Longer hydrogenation duration for large area multi-crystalline silicon solar cells based on high-intensity infrared LEDs

Optics Communications 450 (2019) 252–260

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Longer hydrogenation duration for large area multi-crystalline silicon solar cells based on high-intensity infrared LEDs Jianbo Shao a,b , Xi Xi b,c ,∗, Shaomin Li b,c , Guilin Liu b,c , Ruoying Peng b,c , Chao Li b,c , Guoqing Chen b,c , Rulong Chen d a

School of Internet of things engineering, Jiangnan University, Wuxi, Jiangsu Province, China Jiangsu Provincial Research Center of Light Industrial Optoelectronic Engineering and Technology, Wuxi, Jiangsu Province, China c School of Science, Jiangnan University, Wuxi, Jiangsu Province, China d Wuxi Suntech Power Co., Ltd. Wuxi, Jiangsu Province, China b

ARTICLE

INFO

Keywords: Hydrogenation Multi-crystalline silicon High-intensity infrared LEDs Fe-B pairs Solar cells

ABSTRACT The efficiency improvements of large area (244.34 cm2 ) crystalline silicon solar cells have been receiving significant attention, especially in high-efficiency multi-crystalline silicon (mc-Si) Passivated Emitter and Rear Contact (PERC) cells. The large area boron-doped mc-Si PERC cells were treated with hydrogenation for a different duration based on high-intensity infrared (HI-IR) LEDs of 940 nm. Compared to the boron-doped monocrystalline silicon (mono-Si) PERC cells, different trends of efficiency improvement were displayed after various hydrogenation duration. The results showed that the amplitude of the efficiency improvement was enhanced to the maximum around 8 min but then slowly decreased with the increasing hydrogenation duration. Moreover, we found that the iron-boron (Fe-B) pairs may occupy the dominant role in hindering the improvement of efficiency and the light-induced degradation (LID) after hydrogenation. The trend of efficiency improvement depended on the density of interstitial iron ions (Fe+i ) and the formation and dissociation of Fe-B pairs. Therefore, a longer duration of hydrogenation was required for mc-Si PERC cells than for mono-Si PERC cells to enhance the efficiency and suppress the degradation. Additionally, the duration of hydrogenation was significantly shortened to 1 min when the pre-LID was applied, then the appropriate extension of the hydrogenation duration was more conducive to the efficiency improvement of mc-Si PERC cells.

1. Introduction The efficiency of multi-crystalline silicon (mc-Si) Passivated Emitter and Rear Contact (PERC) cells has reached a high level, especially the p-type mc-Si solar cells have exhibited efficiencies up to 21.6% [1], but the subsequent improvements of efficiency and light-induced degradation (LID) block its further development. Recent work has indicated that silicon solar cells can significantly benefit from bulk hydrogenation [2]. The commercial-grade mc-Si PERC cells modules will exhibit a severe loss of efficiency when they are exposed to light with for a long time. Moreover, the high-intensity illumination and elevated temperature can cause significant degradation for mc-Si cells during the module fabrication with the light welding. Then the electroluminescence images of the modules appear a significant difference in the brightness. Therefore, the commercial-grade mc-Si PERC cells need to be performed the hydrogenation before the module fabrication to avoid the negative effects that were mentioned above. Hydrogenation can also be used to passivate the remaining impurities after gettering [3] and suppress the degradation of mc-Si PERC cells.

Numerous researchers have undertaken detailed defect analysis studies, defect formation and recovery kinetics, which involving in mcSi PERC cells to identify a possible root cause [4–8]. Ramspeck [9] et al. indicated that the formation of boron-oxygen (B-O) defects occurred in boron-doped mc-Si PERC cells, but the root reason of degradation of mc-Si PERC cells cannot be sufficiently attributed to B-O defects. Whatever caused the degradation of the cells, we introduced hydrogenation to improve efficiency and inhibit degradation. The hydrogen from hydrogen-containing dielectric layers, such as hydrogenated silicon nitride (SiNx :H) layer and aluminum oxide (AlOx :H) layer [10, 11], played a significant role in hydrogenation. Hydrogen can be indiffusion from the surface passivation layers throughout the bulk of the silicon solar cell at an appropriate temperature [12] or under light injection. Simultaneously, the defects can be passivated by manipulated charge states of hydrogen, which can effectively extend the stability of solar cells [13]. However, some studies have also indicated that hydrogen or hydrogen-induced defects could induce degradation [14– 16]. Moreover, the SiNx :H passivation layer was found to be unstable

∗ Corresponding author at: School of Science, Jiangnan University, Wuxi, Jiangsu Province, China. E-mail addresses: [email protected] (J. Shao), [email protected], [email protected] (X. Xi), [email protected] (S. Li).

https://doi.org/10.1016/j.optcom.2019.06.011 Received 21 February 2019; Received in revised form 3 June 2019; Accepted 5 June 2019 Available online 10 June 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.

