IEEE JOURNAL OF PHOTOVOLTAICS 1

Advances in Light Trapping for HydrogenatedNanocrystalline Silicon Solar Cells

Laura Sivec, Baojie Yan, Guozhen Yue, Jessica Owens-Mawson, Jeffrey Yang, and Subhendu Guha

Abstract—We optimized Ag/ZnO back reflectors (BR) for hy-drogenated nanocrystalline silicon (nc-Si:H) solar cells by inde-pendently changing the textures of the Ag and ZnO layers. Wefound that Ag/ZnO with textured Ag and thin ZnO provides thehighest nc-Si:H solar cell efficiency. Optimized Ag texture with anrms = 40 nm effectively scatters light without seriously degradingthe nc-Si:H material quality. Using this type of BR and nc-Si:Hcells with ∼1-μm-thick intrinsic layer, we obtained a short-circuitcurrent density Jsc = 24.6 mA/cm2 and conversion efficiency Eff

= 9.47%. By increasing the nc-Si:H layer to ∼3.1 μm, we attaineda Jsc>30 mA/cm2 . In order to increase the Jsc further, we in-creased the texture of the ZnO layer. With highly textured Ag/ZnOBRs, the Jsc was increased. However, the high textures caused poorfill factors, and hence, relatively low efficiency. By using nanocrys-talline silicon-oxide (nc-SiOx:H) to replace both the n-layer anddielectric layer, the texture-induced deterioration of nc-Si:H ma-terial quality was suppressed and the cell structure was simplifiedby removing the ZnO, conventional n-layer, n/i buffer layer, andthe seed layer. A high Jsc over 27 mA/cm2 and high-cell efficiencyof 8.8% were attained using a 2.5-μm-thick nc-Si:H cell with annc-SiOx:H n-layer.

Index Terms—Amorphous semiconductors, back reflector (BR),photovoltaic cells, silicon devices, solar energy, thin-film devices.

I. INTRODUCTION

L IGHT trapping is one critical technique to improve energyconversion efficiency of hydrogenated amorphous silicon

(a-Si:H) and nanocrystalline silicon (nc-Si:H) solar cells. Twotypes of solar cell structures are normally used: either a p-i-n structure on transparent superstrates or an n-i-p structure onnontransparent substrates, where n, i, and p represent n-typedoped layer, intrinsic layer, and p-type doped layer, respec-tively. The light trapping in the p-i-n structure occurs mainlyfrom the texture of the transparent conductive oxide layer, suchas SnO2or ZnO. For n-i-p structures, textured back reflectors(BRs) are commonly used to provide light trapping, becausethe top indium–tin–oxide (ITO) layer is designed to have athickness around a quarter of the wavelength, where the solar

Manuscript received May 23, 2012; revised July 20, 2012 and August 22,2012; accepted August 22, 2012. This paper was supported by the U.S. Depart-ment of Defense through U.S. Army under Subcontract W9132T-11-C-0007.

L. Sivec, B. Yan, G. Yue, J. Yang, and S. Guha are with United Solar OvonicLLC, Troy, MI 48084 USA (e-mail: l.sivec@gmail.com; byan@uni-solar.com;gyue@uni-solar.com; jyang@uni-solar.com; sguha@uni-solar.com).

J. Owens-Mawson was with United Solar Ovonic LLC, Troy, MI 48084 USA(e-mail: jowens@uni-solar.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JPHOTOV.2012.2216509

spectrum has the highest energy flux intensity. For the AM1.5solar spectrum, the ITO thickness is around 65–75 nm, whichis too thin to provide sufficient texture for light scattering. Wehave developed a-Si:H and amorphous silicon germanium (a-SiGe:H) solar cells on flexible stainless steel substrates withboth Al/ZnO and Ag/ZnO BRs [1], [2], where the Al/ZnO BRhas been used in solar laminate products and Ag/ZnO in theresearch and development laboratory for high efficiency.

