlighting the crystal orientation dependent etch mechanism thatreveals the (111) crystallographic planes.

Figure 8 Optical performance (reflectance) for various combina-tions of lithography and etching and comparison with conventionalrandom pyramid texturing. The topography for each plot is shownon the right hand side (color-coded).



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of the number of reflected photons to the total incidentphotons over the wavelength range from 300 to 1170 nm(c-Si absorbance), taking into account the AM1.5 global tiltintensity distribution. The total number of reflected photonsis given by

FR ¼Z lmax

lmin

lhC

� SAM1:5GðlÞ � RðlÞdl; ð2Þ

where FR is the integrated reflected photon flux, l is thewavelength of light (the minimum and maximum valuescorrespond to the wavelength range that silicon absorbs, i.e.,300–1170 nm), h is Planck’s constant, c is the velocity oflight, SAM1.5G is the global tilt solar intensity distribution atthe air mass of 1.5 and R is the measured reflectance value.The same formula can be used for calculating the numberof absorbed photons (integrated absorption) by using themeasured absorption A(l) values. Reflectance and transmis-sion are measured by spectrally resolved measurements,within a wavelength range from 300 to 1170 nm andintegrated over the whole half space using an integratingsphere. For the angular dependence measurements (Fig. 9),R(l)þ T(l) are measured with the sample placed insidethe integrating sphere. Absorption is then extracted asA(l)¼ 100�R(l)� T(l).

The photonic nanostructures from various combinationsof lithography and etching fabricated within the frame of thePhotoNVoltaics project till now, result in a lower integratedreflectance with respect to random pyramid texturing, eventhough the pattern parameters are not the optimal ones.More precisely, compared to random pyramids (11.7%),the integrated reflectance is 10.6% and 10.2% for theperiodic nanopatterns (NILþDry etching and NILþWetetching, respectively) and 10.5% for the random nanopattern(HCLþDry etching). The topography of the fabricatednanostructures is shown on the right side of Fig. 8.

Since the nanostructures have dimensions in the order ofmagnitude relevant to visible light wavelengths, additionalbenefits are observed, such as the low dependence ofabsorption to light incident angle. The integrated absorption(calculated using Eq. (2) for absorption), shown in Fig. 9, for

both periodic and random nanopatterns, remains high forincident angles of light up to 608 while the integratedabsorption of the standard random pyramid texturing reducesfor increasing incident angles of light.

The reason is related to the feature size involved. Moreprecisely, for the micron-scale pyramids, the geometricdescription of light propagation applies. Light reaching thesurface sees a certain crystallographic plane (111) with afixed angle of 54.78 with respect to the horizontal plane.Since the size of the pyramids is an order of magnitude larger(a few micrometres) than the wavelength of light, the in-coupling of light is described by refraction and multiplereflection. On the contrary, for the wavelength-scalednanopatterns, the wave description of light applies, andlight reaching the surface sees an effective overallsurface [64]. The in-coupling of light in this case can bedescribed by a graded index effect. This wave-based light/matter interaction at the nanopatterned surface occurs at anyangle of incidence, with little changes in the integratedabsorption. This is happening thanks to the tolerance of theintegrated absorption with respect to the effective period ofthe nanopatterning [17]. This angular robustness of theperiodic and random nanopatterns can result in a higherannual energy yield of a PV module compared to that ofrandom pyramid texturing, since the loss due to the angularmismatch is reduced.

Having shown the relevance of using photonic nano-structures for patterning the front side of a solar cell, thecontrol on the nanopattern (lithography and etching) will beused to fabricate nanopatterned solar cells with optimalpattern parameters suggested by optical simulations.

3 Toward nanopatterned solar cell devices Asmentioned in the introduction, the optical benefits originat-ing from the use of a nanopattern should not be out-weightedby any degradation of the electronic properties (e.g., increaseof the surface defect density). This trade-off is important forthin-film solar cells since the surface properties dominateover the bulk. Apart from controlling the fabrication of thenanopatterning, the integration of the photonic nano-structures into thin-film solar cells is therefore a criticalpoint. As a proof of concept for the validity of integrating aperiodic nanopattern, the results on micron-thin c-Si cells areconsidered [20, 21]. In order to highlight the connectionbetween the optical and the electrical properties of a solarcell, the fabrication process of the c-Si thin-film solar cells isfirst described, highlighting the conceptual purpose of eachcomponent included in the cell stack. This will facilitate thediscussion on the optical and electrical performance of thefabricated solar cell, as well as on the optical and electricalsimulations of the next section.

The c-Si films were fabricated as described in [7].After the reorganization of cylindrical macropore arraysin silicon, a suspended boron-doped (low p-type dopingconcentration¼ 1015 cm�3) 1.1� 0.1mm film is formed.Before transferring this suspended thin-film from the parentsubstrate to the glass carrier, a 250 nm thick pþ-doped c-Si

Figure 9 Angular robustness of the nanopatterns with respect tolight incidence angle.



