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Materials Science and Engineering B 178 (2013) 645– 650

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B

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hort communication

hin-film silicon solar cells applying optically decoupled back reflectors

. Moulina,∗, U.W. Paetzolda, K. Bittkaua, M. Ermesa, L. Dingb, L. Fannib, S. Nicolayb, J. Kirchhoffa,. Weiganda, A. Bauera, A. Lambertza, C. Ballif b, R. Cariusa

Institute of Energy and Climate Research 5 – Photovoltaics, Research Centre Jülich GmbH, D-52425 Jülich, GermanyEcole Polytechnique Fédérale de Lausanne, Institute of Microengineering, Photovoltaics and Thin Film Electronics Laboratory, 2000 Neuchâtel, Switzerland

r t i c l e i n f o

rticle history:eceived 2 July 2012eceived in revised form 9 October 2012ccepted 11 October 2012vailable online 25 October 2012

a b s t r a c t

Thin-film silicon solar cells often apply a metal back reflector (BR) separated from the silicon layers by athin rear dielectric of thickness around 80 nm or a white paint combined with a thick rear dielectric ofseveral micrometers. In this work, we investigate the optical performance of microcrystalline silicon (�c-Si:H) solar cells applying BRs of various topographies. In contrast to a standard 80 nm-ZnO/Ag BR design,for which the BR nearly strictly follows the texture of the underlying �c-Si:H layers, placing the Ag BR far

eywords:hin-film solar cellight trappingack reflectorlasmonichite paint

from the �c-Si:H layers allows for a variation of the BR topography. Irrespective of the investigated BRtopographies and also for a conventional white paint BR, long distances (of several micrometers) betweenthe BR and the �c-Si:H layers are found to be detrimental for the light trapping. Optical simulations basedon both rigorous and scalar scattering theory have been performed to understand the impact of the diverseBR designs on the optical cell performance.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

The back reflector (BR) is an essential constituent of the light-rapping scheme of thin-film silicon solar cells. In addition to itsrimary role as a reflector, the BR plays a major role regardinghe light scattering within the device. Recently, it was found thathe dominant light-scattering process in thin-film silicon solarells deposited on a randomly textured sputtered ZnO:Al substrateccurs at the rear side of the cells, where scattering angles far abovehe angle of total internal reflection at a flat �c-Si:H/ZnO:Al inter-ace can be obtained [1]. For most of the state-of-the-art thin-filmilicon solar cells, a metal BR separated from the silicon layers by ahin dielectric (e.g. ZnO:Al) of low refractive index (nZnO:Al∼1.8)2–6] or a white paint combined with a micrometer-thick rearnO:B layer is commonly applied as a BR design [7–12]. The thinear ZnO:Al layer (of usual thickness around 80 nm) applied in com-

ination with a metal BR is used to reduce the plasmonic lossesccurring in the metal [13–19]. The main function of the thicknO:B rear dielectric combined with a white paint is to provide

∗ Corresponding author. Tel.: +49 2461 61 2069; fax: +49 2461 61 3735.E-mail addresses: e.moulin@fz-juelich.de (E. Moulin), u.paetzold@fz-juelich.de

U.W. Paetzold), k.bittkau@fz-juelich.de (K. Bittkau), m.ermes@fz-juelich.deM. Ermes), laura.ding@epfl.ch (L. Ding), lorenzo.fanni@epfl.ch (L. Fanni),ylvain.nicolay@epfl.ch (S. Nicolay), j.kirchhoff@fz-juelich.de (J. Kirchhoff),.weigand@fz-juelich.de (D. Weigand), and.bauer@fz-juelich.de (A. Bauer),.lambertz@fz-juelich.de (A. Lambertz), christophe.ballif@epfl.ch (C. Ballif),.carius@fz-juelich.de (R. Carius).

