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Phosphorus treatment to promote crystallinity of the microcrystalline silicon front contact layers for highly efficient heterojunction solar cells
Solar Energy Materials & Solar Cells 209 (2020) 110439
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Phosphorus treatment to promote crystallinity of the microcrystalline silicon front contact layers for highly efficient heterojunction solar cells Chao Lei a, Chen-Wei Peng b, Jun Zhong b, Hongyu Li a, Miao Yang b, Kun Zheng c, Xianlin Qu c, Lili wu a, *, Cao Yu b, **, Yuanmin Li b, Xixiang Xu b a
Institute of Solar Energy Materials and Devices, College of Materials Science and Engineering, Sichuan University, No.29 Wangjiang Road, Chengdu, 610064, China Chengdu Zhufeng Yongming Technology Co., Ltd., Chengdu, Sichuan, 610200, China Beijing Key Lab of Microstructure and Property of Advanced Material, Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing, 100124, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Microcrystalline silicon oxide Phosphorus treatment Crystalline volume fraction SHJ solar cells
The current loss is mainly due to the reflection and the parasitic absorption in the indium tin oxide (ITO) and amorphous silicon (a-Si:H) in the front side of silicon heterojunction (SHJ) solar cells. In this paper, we implemented n-type hydrogenated microcrystalline silicon oxide (n-μc-SiOx:H) as the front surface field (FSF) to improve the short-circuit current density (JSC) of SHJ solar cells. The advantage of employing n-μc-SiOx:H layer is due to its low optical absorption coefficient and tunable refractive index. However, the introduction of carbon dioxide increases light transmission but reduces the crystallinity of n-μc-SiOx:H layer. Meanwhile, inhibiting the incubation layer and increasing microcrystalline/amorphous mixture phase during the growth are critical to the solar cell performance. Therefore, we implemented a high phosphorus-doping seed layer to form a nucleation layer to improve the crystallinity of n-μc-SiOx:H layer. In addition, the plasma enhanced chemical vapor depo sition (PECVD) process parameters of each layer were optimized to obtain good optical and electrical properties of n-μc-SiOx:H layer. Finally, a 242.5 cm2 solar cell had been fabricated with conversion efficiency of 23.87%, open-circuit voltage (VOC) of 739.8 mV, fill factor (FF) of 82.33% and JSC of 39.19 mA/cm2, which was 0.31 mA/ cm2 higher than that of the conventional n type a-Si:H SHJ solar cells.
1. Introduction In recent years, silicon solar cells have made great progress, in which SHJ solar cells have received much attention due to their high efficiency and simple process sequence. Recently, the efficiency of solar cells in many research institutions has exceeded 25% [1,2]. Especially, SHJ solar cells achieved 24.5% (239 cm2) and 25.1% (151.9 cm2) in large effective area by Kaneka [3]. The key point is that the introduction of intrinsic/n-type a-Si:H and intrinsic/p-type a-Si:H layers stacks as passivating and selective contacts on the front and back of crystalline silicon (c-Si) greatly reduces surface carrier recombination [4–7]. Due to its heterojunction structure, it can achieve a very high VOC [8]. However, because of the parasitic optical absorption losses in a-Si:H layers, along with ITO layer and shading losses in the metal grid, espe cially in the uppermost FSF layer, SHJ solar cells experience the lower JSC than conventional aluminum back surface field solar cells [9,10].