J. Shao, X. Xi, S. Li et al.

Optics Communications 450 (2019) 252–260 Table 1 The process sequence of all experimental groups (from Group A to Group F) in this work and the results.

at elevated temperature and illumination during long process times, which dramatically increased the density of mc-Si defects [17]. As a result, the recent research proposed an assumption that the hydrogen could play both roles as follows: When the hydrogenation was activated, one hydrogen atom could activate a defect while a second atom then passivates the active defect [18]. In this work, the mc-Si PERC solar cells required longer hydrogenation duration compared with monocrystalline silicon (mono-Si) PERC cells, which revealed that the mechanism of the light-induced complex in mc-Si PERC and mono-Si PERC cells played different behaviors. Moreover, iron-boron (Fe-B) pairs were considered as a candidate reason for the reversibility of hydrogenation and degradation of mcSi PERC cells in this work. The Fe-B pairs, which was proved to be sensitive to thermal treatment and light, were first involved in hydrogenation and then caused subsequent attenuation, so the hydrogenation conditions of mc-Si PERC cells were difficult to match. Thus, we have to figure out how to make effective hydrogenation for mc-Si PERC in a short time to passivate impurities and defects recombination, avoiding to extend the treatment time of hydrogenation.

Groups Group Group Group Group Group Group

A B C D E F

The order of experimental treatment

Results

Initiala →Pre-LIDa →Dark 300 ◦ Ca →LID Initial→the initial LIDa Initial→Dark 300 ◦ C→LID Initial→Hydrogenation→LID Initial→pre-LID→1 min Hydrogenationa →LID Initial→pre-LID→2 min Hydrogenationa →LID

Fig. Fig. Fig. Fig. Fig. Fig.

5 7 7 7 9 9

a (1)

Initial: the state of silicon solar cells before any treatment; (2) Pre-LID: the previous light-induced degradation before other treatments; (3) Dark 300 ◦ C: the silicon solar cells were heated at 300 ◦ C for 5 min in the dark; (4) the initial LID: the initial performance of LID without any treatment; (5) 1 min Hydrogenation: hydrogenation with a duration of 1 min; (6) 2 min Hydrogenation: hydrogenation with a duration of 2 min.

5 minutes [21], which is seen as a thorough degradation on untreated silicon cells to cause the complete dissociation of Fe-B pairs for producing sufficient iron ions (Fe+ ) and boron ions (B− ). The Xe-lamp (AM i 1.5G) was carried out for the initial LID and LID treatment under the irradiation for 5 h. The Xe-lamp condition was light power 1000 W/m2 , the temperature in the LID chamber ∼45 ◦ C. Particularly, improper cooling methods for mc-Si solar cells may cause ineffective hydrogenation [22]. Hence, Rapid cooling methods were adopted to avoiding the additional effects that were caused by the transitory high-temperature state of the natural cooling process. The final hydrogenation effect can be shortened to one minute with an improvement equivalent to the previous 8 min without pre-LID. The following experiments which were divided into 6 groups (from Group A to Group F) were conducted to study the effect of the formation and dissociation of Fe-B pairs on hydrogenation. Conditions for each group were shown in Table 1.

2. Materials and methods The commercial-grade mc-Si wafers were fabricated into solar cells with the standard PERC processing which sequenced from texture & clean to metallization here. The wafer specification was resistivity 1– 3 Ω cm, thickness 190 μm, and size 156 mm × 156 mm. Prior to deposition of the dielectric passivation layers, wafers were saw-damage etched and surface textured by HNO3 /HF solution, followed by HCl/HF cleaning. The sheet resistance of the emitter was around 90 Ω/sq. the AlOx :H layer was deposited using standard Roth & Rau® remote microwave plasma enhanced chemical vapor deposition (PECVD) systems, the SiNx :H layers were deposited using Centrotherm® direct PECVD systems. The large area boron-doped mc-Si PERC cells (The following were called mc-Si PERC cells) with the same batch of properties were selected from the same efficiency grade after I–V characterization, these cells were divided into several groups for hydrogenation. Moreover, boron-doped mono-Si PERC cells were used as reference groups to compare the trend of mc-Si PERC efficiency improvement. Forty-eight pieces of mono-Si PERC cells with similar properties were divided into eight groups after I–V measurement to assist with the experiment. These mono-Si PERC cells were also fabricated on the boron-doped Czochralski (Cz) wafers and processed in an industrial environment using standard commercial production equipment. The wafer specification was, resistivity 1–3 Ω cm, thickness 200 μm, and size 156.75 mm × 156.75 mm 𝛷205 mm. The AlOx :H layers and SiNx :H layers of cells also were deposited based on the standard processes. The appropriate temperature and illumination were performed to assist the hydrogenation in this work. The illumination was provided by home-built high-intensity infrared (HI-IR) LEDs source platform [19], the intensity of irradiation reaches 20.0 ± 0.5 Suns which was measured and calibrated with the function of photon flux density [20]. The hydrogenation of mc-Si PERC and mono-Si PERC solar cells were performed at 300 ± 5 ◦ C and 240 ± 5 ◦ C, respectively. The temperature of 240 ◦ C has been used for mono-Si PERC since that has been shown to be most effective for passivating the B-O defects which were dominant. Through extensive experiments, the mc-Si PERC solar cells used in this work have been shown to respond best to hydrogenation processes at 300 ◦ C and was therefore used here. The silicon cells were treated with appropriate temperature and illumination in a gradient time under HIIR LEDs in order to observe the effect of hydrogenation. Moreover, some mc-Si PERC cells were heated at 300 ◦ C in the dark (Dark 300 ◦ C) to verify the effect of temperature on the formation of Fe-B pairs under this condition. The pre-LID (previous light-induced degradation) was adopted to investigate a more practical hydrogenation method for mc-Si solar cells. The pre-LID is under HI-IR LEDs (about 20 ± 0.5 Suns at 50 ± 5 ◦ C) for

