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PECVD-SiNx stacks
Materials Science in Semiconductor Processing 100 (2019) 214–219
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Plasma-induced damage and annealing repairing in ALD-Al2O3/PECVD-SiNx stacks
T
Shizheng Lia, Ning Yanga, Xiao Yuana, Xiaojun Yea, Liangxing Wangb, Fei Zhengc, Cui Liua,∗, Hongbo Lia,∗∗ a
School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, China School of Information Science and Technology, ShanghaiTech University, Shanghai, 201210, China c Shanghai Shenzhou New Energy Develepment Co., Ltd., Shanghai, 201112, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silicon passivation Atomic layer deposition Al2O3 Interface defect density Plasma-enhanced chemical vapor deposition Solar cells
We study the effect of plasma-enhanced chemical vapor deposition (PECVD) SiNx process to atomic layer deposited (ALD) Al2O3 films on crystalline silicon surface passivation. The plasma-induced damage on Al2O3 films is affected by the methods of PECVD process (direct or microwave), and the states (as-deposited or annealed) and the thickness of ALD-Al2O3 films. The passivation degradation may be related to the doping of Si, N and H atoms into Al2O3 films during PECVD process, and is mainly manifested in the form of affecting interface defect density rather than negative fixed charge density. In particular, the annealed thick Al2O3 films with direct-PECVD SiNx process show more severe passivation degradation. Anyhow, to the great degree, the passivation quality can be repaired and further improved by post-annealing. Low temperature long time annealing can effectively increase the lifetime of the ALD-Al2O3/PECVD-SiNx stacks, while it cannot be simply replaced by a rapid high temperature firing. By post-annealing repairing, an absolute 0.13% increase in efficiency can be achieved for p-PERC solar cells.
1. Introduction Efficient surface passivation of crystalline silicon solar cells is of great importance to achieve high conversion efficiency [1–3]. In recent years, the Al2O3 passivation film deposited by atomic layer deposition (ALD) has been widely used at the rear surface of p-type passivated emitter and rear contact (p-PERC) solar cells and the p+ emitter of ntype solar cells for its excellent chemical passivation and field-effect passivation [4–9]. Specifically, thin ALD-Al2O3 films (less than 10 nm) are used at the front surface of n-type solar cells, matching with the optically designed plasma-enhanced chemical vapor deposited (PECVD) SiNx to achieve better anti-reflection property [10,11]. Thick ALDAl2O3 films (10–20 nm) are used in p-PERC solar cells, coating with a thick layer of SiNx (∼100 nm or thicker) as a protector against the aluminum paste for preferable passivation performance [12]. In solar cell industry, SiNx layers are commonly deposited by both microwavePECVD (mw-PECVD, called remote-PECVD as well) and direct-PECVD, and the direct-PECVD is increasingly being used in production. Although it has been reported that the passivation quality and the thermal stability can be improved with the mw-PECVD SiNx capping layers, the ∗
relationship between the Al2O3 films and the direct-PECVD process is not fully understood yet [10,13–15]. It has also been reported that the PECVD process, especially the ammonia plasma, will change the structure of Al2O3 films to some extent, which will have a beneficial or detrimental effect on the passivation quality [16–19]. Practically, the high temperature firing process is applied during metallization to improve the passivation quality, whereas the annealing process is usually omitted due to cost consideration. However, for achieving better passivation quality, the damage caused during the PECVD process has to be repaired. In this work, the passivation quality was characterized by elemental distribution of Al2O3/SiNx stacks and studied as a function of the thickness of Al2O3 films with the different methods of the PECVD SiNx deposition. Furthermore, the impact of annealing repairing process on the film properties were studied. Finally, two groups of p-PERC solar cells were prepared with above mentioned annealing repairing process to the Al2O3/SiNx stacks to observe the variation of the electrical parameters. The present work aims to improve the passivation quality of the solar cells and to achieve higher efficiency.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (H. Li).