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layer is epitaxially grown and an aluminium (Al) layer,which serves as the back reflector and back contact, is then e-beam evaporated. The c-Si/Al interface has a high carrierrecombination velocity since it is non-passivated. To screenthe high recombination, the pþ-doped c-Si region serves arethe back surface field (BSF), repelling part of the photo-generated carriers from recombining at the c-Si/Al interface.After transferring the thin-film to the glass carrier, thenanopatterning of the front surface is done. Subsequent tothe nanopatterning step, a 25 nm thick i/nþ aSi:H film isdeposited by PECVD serving as the passivation layer (i-aSi:H) and the heterojunction emitter (nþ-aSi:H). A 75-nm-thickITO film is sputtered, serving both as the antireflectivecoating (ARC) and the transparent conducting oxide (TCO)layer. Finally, the front metallization (Ti/Pd/Ag) is e-beamevaporated through shadow masks.

Regarding the nanopatterning, two different approacheswere used: the first one is a combination of NIL and RIEetching incorporating a deep (550 nm) 2D periodic nano-pattern with a 900 nm pitch. The second one is a combinationof LIL and ICP etching incorporating a shallow (110 nm) 2Dperiodic nanopattern with a 600 nm pitch. The cross sectionof the two nanopatterned cell stacks (excluding the frontmetallization) is shown in the transmission electron micros-copy (TEM) images of Fig. 10. The optical and electrical cellperformances are summarized in Table 1 and Fig. 11.

The photonic nanostructures resulted in a broadbandabsorption enhancement for the nanopatterned cells withrespect to the flat unpatterned cell (Fig. 11a). The absorptionis enhanced on one hand because of better in-coupling oflight in the photoactive material and on the other handbecause of better light-trapping. More precisely, the betterin-coupling of light can be seen for short wavelength photons(that is, a wavelength range of 300–550 nm for the caseof�1mm c-Si layers) which do not reach the back side of thecell. The refractive index mismatch between air and the cell’sfront material stack is reduced for the nanopatterned cellsbecause of a better wave-impedance matching. The betterlight-trapping is obvious for long wavelength photons (thatis, a wavelength range of 550–1170 nm to which a �1mmc-Si slab is actually semi-transparent) which reach the backside of the cell. Light is diffracted inside the photoactivematerial at high angles, leading to total internal reflection

because of the back metal reflector and therefore, thephoton’s traveling distance in the photoactive layer increases(light path enhancement).

The absorption enhancement is translated in an increasein the generated short circuit current (Jsc, shown in Fig. 11band Table 1). This increase is also obvious from the externalquantum efficiency defined as:

EQE ¼ number of carriers collectednumber of incident photons

ð3Þ

of the cells (Fig. 11c), where for both short and longwavelengths the response is better for the nanopatternedcells. Additionally, the decrease in the open circuit voltage(Voc), which is observed for the nanopatterned cells, isattributed to the material degradation during nanopatterning,which affects the minority carrier lifetimes. For shortwavelengths, the material degradation results in a lowerinternal quantum efficiency defined as:

IQE ¼ EQE1� R

¼ number of carriers collectednumber of photons absorbed

ð4Þ

for the nanopatterned cells compared to the flat cell(Fig. 11d). For long wavelengths, the IQE of both the flat

Figure 10 TEM of the final cell stack for the (a) 900 nm pitchdeep and (b) 600 nm pitch shallow corrugation.

Table 1 Performance of unpatterned and periodic nanopatternedsolar cells (area 1 cm� 1 cm).

texturing Jsc[mA cm�2]

Voc

[mV]FF[%]

h[%]

flat 12.5 471 75 4.4900 nm pitch deep 15.3 435 72 4.8600 nm pitch shallow 15.4 403 56 3.5

Figure 11 Optical (a) and electrical (b–d) performances of the flatand nanopatterned thin-film (1mm) c-Si solar cells.



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and the nanopatterned cells overlap, indicating that the non-passivated back side limits the generated minority carrierlifetimes and eventually the overall cell performance.Finally, the obtained fill factor indicates that there arelimitations with respect to contacting, probably related to thepoor conformality of the layers being part of the frontcontacting scheme (ITO and Ti/Pd/Ag) [65].

The obtained enhancement in the integrated absorption(34%) is not equivalent to the enhancement in the short circuitcurrent Jsc (18%) indicating that not all of the absorbed lightgenerates current. The difference is attributed on one hand tothe parasitic (i.e., non useful) absorption in other parts ofthe cells which do not generate electrical carriers, and on theother hand to high minority carrier recombination due to thematerial degradation. The identified issues limiting the Jsc,Voc and FF, namely parasitic absorption, conformality ofcoatings and material degradation, are separately discussed inthe following sections.