Reducing the front metal grid shading is a good way to increase the JSC of SHJ solar cells [11–14]. Heterojunction interdigitated back contact (HJ-IBC) cell with efficiency of 26.6% and the JSC of 42.5 mA/cm2 was achieved by Kaneka corporation in 2017 [15]. However, IBC technology is complex and costly. Therefore, hydrogenated microcrystalline silicon oxide (μc-SiOx:H) has been suggested to overcome these limitations [16]. μc-SiOx:H has remarkable variability in optical and electrical properties. It can obtain a tunable refractive index, a low absorption coefficient, and sufficient electrical conductivity by changing the doping and PECVD parameters. As a result, μc-SiOx:H is widely applicable as a window, back contact (BC) or intermediate reflector (IR) layer in various solar cell concepts [17]. In this study, n-μc-SiOx:H was used as the FSF to replace traditional n-type a-Si:H. Meanwhile, we introduced a precursor layer (the seed layer) for better growth of n-μc-SiOx:H. In order to inhibit the amor phous incubation layer, high power, and high hydrogen dilution ratio
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. wu), [email protected] (C. Yu). https://doi.org/10.1016/j.solmat.2020.110439 Received 29 September 2019; Received in revised form 29 January 2020; Accepted 30 January 2020 Available online 12 February 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Table 1 Summary of deposition parameters for FSF layer by PECVD. Layer
Power (normalized)
Deposition pressure (Torr)
RCO2
RH2
RPH3 (%)
n-type aSi:H Seed layer n-μc-SiOx: H
1
2
-
2.7
1.5
23.3 13.3
3 3
1
473 240
0–15 5
are usually applied to promote the transformation of the amorphous phase to crystalline phase [18–23]. On the basis of the previous description, phosphorus was introduced to be doped in the seed layer to form nucleation sites and accelerate the crystallization procedure. At the same time, we investigated the optical and electrical performances of n-type a-Si:H and different n-μc-SiOx:H films with different content of phosphorus doped seed layers. Then n-μc-SiOx:H was applied to the cell and observed by high-resolution transmission electron microscopy (HR-TEM). Besides, by adjusting the thickness of the ITO layer, more visible light was transmitted. Finally, we reported on the n-μc-SiOx:H FSF layer with a low refractive index of 2.94 at 632 nm, optical band gap of 2.03 eV and crystalline volume fraction (XC) of 36.6%. The best device conversion efficiency was up to 23.87% and its JSC reached 39.19 mA/cm2.
Fig. 2. Absorption coefficient spectra of n-type a-Si:H and different n-μc-SiOx:H films, the inset is a magnification of certain wavelengths.
<1 0 0> orientation) with a thickness of 150 μm (after texturing) and a resistivity of 2–4 Ω⋅cm. The as-cut c-Si substrates were first processed to remove the saw damage. Then, the surfaces were chemically textured to obtain random pyramids with a size of about 5 μm. Before the thin film deposition, all c-Si wafers were cleaned by the post-cleaning procedure. Intrinsic a-Si:H was used as a passivation layer on both sides and ptype a-Si:H was placed as an emitter on the rear of the cell. The FSF was composed of the seed layer and n-μc-SiOx:H layer. More detailed in formation could be found in Table 1. To extract the collected carriers efficiently, 65–90 nm ITO films were sputtered on the front and rear sides of the cells at room temperature, respectively. The thickness of ITO films on the textured wafer was measured by HR-TEM. The size of the cell on the wafer was defined by using a shadow mask during the deposition of the front and back ITO layers. Furthermore, electrical contacts were fabricated on both sides by the screen-printed silver grid, cured and annealed at 200 � C for 30 min. To characterize solar cells, HRTEM was performed to observe the microstructure of the cross-section of the cell [25]. Quantum efficiency (QE) and reflectance measurements were conducted on solar cells using a small light spot between the fin gers with a wavelength between 300 nm and 1200 nm. Moreover, pseudo fill factors (PFF) was measured using a Sinton Suns–VOC in strument. The performance of SHJ solar cells was evaluated by measuring current-voltage (I–V) characteristics under standard test conditions. (AM1.5 G,1 kW⋅m 2, 25 � C, class AAA).