921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.mseb.2012.10.016

a sufficiently high conductivity to fulfill the role of back electrode[20]. In a previous study, it was found that the reflectivity at the backside of thin-film silicon solar cells applying a metal BR can be furtherimproved by replacing the ZnO:Al layer by a loss-free rear dielec-tric (such as SiO2) having a lower refractive index (nSiO2

∼1.45) thanZnO:Al [21]. For detached Ag BRs separated from the silicon lay-ers by a rear dielectric of several micrometers, it was found thatreducing the refractive index of the rear dielectric not only leadsto reduced plasmonic losses in the BR but also results in a notice-able improvement of the light trapping in the �c-Si:H solar cells[22]. Depending on the front texture of the investigated cells, abetter optical cell performance could be obtained with air as a reardielectric rather than with a thin layer of ZnO:Al or SiO2 [21,22].However, reducing the refractive index of the rear dielectric whilemaintaining good electrical cell performance represents a real chal-lenge since, in most of the cases, dielectrics with a lower refractiveindex than ZnO:Al tend to show a significantly lower conductivity.Therefore, alternative approaches have to be considered to improvethe quality of the BR design. For solar cells applying a thin ZnO:Allayer as a rear dielectric, the surface texture of the adjacent metalBR deposited on the ZnO:Al nearly perfectly follows the topogra-phy of the underlying �c-Si:H layers. The surface texture of the BRcan therefore not be varied independently of the �c-Si:H layers. Inorder to address this issue and investigate the optical performance

of thin-film silicon solar cells applying BRs of various topographies,two methods are introduced in this work. The first one is basedon the use of detached BRs combined with a transparent liquidas a rear dielectric. This method, already successfully applied in a

646 E. Moulin et al. / Materials Science and E

Fig. 1. Schematic drawing of thin-film silicon solar cells applying (i) a silver backreflector separated from the �c-Si:H by a thin layer of ZnO:Al and (ii) detached AgBRs combined with methylene iodide MI (nMI ≈ nZnO:Al) as a rear dielectric. Withtti

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his approach (described in more detail in [22]), diverse detached BRs of differentopographies can be placed behind the same solar cell. SEM micrographs of the threenvestigated BR surfaces are shown on the right hand side.

revious work [22], allows for a precise and flexible determinationf the optical impact of BRs of any surface roughness on the sameolar cell. The second method is based on the use of a thick rearielectric of several micrometers made of ZnO:B which is depositedy LPCVD (low pressure chemical vapor deposition) and whichevelops an as-grown surface texture. Using these two methods,e show the influence of diverse surface topographies of Ag BRs

varied in a wide range) on the optical performance of solar cells.he optical response applying a white paint as a BR is also pre-ented. The external quantum efficiency and the absorptance ofhe solar cells are compared with those of a cell applying an 80 nm-nO/Ag BR design. Based on both rigorous and scalar theory, a figuref merit of the light scattering within the device is established forhe diverse types of BR designs, allowing for a clear interpretationf the experimental results.

. Materials and methods

The �c-Si:H thin-film silicon solar cells investigated in this studyre deposited in a superstrate (p-i-n) configuration on a flat glassubstrate (Corning, Eagle 2000) coated with an 800 nm thick sput-ered ZnO layer doped with 1 wt% Al. Prior to the deposition ofhe silicon layers, an etching step exposing the ZnO layer for 50 so 0.5% (w/w) diluted hydrochloric acid (HCl) is carried out. Thistching step induces the formation of crater-like features into thenitially smooth ZnO:Al surface. The p-doped, intrinsic (i-) and-doped layers, deposited by plasma enhanced chemical vaporeposition (PECVD), have a thickness of approximately 20 nm, 1 �mnd 20 nm, respectively. More details about the deposition param-ters are given elsewhere [23]. For the �c-Si:H solar cells applying

thin ZnO layer as a rear dielectric (denoted as reference cells),n 80 nm thick ZnO:Al layer is deposited by rf-sputtering on thec-Si:H prior to the deposition by thermal evaporation of the Agack contact. In order to vary the BR texture independently of thenderlying �c-Si:H layers, two approaches are chosen:

