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Vacuum 86 (2012) 1477e1481

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Vacuum

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Low-temperature preparation of flexible a-Si:H solar cells with hydrogenatednanocrystalline silicon p layer

Ke Tao a,*, Jin Wang b, Hongkun Cai b, Dexian Zhang b, Yanpin Sui b, Yi Zhang a, Yun Sun a

a Institute of Photo-electronics Thin Film Devices and Technique, Nankai University, PR ChinabCollege of Information Technical Science, Nankai University, PR China

a r t i c l e i n f o

Article history:Received 2 August 2011Received in revised form14 January 2012Accepted 16 January 2012

Keywords:Solar cellsp-nc-Si:HAmorphous siliconPENLow temperature

* Corresponding author.E-mail address: patricktao2001@sina.com (K. Tao)

0042-207X/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.vacuum.2012.01.011

a b s t r a c t

In this paper we report on flexible a-Si:H solar cells prepared on polyethylene naphthalate (PEN)substrates using p-type hydrogenated nanocrystalline silicon thin films (p-nc-Si:H) as the window layer.The p-nc-Si:H films were prepared at low temperature (150 �C) using trimethylboron (TMB) as a dopantgas. The influence of the silane concentration (SC) on the electrical and structural properties of ultra-thinp-nc-Si:H as well as the performance of solar cells on PEN was investigated. The results show that thecrystalline fraction and conductivity of p-nc-Si:H thin films diminished, while the deposition rate andRMS roughness of films increased, when the SC increases from 0.53% to 0.8%. For the a-Si:H solar cells onPEN with the non-textured electrodes, the best efficiency of 6.3% was achieved with the p-nc-Si:H thinfilms deposited at SC ¼ 0.67%.

� 2012 Elsevier Ltd. 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-rolldeposition systems and facilitate monolithic interconnection of thecells to produce a photovoltaic module [1,2]. Furthermore, thelightweight, reduced fragility, and flexibility of the modules canreduce the storage and transportation costs. Plastic substrates arealso used in organic solar cells. Several organic polymer films havebeen used as the substrate, like polyimide (PI) [3], polyethyleneterephthalate (PET) [4], polyethylene naphthalate (PEN) [5], andethylene-tetracyclododecene co-polymer (E/TD) [6]. PEN is a semi-crystalline, thermoplastic polyester material, with a Tglass w125 �C,but a working temperature up to 155 �C [7]. It has a high trans-parency, which makes it a feasible candidate for both substrate andsuperstrate structures, and is chemically resistant to most diluteacids and organic solvents.

Extensive research has been carried out to improve the efficiencyof peien structure amorphous silicon solar cells. It has beenreported that p-side illuminated cells have higher efficiency thann-side illuminated cells due to improved collection of holes in theformer structure. It is therefore essential to improve the quality of

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the p-type window layer to obtain high performance solar cells.p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) is a two-phase-mixed materials where silicon nanocrystals are embeddedin an amorphous siliconmatrix [8]. p-nc-Si:Hfilms, whose electricalproperties have been extensively studied [9,10], present an attrac-tive alternative in applications to optoelectronic devices. Comparedwith p-a-Si:H, they have greater doping efficiency, higher electricalconductivity, highermobility and lower light absorption coefficient.

In this paper, fabrication of ultra-thin p-nc-Si:H thin films usingRF-PECVD at different silicon concentrations is described and theinfluence of silicon concentration (SC ¼ SiH4/(SiH4 þ H2)) on themicro-structure and optoelectronic properties of p-nc-Si:H thinfilms is investigated. Also, the photovoltaic characteristics of flex-ible peien structure solar cells on PEN incorporated with p-nc-Si:Hthin film that deposited at different silicon concentrations havebeen studied.

2. Experimental

A serial of p-nc-Si:H thin films were deposited on Eagle2000glass substrates by RF (13.56 MHz) plasma-enhanced chemicalvapor depositionwith different silane concentrations. Silane (SiH4),hydrogen (H2) and 1% trimethylboron (B(CH3)3,TMB) were used asthe source gases. The TMB concentration in silane was 0.5% in gasphase. The substrate temperature Tsub, gas pressure and powerdensity were kept at 150 �C, 200 Pa and 350 mW/cm2. The silane

Table 1The deposition conditions of each layer in the fabricated a-Si:H solar cells. The concentration of silane used in each layer is 20% diluted in hydrogen.

