Efficiency Enhancement of Multicrystalline Silicon Solar Cells by Inserting Two-Step Growth Thermal Oxide to the Surface Passivation Layer

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Research Article

Efficiency Enhancement of Multicrystalline Silicon Solar

Cells by Inserting Two-Step Growth Thermal Oxide to the Surface

Passivation Layer

Shun Sing Liao, Yueh Chin Lin, Chuan Lung Chuang, and Edward Yi Chang

Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu 30010, Taiwan Correspondence should be addressed to Edward Yi Chang; [email protected]

Received 6 June 2017; Revised 6 August 2017; Accepted 7 September 2017; Published 8 October 2017 Academic Editor: Giulia Grancini

Copyright © 2017 Shun Sing Liao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this study, the eﬃciency of the multicrystalline was improved by inserting a two-step growth thermal oxide layer as the surface

passivation layer. Two-step thermal oxidation process can reduce carrier recombination at the surface and improve cell eﬃciency.

Theﬁrst oxidation step had a growth temperature of 780°C, a growth time of 5 min, and with N2/O2gasﬂow ratio 12 : 1. The second

oxidation had a growth temperature of 750°C, growth time of 20 min, and under pure N2gas environment. Carrier lifetime was

increased to 15.45μs, and reﬂectance was reduced 0.52% using the two-step growth method as compared to the conventional

one-step growth oxide passivation method. Consequently, internal quantum eﬃciency of the solar cell increased 4.1%, and

conversion eﬃciency increased 0.37%. These results demonstrate that the two-step thermal oxidation process is an eﬃcient way

to increase the eﬃciency of the multicrystalline silicon solar cells.

1. Introduction

Many methods have been proposed to improve multicrystal-line silicon (mc-Si) solar cell efficiency. The mc-Si wafers usually contain significant amounts of defects and impurities which affect minority carrier lifetime and limit multicrystal-line solar cell efficiency. Solar cell efficiency can also be affected by surface recombination of the carriers. Macdonald et al. [1] and Krotkus et al. [2] successfully enhanced mc-Si solar cell efficiency using several methods, including dry texturing, gettering, selective emitters, hydrogen surface passivation, and using of silicon nitride (SiNx) passivation

layer on the emitter surface.

An enhanced lifetime of the photon-generated carriers has also been realized using SiO2, SiNx, and SiCNxﬁlms as

surface passivation layers [3–5]. Jana et al. [6] showed the interface trap density (Dit) (2.52× 1011cm−2eV−1) at the

SiNx/Si interface. The growth of a high-quality SiO₂ layer with low interface trap density (D_it) on silicon is an effective surface passivation method for the solar cells to further improve cell efficiency [7–9]. Derbali and Ezzaouia [10] reported that thermal SiO2 film is an effective surface

passivation layer for highly phosphorus-doped silicon wafers. Ohtsuka et al. [11] showed the Dit(1.0× 1011cm−2eV−1) at

the SiO2/Si interface. Comparing Dit of without thermal

oxidation (TO) and thermal oxidation (TO) was from 2.52× 1011cm−2eV−1 to 1.0× 1011cm−2eV−1. Kotipallia et al. [12] showed the Dit reduction at the silicon/dielectric

interface by using hydrogen gas annealing to reduce the dan-gling bonds. Schultz et al. [13] showed the inﬂuence of ther-mal oxidation temperature on the D_itof the interface. High temperature (>1000°C) dry oxidation growth of the SiO₂ﬁlm results in low Dit (10

10

cm−2eV−1) at the SiO2/Si interface,

but the process may deteriorate bulk Si quality and reduce bulk carrier lifetime. Reducing the growth temperature to 800°C with wet oxidation (Si + 2H2O→ SiO2+ 2H2) can

increase the bulk carrier lifetime. The eﬃciency of the sample annealed at high temperature (1050°C) was 17.20%, while at lower temperature (800°C) was 17.8%. The open circuit volt-age increased about 16 mV at lower temperature, this is because the low temperature (800°C) passivation has higher bulk lifetime than the high temperature (1050°C) passivation. But using this temperature with dry oxidation process has not been studied. Chen et al. [14] and Hiroshige et al. [15]

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showed that low temperature grown SiO2 layers also have

low interface Ditwith Si wafers when used for surface

passiv-ation. Cai and Rohatgi [16] showed that the most eﬀective annealing temperature after PECVD for multicrystalline solar cell wafers was 650°C to 750°C.