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https://doi.org/10.1016/j.mssp.2019.05.010 Received 27 January 2019; Received in revised form 3 May 2019; Accepted 8 May 2019 Available online 14 May 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
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variation of Al2O3 films and H passivation at the Al2O3/Si interface, and by increasing Qf, respectively [3,13,23–25]. Obviously, the Qf values are relatively close to different thicknesses, so that the greatly reduced Dit values are the major factor of the improved passivation quality with the thicker Al2O3 films. For solar cells, both mw-PECVD and direct-PECVD are used to deposit the SiNx layer which actually always cap the Al2O3 films [10,26]. During the direct-PECVD SiNx process, considerable ion bombardment on the ALD-Al2O3 films occurs, introducing large amounts of plasma damage [27,28]. However, in mw-PECVD, the plasma is excited outside the deposition chamber and the Al2O3 films are not directly contacting with the plasma, thus significantly reducing the plasma-induced damage [27,29]. These two types of PECVD SiNx methods result in the different passivation qualities of the Al2O3/SiNx stacks. Fig. 2 presents the effective lifetime of as-deposited (AS) and annealed (AN) Al2O3 films with the thicknesses of 5 and 20 nm, and those samples after mwPECVD or direct-PECVD SiNx process (PE). As shown in Fig. 2 (a) and (b), the effective lifetime increases after the PECVD process for the effect of annealing with the appropriate deposition temperature (400 °C for mw-PECVD and 480 °C for direct-PECVD) [30]. It is worth noting that although the temperature used in direct-PECVD is also within the range of the appropriate Al2O3 annealing temperature, the effective lifetimes for the as-deposited Al2O3/PECVD SiNx stacks are still relatively lower than the annealed single Al2O3 films when Al2O3 films are thick (20 nm). For annealed Al2O3 films, however, the effective lifetimes are almost unchanged (5 nm, Fig. 2 (c)), or even reduced (20 nm, Fig. 2 (d)), especially on the annealed 20 nm Al2O3 film capped with direct-PECVD SiNx. To improve the passivation quality, samples after PECVD SiNx were post-annealed at a low temperature of 400 °C under the nitrogen atmosphere [15,31] for a short time of 5 min (short time annealing, SA) or for a long time of 30 min (long time annealing, LA), respectively. For the as-deposited 5 nm Al2O3 films, the effective lifetime shows only slight change, indicating that the as-deposited thin Al2O3 films are insensitive to the post-annealing process, and therefore, contribute little to improvement of the passivation qualities. However, for the as-deposited 20 nm Al2O3 films, post-annealing process can further improve the passivation quality to the level of annealed 20 nm Al2O3 films. When the 20 nm Al2O3 films were annealed, the effective lifetimes continuously increased after 5 min and 30 min post-annealing, showing that the post-annealing process can effectively repair the passivation quality for annealed 20 nm Al2O3 films. It should be noticed that to recover the effective lifetime for mw-PECVD, a short time annealing process is enough, but for direct-PECVD process it should be a long time. Fig. 3 presents an additional measure of interface quality including Dit and Qf measured with the same lifetime samples. It can be observed that Qf values of Al2O3/SiNx stacks have a slight decline comparing with that of annealed single Al2O3 films (∼2–3 × 1012 cm−2), due to the positive fixed charge carried by SiNx [30,32,33]. Direct-PECVD SiNx shows more decline, but still over the level of 1 × 1012 cm−2. However, the Dit is relatively high for the Al2O3/SiNx stacks after PECVD comparing with the annealed single 20 nm Al2O3 film (< 1 × 1011 eV−1 cm−2). Particularly for the annealed 20 nm Al2O3/ direct-PECVD SiNx stacks, the Dit is over 1 × 1012 eV−1 cm−2, which is close to the Dit value of the as-deposited 20 nm Al2O3 film, showing severe plasma damage caused by direct-PECVD process. After post-annealing for 5 min or 30 min, Qf increases slightly only, still a little lower than the annealed single Al2O3 films. The as-deposited 5 nm Al2O3 film is an exception in that Dit can be effectively reduced by PECVD process, and insensitive to the post annealing process, showing that the PECVD process is enough for improving the passivation quality. In the other three situations, Dit can be further reduced by post-annealing, consistent to that of effective lifetime shown in Fig. 2. Therefore, Dit is the major factor to affect the passivation quality in ALD-Al2O3/PECVD-SiNx stacks.
2. Experimental details Solar-grade n-type crystalline Czochralski (CZ) Si wafers with the resistivity of 1–7 Ω cm, (1 0 0)-oriented, 180 ± 20 μm thickness, and 156.75 × 156.75 mm2 overall size were used in this study. Surface damage caused during wafer sawing was removed by etching in 15% KOH solution at 80 °C for 15 min (∼15 μm each side) followed by standard RCA cleaning process [20]. Oxidation layer was removed by 5% HF solution prior to the thermal-ALD Al2O3 deposition. Doublesided ALD Al2O3 films with various thicknesses were deposited at 250 °C using trimethylaluminium (Al(CH3)3, TMA, 99.9999%) and deionized water (H2O, DIW) as precursors with the deposition rate of 0.12 nm per cycle and the refractive index n = 1.64 at 632 nm. After Al2O3 deposition, half of these wafers were annealed at 400 °C for 5 min under the nitrogen atmosphere. Then, all of the wafers were taken to deposit an 85 nm SiNx capping layer on both sides by an in-line microwave-PECVD (SiNA XXL, Roth & Rau) at 400 °C, or by an in-line direct-PECVD (PD-380A, S.C New Energy Technology Corporation) at 480 °C, respectively. The applied PECVD SiNx processes in this study were the manufacturing processes, and the effective refractive index of SiNx layers is 2.05 at 632 nm. After PECVD processes, the wafers were post-annealed for a short time of 5 min or for a long time of 30 min at 400 °C under the nitrogen atmosphere. In addition, the annealed 20 nm Al2O3/direct-PECVD SiNx stacks without and with post-annealing were fired at 800 °C for 3 s in air through a rapid thermal annealing system. Two groups of monocrystalline p-PERC solar cells were