3. Results and discussion 3.1. Longer hydrogenation duration demanded to mc-Si PERC by comparing to mono-Si PERC cells Previous investigations have found that the surface passivated mc-Si PERC solar cells were superior to conventional silicon solar cells [23, 24]. The hydrogen with appropriate charge states mainly generated from SiNx :H and AlOx :H layers and then rapidly diffused into the silicon wafer to passivate the surface and internal defects under energy injections such as the thermal treatment and illumination [10]. In this work, the hydrogenation process was carried out for seven groups of commercial-grade mc-Si PERC solar cells. Then the average efficiency of mc-Si PERC cells after hydrogenation of different duration was shown in Fig. 1(a), the testing error has been eliminated by adjusting with independent reference wafers. The results showed that the efficiency of mc-Si PERC cells was improved after the hydrogenation except for the group of one minute. Then the efficiency improvement was gradually increased when the time was extended from 0 min to 8 min. However, the tendency was reversed after 8 min, the efficiency improvement of cells was slightly reduced and then became stable. Compared with the 8-minute treated mc-Si PERC cells, the efficiency improvement after hydrogenation for more than 10 min was decreased, but the efficiency improvement was higher than that of the cells treated in less than 8 min. The efficiency exhibited a slight decline when the mc-Si PERC cells were treated after the hydrogenation for one minute in Fig. 1(a). The reason for this was that impurities and defects were activated and generated in a short time (within 1 min), but the hydrogen with appropriate charge states was difficult to be rapidly generated. Moreover, these hydrogen ions were hard to diffuse into the silicon bulk to passivate the impurities and defects within 1 min because the ionic movements were relied on kinetics to across boundaries. the changes in open-circuit voltage (Uoc ), short-circuit current density (Jsc ) and 253

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Optics Communications 450 (2019) 252–260

Fig. 1. The electrical performance of mc-Si PERC cells at different hydrogenation duration (The testing error has been eliminated by adjusting with independent reference wafers) (a) efficiency; (b) open-circuit voltage; (c) short-circuit current density; (d) fill factor.

0 min to 8 min, and reached the maximum in about 8 min, after which the gain decreased and finally became stable. From Fig. 2, the results were consistent with the previous discussion (Fig. 1(a)). Seven groups of commercial-grade mono-Si PERC cells were conducted with hydrogenation to compare the effect of different hydrogenation duration for mc-Si PERC cells. The efficiency of mono-Si PERC cells was plotted in Fig. 3(a), which can find that the efficiency of the cells instantly increased at approximately one minute. The efficiency improvement seemed to have reached its peak at one minute, while the subsequent hydrogenation had a slight impact on the efficiency improvement after the first minute of hydrogenation. Moreover, the efficiency was stable along with the duration of hydrogenation. The differences in Uoc , Jsc and FF relative to pristine cells (before processing) were also measured and shown in Fig. 3(b), (c) and (d). The results indicated that the Uoc , Jsc , and FF of large area mono-Si PERC cells were all significantly improved, the maximum absolute efficiency improvement of Uoc , Jsc , and FF was 2.30 mV, 0.11 mA/cm2 and 0.44%, respectively. In general, the efficiency improvement of mono-Si PERC cells after hydrogenation was more stable and intuitive. Hydrogenation of crystalline silicon cells was a combination of illumination and thermal treatment [22]. The light injection can improve the relative concentration of minority carriers, and the thermal treatment accelerated the release rate of hydrogen from SiNx :H and AlOx :H layer and also expedited hydrogen migration rate within silicon bulks. For mc-Si PERC cells, the active impurities and defects in the initial state were in low concentration, then most of them would be activated into ionic states and then be passivated during the hydrogenation. Hence, there was a stimulation process to form active impurities and defects which might consume a few seconds or minutes to complete this process in mc-Si PERC cells. In addition, hydrogen ions were probably

