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Solar Energy Materials & Solar Cells 94 (2010) 709–714

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

E-m

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

Development of textured back reflector for n–i–p flexible silicon thin filmsolar cells

Ke Tao a,�, Dexian Zhang b, Linshen Wang b, Jingfang Zhao b, Hongkun Cai b, Yanping Sui b,Zaixiang Qiao b, Qing He a, Yi Zhang a, Yun Sun a

a Institute of Photo-electronics Thin Film Devices and Technique, Nankai University, PR Chinab Department of Electronic Science and Technology, Nankai University, PR China

a r t i c l e i n f o

Article history:

Received 17 September 2009

Accepted 3 November 2009Available online 15 January 2010

Keywords:

Light trapping

Flexible substrate

Zno:Ga

Solar cells

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.solmat.2009.11.001

esponding author. Tel.: +86 1382 0691 521.

ail address: 0110096@mail.nankai.edu.cn (K.

a b s t r a c t

For silicon thin film solar cells, light trapping strategies, to increase the path length of incoming light,

play a decisive role for device performance. In this work, a new way to develop textured back reflectors

for n–i–p flexible solar cells is studied. ZnO:Ga films are deposited by DC magnetron sputtering system

using two facing Ga-doped ZnO ceramic targets at room temperature. The influence of distance

between substrate and plasma on the structure and etching characteristic of ZnO:Ga is investigated.

Polyimide/textured ZnO:Ga/Al structure is used as the back reflector, and the morphological and optical

analyses indicate that it is suitable to be used in the silicon thin film solar cells.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The use of flexible plastic substrates is becoming an issue ofgreat interest in thin film silicon solar cells technology, as theyallow cost reduction in the production process by using roll-to-roll deposition systems and facilitate monolithic interconnectionof the cells to produce a photovoltaic module [1,2]. Furthermore,those lightweight, reduced fragility and flexibility of the modulescan reduce the storage and transportation costs. Plastic substratesare also used in organic solar cells.

The conversion efficiency of hydrogenated amorphous silicon(a-Si:H) and hydrogenated microcrystalline silicon (mc-Si:H) solarcells can be significantly improved by enhancing the lightabsorption in the active layers. For silicon based thin filmtechnologies, one must attach importance to the role of lighttrapping strategies that allow enhanced light absorption inthinner active layers. Light trapping can be achieved by using atextured front contacts (TCO) or/and back reflectors based on TCOand metal layers (Ag or Al). Textured interface can be fabricatedusing either a textured substrate (prepared by sandblast process[3] or hot-embossing lithography (HEL) [4,5]), or a transparentconductive oxide (TCO), which has a roughness surface [6–10]. Atpresent, ZnO and SnO2 are the main TCO materials that are widelyused in the field of a-Si:H and mc-Si:H solar cells.

Several organic polymer films have been used as substrate, likepolyimide (PI), polyethylene terephtalate (PET), polyethylene

ll rights reserved.

Tao).

naphtalate (PEN), etc. while, the melting point and glass transitiontemperature of PI are higher than those of PET and PEN. Thethermal shrinkage of PI is also smaller than that of PET and PEN.Therefore, PI is used as the substrate. However, because of poorlight transmission, like stainless steel foil, n–i–p structures areused to fabricate silicon-based solar cells.

In the present work, in order to enhance the light confinementof the n–i–p solar cells, a preliminary technological step wasdeveloped focusing, on one hand, on the deposition of Ga-dopedZnO (ZnO:Ga) layers on top of PI. On the other hand, the design oflight trapping using PI as substrates will be roughness controlledso that a future use of PI/textured ZnO:Ga/metal/TCO/n–i–p/TCOstructured solar cells can be realized. This technology can transfersuperficial features to the back reflectors, and be compatible toroll-to-roll systems. Thus, the capability to produce repeatablefeatures over a large area makes it a useful technique. In thispaper, the influence of distance between substrate and plasma onthe structure and etching characteristic of ZnO:Ga was investi-gated. The morphological and optical analyses were carried out totest the controllability and the conservation of the texture withthe subsequent depositions.