prepared in this study with the following steps [7,12]. After saw damage etching and textured by NaOH solution, front n+ emitter with a sheet resistance of about 95 Ω/□ was formed via POCl3 diffusion. Then the phosphorosilicate glass (PSG) was removed by dipping in HF solution and the edge isolation and rear surface polishing were processed in the same chain-type wet bench using HNO3-HF-H2SO4 solution. Then the rear surface was passivated by 20 nm ALD Al2O3/100 nm direct-PECVD SiNx stacks and the front surface was passivated by 80 nm microwavePECVD SiNx. Before metallization, half of these solar cells were additionally post-annealed at 400 °C for 30 min under the nitrogen atmosphere, and the other half, which were not additionally post-annealed, were set as the reference. Finally, the metallization process consisted of rear surface laser ablation with 532 nm ps-laser, screen printing and firing processes were used to form Ag/n+-Si contact for front surface, and Al back surface field (Al-BSF) and local Al/p+-Si contact for rear surface. The effective minority carrier lifetime of the bifacial passivated wafers was measured at each step by a Sinton Instruments WCT-120 system in Quasi-Steady State Photoconductance (QSSPC) mode and the values were taken under the injection level of 1 × 1015 cm−3. Negative fixed charge density (Qf) and interface defect density (Dit) were measured by Semilab SDI PV2000 using the Corona oxide characterization of semiconductors (COCOS) contactless measurement method [14,21,22]. The Dit and Qf values were averaged over 9 measuring points on one wafer. The elemental distribution were measured by secondary ion mass spectroscopy (SIMS) to evaluate the elements variation before and after the PECVD process. The electrical parameters of the p-PERC solar cells were measured by a Denken DKSCT-100T-180 IV tester, and the quantum efficiency and reflectance were tested by a Bentham PVE300-IVT system. 3. Results and discussion Fig. 1 (a) and (b) show the effective minority carrier lifetime (τeff) of the silicon wafers passivated by as-deposited and annealed (400 °C for 5 min, N2) single ALD-Al2O3 films. It is clear that the passivation quality improves with the increasing thickness of single Al2O3 films due to the decreasing interface defect density (Dit) and increasing negative fixed charge density (Qf). The annealing process significantly improves the chemical and field-effect passivation by reducing Dit via the structure 215
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Fig. 1. Passivation quality for (a) as-deposited and (b) annealed (400 °C for 5 min, N2) single ALD Al2O3 films with thicknesses of 5, 10, 20 nm.
Fig. 2. The evolution of effective minority carrier lifetime (τeff) for different Al2O3/SiNx stacks. (a) 5 nm as-deposited Al2O3 capped with mw-PECVD SiNx and directPECVD SiNx, (b) 20 nm as-deposited Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (c) 5 nm annealed Al2O3 capped with mw-PECVD SiNx and directPECVD SiNx, (d) 20 nm annealed Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx. The samples were measured right after ALD-Al2O3 deposition (AS), ALD-Al2O3 annealing (AN), mw-PECVD or direct-PECVD SiNx deposition (PE), short time annealing (at 400 °C for 5 min) for the Al2O3/SiNx stacks (SA), and long time annealing (at 400 °C for 30 min) for the Al2O3/SiNx stacks (LA).
When the Al2O3 films are thin (5 nm), PECVD processes can effectively improve the passivation quality, and the annealing repairing process is even not necessary. Furthermore, direct-PECVD is better than mw-PECVD in improving the passivation quality in this case. But for thicker Al2O3 films (20 nm), mw-PECVD shows its advantages of slight plasma-induced damage and that can be easily repaired by a short time post-annealing. For direct-PECVD process, plasma damage requires a long time annealing process to obtain sufficient thermal energy to repair it. More detailed elemental depth profiles were measured by SIMS with different Al2O3/SiNx stacks to analyze the impact of elements distribution. As shown in Fig. 4 (a), it is clear that different PECVD methods take different element doping to the Al2O3 films and the near surface of the Si substrate [19]. With higher deposition temperature, longer time and direct ion bombardment, direct-PECVD process shows heavier doping of Si, N, H to the Al2O3 films and O, N, H to the Si
substrate. Among them, higher H concentration at the Al2O3/Si interface is beneficial to the chemical passivation, due to the fact that more dangling bonds can be passivated by H atoms [17,25,34]. We supposed that the more Si atoms diffused into the Al2O3 film may change the structure of the Al2O3 film to some extent, which can result in the more decline of Qf [35]. Fig. 4 (b) shows the difference between the as-deposited and annealed Al2O3 capped with direct-PECVD SiNx. Obviously, the annealed Al2O3 films has stronger barrier effect on Si and N doping during the direct-PECVD process due to the denser Al2O3 film. Similar to the situation mentioned above, higher H concentration is beneficial to the reducing of Dit and lower doping of Si is conducive to Qf, which is consistent to the trend of effective lifetime, Dit and Qf shown in Figs. 2 and 3. The elements distributions of the annealed Al2O3/direct-PECVD SiNx stacks after long time annealing were also shown in Fig. 4. It can be observed that the H concentration at the Al2O3/Si interface after LA process is significantly increased than that before annealing, which can 216
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Fig. 3. The evolution of interface defect density (Dit) and negative fixed charge density (Qf) for different Al2O3/SiNx stacks. (a) 5 nm as-deposited Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (b) 20 nm as-deposited Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (c) 5 nm annealed Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (d) 20 nm annealed Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx. The samples were measured right after ALD-Al2O3 deposition (AS), ALD-Al2O3 annealing (AN), mw-PECVD or direct-PECVD SiNx deposition (PE), short time annealing (at 400 °C for 5 min) for the Al2O3/ SiNx stacks (SA), and long time annealing (at 400 °C for 30 min) for the Al2O3/SiNx stacks (LA).
