preparation and optimization of a molybdenum electrode for cigs solar cells

In order to obtain lower resistiv-ity, better adhesion and better reflectivresistiv-ity, bilayer films and tri-layer films were both deposited in different mode.. And the tri-layer films

Preparation and optimization of a molybdenum electrode for CIGS solar cells

Feng Jingxue, Wu Zhao, Wang Wei, Yuan Ye, Zhuang Lin, Wang Xin, Hong Ruijiang, Shen Hui, and Michael

Z Q Chen

Citation: AIP Advances 6, 115210 (2016); doi: 10.1063/1.4967427

View online: http://dx.doi.org/10.1063/1.4967427

View Table of Contents: http://aip.scitation.org/toc/adv/6/11

Published by the American Institute of Physics

Preparation and optimization of a molybdenum electrode for CIGS solar cells

Feng Jingxue,1,2Wu Zhao,1Wang Wei,1Yuan Ye,1Zhuang Lin,1, a

Wang Xin,2, aHong Ruijiang,1Shen Hui,1and Michael Z Q Chen3

1School of Physics and Engineering, Institute for Solar Energy Systems, State Key Laboratory

of Optoelectronic Materials and Technologies, Guangdong Provincial Key Laboratory

of Photovoltaics Technologies, Sun Yat-Sen University, Guangzhou 510006, P R China

2School of Materials Science and Engineering, South China University of Technology,

Guangzhou 510640, P R China

3Department of Mechanical Engineering, The University of Hong Kong,

Hong Kong, P R China

(Received 15 June 2016; accepted 24 October 2016; published online 3 November 2016)

Molybdenum (Mo) films were deposited by radio frequency (RF), direct current (DC) and mixed magnetron sputtering, respectively With changing the deposition parameters including deposition pressure and power, the films show different surface morphology and crystallinity Lower resistivity of the films is obtained in the DC mode and better reflectivity of the films is obtained in the RF mode It is shown that the crystallinity increases when the deposition pressure decreases The crystallinity and the grain size both increase as the deposition power increasing The lowest resis-tivity of the single Mo film is 34×10-6Ω·cm when the deposition pressure is 0.1 Pa and the deposition power is 300 W in the DC mode In order to obtain lower resistiv-ity, better adhesion and better reflectivresistiv-ity, bilayer films and tri-layer films were both deposited in different mode They all show good adhesion and low resistivity The

Mo films deposited in mixed mode show better reflectivity It is demonstrated that the resistivity of about 65×10-6Ω·cm is achieved in DC/RF mode and the resistivity of about 61×10-6Ω·cm is achieved in RF/DC/RF mode And the tri-layer films achieved

in RF/DC/RF mode have better reflectivity than bilayer films achieved in DC/RF mode The tri-layer films achieved in RF/DC/RF mode is appropriate for using as

the electrode of CIGS solar cells © 2016 Author(s) All article content, except where

otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4967427]

I INTRODUCTION

Solar energy is sustainable alternative energy sources and solar cell has been studied continually And it is important to reduce production costs.1,2Cu(InGa)Se2solar cells is promising to lower the cost as the new energy for power generation It is reported that the highest efficiency of CIGS solar cell could reach up to 21.7%.3Preparation technology for high efficiency CIGS thin film solar cells include three-stage deposition process, magnetron sputtering and co-evaporation technique.46 The

Mo films as back electrode of CIGS solar cells play an important role in improving the efficiency of the cells The comparison of Mo metal and other materials indicate Mo is an ideal back contact material for CIGS solar cells, because of its inertness and high conductivity.7However, it is difficult to obtain the low resistivity and the good adhesion simultaneously There are many preparation processes of

Mo films which had been attempted to find an appropriate back electrode.7 9P Blosch et al reported that the molybdenum films were deposited on stainless steel foils.10Cherng-Yuh Su et al studied the

a Zhuang Lin and Wang Xin are both corresponding author of this paper Electronic mail: stszhl@mail.sysu.edu.cn and g96217@scut.edu.cn

