ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 93 (2009) 973–975

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

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

Performance of superstrate multijunction amorphous silicon-based solarcells using optical layers for current management

Chandan Das �, Alain Doumit, Friedhelm Finger, Aad Gordijn, Juergen Huepkes, Joachim Kirchhoff,Andreas Lambertz, Thomas Melle, Wilfried Reetz

IEF5-Photovoltaik, Forschungszentrum Juelich GmbH, Leo-brandt str., 52425 Juelich, Germany

a r t i c l e i n f o

Article history:

Received 10 January 2008

Accepted 21 November 2008Available online 13 January 2009

Keywords:

Index matching

Anti-reflection layer

Intermediate-reflector layer

Amorphous silicon

Multijunction

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

016/j.solmat.2008.11.025

esponding author.

ail address: das.chandan@gmail.com (C. Das).

a b s t r a c t

The performance of multijunction amorphous silicon-based thin film solar cells has been reported using

thin layers of TiO2 and SiOx acting as refractive index matching optical layers for different interfaces of

the superstrate device structure. Improvement of short-circuit current from the sub-cells of a-Si/mc-Si

cells is demonstrated with TiO2 as anti-reflection layer at TCO/Si interface and SiOx as intermediate-

reflector layer between two sub-cells. An initial efficiency of 11.8% is achieved by applying both the TiO2

and SiOx optical layers in a-Si/mc-Si solar cell.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The multijunction a-Si-based thin film solar cells have beenreported to reach 15% efficiency in the recent years [1]. Furtherenhancement of efficiency demands improved a-Si-based materi-als as well as efficient device design. In case of a glass superstrate-based device structure, an optical loss occurs due to refractiveindex (n) mismatch between the transparent conducting oxide(n�1.9, viz. ZnO, SnO2) and Si (n�3.5) layers [2]. A layer of TiO2

(n�2.5) at TCO and silicon interface could act as an anti-reflectionlayer for improving the refractive index mismatch [2,3]. Again, byintroducing a refractive index mismatch with an intermediate-reflector layer at the Si/Si interface between two sub-cells of amultijunction cell, preferential improvement in short-circuitcurrent from one sub-cell could be achieved [4,5]. Recently, aPECVD-grown SiOx material is shown to be a good candidate foran intermediate-reflector layer [6,7]. In this article, developedTiO2 and SiOx optical layers have been applied to the a-Si/mc-Sisolar cells and their performance has been discussed on the basisof short-circuit current management.

2. Experimental

2.1. TiO2 thin films

The TiO2 thin films have been developed by RF-magnetronsputtering of TiO2:Nb2O5 targets with 99:1 wt% target using Ar

ll rights reserved.

and O2 gas mixture environment. The TiO2 thin films have beendeposited on Corning 1737 glass. The substrate temperaturehas been kept at room temperature. The deposition pressurewas kept constant at 0.3 Pa. The TiO2 thin films have beencharacterized electrically (co-planar conductivity measurement)and optically (UV–VIS–NIR spectrometry). Further details ofexperimental process and the optimization of the TiO2 filmproperties with electrical and optical characterizations aredescribed elsewhere [8].

2.2. SiOx thin films

The SiOx films have been grown by PECVD technique using H2,SiH4, PH3 and CO2 gas mixtures and the films are n-type in nature.A substrate temperature below 200 1C has been used for thedeposition of SiOx thin films. SiH4, H2, PH3 and CO2 have been usedin the deposition of SiOx films with the following flow ratios:SiH4:H2 ¼ 1:200, PH3:SiH4 ¼ 1:50 and SiH4:CO2 ¼ 2:3. The filmused for the application in the solar cells has a thickness between50 and 100 nm. The electrical and optical properties of the SiOx

films deposited on Corning 1737 glass have been done with co-planar conductivity measurement and photothermal deflectionspectroscopy (PDS), respectively. Further details regarding thedeposition conditions and the layer properties of SiOx could befound elsewhere [6].

2.3. Silicon thin films

The silicon thin films have been deposited by either RF(13.56 MHz) or VHF (95 MHz) PECVD. Both 10�10 cm2 and

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C. Das et al. / Solar Energy Materials & Solar Cells 93 (2009) 973–975974

30�30 cm2 substrate area-compatible PECVD systems have beenused for the thin film deposition process where low- (o1 Torr)and high- (3–10 Torr) pressure regimes in the deposition processhave been used.

2.4. Solar cells

The glass substrate is used in superstrate configuration for thefabrication of p–i–n solar cells. The standard solar cells are depositedeither on commercially available Asahi-U-type glass (SnO2) or onZnO-coated glass where the ZnO is texture-etched with HCl [9]. Incase of the solar cells with anti-reflection layer, the TiO2 layer hasbeen deposited on the texture-etched ZnO and covered with a verythin film of ZnO (10 nm) to protect it from H2 plasma reductionprocess during exposure to PECVD silicon deposition [2]. In case ofthe solar cells with intermediate-reflector layer, the SiOx layer hasbeen deposited at the n/p junction of two consecutive p–i–n sub-cells using both Asahi-U- and ZnO-coated substrates. The backcontact of the solar cells has been fabricated with ZnO–Ag. The cellshave been measured under 1 sun, AM 1.5 illumination. The quantumefficiency (QE) has been evaluated and thereby the integrated short-circuit current, Jsc, has been calculated for the solar cells.

