Degradation studies of transparent conducting oxide: a substrate for microcrystalline silicon thin film solar cells

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Solar Energy Materials & Solar Cells 86 (2005) 207–216

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Degradation studies of transparent conductingoxide: a substrate for microcrystalline silicon thin

film solar cells

Rajesh Das, Tapati Jana, Swati Ray�

Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, India

Accepted 2 July 2004

Available online 11 September 2004

Abstract

Aluminium doped ZnO films have been developed by RF-magnetron sputtering at 350 1C

substrate temperature on glass substrate and commercially available SnO2-coated glass

substrate. The developed ZnO and SnO2/ZnO films can be used as the substrates of

microcrystalline silicon based solar cell. The electrical, optical properties and surface

morphologies of ZnO film and SnO2/ZnO bi-layer films have been investigated and they are

compared with the commercially available SnO2-coated glass substrate. The resistivities of

ZnO and SnO2 films are comparable (�10�4O-cm). Surface morphologies of different

transparent conducting oxide coated substrates before and after H-plasma exposure were

studied by scanning electron microscopy. The optical transmission of ZnO, SnO2/ZnO and

SnO2 films are comparable and varies from 85 to 90% in the visible region. The optical

transmission reduces drastically to less than 20% in SnO2 films and for ZnO film it remains

almost unchanged after H-plasma exposure. For SnO2/ZnO film transmission decreases

slightly but remains considerably high (�80%). The performance of microcrystalline silicon

solar cells fabricated on different transparent conducting oxides as substrates (ZnO/glass,

SnO2/glass and ZnO/SnO2/glass double layer) is investigated in detail.

r 2004 Elsevier B.V. All rights reserved.

Keywords: TCO; Hydrogen plasma degradation; Surface morphology; Solar cell performance

see front matter r 2004 Elsevier B.V. All rights reserved.

.solmat.2004.07.009

nding author.

dress: [email protected] (S. Ray).

www.elsevier.com/locate/solmat

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R. Das et al. / Solar Energy Materials & Solar Cells 86 (2005) 207–216208

1. Introduction

Transparent conducting oxide (TCO) plays the key role for thefabrication of Si-based thin film and other thin film solar cells. TCOmaterial is used to provide a window for the incident illumination to thedevice and also serves as one of the electrodes of solar cell. The ideal TCOprovides high transparency over a wide spectral range with high conductivityand higher carrier mobility. Textured TCO provides light scattering, whichincreases the optical absorption of weakly absorbed light in the solarcells.

Different types of TCOs like Indium tin oxide (ITO), Tin oxide (SnO2)and Zinc oxide have been used in thin film solar cells. Different preparationtechniques, e.g. evaporation, sputtering, chemical vapour deposition,spray pyrolysis, etc. have been used to deposit these transparent conductingoxides. A detailed review on this subject was made by Chopra et al. [1].Indium-doped ZnO films were deposited by spray pyrolysis where 8� 10�4O-cmresistivity and 80% transmission were achieved. Later several groups haveworked on sputtered ZnO films [2–5] and their properties have beeninvestigated. Development of Textured aluminium doped ZnO films havebeen developed by Kluth et al. [6] using magnetron-sputtering technique forapplication in solar cells.

Textured SnO2:F-coated glass is widely used as substrate for amorphoussilicon solar cells. However, in case of microcrystalline Si (mc–Si)-based solar cell[7] ZnO:Al coated glass has been used due to its better stability under H-plasma.In this case, high hydrogen dilution of silane and diborane is used to depositmicrocrystalline p-layer. TCO coating is exposed to intense hydrogen plasmaduring the deposition of microcrystalline p-layer, which is the first layer of p–i–nsolar cell. It was reported by Bannerjee et al. [8] that under hydrogenplasma, the optical transmission as well as the surface of SnO2 coatedsubstrate deteriorate due to its low chemical stability. Major et al.[9] havestudied the effect of hydrogen plasma treatment on different conducting oxidesincluding tin oxide and indium-doped zinc oxide using X-ray photoelectronspectroscopy and corroborated the above observation. These TCOs were depositedon glass by spray pyrolysis technique. However, no detailed studies on hydrogenplasma degradation have been made on Al doped ZnO films deposited by magnetronsputtering.

This paper addresses the development of textured ZnO and SnO2/ZnObi-layer films by RF-magnetron sputtering. In this study, SnO2:F, ZnO:Aland SnO2/ZnO films were exposed in H-plasma at different substratetemperatures. Optical properties and Surface morphologies of these TCOfilms before and after H-plasma exposure have been studied. Finally,mc–Si thin film solar cell has been fabricated on different TCO substrates(SnO2, ZnO and SnO2/ZnO) and the correlations between the substrate surfacemorphology as well as the optical property with the cell performance havebeen investigated.