nc-Si:H as a narrow bandgap material in multijunction thin-film silicon solar cells has shown a significant advantage overa-SiGe:H. First, it exhibits little or no light-induced degradation,especially under red light illumination [3], [4]; second, it has amuch higher spectral response than the a-SiGe:H cell in the longwavelength region of the solar spectrum, and produces high-photocurrent density in the bottom cell of multijunction solarcells. The disadvantage of nc-Si:H is related to the well-knownfact that an indirect bandgap in the crystalline phase leads to arequirement of a thick i-layer for high-current density. In orderto keep the nc-Si:H cell in a reasonable thickness such that thesolar cell can be made within a deposition time acceptable formanufacturing and the recombination loss is not a critical limitfor cell efficiency, improved light trapping is more desirablein nc-Si:H than in a-SiGe:H solar cells. The common wisdomfor improving the light trapping is to increase the texture ofthe TCO in p-i-n solar cells or the texture of the BR in n-i-psolar cells. However, the increased substrate texture normallycauses deterioration of nc-Si:H quality, where the microcracksor microvoids are observed in the sharp valleys or peaks [5]–[7].

We optimized Ag/ZnO BRs for high-efficiency nc-Si:H so-lar cells and found that the Ag/ZnO structure with texturedAg and thin ZnO layers gives high-nc-Si:H solar cell perfor-mance [8]–[12]. The hypothesis was that the texture on theAg layer is more effective than the texture on the ZnO layerfor light scattering; therefore, a relatively low Ag texture maybe effective enough to provide sufficient light scattering forhigh-photocurrent density without a noticeable degradation innc-Si:H material quality. Experimentally, we demonstrated thatthis hypothesis is correct, and showed cell efficiency of 10.6%using a single-junction structure [13]. However, the textured Agmight result in plasmonic absorption losses at the Ag/ZnO in-terface, which limits the gain in photocurrent density. From aplasmonic loss point of view, the optimum light trapping shouldbe achieved using a flat Ag layer and a highly textured ZnOlayer. In reality, we have been facing the paradoxical choice ofwhether texturing the Ag layer or the ZnO layer is better.

P-doped hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) was originally developed for an interreflection layerin multijunction solar cells [14]–[16]. Later, it was shown that

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nc-SiOx:H can be doped into both n-type and p-type and used asthe n-layer or p-layer in nc-Si:H solar cells [17]–[20]. It also hasthe advantage of minimizing the texture-induced deteriorationof nc-Si:H quality [19] and simplifying the cell structure [20].

In this paper, we present the studies of optimization ofAg/ZnO BRs by independently changing the Ag and ZnO tex-ture and the effect of a nc-SiOx:H n-layer on an nc-Si:H solarcell performance.

II. EXPERIMENTAL DETAILS

A. Ag/ZnO BR Deposition and Characterization

We used a sputtering method to deposit Ag about 200-nmthick and ZnO about 140-nm thick on flexible stainless steelsubstrates. The Ag and ZnO textures were varied by changingdeposition conditions and film thickness. In order to expand therange of textures, a precoating layer with high texture was de-posited prior to Ag deposition and was used to increase the Agtexture. The precoating layer is a thick-ZnO layer that was chem-ically etched with 5–10% HCl to increase ZnO texture. Atomicforce microscopy (AFM) was used for surface morphology char-acterization and total/diffuse reflection spectra were measuredfor optical characterization.

B. Solar Cell Deposition and Characterization

We deposited nc-Si:H single-junction solar cells using mul-tichamber systems with very high frequency (VHF) glow dis-charge. The nc-Si:H material quality was improved by optimiz-ing the deposition parameters, such as hydrogen dilution andhydrogen dilution profiling [21]. The cell design with properbuffer and seed layers was used to improve the cell perfor-mance [22]. The nc-Si:H i-layer thickness was varied to studythe light-trapping effect in different solar cells. Conventional nand p layers were deposited using radio frequency glow dis-charge, while nc-SiOx :H n layers were deposited using VHFglow discharge [20]. Photocurrent density versus voltage (J–V)characteristics were measured using an AM1.5 solar simulator at25◦C. External quantum efficiency (QE) spectra were measuredunder shortcircuit (0 V) and a reverse bias (–3 V) conditions.The difference between the QE spectra measured under 0 and–3 V was used to evaluate collection losses (QE loss) in thesolar cells.