wafers to form the grid, and the passivation quality is shown in Fig. 5. It is obvious that the effective lifetime of the sample with post-annealing is much higher than the sample without post-annealing, mainly due to the lower Dit. It shows that the short time high temperature firing process cannot replace the long time low temperature annealing repairing process, and the post-annealing repairing process is necessary for the Al2O3/direct-PECVD SiNx stacks. Based on the discussion above, full size (156.75 × 156.75 mm2) monocrystalline p-PERC solar cells were fabricated with industrycompatible process to study the influence of the post-annealing process after direct-PECVD SiNx. The current-voltage parameters of p-PERC solar cells with additional post-annealing against the reference p-PERC
effectively increase the chemical passivation quality. Comparing with the 20 nm Al2O3 films, the 5 nm Al2O3 samples have the high concentrations of Si, N, and H in the Al2O3 films and Si substrate, showing that thicker Al2O3 films has stronger barrier effect. For the 5 nm Al2O3 films, although the high Si doping in Al2O3 films further reduces the originally low Qf (∼2 × 1012 cm−2 for annealed 5 nm Al2O3 films) to the level of 1.1 × 1012 cm−2, strong H doping during the PECVD-SiNx deposition greatly improves the poor Dit, resulting in the effectively improved passivation quality. The samples passivated by annealed 20 nm Al2O3/direct-PECVD SiNx stacks without and with the post-annealing repairing process were also fired at 800 °C for 3 s in air to simulate the actual firing process of
Fig. 4. Elemental depth profiles for different Al2O3/SiNx stacks: (a) as-deposited 5 nm Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, and the annealed 5 nm Al2O3 capped with direct-PECVD SiNx underwent long time annealing, (b) as-deposited and annealed 20 nm Al2O3 capped with direct-PECVD SiNx, and the annealed 20 nm Al2O3 capped with direct-PECVD SiNx underwent long time annealing. The insets show the elemental distribution in the Al2O3 films. 217
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Fig. 5. The passivation quality for fired (at 800 °C for 3 s in air) 20 nm Al2O3/ direct-PECVD SiNx stacks without and with post-annealing repairing process before firing.
Fig. 7. The external quantum efficiency (EQE) and reflectance for the p-PECR solar cells without and with additional post-annealing before metallization.
0.13% achieved. What is more, the convergence of VOC, JSC and Eff on the post-annealed group is better than the reference group. To further analyze the variation of VOC and JSC, external quantum efficiency (EQE) and reflectance of the two p-PERC solar cells with the average efficiency of each group were measured and shown in Fig. 7. Obviously, the reflectance curves are basically the same but the EQE curves show significant difference in the long wavelength range from 950 to 1180 nm, indicating that the improved EQE curve can be attribute to the further improved rear surface passivation [12,36,37]. Moreover, the improvement in EQE is also consistent with the increase in τeff shown in Fig. 5, which eventually results in the improvement in VOC, JSC, and Eff for p-PERC solar cells.
cells without post-annealing is illustrated in Fig. 6, measured at standard condition of AM 1.5 and 25 °C. It should be noticed that the electrical parameters are the average of 20 pieces solar cells for one group. As Fig. 6 (a) shows, VOC has an increase of 2 mV with the additional annealing process, consistent with the τeff results as shown in Fig. 5. The higher VOC of the solar cells with post-annealing compared with the reference solar cells further confirms that the annealing process effectively reduces the Dit value of the Al2O3/Si interface and improves the passivation performance. Better rear surface passivation also has benefits to JSC as shown in Fig. 6 (b), with an increase of 0.14 mA cm−2. As the processes of metallization are the same, FF is almost unchanged as shown in Fig. 6 (c). Combining with the VOC, JSC and FF, the average efficiency of the p-PERC solar cells with post-annealing reaches 21.67% relative to 21.54% of the reference solar cells, an absolute increase of
Fig. 6. (a) VOC, (b) JSC, (c) FF, and (d) Eff of p-PERC solar cells without and with additional post-annealing before metallization. 218
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4. Conclusions [13]
We have studied the effect of the PECVD SiNx process and the annealing repairing process on passivation quality of the Al2O3/SiNx stacks. Comparing with the annealed single Al2O3 film, an obvious reduction of the effective minority carrier lifetime can be observed due to the remarkably increase of interface defect density caused by plasmainduced damage and the slight reduction of negative fixed charge density. Direct-PECVD process shows more severe plasma-induced damage than the microwave-PECVD process due to the direct ion bombardment on the Al2O3 films, resulting in the more doping of Si, N and H to the bulk of Al2O3 film, Al2O3/Si interface and Si substrate. Simultaneously, thicker Al2O3 films and annealed Al2O3 films show stronger barrier effect to block the doping of Si, N and H, in which H is beneficial to chemical passivation but Si is detrimental to the field-effect passivation. The low temperature post-annealing process can effectively repair the plasma-induced damage. In the case of mw-PECVD SiNx process, low temperature of 400 °C annealing for 5 min is enough for repairing the plasma-induced damage, especially for Dit, but the annealing time should be extended to 30 min for direct-PECVD process. The post-annealing repairing process cannot be replaced by a short time high temperature (800 °C for 3 s) firing process. An absolute increase of 0.13% in efficiency for p-PERC solar cells with the increased VOC and JSC can be achieved by the additional post-annealing repairing process.
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Acknowledgements [24]
This work was supported by the Shanghai Municipal Project (17DZ1201102). The authors would like to thank the Hareon Solar for the Semilab SDI PV2000 system for COCOS measurement.