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R. Das et al. / Solar Energy Materials & Solar Cells 86 (2005) 207–216 209

2. Experimental

Al-doped ZnO films were deposited on corning-7059 glass by RF-magnetronsputtering technique at substrate temperature Ts=350 1C under Ar+O2 gas ambientwith the optimum RF-power, chamber pressure and gas flow rates. Also SnO2/ZnObi-layer films have been developed by depositing �100 nm ZnO layer on SnO2:F-coated glass at the same substrate temperature. Textured SnO2:F-coated glass hasbeen procured from Ashahi, Japan. The ZnO:Al, SnO2:F and SnO2/ZnO films wereexposed under hydrogen plasma with the condition 30W RF-power, 0.5Torrchamber pressure, 100 sccm H2flow where the film was kept at 200 and 300 1C for 2and 30min respectively. Film characterizations were performed before and after theplasma exposure. Resistivity and optical transmission of the TCO films have beenmeasured by Van der Pauw four-probe method and UV-VIS-NIR spectro-photometer, respectively. Surface morphologies have been studied by scanningelectron microscopy. The effect of H-plasma on ZnO films was also investigated byEnergy Dispersive X-ray analysis.

Microcrystalline solar cells have been fabricated in a multichamber PlasmaEnhanced Chemical Vapour Deposition system (PECVD) with diode type parallelplate reactors decomposing different gas mixtures by using very high frequency(VHF) plasma. These textured ZnO:Al, SnO2:F and SnO2/ZnO coated glass wereused as substrate material for the fabrication of microcrystalline silicon thin filmsolar cells. The p–i–n structure was glass/TCO/ p–mc–Si:H/i–mc–Si:H/n–a–Si:H/Al.The performances of the solar cells were studied from current-voltage characteristics,measured under AM 1.5 illumination by a two source solar simulator (WACOM).

3. Results and discussion

3.1. Electrical properties

The resistivities and sheet resistance of as-deposited ZnO:Al film preparedat 350 1C are 8.4� 10�4O-cm and 12O/square respectively. The resistivities andsheet resistance of the SnO2:F film procured from Asahi, Japan, are given by2.3� 10�4O-cm and 8O/square, respectively. As SnO2/ZnO bi-layer film isdeveloped by depositing 100 nm thick ZnO layer on SnO2, the resistivity of SnO2/ZnO bi-layer film is comparable with Ashahi SnO2:F film. The carrier concentrationof as deposited ZnO film is 4.4� 1019 cm�3 and for SnO2 and SnO2/ZnO bi-layerfilms are of the order of 1020 cm�3.These films were exposed under hydrogen plasmaat different temperatures for different times. The film thicknesses of ZnO films beforeand after H-plasma treatment were 1150 and 1100 nm, respectively. The resistivitydecreases but the order remain unchanged and the carrier concentration of ZnO afterH-plasma exposure increases from 4.4� 1019 to 1.3� 1020 cm�3. This is due to theformation of Zn–H-type species by hydrogen adsorption on ZnO film surfaces [10].Here, hydrogen incorporation on ZnO surface may produce extra free carriers. Onthe other hand, SnO2:F films after H-plasma exposure become inhomogeneous and

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the sheet resistance of degraded SnO2:F films increases. But the electrical sheetresistance does not change for the double layer film.

3.2. Optical transmission

Figs. 1–3 show the optical transmission of single layer SnO2:F, ZnO:Al and bi-layer SnO2:F/ZnO:Al films before and after hydrogen plasma exposure. In all thecases, the highest visible transmission was almost same (above 85%) for the asdeposited films and the optical transmission is high over wider range of wavelengthfor both single-layer SnO2:F (shown in Fig. 1 curve 1) and SnO2:F/ZnO:Al (shownin Fig. 3 curve 1) films compared to single-layer ZnO:Al ( shown in Fig. 2 curve 1)film before H-plasma exposure. Although the visible transmission of ZnO:Al film isslightly higher than the other two films, narrower band transmission takes places inthis case. In Fig. 1, curve 2 shows the optical transmission of SnO2:F film after 2minhydrogen plasma exposure at 200 1C whereas curve 3 shows the transmission after30min H-plasma exposure at 300 1C. In the latter case the transmission is less than10% and the film becomes almost black. Even at 200 1C, which is conducive to solarcell fabrication temperature, the transmission in the visible range reduces drastically.Compared to SnO2:F films transmission of ZnO:Al film does not change at all whileexposed to H-plasma for 2min at 200 1C (not shown). Curve 2 of Fig. 2 shows the

Fig. 1. Optical transmission of SnO2:F film, curve 1: as deposited and after H-plasma exposure; curve 2:

for 2min at 200 1C; curve 3: for 30min at 300 1C.

