Efficiency improvement of PERC solar cell using an aluminum oxide passivation layer prepared via spatial atomic layer deposition and post-annealing

Surface & Coatings Technology 358 (2019) 968–975

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Efficiency improvement of PERC solar cell using an aluminum oxide passivation layer prepared via spatial atomic layer deposition and postannealing

T

Chia-Hsun Hsua,b, Chun-Wei Huangc, Yun-Shao Chob, Wan-Yu Wub, Dong-Sing Wuuc, ⁎ Xiao-Ying Zhangd, Wen-Zhang Zhud, Shui-Yang Liena,b, , Chang-Sin Yee,f a

School of Opto-electronic and Communication Engineering, Xiamen University of Technology, Xiamen 361024, China Department of Materials Science and Engineering, Da-Yeh University, Taiwan Department of Materials Science and Engineering, National Chung-Hsing University, Taiwan d School of Opto-electronic and Communication Engineering, Fujian Provincial Key Laboratory of Optoelectronic Technology and Devices, Xiamen University of Technology, Xiamen 361024, China e Metal Industries Research & Development Centre, Taiwan f Metal Industries Research & Development Centre, Opto-Electronics System Section, Taiwan b c

ARTICLE INFO

ABSTRACT

Keywords: Spatial atomic layer deposition Post annealing Aluminum oxide Passivation Passivated emitter and rear contact solar cell

In this study, Al2O3 thin films are deposited on p-type silicon using spatial atomic layer deposition with trimethylaluminum and H2O. The films are annealed in atmosphere (ATM), forming gas (FG) and nitrogen (N2) at temperatures of 300–750 °C. Effects of annealing gas ambient and temperature on structural, electrical, and passivation properties of Al2O3/Si are systematically investigated. The experimental results show that the ATM annealing leads to an interfacial silicon oxide more than two times thicker than that of the samples annealed in FG and N2. The ratio of tetrahedral AlO4 to octahedral AlO6 can be correlated to the negative fixed oxide charge density (Qf) near the interface of Al2O3/Si. The 600 °C ATM-annealed sample has the highest Qf of 3.23 × 1012 cm−1 and thus gives the best field effect passivation. The sample annealed in FG at 450 °C has the lowest interface trap density (Dit) of 3.98 × 1011 eV−1 cm−2, indicating that the hydrogen is more effective than oxygen for chemical passivation. Overall, the FG-annealed sample has the highest lifetime of 933.8 μs, showing that chemical passivation is the primary consideration. The ATM annealing requires higher temperatures (~600 °C) to reach the optimal passivation, whereas the FG and N2 annealing temperature is limited to 450 °C to avoid dehydrogenation. Finally, for passivated emitter and rear contact cell fabrication, the cell with FG annealing has the best conversion efficiency of 21.43%, which is 0.17 and 0.54 percentage point higher than that of the cells with ATM and N2 annealing, respectively.

1. Introduction Passivated emitter and rear contact (PERC) solar cell has emerged as a promising technology with higher efficiency in recent years. The most remarkable feature of PERC cells is the rear passivation structure that greatly reduces the dangling bonds and surface recombination at the rear of silicon wafers. Aluminum oxide (Al2O3) is considered as an ideal materials due to its negative oxide charges and chemical passivation [1]. Al2O3 can be deposited by various techniques such as plasma enhanced chemical vapor deposition (PECVD) [2], sputtering [3] and atomic layer deposition (ALD) [4]. In ALD, the film deposition cycle by cycle at a relative low substrate temperature. This leads to a self-

⁎

limiting film growth with high conformity, large-area uniformity and accurate control of film thickness [5]. Till now, many efforts have been made to optimize the deposition of Al2O3. Properties of Al2O3 films deposited by the ALD process using Al(CH3)3 (TMA) and O3 are reported [6,7]. Dueñas et al. reported the electrical characterization of Al2O3-based metal–insulator–semiconductor structures prepared by ALD using AlCl3 and H2O [8]. The mostly used precursors for ALD are TMA and H2O as they are inexpensively volatile liquids and thus easy to handle. It is found that the H2O-based ALD process of Al2O3 results in strong hydrogenation of Si wafers, and thus an interfacial SiOx layer is formed for most of the deposition processes [9]. This interfacial layer is found to play an important role of chemical passivation of Al2O3/Si

Corresponding author. E-mail address: [email protected] (S.-Y. Lien).

https://doi.org/10.1016/j.surfcoat.2018.12.016 Received 31 August 2018; Received in revised form 13 November 2018; Accepted 5 December 2018 Available online 06 December 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.