(i) The first is based on the use of a local rear contact schemethat allows the use of detached BRs combined with transparent

liquids (cf. Fig. 1). This method, already introduced in a pre-vious work and successfully adopted, enables us to study theimpact of BRs of any desired topography in a flexible and pre-cise way since the BRs can simply be pressed against the liquid

ngineering B 178 (2013) 645– 650

at the rear side of the same solar cell. In this work, methy-lene iodide (MI) is used as a rear dielectric since the refractiveindex of this liquid shows similar values to that of ZnO:Al(nMI ≈ 1.74 ≈ nZnO:Al ≈ 1.8). The detached BRs are mechanicallypressed against the rear side of the solar cell to keep the liq-uid in good contact with silicon and avoid any air gap betweenthe �c-Si:H, the liquid and the BR. The 20 nm thick �c-Si:Hn-doped layer provides a sufficiently high lateral conductiv-ity to enable external quantum efficiency (EQE) measurementsat a relatively low bias voltage (−0.25 V) of solar cells with anon-conductive rear dielectric and a detached BR. The activearea of the finalized cells is defined by laser scribing and setto 1.1 cm × 1.1 cm. As the detached BRs are used together withmethylene iodide, a 160 nm thick ZnO layer is deposited ontothe Ag surface in order to prevent any mechanical or chemicalreaction with methylene iodide.

(ii) The second approach adopted to obtain BR textures differentthat the �c-Si:H rear-layer textures relies on the use of ZnO:Bas a rear dielectric deposited by LPCVD (cf. Fig. 3). This typeof layers shows a high transparency and develops an as-grownsurface texture whose roughness can be varied independentlyof the topography of the �c-Si:H rear side [7–9,20,24]. A ther-mally evaporated silver layer or a commercially available whitepaint (from the company Marabu) is then deposited on thehighly conductive LPCVD ZnO:B and used as a BR.

The absorptance of the solar cells has been deduced fromreflectance measurements performed with a spectrophotometerLAMBDA 950 (Perkin Elmer) equipped with an integrating sphere.For the EQE measurements, the size of the light spot (of only a fewsquare millimeters) is significantly smaller than the area definedby the rear Ag grid lines. The light spot is positioned such that lightreaching the rear side of the cell does not interact with the grid.Since a large fraction of the visible light reaches the rear side, thelight spot can easily be adjusted. For thin-film solar cells with suchan active area measured with a smaller illumination spot, the lat-eral leakage of light in glass can lead to a significant underestimateof EQE at various wavelengths, compared with a full-scale thin-filmsolar cell [25,26]. However, owing to the relatively small thicknessof the glass substrate employed in this study (around 1 mm) and thevery small size of the illumination spot (Ø ∼2 mm), discrepanciesin EQE significantly lower than 5% are expected [26]. This approachshould therefore provide a good estimate of the photocurrent thatwould actually be measured with a full-scale thin-film solar cell.

3. Results and discussion

3.1. Solar cell with detached Ag BRs of various surfacetopographies compared to the reference cell

In order to investigate the impact of various topographies ofBRs while keeping the other geometric parameters of the cell con-stant, detached BRs have been applied combined with methyleneiodide as a rear dielectric (Fig. 1). The surface topography of thedetached BRs is varied from a flat surface (Fig. 1a) to a surface ofmoderate texture roughness (Fig. 1b) and a surface of high tex-ture roughness (Fig. 1c). The detached BRs cover a range of rms(root mean square) roughnesses varied from a few nanometers, toaround 80 nm and 130 nm. The rms values are determined fromatomic force microscopy (AFM) images analysis. Fig. 2 shows theEQE and the cell absorptance of the solar cell applying the differ-

ent detached BRs. Despite the fact that the thickness of the reardielectric cannot be perfectly controlled, a high reproducibility inthe measurements has been observed, suggesting that the largedistance between the �c-Si:H and the BR (of a few micrometers)