Fig. 2. The PL images and efficiency changes of mc-Si solar cells after different hydrogenation duration.

fill factor (FF) relative to initial values were plotted in Fig. 1(b), (c) and (d) where the value of Uoc and Jsc always were enhanced after the hydrogenation. However, the value of FF was decreased when the hydrogenation time was less than 5 min, once the hydrogenation time exceeded 5 min, the FF increased again. Among those parameters, the Uoc was significantly increased in the different duration of hydrogenation and gradually became stable as the hydrogenation lasted more than 8 min. The efficiency of mc-Si PERC cells achieved to the maximum as the duration of hydrogenation was around 8 min. Eight groups of solar cells were measured by Photoluminescence (PL) to investigate the hydrogenation effect in depth, PL images of a cell under each condition were in Fig. 2. The contrast of PL images was from shallow to deep from 254

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Optics Communications 450 (2019) 252–260

Fig. 3. The electrical performance of mono-Si PERC cells at different hydrogenation duration (The testing error has been eliminated by adjusting with independent reference wafers) (a) efficiency; (b) open-circuit voltage; (c) short-circuit current density; (d) fill factor.

Fig. 4. The LID of silicon solar cells followed by hydrogenation with different processing duration (a) mc-Si solar cells; (b) mono-Si solar cells.

insufficient at the beginning of the hydrogenation process because most hydrogen ions were bonded in the SiNx :H and AlOx :H layers. As we have described above, the generation and diffusion of hydrogen with appropriate charge states required sufficient time and kinetics which finally migrated into silicon wafers. It was worth noting that hydrogen ions inside the silicon wafer may stimulate the impurities and defects at grain boundaries instead of passivating them [18]. As a result, when the duration of hydrogenation on mc-Si PERC cells was only one minute, an absolute loss in efficiency was observed in Fig. 1(a). As the time of hydrogenation extended, the passivation effect on impurities and defects would play a dominant role rather than activating impurities and defects, leading to an upward efficiency. The efficiency was gradually improved before eight minutes because the impurities

and defects were passivated to form the stable substance which can hardly be activated again. Thus, the effect of passivated impurities and defects can be neglected in the following hydrogenation process. Then the decrease of the efficiency gain after 8 min might be related to another impurities or defects. As the light injection continued, the minority carriers gradually accumulated to form excess carriers which may stimulate other impurities and defects, dissociating to form corresponding interstitial ions. Hence, the efficiency of hydrogenation after 10 min was lower than that of 8-min treatment. According to references [12,25], the main impurities in mono-Si PERC cells are B-O pairs. For mono-Si PERC cells, the positive charged B-O pairs (BO+ ) were attracted by the hydrogen with the negative charge state (H− ). Thus the activity of B-O pairs can be reduced by 255

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Optics Communications 450 (2019) 252–260

According to the Fermi level diagram [30] of the silicon band gap, both Cr-B and Fe-B may be the reason for the decreased effect of hydrogenation for mc-Si PERC solar cells. In the middle of the band gap between silicon, such as Vanadium (V) [31], Chromium (Cr) [32] and Manganese (Mn), the donor level of metal impurities was high [33]. Therefore, it was relatively difficult to activate or passivated such metal impurities. Compared with the content and the energy level [32,34], the Fe has a higher possibility than Cr to play a dominant role in making mc-Si PERC cells take longer time for hydrogenation and LID. The relationship among Fe+ , B− and Fe-B pairs was accord with this i conjecture of ‘‘other impurities’’ mentioned above that was shown in Eq. (1),