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AlO6

AlO4

AlO4

[11,12]. It has been reported that the minority carrier lifetime can reach 0.8–8 ms for a p-type FZ wafer passivated by vacuum ALD Al2O3 [13–16] or spatial ALD Al2O3 [17–19]. The passivation quality for ptype CZ wafers is lower, in the range of 0.1–2 ms [20,21]. Post-deposition annealing either in nitrogen gas ambient or in forming gas at 400–500 °C has been shown to significantly increase the wafer lifetime [6,22]. However, a comparison of passivation quality between different annealing gases is rarely reported. In this study, Al2O3 films are prepared on Si by spatial ALD using TMA and H2O as precursors with a deposition rate of 0.2 nm/min. The Al2O3 films are annealed in atmosphere (ATM), forming gas (FG) and nitrogen (N2) at temperatures of 300–750 °C. Effects of annealing ambient and temperature on silicon wafer passivation quality are systematically investigated. A comparison of electrical and structural properties of the annealed Al2O3/Si between these annealing gases is also discussed. Finally, performance of PERC solar cells with these different annealing processes is presented.

Si-O-Si

(a) ATM

Intensity (a. u.)

750°C

600°C

450°C

300°C

2. Experimental

(b) FG

Intensity (a. u.)

750°C

Boron-doped p-type (100) Czochralski silicon wafers with a thickness of 200 μm and a resistivity of 1 Ω-cm were used. The wafers were cleaned using a standard RCA process, and blow-dried with nitrogen. After cleaning the wafers were dipped in hydrogen fluoride to remove native oxide on silicon. Al2O3 films of 130 cycles (about 18 nm in thickness) were grown by a spatial ALD system, which was equipped with a multiple slit gas source head for precursors, inert gas, and the exhaust. TMA and H2O were used as precursors to deposit Al2O3 films. Nitrogen gas was used as a carrier gas and an inert separator gas. A movable substrate holder was set 1 mm below the gas delivery head. The TMA and H2O were kept at 17.5 °C and 30 °C, respectively. The gas delivery pipes were kept at 45 °C to avoid the recondensation of the precursors. The substrate temperature was kept at 150 °C. For minority carrier lifetime measurement, Al2O3 films were deposited on both sides of wafers, followed by annealing in a furnace with ATM, FG or N2 for 20 min. The annealing temperature was varied from 300 to 750 °C. For PERC fabrication, the wafers were textured using alkaline solution. A pn junction was formed by POCl3 diffusion in a standard tube furnace at 800 °C. The sheet resistance of the emitter was 75 Ω/square. Afterwards, a SiNx:H anti-reflective layer of 80 nm thickness was deposited on the front of the wafer by a 13.56 MHz radio-frequency inductively coupled plasma chemical vapor deposition (ICPCVD) system using a gas mixture of tetramethylsilane (TMS) and ammonia (NH3). The flow rates of TMS and NH3 were 35 and 25 sccm, respectively. The RF power was 1200 W. The deposition pressure was 5 mTorr. The substrate temperature was kept to 120 °C. The rear of the wafer were polished by 20% KOH solution at 70 °C for 3 min. An Al2O3 layer of 18 nm in thickness was deposited using spatial ALD at 160 °C. A SiNx film with a thickness of 120 nm was deposited on the Al2O3 layer at 120 °C. The samples were annealed in different annealing gases for 20 min. The rear local openings were created by laser scribing with a wavelength of 532 nm. The pitch and diameter of local contacts were 260 and 40 μm, respectively. Finally, rear aluminum paste and front silver paste was screen-printed and co-fired at a peak temperature of 860 °C. The minority carrier lifetime and one-sun implied Voc of the wafer has been determined using Sinton WCT-120 lifetime tester. The CV measurement was carried out by HP 4284A LCR meter. To fabricate metal-oxide-semiconductor (MOS) capacitors for CV measurements, the Al2O3 with a thickness of 18 nm was deposited on c-Si wafers by spatial ALD. The samples were then annealed in different gases. An Al layer with a thickness of 500 nm was evaporated on Al2O3 using a mask defining the electrode diameter of 2 mm. A 500 nm-thick planar Al was evaporated as a back electrode of the MOS capacitors. The current density-voltage (J-V) measurements were performed by a dual light source type solar simulator (Wacom Co., Japan) using both xenon lamp and halogen lamp with a calibrated class A AM 1.5G simulated light

