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Textured surface boron-doped ZnO transparent conductive oxides on polyethyleneterephthalate substrates for Si-based thin film solar cells

Xin-liang Chen ⁎, Quan Lin, Jian Ni, De-kun Zhang, Jian Sun, Ying Zhao, Xin-hua Geng

Institute of Photo-electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, People's Republic of China

Tianjin Key laboratory of Photo-electronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, People's Republic of China

Key laboratory of Opto-electronic Information Science and Technology for Ministry of Education, Nankai University, Tianjin 300071, People's Republic of China

a b s t r a c ta r t i c l e i n f o

Available online 5 May 2011

Keywords:

Flexible substrates

LP-MOCVD

ZnO thin films

Textured surface

Solar cells

Textured surface boron-doped zinc oxide (ZnO:B) thin films were directly grown via low pressure metalorganic chemical vapor deposition (LP-MOCVD) on polyethylene terephthalate (PET) flexible substrates at

low temperatures and high-ef ficiency flexible polymer silicon (Si) based thin film solar cells were obtained.

High purity diethylzinc and water vapors were used as source materials, and diborane was used as an n-type

dopant gas. P-i-n silicon layers were fabricated at ~398 K by plasma enhanced chemical vapor deposition.

These textured surface ZnO:B thin films on PET substrates (PET/ZnO:B) exhibit rough pyramid-like

morphology with high transparencies (T ~80%) and excellent electrical properties (Rs ~ 10 Ω at

d ~1500 nm). Finally, the PET/ZnO:B thin films were applied in flexible p-i-n type silicon thin film solar

cells (device structure: PET/ZnO:B/p-i-n a-Si:H/Al) with a high conversion ef ficiency of 6.32% (short-circuit

current density J SC=10.62 mA/cm2, open-circuit voltage V OC=0.93 V and fill factor=64%).

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Flexible thin film solar cells have recently gained great interest

because of light-weight, low-cost, flexibility, and easy scale-up to

large format for large volume roll-to-roll production [1]. Flexible

transparent conductive oxides (TCOs) are the key part in these thin

film solar cells. The polymer substrate materials used for flexible solar

cell applications include polycarbonate (PC), polyethylene-naphtalate

(PEN), polyimide (PI), polyethylene terephthalate (PET), and so on

[2–6]. PET substrates are relatively cheaper than the other flexible

polymer materials and they have high transmittance in a wide

spectral range.

In the reported work, magnetron sputtering is the main deposition

technique for the ZnO–TCO thin films on PET substrates [7–11]. There

are few reports on growing ZnO thin films on PET substrates via low

pressure metal organic chemical vapor deposition (LP-MOCVD). Using

LP-MOCVD, ZnO thin films with textured surface and some light

scattering can be obtained on glass substrates at low deposition

temperatures [12]. The basic properties of ZnO–TCO thin films used as

front electrodes in p-i-n type Si-based thin film solar cells are high

optical transparency in the required spectral range (λ~400–1200 nm),

high electrical conductivity and their good scattering abilities to

enhance the path of the light inside the solar cells.

In this work, the microstructural, optical and electrical properties of

un-doped and boron-doped ZnO–

TCO thin films on PET substratesprepared using LP-MOCVD tec hnique were investigated.

Textured surface boron-doped ZnO–TCO thin films on PET substrates

(PET/ZnO:B) were fabricated and preliminary results on flexible p-i-n

type silicon thinfilm solar cells (device structure:PET/ZnO:B/p-i-n a-Si:

H/Al) were obtained.

2. Experimental details

ZnO–TCO thin films were deposited by LP-MOCVD technique on PET

flexible substrates with an area of 50 mm×50 mm. The deposition

temperatures during process varied from 383 K to 428 K. Diethylzinc

(DEZn, purity: 99.995%) and water vapors carried by purified Ar gas

(purity: 99.999%) were used as reactant gases, and their temperatures

were kept at 318 K and 333 K, respectively. Diborane (B2H6), 1% diluted

in hydrogen, was used as the doping gas. The working pressure was set

at 270 Pa.The crystallinity of the ZnOfilms was determined using X-ray

diffraction measurement (XRD, Rigaku D/max-2500) with Cu Kα(λ=0.1542 nm) in the θ/2θ mode and the angle 2θ ranged from 10°