2. Experimental

The ZnO:Ga films were deposited by DC magnetron sputteringsystem using two facing Ga-doped ZnO ceramic targets in anargon atmosphere at room temperature on PI and Corning 7059glass on the same run [11]. Each target was sintered from zincoxide (99.99%) and gallium oxide (99.99%) mixture with gallium

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Fig. 1. Schematic drawing for the facing targets, the solid arrow represents the

moving direction of substrates.

7 14 21 28 35

32

36

40

44

48

depo

sitio

n ra

te (n

m/m

in)

distance between substrate and plasma (mm)

Fig. 2. The deposition rate dependence of the distance between substrate and

plasma. The line is drawn to guide the eye.

K. Tao et al. / Solar Energy Materials & Solar Cells 94 (2010) 709–714710

content of 2 wt%. The substrate was placed outside the plasma toreduce the effect of bombardment of the plasma on the surface ofthin films, thus, high quality ZnO:Ga thin films can be obtained.The schematic drawing for the ZnO:Ga deposition system wasshown in Fig. 1. When the sputtering chamber was evacuated to abase pressure of 3�10�3 Pa, the controlled flux of pure argon(99.999%) was introduced into the chamber to act as sputter gas.The DC sputtering current applied on both targets was kept at400 mA, while, the gas pressure was fixed at 0.6 Pa, and thedistance between substrate and plasma (DBSP) changed from 35to 7 mm. The thickness of all the samples was controlled to about900 nm by adjusting the sputtering time. After deposition theinitially smooth films were etched in diluted hydrochloric acid(0.5% HCl). The etching time was adjusted to obtain ZnO filmswith different roughness.

The thickness of films was measured using a surface profiler(AMBIOS XP-2), thereafter the deposition rate as well as the etchrate can be obtained. The micro-structure of the ZnO films wascharacterized by X-ray diffractometer (XRD, Philips PANalyticalX’Pert, CuKa). The surface morphologies were observed usingatomic force microscopy (AFM). The optical reflectance spectra ofPI/ZnO:Ga (chemical etched)/metal/air system were measured bythe UV–vis–IR spectrophotometer (VAR-IAN CARY5000),equipped with an integrating sphere at wavelengths rangingfrom 200 to 2000 nm. The haze parameter, which is the ratio ofthe scattered part of the reflected light to the total reflected light,was used to describe the scattering properties of the roughinterfaces [12].

3. Results and discussion

3.1. ZnO:Ga deposited on PI

The deposition rate of ZnO:Ga films on glasses as a function ofDBSP was shown in Fig. 2. The deposition rate increases almostlinearly from 32.80 to 49.38 nm/min when the DBSP decreasesfrom 35 to 7 mm. This increase indicates that the number ofsputtered particles that arrived at the substrate is proportional tothe DBSP. As described in Fig. 1, when the substrate moves closeto the plasma, on the one hand, more and more sputteredparticles can arrive at the substrate, which can contribute to theincrease of the deposition rate. On the other hand, the sputteredparticles would have a small number of collisions with gasmolecules on their way from the targets to the substrate. Thus,the lost energy of the sputtered particles diminishes. It is alsoexpected that as the DBSP decreases, the effect of energeticelectron bombardment on the growing film will increase,providing in the form of thermal energy. At the same time, asthe DBSP decreases, the uniformity of the ZnO:Ga films would be

improved largely because more and more particles can arrive atthe substrate without diffraction. Fig. 3 has shown the XRDspectrum for ZnO:Ga films deposited on PI with different DBSPand there were only (0 0 2) and (0 0 4) diffraction peaks observed,which indicates that all of the obtained films are polycrystallinewith the hexagonal wurtzite structure and have a preferredorientation with the c-axis perpendicular to the substrates.Neither metallic zinc or gallium characteristic peaks nor galliumoxide peak is observed from the XRD patterns, which indicatesthat gallium atoms replace zinc in the hexagonal lattice or galliumatoms segregate to the noncrystalline region in grain boundary.On decreasing the DBSP, the intensities of the (0 0 2) peakincrease, which means that the crystallinity of the ZnO:Ga filmsimprove [13]. It is noteworthy that the intensity of (0 0 2) peakdecreases when the DBSP is less than 28 mm. It is mainly due tothe very high deposition rate that causes a degradation of thepreferred orientation.