2. Experimental details n-μc-SiOx:H and n-type a-Si:H films were deposited by the PECVD system using an excitation frequency of 40.68 MHz. The precursor gas mixture consisted of silane (SiH4), hydrogen (H2), phosphine (PH3), and carbon dioxide (CO2), in which CO2 acted as the major oxygen source. The PH3 was diluted in hydrogen gas and its concentration was 5%. The CO2 flow ratio RCO2 ¼ CO2/SiH4 and PH3 flow ratio RPH3 ¼ PH3/SiH4, while RH2 ¼ H2/SiH4. Table 1 shows the deposition parameters for FSF layers. n-μc-SiOx:H and n-type a-Si:H films were deposited on glass substrates. The substrate was previously deposited with 5 nm intrinsic aSi:H. For the convenience of measurement, the film thickness was three times than the film on the cell. The thickness and refractive index (n) of the film were measured by Spectroscopic Ellipsometer (SE, CCD type) and dark conductivity was measured in the vacuum using mercury coplanar electrodes at room temperature. Optical absorption coefficient characterized by fitting the measured reflection and transmission spectra with the Tauc–Lorentz dispersion model, whereas XC was eval uated by Raman scattering spectroscopy with 514 nm laser [24]. The optical band gap (E04) was defined by the photon energy for which an optical absorption coefficient of 104 cm 1 was obtained. Solar cells were made from n-type Czochralski silicon (as-cut, 8-inch,
Fig. 1. Cross-sectional view of textured SHJ solar cells in rear emitter configuration. Three different FSF stacks: structure A with conventional n-type a-Si:H; structure B with seed layer (PH3 free) þ n-μc-SiOx:H; structure C with seed layer (PH3 doping 10%) þ n-μc-SiOx:H. 2
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layer changed, other parameters of the FSF layer did not change, espe cially RH2. Fig. 2 shows absorption coefficient spectra of the n-type a-Si:H and nμc-SiOx:H films. The absorption coefficient of n-type a-Si:H is much higher than that of n-μc-SiOx:H. As the amount of PH3 doping increases in the seed layer, the absorption coefficient of n-μc-SiOx:H increases a little and the E04 moves from 2.03 to 2.00 eV. It indicates that the doping content of PH3 does affect the growth of the subsequent n-μc-SiOx:H layer. Raman spectroscopy data of different n-μc-SiOx:H films are reported in Fig. 3. The spectrum is decomposed into three components: the crystalline component peaked at 520 cm 1, the amorphous component peaked at 480 cm 1, and an intermediate component peaked at 494–507 cm 1 which is associated with bond dilation at grain bound aries [26]. Its XC can be calculated as: XC¼(I520þI510)/(I480þI510þI520), the XC is 16.2%/31.4%/33.5%/36.6% respectively. It has a great in fluence on the Xc with or without PH3 in the seed layer, and with the increase of RPH3, the XC increase slightly. This has been attributed to the tetrahedral configuration of phosphorus atoms in the Si matrix, making them act as crystallization centers, therefore, it contributes to the growth of subsequent crystallites [27–29]. In the actual deposition process, we found that the parameters of the seed layer did not form a film on the glass because of its high hydrogen dilution ratio. Therefore, PH3 may act on several atomic layers of the intrinsic a-Si:H layer to form a nucleation layer. The thickness and refractive index (n) of the n-μc-SiOx:H and n-type a-Si:H films are measured by SE and other related performances are summarized in the Table 2. The 43 nm-thick films are used to measure Raman spectra because 13 nm-thick film cannot provide reliable Raman signals due to the low signal/noise ratio.
