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The investigation on the front surface field of aluminum rear emitter N-type silicon solar cells
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ScienceDirect Solar Energy 114 (2015) 198–205 www.elsevier.com/locate/solener
The investigation on the front surface field of aluminum rear emitter N-type silicon solar cells Xi Xi a,b,c, Xiaojing Chen b, Song Zhang b, Wenjia Li a,b, Zhengrong Shi a,b,c, Guohua Li a,c,⇑ a
School of Science, Jiangnan University, Wuxi, China b Suntech Power Co., Ltd., Wuxi, China c Optoelectronic Engineering and Technology Research Center, Jiangsu Province, China Received 5 November 2014; received in revised form 15 January 2015; accepted 17 January 2015
Communicated by: Associate Editor Igor Tyukhov
Abstract N-type solar cells with aluminum rear emitter provide high power conversion efficiency and overcome light induced degradation. Meanwhile, since the processes are realized on the existing P-type cells, the fabrication cost gets lower for N-type solar cells. Front surface field (FSF) is an important part in this structure. The functions of FSF are similar to that of Al back surface field in P-type cells. FSF enhances carrier collection ability, increases the cells’ open-circuit voltage, as well as increases the efficiency. In this paper, the FSF profiles were investigated. The appropriate surface concentration was around 4 1020 cm3, meanwhile the depth should be kept between 0.25 lm and 0.3 lm from the experiment. A too low concentration cannot provide a strong field effect passivation, while a too high concentration will introduce much more recombination centers. A thin FSF layer increased the resistance, while a thick FSF layer made the short-circuit current decrease, because of the degradation of photoelectric response at the short wavelength. Thermal oxidation was introduced in the process. The FSF profiles were also studied. The minority lifetime and efficiency were enhanced after oxidation. The surface concentration of FSF should be adjusted to around 3 1019 cm3. The best cell efficiency has reached to 19.93%, tested by 18th Institute of China Electronics Group Corporation. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Solar cells; N-type; Aluminum rear emitter; Front surface field; Surface concentration
1. Introduction Majority of commercial solar cells are based on P-type doped silicon material. Almost all production solar cells have a homogeneous emitter, a PECVD deposited SiNx antireflective layer, and screen-printed contacts on both sides. Currently there are increasing interests in the development of N-type solar cells. N-type silicon has higher ⇑ Corresponding author at: School of Science, Jiangnan University, Wuxi, China. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.solener.2015.01.019 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
lifetime for minority carriers with small capture cross section of metallic impurities compared to P-type Si (Cotter et al., 2006; Macdonald and Geerligs, 2004; Schmidt et al., 1997). Meanwhile, N-type cells overcome the light induced degradation (LID) problems, which are attributed to boron–oxygen complexes (Bothe et al., 2005; Dubois et al., 2012). In recent years, several techniques have been developed to fabricate N-type silicon solar cells (Book et al., 2011; Hermle et al., 2011; Rauer et al., 2010; Ru¨diger et al., 2012; Schmiga et al., 2010; Sheoran et al., 2012; Su et al., 2012a; Sugianto et al., 2010). Many products of N-type based cells have been spread into the
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market, such as Yingli’s Panada (Romijn et al., 2012), Panasonic’s HIT (Dwivedi et al., 2013; Rawat et al., 2014; Shen et al., 2013; Wen et al., 2013; Yano et al., 2013) and Sunpower’s IBC (Smith et al., 2013). However, the processes of these cells are much different from the existing P-type cells’ fabrication processes. New equipments investments and existing facilities alteration make the costs of these N-type cells be higher. Fast and low cost processes for N-type cells based on the normal P-type cell production line are achieved by using the structure of aluminum rear emitter. In this structure, both the P–N junction and the rear metal contact are simultaneously formed by the use of well-established screen-printing techniques and the firing of the aluminum paste. At the front surface, an N+ doping field is formed by phosphorous diffusion. High power conversion efficiencies of Al rear emitter N-type cells have been achieved. Christian Schmiga et al. reported that they got an efficiency of 19.3% in the lab with Ag-plated contact finger on aerosol seed layer (Schmiga et al., 2012). Topcell Solar International fabricated an efficiency of 19.2% N-type Al rear emitter cell on large area wafers (239 cm2), which phosphorous-implanted selective front surface field was introduced (Su et al., 2012b). Suntech Power Co., Ltd. got the efficiency of 19.8% on 5-in. Cz wafer (148.58 cm2) by using PLUTO technology (Shi et al., 2009a,b). Recently, this efficiency in Suntech has been increased to 19.93% based on 6-in. wafer (238.95 cm2), tested by 18th Institute of China Electronics Group Corporation (CETC) (Voc = 647.2 mV, Jsc = 38.01 mA/cm2 FF = 81.0%). Front surface field (FSF) is formed by a phosphors doped N+ layer. It is an N+–N high-low junction, and is similar to the Al back surface field (Al-BSF) in P-type cells. FSF can enhance the open-circuit voltage (Voc), improve the carrier collection and reduce the transport resistance. Most of the investigation on Al rear emitter N-type solar cell was focused on the rear surface, such as the selection of Al paste, the formation and optimization of rear emitter and the metallization. The study on the FSF was rarely reported. The function of FSF in N-type cells is similar to Al-BSF in P-type cells. For P-type cells, the performances are better when the BSF layer is thicker. From this point, the depth of FSF should be as thicker as possible. However, FSF is at the irradiated surface. The position of FSF is the same with the emitter of P-type silicon cells. The emitter depth in P-type silicon cells is strictly controlled, since the depth strongly influences the photon absorption and response. From the experience of the emitter in P-type cells, the thicker FSF layer is not needed. The appropriate depth of FSF layer was studied in this paper. The relationship between the thickness of FSF layer and the cells’ performances was also investigated. The front surface concentration of doped phosphors is another important parameter. For P-type solar cells, lightly doped emitter (LDE) is very popular today. And it really enhances the cells’ performances. The necessity of lightly doped FSF was also studied. The appropriate front surface concentration of FSF layer was shown in this paper.