3.1 Parasitic absorption Even if the experimentallymeasured absorption of a solar cell reveals certaininformation regarding its overall optical performance, suchmeasurement is not sufficient to determine the usefulabsorption of the cell, i.e., the absorption in only thephotoactive layer that generates electrical carriers. Materialslike ITO, aSi or Al exhibit a high parasitic absorption due tonon-negligible extinction coefficients. In particular, a thin-layer of ITO and aSi:H deposited at the front of the cellabsorbs photons in the blue part of the solar spectrum whilethe Al reflector can absorb photons in the red part whichreach the back side of the cell. In addition, in the case ofhighly doped regions (like the BSF), the charges created inthis part of the cell are lost due to high recombination rates.

In the theoretical case of an IQE equal to unity (thismeans that every photon that is absorbed, generates carriers),optical simulations, as described further, allow us todetermine the maximal value of the short-circuit currentdensity (Jsc) that may be expected from solar cells. An uppervalue can be estimated by calculating specifically theabsorption within the active material. Parasitic absorption invarious layers at the front or the back of the cell can thus bedisregarded.

For instance, if we consider a flat 1.2mm thick c-Si solarcell, the simulated absorption spectra from the differentlayers of the cell are presented in Fig. 12. A large amount ofshort wavelength photons is absorbed in the top layers of thecell (ITOþ a-Si:H), while the back metal absorbs many longwavelength photons.

The integrated photon absorption of the whole stack is54.6% for the solar light (i.e., from 300 to 1100 nm andby taking into account the AM1.5G solar spectrum), whichis in good agreement with the measured value shown inFig. 11a. However, this value contains about 50% ofparasitic absorption, so that the absorption in the active layeris only 27.8%. This corresponds to an achievable Jsc value of12.1mA cm�2 which is comparable to the current obtainedexperimentally. For the nanopatterned cells, the contribution

of the parasitic absorption increases even further [21]. Itshould be noted here that nanocrystalline silicon solar cellshave been reported to show higher Jsc [2]. However, thosefilms are usually thicker and feature a higher absorptioncoefficient than the 1mm monocrystalline silicon which isreported here.An optimized nanopatterned monocrystallinesilicon solar cell has the potential to reach such high Jsc butwith higher Voc and FF.

3.2 Conformality of thin-film coatings The factthat the surface topography of silicon is altered throughnanopatterning might cause the subsequently deposited thin-film coatings to be non-conformal. This means that the thin-films will not be able to follow exactly the nanopatternedtopography of silicon, leaving exposed parts of the surfaceor thickness non-uniformities, and therefore altering itsproperties.

For the fabricated cells presented earlier, the PECVDdeposited aSi:H films (Fig. 13a and b) are rather conformal,with thickness variation at the sidewalls of the nanopattern.On the other hand, the sputtered ITO film (Fig. 13a and b) andthe e-beam evaporated front-sidemetallization layer (Fig. 13cand d) are not conformal. The existing discontinuities arerestricted to the sidewalls of the nanopattern.

Figure 12 Simulated absorption per layer for a flat 1.2mm c-Sisolar cell highlighting the parasitic absorption of other films thanthe photoactive layer [21].

Figure 13 TEM images of (a and b) PECVD a-Si:H and sputteredITO films and (c and d) cross section SEM e-beam evaporatedmetallization, on top of a nanopatterned silicon surface. (a) and (c)correspond to the deep etch while (b) and (d) correspond to theshallow etch.



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The low FF values (Table 1) related to contacting issuescould be attributed to the partial or non-conformaldeposition of the ITO and of the metal films [65]. However,the fact that the deeper corrugations have higher FF is yet tobe clarified.

3.3 Recombination of electrical carriers In orderto study the effect of nanopatterning on the materialquality [46], 280mm thick, 3� 2V cm, p-type, mirrorpolished float-zone (FZ) wafers were used. Nanopatterningwas performed by NIL followed by various etchingtechniques. For the passivation of the wafers, 25 nm ofintrinsic and nþ-doped amorphous silicon (i/nþ-a-Si:H) wasdeposited on both sides by PECVD. The minority carrierlifetime results obtained by photoconductance decaymeasurements (the values correspond to an excess carrierconcentration of 1015 cm�3) are presented in Fig. 14.

The surface recombination velocities (S) were calculatedfrom the formula

SFeff �d

teff� SBeff ; ð5Þ

where SFeff and SBeff are the effective surface recombinationvelocities at the front and the back surfaces of the wafers,respectively, d is the wafer thickness (280mm) and teff is themeasured effective minority carrier lifetime. Since only oneof the surfaces is patterned (the front one), SBeff was calculatedfrom the measured effective minority carrier lifetime of theflat sample using the formula [66]

1teff

¼ 1tbulk

þ SFeff þ SBeff� �

d; ð6Þ

assuming that the bulk lifetime tbulk is infinite.Following this methodology, since all recombination

events have been attributed to the surfaces by assuming norecombination in the bulk (tbulk¼ infinite), the calculatedsurface recombination velocities (Table 2) are the maximumvalues. In fact, bulk recombination, dominated by Augerrecombination, is in reality not negligible, and the realsurface recombination velocities values are actually smaller.Moreover, the thinner the photoactive layer gets, the lessAuger recombination we have in the bulk of the material andthe more important the surfaces become. This argumentbecomes essential when studying thin-film substrates.