Fig. 3. Raman spectra of different n-μc-SiOx:H films. Table 2 Optical and electrical properties of n-type a-Si:H and different n-μc-SiOx:H films. Film
Seed layer
Thickness (nm)
E04
n at 632 nm
n-μcSiOx:H n-μcSiOx:H n-μcSiOx:H n-μcSiOx:H n-type aSi:H
PH3 free
43.2
2.03
2.968
doping 5% doping 10% doping 15% -
43.4
2.01
2.944
43.1
2.00
2.943
43.3
2.00
2.941
12
1.83
4.215
σdark (S/
Xc (%)
7.2 � 10 7 3.3 � 10 6 6.4 � 10 6 5.2 � 10 6 -
16.2
cm)
31.4 33.5
3.2. Cell performance
36.6
Fig. 4 shows the Cross-sectional HR-TEM micrograph of the front side cell stack. Fig. 4(a) and Fig.4(b) represent the FSF layer of structure B (seed layer PH3 free) and structure C (seed layer PH3 doping 10%), respectively. Comparing the individual crystals marked by the yellow curve of two figures, it is obvious that the seed layer doped with PH3 contains more microcrystalline phase. Meanwhile, the n-μc-SiOx:H which evolves from the amorphous phase grows in columns and has different orientations. However, the incubation zone and the amorphous passivation layer cannot be distinguished from the micrograph. B and C in Fig. 4 contain both a lattice of crystals (A in Fig. 4) and an amorphous diffraction ring. It indicates that it is not a pure crystal, but a mixed phase of amorphous and microcrystalline. This is because the intro duction of oxygen destroys the Si–Si bond, making it difficult to grow individual crystals. Four kinds of cells are fabricated according to Fig. 1, cell D only
-
3. Results and discussion 3.1. The electrical and optical performances of films To evaluate the electrical and optical properties of n-type a-Si:H and n-μc-SiOx:H films, five different kinds of films were deposited on glass substrates. For simulating the real solar cell structure, a 5 nm intrinsic aSi:H was previously deposited on the glass substrate. n-μc-SiOx:H films had the same structure as structure B in Fig. 1, but the RPH3 in the seed layer were 0%, 5%, 10%, 15%, respectively. When the RPH3 in the seed
Fig. 4. Cross-sectional HR-TEM micrograph of the front side cell stack (ITO/(n) n-μc-SiOx:H /(i) a-Si:H/c-Si) of structure B and C. (a) structure B (seed layer without PH3 doping) (b) structure C (seed layer PH3 doping 10%). The zone axis orientation for the Si substrate is<101> in both micrographs. The small insets are the Fourier transforms calculated in the area indicated by the letters. Yellow lines highlight the boundaries of the passivation layer þ incubation phase and curves individual crystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
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coefficient cannot explain the JSC variation from B to C & D since the Jsc increases with the absorption coefficient increase. After analyzing, we think the absorption coefficient cannot be simply related with Jsc because the absorption difference among the n-μc-SiOx:H films is so small. Furthermore, the following two points should be considered. First, the Jsc is a complex result of photo absorption, carrier separation and collection processes. The change in absorption coefficient of a film has an impact indeed, but it is not the only factor. The separation and collection process of the carriers are equally important. Tucci M. et al. reported that the PH3 doping might reduce the defect density at the interface, resulting in the increase of JSC and VOC [30]. This is consistent with our experimental results. Second, the n-μc-SiOx:H film used for the absorption coefficient measurement is grown on smooth glass sub strates, while the n-μc-SiOx:H film in the cell is grown on the textured silicon wafers. The penetration path of photons in the film on glass is obviously different from that on the wafer. For photons, every interface they pass through is important. Therefore, the actual reason for the Jsc variation among Cell B, C, D is not clear and further investigation is needed.
Table 3 The details of cell type. FSF RPH3 of the seed layer
Cell A
Cell B
Cell C
Cell D
n-type a-Si: H -
n-μc-SiOx: H 0%
n-μc-SiOx: H 10%
n-μc-SiOx: H 15%
changes the RPH3 of the seed layer based on the structure C. Each group contains four solar cells of the same batch and is named cell A, B, C, D. More details are shown in Table 3. Fig. 5 illuminates the different performances of the cell. Comparing with the traditional n-type a-Si:H (cell A) FSF, the n-μc-SiOx:H (cell B, C, D) has a large increase in JSC about 0.31 mA/cm2. This may be due to the lower absorption coefficient of the n-μc-SiOx:H than that of n-type a-Si: H, which makes more lights pass through. For cell B, cell C, and cell D, the JSC increases a little bit as the seed layer PH3 doping increases. The median JSC of cell C is 0.025 mA/cm2 higher than that of cell B, and cell D is 0.019 mA/cm2 higher than cell C. However, the absorption
Fig. 5. Cell parameters for SHJ solar cells. The three kinds of cells (A/B/C) adopt structure A /B/C respectively. Cell D uses the same structure as structure C except that the Seed layer is doped with 15% PH3. (a) conversion efficiency, (b) Open circuit voltage, (c) short circuit current density, (d) fill factor.