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A selective FSF, which phosphors are heavily doped under the front electrodes, is good for cells’ performances (Su et al., 2012b; Sugianto et al., 2009; Xi et al., 2012). In this paper, the heavily doped area was fixed, and was not the investigation emphasis. 2. Experiment The experiments were based on 6-in. Cz mono crystalline N-type Si wafers with the resistivity about 5 X cm. The process started with an alkaline texture and acid cleaning. And then the wafers were diffused with POCl3 for the FSF formation. The sheet resistance after diffusion was nearly 50 X/h. The residual N+ layer at the side and rear surface were removed by chemical etch (HNO3/HF). A standard SiNx deposition on the front surface was followed. Al paste was printed and fired on the rear surface for doping. After Al paste firing, a phosphorous source was sprayed on the front surface. A laser process was applied sequentially. After this process, the SiNx layers at the places of designed fingers and bus-bars were removed. Meanwhile, phosphorous was also heavily doped at these places. A structure of selective-FSF was formed (Sugianto et al., 2009; Xi et al., 2012). The sheet resistance of the laser doped region was 15 X/h.Then, light-inducedplating (LIP, PLUTO technology of Suntech) technology was used (Shi et al., 2009a,b; Sugianto et al., 2009; Xi et al., 2012). Ni, Cu and Ag were sequentially plated on the Si surface, where had been scanned by laser before. The cells’ structure is shown in Fig. 1. For some batches, thermal oxidation was used to passivate the surface before SiNx deposition. Since the doping profiles are necessary in the investigation, electrochemical capacitance voltage (ECV) test was used. The PSG layer should be removed before the measurement. The measurement was carried on automatically by the equipment. The solution in the ECV equipment was NH4HF2 (0.1 mol/L). A Schottky barrier formed by the etching solution and the semiconductor is utilized in the ECV measurement. And this is the basic principle of ECV test. A bias voltage is added on this barrier. And the voltage is continuous adjusted by the calculation software to keep a constant etching current. The charge concentration is obtained by the differential capacity of electrolyte-semiconductor Schottky barrier. The charge concentration is written as (Kinder et al., 1999; Udhayasankar et al., 2004), 1 C3 N¼ 2 ee0 er A dc=dV where e0 is the dielectric constant in vacuum; er is the dielectric constant of semiconductor; e is the electron charge; A is the Schottky barrier area; C is the capacitance. The process of etching-measuring and calculation is repeated again and again. Then the relationship between the charge concentration and the depth will be obtained.
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Fig. 1. Schematic cross section of N-type Al rear emitter cell with laser doped selective FSF and LIP front contact.
3. Results and discussion The function of FSF in Al rear emitter N-type solar cells is similar to Al-BSF in normal P-type cells. The thickness of Al-BSF is a crucial parameter for P-type cells. The investigation on FSF is also started from the depth. Usually, the front surface concentration of phosphors after POCl3 diffusion and HNO3/HF rear and edge etch is at the order of magnitude of 1020 cm3. Here, the surface concentration was managed to control between 3 1020 cm3 and 4 1020 cm3. Different diffusion processes and chemical etch processes were used to control the surface concentration, and got different depths. ECV test can give the doping profiles. These profiles are shown in Fig. 2. These profiles were tested after the etching of rear surface and edge, before the SiNx film deposition. The N-type Si base was doped about 3 1016 cm3 via ECV test. So the FSF layer starts from the front surface, and ends at the depth where the concentration is about 3 1016 cm3. Thermal oxide process was not applied in these batches of cells. The characteristics of these cells are shown in Table 1. The open-circuit voltages of the cells in Table 1 are nearly stable. It means the thickness of FSF layer has little influence to the Voc. However, the short-circuit currents increase with the decreasing of the depth. From this point, it is opposite to the Al-BSF. The lower Jsc exhibits when the FSF layer becomes thicker. This difference between FSF and BSF attributes to the position of the field inside the
cells. The FSF is at the irradiated surface. The decrease of Jsc is due to the degradation of the photoelectric response at the short wavelength. Fig. 3 shows the internal quantum efficiency (IQE) curves of the cells with different FSF depths. It is obvious that the IQE at the short wavelength gets lower when the FSF depth increases. The photons with short wavelength sharply attenuate after entering the wafers. Most of these photons are absorbed at the front surface or within a very short depth. The FSF layer is formed by phosphors doping. More phosphor atoms are needed to dope for a thicker FSF layer, and meanwhile more recombination centers are introduced. Unfortunately, the carriers excited by the photons with short wavelengths generate in the FSF layer. Thus, more carriers will be recombined in the thicker FSF layer, and a lower IQE exhibits. Since one of the most crucial functions for FSF is carrier collection, the sheet resistance is important. A thicker layer gives a lower sheet resistance. The resistance data after the chemical etching of rear surface and edge for the cells are also shown in Table 1. A lower sheet resistance is better for carrier collection and provides a lower series resistance of the cells and a higher fill factor (FF). A thinner FSF layer is good for Jsc, while it is disadvantageous to FF. A general conclusion from the cell efficiency is that the appropriate depth of FSF is in the range between 0.25 lm and 0.3 lm. The surface doping concentration is another focused point. Increasing the diffusion temperature step by step,
Fig. 2. The FSF profiles with different depths, the surface concentration is fixed on the same level. The depths are 0.17 lm, 0.21 lm, 0.26 lm, 0.29 lm and 0.33 lm for Group A, B, C, D, and E, respectively. The characteristics of these cells are shown in Table 1.