600°C

450°C

300°C

(c) N2

Intensity (a. u.)

750°C

600°C

450°C

300°C

-1

Wavenumber (cm ) Fig. 1. Fourier transform infrared spectra of the Al2O3/Si samples annealed in (a) ATM, (b) FG and (c) N2 at different temperatures.

system. At the same time, post-annealing is required for Al2O3 as this process significantly improves passivation. The energy provided by annealing can reorganize the structure at the Al2O3/Si interface, and this increases the negative fixed oxide charge and decreases interface trap density [10]. Spatial ALD Al2O3 films have been extensively studied in recent years due to their higher deposition rate (0.03–1.2 nm/s) compared to that of the conventional vacuum-type ALD (< 0.03 nm/s) 969

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spectrum. The bonding configuration of the samples was obtained by Fourier transformation infrared (FTIR) absorption spectroscopy (Tensor 27, Bruker Optics, Germany) in 400–1200 cm−1 at room temperature. The cross-sectional images of the samples were observed using a transmission electron microscope (TEM, JEM-3000F, JEOL, Japan) with an acceleration voltage of 300 kV. The hydrogen atomic depth profiles were measured by secondary ion mass spectroscopy (SIMS, TOF-SIMS IV, Ion-ToF GmbH, Germany) with 5 keV Cs+ as primary ions.

AlO4/AlO6

(a)

3. Results and discussion Fig. 1 shows the FTIR spectra of Al2O3/Si annealed in ATM, FG and N2 at temperatures of 300 °C to 750 °C. The spectra have been convoluted into several peaks. The peak at 1080 cm−1 is attributed to SieOeSi bonds [23], indicating that SiOx presents at the Al2O3/Si interface. Note that for Al2O3 deposition in this study, H2O was used in the first half-reaction. An interfacial SiOx layer might formed in the first few cycles [24]. Post-annealing may also contribute to growth of the interfacial SiOx layer [24]. The peak at 570 cm−1 corresponds to octahedral AlO6 units, while the peaks at 670 cm−1 and 740 cm−1 can be assigned to tetrahedral AlO4 groups [25]. These two types of structures are the main components in Al2O3, but AlO4 units which is negatively charged is favorable for field-effect passivation on p-type wafers. Fig. 2a shows the peak intensity ratio of AlO4/AlO6, defined as peak intensity (I670 + I740) divided by I570, as a function of annealing temperature for different annealing gases. For ATM annealing, the ratio increases from 2.45 to 3.54 with increasing the temperature from 300 to 600 °C, and then drops to 2.9 when the temperature further increases to 750 °C. Similar trends can be seen in the cases of FG and N2 annealing, but the ratios are lower as compared to that of ATM annealing at all the temperatures. Fig. 2b shows the SieOeSi peak intensity as a function of temperature for different gases. The peak intensity of ATM

Si-O-Si intensity

(b)

Annealing temperature (°C) Fig. 2. (a) Ratio of AlO4/AlO6 and (b) SieOeSi peak intensity of the annealed Al2O3/Si samples as a function of annealing temperature.