III. RESULTS AND DISCUSSION

A. Optimization for Ag Texture

Our previous studies showed that an Ag/ZnO BR structurewith a moderately textured Ag layer and a thin (110–140 nm)ZnO layer resulted in optimized nc-Si:H solar cell performancebecause low-Ag texture is effective for light scattering, but doesnot significantly degrade the subsequently deposited nc-Si:Hmaterial [12]. Here, we present the results of structural and op-tical characterizations of Ag/ZnO BRs with various Ag texturesand the same thin ZnO layer. The Ag texture was widely var-ied. Fig. 1 shows AFM images of two BRs, where (a) is with arelatively flat Ag layer and its surface features are mainly from

Fig. 1. Example of AFM images of Ag/ZnO BRs, where (a) has an rms =17.0 nm and (b) has an rms = 118.0 nm.

Fig. 2. Total (solid) and diffuse (dashed) reflection spectra of three Ag/ZnOBRs with rms = 17 nm (black), 40.0 nm (blue), and 69.5 nm (red).

the ZnO layer, and (b) has large features resulting from the Aglayer and small surface features from the ZnO. It shows that wecan effectively change the Ag surface textures in a wide range.

Fig. 2 shows the total and diffuse reflection spectra of threeAg/ZnO BRs with different rms roughness values. First, we ob-served that the total reflectance is reduced with increasing tex-ture. We see two possible mechanisms to explain the reducedtotal reflectance: one is absorption at the Ag/ZnO interface, atwhich localized interface plasmonic absorption could increasewith the increase in Ag roughness, and another mechanism is

SIVEC et al.: ADVANCES IN LIGHT TRAPPING FOR HYDROGENATED NANOCRYSTALLINE SILICON SOLAR CELLS 3

Fig. 3. QE spectra of four nc-Si:H solar cells, where three solar cells (A–C)were deposited on Ag/ZnO BRs with different rms values and one solar cell onflat stainless steel.

light trapping in the ZnO layer because the optical index inZnO is larger than that in air. Second, we observed that diffusereflectance increased significantly with the increase of Ag tex-ture. We made nc-Si:H solar cells with an i-layer thickness of∼1 μm. Fig. 3 shows QE curves from three nc-Si:H solar cellsmade with the same recipe but on different substrates. From thisplot, we observe the following: First, the nc-Si:H cell on thestainless steel substrate has a short-circuit current density Jscof 15.12 mA/cm2 , calculated from the integral of the QE curvewith respect to the AM1.5 spectrum. The nc-Si:H cell (B) on anAg/ZnO BR with an rms = 40 nm has a Jsc = 24.62 mA/cm2 .The gain is 63% in Jsc , which indicates very effective lighttrapping by the Ag/ZnO BR. Second, significant interferencefringes are observed in the nc-Si:H cell (A) on the Ag/ZnO withan rms = 17 nm, indicating a large portion of directly reflectedlight. With the increase of Ag/ZnO rms, the interference fringesare reduced, indicating a reduction of direct reflection. Third,comparing samples (B) and (C), we observe that increasing therms further results in reduced interferences fringes, but does notincrease the QE response, indicating a saturation of photocur-rent gain. We also found that the FF is reduced and the QE lossis increased by the increase of rms, indicating a deteriorationof nc-Si:H quality and cell performance. From the standpointof optimizing the overall cell performance, we conclude theroughness of Ag/ZnO with a textured Ag and thin ZnO shouldnot exceed an rms of 40 nm. With such a BR, we obtained anefficiency Eff of 9.47% in a nc-Si:H single-junction solar cellwith an i-layer thickness of ∼1 μm. The J–V parameters areJsc = 24.62 mA/cm2 , open-circuit voltage Voc = 0.541 V andfill factor FF = 0.711. Using the same BR, we made a thickercell (∼3.1 μm), and attained a Jsc = 30. 47 mA/cm2 , Voc =0.488 V, FF = 0.646 and Eff = 9.61%.