The low lifetimes values measured for the dry etchingare attributed to the severe damage on the materialquality [45] from the etching process (ion bombardment).The material quality remains significantly better whenwet chemical etching is used instead of dry etching, tonanopattern the surface [46], revealing much less surfacedegradation due to the wet chemical etching.

In order to tackle those limitations, certain designcosiderations could be taken into account. More precisely,using an optical buffer layer, for example, an ITO filmbetween c-Si and the back reflector, would result in lessparasitic absorption in the metal layer. Moreover, using wetetching techniques in order to etch the c-Si photoactive layerwould result in minimal material degradation, and thusin higher minority carrier lifetimes. Finally, improving theconformality of the contacting scheme (aSi/ITO/Front metalcontacts) would reduce the contact resistance losses andimprove the cell’s fill factor. Overall, a current reachingvalues up to 23–24mA cm�2 could be achieved.

The choice of the relevant etching technique, alongwith minimizing the parasitic absorption and solving thecontacting issues, will be taken into account for futureintegration of the nanopatterns.

4 Toward optimized photonic nanostructuresNumerical simulations are needed in order to theoreticallystudy the optical and optoelectrical properties of corrugatedsolar cells and optimize their parameters. For the opticalproperties, the goal is to find the best parameters for thenanopatterns that maximize the absorption in the active layerof thin-film solar cells. For the optoelectrical properties, thegoal is to find the best parameters that maximize the carriers’generation when integrating photonic nanostructures in thin-film solar cells. The combination of the two will enable usmaximizing the energy conversion efficiency of thin-filmsolar cells.

Many theoretical works, sometimes confirmed byexperimental verifications, have been done in this field [23,35, 67–71]. In order to optically optimize the patternparameters, two different tools were used: 1. 3D finitedifference time-domain (FDTD) simulations [72] (simula-tions were carried out using the software Lumerical FDTDSolutions) and 2. 3D rigorous coupled wave analysis(RCWA) [73]. All simulations consider non-polarized lightat normal incidence and take into account the chromaticdispersion of the optical indices of each material, mainly

Figure 14 Effective minority carrier lifetimes (teff) for the flat,RIE and ICP plasma etching, and TMAH wet chemical etching.

Table 2 Minority carrier lifetimes and surface recombinationvelocities (S) of periodic nanopatterned cells.

etching lifetime @ 1015 cm�3

(ms)Smax @ 1015 cm�3

(cm s�1)

flat 2200 6.4RIE 170 160ICP 44 630TMAH 930 24



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obtained from experimental measurements. For the opto-electical simulations, Silvaco [74] was used.

We will present here some simulation results for periodicand random patterns. The influence of the shape of thecorrugations on their optical performance and the optimiza-tion of the pattern parameters using a genetic algorithmapproach will first be shown in bare silicon slabs. Theoptimization results of the pattern parameters on the multi-layered full cell stack will be presented, in order to take intoaccount in the simulations the parasitic absorption in othercomponents of the cells. Finally, design considerationsrelated to the electrical properties of thin-film solar cells,such as the type of the absorber and the position of thenanopattern with respect to the position of the junction, willbe discussed based on optoelectrical simulation results.

4.1 Influence of the corrugation shape Severalstudies have shown that the shape of corrugations fabricatedat the front surface of the photoactive layer has a stronginfluence on the optical performances of the solar cell [23,68, 25, 75–77]. The corrugations, as already discussed,consist of an array of holes with various shape profilesallowing a graded variation of the refractive index from thetop of the solar cell to the active layer. This leads to animpedance matching effect that increases the absorption atshort wavelengths due to the antireflection effect. Further-more, corrugations should allow, at the same time, anincrease of the absorption at long wavelengths due tocoupling of incident light with quasi-guided modes [23, 25,77]. In order to numerically describe a large range of holeshapes, a generic mathematical profile function (super-Gaussian) can be used [23]. This function is given by

f ðrÞ ¼ exp � R2

2s2

� �m� �; ð7Þ

where f(r) represents the value of the super-Gaussian profilefunction at a given radius R from the center of the hole. Thereal number,m, describes the shape of the hole (see examplesof various hole shapes in Fig. 15). The parameter s dependson the radius (R) of the hole at the surface:

s2 ¼ R2

2ln

T

T � h

� �� mð8Þ

where T is the thickness of the active layer and h is the depthof the hole (see Fig. 15c).