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Plasma-induced damage and annealing repairing in ALD-Al2O3/PECVD-SiNx stacks
T
Shizheng Lia, Ning Yanga, Xiao Yuana, Xiaojun Yea, Liangxing Wangb, Fei Zhengc, Cui Liua,∗, Hongbo Lia,∗∗ a
School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, China School of Information Science and Technology, ShanghaiTech University, Shanghai, 201210, China c Shanghai Shenzhou New Energy Develepment Co., Ltd., Shanghai, 201112, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silicon passivation Atomic layer deposition Al2O3 Interface defect density Plasma-enhanced chemical vapor deposition Solar cells
We study the effect of plasma-enhanced chemical vapor deposition (PECVD) SiNx process to atomic layer deposited (ALD) Al2O3 films on crystalline silicon surface passivation. The plasma-induced damage on Al2O3 films is affected by the methods of PECVD process (direct or microwave), and the states (as-deposited or annealed) and the thickness of ALD-Al2O3 films. The passivation degradation may be related to the doping of Si, N and H atoms into Al2O3 films during PECVD process, and is mainly manifested in the form of affecting interface defect density rather than negative fixed charge density. In particular, the annealed thick Al2O3 films with direct-PECVD SiNx process show more severe passivation degradation. Anyhow, to the great degree, the passivation quality can be repaired and further improved by post-annealing. Low temperature long time annealing can effectively increase the lifetime of the ALD-Al2O3/PECVD-SiNx stacks, while it cannot be simply replaced by a rapid high temperature firing. By post-annealing repairing, an absolute 0.13% increase in efficiency can be achieved for p-PERC solar cells.
1. Introduction Efficient surface passivation of crystalline silicon solar cells is of great importance to achieve high conversion efficiency [1–3]. In recent years, the Al2O3 passivation film deposited by atomic layer deposition (ALD) has been widely used at the rear surface of p-type passivated emitter and rear contact (p-PERC) solar cells and the p+ emitter of ntype solar cells for its excellent chemical passivation and field-effect passivation [4–9]. Specifically, thin ALD-Al2O3 films (less than 10 nm) are used at the front surface of n-type solar cells, matching with the optically designed plasma-enhanced chemical vapor deposited (PECVD) SiNx to achieve better anti-reflection property [10,11]. Thick ALDAl2O3 films (10–20 nm) are used in p-PERC solar cells, coating with a thick layer of SiNx (∼100 nm or thicker) as a protector against the aluminum paste for preferable passivation performance [12]. In solar cell industry, SiNx layers are commonly deposited by both microwavePECVD (mw-PECVD, called remote-PECVD as well) and direct-PECVD, and the direct-PECVD is increasingly being used in production. Although it has been reported that the passivation quality and the thermal stability can be improved with the mw-PECVD SiNx capping layers, the ∗
relationship between the Al2O3 films and the direct-PECVD process is not fully understood yet [10,13–15]. It has also been reported that the PECVD process, especially the ammonia plasma, will change the structure of Al2O3 films to some extent, which will have a beneficial or detrimental effect on the passivation quality [16–19]. Practically, the high temperature firing process is applied during metallization to improve the passivation quality, whereas the annealing process is usually omitted due to cost consideration. However, for achieving better passivation quality, the damage caused during the PECVD process has to be repaired. In this work, the passivation quality was characterized by elemental distribution of Al2O3/SiNx stacks and studied as a function of the thickness of Al2O3 films with the different methods of the PECVD SiNx deposition. Furthermore, the impact of annealing repairing process on the film properties were studied. Finally, two groups of p-PERC solar cells were prepared with above mentioned annealing repairing process to the Al2O3/SiNx stacks to observe the variation of the electrical parameters. The present work aims to improve the passivation quality of the solar cells and to achieve higher efficiency.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (H. Li).
∗∗
https://doi.org/10.1016/j.mssp.2019.05.010 Received 27 January 2019; Received in revised form 3 May 2019; Accepted 8 May 2019 Available online 14 May 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
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variation of Al2O3 films and H passivation at the Al2O3/Si interface, and by increasing Qf, respectively [3,13,23–25]. Obviously, the Qf values are relatively close to different thicknesses, so that the greatly reduced Dit values are the major factor of the improved passivation quality with the thicker Al2O3 films. For solar cells, both mw-PECVD and direct-PECVD are used to deposit the SiNx layer which actually always cap the Al2O3 films [10,26]. During the direct-PECVD SiNx process, considerable ion bombardment on the ALD-Al2O3 films occurs, introducing large amounts of plasma damage [27,28]. However, in mw-PECVD, the plasma is excited outside the deposition chamber and the Al2O3 films are not directly contacting with the plasma, thus significantly reducing the plasma-induced damage [27,29]. These two types of PECVD SiNx methods result in the different passivation qualities of the Al2O3/SiNx stacks. Fig. 2 presents the effective lifetime of as-deposited (AS) and annealed (AN) Al2O3 films with the thicknesses of 5 and 20 