B. Optimization for ZnO Texture

Next, we investigated the effect of ZnO texture on nc-Si:H so-lar cell performance. The points to be considered are 1) whetherwe need textured Ag if the ZnO layer is textured enough toscatter the light, and 2) whether we can achieve a higher pho-

Fig. 4. Example of AFM images of Ag layers, where (a) is the flattest Aglayer with rms = 5 nm, (b) rms = 15–17 nm, and (c) rms = 30–40 nm.

tocurrent density using highly textured ZnO than the optimizedBR with moderately textured Ag and thin ZnO layers (shownin the previous section, and called standard BR in the followingsections). We chose three Ag textures as shown in Fig. 4, wherethe AFM images were taken on the samples of Ag/ZnO BRsafter chemically removing the ZnO layers, which ensure themeasured roughness of Ag is the same as in the Ag/ZnO BRsbecause the deposition of ZnO could change the Ag textures.Sample (A) is the flattest (rms = 5.0 nm) Ag layer that has thesame roughness as the stainless steel substrate, and sample (B)is a low-textured Ag with rms = 15–17 nm, (C) is the texturedAg layer (rms = 30–40 nm) used in our standard Ag/ZnO. Tovary the ZnO texture, we deposited a thick ZnO layer on eachAg layer. Two sets of ZnO layers were made, one set with 1 μmand the other with 2 μm. Then, we etched the ZnO using 5–10%HCl to increase texture.

We first made a set of nc-Si:H solar cells on the BRs withdifferent chemical etching times, where the underneath Ag isthe flattest layer having the same rms (5 nm) as the stainlesssubstrate. We observed the following phenomena. First, the Voc

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Fig. 5. (a) Photocurrent density, (b) QE loss, and (c) Pm ax of nc-Si:H solarcells as a function of rms roughness measured on the ZnO layer. The Ag is theflattest layer with an rms = 5 nm.

does not show a clear decrease with the increase of rms withinthe experimental accuracy. The FF decreases with the increaseof rms. The major focus is on the photocurrent density becauseit reflects the effectiveness of light trapping. Fig. 5(a) plotsthe photocurrent density, obtained from the integral of the QEcurves and the AM1.5 solar spectrum, of the nc-Si:H solar cellsas a function of the rms roughness of the ZnO layer, where theinitial ZnO thickness was 1.0 μm. Two sets of QE curves wereused: One was measured at –3 V and one at 0 V. The data inthe square are from the cell on the Ag/ZnO BR with flat Agand thick ZnO without chemical etching; the data in the oval

Fig. 6. QE spectra measured under –3 and 0 V of three nc-Si:H solar cells.Two cells on Ag/ZnO BRs with a flat (rms = 5 nm) Ag layer and less texturedZnO (rms = 25 nm) and more textured ZnO (rms = 65 nm). As a comparison,one nc-Si:H cell on the standard (STD) BR is included.