(a)

300°C

C

450°C

600°C

Al2O3 c-Si

SiO2 (4.1 nm)

c-Si

300°C

10 nm

450°C Al2O3

C

c-Si SiO2 (1.8 nm)

C

c-Si SiO2 (2 nm)

Al2O3

C

Al2O3

C

10 nm

10 nm

10 nm

750°C Al2O3

C

Al2O3

C

c-Si SiO2 (2 nm) 10 nm

Fig. 3. TEM images of the Al2O3/Si samples annealed in (a) ATM, (b) FG and (c) N2. 970

C

c-Si SiO2 (2 nm)

c-Si SiO2 (2 nm) 10 nm

Al2O3

C

600°C

c-Si SiO2 (2 nm)

c-Si SiO2 (1.7 nm)

Al2O3 c-Si SiO2 (2 nm)

450°C

300°C

10 nm

750°C

10 nm

10 nm

(c)

10 nm

600°C Al2O3

C SiO2 Al2O3 c-Si (5.9 nm)

SiO2 Al2O3 c-Si (5.5 nm)

SiO2 Al2O3 (4.7 nm)

10 nm

(b)

750°C C

C

10 nm

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oxygen, the thickness of the interfacial SiOx layer (consisting of mainly [SiO4] tetrahedra) is thicker compared to that annealed in FG and N2 gas. More Al can substitute in Si site to form AlO4 at the Al2O3/SiO2 interface region [27]. Therefore, AlO4/AlO6 ratio is higher for the sample annealed in ATM. This result predicts that ATM annealing can obtain the highest amount of fixed negative oxide charges, as the fixed negative charge has been considered to originate from the AlO4 at the interface [19,28]. To confirm the interfacial oxide thickness for the different annealing gases, the TEM images of the Al2O3/Si samples are shown in Fig. 3. An interfacial SiOx layer between Al2O3 and c-Si can clearly be seen, and its thickness is indicated in the parenthesis. It can be seen that the interfacial oxide thickness insignificantly varies from 1.7–2 nm for FG and N2 annealing. Whereas the interfacial oxide thickness of the ATM annealed samples monotonically increase from 4.1 to 5.9 nm when the temperature increases from 300 to 750 °C. As the silicon wafer oxidation in the post-annealing process requires oxygen atoms reacting with silicon substrate surface to form SieO bonds, this increased thickness confirms that oxygen in ATM is able to pass through the Al2O3 layer and reach the wafer surface to form SiOx during the annealing process. Fig. 4 shows the C-V curves for Al/Al2O3/Si MOS samples annealed in different gases at 300–750 °C. The capacitance has been normalized to the maximum capacitance Cox. Generally, the flatband voltage (Vfb) in a C-V curve would fall in the depletion region, and equal to the work function difference between metal and semiconductor (Vms) if there is no Qf presented. The Vfb in this study is about −0.9 V, and all the curves shift to a higher voltage. This indicates the negative charges existing in the annealed films. The density of the negative charge can be further calculated using the following equation [29]

C/Cox

(a) ATM

C/Cox

(b) FG

Qf =

(Vms

V

fb ) Cox

qA

(1)

where q is the electron charge and A is the electrode area of MOS samples. V′fb is the flatband voltage in the case of presence of oxide charges, and is the voltage at which the measured capacitance equals to flatband capacitance (Cfb) as given by [29]

(c) N2

C/Cox

Cfb = =

Cox ( s A/ ) Cox + ( s A/ ) s kT q2 N

(2) (3)