nm, and those samples after mwPECVD or direct-PECVD SiNx process (PE). As shown in Fig. 2 (a) and (b), the effective lifetime increases after the PECVD process for the effect of annealing with the appropriate deposition temperature (400 °C for mw-PECVD and 480 °C for direct-PECVD) [30]. It is worth noting that although the temperature used in direct-PECVD is also within the range of the appropriate Al2O3 annealing temperature, the effective lifetimes for the as-deposited Al2O3/PECVD SiNx stacks are still relatively lower than the annealed single Al2O3 films when Al2O3 films are thick (20 nm). For annealed Al2O3 films, however, the effective lifetimes are almost unchanged (5 nm, Fig. 2 (c)), or even reduced (20 nm, Fig. 2 (d)), especially on the annealed 20 nm Al2O3 film capped with direct-PECVD SiNx. To improve the passivation quality, samples after PECVD SiNx were post-annealed at a low temperature of 400 °C under the nitrogen atmosphere [15,31] for a short time of 5 min (short time annealing, SA) or for a long time of 30 min (long time annealing, LA), respectively. For the as-deposited 5 nm Al2O3 films, the effective lifetime shows only slight change, indicating that the as-deposited thin Al2O3 films are insensitive to the post-annealing process, and therefore, contribute little to improvement of the passivation qualities. However, for the as-deposited 20 nm Al2O3 films, post-annealing process can further improve the passivation quality to the level of annealed 20 nm Al2O3 films. When the 20 nm Al2O3 films were annealed, the effective lifetimes continuously increased after 5 min and 30 min post-annealing, showing that the post-annealing process can effectively repair the passivation quality for annealed 20 nm Al2O3 films. It should be noticed that to recover the effective lifetime for mw-PECVD, a short time annealing process is enough, but for direct-PECVD process it should be a long time. Fig. 3 presents an additional measure of interface quality including Dit and Qf measured with the same lifetime samples. It can be observed that Qf values of Al2O3/SiNx stacks have a slight decline comparing with that of annealed single Al2O3 films (∼2–3 × 1012 cm−2), due to the positive fixed charge carried by SiNx [30,32,33]. Direct-PECVD SiNx shows more decline, but still over the level of 1 × 1012 cm−2. However, the Dit is relatively high for the Al2O3/SiNx stacks after PECVD comparing with the annealed single 20 nm Al2O3 film (< 1 × 1011 eV−1 cm−2). Particularly for the annealed 20 nm Al2O3/ direct-PECVD SiNx stacks, the Dit is over 1 × 1012 eV−1 cm−2, which is close to the Dit value of the as-deposited 20 nm Al2O3 film, showing severe plasma damage caused by direct-PECVD process. After post-annealing for 5 min or 30 min, Qf increases slightly only, still a little lower than the annealed single Al2O3 films. The as-deposited 5 nm Al2O3 film is an exception in that Dit can be effectively reduced by PECVD process, and insensitive to the post annealing process, showing that the PECVD process is enough for improving the passivation quality. In the other three situations, Dit can be further reduced by post-annealing, consistent to that of effective lifetime shown in Fig. 2. Therefore, Dit is the major factor to affect the passivation quality in ALD-Al2O3/PECVD-SiNx stacks.
2. Experimental details Solar-grade n-type crystalline Czochralski (CZ) Si wafers with the resistivity of 1–7 Ω cm, (1 0 0)-oriented, 180 ± 20 μm thickness, and 156.75 × 156.75 mm2 overall size were used in this study. Surface damage caused during wafer sawing was removed by etching in 15% KOH solution at 80 °C for 15 min (∼15 μm each side) followed by standard RCA cleaning process [20]. Oxidation layer was removed by 5% HF solution prior to the thermal-ALD Al2O3 deposition. Doublesided ALD Al2O3 films with various thicknesses were deposited at 250 °C using trimethylaluminium (Al(CH3)3, TMA, 99.9999%) and deionized water (H2O, DIW) as precursors with the deposition rate of 0.12 nm per cycle and the refractive index n = 1.64 at 632 nm. After Al2O3 deposition, half of these wafers were annealed at 400 °C for 5 min under the nitrogen atmosphere. Then, all of the wafers were taken to deposit an 85 nm SiNx capping layer on both sides by an in-line microwave-PECVD (SiNA XXL, Roth & Rau) at 400 °C, or by an in-line direct-PECVD (PD-380A, S.C New Energy Technology Corporation) at 480 °C, respectively. The applied PECVD SiNx processes in this study were the manufacturing processes, and the effective refractive index of SiNx layers is 2.05 at 632 nm. After PECVD processes, the wafers were post-annealed for a short time of 5 min or for a long time of 30 min at 400 °C under the nitrogen atmosphere. In addition, the annealed 20 nm Al2O3/direct-PECVD SiNx stacks without and with post-annealing were fired at 800 °C for 3 s in air through a rapid thermal annealing system. Two groups of monocrystalline p-PERC solar cells were prepared in this study with the following steps [7,12]. After saw damage etching and textured by NaOH solution, front n+ emitter with a sheet resistance of about 95 Ω/□ was formed via POCl3 diffusion. Then the phosphorosilicate glass (PSG) was removed by dipping in HF solution and the edge isolation and rear surface polishing were processed in the same chain-type wet bench using HNO3-HF-H2SO4 solution. Then the rear surface was passivated by 20 nm ALD Al2O3/100 nm direct-PECVD SiNx stacks and the front surface was passivated by 80 nm microwavePECVD SiNx. Before metallization, half of these solar cells were additionally post-annealed at 400 °C for 30 min under the