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Table 1 The characteristics of the cells with different FSF layer thicknesses. Depth (lm)
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
Rseries (X cm2)
RSheet (X/h)
0.17 0.21 0.26 0.29 0.33
640.2 640.6 641.2 640.8 641.1
37.584 37.502 37.404 37.295 36.946
78.92 79.63 80.39 80.59 80.68
18.99 19.13 19.28 19.26 19.11
1.34 1.00 0.74 0.67 0.57
135 121 101 94.8 80.3
(20 pcs) (20 pcs) (20 pcs) (20 pcs) (20 pcs)
Fig. 3. The IQE curves of the cells with different FSF layer thicknesses.
and adjusting the time of diffusion and wet chemical etch, the depth was fixed around 0.27 lm. The surface concentration was changed from 1 1020 cm3 to 8 1020 cm3. Here the thermal oxidation was also not introduced. The surface concentration and depth were measured after rear and edge etching processes, before SiNx deposition. The ECV profiles are shown in Fig. 4, and the cells’ performances are exhibited in Table 2. From Table 2, the Jsc of the cells are at the same level. As discussed above, the FSF depths are at the same level. The surface doping concentration mostly influences the Voc of the cells in this batch. The highest Voc appears at the concentration of about 4 1020 cm3. Either too low or too high of the concentration is not appropriate. FSF is a high-low junction. The energy level sketch is shown in Fig. 5.
The potential barrier DU in Fig. 5 is related to the doping level. In other words, the surface concentration can strongly influence this energy level difference (DU). The larger DU exists, the higher Voc appears. Meanwhile, the larger DU means a higher build-in field of FSF. This field can drive electrons moving out of the surface quickly and reduce the probability of electron and hole recombination. Thus, the surface recombination velocity drops, and the minority lifetime increases. These result in a high Voc. From these two opinions, high Voc needs a heavily doping FSF. It is true that the Voc grows up with the surface concentration increasing when the concentration is at a low level. However, the actual experiments are different when the doping level is too high. The Voc drops with the concentration increasing. Phosphors doping is used to form a FSF
Fig. 4. The ECV profiles of FSF (fixed on the same depth). The surface concentration are 1.1 1020 cm3, 3.2 1020 cm3, 4.5 1020 cm3, 6.2 1020 cm3 and 8.1 1020 cm3 for Group A, B, C, D, and E, respectively. The characteristics of these cells are shown in Table 2.
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Table 2 The performances of the cells with different FSF’s surface concentrations, fixed on the same depth. Surface concentration (cm3) 20
1.1 10 3.2 1020 4.5 1020 6.2 1020 8.1 1020
(20 pcs) (20 pcs) (20 pcs) (20 pcs) (20 pcs)
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
Rseries (X cm2)
RSheet (X/h)
635.0 640.5 640.7 635.2 631.6
37.394 37.402 37.400 37.401 37.401
80.48 80.52 80.58 80.65 80.69
19.11 19.29 19.31 19.16 19.06
1.10 0.72 0.67 0.60 0.50
107 99.2 96.7 88.6 82.3
Fig. 5. The energy level sketch of FSF.
Table 3 The minority lifetime of the wafers with different surface concentrations. The depth of FSF is fixed on 0.27 lm. In order to measure the minority lifetime, SiNx films of 75 nm were deposited on both front and rear sides by PECVD after wet chemical etch. Surface concentration (cm3) 20
1.1 10 3.2 1020 4.5 1020 6.2 1020 8.1 1020
Minority carrier lifetime (ls) 204 259 267 196 123
layer. The build-in field of FSF can give a field-effect passivation, and can increase the lifetime. However, phosphors atom itself also acts as a recombination center. When the doping level is not too high, the function of field-effect passivation is stronger than the function of the phosphors recombination center. At this time, the Voc still can exhibit increasing with the surface concentration growing up. When the doping level is too high, the function of fieldeffect passivation is weakened by the high concentration of phosphors recombination center. It results in the lower Voc. This discussion can also be described by the minority lifetime measurement. The minority lifetime of the wafers was measured by SEMLAB WT2000. In order to measure
the minority lifetime, SiNx films of 75 nm were deposited on both front and rear sides by PECVD after wet chemical etch. Table 3 shows the lifetime data. All the wafers are corresponding to the cells in Table 2. The highest lifetime of the wafer appears when the surface concentration is 4.5 1020 cm3. And the lifetime data all agree with the Voc and efficiencies of the cells. Thermal oxidation is one of the ways to provide good surface passivation and to enhance the efficiency. During the oxidation, a very thin silicon layer at the front surface will be oxidized; meanwhile the phosphors atoms will also be re-distributed. Some phosphors atoms will be driven deeper inside the silicon, while lots of phosphors atoms at the front surface will move into the SiO2 layer, since the diffusion speed of phosphors inside SiO2 is faster than that of the speed inside Si. Because of this, the surface concentration of FSF will drop, and the depth will increase a little. The minority lifetime data of the wafers are shown in Table 4. Additionally, SiNx films of 70 nm were deposited on both front and rear sides by PECVD after thermal oxidation. These lifetime data are also exhibited in Table 4. The oxidation temperature was 900 °C and lasted 15 min with an optimized oxidation process (Chen et al., 2012). Higher oxidation temperature can provide a compacter SiO2 layer and can show a higher minority lifetime. But the minority lifetime will be not stable if the temperature is over 920 °C, since the impurity may be activated and influence the quality of passivation. In this process, the oxidation condition was fixed, only the diffusion and wet chemical etch processes were adjusted. The depth of FSF was also kept at around 0.27 lm. The cells’ performances with different FSF surface concentrations are shown in Table 4. And the FSF profiles with oxidation are shown in Fig. 6.