The main advantage of optimizing a super-Gaussianprofile is that it allows scanning a large range of shapes bychanging only the value of the parameter m.

As mentioned earlier, the goal is to find the parametersthat maximize the solar cell performances. In the following,we will use the so-called integrated quantum efficiency(hoptical). This quantity represents the percentage of incidentphotons that are absorbed in the active layer of a solar celland is defined by

hoptical ¼R

lhc SðlÞAðlÞdlR

lhc SðlÞdl

; ð9Þ

where h is the Planck constant, c the speed of light, l theincident wavelength, A(l) the absorption spectrum of theactive layer, and S(l) the normalized solar spectrum(AM1.5G). The integration is performed for the wavelengthrange that silicon absorbs (300–1170 nm).

In order to study the influence of the corrugation shapeson the optimization, front-side corrugated c-Si slabs ofextreme thicknesses (1mm and 700mm-thick) are studied[23]. The corrugations follow a super-Gaussian profile(Eq. (7)). The parameters to be optimized are the period ofthe square array of the holes (P, varying from 250 to1250 nm), the depth (h, varying from 50 to 700 nm) and theshape of the holes (m). The diameter (d) of the holes was alsooptimized but the result is not shown here. Independentlyof the thickness of the slab and for all realistic ranges of theother parameters, the optimal diameter should be as close aspossible to the period (d � 0:9� P for practical reasons) [23,75]. For such a value of d, only a small quantity of silicon ispresent at the top surface. This value of the hole diameter,associated with an adequate shape of the holes, allows asmooth refractive index transition between air and silicon(graded index effect).

Figure 16 shows the maps of hoptical for two thicknessesof c-Si slabs (T¼ 1 and 700mm) and according to thevarious parameters. For both thicknesses, the optimal period(Popt) is of the order of the average between the free spacewavelength and the wavelength inside silicon (Popt¼ 500nm for the 1mm-thick slab and Popt¼ 750 nm for the 700-mm-thick slab). Furthermore, in order to achieve diffraction

Figure 15 A large range of hole shapes (b–e) are scanned thanks to the use of a super–Gaussian profile function in the optimizationprocess. The geometrical parameters used to define the super-Gaussian function are indicated in (c) while (a) shows a top view of thesimulated sample.



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into the c-Si slab and interferences between the diffractedorders in the most efficient way, both the optimal period Pand depth h increase with the slab thickness T [25, 78, 79].

For a given slab thickness, the increase of the periodleads to the shrinking of the size of the optimal hoptical region.With increasing period, the optimal depth also increases. Fora given period, the shape of the holes (m) has a stronginfluence on the size of the optimal hoptical region. Therefore,the shape of the holes should not be neglected and certaintypes of corrugations should be fabricated according to thesimulation output, aiming at the highest possible efficiency.

4.2 Optimization of periodic corrugations by thegenetic algorithm In order to enable a broader explora-tion of the parameters and to achieve even higherefficiencies, we used a genetic algorithm approach [80–82]. The parameters to be optimized in this case are theperiod P, the thickness T, the depth of the holes h, the area fillfactor a (ratio between the hole diameter and the perioda ¼ d=P), and the exponent m of the super-Gaussian. For theimplementation of the genetic algorithm, we considered apopulation of 100 individuals. Each individual is associatedwith a given set of values of the parameters to be optimized.These parameters are actually represented by a string of bits(the “DNA”). The efficiency hoptical associated with a givenset of parameters is the “fitness” that the genetic algorithmseeks at optimizing. The idea consists in selecting the bestindividuals in the population, breeding them in order todetermine individuals for the next generation, and playingthis game of selection from generation to generation in orderto find the global optimum for the efficiency hoptical. Randombit flipping in the DNA (“mutation”) is also implemented inthe exploration of parameters.

The initial population consists of 100 individuals with arandom DNA. In order to determine the individuals ofthe next generation, we select 25 pairs of parents by aclassification-based “roulette wheel selection” (this proce-dure gives individuals with a higher fitness more chanceto be selected). Each pair of selected parents is then

automatically transferred to the next generation. In addition,they determine a new pair of individuals (the children),which are either obtained by (i) a one-point crossing of theDNA of the two parents (probability of 90%) or (ii) a simplecopy of the two parents (probability of 10%). Mutation isimplemented for the children (each bit in the DNA has aprobability of 0.001 to be reverted). Finally, the individualswhose fitness value turns out to rank in the bottom 10%are replaced by random individuals. We proceed in thisway through a maximum of 100 generations unless earlyconvergence is achieved.