nitrogen atmosphere, and the other half, which were not additionally post-annealed, were set as the reference. Finally, the metallization process consisted of rear surface laser ablation with 532 nm ps-laser, screen printing and firing processes were used to form Ag/n+-Si contact for front surface, and Al back surface field (Al-BSF) and local Al/p+-Si contact for rear surface. The effective minority carrier lifetime of the bifacial passivated wafers was measured at each step by a Sinton Instruments WCT-120 system in Quasi-Steady State Photoconductance (QSSPC) mode and the values were taken under the injection level of 1 × 1015 cm−3. Negative fixed charge density (Qf) and interface defect density (Dit) were measured by Semilab SDI PV2000 using the Corona oxide characterization of semiconductors (COCOS) contactless measurement method [14,21,22]. The Dit and Qf values were averaged over 9 measuring points on one wafer. The elemental distribution were measured by secondary ion mass spectroscopy (SIMS) to evaluate the elements variation before and after the PECVD process. The electrical parameters of the p-PERC solar cells were measured by a Denken DKSCT-100T-180 IV tester, and the quantum efficiency and reflectance were tested by a Bentham PVE300-IVT system. 3. Results and discussion Fig. 1 (a) and (b) show the effective minority carrier lifetime (τeff) of the silicon wafers passivated by as-deposited and annealed (400 °C for 5 min, N2) single ALD-Al2O3 films. It is clear that the passivation quality improves with the increasing thickness of single Al2O3 films due to the decreasing interface defect density (Dit) and increasing negative fixed charge density (Qf). The annealing process significantly improves the chemical and field-effect passivation by reducing Dit via the structure 215
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Fig. 1. Passivation quality for (a) as-deposited and (b) annealed (400 °C for 5 min, N2) single ALD Al2O3 films with thicknesses of 5, 10, 20 nm.
Fig. 2. The evolution of effective minority carrier lifetime (τeff) for different Al2O3/SiNx stacks. (a) 5 nm as-deposited Al2O3 capped with mw-PECVD SiNx and directPECVD SiNx, (b) 20 nm as-deposited Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (c) 5 nm annealed Al2O3 capped with mw-PECVD SiNx and directPECVD SiNx, (d) 20 nm annealed Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx. The samples were measured right after ALD-Al2O3 deposition (AS), ALD-Al2O3 annealing (AN), mw-PECVD or direct-PECVD SiNx deposition (PE), short time annealing (at 400 °C for 5 min) for the Al2O3/SiNx stacks (SA), and long time annealing (at 400 °C for 30 min) for the Al2O3/SiNx stacks (LA).
When the Al2O3 films are thin (5 nm), PECVD processes can effectively improve the passivation quality, and the annealing repairing process is even not necessary. Furthermore, direct-PECVD is better than mw-PECVD in improving the passivation quality in this case. But for thicker Al2O3 films (20 nm), mw-PECVD shows its advantages of slight plasma-induced damage and that can be easily repaired by a short time post-annealing. For direct-PECVD process, plasma damage requires a long time annealing process to obtain sufficient thermal energy to repair it. More detailed elemental depth profiles were measured by SIMS with different Al2O3/SiNx stacks to analyze the impact of elements distribution. As shown in Fig. 4 (a), it is clear that different PECVD methods take different element doping to the Al2O3 films and the near surface of the Si substrate [19]. With higher deposition temperature, longer time and direct ion bombardment, direct-PECVD process shows heavier doping of Si, N, H to the Al2O3 films and O, N, H to the Si
substrate. Among them, higher H concentration at the Al2O3/Si interface is beneficial to the chemical passivation, due to the fact that more dangling bonds can be passivated by H atoms [17,25,34]. We supposed that the more Si atoms diffused into the Al2O3 film may change the structure of the Al2O3 film to some extent, which can result in the more decline of Qf [35]. Fig. 4 (b) shows the difference between the as-deposited and annealed Al2O3 capped with direct-PECVD SiNx. Obviously, the annealed Al2O3 films has stronger barrier effect on Si and N doping during the direct-PECVD process due to the denser Al2O3 film. Similar to the situation mentioned above, higher H concentration is beneficial to the reducing of Dit and lower doping of Si is conducive to Qf, which is consistent to the trend of effective lifetime, Dit and Qf shown in Figs. 2 and 3. The elements distributions of the annealed Al2O3/direct-PECVD SiNx stacks after long time annealing were also shown in Fig. 4. It can be observed that the H concentration at the Al2O3/Si interface after LA process is significantly increased than that before annealing, which can 216
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Fig. 3. The evolution of interface defect density (Dit) and negative fixed charge density (Qf) for different Al2O3/SiNx stacks. (a) 5 nm as-deposited Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (b) 20 nm as-deposited Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (c) 5 nm annealed Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, (d) 20 nm annealed Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx. The samples were measured right after ALD-Al2O3 deposition (AS), ALD-Al2O3 annealing (AN), mw-PECVD or direct-PECVD SiNx deposition (PE), short time annealing (at 400 °C for 5 min) for the Al2O3/ SiNx stacks (SA), and long time annealing (at 400 °C for 30 min) for the Al2O3/SiNx stacks (LA).