From Fig. 4, it can be seen that the XRD diffraction 2y angle ofZnO:Ga films with (0 0 2) orientation as a function of the DBSP. Itis observed that the peak positions of all samples occur at angleslower than the JCPDS value of 34.421, and the interplanar spacingof all samples are larger than the JCPDS value of 2.603 A. As DBSPdecreases gradually, the peak position shifts from 34.061 to34.271, and the interplanar spacing decreases from 2.633 to2.617 A. This peak shift with respect to the bulk value has beenobserved by a number of authors [14–17]. Kappertz et al.attributed the lower diffraction angles shift to the stresses thatoriginated from the implantation of particles sputtered fromtarget into the growing films. However, their explanation cannotbe used to explain what was observed in this work. Given theother parameters unchangeable, and just decreasing the DBSPvalue, the energy of the sputtered particles enhance when theyarrive at the growing surface of the films, according to discussionabove. Thus, the bombardment of the energy particles on thesurface of the films increases, other than reduce. Gupta et al.observed that sputtered ZnO:Al films are in a uniform state ofstress with tensile components parallel to c-axis that would resultin the lower diffraction angles shift. Chen et al. observed that thestress can be reduced by heating the substrate, and attributed thestress to the structural defects that generated during deposition atlow temperature. In this work, according to the discussion above,the deviation of the diffraction peak position can be attributed tothe existence of tensile stress parallel to c-axis. In other words,

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20 40 60 80 100

0

60000

120000

180000

240000

Inte

nsity

(a.u

.)

2θ (degree)

7mm

14mm

21mm

28mm35mm

(002)

(004)

Fig. 3. The XRD measurement of ZnO:Ga films deposited on PI with different distance between substrate and plasma.

0 7 14 21 28 35 42

34.05

34.10

34.15

34.20

34.25

34.30

2.615

2.620

2.625

2.630

2.635

2θ (d

egre

e)

Distance between substrate and plasma (mm)

Inte

rpla

nar s

paci

ng (1

0-1nm

)

Fig. 4. Interplanar spacing and peak position determined from (0 0 2) diffraction peak in Fig. 3 as a function of the distance between substrate and plasma. The lines are

drawn to guide the eye.

K. Tao et al. / Solar Energy Materials & Solar Cells 94 (2010) 709–714 711

with decreasing the DBSP value a shift of the (0 0 2) peak positionto higher angles indicates decreasing stress. This can be explainedin terms of the increased effect of energetic electronbombardment, providing in form of thermal energy.

Keeping the DBSP 35 mm unchangeable, Ga doped ZnO filmswere deposited on glasses and etched by diluted HCl (0.5%) fordifferent time. Fig. 5 has shown the AFM images of the ZnO:Gafilms etched for (a) 15 s, (b) 20 s, (c) 25 s and (d) 30 s, respectively.It can be seen that these ZnO:Ga films developed crater-likestructure during the etching step, but the size of the craterschanges with the etching time. With the increase of etching timefrom 15 to 30 s, the RMS roughness (drms) is enhancedmonotonously from 34.32 to 46.68 nm. Fig. 6 has shown thesurface morphology of ZnO:Ga films etched for 30 s, which wereprepared with various DBSP: (a) 35 mm and (b) 14 mm. With thesame deposition parameter, sample with DBSP at 14 mm has alarger RMS roughness than that of sample with DBSP at 35 mm,and the size of craters is up to 1.2mm for DBSP at 14 mm compareto 0.6mm for DBSP at 35 mm. This suggests a different resistance

for ZnO samples prepared with different DBSP against etching.Probably, when the DBSP was smaller, the precursors arrived atthe surface of substrate with a higher energy. The increasedimpinging effect leads to a more compact and denser structure.

3.2. Back reflectors

Usually, the back reflectors of n–i–p structure solar cells areconstituted by two layers (Al or Ag and doped ZnO). However, ifthe electrode materials are deposited directly on the substrate,the surface morphology of the back reflectors are smooth, like theFig. 7(a) and (c), and the light scattering effect is very weak.According to the discussion in Section 3.1, a new way to enhancethe scattering effect of reflectors for n–i–p flexible silicon basedsolar cells was developed using polyimide/textured ZnO:Ga/Al/ZnO:Ga structure. Fig. 7(b) has shown the surface morphology ofPI/textured ZnO:Ga, where the ZnO:Ga films are prepared withDBSP at 14 mm and etched for 30 s. After this process, the RMS

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Fig. 5. AFM image of ZnO:Ga films deposited on glass substrates after etching in 0.5% HCl for (a) 15 s, drms=34.32 nm, (b) 20 s, drms=39.93 nm, (c) 25 s, drms=44.80 nm and

(d) 30 s, drms=46.68 nm, respectively.