Fig. 6. Series resistance and pseudo fill factor for solar cells. The type of cells representative has been described in Fig. 5. (a) series resistance, (b)pseudo fill factor. 4
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Fig. 8. Best experimental SHJ solar cell I–V characteristics under AM 1.5G illumination condition.
layer. As shown in Fig. 6, the resistivity and PFF of the seed layer without PH3 doping (cell B) are worse than that of n-type a-Si:H (cell A). This may be because the incorporation of oxygen breaks down the original network structure and the presence of the incubation layer in creases the density of defects. This can also explain the decline of VOC. However, as the seed layer is doped with PH3 (cell C and cell D), the Xc of the microcrystalline layer increases, and both Rs and PFF develop in a good direction. The difference in FF between cell C and cell D is mainly from PFF, which may be related to the contact of microcrystal and ITO. In the end, cell C shows the best performance, reaching 23.68% efficiency. Fig. 7 shows the QE performance of the best cell. Comparing the QE of cell A and cell C1, cell C1 has a higher spectral response in 500–700 nm of EQE due to the lower absorption coefficient of μc-SiOx:H. How ever, due to the introduction of microcrystal, the E04 and refractive index have changed, and the reflection of the cell C1 moves towards the red light. As a result, EQE and IQE are not improved significantly. In the end, we induce the reflection to move towards blue light by reducing the thickness of the ITO, allowing more visible light to pass through. As a result, the EQE of cell C2 increases in the range of 400–650 nm, and also increases in the range of 100–1200 nm due to the decrease of ITO parasitic absorption. Finally, the JSC-EQE reaches 40.98 mA/cm2. The I–V curve of the best cell (242.5 cm2) is shown in Fig. 8. Using the n-μc-SiOx:H as the FSF layer of SHJ cell, through the final optimi zation, its conversion efficiency reached 23.87%, VOC ¼ 739.8 mV, FF ¼ 82.33%, JSC ¼ 39.19 mA/cm2. 4. Conclusion In this paper, we demonstrated the seed layer doped different con tents of PH3 could modify the XC of n-μc-SiOx:H layer. The Jsc of n-μcSiOx:H FSF layer SHJ solar cell was improved about 0.31 mA/cm2 higher than n-type a-Si:H and this attributed to n-μc-SiOx:H had lower absorption coefficient than n-type a-Si:H. However, n-μc-SiOx:H FSF layer of the seed layer without doping PH3 was at a disadvantage in terms of FF. This might be blamed for too low XC. The optical and electrical properties of the n-μc-SiOx:H FSF layer were improved by introducing PH3 in seed layer to promote the crystallization of micro crystalline silicon. The FF of n-μc-SiOx:H FSF layer kept the same level compared to n-type a-Si:H. By decreasing ITO thickness, the cell ob tained good optical matching. Finally, the best cell had a conversion efficiency of 23.87% (242.5 cm2), VOC ¼ 739.8 mV, FF ¼ 82.33%, JSC ¼
Fig. 7. Cell parameters of the best SHJ solar cells. (a) External quantum effi ciency (EQE), the current density in the figure is the integrated current of EQE. (b) reflection (C) IQE calculated from the 1-R and EQE spectra. Cell A repre sents a conventional n-type a-Si:H FSF layer, Cell C1 adopts the structure of the n-μc-SiOx:H FSF layer (structure C), Cell C2 reduces the thickness of front ITO layer by 10% based on Cell C1.