the matched hydrogen ions and no longer affect the minority carriers. The efficiency was stable after 1-min hydrogenation for the mono-Si PERC, indicating that the passivated B-O pairs may not be reactivated by the excess carrier. For mc-Si PERC cells, the interstitial oxygen concentration was lower than that of Cz silicon [26], and the formation of B-O defects in mc-Si PERC cells may require a much longer duration scale than in Cz silicon [27]. Therefore, the formation of B-O pairs has been discarded as the primary reason that caused the negative effect in mc-Si PERC cells. Therefore, the decline of efficiency growth in mcSi PERC after the long period of hydrogenation was caused by ‘‘other impurities’’ rather than B-O pairs. The relative efficiency changes after LID were shown in Fig. 4(a) where the LID of mc-Si PERC cells was improved with the increase of hydrogenation duration. The results indicated that the loss in efficiency of untreated mc-Si PERC cells after LID was about −3.0 ± 0.2%. The hydrogenation effect was not remarkable in improving LID as the duration was less than 8 min. When the hydrogenation duration was about one minute, the degradation was close to −5.0 ± 0.2% which was larger than degradation without hydrogenation (−3.0 ± 0.2%). Recent investigations have indicated that some impurities can only be stimulated by illumination, while others can only be activated by annealing in the dark [22,27] and [28]. In addition, impurities can be activated to form impurity precursors which were difficult to be stimulated again by annealing in the dark, while can be fully activated by high-intensity illumination, affecting the efficiency of mc-Si PERC cells [28,29]. Another possible reason for mc-Si PERC solar cells was that a preparatory process remodeled impurities in the early stage of hydrogenation, and then formed recombination centers which decreased the efficiency. Because the combined effect of illumination and thermal treatment, original light-insensitive defects may transform to another light-sensitive state. Therefore, the mc-Si PERC solar cells with hydrogenation can generate abundant defects compared with the cells without hydrogenation as the hydrogenation time was less than 1 min, then most defects were activated in light soaking of the LID. After the extension of hydrogenation time, the degradation was improved to approximately −2.0 ± 0.2%. The subsequent corresponding LID of the passivated mono-Si PERC cells was also processed. As shown in Fig. 4(b), there was a slight difference for mono-Si PERC cells and relatively stable after more than the one-minute duration of hydrogenation. The results indicated that the initial LID of mono-Si PERC cells was about −2.5 ± 0.2%, and then the LID of mono-Si solar cells was decreased to around −0.8 ± 0.1% with the hydrogenation duration increasing. Overall, the improvement of LID also required a longer time of hydrogenation than that of mono-Si PERC cells.

𝑎

𝐹 𝑒𝑖 + + 𝐵 − ↔ 𝐹 𝑒𝐵 𝑏

(1)

where a (towards the right) is the condition of thermal treatment at an appropriate temperature in the dark; b (towards the left) is the condition of illumination (h𝜈 ≥ 1.1 eV) or ∼200 ◦ C thermal. According to Eq. (1), Fe+ can bond with B− to form relatively i stable Fe-B pairs under the condition of suitable thermal treatment without the high-intensity illumination. In contrast, the Fe-B pairs also can be dissociated to become Fe+ and B− by the single effect of i illumination. However, hydrogenation was a combination of illumination and thermal treatment, so the formation and dissociation of Fe-B pairs simultaneously occurred during the process of hydrogenation. Therefore, the hydrogenation process covered the dissociation of FeB pairs, the passivation of Fe+ and B− and the formation of Fe-B i pair. With the dissociation of Fe-B pairs during hydrogenation, the minority carriers that have migrated into the mc-Si PERC cells can bond with the corresponding Fe+ and B− until completely passivating i the dissociated Fe-B pairs. Then the effect of hydrogenation would be improved slowly and can be more stable than previous states. During the LID treatment, when the effect of illumination was superior to that of thermal treatment provided by light, the reaction would in and B− . The presence of Fe+ be more favorable to generate Fe+ i i silicon can reduce the non-equilibrium carrier lifetime, even at low concentrations [35]. The illumination caused the dissociation of Fe-B pairs to generate Fe+ and B− which decreased the efficiency of mc-Si i PERC solar cells. Therefore, both the treatment of hydrogenation and LID for mc-Si PERC cells was slower than that for mono-Si PERC cells, resulting in longer hydrogenation duration for the mc-Si PERC cells. The reversible process of Eq. (1) can be verified by the following experiments in Fig. 5. As shown in Fig. 5, six pieces mc-Si PERC cells with similar efficiency were performed with pre-LID to dissociate Fe-B pairs (as the process b in Eq. (1)), and then the mc-Si PERC cells were treated in the process of Dark 300 ◦ C (as the process a in Eq. (1)). The efficiency of the mc-Si PERC cells recovered to the initial state after Dark 300 ◦ C, which once again degraded to a level similar to that of the pre-LID after the followed LID. The results indicated that the efficiency loss after LID with Dark 300 ◦ C (the Cyan box plot of Fig. 5) was almost equal to that of after pre-LID (the Green box plot of Fig. 5), and the degradation was still existent without any improvement in mc-Si PERC cells. Then, some electrical characteristics after LID were measured, which were similar to the results of pre-LID as shown in Table 2. Although a slight improvement after the process of Dark 300 ◦ C was existent, the effect was reversible that only thermal treatment may not completely overcome LID. The phenomenon of formation and recovery was consistent with the previous discussion of Fe-B pairs (just like Eq. (1)). Therefore, the Fe-B pairs could be the dominated reason for the degradation and recovery of efficiency in mc-Si PERC solar cells. Zoth and Bergholz [36] indicated that the low-injection diffusion length before (Lbefore ) and after (Laf ter ) pair dissociation could be measured by assuming other recombination processes remain unchanged, so the iron has been detected according to [35] and the iron concentration (cm−3 ) calculated by Eq. (1),