Table 4 The minority lifetime data and cells’ performances with different FSF surface concentration introducing thermal oxidation process (the depth is fixed around 0.27 lm). Surface concentration (cm3)
Lifetime after oxidation ls
Lifetime with SiO2/SiNx (ls)
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
Rseries (X cm2)
RSheet (X/h)
5.6 1018 1.2 1019 3.2 1019 4.8 1019 7.6 1019
109 113 132 96 78
596 603 672 328 169
643.1 645.9 646.3 643.7 641.9
37.535 37.545 37.662 37.533 37.441
80.41 80.53 80.58 80.63 80.68
19.41 19.53 19.61 19.48 19.39
1.00 0.91 0.84 0.74 0.69
186 158 145 136 128
a
(20 pcs) (20 pcs) (20 pcs)a (20 pcs) (20 pcs)
The best cell is in this batch. The efficiency is 19.93%, tested by 18th Institute of China Electronics Group Corporation (CETC) (Voc = 647.2 mV, Jsc = 38.01 mA/cm2, FF = 81.0%).
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Fig. 6. The FSF profiles with oxidation process. The surface concentration are 5.6 1018 cm3, 1.2 1019 cm3, 3.2 1019 cm3, 4.8 1019 cm3 and 7.6 1019 cm3 for Group A, B, C, D, and E, respectively. They are fixed on the same depth. The characteristics of these cells are shown in Table 4.
Fig. 7. The I–V curve of the best cell, tested by 18th Institute of CETC.
Higher lifetime data are shown in Table 4 than that of the data in Table 3. High lifetime is essential for high conversion efficiency solar cell production. The rule of Voc in Table 4 is the same with the Voc in Table 2. Here SiO2 layer provides a perfect surface passivation, plus the fieldeffect passivation of FSF, the cells in Table 4 show high Voc. Since the prefect passivation at the surface, only a very small quantity of recombination centers can weaken this passivation. The appropriate surface concentration should be 3 1019 cm3. When the surface concentration
is extraordinary low, such as 1 1019 cm3, the Rs will sharply increase, and FF will rapidly decrease. So the efficiencies of the cells shown in the first two lines in Table 4 are lower, because of a poor FF. And the Voc of these cells are also not so good. Appropriate surface concentration is concluded at about 3 1019 cm3, and the depth should be kept between 0.25 lm and 0.3 lm. The best cell efficiency is 19.93% (Voc = 647.2 mV, Jsc = 38.01 mA/cm2 FF = 81.0%) (see Fig. 7).
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4. Conclusion FSF in N-type Al rear emitter solar cells enhances carrier collection ability, increases the cells Voc as well as the power conversion efficiency. From the investigation, the status of FSF strongly influences the cells’ performances. In order to utilize the FSF advantages, the profiles of FSF must be controlled. The depth and surface concentration of FSF were studied in this paper. A thicker FSF layer exhibited a lower Jsc. Although a very thin FSF layer can provide a high Jsc, the FF of the cells was poor and resulted in a bad efficiency. From the experiments, the FSF depth should be kept between 0.25 lm and 0.3 lm. The Voc were getting higher with the increasing of surface concentration. However, when the concentration was too high, the recombination velocity was sharply increased, and the field-effect passivation of FSF was weakened. The appropriate surface concentration was around 4 1020 cm3. SiO2 layer provided a perfect surface passivation. Since this perfect passivation at the surface, only a very small quantity of recombination centers will weaken the passivation effect. So the surface concentration should not be very high. From the experiments, the appropriate surface concentration should be about 3 1019 cm3. The best cell efficiency tested by 18th Institute of CETC is 19.93%. The FSF’s surface concentration and depth of this batch of cells are 3.2 1019 cm3 and 0.272 lm, respectively. Reference Book, F., Wiedenmann, T., Gloger, S., Raabe, B., Schubert, G., Plagwitz, H., Hahn, G., 2011. Analysis of processing steps for industrial large area n-type solar cells with screen printed aluminum-alloyed rear emitter and selective FSF. In: 26th European Photovoltaic Solar Energy Conference and Exhibition, pp. 1160–1163. Bothe, K., Sinton, R., Schmidt, J., 2005. Fundamental born-oxygenrelated carrier lifetime limit in mono and multicrystalline silicon. Prog. Photovoltaics: Res. Appl. 13, 287–296. Chen, L., Xi, X., Wu, W., Gao, F., Xu, J., Wang, Z., Yu, Z., Lu, Q., Zhang, S., Zhu, H., Chen, R., Yang, J., Ji, J., Shi, Z., 2012. The investigation on the front surface oxidation for aluminum rear emitter N-type solar cells. In: 38th IEEE Photovoltaic Specialists Conference (PVSC), pp. 002142–002144. Cotter, J.E., Guo, J.H., Cousins, P.J., Abbott, M.D., Chen, F.W., Fisher, K.C., 2006. P-type versus n-type silicon wafers: prospects for highefficiency commercial silicon solar cells. IEEE Trans. Electron Dev. 53, 1893–1901. Dubois, S., Tanay, F., Veirman, J., Enjalbert, N., Stendera, J., Butte´, S., Pochet, P., Caliste, D., Mao, Y., Timerkaeva, D., Blanc-Pe´lissier, D., Fraser, K., Lemiti, M., Palais, O., Pe´richaud, I., Mong-The-Yen, V., Pasquinelli, M., Gerard, M., Le-Quang, N., 2012. The BOLID project: suppression of the boron-oxygen related light-induced-degradation presentation – main results. In: 27th European Photovoltaic Solar Energy Conference and Exhibition, pp. 749–754. Dwivedi, N., Kumar, S., Bisht, A., Patel, K., Sudhakar, S., 2013. Simulation approach for optimization of device structure and thickness of HIT solar cells to achieve 27% efficiency. Sol. Energy 88, 31– 41. Hermle, M., Benick, J., Ru¨diger, M., Bateman, N., Glunz, S.W., 2011. Ntype silicon solar cells with implanted emitter. In: 26th European Photovoltaic Solar Energy Conference and Exhibition, pp. 875–878.