The parameter optimization using a genetic algorithmcan be ultimately refined by a local optimization using themethod of conjugated gradients. This enables a refinement ofthe parameters found by the genetic algorithm. Starting fromthe best parameters established via Fig. 16 for a slabthickness (T) of 1mm (P¼ 500 nm, h¼ 500 nm, a¼ 0.9, andm¼ 2 giving h¼ 0.52) and applying a local optimization, weachieved an efficiency hoptical (see Eq. 4) of 0.56 withP¼ 586 nm, T¼ 1020 nm, h¼ 451 nm, a¼ 0.9, and m¼ 2.When restricting the genetic algorithm to consider onlythickness values between 0 and 2mm, we obtained anefficiency hoptical of 0.59 with P¼ 620 nm, T¼ 1997 nm,h¼ 1123 nm, a¼ 0.93, and m¼ 6.3. When relaxing thiscondition to thickness values between 0 and 10mm, weobtained an efficiency hoptical of 0.70 with P¼ 1061 nm,T¼ 9149 nm, h¼ 2091 nm, a¼ 0.94, and m¼ 9.7. Theseresults (summarized in Table 3) demonstrate the advantage

Figure 16 Maps of the integrated quantum efficiency (hoptical) according to the depth of the holes (h) and to the shape of the super-Gaussian profile (m) for different periods (P) of the front-side corrugations and for two slab thicknesses (T¼ 1mm (first row) andT¼ 700mm (second row)).

Table 3 Genetic algorithm optimization of pattern parameters onbare c-Si slabs.

thickness(nm)

period(nm)

depth(nm)

fillfactor

shape(m-value)

hoptical

1000 500 500 0.9 2 0.521020 586 451 0.9 2 0.561997 620 1123 0.93 6.3 0.599149 1061 2091 0.94 9.7 0.70



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of using genetic algorithms for the optimization ofcorrugation parameters.

4.3 Optimization of periodic corrugations –

multi-layered solar cell stack FDTD simulations arerealized for a simple multi-layered structure made of a c-Silayer (1.1mm thick) and a shallow nanopattern (shown inFig. 10b), the parameters of which are the diameter of thehole, the etching depth and the period of the photonicnanostructures. In order to optically determine the beststructure, we can add as parameters the thickness of thepassivation layer (a-Si:H), of the top TCO/ARC (ITO) and ofthe optical spacer as depicted on Fig. 17.

A protocol has been set to avoid a time consuming fullsweep of all the parameters simultaneously, as well as the useof an optimization algorithm, inefficient for such a complexoptimization. The first step consists in roughly finding thebest thickness of each of the three layers (for the unpatternedstructure) by scanning these three parameters together. In asecond step, we can scan the nanopattern parameters (holediameter, etching depth, and period). After this second scan,the three thicknesses and the nanopattern parameters arerefined, but within a small range. Finally, after this first setof iterations (four runs of multi-parameter sweeps), wemaximize the absorption for each parameter independently.Similar refinements are performed until absorption isoptimized in the c-Si layer.

Such an optimization has been done on such a structure(with the thicknesses of the passivation layer and of theTCO fixed) and led to a short circuit current density value of18.0mA cm�2 for a 1.1mm thick c-Si cell, whichcorresponds to an increase of 7.2mA cm�2 (þ65% ofrelative enhancement) compared to the optimized unpat-terned reference. Slanted corrugation profiles and introduc-tion of disorder are expected to enable reaching highervalues.

4.4 Optimization of random corrugations –

multi-layered solar cell stacks Numerous studies haveshown that randomly nanostructured solar cells couldprovide better absorption performances than periodicones [83–85]. A way to simulate such a structure, given

the practical impossibility of modeling numerically a fullyrandom cell, is to use the supercell principle which consistsof modeling a finite disordered cell that is periodicallyrepeated (pseudo-periodic cell). The main goal is to optimizethe absorption of these random structures. Before startingthe optimization, it is important to study the influence of thesupercell period on the random behavior of the cell.

RCWA simulations are realized for a simple multi-layered structure made of a c-Si layer (2mm thick), an anti-reflective coating (ITO – 70 nm thick) and a back reflector(Al – 2mm thick) as presented in Fig. 18(a1) and (b1).

The thickness of each layer is set arbitrarily but is a fairrepresentation of the current state of the art [55]. Regardingthe corrugation pattern, cylindrical holes are consideredfor simplicity (depth of 500 nm). The idea is to keep thehole radius (Rhole) and the area fill factor (a) constant forevery studied cell (here, Rhole¼ 170 nm and a¼ 0.36). Theinvestigated periods (P) are 500 nm for the referenceperiodic cell and 1, 1.5, 2, and 2.5mm for the four studiedsupercells. Due to the random nature of supercells, astatistical study is required. Therefore, for each case, 25simulations are made where the hole positions are changedrandomly.