3.2. The theoretical hypothesis of Fe-B pairs and the concentration calculation of Fe+ i Bredemeier [4] et al. have proposed some assumptions about ‘‘other impurities’’ which affect the recombination inside of silicon bulks. We also found that hydrogen can accelerate defects formation and extended degradation in the early stages of hydrogenation. The decrease in efficiency was probably that the defect precursors increased because of the thermal treatment [29], leading to an increase of active defects after light soaking. The defect precursors were unstable potential defects which were hard to be activated by thermal treatment in the dark, while it seemed to be activated by illumination, causing extended degradation. During the hydrogenation, thermal treatment accelerated the formation of defects precursors and the light further stimulated the precursors to form the activated defect. The iron concentration of commercial-grade mc-Si PERC cells has been detected and effectively controlled by silicon wafer manufacturers before and after gettering, while the iron can never sufficiently eliminated which would exist in the mc-Si PERC cells at low concentration. Thus, the presence of iron in mc-Si PERC cells was known. A speculate was proposed for the root cause that Fe-B pairs as one of the defect precursors may affect the hydrogenation and degradation of mc-Si PERC solar cells in this work.

[Fe] = A(

256

1 1 − ) 𝐿2𝑎𝑓 𝑡𝑒𝑟 𝐿2𝑏𝑒𝑓 𝑜𝑟𝑒

(2)

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Optics Communications 450 (2019) 252–260

Table 2 The average change in solar cell parameters (%rel. ) after processing of dark annealing at 300 ◦ C relative to original values and remeasurement (Brackets). Pre-LID 𝛥Uoc (%rel. ) 𝛥Jsc (%rel. ) 𝛥FF (%rel. ) 𝛥Eff. (%rel. )

−0.35, −0.72, −1.12, −2.28,

(0.2) (0.1) (−0.1) (0.2)

Dark 300 ◦ C

LID

0.12, (0.1) 0.09, (−0.1) −0.28, (0.1) 0.08, (−0.1)

−0.25, −0.72, −0.99, −2.22,

Table 3 Diffusion length (μm) (the deviation range between 0 μm and 100 μm) measured by Semilab’s after each process relative to original values.

(−0.2) (0.1) (0.2) (−0.1)

Initial (μm) Pre-LID (μm) Hydrogenation (μm) LID (μm)

1#

2#

3#

4#

5#

6#

1297.30 596.10 1118.90 666.90

2280.00 613.21 932.22 787.04

1153.90 475.87 941.99 653.69

1178.50 542.69 1195.10 712.25

940.95 473.56 803.64 705.94

1379.90 481.89 866.09 717.58

3.3. The further demonstration of the possibility of Fe-B pairs In this work, the hydrogenation of mc-Si solar cells before any treatments was called the direct hydrogenation. Compared with the process of Dark 300 ◦ C, hydrogenation involves more hydrogen which was produced by light. However, whether the effect of direct hydrogenation can be better reflected has been questioned because of the reversible property of Fe-B pairs, then some verification experiments were performed. The Group B (in Fig. 7) of mc-Si PERC cells was set as a reference group which was treated with Xe-lamp at 5 h to obtain the initial LID for comparing with final LID. Group C (in Fig. 7(a)) and Group D (in Fig. 7(b)) of mc-Si PERC cells (same electrical properties as Group B) were treated by different treatment: (1) Group C: initial→Dark 300 ◦ C→LID; (2) Group D: initial→Hydrogenation→LID. Group C performed with five-minute Dark 300 ◦ C was aimed to prove that the only thermal process had a slight improvement in efficiency and degradation. Based on the results of the experiment of Group C, the efficiency after Dark 300 ◦ C was equal to that of the initial state, as shown in Fig. 7(a). Because the Fe-B pairs were hard to stimulate under the dark annealing process, and even some interstitial Fe+ or B− i − ions existed would be forced to form Fe-B pairs, only rare Fe+ or B i in silicon bulks. Even though hydrogen ions have migrated into silicon + − cells, only a few of the remaining Fei and B ions can be passivated. The subsequent LID (in Fig. 7(a)) showed that the degradation of mc-Si PERC cells was also slightly improved compared to the initial LID. This result indicated that only a few Fe+ and B− were passivated to form i stable substance during the previous Dark 300 ◦ C, so the process of Dark 300 ◦ C was recoverable. Moreover, the results of the PL images of Group C as shown in Fig. 8 indicated that there was a bit different in the PL images after Dark 300 ◦ C by comparing to the initial state, which strengthened the suspicion that Fe-B pairs were responsible for the hydrogenation and degradation of mc-Si PERC cells. In Fig. 7(b), the hydrogenation duration was adjusted to eight minutes with illumination and thermal treatment according to previous experiments. The concentration of interstitial Fe+ and B− in the silicon i cells was low in the initial phase of the experiment because the Fe-B pairs were not excited in advance. But the dissociation of a few Fe-B pairs can still occur in the presence of light during hydrogenation, even if the thermal treatment would dominate to promote the formation of Fe-B pairs. Then the hydrogen ions diffused into the silicon and can only passivate a limited number of Fe+ and B− , while the majority i − would still recombine into Fe-B pairs since the thermal Fe+ and B i treatment. Once, the Fe+ was bound by the corresponding hydrogen i ions to form the stable substance, which was difficult to be activated by light. The Fe-B pairs formed by thermal treatment can be reactivated to generated Fe+ and B− , then reducing the efficiency of the cells. i Thus, the efficiency and LID can only have a slight improvement after the hydrogenation (Fig. 7(b)). The PL images in Fig. 8 exhibited the performance of mc-Si PERC cells after Dark 300 ◦ C for 5 min or hydrogenation for 8 min, which both became better than before. Moreover, the improvement of hydrogenation and LID was a bit higher than that of under Dark 300 ◦ C. Hence, the Dark 300 ◦ C was hard to achieve efficiency improvement in this work. Moreover, even though hydrogenation involved more hydrogen participation, it was still difficult to demonstrate the desired effect of hydrogenation because of the reversible property of Fe-B pairs in mc-Si PERC cells.