115210-2 Jingxue et al. AIP Advances 6, 115210 (2016)

deposition parameters have big impacts on various properties of Mo thin films.11Karthikeyan et al used a powder target to deposit the Mo films by DC magnetron sputtering.12K Aryal et al made a comparison of monolayer Mo film between DC mode and RF mode.13The preparation of bilayer Mo films was reported and the adhesion of Mo film and substrate was solved.8However, the adhesion of

Mo film and CIGS cells and the reflectivity of the back contact are also important parameters which have not been solved yet

In this study, in order to improve the adhesion of the Mo film and CIGS solar cells, bilayer films and tri-layer films were both deposited in DC and DC/RF mixed mode, respectively Both the films show good adhesion and low resistivity And tri-layer Mo films obtained in RF/DC/RF mode show better reflectivity

II EXPERIMENTAL PROCEDURE

Make the title as concise as possible but informative enough to facilitate information retrieval Only the most common acronyms and abbreviations are allowed in the title Use acronyms with considerable moderation and always define at first use

A Molybdenum deposition

Soda-lime glass (SLG) substrates were cleaned using deionized water, methanol, acetone and distilled water sequently And then the substrates were dried by N2 and placed in the deposition chamber The Mo thin films on SLG substrates was deposited using a circular Mo target (75 mm diameter) with a high purity of 99.95% Mo thin films were deposited using RF, DC and mixed DC/RF magnetron sputtering, respectively The procedure for depositing all the films is as follows: (a) the pre-sputtered was given first in an Argon atmosphere for about 15 min so that any oxide layer remained on the surface of the target can be removed; (b) the chamber needed a base pressure of 4.0×10-4 Pa; (c) the chamber introduced argon flow of 99.99% pure The flow rate of Argon was 20.6 sccm and the working gas pressure was 0.1∼1.2 Pa; (d) the working power was changed from

100 to 300 W

B Charaterization

The thickness and the electrical resistivity of the Mo films were measured using a 3D-profiler gauge and a four-point probe (Napson Cresbox), respectively When measure the resistivity by using

a four-point probe, the size of all samples is 1 cm×1 cm The arrangement array of the four probes

is linear and the spacing distance between the four probes is s=1mm The formula to calculate the resistivity is

R=V

I ·4.532 · Thick (cm) · Temp Correction · Circum Correction

The reflectivity was measured by UV-visible spectrophotometer (Hitachi U-4100) in the wavelength range of 300–1100 nm Scanning Electron Microscopy (Multimode), Atomic Force Microscopy (EVO18) and X-ray Diffraction (Empyrean) were employed to analyze the crystallinity and morphological properties of the Mo films The adhesiveness of Mo thin film was simply measured using a 3M tape

III RESULTS AND DISCUSSION

A The deposited single layer Mo films

1 Crystal structure

The crystallinity of the Mo films deposited in DC mode is better than the films deposited in RF mode in Fig.1(a) A strong peak along the (110) plane and a weak peak along the (211) plane are shown in the DC mode deposited films (DC films) Fig.1(b)shows that the intensity of the (110) peak increases significantly as the deposition power increases Fig.1(c)shows the XRD patterns of the Mo films sputtered at different deposition pressure All the Mo films have a sharp (110) peak

FIG 1 XRD patterns of Mo films deposited in different deposition conditions: (a) different deposition mode; (b) different deposition power; (c) different deposition pressure.

revealing good crystallinity of the films Besides, the intensity of the (110) peak decreases and its half-peak width widens with the increasing of the deposition pressure It reveals that the crystallinity

of the Mo films is inversely proportional to the deposition pressure When the deposition pressure increases, the kinetic energy of Mo ions reduces because of the increased particle scattering; when the deposition power increases, the kinetic energy of Mo ions also increases

FIG 2 AFM images of Mo thin films deposited in (a) RF mode and (b) DC mode; SEM images of Mo thin films deposited

at the deposition power of (c) 100 W and (d) 200 W; SEM images of Mo thin films deposited at the pressure of (e) 0.1 Pa and (f) 0.6 Pa.