Fig. 6. AFM 10�10mm2 plots of the surface morphology of ZnO:Ga films prepared on glass with various DBSP: (a) 35 mm, drms=46.68 nm and (b) 14 mm, drms=75.43 nm,

respectively, both etched for 30 s.

K. Tao et al. / Solar Energy Materials & Solar Cells 94 (2010) 709–714712

roughness of PI/ZnO is enhanced drastically from 2.45 to56.27 nm. However, the RMS roughness of PI/ZnO is a littlelower than that for glass/ZnO. It is probably due to the moredefects thus less compactness of ZnO on PI. When an Al layerabout 500 nm was deposited on the surface of PI/ZnO, the RMSroughness decreased slightly to 51.21 nm, as shown in Fig. 7(d),and it is clear to see that the films tend to grow along the primarysurface morphology, which was also discussed by authors [4,18].Therefore, it could be concluded that the deposition of an Alreflector on the top of the PI/textured ZnO:Ga do not affectsignificantly the roughness of the whole structure.

Fig. 8 had shown the scattering reflectance of PI/etched-ZnO/Alas a function of the etching time, respectively. It can be seen thatthe scattering reflectance of the back reflector was drasticallyenhanced by introducing a textured ZnO:Ga film, however, therewas little scattering reflectance when a smooth ZnO:Ga films wasintroduced. Furthermore, the light scattering effect of thereflectors increased when the etching time was prolonged,especially for the spectrum in the range from 400 to 1000 nm.The haze parameter of PI/textured ZnO/Al/air system was plottedin Fig. 9. The figure shown that with the increase of wavelength oflight, the haze decreased. Higher haze at shorter wavelengths

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Fig. 7. AFM 10�10mm2 plots of the surface morphology for (a) PI/ZnO, drms=2.45 nm, (b) PI/textured ZnO, drms=56.27 nm, (c) PI/Al, drms=5.76 nm and (d) PI/textured ZnO/

Al, drms=51.21 nm.

400 600 800 1000 1200 1400 1600 1800 2000

0

10

20

30

40

50

60

Sca

tterin

g R

efle

ctan

ce (%

)

Wavelength (nm)

Glass/smooth ZnO/Al PI-20secPI-30sec

PI-40secPI-50sec

Fig. 8. The scattering reflectance of PI/etched-ZnO/Al at different etching time. For

compare, the scattering reflectance of glass/smooth ZnO/Al is also included in the

figure.

400 600 800 1000 1200 1400 1600 1800 2000

0.0

0.2

0.4

0.6

0.8

1.0

PI-20secPI-30secPI-40secPI-50secGlass/smooth ZnO/Al

Haz

e (%

)

wavelength (nm)

Fig. 9. Wavelength-dependent haze parameter of PI/etched-ZnO/Al/air system for

different etching time. For compare, the haze of glass/smooth ZnO/Al/air system is

also included in the figure.

K. Tao et al. / Solar Energy Materials & Solar Cells 94 (2010) 709–714 713

indicated that the short-wavelength light scattered moreefficiently since the wavelength of the light in the layersbecame comparable to the interface roughness. It also can beseen from the figure that the haze was enhanced when theetching time increased for the whole wavelength range, whichwas consistent with the change of scattering reflectance.

Results presented above suggested that the new way fordesigning back reflector on polyimide substrate is feasible for

silicon based thin film solar cells. Depending on the structuralproperties and the etching time, the surface roughness of thereflector can be tuned over a wide range. This way for preparingtextured back reflector is effectual for other substrates with poorlight transmittance. Further work needs to be done to identify theoptimal roughness for the flexible amorphous and microcrystal-line silicon thin film solar cells.