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ScienceDirect Solar Energy 114 (2015) 198–205 www.elsevier.com/locate/solener
The investigation on the front surface field of aluminum rear emitter N-type silicon solar cells Xi Xi a,b,c, Xiaojing Chen b, Song Zhang b, Wenjia Li a,b, Zhengrong Shi a,b,c, Guohua Li a,c,⇑ a
School of Science, Jiangnan University, Wuxi, China b Suntech Power Co., Ltd., Wuxi, China c Optoelectronic Engineering and Technology Research Center, Jiangsu Province, China Received 5 November 2014; received in revised form 15 January 2015; accepted 17 January 2015
Communicated by: Associate Editor Igor Tyukhov
Abstract N-type solar cells with aluminum rear emitter provide high power conversion efficiency and overcome light induced degradation. Meanwhile, since the processes are realized on the existing P-type cells, the fabrication cost gets lower for N-type solar cells. Front surface field (FSF) is an important part in this structure. The functions of FSF are similar to that of Al back surface field in P-type cells. FSF enhances carrier collection ability, increases the cells’ open-circuit voltage, as well as increases the efficiency. In this paper, the FSF profiles were investigated. The appropriate surface concentration was around 4 1020 cm3, meanwhile the depth should be kept between 0.25 lm and 0.3 lm from the experiment. A too low concentration cannot provide a strong field effect passivation, while a too high concentration will introduce much more recombination centers. A thin FSF layer increased the resistance, while a thick FSF layer made the short-circuit current decrease, because of the degradation of photoelectric response at the short wavelength. Thermal oxidation was introduced in the process. The FSF profiles were also studied. The minority lifetime and efficiency were enhanced after oxidation. The surface concentration of FSF should be adjusted to around 3 1019 cm3. The best cell efficiency has reached to 19.93%, tested by 18th Institute of China Electronics Group Corporation. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Solar cells; N-type; Aluminum rear emitter; Front surface field; Surface concentration
1. Introduction Majority of commercial solar cells are based on P-type doped silicon material. Almost all production solar cells have a homogeneous emitter, a PECVD deposited SiNx antireflective layer, and screen-printed contacts on both sides. Currently there are increasing interests in the development of N-type solar cells. N-type silicon has higher ⇑ Corresponding author at: School of Science, Jiangnan University, Wuxi, China. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.solener.2015.01.019 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
lifetime for minority carriers with small capture cross section of metallic impurities compared to P-type Si (Cotter et al., 2006; Macdonald and Geerligs, 2004; Schmidt et al., 1997). Meanwhile, N-type cells overcome the light induced degradation (LID) problems, which are attributed to boron–oxygen complexes (Bothe et al., 2005; Dubois et al., 2012). In recent years, several techniques have been developed to fabricate N-type silicon solar cells (Book et al., 2011; Hermle et al., 2011; Rauer et al., 2010; Ru¨diger et al., 2012; Schmiga et al., 2010; Sheoran et al., 2012; Su et al., 2012a; Sugianto et al., 2010). Many products of N-type based cells have been spread into the
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market, such as Yingli’s Panada (Romijn et al., 2012), Panasonic’s HIT (Dwivedi et al., 2013; Rawat et al., 2014; Shen et al., 2013; Wen et al., 2013; Yano et al., 2013) and Sunpower’s IBC (Smith et al., 2013). However, the processes of these cells are much different from the existing P-type cells’ fabrication processes. New equipments investments and existing facilities alteration make the costs of these N-type cells be higher. Fast and low cost processes for N-type cells based on the normal P-type cell production line are achieved by using the structure of aluminum rear emitter. In this structure, both the P–N junction and the rear metal contact are simultaneously formed by the use of well-established screen-printing techniques and the firing of the aluminum paste. At the front surface, an N+ doping field is formed by phosphorous diffusion. High power conversion efficiencies of Al rear emitter N-type cells have been achieved. Christian Schmiga et al. reported that they got an efficiency of 19.3% in the lab with Ag-plated contact finger on aerosol seed layer (Schmiga et al., 2012). Topcell Solar International fabricated an efficiency of 19.2% N-type Al rear emitter cell on large area wafers (239 cm2), which phosphorous-implanted selective front surface field was introduced (Su et al., 2012b). Suntech Power Co., Ltd. got the efficiency of 19.8% on 5-in. Cz wafer (148.58 cm2) by using PLUTO technology (Shi et al., 2009a,b). Recently, this efficiency in Suntech has been increased to 19.93% based on 6-in. wafer (238.95 cm2), tested by 18th Institute of China Electronics Group Corporation (CETC) (Voc = 647.2 mV, Jsc = 38.01 mA/cm2 FF = 81.0%). Front surface field (FSF) is formed by a phosphors doped N+ layer. It is an N+–N high-low junction, and is similar to the Al back surface field (Al-BSF) in P-type cells. FSF can enhance the open-circuit voltage (Voc), improve the carrier collection and reduce the transport resistance. Most of the investigation on Al rear emitter N-type solar cell was focused on the rear surface, such as the selection of Al paste, the formation and optimization of rear emitter and the metallization. The study on the FSF was rarely reported. The function of FSF in N-type cells is similar to Al-BSF in P-type cells. For P-type cells, the performances are better when the BSF layer is thicker. From this point, the depth of FSF should be as thicker as possible. However, FSF is at the irradiated surface. The position of FSF is the same with the emitter of P-type silicon cells. The emitter depth in P-type silicon cells is strictly controlled, since the depth strongly influences the photon absorption and response. From the experience of the emitter in P-type cells, the thicker FSF layer is not needed. The appropriate depth of FSF layer was studied in this paper. The relationship between the thickness of FSF layer and the cells’ performances was also investigated. The front surface concentration of doped phosphors is another important parameter. For P-type solar cells, lightly doped emitter (LDE) is very popular today. And it really enhances the cells’ performances. The necessity of lightly doped FSF was also studied. The appropriate front surface concentration of FSF layer was shown in this paper.