E. Moulin et al. / Materials Science and Engineering B 178 (2013) 645– 650 647

Fig. 2. EQE and absorptance A as function of wavelength of a �c-Si:H p-i-n solar cellapplying diverse detached silver reflectors of various surface topographies com-bined with methylene iodide as a rear dielectric. The root mean square roughnessot8

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Fig. 3. Schematic drawing of thin-film silicon solar cells applying a silver back reflec-

the �c-Si:H layers from the BR on the optical performance of thin-film silicon solar cells, rigorous simulations – based on the FDTD(finite difference time domain) method – of a complete solar cellhave been performed using the open source program Meep [29].

f the detached Ag BRs is varied from a few nanometers to approximately 80 nm upo 130 nm. The EQE and absorptance of a solar cell (deposited in parallel) with an0 nm-ZnO/Ag back reflector design are plotted for comparison (dashed line).

rohibits optical coupling effects. Increasing the surface roughnessf the detached BRs leads to a simultaneous enhancement of theQE and the cell absorptance (see arrows in Fig. 2). This result indi-ates an improvement of the light trapping within the cell, causedy an increase in the light scattering at the rough silver BRs. Nev-rtheless, even for the detached BR of high surface roughness (inhe same order than that of the �c-Si:H rear side), a higher EQEs found for the reference cell (Fig. 2, dashed line). Parasitic lossesn the MI liquid and in the additional ZnO layer (used as a pro-ective layer between the MI and the Ag BR) certainly contributeo lower the EQE of the cell with detached BRs. Interestingly, inpite of these additional parasitic losses, the absorptance of theell with detached BRs remains significantly lower than that of theeference cell. Therefore, these additional parasitic losses cannotolely explain the lower EQE. The lower absorptance of the cellith detached BRs suggests an alteration of the light-trapping prop-

rties, likely caused by the long distance (of a few micrometers)eparating the �c-Si:H layers from the Ag BR. This alteration mighte partly compensated by using BRs with a higher surface rough-ess (rms > 130 nm) [27] since such BRs should provide even better

ight scattering properties [28]. Furthermore, beyond a certain sur-ace roughness, plasmonic losses at a textured Ag reflector becomeess significant as the surface roughness increases [18]. Therefore,t is expected that the optical performance of the cell should be fur-her improved by further increasing the roughness of the detachedR. In the next section, we present the optical performance of cellspplying a BR of larger rms surface roughness.

.2. Solar cell with a 5 �m-ZnO/back-reflector compared to theeference cell

The surface roughness of the BR has been further increasedy using an LPCVD ZnO:B layer as a rear dielectric showing ans-grown texture. In addition to the fact that LPCVD can lead toery large feature sizes of the rear ZnO:B, this approach also pro-ides a realistic BR design configuration for solar cell applicationss the BR is attached to the rear side of the cell (cf. Fig. 3). Fig. 4resents the EQE and cell absorptance of a �c-Si:H solar cell apply-

ng a 5 �m thick LPCVD ZnO:B showing an as-grown texture of veryigh roughness (rms ≈ 180 nm) covered with a silver BR (stars).espite the high feature sizes of the textured silver BR deposited

n the thick ZnO:B – leading to a strong light scattering in theear-infrared wavelength region – the solar cell exhibits a signifi-antly lower EQE and cell absorptance than the reference cell (Fig. 4,ashed line), suggesting poorer light-trapping properties.

tor separated from the �c-Si:H by a thin layer of ZnO:Al (left), a 5 �m thick LPCVDZnO:B of very high surface roughness combined with Ag (middle) and with a whitepaint (right).