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K. Tao et al. / Solar Energy Materials & Solar Cells 94 (2010) 709–714714

4. Conclusion

In this paper, a new way to design textured back reflector on PIsubstrates for silicon based solar cells has been studied. ZnO:Gafilms have been deposited by DC magnetron sputtering systemusing two facing Ga-doped ZnO ceramic targets at roomtemperature. The influence of the distance between substrateand plasma on the structure and the etching characteristic ofZnO:Ga films was investigated. With decreasing the DBSP, thestress in the film was reduced, and the structure of ZnO:Ga turnsto be more compact and denser. The surface morphology andoptical reflectance measurements indicated that the PI/texturedZnO:Ga/Al structure is suitable to be used as back reflector in solarcells. Moreover, the way for developing textured back reflectorcan be promoted to other plastic substrate.

Acknowledgements

The authors are grateful to Changjian Li, Weiyu He and Bo Liufor helpful discussions and AFM measurements. Also, the authorshighly appreciate China National high-tech research developmentplan (2006AA05Z422), China Tianjin applied basic researchprojects and cutting-edge technology (08JCYBJC13100) and ChinaTianjin Science and Technology research plan (06YFGPGX08000).

References

[1] Yukimi Ichikawa, Takashi Yoshida, Toshio Hama, Hiroshi Sakai, KouichiHarashima, Sol. Energy Mater. Sol. Cells 66 (2001) 107.

[2] Masat Izu, Tim Ellison, Sol. Energy Mater. Sol. Cells 78 (2003) 613.[3] H. Taniguchi, H. Sannomiya, K. Kajiwara, K. Nomoto, Y. Yamamoto, K. Hiyoshi,

H. Kumada, M. Murakami, Sol. Energy Mater. Sol. Cells 49 (1997) 101.[4] M. Fonrodona, J. Escarre, F. Villar, D. Soler, J.M. Asensi, J. Bertomeu, J. Andreu,

Sol. Energy Mater. Sol. Cells 89 (2005) 37.[5] J. Escarre, F. Villar, M. Fonrodona, Sol. Energy Mater. Sol. Cells 87 (2005) 333.[6] J. Muller, B. Rech, J. Springer, M. Vanecek, Sol. Energy 77 (2004) 917.[7] S. Fay, L. Feithnecht, R. Schluchter, U. Kroll, Sol. Energy Mater. Sol. Cells 90

(2006) 2960.[8] J. Hupkes, B. Rech, O. Kluth, T. Repmann, B. Zwaygardt, J. Muller, R. Drese, M.

Wuttig, Sol. Energy Mater. Sol. Cells 90 (2006) 3054.[9] T. Tohsophon, J. Hupkes, H. Siekmann, B. Rech, M. Schultheis, N. Sirikulrat,

Thin Solid Films 516 (2008) 4628.[10] Sung Ju Tark, Min Gu Kang, Sungeun Park, Ji Hoon Jang, Jeong Chul Lee, Mon

Wok Kim, Joon Sung Lee, Konghwan Kim, Curr. Appl. Phys. 9 (2009) 1318.[11] Wei Li, Yun Sun, Yaxin Wang, Hongkun Cai, Fangfang Liu, Qing He, Sol. Energy

Mater. Sol. Cells 91 (2007) 659.[12] J. Krc, M. Zeman, F. Smole, M. Topic, J. Appl. Phys. 92 (2002) 749.[13] Jaehyeong Lee, Dongjin Lee, Donggun Lim, Keajoon Yang, Thin Solid Films 515

(2007) 6094.[14] T. Tsuji, M. Hirohashi, Appl. Surf. Sci. 157 (2000) 47.[15] V. Gupta, A. Mansingh, J. Appl. Phys. 80 (1996) 1063.[16] O. Kappertz, R. Drese, M. Wuttig, J. Vac. Sci. Technol. A 20 (2002) 2084.[17] M. Chen, Z.L. Pei, X. Wang, C. Sun, L.S. Wen, J. Vac. Sci. Technol. A 19 (2001)

963.[18] J. Hupkes, S.E. Pust, W. Bottler, A. Gordijn, N. Wyrsch, D. Guttler, A.N. Tiwari,

I. Gordon, Y. Qiu, in: Proceeding of 24th EU PVSEC, Hamburg, Germany,September 21-25, 2009, p. 2766.