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All issues Volume 3 (2012) EPJ Photovolt., 3 (2012) 35001 Full HTML
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EPJ Photovolt.
Volume 3, 2012
Topical issue: Photovoltaic Technical Conference (PVTC 2011)
Article Number 35001
Number of page(s) 5
DOI https://doi.org/10.1051/epjpv/2012002
Published online 26 June 2012

© EDP Sciences 2012

1 Introduction

Silicon thin-film solar cells are a promising candidate for future photovoltaic power generation. The most advanced approach employs hydrogenated amorphous and microcrystalline silicon (a-Si :H, μc-Si :H) as active layers in single and multi-junction cells, which are fabricated using a very-high-frequency (VHF) plasma-enhanced chemical vapor deposition method (PE-CVD) from a mixture of SiH4 and H2[1, 2]. Silicon thin-film solar cells in a p-i-n (superstrate) configuration require a transparent conductive oxide (TCO) films as a front contact. Such contacts must have a low series resistance and a high transparency in the visible and near-infrared regions. Furthermore, a surface topography is required to ensure light-scattering and subsequent light trapping inside the silicon solar cell structure [3, 4]. Among various TCOs, impurity-doped zinc oxide, ZnO, films such as ZnO :Al (AZO) and ZnO :Ga (GZO) are attractive because of their low material cost, non-toxicity, relatively low resistivity, and high visible transmission. In addition, AZO can be easily textured by wet chemical etching with diluted hydrochloric acid (0.5% HCl). Recently, there has been particular interest in improving of the properties of impurity-doped ZnO, i.e., the suppression of the free-carriers in multi-junction Si thin-film solar cells [5]. Deposition of AZO and GZO films as a TCO have been extensively studied using several fabrication techniques, i.e., metal-organic chemical vapor deposition (CVD), magnetron sputtering, and sol-gel methods [6, 7, 8, 9, 10], followed by subsequent thermal annealing in vacuum to improve their optical and electrical properties. Among them, the thermal annealing of AZO with n+-a-Si capping layer on top of AZO is one of interests for further improvement of the performance of TCO films [11, 12]. In addition, for further high efficiencies of single- and multi-junction solar cells, a new Si thin-film technology is needed, i.e., polycrystalline Si thin film and its alloys [13, 14].

In this study, we demonstrate the rapid thermal annealing for AZO film with and without an a-Si capping layer using an inductively-coupled radio-frequency (rf) argon thermal plasma jet (TPJ) at atmospheric pressure and applied for the n-i-p microcrystalline Si (μc-Si) thin-film solar cells.

2 Experimental details

The 800-nm-thick AZO films used in this study were fabricated on corning 1737 glass (25 × 15mm2) using a rf magnetron sputtering technique in argon and a 99.99% AZO ceramic targets with 2-wt%-Al2O3 at a substrate temperature Ts of 300 °C and a working pressure of 0.9 Pa. The AZO was textured by wet chemical etching with diluted hydrochloric acid (0.5% HClaq) [15]. The resistivity of the as-deposited AZO films was approximately 3−6 × 10-4Ω cm. 30-nm-thick n+- and p+-a-Si :H layers were fabricated on flat and textured AZO-coated glass using a capacitively-coupled rf PE-CVD of a dichlorosilane (SiH2Cl2) and H2 mixture with and without an additional 1-%-PH3(B2H6), respectively, at a Ts of 250 °C. In the subsequent solar cell processing steps, much higher temperatures would be reached during thermal plasma exposure. Therefore, the a-Si :H films were pre-heated to 400 °C for 90 min prior to the thermal plasma annealing to remove the residual hydrogen from the films. The TPJ-annealing of ZnO and AZO films with and without an a-Si :H capping layer was performed with a variable substrate stage velocity vsub and a 2 KW inductively-coupled rf plasma source in a tri-axial quartz tube having an inner diameter of 10 mm. The maximum surface temperature Tmax was controlled by adjusting vsub. The quartz tube-substrate to substrate spacing was 15 mm. The surface temperature profiles during TPJ annealing was monitored using a non-contact thermometry [16].

TPJ-annealed AZO and crystallized Si films were characterized by grazing incidence X-ray diffraction (GI-XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and spectroscopic ellipsometry (SE). The Hall mobility μ and carrier density Ne of the TPJ-annealed ZnO and AZO films were determined by a standard Van der Pauw method. The optical characterization of the textured AZO before and after TPJ annealing was performed using a double-beam UV-visible spectrometer equipped with an integrating sphere (Perkin Elmer, Lamda 35) at wavelengths ranging from 250 to 1800 nm. The Si thin-film solar cells were fabricated with a single-junction n-i-p μc-Si solar cell structure. The 2-μm-thick μc-Si layer was fabricated from a SiH2Cl2 and H2 mixture at 500 °C.

thumbnail Fig. 1

Resistivity vs. maximum surface temperature Tmax of AZO films with and without a 30-nm-thick n+-a-Si capping layer after TPJ annealing.