Replacing the Ag BR by a white paint (one concept used in theindustry) results in a noticeable increase in the EQE (Fig. 4, circles).The white paint provides a high EQE, showing the relevance of thisBR for industrial applications. Since this increase is associated witha decrease in the cell absorptance, a reduction of the parasitic lossesoccurring in the BR is taken responsible for this effect. Using a whitepaint of high reflectivity allows one to circumvent the problem ofplasmonic losses and to provide a high reflectance (higher than95% in air at wavelengths longer than 500 nm). Nevertheless, inspite of the efficient light scattering and the high reflectance pro-vided by the white paint, the EQE and the absorptance of the cellremain below those of the reference cell, indicating a poorer lighttrapping. Again, the optical decoupling of the BR from the siliconlayers leads to a deterioration of the light-trapping properties. Insummary, irrespective of the investigated BRs, a better optical per-formance has been found for the reference solar cell than for cellsapplying BRs separated from the �c-Si:H by several micrometers.

3.3. FDTD simulations

To better understand the influence of the distance separating

Fig. 4. EQE and absorptance A as function of wavelength of a �c-Si:H p-i-n solar cellapplying a 5 �m thick LPCVD ZnO:B of very high surface roughness combined witha silver back reflector (stars) and a white paint (circles). The EQE and absorptance ofa solar cell (deposited in parallel) with an 80 nm-ZnO/Ag back reflector design areplotted for comparison (dashed line).

648 E. Moulin et al. / Materials Science and Engineering B 178 (2013) 645– 650

Fig. 5. Absorptance in the �c-Si:H layer obtained by FDTD simulations of a completesolar cell applying a perfect reflector separated from the �c-Si:H layers by a loss-freerear ZnO of thickness 80 nm (solid line) and 5 �m (dashed line). An AFM micrographoam

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Fig. 6. Normalized angular intensity distribution (AID) at � = 750 nm of the scatteredlight transmitted from the rear ZnO into the �c-Si:H for an angle of incidence of 10◦

(triangles) and 70◦ (circles). The horizontal arrows indicates that by increasing ˇ, the

f the �c-Si:H rear side is used to model all the interfaces of the solar cell as wells the texture of the back reflector. The illustration depicts the three-dimensionalodel used to simulate the solar cell.

he reliability and relevance of the FDTD method, as applied in theresent work, have already been demonstrated in a previous workhere the electromagnetic field distribution at a textured interface

determined by scanning near-field microscopy (SNOM) measure-ents – was adequately reproduced by the simulations [30]. The

urface topography of the �c-Si:H rear side, measured by AFM, haseen implemented to accurately model the interfaces of the solarell in the three-dimensional simulation. Periodic boundary condi-ions with a period of 10 �m in both the x and y directions and aesolution of 20 nm have been used in the model. Since the struc-ure sizes at the surface of the �c-Si:H are in the micrometer range,

scan size of 10 �m × 10 �m representatively describes the surfaceopography. We use the electromagnetic field distribution rightfter the light source for calculation of the diffuse scattered light.ince light incidence is perpendicular to the plane of the cell, all dif-use light must result from reflection. The absorptance in each layers obtained by calculating the absorption in each grid domain of the

odel from the Poynting vector and by integrating the absorptionver the whole volume of the layer [31]. In contrast to the experi-ental approach, simulations allow for solely varying the distance

etween the BR and the �c-Si:H layers without modifying othereometric parameters, such as the topography of the interfaces.onformal interfaces can thus be simulated, as shown in Fig. 5. Inrder to avoid long computational times, the 1 mm thick glass sub-trate has been replaced by a glass half space. Light thus directlyropagates in the glass before reaching the front ZnO. In order tonly account for the effect of the rear-ZnO thickness on the light-rapping properties of the device, absorption losses in the rear ZnOre neglected by setting the extinction coefficient of the ZnO layero zero. Plasmonic losses in the metal BR are also not taken intoccount. Instead, a perfect conductor is chosen for the BR. Finally,he thin p- and n-doped layers of thickness around 20 nm are notonsidered in the model.