In theory, the FF of solar cells with microcrystalline silicon layer is higher than that of solar cells with a-Si:H layer. However, in order to increase the light transmission, oxygen is incorporated into the micro crystalline silicon layer, which reduces the conductivity of μc-SiOx:H 5
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39.19 mA/cm2.
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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. CRediT authorship contribution statement Chao Lei: Conceptualization, Writing - original draft, Validation. Chen-Wei Peng: Conceptualization. Jun Zhong: Investigation. Hongyu Li: Data curation, Visualization. Miao Yang: Investigation. Kun Zheng: Investigation. Xianlin Qu: Investigation. Lili wu: Visualization, Su pervision. Cao Yu: Supervision. Yuanmin Li: Supervision, Project administration. Xixiang Xu: Supervision, Project administration. Acknowledgment The authors would like to thank the engineers in other groups in Chengdu Zhufeng Yongming Technology Co., Ltd., peculiarly Gangqiang Dong, Shi Yin in PVD group, Guofu Peng in screen printing group. The authors thank Zhiwei Shi (College of Materials Science and Engineering of Beihang University) for Raman measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110439. References [1] K. Masuko, M. Shigematsu, T. Hashiguchi, D. Fujishima, M. Kai, N. Yoshimura, T. Yamaguchi, Y. Ichihashi, T. Mishima, N. Matsubara, T. Yamanishi, T. Takahama, M. Taguchi, E. Maruyama, S. Okamoto, Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell, IEEE J. Photovoltaics 4 (2014) 1433–1435. [2] F. Haase, R. Peibst, 26.1% Record Efficiency for P-type Crystalline Si Solar Cells, ISFH, Emmerthal/Hannove, 2018. [3] D. Adachi, J.L. Hern� andez, K. Yamamoto, Impact of carrier recombination on fill factor for large area heterojunction crystalline silicon solar cell with 25.1% efficiency, Appl. Phys. Lett. 107 (2015), 233506. [4] L. Mazzarella, A.B. Morales-Vilches, L. Korte, R. Schlatmann, B. Stannowski, Ultrathin nanocrystalline n-type silicon oxide front contact layers for rear-emitter silicon heterojunction solar cells, Sol. Energy Mater. Sol. Cell. 179 (2018) 386–391. [5] A. Kanevce, W.K. Metzger, The role of amorphous silicon and tunneling in heterojunction with intrinsic thin layer (HIT) solar cells, J. Appl. Phys. 105 (2009), 094507. [6] T. Mueller, J. Wong, A.G. Aberle, Heterojunction silicon wafer solar cells using amorphous silicon suboxides for interface passivation, Energy Procedia 15 (2012) 97–106. [7] M. Despeisse, C. Battaglia, M. Boccard, G. Bugnon, M. Charri� ere, P. Cuony, S. H€ anni, L. L€ ofgren, F. Meillaud, G. Parascandolo, Optimization of thin film silicon solar cells on highly textured substrates, Phys. Status Solidi 208 (2011) 1863–1868. [8] M. Taguchi, A. Yano, S. Tohoda, K. Matsuyama, Y. Nakamura, T. Nishiwaki, K. Fujita, E. Maruyama, 24.7% record efficiency HIT solar cell on thin silicon wafer, IEEE J. Photovoltaics 4 (2013) 96–99.