where εs is the dielectric constant of silicon, λ is the Debye length, k is the Boltzmann constant, T is the absolute temperature, and N is the doping concentration of silicon wafer. The calculated Qf values for the samples annealed in different gases at the temperatures of 300–750 °C are shown in Fig. 5a. In the case of ATM annealing, Qf increases from −4.19 × 1011 to −3.23 × 1012 cm−3 with increasing the annealing temperature from 300 °C to 600 °C, and then decreases to −1.34 × 1012 cm−3 at the temperature of 750 °C. The FG and N2 annealing show smaller Qf values. The trend of Qf values confirms the prediction made from the FTIR result that samples with ATM annealing may have a higher fixed negative charge compared to the samples annealed in N2 and FG. On the other hand, Dit values can also be calculated from C-V measurement by using Terman method [30], as shown in Fig. 5b. The lowest Dit for ATM annealing is 7.21 × 1011 eV−1 cm−2 at 600 °C, while for FG and N2 annealing the lowest Dit values are respectively 3.98 × 1011 and 1.2 × 1012 eV−1 cm−2 at 450 °C. The oxygen in ATM and hydrogen in FG can bond with the dangling bonds, so the Dit values of these two annealing gases are significantly lower than N2 annealing. Passivation by ATM annealing mostly depends on the interfacial SiOx quality. A higher temperature improves the density of this layer. In contrast, hydrogen passivation requires temperatures lower than 550 °C to avoid dehydrogenation at the interface [31]. The optimal temperature of the ATM annealing is therefore higher than that of FG annealing. Overall, in the aspect of decreasing Dit, hydrogen

Voltage (V) Fig. 4. C-V curves for Al/Al2O3/Si MOS samples annealed in (a) ATM, (b) FG and (c) N2.

annealing increases with the temperature, whereas the peak intensities are smaller and only slightly change in FG and N2 annealing. It is reported that the oxygen from the annealing gas can interchange with the oxygen atoms of Al2O3, producing mobile oxygen atoms which go deeper and repeat the interchange process [26]. Eventually the oxygen propagation front could reach the Al2O3/Si interface. Therefore, the oxygen in ATM annealing could reach the Al2O3/Si interface, and enhance the thickness of the interfacial SiOx. The other two annealing gases would have no significant change in the interfacial oxide due to the absence of the oxygen. Furthermore, as the ATM annealing contains 971

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(a) ATM

-2

-Qf (cm )

Lifetime (µs)

(a)

Lifetime (µs)

(b) FG

1E+11

-2

Dit (eV-1cm )

(b)

Lifetime (µs)

(c) N2

Annealing temperature (°C) Fig. 5. (a) −Qf and (b) Dit for the annealed samples as a function of annealing temperature.

-3

Injection Level (cm ) Fig. 7. Injection-level dependent minority carrier lifetime of the Al2O3/Si samples annealed in (a) ATM, (b) FG and (c) N2.

(a) ATM Intensity

passivation provided by FG annealing is more effective than SiOx passivation provided by ATM annealing. Fig. 6 shows the SIMS profile of hydrogen in Al2O3/Si samples annealed in different gases at temperatures of 300–750 °C. The depth around 18 nm corresponds to the interface between Al2O3 and Si. All the samples show a maximal intensity at the interface. It is known that Al2O3 may have about 2%–3% hydrogen on after film deposition, and during annealing process the hydrogen in the film can be released and diffuse towards the interface to react with silicon dangling bonds [32]. Besides the hydrogen provided by Al2O3 films, the significantly higher SIMS intensity for FG annealing confirms that hydrogen provided in FG can also diffuse into the Al2O3 layer. This enhancement of the hydrogen

Minority carrier lifetime (µs)

Intensity

(b) FG

Intensity

(c) N2

Annealing temperature (°C)

Depth (nm)

Fig. 8. Minority carrier lifetime at injection-level of 1 × 1015 cm−3 for the annealed samples as a function of annealing temperature.

Fig. 6. SIMS depth profiles of hydrogen in the Al2O3/Si samples annealed in (a) ATM, (b) FG and (c) N2. 972

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c-Si Chemical Passiva on

SiO2 (2-5 nm) Field-Effect Passiva on

O H

Al2O3 film SiO2 film O H Metal

Repelled due to Nega ve Fixed Charge (-Qf)

Ec Dangling bond p-type c-Si

Al2O3 (~18 nm)

Ev

Fig. 9. Mechanism of chemical and field-effect passivation for Al2O3/Si system.