Fig. 5 shows the absorptance in the �c-Si:H i-layer (equivalento the EQE in the previous sections) for thicknesses of the rear ZnOf 80 nm and 5 �m. The simulations show that, in the whole spec-ral range where light trapping is important, a significantly higherbsorptance is achieved in the i-layer for a rear ZnO of 80 nm thanor a rear ZnO of several micrometers. Since absorption losses in the

ear ZnO and in the reflector are not taken into account, this differ-nce can only be explained by a modification of the light-trappingroperties. The optical decoupling of the interfaces �c-Si:H/rear-nO and rear-ZnO/BR induced by the longer distance between the

AID in the �c-Si:H continuously shifts toward larger angles. The angular intensitydistribution of the light reflected in the �c-Si:H is depicted for comparison (dashedline).

BR and the �c-Si:H layers results in an alteration of the light-trapping properties within the absorber layer of the cell.

The FDTD simulation represents an extensive and time-consuming approach that does not allow for a systematical andexhaustive study of all possible BRs (i.e. of any arbitrary surfacetopography). The question arises whether for the considered tex-ture of the �c-Si:H rear side, an optically decoupled BR can be foundthat would provide a better light trapping than a BR adjacent to the�c-Si:H. To answer this question, a scalar theory approach based oncalculations of the angular intensity distribution (AID) within the�c-Si:H absorber layer using an extension of the model developedby Dominé et al. [32] has been adopted.

3.4. Calculations of the angular intensity distribution in the�c-Si:H

The model developed by Dominé represents a fast method forwhich a figure of merit of the light scattering in the device canbe obtained. In contrast to the FDTD approach, only one inter-face of the solar cell is considered in this model. The local phaseshift exp[ik·z(x,y)·(n1 − n2)] of the electromagnetic wave travers-ing the roughness zone (here the �c-Si:H/rear-ZnO) defines thescattering, where n1 and n2 are the refractive indices of the twohalf spaces forming the interface �c-Si:H/ZnO, k the wave vectorin vacuo and z(x,y) the surface topography. This method enables usto assess the angular distribution of the scattered light for any angleof incidence at the interface. Therefore, it can be used to evaluatethe light scattering within the cell for all possible BRs and asso-ciated light scattering scenarios. Two extensions have been madein comparison to the formulas introduced by Dominé et al.: (i) theapplication of the model to the reflection [1,33] and the model-ing of light scattering at finite angle of incidence [33]. Again, theAFM scan of the �c-Si:H rear side of the solar cell is used for thecalculations. The two interfaces �c-Si:H/rear-dielectric and rear-dielectric/BR are considered to be independent of each other. Thisapproach may be realistic as long as the distance between the BRand the �c-Si:H layer is long enough (∼several micrometers) forthe light to lose its spatial coherence.

Fig. 6 presents the AID at 750 nm of the scattered light trans-mitted from the rear dielectric (i.e. a half space ZnO representing

the thick ZnO rear dielectric) into the �c-Si:H for different anglesof incidence in this rear dielectric. For increasing angle ˇ, the AIDof the refracted light in the �c-Si:H shifts toward higher angles ˛(indicated by the arrow in Fig. 6). For an angle of incidence of

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0◦ in the rear dielectric, the AID within the �c-Si:H exhibits a nar-ow shape with a maximum at around 10◦ (cf. Fig. 6, triangles). Forn angle of incidence of 70◦, the AID peaks at an angle of around7◦ in the �c-Si:H, rendering the AID more appropriate for a good

ight trapping in the absorber layer of the cell (cf. Fig. 6, circles).owever, light that is directly reflected in the �c-Si:H at the �c-i:H/rear-dielectric interface is scattered at larger angles than lightoming from the rear dielectric. Indeed, the AID of the light reflectedn the �c-Si:H (with normal incidence with respect to the plane ofhe textured surface) peaks at larger angles and extends to mucharger angles than that of the light coming from the thick rear ZnOayer (cf. Fig. 6, dashed line).