3 Results

3.1 TPJ annealing of ZnO and AZO films with and without an a-Si capping layer

Figure 1 shows the resistivity of AZO films with and without capping 30-nm-thick n+-a-Si layer upon the TPJ annealing plotted against Tmax. Here, the TPJ annealing was performed on the identical region by repeating the plasma exposure at vsubs from 55 down to 10 mm/s to minimize the thermal damage of previous annealing history. The resistivity increased markedly from 10-4 to 10-2Ω cm at Tmaxs above 650 °C for bare AZO films on glass. On the other hand, it increased slightly and/or was almost independent of vsub of an order of 10-4Ω cm up to a Tmax of 825 °C (vsub: 25 mm/s) for AZO films with a 30-nm-thick n+-a-Si capping layer. The film crystallization of both a-Si and AZO layers was observed at Tmax above 740 °C. These results agreed qualitatively with those of conventional solid-phase-crystallization (SPC) of n+-a-Si coated AZO films at 600 °C for 22 h [11, 12]. Similar result was also obtained in the AZO films capping p+-a-Si layer. Similar results were also observed in the boron-doped and intrinsic a-Si layers.

thumbnail Fig. 2

(a) Hall mobility μ and (b) carrier density Ne of TPJ-annealed AZO films at different Tmaxs. The n+-Si layer on top of AZO was etched out using reactive ion etching after TPJ annealing. The TPJ annealing was performed by changing vsub from 55 to 22 mm/s.

thumbnail Fig. 3

Depth profiles of atomic concentrations, Si, Zn, Al, and O for n+-a-Si/AZO heterostructures before and after TPJ annealing monitored by XPS. The measurement was performed each after 5-nm-thick etching by Ar plasma.

thumbnail Fig. 4

Total transmittance Ttotal and haze spectra of TPJ-annealed AZO films with different surface morphology. The wet chemical etching was conducted after TPJ annealing by a 0.5% HCl at different dipping times. The TPJ annealing was performed at a vsub of 25 mm/s and a Tmax of 650 °C. The inset shows the AFM images of the 80 and 100 s chemically etched AZO film.

In Figure 2, the μ and Ne of TPJ-annealed ZnO and AZO layers with and without a-Si capping layers are summarized as a function of Tmaxs. The Hall measurement was performed for TPJ-annealed AZO films on flat glass after etching out top n+-μc-Si layer. The μ and Ne in bare ZnO films decreased to 13–18 cm2/V s and 3 × 1020cm-3, respectively, with increasing Tmax. On the other hand, the μ increased from 30 to 39 cm2/V s for AZO with n+-a-Si capping layer. While, the Ne was almost independent of vsub and remained on the order of 1020cm-3 even after TPJ annealing at a Tmax of 825 °C. These results suggest that the film crystallization of AZO layer promoted with any increase in the O and Zn related defects. The PL characterization also revealed that the generation of O and Zn related defects was suppressed by the a-Si capping layer.

Depth profiles of the atomic concentrations of Si, Zn, Al, and O of the AZO a 30-nm-thick n+-a-Si capping layer are shown in Figure 3, as measured by XPS before and after TPJ annealing. No significant changes in the depth profiles of Al, Zn, or Si and thicknesses were observed in the Si/AZO heterostructure except for the formation of an oxidized Si layer at the top surface. In addition, the SE characterization also revealed that the film crystallization of a-Si ansd AZO layers was enhanced and no significant generation of intermixing layer were formed at a-Si and AZO interface. These findings imply that the film crystallization of both AZO and Si layers was enhanced without significant oxygen diffusion from underlying AZO to top a-Si layer.

3.2 Texturing of AZO and its effect of the crystallization of a-Si capping layer

Figure 4 shows the total transmittance Ttotal and haze spectra of TPJ-annealed AZO films with different surface morphology. The TPJ annealing was conducted at a Tmax of 650 °C and subsequently, the wet chemical etching using a 0.5% HCl was carried out at different dipping times for 2-μm-thick AZO films. The AFM image of corresponding textured AZO is also shown on the top. The uniform crater structures were formed by adjusting the HCl rinse time. High haze values were obtained of above 70% in entire spectra range for  ~ 100 s (800 nm thickness) etched AZO films. The Ttotal in the 1000–1200 nm regions corresponding to the free carrier absorption was also suppressed with increasing the HCl etching time. These results originate from less free carrier density. The Hall measurement revealed that the μ and Ne were 45 cm2/V s and 1020cm-3 in the 100 s etched AZO films. On the other hand, the TPJ annealing of HCl-etched AZO films showed relatively high resistivity because the etching speed was higher. Thus, the texturing of AZO was conducted after TPJ annealing to design the optical management without deteriorating electrical properties.

thumbnail Fig. 5

GI-XRD patterns of TPJ-annealed 30-nm-thick n+-, p+-, and intrinsic- crystallized Si films on textured AZO-coated glass substrates after TPJ annealing at a Tmax of 825 °C. The result of intrinsic- crystallized Si on flat AZO is also shown as a reference.