Narasimha and Rohatgi [17] showed that growth of the SiO₂ layer by rapid thermal oxidation had a low surface recombination velocity (S = 10 cm/s) on the silicon surface, and Chen et al. [18, 19] showed that a SiO2layer deposited

on Si wafer by PECVD followed by rapid thermal annealing could produce lower S and higher carrier lifetime. In this work, the use of lower temperature (750°C–780°C) two-step grown SiOx ﬁlm as passivation layers using dry oxidation (Si + O₂→ SiO₂) process was performed and studied. The goal was to improve bulk Si crystalline quality and the bulk

carrier lifetime after lower temperature thermal oxidation process and to compare the crystalline quality and carrier lifetime with high temperature (>1000°C) grown oxide. The efficiency enhancement of the mc-Si solar cells was observed after by applying silicon oxide passivation layer grown by two-step thermal oxidation (TO) method. The TO process influence on mc-Si solar cell electrical properties is also investigated and found that by proper design of the oxidation process, the solar cell electrical properties can be effectively improved.

2. Experimental

Figure 1(a) shows the sample structure of the solar cell in this study, fabricated on a p-type mc-Si wafer, and Figure 1(b)

Ag

n-type emitter

Back surface field

Ag/Al Al

Silicon oxide SiNx

p-type multicrystalline silicon

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Sample B process flow Acid texture

POCl₃ phosphorus diffusion

Removal phosphosilicate glass

Wet chemical cleaning

Thermal oxidation process

Deposit SiNx antirefection layer

Screen-printing Ag and drying Screen-printing AgAl and drying

Screen-printing Al and drying

Sintering I-V testing Sample A process flow

Acid texture

POCl₃ phosphorus diffusion

Removal phosphosilicate glass

Wet chemical cleaning

Deposit SiNx antirefection layer

Screen-printing Ag and drying

Screen-printing AgAl and drying

Screen-printing Al and drying

Sintering I-V testing

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Figure 1: (a) The cross section of the completed multicrystalline silicon solar cell and (b) fabrication process ﬂow of the multicrystalline silicon solar cells.

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shows the fabrication process, consisting of Al/p-Si/n-Si/ SiO2/SiNx/Ag layers. The fabrication process commenced with acid texturing, followed by a shallow diﬀusion layer formed by POCl3 phosphorus diﬀusion process, resulting

in 82 Ω/sq sheet resistance. The phosphosilicate glass was then removed by the wet chemical method, and each wafer was divided into sample A (without TO process) and sam-ple B (with TO process). A high-quality batch-type quartz tube furnace system was used for the two-step TO process, with a temperature proﬁle as shown in Figure 2 to grow oxide on the wafer. The ﬁrst oxidation step had a growth temperature of 780°C, with growth time 5 min, and N2/O2

gas ﬂow ratio 12 : 1. The second oxidation step had a growth temperature of 750°C, growth time 20 min, and with pure N₂ gas environment. To improve incident light absorption, PECVD SiNxﬁlm was deposited on both sam-ples after oxidation.

Screen printing was employed to deposit Ag glue on the front side SiNx layer and AgAl glue on the back side of the wafer, with an intermediate drying process. Finally, the samples were annealed at 750–850°_{C to complete the solar}

cell fabrication.

3. Results and Discussion

Texturizing of the solar cell surface was performed with HF/HNO3/H2SO4 and deionized water mixture to form a

randomly oriented micron-size texture. Figures 3(a) and

3(b) show plane-view optical microscopic (OM) images of pre- and postetched wafer surfaces at 100x magniﬁcation.

Schmidt and Aberle [20] showed that the carrier lifetime can be measured by the microwave photoconductance decay (Semilab system, Model WT-2000). Both samples were mea-sured; Figures 4(a) and 4(b) show the carrier lifetime for sam-ples A and B (with and without oxide layer, resp.), and average carrier lifetimes were 5.55 and 21μs, respectively. Thus, carrier lifetime was signiﬁcantly improved by the pro-posed TO process. Lee and Glunz [21] showed that improve carrier lifetime also improved the solar cell eﬃciency.

Surface reflectance was measured using a UV-VIS-NIR spectrophotometer (HMT, Model MFS-630) in the spectral range 350–1050 nm; after SiNx film deposition are shown in Figure 5(a) test data and summarized in Table 1. Average reflectances were 3.28% and 2.76% for samples A and B, respectively; sample B (with TO) had a lower reflectance of 0.52%. Lee et al. [22] showed that the solar cell absorption band was in the range 400–1200 nm. Thus, the lower average reflectance of sample B results in higher efficiency of the solar cell. Meziani et al. [23] showed that reflectance were 2.77% and 0.94% at 600–800 nm range for SiNx and SiNx/SiO2

pas-sivation, respectively. The refractive index (n) of SiNx and SiO₂were 2 and 1.45, respectively. Thus, adding SiO₂ ﬁlm of low refractive index (n) can decrease reﬂectance of the light (Figure 6).

Internal quantum eﬃciency (IQE) was measured using a Solar Cell Scan 100 quantum eﬃciency measurement system 650 780 780 750 750 700 500 550 600 650 700 750 800 Profile Time (Min) T em p era tur e ( °C) N₂_{: O}₂_{= 12 : 1} N₂_{: O}₂_{= 1 : 0} N₂_{: O}₂_{= 1 : 0 N}₂ : O₂ = 12 : 0 N₂ : O₂ = 1 : 0 Oxidation growth Atomic rearrangement 0 10 15 20 40 50 Phase change Phase equilibrium

Figure 2: Two-step thermal oxidation temperature versus time proﬁle.