Implied Voc (mV)

using the following equation [33] Al2O3 (ATM) Al2O3 (FG) Al2O3 (N2)

Al2O3+Si3N4 (ATM) Al2O3+Si3N4 (FG) Al2O3+Si3N4 (N2)

implied Voc =

kT ln q

n (p + ni 2

n) (4)

where Δn is the minority carrier concentration measured by WCT-120 at one sun condition, p is the hole concentration, and ni is the intrinsic carrier concentration. As this measurement is performed before metallization, this value reflects the maximum Voc that can be achieved purely based on the intrinsic material quality and passivation quality, assuming no optical losses nor losses caused by imperfect contact architectures. The implied Voc values basically follow the trends of the minority lifetime values. The highest implied Voc of the samples annealed in FG and N2 are respectively 689 and 669.1 mV at 450 °C, while for ATM annealing the maximum value is 681.2 mV occurs at 600 °C. In order to be more close to the practical PERC solar cells with Si3N4/ Al2O3 rear passivation structure, we deposited the double-layer passivation structure on the best sample of each type of annealing gas. The corresponded implied Voc values are labeled as open symbols. It is seen that the implied Voc of the ATM, FG and N2 annealed samples with double-layer passivation elevate to 699, 703 and 686.9 mV, respectively. The improved Voc values are due to the enhancement of the passivation brought by the Si3N4 layer. It is reported that PECVD SiNx deposited at ~400 °C and stacked on a thin Al2O3 (< 10 nm) is able to improve silicon wafer passivation after annealing or firing [14,34–36]. A hydrogen release of the SiNx layer, which improves the interface hydrogenation, is considered as a possible explanation [37]. In this work, SiNx films were deposited by ICPCVD at 120 °C using a gas mixture of Si(CH3)4 and NH3. The hydrogen atoms were incorporated in the films during deposition. After the annealing process, the wafer passivated by SiNx/Al2O3 is higher than the wafer passivated by Al2O3 single layer. In addition, from SIMS measurement (not shown here), it is observed that after annealing, hydrogen at interface is higher for the wafer with SiNx/Al2O3 than for the wafer with Al2O3 single layer, and this might be a possible reason for the improved lifetime. The fabricated six-inch PERC cells with different annealing gases are shown in Fig. 11. The extracted Voc, Jsc, FF and η are listed in Table 1. The Jsc and FF values of the cells range between 39.3 and 39.42 mA/ cm2 (0.3% of difference) and between 0.813 and 0.814 (0.12% of difference), respectively, indicating the minor effect of the annealing gas ambient on Jsc and FF. The change in Voc is more obvious. The Voc is 663 mV for ATM annealing, 668 mV for FG annealing and 653 mV for N2 annealing. Overall, the FG annealed cell can have best cell conversion efficiency of 21.43%, about 0.17 percentage points higher than that of the ATM annealed cell and 0.54 percentage point higher than that of the N2 annealed cell.

Annealing temperature (°C) Fig. 10. Implied Voc of the annealed samples. The open symbols correspond to the Al2O3 passivation layer capped with a Si3N4 layer.