to 90°. The surface morphology of ZnO thinfilms was observed by field

emission scanning electron microscope (FE-SEM, JSM-6700) using the

10.0 kV operating voltage. The thicknesses of these thin films were

measured by a step profilometer (AMBIOS-XP2). Sheet resistance was

determined by four-point probe measurements. Carrier concentrations

and electron mobilities were determined by hall measurement (Accent

HL5500 PC) using the van der Pauw configuration. Optical trans-

mittances, both specular and total, were recorded with a double beam

Thin Solid Films 520 (2011) 1263–1267

⁎ Corresponding author at: Institute of Photo-electronic Thin Film Devices and

Technology, Nankai University, No. 94 Weijin Road, Nankai District, Tianjin 300071,

People's Republic of China.

E-mail address: [email protected] (X.L. Chen).

0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2011.04.199

Contents lists available at ScienceDirect

Thin Solid Films

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

http://dx.doi.org/10.1016/j.tsf.2011.04.199



mailto:[email protected]


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spectrometer with an integrating sphere (Varian-Cary 5000). The haze

was calculated by the ratio of diffuse to specular transmittance: Haze=

(Diffuse Transmittance)/(Total Transmittance)×100%.

Single junction p-i-n type hydrogenated amorphous silicon (a-Si:

H) thin film solar cells were prepared in multi-chamber vacuum

system. The silicon layers were deposited by plasma enhanced

chemical vapor deposition (PECVD) with diode-type reactors using

RF excitation frequencies ( f =13.56 MHz) and gas mixtures of silane

(SiH4) and hydrogen (H2). Trimethylboron (TMB) and methane (CH4)were added into process gases to deposit the p-type a-SiC:H layers.

Phosphine (PH3) was used as the doping gas for the n-type a-Si:H layer

deposition. The deposition temperature was set at 398 K for all a-Si:H

layers. The structure of solar cells was glass/TCO [(PET/ZnO:B) or

(glass/SnO2)]/p-a-SiC:H/buffer/i-a-Si:H/n-a-Si:H/Al. Here, the graded

band gap buffer layer at p/i interface was formed by sudden close of the

mass flow controller for the boron source and carbon source during the

deposition of the p-a-SiC:H window layer. Solar cells were characterized

by current–voltage ( J –V ) measurements (Changchun Institute of Optics,

Fine Mechanicsand Physics, Chinese Academy of Sciences,SolarSimulator

System-300 SQ) under 1 sun (AM 1.5, 100 mW/cm2) illumination,

including open-circuit voltage (V OC), fill factor (FF), short-circuit current

density ( J SC), and ef ficiency.

3. Results and discussion

3.1. Temperature-dependent growth of PET/ZnO thin films

Fig. 1 shows the XRD patterns of un-doped ZnO thin films on PET

substrates (PET/ZnO) at different deposition temperatures in the range

from 383 K to 428 K. From the XRD patterns, one can see that ZnO thin

films exhibit the (002) preferred orientation in the range from 383 K to

398 K and the intensity of the (002) peak reaches a relatively higher

value at 398 K. This indicates that the ZnOthinfilms havea c -axishighly

preferred orientation perpendicular to the substrate. When the

deposition temperature is further increased to 423 K, the intensity of

(002) peak decreases significantly and a strong (110) peak appears in

the XRD pattern. However, the intensity of the (110) peak weakens and

intensity of the (100)peakrelativelystrengthenswhen the temperatureis increased to 428 K. The above experimental results of PET/ZnO thin

filmsare similar to those reported for glass/ZnOsamples [12]. The above

microstructural changes of ZnO thin films can be attributed to the

different surface free energies and hence the substrate temperature

activates the ZnOfilm growth from relative lower surface free energy to

higher surface free energy (1.6 J/m2 for (002), 2.0 J/m2 for (110), and

3.4 J/m2 for (100), respectively [13,14]).

The corresponding SEM images of PET/ZnO thin films are shown in

Fig. 2 (a–d). From the SEM images, we can see that the ZnO thin film

prepared at low deposition temperature of 383 K exhibits a smooth

surface with crystal grain size of about 30–80 nm. When the deposition

temperature is increased to 398 K, the surface morphology of the ZnO

thin films is rough with a mixture of some sphere-like and irregular

structures. With the deposition temperature further increasing to 423 Kand 428 K, sphere-like structures disappear and the crystal grain size

increases up to ~300–500 nm with typical pyramid shape structure.