2 휇m

(a)

2 휇m

(b)

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(Model QEX10) at 350–1050 nm spectral range as shown in Figure 5(b), and the results are summarized in Table 1. Aver-age IQE data were 84.62% and 88.72% for samples A and B, respectively, an increase of 4.1% for sample B.

The IQE of a solar cell with and without TO passivation can provide information on the passivation effect from the surface and bulk recombinations. For wavelengths< 800 nm, IQE provides information related to S, while for wave-lengths> 800 nm, IQE provides information related to τ. Figure 5(b) shows that for sample B, IQE increased mostly in the spectral range 600–1050 nm, with its reflectance decrease over that range due to the SiOx/SiNx antireflection

coating. Thus, the treatments eﬀectively increased light absorption of the solar cell.

Thus, sample B has enhanced IQE over sample A. Since the major IQE enhancement is in the 600–1050 nm range, it is mainly due to reduced reﬂectance.

Morales-Acevedo and Pérez-Sánchez [24] showed that improve eﬃciency of without TO and with TO (SiO2) were

11.37% and 11.53%, respectively, and conversion efficiency increased 0.16%. Mack et al. [25] showed that improve e ffi-ciency of without TO and with TO (SiO₂) were 17.8% and 18.1%, respectively, and conversion efficiency increased 0.3%. Gatz et al. [26] showed that improve efficiency of back

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Figure 4: Carrier lifetime maps with respect to time: (a) sample A (without thermal oxidation) average carrier lifetime was 5.5 μs and

(b) sample B (with thermal oxidation) average carrier lifetime was 21μs.

0 450 350 550 650 750 850 950 1050 2 4 6 8 10 12 14 16 18 20 22 Reflectivity (%) Wavelength (nm) Sample A Sample B (a) 50 55 60 65 70 75 80 85 90 95 100 IQE (%) 450 550 650 750 850 950 1050 Wavelength (nm) Sample A Sample B (b)

Figure 5: The solar cell data (a) reﬂectance and (b) internal quantum eﬃciency (IQE) curves. Table 1: Measurement results of the samples of the multicrystalline silicon solar cells.

Samples Carrier lifetime (μs) Reﬂectance (%) IQE (%) _{Eﬃciency (%)}I-V characterization

VOC(V) ISC(A)

A 5.55 3.28 84.62 17.27 0.623 8.59

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side SiNy/SiNx stacks for the surface passivation was from 17.9% to 18.3%, and conversion efficiency increased 0.4%. The absolute efficiency gain by surface passivation in single crystalline from 2006 to 2012 was only 0.16%~0.4% as shown in Figure 7 [24–26]. Thus, the increase of 0.4% is quite significant.

In this study, the I-V characteristics of the samples were measured under illumination (Berger Lichttechnik, Model PSS 10 II), as summarized in Table 1. The efficiencies of sample A and sample B were 17.27% and 17.64%, respec-tively. The conversion efficiency increase of 0.37% is signif-icant. Sample B efficiency was 2.14% higher than that of sample A, open circuit voltage (Voc) was 1.44% higher,

and short circuit current (Isc) was 1.51% higher than that

of sample A. Thus, the proposed two-step TO process is an eﬀective way to increase mc-Si solar cell eﬃciency and related characteristics.

4. Conclusions

This study successfully demonstrates enhanced multicrystal-line silicon solar cell efficiency by inserting a two-step growth thermal oxide layer to the SiNx passivation layer. Thefirst oxidation step had a growth temperature of 780°C, growth time of 5 min, and with N₂/O₂gasflow ratio 12 : 1; the second step had a growth temperature of 750°C, growth time of 20 min, and under pure N2gas environment. The wafers were

annealing at the temperature of 750°C to 780°C after the dry

oxidation process. The goal was to improve bulk Si crystalline quality and bulk carrier lifetime after thermal oxidation. The proposed two-step process retained thin oxide layers, pre-venting eﬀect formation of SiNx layer optical characteristics, which can reduce eﬃciency.

The mc-Si solar cell eﬃciency improved to 17.64% from after inserting two-step growth thermal oxide layer between the p-type Si/n-type Si and SiN_xpassivation layer. The resul-tant silicon oxide layer increased the absorption of the light in the spectral range 400–1050 nm by 4.1% and thus improved the total absolute eﬃciency of the mc-Si solar cell.

Conflicts of Interest

The authors declare that they have no conﬂicts of interest.

Acknowledgments

This work was sponsored by TSMC, NCTU-UCB I-RiCE program and Ministry of Science and Technology, Taiwan, under no. MOST106-2911-I-009-301.

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