Parameters p layer Buffer i layer n layer

nc-Si:H a-SiC:H a-Si:H nc-Si:H

Tsub (�C) 150 150 150 150Flow rate (sccm) SiH4þH2 ¼ 120, SC: 0.53%e1.0% SiH4:CH4:H2 ¼ 20:2.6:20 SiH4:H2 ¼ 20:30 SiH4:PH3:H2 ¼ 3.3:3.3:76.8Gas pressure (Pa) 200 200 50 200Power density (mW/cm2) 350 140 120 140Thickness (nm) 25 3 400 35

K. Tao et al. / Vacuum 86 (2012) 1477e14811478

concentration (SC) was varied from 0.53% to 1.0%, and the thicknessof the doped samples was kept at about 60 nm by adjusting thedeposition time.

The thickness of films was measured using a surface profiler(AMBIOS XP-2), thereafter the deposition rate can be obtained. Themicro-structure of the p-nc-Si:H films was characterized using anX-ray diffractometer (XRD, Philips PANalytical X’Pert, CuKa). Theaverage crystallite size D of nc-Si:H can be estimated according toDebyeeScherrer formula [11]:

D ¼ Kl=bcos q

where K (z1) is a constant, l (¼0.154187 nm) is the wavelength ofX-ray, b is the full width at half maximum (FWHM) of the diffrac-tion peak, and the q is the diffraction angle. Raman scattering wasperformed to characterize the structure of p-nc-Si:H films. The514 nm laser line was used for the excitation. AFM measurementwas also used to characterize the surface morphology of p-nc-Si:Hfilms. The electrical conductivity of the films was measured usingan Al coplanar contact electrode using a Keithley 6430 Source/measure unit at room temperature.

Single junction peien solar cells were fabricated on ZnO:Gacoated PEN substrates in a superstrate configuration i.e., PEN/ZnO:Ga/p-nc-Si:H/i-a-Si:H/n-nc-Si:H/Al [12]. The thickness of thePENsubstratewas 50mm,and thedeposition conditions of each layerare summarized inTable 1. The illuminated JeV characteristics of thesolar cells were measured under an AM1.5 solar simulator at 25 �C.

3. Results and discussions

3.1. p-nc-Si:H materials

The deposition rate and the conductivity of p-nc-Si:H films onglasses as a function of SC are shown in Fig. 1. The deposition rate

0.5 0.6 0.7 0.8 0.9 1.02.0

2.5

3.0

3.5

4.0

4.5

1E-9

1E-7

1E-5

1E-3

0.1

Con

duct

ivity

(S/c

m)

deposition rate

Dep

ositi

on ra

te (n

m/m

in)

SC (%)

dark-conductivity photo-conductivity

Fig. 1. The deposition rate, dark-conductivity and photo-conductivity of p-nc-Si:H thinfilms on glass as a function of the silane concentration. The lines were drawn to guidethe eye.

increased almost linearly from 2.26 nm/min to 4.44 nm/min whenthe SC varied from 0.53% to 1.0%. However, both the dark-conductivity and photo-conductivity of those films decreasedmonotonously as the SC increased and the dark-conductivitywas only about 10�6 S/cm when the SC had a value up to 0.93%.In addition, it can be seen from Fig. 1 that the ratio of photo-conductivity to dark-conductivity became large as the SCincreased. This indicated that the doping efficiency of boron inthe silicon matrix decreased.

The evolution of layer crystallinity of p-nc-Si:H thin filmsdeposited under different SC can be demonstrated from the XRDspectra. From the result of XRD given in Fig. 2, it can be seen that theX-ray diffraction signal became weaker as the SC increased.Furthermore, the diffraction peak at the (311) plane disappearedwhen the SC was up to 0.80% and the diffraction peak at the (111)plane disappeared when the SC was 0.93%. Indeed, the decrease inthe diffraction signal at high SC must be related to the decrease incrystallinity. The crystallite size calculated from the (111) peakbecame smaller from 11.1 nm to 7.2 nmwhen the SC increased from0.53% to 0.80%, and the film was completely amorphous when theSC increased up to 0.93%.