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Phosphorus treatment to promote crystallinity of the microcrystalline silicon front contact layers for highly efficient heterojunction solar cells Chao Lei a, Chen-Wei Peng b, Jun Zhong b, Hongyu Li a, Miao Yang b, Kun Zheng c, Xianlin Qu c, Lili wu a, *, Cao Yu b, **, Yuanmin Li b, Xixiang Xu b a
Institute of Solar Energy Materials and Devices, College of Materials Science and Engineering, Sichuan University, No.29 Wangjiang Road, Chengdu, 610064, China Chengdu Zhufeng Yongming Technology Co., Ltd., Chengdu, Sichuan, 610200, China Beijing Key Lab of Microstructure and Property of Advanced Material, Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing, 100124, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Microcrystalline silicon oxide Phosphorus treatment Crystalline volume fraction SHJ solar cells
The current loss is mainly due to the reflection and the parasitic absorption in the indium tin oxide (ITO) and amorphous silicon (a-Si:H) in the front side of silicon heterojunction (SHJ) solar cells. In this paper, we implemented n-type hydrogenated microcrystalline silicon oxide (n-μc-SiOx:H) as the front surface field (FSF) to improve the short-circuit current density (JSC) of SHJ solar cells. The advantage of employing n-μc-SiOx:H layer is due to its low optical absorption coefficient and tunable refractive index. However, the introduction of carbon dioxide increases light transmission but reduces the crystallinity of n-μc-SiOx:H layer. Meanwhile, inhibiting the incubation layer and increasing microcrystalline/amorphous mixture phase during the growth are critical to the solar cell performance. Therefore, we implemented a high phosphorus-doping seed layer to form a nucleation layer to improve the crystallinity of n-μc-SiOx:H layer. In addition, the plasma enhanced chemical vapor depo sition (PECVD) process parameters of each layer were optimized to obtain good optical and electrical properties of n-μc-SiOx:H layer. Finally, a 242.5 cm2 solar cell had been fabricated with conversion efficiency of 23.87%, open-circuit voltage (VOC) of 739.8 mV, fill factor (FF) of 82.33% and JSC of 39.19 mA/cm2, which was 0.31 mA/ cm2 higher than that of the conventional n type a-Si:H SHJ solar cells.
1. Introduction In recent years, silicon solar cells have made great progress, in which SHJ solar cells have received much attention due to their high efficiency and simple process sequence. Recently, the efficiency of solar cells in many research institutions has exceeded 25% [1,2]. Especially, SHJ solar cells achieved 24.5% (239 cm2) and 25.1% (151.9 cm2) in large effective area by Kaneka [3]. The key point is that the introduction of intrinsic/n-type a-Si:H and intrinsic/p-type a-Si:H layers stacks as passivating and selective contacts on the front and back of crystalline silicon (c-Si) greatly reduces surface carrier recombination [4–7]. Due to its heterojunction structure, it can achieve a very high VOC [8]. However, because of the parasitic optical absorption losses in a-Si:H layers, along with ITO layer and shading losses in the metal grid, espe cially in the uppermost FSF layer, SHJ solar cells experience the lower JSC than conventional aluminum back surface field solar cells [9,10].
Reducing the front metal grid shading is a good way to increase the JSC of SHJ solar cells [11–14]. Heterojunction interdigitated back contact (HJ-IBC) cell with efficiency of 26.6% and the JSC of 42.5 mA/cm2 was achieved by Kaneka corporation in 2017 [15]. However, IBC technology is complex and costly. Therefore, hydrogenated microcrystalline silicon oxide (μc-SiOx:H) has been suggested to overcome these limitations [16]. μc-SiOx:H has remarkable variability in optical and electrical properties. It can obtain a tunable refractive index, a low absorption coefficient, and sufficient electrical conductivity by changing the doping and PECVD parameters. As a result, μc-SiOx:H is widely applicable as a window, back contact (BC) or intermediate reflector (IR) layer in various solar cell concepts [17]. In this study, n-μc-SiOx:H was used as the FSF to replace traditional n-type a-Si:H. Meanwhile, we introduced a precursor layer (the seed layer) for better growth of n-μc-SiOx:H. In order to inhibit the amor phous incubation layer, high power, and high hydrogen dilution ratio
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. wu), [email protected] (C. Yu). https://doi.org/10.1016/j.solmat.2020.110439 Received 29 September 2019; Received in revised form 29 January 2020; Accepted 30 January 2020 Available online 12 February 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Table 1 Summary of deposition parameters for FSF layer by PECVD. Layer
Power (normalized)
Deposition pressure (Torr)
RCO2
RH2
RPH3 (%)
n-type aSi:H Seed layer n-μc-SiOx: H
1
2
-
2.7
1.5
23.3 13.3
3 3
1
473 240
0–15 5
are usually applied to promote the transformation of the amorphous phase to crystalline phase [18–23]. On the basis of the previous description, phosphorus was introduced to be doped in the seed layer to form nucleation sites and accelerate the crystallization procedure. At the same time, we investigated the optical and electrical performances of n-type a-Si:H and different n-μc-SiOx:H films with different content of phosphorus doped seed layers. Then n-μc-SiOx:H was applied to the cell and observed by high-resolution transmission electron microscopy (HR-TEM). Besides, by adjusting the thickness of the ITO layer, more visible light was transmitted. Finally, we reported on the n-μc-SiOx:H FSF layer with a low refractive index of 2.94 at 632 nm, optical band gap of 2.03 eV and crystalline volume fraction (XC) of 36.6%. The best device conversion efficiency was up to 23.87% and its JSC reached 39.19 mA/cm2.