are from the cells on the standard BR described in previoussection. Although photocurrent density is increased as the ZnOtexture increases, no matter how highly we textured the ZnOlayer, the measured photocurrent density of the nc-Si:H cellswas lower than that from the nc-Si:H cells made using the samerecipe but on the standard BR (the data in the oval). This resultindicates that 1) the scattering from the Ag layer is needed, and2) although there could be a plasmonic interfacial loss whenthe Ag is textured, such loss is not very significant for low-rmsvalues. Therefore, a moderately textured Ag is still the choiceat the current stage. Fig. 5(b) show the QE loss as a functionof rms, where the QE loss is defined by the relative differencebetween the current density obtained using the integral of theAM1.5 solar spectrum and QE spectra measured under –3 Vbias and 0 V as shown in Fig. 6. The nc-Si:H solar cells showan increased QE loss with the increase of rms roughness. Theenhanced QE loss by the high texture indicates a poor nc-Si:Hmaterial quality, which is consistent with previous observations.From Fig. 6, one may notice that the QE curves of the nc-Si:Hsolar cell on flat Ag and less textured (rms = 25 nm) ZnO havesignificant interference fringes, reflecting insufficient scattering,and the QE curves measured under 0 and –3 V are identical,indicating no QE loss. The nc-Si:H cell on the highly texturedBR has significantly reduced interference fringes, indicating aneffective scattering by the textured ZnO layer. However, theinterference fringes are not completely removed because evenfor fully randomized scattering there is always some light, whichis scattered in a small angle to the normal direction forminginterference fringes with the reflected light from the top surface.For some nc-Si:H solar cells on BRs with very large surfacefeatures, the interference fringes indeed disappear, indicatinga suppressed scattering in the normal direction. Unfortunately,the QE curves of the solar cells on the very textured BR havelower values than those from the cell on the standard BR. Fromthis comparison, we conclude that a strong scattering may be a

SIVEC et al.: ADVANCES IN LIGHT TRAPPING FOR HYDROGENATED NANOCRYSTALLINE SILICON SOLAR CELLS 5

Fig. 7. (a) Current density, (b) QE loss, and (c) Pm ax of nc-Si:H solar cellsas a function of rms of the Ag/ZnO BRs. The BRs have the same Ag layer withan rms = 15–17 nm.

necessary condition for effective light trapping, but it may notresult in enhanced efficiency. Overall, the reduced FF by thetexture and insufficient gain in Jsc result in the solar cells on BRwith flat Ag and very textured ZnO to have lower efficienciesthan the solar cells on the standard Ag/ZnO BRs.

Next, we made another set of nc-Si:H solar cells on Ag/ZnOBRs with a slightly textured (rms = 15-17 nm) Ag layer. In

Fig. 8. Comparison of AFM images of a Ag/ZnO BR before (top, rms =83 nm) and after (bottom, rms = 67 nm) nc-SiOx :H coating.

this set of experiments, we used two ZnO layers of 1 and 2 μmto increase the range of ZnO textures after chemical etching.Trends similar to the previous set are observed in Voc and FFversus the rms values, which implies that the ZnO layer texturesdegrade nc-Si:H material quality no matter if the textures arefrom the underneath Ag layer or the ZnO itself. However, thephotocurrent density continuously increases with ZnO rough-ness as shown in the top plot of Fig. 7(a), where the data inthe oval are from the control samples on the standard BR (asdescribed in the previous section). The increase of QE currentvalues, especially those measured using –3 V bias, indicatesthe texture of the ZnO can increase light trapping and enhancephotocurrent density further. With both the scattering from theAg and ZnO textures, about a 5% gain in photocurrent density,especially measured at –3 V, was obtained as shown in Fig. 7(a).However, the increased rms degraded nc-Si:H material qualityand caused increased QE losses as shown in Fig. 7(b) and re-duced FF. Therefore, the overall cell performance still does notexceed the cells on the standard BR as shown in Fig. 7(c).

In the experiments with a thick ZnO layer, there could bea potential issue of free electron absorption in the ZnO layer,which has been very well identified in Al or Ga doped ZnO.The free-electron absorption could make the interpretation ofFig. 7(a) become complicated because the chemical etchingincreases the texture and reduces the ZnO thickness. Both effectscould increase photocurrent density. However, we used undopedZnO, where the free-electron density is much lower than dopedZnO. In addition, we found the reducing the ZnO thicknessby chemical etching does not always increase the photocurrentnoticeable as shown in Fig. 5(a). We believe the free-electronabsorption is not the dominant factor for light trapping in our

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TABLE IJ–V CHARACTERISTICS OF NC-SI:H SOLAR CELLS ON VARIOUS SUBSTRATES

experiments. However, it could be a critical issue when dopedZnO is used.