In Figure 5, the GI-XRD patterns are shown for the 30-nm-thick n+-, p+-, and intrinsic-crystallized Si films fabricated on textured AZO-coated glass substrates after TPJ annealing. The result of intrinsic-crystallized Si film on flat AZO is also shown as a reference. Apparently, both Si and AZO layers were crystallized by the TPJ annealing for all samples, although the degree of the film crystallization depended on the impurity concentration and film thickness. In addition, the Si (111) diffraction peak intensities in intrinsic-, n+-, and p+-a-Si films were enhanced on textured AZO rather than the flat. Similar results were also reported elsewhere for p-i-n μc-Si thin-film solar cells [17]. These results originate from that SPC of a-Si by TPJ annealing is promoted not from the top surface but the bottom. These results suggest that the texturing AZO surface promotes the inhomogeneous temperature profile toward the film depth, which is a possible origin to enhance the nucleation and grain growth rather than the flat. Therefore, the use of texturing AZO is effective to promote the film crystallization of top a-Si layer, although its degree depends on impurity type, P and B, and their concentrations in a-Si precursors. These findings imply that the TPJ annealing of n+-a-Si/textured AZO heterostructure is a possible technique for the optical management for Si thin-film solar cells.

Figure 6 shows the current-voltage, I-V curve and quantum efficiency (QE) spectra of μc-Si single-junction solar cells using textured AZO. Inset shows the corresponding μc-Si film. The current density Jsc improved markedly with no decrease in open circuit voltage Voc by introducing the textured AZO because of better light scattering. The efficiency of 8.2% (Voc:542 mV, Jsc: 25.4 mA/cm2, FF : 68%) has been obtained for μc-Si :H n-i-p solar cell with 2 μm i-layer and 0.253 cm2 area at a deposition rate of 5 Å/s. These findings suggest that the TPJ annealing of AZO with n+-a-Si capping layer is a possible method for further high performance of μc-Si thin-film solar cells at high temperature regime.

thumbnail Fig. 6

I-V characteristics and QE spectra of n-i-p μc-Si thin-film solar cells with textured AZO. The intrinsic μc-Si :H :Cl layer used was 2 μm thickness and fabricated from a SiH2Cl2 and H2 mixture by rf PE-CVD. Inset shows the XRD pattern of corresponding μc-Si films.

4 Conclusions

We demonstrated the effects of a-Si capping layers on ZnO :Al (AZO) during rf thermal plasma jet annealing. Both AZO and a-Si layers were crystallized by the TPJ annealing at a maximum surface temperature Tmax above 825 °C. In addition, the film crystallization of a-Si layer was promoted efficiently on textured AZO by TPJ annealing with no significant diffusion of oxygen at the interface. The role of a-Si capping layer on AZO during TPJ annealing is the suppression of the fluctuation of the band potential due to the segregation of Al2O3 phase in ZnO. Higher haze values above 70% were obtained in the entire spectra range from 400 to 1500 nm maintaining.

Acknowledgments

This work was supported partially by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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All Figures

thumbnail Fig. 1

Resistivity vs. maximum surface temperature Tmax of AZO films with and without a 30-nm-thick n+-a-Si capping layer after TPJ annealing.
In the text
thumbnail Fig. 2

(a) Hall mobility μ and (b) carrier density Ne of TPJ-annealed AZO films at different Tmaxs. The n+-Si layer on top of AZO was etched out using reactive ion etching after TPJ annealing. The TPJ annealing was performed by changing vsub from 55 to 22 mm/s.
In the text
thumbnail Fig. 3

Depth profiles of atomic concentrations, Si, Zn, Al, and O for n+-a-Si/AZO heterostructures before and after TPJ annealing monitored by XPS. The measurement was performed each after 5-nm-thick etching by Ar plasma.
In the text
thumbnail Fig. 4

Total transmittance Ttotal and haze spectra of TPJ-annealed AZO films with different surface morphology. The wet chemical etching was conducted after TPJ annealing by a 0.5% HCl at different dipping times. The TPJ annealing was performed at a vsub of 25 mm/s and a Tmax of 650 °C. The inset shows the AFM images of the 80 and 100 s chemically etched AZO film.
In the text
thumbnail Fig. 5

GI-XRD patterns of TPJ-annealed 30-nm-thick n+-, p+-, and intrinsic- crystallized Si films on textured AZO-coated glass substrates after TPJ annealing at a Tmax of 825 °C. The result of intrinsic- crystallized Si on flat AZO is also shown as a reference.
In the text
thumbnail Fig. 6

I-V characteristics and QE spectra of n-i-p μc-Si thin-film solar cells with textured AZO. The intrinsic μc-Si :H :Cl layer used was 2 μm thickness and fabricated from a SiH2Cl2 and H2 mixture by rf PE-CVD. Inset shows the XRD pattern of corresponding μc-Si films.
In the text

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