content near the interface region accounts for the low Dit value observed in 450 °C FG annealing. The hydrogen intensity reduces at 600 °C and 750 °C as SieH bonds start to break when the temperature exceeds 550 °C [31]. Fig. 7 shows the injection-level dependent minority carrier lifetime for the samples annealed in different gases at temperature of 300–750 °C. The carrier lifetime values at injection level of 1 × 1015 cm−3 are extracted for the purpose of comparison as shown in Fig. 8. It can be seen that ATM annealing has a maximum lifetime of 737.1 μs at 600 °C, while the maximum lifetimes of FG and N2 occur at 450 °C. Overall, among the used annealing gases, FG gives the best lifetime of 933.8 μs. The highest lifetime of the samples annealed in N2 only reaches 497.2 μs. This indicates that annealing in gases containing passivating sources such as oxygen or hydrogen is strongly required as compared to annealing in inert gas. Based on the above results, the mechanism of passivation by the ATM, FG and N2 annealing is discussed, as shown in Fig. 9, in terms of chemical passivation and field-effect passivation. The former is associated with reduction of dangling bonds on wafer surface, and can be evaluated by Dit. The field-effect passivation is related to Qf in the Al2O3 layer that causes band bending of the conduction band of silicon, repelling electrons away from the silicon surface. We can compare two samples, one is annealed in FG at 450 °C with the best chemical passivation, and the other is annealed in ATM at 600 °C having the best fieldeffect passivation. It is seen that the FG annealed sample has a Dit 1.8 times smaller than the ATM annealed sample, while the latter has two times higher Qf. As minority carrier lifetime is a combination of the results of chemical passivation and field-effect passivation, the higher lifetime of the FG annealed sample clearly shows that chemical passivation is the primary consideration. Fig. 10 shows the one sun implied Voc of the samples annealed in ATM, FG and N2 as a function of annealing temperature. It is calculated

4. Conclusion The Al2O3/Si samples are annealed in different gases at temperatures of 300–750 °C. The Qf can be positively correlated to AlO4/AlO6 intensity ratio. The sample annealed in ATM at 600 °C has the highest 973

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Current density (mA/cm2)

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Voltage (V) Fig. 11. J-V curves of the PERC solar cells with the Al2O3 layer annealed in ATM, FG and N2.

Table 1 Detail parameters of the fabricated PERC solar cells with ATM, FG and N2 annealing.

Voc (mV) Jsc (mA/cm2) FF (%) η (%)

ATM

FG

N2

663 39.4 81.4 21.26

668 39.42 81.4 21.43

653 39.3 81.4 20.89

Qf of 3.23 × 1012 cm−3. Only the samples annealed in ATM show widening of the interfacial SiOx layer. Annealing in FG at 450 °C leads to the lowest Dit of 3.98 × 1011 cm−3. The ATM and FG annealing can significantly improve carrier lifetime as compared to the N2 annealing. The hydrogen is more effective than oxygen to passivate the dangling bonds on wafers. The best minority carrier lifetime is 933.8 μs for the FG annealed sample, indicating that chemical passivation is the predominating factor of passivation. The implied Voc can reach 689 mV for the FG annealed sample, and it can further increase up to 703 mV when Si3N4 is stacked on Al2O3. Finally, the PERC cell fabricated with FG annealing has the best conversion efficiency of 21.43%, compared with that fabricated with ATM and N2 annealing which were 21.26% and 20.89%, respectively. Acknowledgement This work is sponsored by the Ministry of Science and Technology of the Republic of China under the grants No. 105-2632-E-212-001, 1062622-E-212-007-CC3 and 104-2221-E-212-002-MY3. This work is also supported by the National Natural Science Foundation of China (no. 61474081, 61534005 and 61307115), the Science Technology innovation project of Xiamen (no. 3502Z201730404) and the Fundamental Research Funds for the Central Universities (no. 20720150028). References [1] G. Dingemans, W.M.M. Kessels, Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells, J. Vac. Sci. Technol. A 30 (2012) 40802, https://doi.org/10.1116/1.4728205. [2] S. Kühnhold, B. Kafle, L. Kroely, P. Saint-Cast, M. Hofmann, J. Rentsch, R. Preu, Impact of thermal treatment on PECVD Al2O3 passivation layers, Energy Procedia 27 (2007) 273–279, https://doi.org/10.1016/j.egypro.2012.07.063. [3] J.A. García-Valenzuela, R. Rivera, A.B. Morales-Vilches, L.G. Gerling, A. Caballero, J.M. Asensi, C. Voz, J. Bertomeu, J. Andreu, Main properties of Al2O3thin films deposited by magnetron sputtering of an Al2O3ceramic target at different radiofrequency power and argon pressure and their passivation effect on p-type c-Si wafers, Thin Solid Films 619 (2016) 288–296, https://doi.org/10.1016/j.tsf.2016. 10.049.

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