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Fig. 2. Optical transmission of ZnO:Al film, curve 1: as deposited; curve 2: after H-plasma exposure for

30min at 300 1C.


transmission of the ZnO film after 30min hydrogen plasma exposure at 300 1C. Filmwas slightly greenish after the long H-plasma exposure but the change intransmission is very little. Similar results were observed by Major et al. [9] wherethe spray-deposited indium-doped ZnO film showed no changes in conductivity oroptical characteristics after H-plasma exposure. In the case of SnO2:F/ZnO:Al bi-layer film (shown in Fig. 3), the optical transmission shows very little change after2min of hydrogen plasma exposure at 200 1C; only visible transmission decreasesfrom 85 to 78% after 30min of exposure at 300 1C. Another advantage of thisdouble-layer film is that the transmission in the longer wavelength region in this caseis much better than that of single-layer ZnO film. This may be beneficial for itsapplication in solar cell as window layer.

3.3. Surface morphology by scanning electron microscopy

Figs. 4a–c show the surface morphologies of as deposited SnO2;F, ZnO:Al andSnO2/ZnO bi-layer films. Granular structure has been observed for the SnO2 filmwhereas for ZnO film the grains are somewhat elongated. For SnO2/ZnO double-layered film larger round-shaped granular texture has been observed.

After 30min of plasma exposure, the scanning electron micrographs of hydrogenplasma exposed TCO films are different and are shown in Figs. 5a–c. The grains with

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Fig. 3. Optical transmission of SnO2:F/ZnO:Al film curve 1 as deposited and after H-plasma exposure for

curve 2: 2min at 200 1C; curve 3: 30min at 300 1C.


size �60 nm are observed in plasma degraded SnO2 film. As studied under AugerElectron Spectroscopy by Bannerjee et al. [8] the stoichiometry of the film changes.Under hydrogen plasma atmosphere, H+ ions react with lattice oxygen atoms andreduces the surface which results the stoichiometric variation on the film surface.Auger Survey Scan of SnO2:F film surface of unexposed and exposed to hydrogenplasma shows that oxygen signal drops more sharply at the spot compared to the Snsignal i.e. Sn:O ratio increases in degraded SnO2:F film as compared with that inuntreated film. The damage is superficial and decreases in the bulk. Presence ofmetallic Sn was also reported by Major et al. in H-plasma-treated SnO2:F film fromthe analysis of XPS spectra [9].

Surface morphology of ZnO:Al film after 30min exposure to H-plasma at 300 1Cis shown in Fig. 5b. Although slight etching of film surface is evident from SEMphotograph, energy dispersive X-ray analysis (EDX) has shown no significantchanges in Zn:O ratio before and after H-plasma exposure. The composition of asdeposited and hydrogen plasma exposed ZnO film has been given in Table 1. It maybe noted that the film surface does not change after 2min exposure to plasma at200 1C (not shown). The excitonic binding energy of ZnO is 60meV [11]. Here, dueto stronger bonding between Zn and O compared to Sn and O, reduction of oxygenby hydrogen is not significant. Stability of transmission spectra (Fig. 2) underH-plasma corroborates this result. In spray-deposited indium-doped ZnO film,

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Fig. 4. Surface morphology of as-deposited (a) SnO2:F; (b) ZnO:Al and (c) SnO2:F/ZnO:Al films.


it was also observed from XPS analysis that Zn exists only in oxidized state even inplasma-treated film.

Fig. 5c shows the surface morphology of SnO2/ZnO bi-layer film after H-plasmaexposure. Although the surface etches after long plasma exposure (30min, 300 1C),the chemical composition of ZnO film and transmission do not change if it is exposedto H-plasma for 2min at 200 1C. The material properties of H-plasma exposedSnO2:F/ZnO:Al bi-layer films can be described as follows. As ZnO does not degradeunder hydrogen plasma, a thin ZnO layer acts as protective layer on SnO2:Fsurfaces. This bi-layer film can be exposed to H-plasma without any major change inoptical transmission. As the p-layer for solar cell is deposited usually at p200 1C andthe TCO layer becomes covered by p-type silicon layer by 2min, SnO2/ZnO as wellas ZnO-coated glass can be used as substrate for microcrystalline silicon solar cell.