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A selective FSF, which phosphors are heavily doped under the front electrodes, is good for cells’ performances (Su et al., 2012b; Sugianto et al., 2009; Xi et al., 2012). In this paper, the heavily doped area was fixed, and was not the investigation emphasis. 2. Experiment The experiments were based on 6-in. Cz mono crystalline N-type Si wafers with the resistivity about 5 X cm. The process started with an alkaline texture and acid cleaning. And then the wafers were diffused with POCl3 for the FSF formation. The sheet resistance after diffusion was nearly 50 X/h. The residual N+ layer at the side and rear surface were removed by chemical etch (HNO3/HF). A standard SiNx deposition on the front surface was followed. Al paste was printed and fired on the rear surface for doping. After Al paste firing, a phosphorous source was sprayed on the front surface. A laser process was applied sequentially. After this process, the SiNx layers at the places of designed fingers and bus-bars were removed. Meanwhile, phosphorous was also heavily doped at these places. A structure of selective-FSF was formed (Sugianto et al., 2009; Xi et al., 2012). The sheet resistance of the laser doped region was 15 X/h.Then, light-inducedplating (LIP, PLUTO technology of Suntech) technology was used (Shi et al., 2009a,b; Sugianto et al., 2009; Xi et al., 2012). Ni, Cu and Ag were sequentially plated on the Si surface, where had been scanned by laser before. The cells’ structure is shown in Fig. 1. For some batches, thermal oxidation was used to passivate the surface before SiNx deposition. Since the doping profiles are necessary in the investigation, electrochemical capacitance voltage (ECV) test was used. The PSG layer should be removed before the measurement. The measurement was carried on automatically by the equipment. The solution in the ECV equipment was NH4HF2 (0.1 mol/L). A Schottky barrier formed by the etching solution and the semiconductor is utilized in the ECV measurement. And this is the basic principle of ECV test. A bias voltage is added on this barrier. And the voltage is continuous adjusted by the calculation software to keep a constant etching current. The charge concentration is obtained by the differential capacity of electrolyte-semiconductor Schottky barrier. The charge concentration is written as (Kinder et al., 1999; Udhayasankar et al., 2004), 1 C3 N¼ 2 ee0 er A dc=dV where e0 is the dielectric constant in vacuum; er is the dielectric constant of semiconductor; e is the electron charge; A is the Schottky barrier area; C is the capacitance. The process of etching-measuring and calculation is repeated again and again. Then the relationship between the charge concentration and the depth will be obtained.
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Fig. 1. Schematic cross section of N-type Al rear emitter cell with laser doped selective FSF and LIP front contact.
3. Results and discussion The function of FSF in Al rear emitter N-type solar cells is similar to Al-BSF in normal P-type cells. The thickness of Al-BSF is a crucial parameter for P-type cells. The investigation on FSF is also started from the depth. Usually, the front surface concentration of phosphors after POCl3 diffusion and HNO3/HF rear and edge etch is at the order of magnitude of 1020 cm3. Here, the surface concentration was managed to control between 3 1020 cm3 and 4 1020 cm3. Different diffusion processes and chemical etch processes were used to control the surface concentration, and got different depths. ECV test can give the doping profiles. These profiles are shown in Fig. 2. These profiles were tested after the etching of rear surface and edge, before the SiNx film deposition. The N-type Si base was doped about 3 1016 cm3 via ECV test. So the FSF layer starts from the front surface, and ends at the depth where the concentration is about 3 1016 cm3. Thermal oxide process was not applied in these batches of cells. The characteristics of these cells are shown in Table 1. The open-circuit voltages of the cells in Table 1 are nearly stable. It means the thickness of FSF layer has little influence to the Voc. However, the short-circuit currents increase with the decreasing of the depth. From this point, it is opposite to the Al-BSF. The lower Jsc exhibits when the FSF layer becomes thicker. This difference between FSF and BSF attributes to the position of the field inside the
cells. The FSF is at the irradiated surface. The decrease of Jsc is due to the degradation of the photoelectric response at the short wavelength. Fig. 3 shows the internal quantum efficiency (IQE) curves of the cells with different FSF depths. It is obvious that the IQE at the short wavelength gets lower when the FSF depth increases. The photons with short wavelength sharply attenuate after entering the wafers. Most of these photons are absorbed at the front surface or within a very short depth. The FSF layer is formed by phosphors doping. More phosphor atoms are needed to dope for a thicker FSF layer, and meanwhile more recombination centers are introduced. Unfortunately, the carriers excited by the photons with short wavelengths generate in the FSF layer. Thus, more carriers will be recombined in the thicker FSF layer, and a lower IQE exhibits. Since one of the most crucial functions for FSF is carrier collection, the sheet resistance is important. A thicker layer gives a lower sheet resistance. The resistance data after the chemical etching of rear surface and edge for the cells are also shown in Table 1. A lower sheet resistance is better for carrier collection and provides a lower series resistance of the cells and a higher fill factor (FF). A thinner FSF layer is good for Jsc, while it is disadvantageous to FF. A general conclusion from the cell efficiency is that the appropriate depth of FSF is in the range between 0.25 lm and 0.3 lm. The surface doping concentration is another focused point. Increasing the diffusion temperature step by step,
Fig. 2. The FSF profiles with different depths, the surface concentration is fixed on the same level. The depths are 0.17 lm, 0.21 lm, 0.26 lm, 0.29 lm and 0.33 lm for Group A, B, C, D, and E, respectively. The characteristics of these cells are shown in Table 1.