In addition, thin film thicknesses (d) gradually increase from ~560 nm

to ~ 1250 nm as the deposition temperatures increase from 383 K to

428 K. The lowest sheet resistance, Rs~250 Ω, was obtained at the

deposition temperature of 423 K.

Fig. 3 shows theopticaltransmittancespectra in thewavelength range

of 300–1500 nm for the PET/ZnO thin films deposited at various

deposition temperatures. The transmittance spectra show that the ZnO

thin films grown below423 K exhibit a high transmittance of ~80% in the

400–1100 nm range. However, the transmittance drops very sharply in

the UV region due to the onset of fundamentalabsorption; the absorption

edge is about 380 nm. When the deposition temperature is increased to

423 K, the transmittance decreases distinctly in the visible spectra range

resulting from the higher thickness of these ZnO films and the light-

scattering effects associated to their rough surface.

3.2. Boron-doped ZnO thin films on PET substrates (PET/ZnO:B)

Fig. 4 shows SEM images of PET/ZnO:B thin films at light ~ 3 sccm

and heavy ~20 sccm nominal doping levels, respectively. It can be

seen that the lightly doped PET/ZnO:B thin films have large crystal

grain sizes ~300–500 nm, while the crystal grain size is lower for

heavily dopedfilms. It can be speculated that more boron atoms enterinto the interstitial position in the ZnO lattice during the thin film

growth process, and thus disturb the crystal grain nucleation and

growth.

Table 1 shows the electrical properties of typical PET/ZnO:B and

glass/ZnO:Bthinfilmsobtained inthesame depositionprocess at theB2H6

flow rate of ~3 sccm and temperature of 423 K. The glass/ZnO:B thin film

has higher electron mobility (~40.1 cm2/Vs) and lower sheet resistance

(~7.8 Ω) thanthe PET/ZnO:B thinfilm(~22.2 cm2/Vsfor electron mobility

and ~13.44 Ω for sheet resistance, respectively). The resistivity, ρ, can be

calculated by the formula ρ=Rs⋅d. Consequently, the glass/ZnO:B thin

film exhibits relatively lower resistivity ~1.05× 10−3Ωcm than the

PET/ZnO:B thinfilm ~1.81×10−3Ωcm. The reason for a relatively higher

resistivity of PET/ZnO:B thin films than that of glass/ZnO:B sample is

because PET substrates have low stability at a certain deposition

10 20 30 40 50 60 70 80 90

I n t e n s i t y ( a r b . u n i t s )

PET

2 Theta (degree)

T =383 K

( 0 0 2 )

PET/ZnO

T =398 K

PET/ZnO

( 1 1 0 )

PET/ZnO

( 1 0 0 )

( 0 0 2 )

( 1 1 0 )

T =423 K

PET/ZnO

T =428 K

( 1 0 0 )

Fig. 1. XRD patterns of PET/ZnO thin films at different deposition temperatures.

1264 X.L. Chen et al. / Thin Solid Films 520 (2011) 1263–1267



temperature (above 393 K),which influencesthe crystal growth in ZnO:B

thin film deposition. Therefore, we think that glass/ZnO:B thin films havebetter crystal quality than PET/ZnO:B thin films. In addition, it has been

suggested that the low resistivity of ZnO:B thin films is mainly governed

by extrinsic donors on substitutional sites (B3+ in Zn2+ site), when no

native defects are present in the ZnO thin films [15]. Whereas, n-type

conductivity of un-doped ZnO thin films is considered originating from

native defects such as oxygen vacancies and zinc interstitials.

Fig. 5 gives the optical transmittance in the wavelengths of

300–1500 nm for the PET/ZnO:B thin films deposited at 423 K.

The total transmittance shows that the PET/ZnO:B thin films exhibit a

high transmittance of ~80% in the 400–1100 nm range and the hazevalue at 550 nm wavelength of PET/ZnO:B sample is 11.2%. This means

that the textured PET/ZnO:B samples have certain light-scattering

capacity in thevisible spectra range. However, theZnO:B thinfilmshave

relatively lower transmittance in the near infrared region due to the

free-carrier absorption.