The results are summarized in Fig. 18d in terms of theoptical efficiency hoptical (Eq. (9)) of the whole structure.These results allow us to highlight three important points.First, as we expected, an important improvement of the

Figure 18 Schematic diagrams of solar cells with square latticeperiodic unit cells (a), pseudo-random supercells of various periodsand uniform hole diameter (b and c). In each case, the thickness ofthe various layers (ITO, cSi, and Al), the hole depth (500 nm), thehole radius (170 nm) and the fill factor (0.36) are constant. Meanabsorption efficiency and standard deviation for each structure (d).

Figure 17 Cross section of the multi-layered solar cell stack andthe parameters to be optimized.



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absorption efficiency is obtained by using a randomlypatterned cell (hoptical� 82.5%) instead of a periodic one(hoptical� 75.5%). However it should be noted that theperiodic structure was not optimized and is shown here onlyas a reference. Secondly, regarding the efficiency of the foursupercells, it is nearly constant, whatever the period of thesupercell is. Thirdly, the standard deviation increases whenthe supercell size decreases. The two last points are veryimportant regarding the correct modeling of a randomlypatterned cell. Indeed, it is possible to simulate such astructure by modeling either a small supercell or a big one. Inthe first case, in order to reduce the standard deviation, manysimulations are required. In the second case, just a fewsimulations are needed but each of them requires a longercomputation time (in RCWA, it is due to the requirednumber of Fourier harmonics which increases dramaticallywith increasing the cell period [86]).

All these considerations show the need for severalsupplementary optimization loops, especially regarding theinfluence of the fill factor on the absorption efficiency. Otherparameters such as the hole shape can, of course, be takeninto account.

4.5 Optoelectrical simulations In thin-film solarcells, the difference in the concentration of photogeneratedcarriers in the bulk is less prominent between top and bottomsurfaces, compared to a standard 200mm thick solar cell. Asa consequence, the bottom part of the device, where a BSFis often used to prevent the photogenerated carriers frombeing recombined on a poorly passivated surface, must bedesigned carefully.

In Fig. 19, we compare different schemes for a thin-filmsolar cell (with a constant thickness of 1.4mm) consisting ofa top ARC (75 nm of ITO), a 12 nm thick n-type a-Si emitter(doped at 2� 1021 cm�3) and a 12 nm thick intrinsic layerof a-Si:H (passivation). The absorber is made of p-typecrystalline silicon, and the BSF aims at repelling the carriersfrom recombining at the bottom aluminium contact. Forthe following simulations, a back surface recombinationvelocity of 107 cm s�1 is taken into account.

Due to the high recombination rate inside the BSF, thethickness of this region should be kept as thin as possible. Adecrease of 100 nm of the BSF thickness leads to an increaseof approximately 0.3mA cm�2 (Fig. 19a) with a lowlydoped absorber. When the doping level of the absorberincreases, the short circuit current density decreases due tobulk recombination. However, different behaviours areobserved: if the BSF is not efficient (black curve), the solarcell is more sensitive to the absorber doping level. A BSFdoping level up to 1020 cm�3 for the red and blue lines(instead of 2� 1019 cm�3 for the black one) leads to a morerobust device.

Taking into account the band-gap narrowing (BGN)effect [87] in both the absorber and the BSF, we observe(Fig. 19b) a constant Voc when the absorber doping is not toohigh (less than �1017 cm�3). In the case of an inefficientBSF (black line), the Voc tends to increase with the doping

level of the absorber. Therefore, the BSF must be thickenough to get a maximum Voc by comparison of the red andblue lines of Fig. 19b.

Fig. 19c represents the maximum efficiency of thesethree different cases. The cases with an intermediatethickness of the BSF correspond to the best efficiencies,whatever the doping level of the absorber is. Even if thesethree cases are not optimized, we can deduce that the BSFmust be heavily doped, but its thickness must be carefullychosen since a compromise is necessary between the value ofthe Voc and the amount of carriers that can recombine insideits volume.

Thanks to the nanopatterning, a significantly higherphotogeneration rate in the solar cell is expected. This shouldlead to a higher Jsc (and a slight increase of the Voc).However, the effective surface area that needs to be

Figure 19 Influence of the doping level of the absorber on solarcell performance parameters for different BSF thicknesses anddoping levels: a 250 nm thick BSF at a doping level of2� 1019 cm�3 (black lines), a 150 nm thick BSF at a doping levelof 1020 cm�3 (blue lines), a 50 nm thick BSF at a doping level of1020 cm�3 (red lines).



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passivated is larger compared to flat solar cells. Ideally, the a-Si:H layer should be conformal. As already discussed, thismight not always be the case and therefore, the influence ofsurface recombination on the pattern of the surfaces shouldbe studied for sidewalls that are not efficiently covered by theintrinsic and n-doped a-Si:H.

The optoelectrically simulated (in Silvaco) structureis represented in Fig. 20. The left half-part of the figureconsiders the case where the absorber is p-type withthe junction at the top and the right half-part of the figureconsiders the case where the absorber is n-type with thejunction at the bottom.We observe that the electric fieldmapsare very different depending on the type of the absorber.