Irrespective of the angle of incidence in the half space ZnO,ight directly reflected in the �c-Si:H is scattered at larger angles,

aking the AID more appropriate for an optimal light trapping.his indicates that a BR in direct contact with the �c-Si:H layer ordjacent to it – leading to a direct coupling of the light back intohe cell – should provide better light-trapping properties than anyR separated from the �c-Si:H by a thick ZnO rear dielectric. Thisxplains why in the previous sections cells with detached Ag BRsnd BRs deposited on a thick LPCVD ZnO:B have shown a lower EQEnd absorptance than the cell applying a thin ZnO:Al layer. Thislso provides an explanation for the FDTD simulations presentedbove predicting a lower absorptance for optically decoupled BRsf conformal surface texture. This result is due to the very appro-riate texture of the considered �c-Si:H/rear-ZnO interface that

eads to a back reflectance at very large angles within the absorberayer of the cell. For less suitable textures of the �c-Si:H rear sidee.g. of lower surface roughness), an optically decoupled BR of highurface roughness might lead to a better light trapping than an0 nm-ZnO/Ag BR design. Excellent light trapping properties wereeported with a thick LPCVD rear ZnO used in conjunction withhite paint for solar cells deposited on rougher front ZnO elec-

rodes, indicating that the effectiveness of the BR is linked to theature of the front electrode [34,35].

. Conclusions

We have compared the optical performance of �c-Si:H solarells applying BRs of various topographies with solar cells usingn 80 nm-ZnO/Ag BR design. In order to study the impact of BRsaving a topography different than that of the underlying �c-Si:H

ayers, two approaches have been presented: the first is based onhe utilization of detached BRs combined with a transparent liq-id having a refractive index similar to that of a conventionallysed rear ZnO. With this method, the effect of BRs with rms rough-esses varied from a few nanometers (glass/Ag) to 80 nm (surfacef moderate roughness) and 130 nm (surface of high roughness) haseen investigated. The results show that by increasing roughnessf the BR and consequently the light scattering at the rear side ofhe cell, a better light trapping within the device is obtained, lead-ng to an enhanced EQE. The second approach is realized by usingn LPCVD rear-ZnO:B layer developing an as-grown surface tex-ure. As an example, a 5 �m thick LPCVD ZnO:B showing a surfaceoughness of approximately 180 nm has been applied in combi-ation with silver and a white paint. Due to a higher reflectancend probably a better light scattering at the rear side of the cell,hite paint as a BR exhibits an improved optical performance com-ared to Ag. Nevertheless, also for the BR design applying the whiteaint, a lower EQE is achieved compared to a BR adjacent to thec-Si:H layers. FDTD simulations have shown that by increasing

he distance between the BR and the �c-Si:H layers and keep-ng all other geometric parameters constant, a lower absorptancen the i-layer is found. This indicates that for this particular tex-ure of the �c-Si:H rear layer, the optical decoupling of the two

[

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ngineering B 178 (2013) 645– 650 649

interfaces (�c-Si:H/rear-ZnO and rear-ZnO/BR) is detrimental forthe light trapping. Calculations of the angular intensity distribu-tion have demonstrated that for this specific texture of the �c-Si:Hrear side, a better optical performance should be achieved with aBR adjacent to the �c-Si:H than with any optically decoupled BR.

Acknowledgements

The authors would like to thank T. Birrenbach, W. Böttler, T. Guo,J. Hotovy, W. Reetz, H. Siekmann, D. Wippler, J. Worbs, and W. Zhangfor technical assistance as well as A. Hoffmann, B.E. Pieters and U.Rau for helpful suggestions. The authors express their gratitude tothe partners of the SUNPLAS project for valuable discussions. Wegratefully acknowledge financial support from the German FederalMinistry of Education and Research (grant 03SF0354D).

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