3.3. Application of textured surface PET/ZnO:B in a-Si:H thin film solar cells

As wasshownabove, textured surface PET/ZnO:B thinfilmswith low

sheet resistance and high transmittance can be obtained via LP-MOCVD

technique. These textured ZnO:B thin films were used in the fabrication

of flexible p-i-n type silicon thin film solar cells (device structure:

PET/ZnO:B/p-a-SiC:H/buffer/i-a-Si:H/n-a-Si:H/Al) with an area of

0.25 cm2

(S =5 mm× 5 mm). P-i-n type a-Si:H thin film solar cellswere deposited by RF-PECVD process at a low substrate temperature of

398 K, which is compatible with low-cost PET plastic substrates. Wide

band gap (E opt N1.88 eV) intrinsic a-Si:H films were achieved before the

onset of the microcrystalline regime by changing the hydrogen dilution

ratios. The structural, optical and electrical properties of p-type p-a-SiC:

H window layers have been optimized at 398 K.

It hasbeen previouslymentioned thatglass/ZnO:B thinfilms present

equivalent performances to Asahi U-type glass/SnO2 thin films [15]. As

shown in Fig. 6, a-Si:H thinfilm solar cells on PET/ZnO:B and glass/SnO2substrates show ef ficiencies of 6.32% ( J SC=10.62 mA/cm2, V OC=0.93 V

and FF=64%) and 7.50% ( J SC= 11.60 mA/cm2, V OC=0.97 V and

FF=67%), respectively. Generally, due to the higher contact potential

of ZnO/p a-SiC:H compared with SnO2/p a-SiC:H, a-Si:H solar cells

prepared on ZnO suffer from reduced FF and Voc [16]. Series resistances

Fig. 2. SEM images of PET/ZnO thin films at different deposition temperatures: (a) 383 K, Rs ~10 k Ω at d ~560 nm, (b) 398 K, Rs ~2 k Ω at d ~720 nm, (c) 423 K, Rs ~250 Ω at

d ~1080 nm, and (d) 428 K, Rs ~300 Ω at d ~1250 nm.

300 400 500 600 700 800 900 100011001200130014001500

0

10

20

30

40

50

6070

80

90

100

T r a n s m i t t a n c e ( %

)

Wavelength (nm)

PET

383 K

403 K

423 K

Fig. 3. Transmittance spectra of PET/ZnO thin films at different deposition

temperatures.


http://localhost/var/www/apps/conversion/tmp/scratch_4/image%20of%20Fig.%E0%B3%80




































of a-Si:H thin film solar cells calculated from the reciprocal slope of the

J –

V curve at the Voc point are 12.2 Ω and 12.8 Ω on glass/SnO2 andPET/ZnO:B, respectively. Such a little change in series resistance should

not have a significant impact on the fill factors of a-Si:H solar cells.

Consequently, the reduction of shunt resistance from the deteriorated

p–n junctioncharacteristics of solar cells on PET/ZnO:B substrates could

be the main reason for the decrease of FF values. In addition, it can be

seen from J –V curves that a-Si:H thin film solar cell on PET/ZnO:B

substrate shows relatively lower J SC value resulting from lower

transmittance of PET substrate.

4. Conclusions

In summary, the surface morphology of PET/ZnO via low pressure

metal organic chemical vapor deposition (LP-MOCVD) on PET flexiblesubstrates depends strongly on the deposition temperatures. Heavy

boron-doping in PET/ZnO:B thin films deteriorates the crystal grain

nucleation and growth. PET/ZnO:B thinfilmswith textured surface, high

transmittance (T ~80%) and low sheet resistance (Rs~ 10 Ω) were

directly grown at low temperature 423 K. A flexible a-Si:H thin film

solar cell ef ficiency of 6.32% with the J SC=10.62 mA/cm2, V OC=0.93 V

and FF =64% was obtained on this PET/ZnO:B substrate.

Acknowledgment

This work describedin this paper is supported by Tianjin AppliedBasic

Research Project and Cutting-edge Technology Research Plan

(No. 09JCYBJC06900), the State Key Development Program for Basic

Research of China (Nos. 2011CB201605, 2011CB201606 and 2011CB20

1607), the Fundamental Research Funds for the Central Universities(No. 65010341), and the International Cooperation Project between

China–Greece Government (No. 2009DFA62580).