Fig. 5. The efficiency changes of Group A after different experimental treatment. The order of experimental treatment for Group A: Initial→Pre-LID→Dark 300 ◦ C→LID. ‘‘Initial’’: the state of silicon solar cells before any treatment; ‘‘Pre-LID’’: the previous light-induced degradation before other treatments; ‘‘Dark 300 ◦ C’’: the silicon solar cells were heated at 300 ◦ C for 5 min in the dark; ‘‘LID’’: the final LID after a series of processes . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. The concentration of iron in mc-Si PERC cells during different processes.

The prefactor A was determined to be around 1.05 × 1016 μm2 cm−3

for silicon wafers of dopant density 1 − 3 × 1015 cm−3 (resistivity

range from 5 to 15 Ω cm) [35]. Diffusion lengths before and after different experiments were measured as shown in Table 3, then the concentration of Fe+ in mc-Si PERC cells could calculated by Eq. (2) i as shown in Fig. 6. The results showed that the concentration of Fe+ i significantly decreased after hydrogenation, indicating that most Fe+ i were bound to form compound substances, rather than in the form of Fe+ . Then, some Fe+ could be reactivated to recover to a partial i i level after the subsequent LID treatment as shown in Fig. 6. The results further indicated that the initial Fe+ was partially deactivated by i hydrogen to form similar Fe-H pairs which were hard to be stimulated during the LID process. Meanwhile, the increase of Fe+ concentration i was the contributed by the dissociation of Fe-B pairs. 257

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Fig. 7. The efficiency improvement of Group C and D after different experimental treatment relative to the initial state (a) the thermal treatment at 300 ◦ C for 5 min in the dark; (b) the direct hydrogenation. The order of experimental treatment for Group B: Initial→the initial LID; Group C: Initial→Dark 300 ◦ C→LID; Group D: Initial→Hydrogenation→LID. ‘‘the initial LID’’: the performance of LID without any treatment; ‘‘Hydrogenation’’: the direct hydrogenation for untreated mc-Si PERC cells about 5 min.