wafers to form the grid, and the passivation quality is shown in Fig. 5. It is obvious that the effective lifetime of the sample with post-annealing is much higher than the sample without post-annealing, mainly due to the lower Dit. It shows that the short time high temperature firing process cannot replace the long time low temperature annealing repairing process, and the post-annealing repairing process is necessary for the Al2O3/direct-PECVD SiNx stacks. Based on the discussion above, full size (156.75 × 156.75 mm2) monocrystalline p-PERC solar cells were fabricated with industrycompatible process to study the influence of the post-annealing process after direct-PECVD SiNx. The current-voltage parameters of p-PERC solar cells with additional post-annealing against the reference p-PERC
effectively increase the chemical passivation quality. Comparing with the 20 nm Al2O3 films, the 5 nm Al2O3 samples have the high concentrations of Si, N, and H in the Al2O3 films and Si substrate, showing that thicker Al2O3 films has stronger barrier effect. For the 5 nm Al2O3 films, although the high Si doping in Al2O3 films further reduces the originally low Qf (∼2 × 1012 cm−2 for annealed 5 nm Al2O3 films) to the level of 1.1 × 1012 cm−2, strong H doping during the PECVD-SiNx deposition greatly improves the poor Dit, resulting in the effectively improved passivation quality. The samples passivated by annealed 20 nm Al2O3/direct-PECVD SiNx stacks without and with the post-annealing repairing process were also fired at 800 °C for 3 s in air to simulate the actual firing process of
Fig. 4. Elemental depth profiles for different Al2O3/SiNx stacks: (a) as-deposited 5 nm Al2O3 capped with mw-PECVD SiNx and direct-PECVD SiNx, and the annealed 5 nm Al2O3 capped with direct-PECVD SiNx underwent long time annealing, (b) as-deposited and annealed 20 nm Al2O3 capped with direct-PECVD SiNx, and the annealed 20 nm Al2O3 capped with direct-PECVD SiNx underwent long time annealing. The insets show the elemental distribution in the Al2O3 films. 217
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Fig. 5. The passivation quality for fired (at 800 °C for 3 s in air) 20 nm Al2O3/ direct-PECVD SiNx stacks without and with post-annealing repairing process before firing.
Fig. 7. The external quantum efficiency (EQE) and reflectance for the p-PECR solar cells without and with additional post-annealing before metallization.
0.13% achieved. What is more, the convergence of VOC, JSC and Eff on the post-annealed group is better than the reference group. To further analyze the variation of VOC and JSC, external quantum efficiency (EQE) and reflectance of the two p-PERC solar cells with the average efficiency of each group were measured and shown in Fig. 7. Obviously, the reflectance curves are basically the same but the EQE curves show significant difference in the long wavelength range from 950 to 1180 nm, indicating that the improved EQE curve can be attribute to the further improved rear surface passivation [12,36,37]. Moreover, the improvement in EQE is also consistent with the increase in τeff shown in Fig. 5, which eventually results in the improvement in VOC, JSC, and Eff for p-PERC solar cells.
cells without post-annealing is illustrated in Fig. 6, measured at standard condition of AM 1.5 and 25 °C. It should be noticed that the electrical parameters are the average of 20 pieces solar cells for one group. As Fig. 6 (a) shows, VOC has an increase of 2 mV with the additional annealing process, consistent with the τeff results as shown in Fig. 5. The higher VOC of the solar cells with post-annealing compared with the reference solar cells further confirms that the annealing process effectively reduces the Dit value of the Al2O3/Si interface and improves the passivation performance. Better rear surface passivation also has benefits to JSC as shown in Fig. 6 (b), with an increase of 0.14 mA cm−2. As the processes of metallization are the same, FF is almost unchanged as shown in Fig. 6 (c). Combining with the VOC, JSC and FF, the average efficiency of the p-PERC solar cells with post-annealing reaches 21.67% relative to 21.54% of the reference solar cells, an absolute increase of
Fig. 6. (a) VOC, (b) JSC, (c) FF, and (d) Eff of p-PERC solar cells without and with additional post-annealing before metallization. 218
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4. Conclusions [13]
We have studied the effect of the PECVD SiNx process and the annealing repairing process on passivation quality of the Al2O3/SiNx stacks. Comparing with the annealed single Al2O3 film, an obvious reduction of the effective minority carrier lifetime can be observed due to the remarkably increase of interface defect density caused by plasmainduced damage and the slight reduction of negative fixed charge density. Direct-PECVD process shows more severe plasma-induced damage than the microwave-PECVD process due to the direct ion bombardment on the Al2O3 films, resulting in the more doping of Si, N and H to the bulk of Al2O3 film, Al2O3/Si interface and Si substrate. Simultaneously, thicker Al2O3 films and annealed Al2O3 films show stronger barrier effect to block the doping of Si, N and H, in which H is beneficial to chemical passivation but Si is detrimental to the field-effect passivation. The low temperature post-annealing process can effectively repair the plasma-induced damage. In the case of mw-PECVD SiNx process, low temperature of 400 °C annealing for 5 min is enough for repairing the plasma-induced damage, especially for Dit, but the annealing time should be extended to 30 min for direct-PECVD process. The post-annealing repairing process cannot be replaced by a short time high temperature (800 °C for 3 s) firing process. An absolute increase of 0.13% in efficiency for p-PERC solar cells with the increased VOC and JSC can be achieved by the additional post-annealing repairing process.
[14]
[15]
[16]
[17]
[18]
[19]
[20] [21]
[22]
[23]
Acknowledgements [24]
This work was supported by the Shanghai Municipal Project (17DZ1201102). The authors would like to thank the Hareon Solar for the Semilab SDI PV2000 system for COCOS measurement.
[25]
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