References

[1] Y.-S. Park, H.-K. Kim, S.-W. Jeong, W.-J. Cho, Thin Solid Films 518 (2010) 3071.[2] L. Gong, J. L., Z.Z. Ye, Sol. Energy Mater. Sol. Cells 94 (2010) 937.[3] T. Soderstrom, F.-J. Haug, X. Niquille,C. Ballif, Prog. Photovolt:Res.Appl.17 (2009)

165.[4] H. Zhang, C. Lei, H. Liu, C. Yuan, Appl. Sur. Sci. 255 (2009) 6054.[5] K. Tao, D.X. Zhang, L.S. Wang, J.F. Zhao, H.K. Cai, Y.P. Sui, Z.X. Qiao, Q. He, Y. Zhang,

Y. Sun, Sol. Energy Mater. Sol. Cells 94 (2010) 709.[6] J.-M. Kim, P. Thiyagarajan, S.-W. Rhee, Thin Solid Films 518 (2010) 5860.[7] A.N. Banerjee, C.K. Ghosh, K.K. Chattopadhyay, H. Minoura, A.K. Sarkar, A. Akiba, A.

Kamiya, T. Endo, Thin Solid Films 496 (2006) 112.[8] B.-G. Kim, J.-Y. Kim, S.-J. Lee, J.-H. Park, D.-G. Lim, M.-G. Park, Appl. Surf. Sci. 257

(2010) 1063.

Fig. 4. SEM images of PET/ZnO:B thin films at (a) light doping level ~3 sccm, Rs~ 10 Ω;

and (b) heavy doping level ~20 sccm, Rs ~20 Ω. The deposition temperature was set at

423 K and the thin film thickness (d) is ~1500 nm.

Table 1

Typical electrical properties of PET/ZnO:B and glass/ZnO:B thin films prepared at 423 K

with a nominal doping level corresponding to ~3 sccm.

d/nm Rs/Ω Ns/cm−2 μ / cm2 V −1 s−1

PET/ZnO:B ~1350 13.44 2.093E16 22.2

Glass/ZnO:B ~1350 7.80 1.977E16 40.1

300 400 500 600 700 800 900 100011001200130014001500

0

10

20

30

40

50

60

70

80

90

100

Haze550nm=11.2% T r a n

s m i t t a n c e ( % )

Wavelength (nm)

Direct Transmittance

Total Transmittance

Diffuse Transmittance

PET

Fig. 5. Optical transmittance spectra of the PET/ZnO:B thin films deposited at a typical

nominal doping level ~3 sccm and temperature of 423 K.

0.0 0.2 0.4 0.6 0.8 1.0-8

-6

-4

-2

0

2

4

6

8

1012

14

i layer thickness = 360 nm

J s c ( m A / c m

2 )

U-Type SnO2

Jsc = 11.60 mA/cm2

Voc = 0.97 V

FF = 0.67

Efficiency = 7.50 %

PET/ZnO

Jsc = 10.62 mA/cm2

Voc = 0.93 V

FF = 0.64

Efficiency = 6.32 %

Voltage (V)

Glass/U-Type SnO2

PET/ZnO:B

Fig. 6. J –V curves of a-Si:H thin film solar cells deposited on glass/SnO2 and PET/ZnO:B

substrates prepared at nominal typical doping level ~3 sccm and temperature of 423 K.









































[9] Lee, D. Lee, D. Lim, K. Yang, Thin Solid Films 515 (2007) 6094.[10] Z.L. Pei, X.B. Zhang, G.P. Zhang, J. Gong, C. Sun, R.F. Huang, L.S. Wen, Thin Solid

Films 497 (2006) 20.[11] S. Fernández, A. Martínez-Steele, J.J.Gandía,F.B. Naranjo,ThinSolid Films 517(2009)

3152.[12] X.L. Chen, X.H. Geng, J.M. Xue, D.K. Zhang, G.F. Hou, Y. Zhao, J. Cryst. Growth 296

(2006) 43.

[13] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 (1993) 269.[14] X. Jiang, C.L. Jia, B. Szyszka, Appl. Phys. Lett. 80 (2002) 3090.[15] X.L. Chen, J.M. Xue, J. Sun, Y. Zhao, X.H. Geng, Chin. J. Semicond. 28 (2007) 1072.[16] J.C. Lee, V. Dutta, J. Yoo, J. Yi, J. Song, K.H. Yoon, Superlattices Microstruct. 42

(2007) 369.


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