115210-4 Jingxue et al. AIP Advances 6, 115210 (2016)

Therefore, the kinetic energy of Mo ions is higher when the deposition pressure is lower and the deposition power is higher And the Mo crystal can be deposited layer by layer on the substrate The crystallinity of the Mo films is significantly influenced by the deposition power and the deposited pressure, which are shown in Fig.1(b)and Fig.1(c)

2 Surface morphology

From the AFM pictures shown in Fig.2, the DC films show larger grain size than the RF mode deposited films (RF films) It may be caused by the higher deposition rate of the DC films which makes lower foreign atoms incorporation And the DC films show relatively rougher surface and both the films have uniform grain distribution As shown in TABLEI, the root-mean-square (RMS) surface roughness of the DC films and the RF films is 5.014 nm and 3.221 nm, respectively This result is not in accordance with the data13reported in which the RF films have a wider particle size distribution It may be explained that a rotating platform is used in the deposition chamber and the pressure and power remain unchanged in our experiment

From Fig.2(c)and Fig.2(d), the grain size also increases as the deposition power increases Vink et al indicates that films deposited at lower power exhibit porous microstructures while films deposited at higher power exhibit dense microstructures.14 However, no porous microstructure is observed And Fig.2(c)and Fig.2(d)both show dense microstructure The films prepared at higher deposition power produce larger crystalline grains

It is reported that the films obtained from higher deposition power are smoother.15It is explained that the higher energy of Mo particles causes that the surface diffusion of the growing Mo thin film strengthens.16While our result observed is not in accordance with the reports mentioned above It is seen that the film in Fig.2(d)is rougher than that in Fig.2(c) In order to give a further explanation, the RMS surface roughness of these films is measured The RMS increases as the deposition power increases which is shown in TABLEII It is explained that the surface of the SLG substrate is sputtered

a great deal of Mo molecules The increasing of the sputtering power also arouses the raise of the deposition rate

The possibility of collision between Ar ions and Mo atoms is different under various pressure due

to the distinction of collision frequency As a result, the growth mechanism and morphologies of the

Mo films are dissimilar Fig.2(e)shows that the Mo thin films have small crystal and dense microstruc-ture at the low deposition pressure of 0.1 Pa However, the surface of the Mo films deposited at 0.6 Pa show loose and fish-like shape grains as shown in Fig.2(f), which was also observed by Kadam

et al.17

3 Resistivity

The RF films possess higher resistivity compared with the DC films, which is shown in TableI Fig.3(a)indicates the resistivity of Mo films reduces with the increasing of the deposition power The

TABLE I Different performance of Mo thin films at different deposition mode.

TABLE II Different performance of Mo thin films in different deposition power.

FIG 3 The resistivity of Mo thin films at different deposition conditions: (a) different deposition power; (b) different deposition pressure.

reduction of the resistivity could be attributed to an increment of the grain size When the grain size

of the Mo films increases, the number of grain boundaries and the potential barrier’s height of grain boundary are reduced The transportation of the carrier can be easier during the electronic transport process Therefore the carrier mobility of the Mo films increases and the resistivity of the Mo films decreases

It is presumable that the presence of loose structure results in the increasing resistivity of Mo films As shown in Fig.3(b), the lowest resistivity of Mo films is 34×10-6Ω·cm with the deposition pressure of 0.1 Pa, and the resistivity increases as the increasing of deposition pressure However, when the pressure is 0.1 Pa, the adhesion test is failed Wang et al reported the residual stress will make effect on the adhesion of the Mo films.18 And Yoon et al explained the effect of working pressure on the residual stress for the sputtered films.19 Films deposited at lower pressure exhibit compressive stress and the compressive stress increases with the decreasing of the deposition pressure The adhesion between the films and the substrate is poor which is caused by the redundant compressive stress

B The deposited multilayer Mo films

1 The deposited bilayer Mo films

From the above results, it is found that the Mo films deposited under lower deposition pressure

in DC mode possess lower resistivity and worse adhesion, and the Mo films deposited under a higher deposition pressure in DC mode possess higher resistivity and better adhesion In order to obtain the optimized Mo films with lower resistivity, better reflectivity and better adhesion between the Mo films and the glass, bilayer Mo films using a sequentially different deposition pressure are sputtered in DC/DC mode and DC/RF mode, respectively The bottom layer is deposited at high deposition pressure to obtain good adhesion and the top layer is formed at low deposition pressure to obtain low resistivity It is shown in Fig.4 The bottom layers are both about 100 nm deposited under 0.3 Pa and the top layers are both about 800 nm deposited under 0.1 Pa in DC mode