The degree of crystallinity in the p-nc-Si:H films prepared atdifferent SC was investigated by Raman studies, as shown in Fig. 3.There was a shoulder peak in the transverse optical (TO) mode forall the samples. However, with an increase in the SC, this shoulderpeak became weak and the position of this peak shifted graduallyfrom 516 cm�1 to 512 cm�1. This indicated that the crystallinefraction originating from nanocrystals decreased gradually, andthe micro-structure of the thin films translated from mixed-phase (crystalline þ amorphous) to the amorphous phase. Usinga Gaussian 3-peaks fit, the crystalline volume fraction Xc can beestimated from the Raman spectra according to the followingformula [13]: Xc¼ (I500þ I520)/(I480 þ I500 þ I520). Here I480, I500, I520were the areas of the corresponding Gaussian peaks used to fit

Fig. 2. The XRD measurement of p-nc-Si:H thin films deposited on glass underdifferent silane concentration. The dashed lines were drawn to guide the eye.

100 200 300 400 500 600 700

0

2000

4000

6000

8000

10000

12000

14000

0.80%

0.73%

0.67%

0.60%

Inte

nsity

(a.u

.)

Raman shift (cm )

0.53%

c-Sia-Si

Fig. 3. The Raman scattering spectra of the p-nc-Si:H thin films deposited on glassunder different silane concentrations.

K. Tao et al. / Vacuum 86 (2012) 1477e1481 1479

the measured Raman spectra near 480 cm�1 (associated with theamorphousphase), and500 cm�1 and520 cm�1 (associatedwith thecrystalline phase), respectively. In fact, Xc decreased gradually from46.1% to lower than 8% when the SC increased from 0.53% to 0.80%.

In Fig. 4, the surface topology of a 5� 5 mm2 scan provided by anAFM in non-contact mode is shown for p-nc-Si:H thin filmsdeposited for various SC. It can be seen that, when SC increased

Fig. 4. 5 � 5 mm2 AFM images of p-nc-Si:H thin films dep

from 0.53% to 0.80%, the surface became markedly rough. Fig. 5shows the RMS roughness of samples calculated from AFM dataas a function of the SC. The surface RMS roughness increasedmonotonically from 3.23 nm at SC¼ 0.53% to 5.60 nm at SC¼ 0.80%.

We know that the growth of the microcrystalline silicon thinfilms starts with an incubation layer that is an amorphous phase.Then crystalline nuclei form in the incubation layer and graduallygrow into small grains embedded in the amorphous phase, which isa mixed-phase. Afterward, the grains grow larger and larger untilthey come into contact with each other, and, finally, the very largeconglomerates of microcrystalline silicon, sometimes labeled as“columns”, form the single crystalline phase [14,15]. In this process,the surface RMS roughness measured by AFM increased first due tothe presence of isolated large grains and reached amaximumvalue.Later, it decreased and tended to a saturated value [16,17]. For p-nc-Si:H thin films, the growth mechanism was similar to that ofintrinsic microcrystalline silicon thin films. However, due to theincorporation of boron atoms, the bond-angle and bond-lengthfluctuations were increased, causing the degradation of the short-range order [18,19]. Although, the micro-structure of p-nc-Si:Hfilms was affected, it can be compensated by reducing the SC [20].On one hand, when the SC was lower, the atomic hydrogen densitywas larger enough to cover the growth surface, and the surfacediffusion probability of gas precursors was larger. Thus, the gasprecursors can diffuse for a long distance on the surface to findenergetically stable sites to form an ordered structure. On the other

osited on glass under different silane concentrations.

0.50 0.55 0.60 0.65 0.70 0.75 0.80

5.0

5.5

6.0

10

11

12

13

14

15

0.40

0.45

0.50

0.55

0.60

0.80

0.85

0.90

0.95

FFVo

c (V

)

Jsc

(mA/

cm2 )

Eff (

%)

SC (%)

Fig. 6. Output performance of a-Si:H solar cells on PEN as a function of silaneconcentration. The lines were drawn to guide the eye.