From previous experiments in Fig. 7, the hydrogenation process has shown an unsatisfactory effect on efficiency improvement. Because the thermal treatment accelerated the formation of Fe-B pairs as we have described, the hydrogen with appropriate charge states diffused into silicon cells and can only passivate a few active Fe+ or B− . Therei fore, accelerating the formation of defects before hydrogenation would be vital in achieving rapid defects passivation [37]. Consequently, a pre-LID process before hydrogenation was required to apply for the mc-Si PERC cells. As shown in Fig. 9, the mc-Si PERC solar cells were treated to sufficiently dissociate Fe-B pairs by pre-LID before hydrogenation. After the hydrogenation, the efficiency of mc-Si PERC cells also can recover to the initial state in Group E (Fig. 9(a)) & Group F (Fig. 9(b)), the average efficiency improvement of Group E and Group F were 0.02 ± 0.02%abs. and 0.10 ± 0.02%abs. , respectively. Fe-B pairs have been dissociated to generate Fe+ and B− during the prei LID which would be passivated by the migrated hydrogen. Meanwhile, and B− would since thermal induced Brownian movements, part of Fe+ i be recombined to Fe-B pairs. The results indicated that the passivated proportion of Fe+ and B− was higher in Group E & Group F than that of i Group D (Fig. 7(b)) because most Fe-B pairs have dissociated into Fe+ i and B− at the beginning of hydrogenation. The relative loss of average efficiency of Group D after the LID was −2.18% ± 0.02%rel. , while that of Group E was only −1.62% ± 0.02%rel. . According to Fig. 9(b), extending the hydrogenation duration to 2 min made the effect of hydrogenation further promoted. After adjusting the hydrogenation time to 2 min, the relative loss of average efficiency of Group F was only −1.20% ± 0.02%rel. . As shown in Fig. 10, the External Quantum Efficiency (EQE) was measured, which was higher than the pristine EQE after the hydrogenation. The results in Fig. 10 indicated that the light absorption coefficient of silicon was stronger in short wavelength range but slightly lower in the long wavelength range. Therefore, the light of the short wavelength segment has been absorbed on the front surface, while the long-wavelength light can travel to the near back side where the absorption of long-wavelength photons occurred. The short waveband and long waveband segment of EQE can reflect the passivation effect of the front surface and the rear surface respectively, while the middle waveband can represent the defect at junction and bulks. Both Fig. 10(a) and (b) can prove the improvement of EQE at the middle waveband but be in different magnitude, which pointed out that hydrogenation had a passivation effect on internal defects. The results also demonstrated that the longer hydrogenation duration had a better effect for mc-Si PERC cells in this work, not only in the efficiency of hydrogenation but also in the LID.

Fig. 8. The PL image of Group C and Group D after each process.

duration. The mc-Si PERC cells required longer hydrogenation time than that of mono-Si PERC cells to achieve a better hydrogenation effect. The efficiency of mc-Si PERC cells appeared a loss at the initial period of hydrogenation (the condition in this paper was below 1 min). The reason for this was that impurities and defects needed time to be stimulated, and the diffusion and migration of hydrogen also required a certain amount of time. When the active impurities and defects were hard to be passivated by suitable hydrogen ion in time, the efficiency would decrease. The efficiency increased to the maximum as the time extending when the hydrogenation duration was less than 8 min, while the efficiency improvement after 8 min would be lower than that before 8 min. The active impurities and defects were continuously passivated by corresponding hydrogen ions, causing the efficiency improvement. Then another impurities or defects might be induced by excess hydrogen ions after long hydrogenation time, which might be different from the previous defect type that decreased the efficiency improvement. The reason why mc-Si PERC needed a longer duration for hydrogenation has attributed to Fe-B pairs in this work. Fe+ and B− were i bound to form the Fe-B pairs due to heating, while the light would dissociate the Fe-B pairs into Fe+ and B− which can be passivated by i hydrogen ions. Passivating Fe+ and B− by hydrogen ions were essential i to overcome the thermally induced movements and considered the rate of the dissociation and the recombination of Fe-B pairs at boundaries, so the duration of hydrogenation for mc-Si PERC was longer. Moreover, a pre-LID method before hydrogenation for mc-Si PERC cells was performed, then the hydrogenation time can be shortened directly to 1–2 min which was better than that of direct hydrogenation (8 min) under the same conditions.

4. Conclusions In this paper, large area boron-doped mc-Si PERC cells have exhibited different efficiency improvements after different hydrogenation 258

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Fig. 9. The efficiency improvement after different hydrogenation duration with pre-LID: (a) Group E for 1 min; (b) Group F for 2 min. The order of experimental treatment for Group E: Initial→Pre-LID→1 min Hydrogenation→LID; Group F: Initial→Pre-LID→2 min Hydrogenation→LID.

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Fig. 10. The EQE values of Group E & Group F before and after hydrogenation. The hydrogenation duration of group E and F was 1 min and 2 min, respectively.

CRediT authorship contribution statement Jianbo Shao: investigation, methodology, performed and analyzed the experiments, conceived the transient experiments and contributed analysis, wrote the paper. Xi Xi: performed and analyzed the experiments, wrote the paper. Shaomin Li: investigation, methodology. Guilin Liu: conceived the transient experiments and contributed analysis. Ruoying Peng: investigation, methodology. Chao Li: contributed to the device fabrication and characterization. Guoqing Chen: supervised and provided resources for the experiments. Rulong Chen: supervised and provided resources for the experiments. Acknowledgments Thanks to Wuxi Suntech Power Co., Ltd. for providing the test silicon cells. Funding This research was supported by ‘‘the National Natural Science Foundation of China (Grant No. 61804066)’’ & ‘‘the Natural Science Foundation of Jiangsu Province (Grants No. BK20180596, BK20180601)’’ & ‘‘the Fundamental Research Funds for the Central Universities (JUSRP11830)’’ & ’’Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grants No. KYCX19_1858)’’. 259

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