The bilayer films both show good adhesion and low resistivity The resistivity of the films in Fig 4(a)is 70×10-6 Ω·cm and the resistivity of the films in Fig.4(b) is 65×10-6 Ω·cm Reflec-tivity of the back contact is an important parameter in improving the efficiency of CIGS solar cells.9 Fig.5(a)shows that the single films deposited in RF mode have better reflectivity than those deposited in DC mode And Fig.5(b)shows that the bilayer Mo films deposited in DC/RF mode has slightly better reflectivity than those deposited in DC/DC mode because of the bottom Mo films by

RF mode

2 The deposited tri-layer Mo films

In order to obtain the optimized Mo films with lower resistivity, better reflectivity and better adhesion between the Mo films and the CIGS layer, tri-layer Mo films using a sequentially different

115210-6 Jingxue et al. AIP Advances 6, 115210 (2016)

FIG 4 SEM micrographs of cross-sections of bilayer Mo films: (a) DC/DC mode; (b) DC/RF mode.

deposition pressure is sputtered in DC/DC/DC mode and RF/DC/RF mode, respectively The first layer is deposited at high deposition pressure to obtain good adhesion between the Mo films and the glass The second layer is formed at low deposition pressure to obtain low resistivity The third layer is also deposited at high deposition pressure to obtain good adhesion between the Mo films and the CIGS layer It is apparent that a sandwich structure can be seen from Fig.6 The first layer and the third layer are both about 100 nm deposited under 0.3 Pa and the second layer is about 800 nm deposited under 0.1 Pa in DC mode

The tri-layer films both show good adhesion and low resistivity The resistivity of the films in Fig.6(a) is 67×10-6 Ω·cm and the resistivity of the films in Fig 6(b)is 61×10-6 Ω·cm Besides, the tri-layer Mo films in RF/DC/RF mode have much better reflectivity than those in DC/DC/DC mode and DC/RF mode shown in Fig.7(a)and Fig.7(b), respectively It should attribute to the first layer and third layer deposited in RF mode, which have better reflectivity than those deposited in DC mode

FIG 5 The reflectivity of Mo films: (a) single Mo films; (b) bilayer Mo films.

FIG 6 SEM micrographs of cross-sections of three-layer Mo films: (a) DC/DC/DC mode; (b) RF/DC/RF mode.

FIG 7 The reflectivity of Mo films: (a) tri-layer Mo films; (b) Mo films deposited by mixed mode.

IV CONCLUSIONS

Rougher film morphology, higher deposition rate and lower resistivity are obtained in DC films And the RF films possess higher resistivity compared with the DC films The crystallinity, deposition rate, RMS and grain size of Mo films are all proportional to the deposition power While the resis-tivity of Mo films is inversely proportional to the deposition power The crystallinity of Mo films is inversely proportional to the deposition pressure The Mo films have a small, densely packed grain microstructure at low deposition pressure of 0.1 Pa and the surface of the Mo films deposited at 0.3 and 0.6 Pa shows loose and elongated grains The lowest resistivity of Mo films is 34×10-6Ω·cm with the deposition pressure of 0.1 Pa, but it is failed in the adhesion test The bilayer molybdenum films and tri-layer molybdenum films all show good adhesion and low resistivity The Mo films deposited

in mixed mode show better reflectivity and the tri-layer films achieved in RF/DC/RF mode have better reflectivity than bilayer films achieved in DC/RF mode It is demonstrated that the resistivity of about 65×10−6Ω·cm is achieved in DC/RF mode and the resistivity of about 61×10-6Ω·cm is achieved in RF/DC/RF mode, which is appropriate for using as the electrode of the CIGS solar cells

115210-8 Jingxue et al. AIP Advances 6, 115210 (2016)

ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (Grant No 61376014 and No 60906005), the Guang Dong Provincial Natural Science Foundation of China (Grant No S2013010016010 and No 9451027501002848) and the Hong Kong Research Grants Council under the GRF (Grant NO 17205414)

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