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Table 1 The characteristics of the cells with different FSF layer thicknesses. Depth (lm)
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
Rseries (X cm2)
RSheet (X/h)
0.17 0.21 0.26 0.29 0.33
640.2 640.6 641.2 640.8 641.1
37.584 37.502 37.404 37.295 36.946
78.92 79.63 80.39 80.59 80.68
18.99 19.13 19.28 19.26 19.11
1.34 1.00 0.74 0.67 0.57
135 121 101 94.8 80.3
(20 pcs) (20 pcs) (20 pcs) (20 pcs) (20 pcs)
Fig. 3. The IQE curves of the cells with different FSF layer thicknesses.
and adjusting the time of diffusion and wet chemical etch, the depth was fixed around 0.27 lm. The surface concentration was changed from 1 1020 cm3 to 8 1020 cm3. Here the thermal oxidation was also not introduced. The surface concentration and depth were measured after rear and edge etching processes, before SiNx deposition. The ECV profiles are shown in Fig. 4, and the cells’ performances are exhibited in Table 2. From Table 2, the Jsc of the cells are at the same level. As discussed above, the FSF depths are at the same level. The surface doping concentration mostly influences the Voc of the cells in this batch. The highest Voc appears at the concentration of about 4 1020 cm3. Either too low or too high of the concentration is not appropriate. FSF is a high-low junction. The energy level sketch is shown in Fig. 5.
The potential barrier DU in Fig. 5 is related to the doping level. In other words, the surface concentration can strongly influence this energy level difference (DU). The larger DU exists, the higher Voc appears. Meanwhile, the larger DU means a higher build-in field of FSF. This field can drive electrons moving out of the surface quickly and reduce the probability of electron and hole recombination. Thus, the surface recombination velocity drops, and the minority lifetime increases. These result in a high Voc. From these two opinions, high Voc needs a heavily doping FSF. It is true that the Voc grows up with the surface concentration increasing when the concentration is at a low level. However, the actual experiments are different when the doping level is too high. The Voc drops with the concentration increasing. Phosphors doping is used to form a FSF
Fig. 4. The ECV profiles of FSF (fixed on the same depth). The surface concentration are 1.1 1020 cm3, 3.2 1020 cm3, 4.5 1020 cm3, 6.2 1020 cm3 and 8.1 1020 cm3 for Group A, B, C, D, and E, respectively. The characteristics of these cells are shown in Table 2.
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Table 2 The performances of the cells with different FSF’s surface concentrations, fixed on the same depth. Surface concentration (cm3) 20
1.1 10 3.2 1020 4.5 1020 6.2 1020 8.1 1020
(20 pcs) (20 pcs) (20 pcs) (20 pcs) (20 pcs)
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
Rseries (X cm2)
RSheet (X/h)
635.0 640.5 640.7 635.2 631.6
37.394 37.402 37.400 37.401 37.401
80.48 80.52 80.58 80.65 80.69
19.11 19.29 19.31 19.16 19.06
1.10 0.72 0.67 0.60 0.50
107 99.2 96.7 88.6 82.3
Fig. 5. The energy level sketch of FSF.
Table 3 The minority lifetime of the wafers with different surface concentrations. The depth of FSF is fixed on 0.27 lm. In order to measure the minority lifetime, SiNx films of 75 nm were deposited on both front and rear sides by PECVD after wet chemical etch. Surface concentration (cm3) 20
1.1 10 3.2 1020 4.5 1020 6.2 1020 8.1 1020
Minority carrier lifetime (ls) 204 259 267 196 123
layer. The build-in field of FSF can give a field-effect passivation, and can increase the lifetime. However, phosphors atom itself also acts as a recombination center. When the doping level is not too high, the function of field-effect passivation is stronger than the function of the phosphors recombination center. At this time, the Voc still can exhibit increasing with the surface concentration growing up. When the doping level is too high, the function of fieldeffect passivation is weakened by the high concentration of phosphors recombination center. It results in the lower Voc. This discussion can also be described by the minority lifetime measurement. The minority lifetime of the wafers was measured by SEMLAB WT2000. In order to measure
the minority lifetime, SiNx films of 75 nm were deposited on both front and rear sides by PECVD after wet chemical etch. Table 3 shows the lifetime data. All the wafers are corresponding to the cells in Table 2. The highest lifetime of the wafer appears when the surface concentration is 4.5 1020 cm3. And the lifetime data all agree with the Voc and efficiencies of the cells. Thermal oxidation is one of the ways to provide good surface passivation and to enhance the efficiency. During the oxidation, a very thin silicon layer at the front surface will be oxidized; meanwhile the phosphors atoms will also be re-distributed. Some phosphors atoms will be driven deeper inside the silicon, while lots of phosphors atoms at the front surface will move into the SiO2 layer, since the diffusion speed of phosphors inside SiO2 is faster than that of the speed inside Si. Because of this, the surface concentration of FSF will drop, and the depth will increase a little. The minority lifetime data of the wafers are shown in Table 4. Additionally, SiNx films of 70 nm were deposited on both front and rear sides by PECVD after thermal oxidation. These lifetime data are also exhibited in Table 4. The oxidation temperature was 900 °C and lasted 15 min with an optimized oxidation process (Chen et al., 2012). Higher oxidation temperature can provide a compacter SiO2 layer and can show a higher minority lifetime. But the minority lifetime will be not stable if the temperature is over 920 °C, since the impurity may be activated and influence the quality of passivation. In this process, the oxidation condition was fixed, only the diffusion and wet chemical etch processes were adjusted. The depth of FSF was also kept at around 0.27 lm. The cells’ performances with different FSF surface concentrations are shown in Table 4. And the FSF profiles with oxidation are shown in Fig. 6.