We further increased the Ag texture to the level used in thestandard Ag/ZnO BRs (described in the previous section, wherethe rms is in the range of 30–40 nm). Similar results to the caseof slightly textured Ag (rms = 15–17 nm) were obtained, whichmeans that with the highly textured ZnO layer, increasing the Agtexture further does not produce more photocurrent. Comparingall of the results, we conclude that for our current nc-Si:H solarcells, the optimized Ag/BR is constructed with textured Ag andthin (110–140 nm) ZnO layers, where the Ag texture has an rmsaround 30–40 nm. The optimized Ag texture can be reduced toaround 15–17 nm when highly textured ZnO is used. However,the high texture on the ZnO layer causes a poor FF and a largeQE loss in nc-Si:H cells. The overall solar cell efficiency resultsare not improved by the increased ZnO texture as shown inFig. 7(c). To overcome the texture-induced degradation in nc-Si:H solar cells, use of polished a-Si:H to fill the valleys on theBRs was proposed to make an optically textured and physicallyflat surface for nc-Si:H deposition [23]–[25]. However, it hascaused additional absorption loss by the a-Si:H filling layer.

C. Improvement From nc-SiOx :H n-Layer

Hydrogenated nc-SiOx :H has been used as an interreflectionlayer for increasing the top and middle cell current in multi-junction solar cells and as the regular n-layer in single-junctionsolar cells. When it is used as the n-layer in an n-i-p structure,it not only serves as the doped layer for Fermi-level spilt, but itcan also replace the ZnO layer as the dielectric layer for lighttrapping. In addition, the nc-SiOx :H n-layer can serve as theseed layer and simplify the cell structure significantly. Recently,it has been found that the nc-SiOx :H layer can improve the FFfor nc-Si:H solar cells on very textured BRs.

Fig. 8 shows the comparison of AFM images of Ag/ZnOBRs with a 2-μm thick ZnO layer before and after a nc-SiOx :Hn layer coating. It clearly shows that the nc-SiOx :H n layersmoothes the texture of the ZnO layer. The rms is reduced from83 to 67 nm, and the feature shape changes from sharp peaksto rounded peaks. If one believes that the sharp peaks causethe deterioration of nc-Si:H quality, one would expect nc-Si:Hsolar cells on the highly textured BR might give an improvedperformance when an nc-SiOx :H n-layer is used.

SIVEC et al.: ADVANCES IN LIGHT TRAPPING FOR HYDROGENATED NANOCRYSTALLINE SILICON SOLAR CELLS 7

Fig. 9. QE spectra measured under –3 and 0 V of two nc-Si:H solar cells.Sample 7 is a conventional nc-Si:H n-i-p cell deposited on standard Ag/ZnOBR, sample 4 is a nc-Si:H cell directly deposited on very textured Ag with annc-SiOx :H n-layer without a ZnO layer.

We made a set of nc-Si:H solar cells with the i-layer thicknessaround 2.5 μm. As listed in Table I, three kinds of solar cells weremade. First, two conventional n-i-p solar cells were deposited onthe standard BR as the control sample (samples 1 and 7). Onecan see that the reproducibility is reasonably good. Second,one conventional n-i-p solar cell (sample 2) was deposited ona Ag/ZnO BR with textured Ag and thick ZnO (2.5 μm, theoverall rms is 89 nm). One can see that the solar cell on thehighly textured Ag/ZnO BR has much poorer FF and higher QEloss than the cells on the standard BR. This result is consistentwith the observations in the previous section. Third, we made nc-Si:H solar cells directly on very textured Ag with an nc-SiOx :Hn-layer. As discussed earlier, when using an nc-SiOx :H n-layer,we can simplify the cell structure by removing the ZnO layer, theregular n-layer, the n/i buffer layer, and the seed layer. Four cellswith different nc-SiOx :H layer thicknesses were used as listedin Table I. From the solar cell results, we make the followingobservations. First, the nc-Si:H solar cells, which have an nc-SiOx :H n-layer directly on Ag without a ZnO layer, have similarphotocurrent current as the control sample (at –3 V). It meansthat the nc-SiOx :H has the same function as the ZnO layer forlight trapping. The function of the ZnO layer is twofold: (a) Itcan act as a buffer layer to avoid intermixing between silicon andsilver, and b) it moves the plasmonic frequency at the interfaceto a lower wavelength; since the low wavelength photons areabsorbed in the i-layer and do not reach the interface, there is nointerface absorption loss. It appears that the nc-SiOx :H servesboth purposes. Therefore, when a nc-SiOx :H n-layer is used, theZnO layer is not needed anymore. Second, the solar cells withthe nc-SiOx :H n-layer have a FF and Voc similar to or even betterthan that of the control cells. It has the implication that the nc-SiOx :H n-layer indeed prevents the texture-induced degradationin FF and Voc . It has been shown that the nc-SiOx :H layerremoves the defects in the nc-Si:H i-layer [19] nearby the sharpvalleys. Third, the cell performance does not show a significantchange when the nc-SiOx :H n-layer thickness increased from