0.50 0.55 0.60 0.65 0.70 0.75 0.803.0

3.5

4.0

4.5

5.0

5.5

6.0

RM

S-ro

ughn

ess

(nm

)

SC (%)

Fig. 5. RMS roughness of samples calculated from AFM as a function of silaneconcentration. The line was drawn to guide the eye.

K. Tao et al. / Vacuum 86 (2012) 1477e14811480

hand, the concentration of the precursors (e.g., SiH3, SiH2) waslower, that resulted in a smaller deposition rate. As the SCincreased, parts of the atomic hydrogen were depleted by thereaction [21]:

SiH4 þ H ¼ SiH3 þ H2 (1)

This means that the generation of themain deposition precursorSiH3 increased, and the atomic hydrogen density is reduced. Theatomic hydrogen is understood to play a prominent role in thepreferential elimination of the weak and disordered bonds bya chemical etching mechanism [22]. It needs to exceed a certainthreshold value to form the crystallinity. As a result, the depositionrate of films increased gradually, whereas, the crystalline volumefraction of films was reduced. In addition, the dark-conductivitywas smaller due to a large amorphous tissue fraction surroundingthe conductive crystalline grains.

When the thickness of the film is small, the crystalline nucleicannot make contact with each other and the crystallite size isdependent on the thickness of film. So, given the fixed thickness(e.g., 60 nm), samples deposited at lower SC had larger crystallitesize and crystalline volume fraction than samples at higher SC.Thus, the results shown in Figs. 4 and 5 can be explained by thefollowing: the crystalline grains of the samples at low SC (e.g.,0.53%, 0.6%) have contacted with each other, and the gaps betweengrains have been filled gradually. Whereas, for samples depositedat high SC (e.g., 0.73%, 0.8%), the grains just partly aggregate and didnot make contact with each other. Those films thus had largesurface roughness.

3.2. Solar cells

Pin-structure a-Si:H solar cells were fabricated with the non-textured electrodes, and 25 nm-thickness p-nc-Si:H thin filmsdeposited at different SC were used as the window layer. A 3 nmbuffer layer (Table 1) of wide band-gap a-SiC:Hwas placed betweenthe i and p interface. The role of the buffer layer in the solar cellstructure was to suppress electron back diffusion caused by theband offset at the conduction band edge between the nano-crystalline p layer and the amorphous i layer [23,24]. Fig. 6 showsthe initial output performance of a-Si:H solar cells as a function ofthe SC in p layer. One can see that, with increase in the SC in the player, the open circuit voltage (Voc) increased monotonically, while

the fill factor (FF) and the short circuit current density (Jsc) firstincreased then decreased, and reached a maximum at SC ¼ 0.6%and 0.73%, respectively. As a result, the initial efficiency (Eff) of thesolar cells reached a maximum at SC ¼ 0.67%. According to thediscussion above, the crystalline volume fraction of the p layerdecreased gradually with increasing the SC, thus, the band gap ofthe p layer increased. As a result, the Voc increased. When the SCwas lower, a larger mismatch of band gap between p layer and ilayer induces a larger carrier recombination at the i/p interface [17]and, as a result, the FF was smaller. When the SC was high, theconductivity of the p layer was low, as described above. Thus thecontact resistance between p layer and TCO increased, whichinduced a low Jsc. The best solar cells on PEN with a SC of p layer at0.67% had an efficiency of 6.3%.

4. Conclusion

The SC dependence of micro-structure and optoelectronicproperties of ultra-thin p-nc-Si:H thin films has been studied. Thecrystalline fraction and conductivity of p-nc-Si:H thin filmsdiminished, whereas, the deposition rate and RMS roughness offilms increased, when the SC increases from 0.53% to 0.8%. Whenp-nc-Si:H thin films deposited at different SC were incorporatedinto flexible peien structure a-Si:H solar cells, the best perfor-mance of solar cells was obtained with the p layer prepared at aSC ¼ 0.67%, and the conversion efficiency was 6.3%.

Acknowledgments

The authors are grateful to Changjian Li, Yanfeng Ma and Bo Liufor helpful discussions and AFM, Raman measurements. Also, theauthors highly appreciate the 973 Project of China (2010CB933803),the China Tianjin Applied Basic Research Projects and Cutting-edgeTechnology (08JCYBJC13100) and the China Tianjin Science andTechnology Research Plan (06YFGPGX08000).

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