Figure 21 represents the influence of surface recombi-nation on either sides of the absorber (both for n- and p-typewith the junction located at the rear and front side,respectively), for a continuous or discontinuous a-Si:Hlayer on the sidewalls. As we can observe in Fig. 21a, solarcells are very sensitive to surface recombination on thepattern when a discontinuous a-Si:H layer is present (dashedlines). Consequently, the shape of the patterns is veryimportant and it should allow a conformal deposition of thea-Si:H, without any discontinuities. If the junction is locatedon the pattern side, the device is even more sensitive tosurface defects (blue dashed line vs. red dashed line). In casethe sidewalls are perfectly covered by a-Si:H (solid lines),there is almost no difference in terms of robustness betweena n-type absorber/bottom junction and a p-type absorber/topjunction. On the contrary, if the a-Si:H deposited layer is notperfectly covering the sidewalls of the pattern, it is relevantusing the solution with a bottom junction.

Besides, if the interface between the BSF and theabsorber leads to surface recombination (for instance, ifthe BSF results from imperfect epitaxial growth), then, the

p-type absorber/top junction is a little bit more sensitive typeof the absorber.

Consequently, the surfaces which are more sensitive tosurface recombination are those located on the opposite sideof the junction. To get a robust device regardless offabrication defects on the patterns, an n-type absorber forwhich the electrical junction is located at the bottom part ofthe device should be preferred.

5 Conclusions We report on the fabrication, charac-terization, integration and optimization of 2D photonicnanostructures for thin-film crystalline silicon (c-Si) solarcells. We propose a fabrication process consisting of acombination of lithography and etching. Regarding thedefinition of the nanopattern, nanoimprint lithography(NIL), and laser interference lithography (LIL) are used forperiodic nanopatterns and hole mask colloidal lithography(HCL) for random nanopatterns. Regarding the etching ofsilicon, either dry plasma etching (reactive ion etching orinductively coupled plasma etching) or wet chemicaletching (TMAH etch) are used to transfer the pattern to thephotoactive layer. Optically, those nanostructures behaveso far comparably to the state-of-the-art light trappingscheme (random pyramid texturing) while offering theextra advantage of a robust behavior with respect to lightincidence angle and of minimal material consumption.From the integration of periodic nanopatterns on 1mm c-Si

Figure 20 Schematic view of the simulated device for a non-conformal top a-Si:H layer. The left half-part considers the casewhere the absorber is p-type with the junction at the top, and theright half-part considers the case where the absorber is n-type withthe junction at the bottom.

Figure 21 Variation of the EQE of the device as a function of thesurface recombination velocity. (a) Influence of the a-Si:H/c-Siabsorber interface and the sidewalls of the patterns and (b) influenceof the c-Si absorber/c-Si BSF interface.



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cells, we have identified certain issues affecting the cellperformance. The parasitic absorption in layers other thanthe photoactive layers, the conformality of thin-filmcoatings on top of the nanopattern and the materialdegradation after etching contribute to limitations on theenergy conversion efficiencies. By solving these processingissues, a doubling of the Jsc could be expected. Then,further optimization guidelines can be proposed in a nextstage. From optical simulations (FDTD and RCWA), thepattern parameters which could maximize the absorption oflight in a thin-film solar cell, for both a periodic and arandom pattern, are identified. From optoelectrical simu-lations (Silvaco), an n-type absorber for which the electricaljunction is located at the bottom part of the device should bepreferred. Given the achieved control on the patterning andthe etching of c-Si, integrating the optimal parameters on ac-Si thin-film cell, keeping in mind the identified issues(i.e., minimizing the parasitic absorption, solving thecontacting issue, and using the etching which results inminimal material degradation) may enable us to reachenergy conversion efficiencies above 20% with only a fewmicrometers of c-Si.

Acknowledgments This project has received funding fromthe European Union’s Seventh Programme for research, techno-logical development and demonstration under grant agreement No309127, PhotoNVoltaics (Nanophotonics for ultra-thin crystallinesilicon photovoltaics).

Alexandre Mayer is funded as Research Associate by theNational Fund for Scientific Research (FNRS) of Belgium. He ismember of the Namur Center for Complex Systems (NAXYS). Heacknowledges D. Nicolay and T. Carletti for useful discussions ongenetic algorithms. Part of this research used resources of the“Plateforme Technologique de Calcul Intensif (PTCI)” (http://www.ptci.unamur.be) located at the University of Namur, Belgium,which is supported by the F.R.S.-FNRS under the conventionNo. 2.4520.11. The PTCI is member of the “Consortium desÉquipements de Calcul Intensif (CÉCI)” (http://www.ceci-hpc.be.

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Photonic nanostructures for advanced light trapping in thin crystalline silicon solar cells

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