Table 4 The minority lifetime data and cells’ performances with different FSF surface concentration introducing thermal oxidation process (the depth is fixed around 0.27 lm). Surface concentration (cm3)
Lifetime after oxidation ls
Lifetime with SiO2/SiNx (ls)
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
Rseries (X cm2)
RSheet (X/h)
5.6 1018 1.2 1019 3.2 1019 4.8 1019 7.6 1019
109 113 132 96 78
596 603 672 328 169
643.1 645.9 646.3 643.7 641.9
37.535 37.545 37.662 37.533 37.441
80.41 80.53 80.58 80.63 80.68
19.41 19.53 19.61 19.48 19.39
1.00 0.91 0.84 0.74 0.69
186 158 145 136 128
a
(20 pcs) (20 pcs) (20 pcs)a (20 pcs) (20 pcs)
The best cell is in this batch. The efficiency is 19.93%, tested by 18th Institute of China Electronics Group Corporation (CETC) (Voc = 647.2 mV, Jsc = 38.01 mA/cm2, FF = 81.0%).
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Fig. 6. The FSF profiles with oxidation process. The surface concentration are 5.6 1018 cm3, 1.2 1019 cm3, 3.2 1019 cm3, 4.8 1019 cm3 and 7.6 1019 cm3 for Group A, B, C, D, and E, respectively. They are fixed on the same depth. The characteristics of these cells are shown in Table 4.
Fig. 7. The I–V curve of the best cell, tested by 18th Institute of CETC.
Higher lifetime data are shown in Table 4 than that of the data in Table 3. High lifetime is essential for high conversion efficiency solar cell production. The rule of Voc in Table 4 is the same with the Voc in Table 2. Here SiO2 layer provides a perfect surface passivation, plus the fieldeffect passivation of FSF, the cells in Table 4 show high Voc. Since the prefect passivation at the surface, only a very small quantity of recombination centers can weaken this passivation. The appropriate surface concentration should be 3 1019 cm3. When the surface concentration
is extraordinary low, such as 1 1019 cm3, the Rs will sharply increase, and FF will rapidly decrease. So the efficiencies of the cells shown in the first two lines in Table 4 are lower, because of a poor FF. And the Voc of these cells are also not so good. Appropriate surface concentration is concluded at about 3 1019 cm3, and the depth should be kept between 0.25 lm and 0.3 lm. The best cell efficiency is 19.93% (Voc = 647.2 mV, Jsc = 38.01 mA/cm2 FF = 81.0%) (see Fig. 7).
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4. Conclusion FSF in N-type Al rear emitter solar cells enhances carrier collection ability, increases the cells Voc as well as the power conversion efficiency. From the investigation, the status of FSF strongly influences the cells’ performances. In order to utilize the FSF advantages, the profiles of FSF must be controlled. The depth and surface concentration of FSF were studied in this paper. A thicker FSF layer exhibited a lower Jsc. Although a very thin FSF layer can provide a high Jsc, the FF of the cells was poor and resulted in a bad efficiency. From the experiments, the FSF depth should be kept between 0.25 lm and 0.3 lm. The Voc were getting higher with the increasing of surface concentration. However, when the concentration was too high, the recombination velocity was sharply increased, and the field-effect passivation of FSF was weakened. The appropriate surface concentration was around 4 1020 cm3. SiO2 layer provided a perfect surface passivation. Since this perfect passivation at the surface, only a very small quantity of recombination centers will weaken the passivation effect. So the surface concentration should not be very high. From the experiments, the appropriate surface concentration should be about 3 1019 cm3. The best cell efficiency tested by 18th Institute of CETC is 19.93%. The FSF’s surface concentration and depth of this batch of cells are 3.2 1019 cm3 and 0.272 lm, respectively. Reference Book, F., Wiedenmann, T., Gloger, S., Raabe, B., Schubert, G., Plagwitz, H., Hahn, G., 2011. Analysis of processing steps for industrial large area n-type solar cells with screen printed aluminum-alloyed rear emitter and selective FSF. In: 26th European Photovoltaic Solar Energy Conference and Exhibition, pp. 1160–1163. Bothe, K., Sinton, R., Schmidt, J., 2005. Fundamental born-oxygenrelated carrier lifetime limit in mono and multicrystalline silicon. Prog. Photovoltaics: Res. Appl. 13, 287–296. Chen, L., Xi, X., Wu, W., Gao, F., Xu, J., Wang, Z., Yu, Z., Lu, Q., Zhang, S., Zhu, H., Chen, R., Yang, J., Ji, J., Shi, Z., 2012. The investigation on the front surface oxidation for aluminum rear emitter N-type solar cells. In: 38th IEEE Photovoltaic Specialists Conference (PVSC), pp. 002142–002144. Cotter, J.E., Guo, J.H., Cousins, P.J., Abbott, M.D., Chen, F.W., Fisher, K.C., 2006. P-type versus n-type silicon wafers: prospects for highefficiency commercial silicon solar cells. IEEE Trans. Electron Dev. 53, 1893–1901. Dubois, S., Tanay, F., Veirman, J., Enjalbert, N., Stendera, J., Butte´, S., Pochet, P., Caliste, D., Mao, Y., Timerkaeva, D., Blanc-Pe´lissier, D., Fraser, K., Lemiti, M., Palais, O., Pe´richaud, I., Mong-The-Yen, V., Pasquinelli, M., Gerard, M., Le-Quang, N., 2012. The BOLID project: suppression of the boron-oxygen related light-induced-degradation presentation – main results. In: 27th European Photovoltaic Solar Energy Conference and Exhibition, pp. 749–754. Dwivedi, N., Kumar, S., Bisht, A., Patel, K., Sudhakar, S., 2013. Simulation approach for optimization of device structure and thickness of HIT solar cells to achieve 27% efficiency. Sol. Energy 88, 31– 41. Hermle, M., Benick, J., Ru¨diger, M., Bateman, N., Glunz, S.W., 2011. Ntype silicon solar cells with implanted emitter. In: 26th European Photovoltaic Solar Energy Conference and Exhibition, pp. 875–878.
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