100 to 300 nm, indicating the absorption in the nc-SiOx :H n-layer is negligible. Fourth, the cell efficiency is similar to orslightly better than the control cells. Fifth, the QE loss in thecells with the nc-SiOx :H n-layer is higher than the control cellsas shown in Table I and Fig. 9. One might argue that the nc-Si:Hi-layer still has a poor quality, which causes a recombinationloss. Because the FF is actually not reduced by the high texture,we speculate the QE loss is caused by something else. Ourprevious studies showed that the long-wavelength loss could becaused by incorporation of n-type doping, such as P, O, or N. Inthis set of solar cells, no a-Si:H buffer layer was used betweenthe nc-SiOx :H n-layer and the nc-Si:H i-layer. We speculate thata small amount of P might get into the i-layer either during thedeposition of the i-layer or diffusion into the i-layer. Furtherstudies with an n/i buffer layer and SIMS analysis will providemore insight.

IV. CONCLUSION

We have carried out three types of studies of light trappingin n-i-p structured nc-Si:H single-junction solar cells on flexi-ble stainless steel substrates. First, we studied the effect of Agtexture on nc-Si:H solar cell performance. Because the textureof Ag is more effective than the texture of ZnO, with moderatetexture such as the rms around 40 nm, optimal light trappingis achieved in nc-Si:H solar cells. Increasing the Ag texturefurther can increase the light scattering, but does not increasethe photocurrent density. It may imply that the light trappinghas reached the practical limit, which is the classical limit withreflection and absorption losses. With such optimized BR, weattained a Jsc>30 mA/cm2 , which is a significant milestone forthin-film silicon solar cells. Second, we studied the effect ofZnO texture. We found that with a flat Ag layer, no matter howmuch the ZnO is textured, the photocurrent of nc-Si:H cells onsuch BR is always lower than the cells on the standard BR witha moderately textured Ag layer and a thin-ZnO layer. It impliesthat the interface plasmonic loss at the Ag/ZnO interface may notbe a serious issue, because the low optical index of ZnO movesthe plasmonic resonance frequency to short wavelengths. Usingtextured Ag and highly textured ZnO can increase the photocur-rent slightly, but it causes a reduction in FF, and therefore cannotimprove the solar cell efficiency. Third, using an nc-SiOx :H n-layer can replace the combination of the ZnO layer, conventionaln-layer, n/i buffer layer, and seed layer. It significantly simplifiesthe cell structure, which can have a remarkable impact on themanufacturing simplicity and cost reduction. In addition, the nc-SiOx :H n-layer smoothes sharp features on the highly texturedBRs and reduces the texture-induced degradation in nc-Si:Hquality and solar cell performance. The increased QE loss in thenc-Si:H cells with an nc-SiOx :H n-layer could be caused by asmall amount of n-type doping in the i-layer, which could bemitigated by optimizing the nc-SiOx :H deposition parametersand cell structure such as adding an interface layer.

ACKNOWLEDGMENT

The authors would like to thank all team members in theAdvanced Technology Group for their dedicated work

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Authors’ photographs and biographies not available at the time of publication.