3. Results and discussions

In Fig. 1, the effect of O2 in Ar gas in the sputter depositionprocess has been studied for optical properties of the TiO2 filmsand the absorption (A ¼ 1�T�R) in the films has been plottedagainst wavelength as measured by UV–VIS–NIR spectroscopy.The inset shows the conductivity of the TiO2 films plotted as afunction of fraction of O2 for several deposition runs. Interestingly,with the incorporation of O2 in the deposition process, theabsorption in the TiO2 films decreases. Again, the conductivity ofthe TiO2 films shows high values (in the range of 10�4–10�1 Scm�1) without incorporation of O2. However, the conductivitydecreases by several orders of magnitude even with slightincorporation of O2 in the deposition process. A conductivity of�10�7 S cm�1 could be achieved with 0.2% O2 in Ar environment.Thus, the incorporation of O2 in the deposition process affectsreversely the electrical and optical properties of the TiO2 thinfilms and an optimum value of O2 (0.2%) has been chosen for thedevice application.

Fig. 1. Absorption from the TiO2 films against wavelength for different O2 (%) flow

in the Ar gas during sputtering deposition process. Inset shows the variation of co-

planar conductivity of the TiO2 films against the variation of O2 (%) flow in Ar.

In Fig. 2, the refractive index, n (as measured from PDS) and theconductivity of the SiOx films have been plotted as a function ofCO2 flow during the deposition process. A wide range of refractiveindex could be achieved with variation of CO2 in the gas phase.The co-planar conductivity of the SiOx films also varies within awide range with variation of flow of CO2. Both the refractive indexand conductivity of the SiOx films decrease with the increase ofCO2 flow and for the device application n�2.2 has been chosen inthis work.

The TiO2 film as anti-reflection layer has been appliedfollowing the device design: glass/ZnO(texture-etched)/TiO2/ZnO(10 nm)/a-Si(p–i–n)/mc-Si(p–i–n)/ZnO–Ag. In Table 1, theperformance with TiO2 is compared with a standard cell usingZnO front TCO. The bottom cell current increases by 7% and thetop cell current decreases slightly with TiO2 anti-reflection layer.

Again, with the incorporation of SiOx as intermediate-reflectorlayer, following a design, a-Si(p–i–n)/n-SiOx/mc-Si(p–i–n), com-pared with the standard one an increase of Jsc from the top cell hasbeen observed. The performance with SiOx is shown andcompared with a standard cell in Table 1 using SnO2 and ZnOfront TCOs. The top cell Jsc increases by �12% and �4% using SnO2

and ZnO front TCOs, respectively. However, the increase in Jsc fromthe top cell is counterpoised with a successive decrease in thebottom cell using both types of the TCO.

In the next step, both TiO2 and SiOx have been incorporated inthe device design and the performance of solar cells has beendemonstrated in Table 1 using ZnO-coated substrate. In this case,an increase (4.5%) of Jsc from the top cell is achieved and thedecrease in Jsc from bottom cell due to application of only SiOx is

Fig. 2. Refractive index (dot) and co-planar conductivity (star) of the SiOx films

plotted as a function of CO2 gas flow in the PECVD deposition process.

Table 1Performance of a-Si/mc-Si double-junction cells for standard, with TiO2, with SiOx,

with both TiO2 and SiOx using ZnO or SnO2 as TCO.

TCO Voc (V) FF Jsc (mA/cm2) Z (%)

Top Bottom

Standard ZnO 1.41 0.73 11.0 11.4 11.3

Standard SnO2 1.36 0.73 10.0 12.7 9.9

With TiO2 ZnO 1.42 0.72 10.8 12.2 11.0

With SiOx SnO2 1.37 0.75 11.2 11.2 11.5

With SiOx ZnO 1.42 0.71 11.4 10.7 10.8

With TiO2 and SiOx ZnO 1.42 0.74 11.5 11.2 11.8

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recovered. This corresponds to an improvement of efficiency up to11.8% in the a-Si/mc-Si solar cell using the optical layers.

4. Conclusions

�

The TiO2 as anti-reflection layer and SiOx as intermediate-reflector layer have been developed and incorporated into themultijunction a-Si-based solar cell design. � The application of TiO2 and SiOx has been demonstrated to

improve the efficiency of the a-Si/mc-Si solar cells with aprecise current management of the sub-cells. An efficiency of11.8% has been achieved in this program of work.

Acknowledgements

This work has been done under the European Union projectATHLET, Contract no. 019670. The authors would like to thank R.van Aubel, J. Klomfass, H. Siekmann and J. Worbs for theirassistance in the experiments.

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