3.4. Role of different TCO coated substrate on the performance of solar cell

Single junction solar cells with the structure p–mc–Si:H/i–mc–Si:H/n–a–Si:H/Alhave been fabricated on different TCO substrates (glass/ZnO, glass/SnO2 and glass/SnO2/ZnO bi-layer). The I–V characteristics of these microcrystalline silicon basedcells are shown in Fig. 6. Open circuit voltage (Voc), short circuit current (Isc), fillfactor (FF) and efficiency of these cells are depicted in Table 2. It may be seen that

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Fig. 5. Surface morphology of (a) SnO2:F; (b) ZnO:Al and (c) SnO2:F/ZnO:Al films after 30min

H-plasma exposure at 300 1C.

Table 1

Summery of EDX result of as-deposited and hydrogen plasma exposed ZnO:Al films

Elements Composition (At%) for ZnO:Al films

As-deposited Hydrogen plasma exposed

O 52.01 51.49

Al 2.00 2.15

Zn 45.99 46.36


both Isc and fill factor are low in case of SnO2:F-coated substrate. For ZnO-coatedsubstrate fill factor and efficiency is highest. However, for the cell deposited on SnO2/ZnO-coated glass substrate highest short circuit current (Isc) has been achieved. Theresults obtained can be explained as follows. The sheet resistance of the transparentconducting oxides (TCOs) are comparable (8–12O/&) and the transmissions for allthe TCOs are same in the visible range but in the higher wavelength region themaximum transmission is observed in SnO2:F-coated glass. But the cell prepared onSnO2:F-coated glass substrate (Cell 1) shows a very low efficiency value along with alow short circuit current density (Jsc), open circuit voltage (Voc) and Fill factor. This

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Fig. 6. I–V characteristics of hydrogenated microcrystalline Si thin film solar cells.

Table 2

Microcrystalline silicon (mc–Si) solar cell performance deposited on different substrates

Substrate Jsc (mA/cm2) Voc (V) Fill factor (F.F.) % Efficiency (Z)

Glass/SnO2 (Cell-1) 18.2 0.40 0.50 3.7

Glass/ZnO (Cell-2) 19.8 0.48 0.66 6.3

Glass/SnO2/ZnO (Cell-3) 22.1 0.46 0.60 6.1


is due to the degradation of the SnO2:F film under high hydrogen plasma, which isessential for the fabrication of p–mc–Si:H layer. The optical transmission of SnO2:Ffilms reduces drastically under H-plasma during fabrication of p layer and themaximum part of incident light will be absorbed inside the TCO i.e. effective lightabsorption in the active layer is reduced. As a result short circuit current decreases.The low open circuit voltage (Voc) and Fill Factors are due to defective interface ofTCO and p-layer. The maximum efficiency is achieved using ZnO-coated glasssubstrate along with the highest Fill Factor value [12]. But Jsc was less for the Cell 2compared to Cell 3 due to narrower range of transmitted wavelength.

The optical transmission of SnO2/ZnO double layer is higher than the ZnO coatedglass substrate over wide range of wavelength and due to ZnO protective layer the

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degradation of the film is almost zero. So short circuit current was maximum for thecell prepared (Cell 3) on SnO2/ZnO coated glass substrate.

4. Conclusion

The sheet resistance and transmission in the visible range of different transparentconducting oxides e.g. SnO2:F, ZnO:Al and SnO2/ZnO bi-layer films are comparablealthough transmission in the near infrared region is lower in the case of ZnO film.Degradation studies show that ZnO:Al film does not degrade due to reducing effectof hydrogen plasma even after 30min of exposure whereas SnO2:F strongly degradesdue to reducing effect of hydrogen. Degradation of SnO2/ZnO double-layer film isnot significant after H-plasma exposure for 2min at 200 1C and its advantage overZnO film is higher transmission in the longer wavelength region. Surfacemorphologies are different for different TCOs. Even ZnO film is etched by hydrogenplasma although composition does not change significantly.

Effects of using different TCO layers on the performance of microcrystallinesilicon solar cell have been studied. Efficiency is lower for microcrystalline siliconsolar cell deposited on SnO2-coated glass as the TCO degrades under H-plasma. Fillfactor and efficiency are highest when the microcrystalline silicon-based solar cell isfabricated on ZnO. For the SnO2/ZnO coated substrate ZnO acts as protective layerfor plasma degradation. Moreover, due to higher transmission range compared tosingle layer ZnO, the cell on this bi-layer TCO shows highest short circuit currentand reasonably good efficiency.

Acknowledgement

This work has been done under a project funded by the Ministry of Non-Conventional Energy Sources, Government of India.

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Degradation studies of transparent conducting oxide: a substrate for microcrystalline silicon thin film solar cells