Fig. 2. Absorption coefficient spectra of n-type a-Si:H and different n-μc-SiOx:H films, the inset is a magnification of certain wavelengths.
<1 0 0> orientation) with a thickness of 150 μm (after texturing) and a resistivity of 2–4 Ω⋅cm. The as-cut c-Si substrates were first processed to remove the saw damage. Then, the surfaces were chemically textured to obtain random pyramids with a size of about 5 μm. Before the thin film deposition, all c-Si wafers were cleaned by the post-cleaning procedure. Intrinsic a-Si:H was used as a passivation layer on both sides and ptype a-Si:H was placed as an emitter on the rear of the cell. The FSF was composed of the seed layer and n-μc-SiOx:H layer. More detailed in formation could be found in Table 1. To extract the collected carriers efficiently, 65–90 nm ITO films were sputtered on the front and rear sides of the cells at room temperature, respectively. The thickness of ITO films on the textured wafer was measured by HR-TEM. The size of the cell on the wafer was defined by using a shadow mask during the deposition of the front and back ITO layers. Furthermore, electrical contacts were fabricated on both sides by the screen-printed silver grid, cured and annealed at 200 � C for 30 min. To characterize solar cells, HRTEM was performed to observe the microstructure of the cross-section of the cell [25]. Quantum efficiency (QE) and reflectance measurements were conducted on solar cells using a small light spot between the fin gers with a wavelength between 300 nm and 1200 nm. Moreover, pseudo fill factors (PFF) was measured using a Sinton Suns–VOC in strument. The performance of SHJ solar cells was evaluated by measuring current-voltage (I–V) characteristics under standard test conditions. (AM1.5 G,1 kW⋅m 2, 25 � C, class AAA).
2. Experimental details n-μc-SiOx:H and n-type a-Si:H films were deposited by the PECVD system using an excitation frequency of 40.68 MHz. The precursor gas mixture consisted of silane (SiH4), hydrogen (H2), phosphine (PH3), and carbon dioxide (CO2), in which CO2 acted as the major oxygen source. The PH3 was diluted in hydrogen gas and its concentration was 5%. The CO2 flow ratio RCO2 ¼ CO2/SiH4 and PH3 flow ratio RPH3 ¼ PH3/SiH4, while RH2 ¼ H2/SiH4. Table 1 shows the deposition parameters for FSF layers. n-μc-SiOx:H and n-type a-Si:H films were deposited on glass substrates. The substrate was previously deposited with 5 nm intrinsic aSi:H. For the convenience of measurement, the film thickness was three times than the film on the cell. The thickness and refractive index (n) of the film were measured by Spectroscopic Ellipsometer (SE, CCD type) and dark conductivity was measured in the vacuum using mercury coplanar electrodes at room temperature. Optical absorption coefficient characterized by fitting the measured reflection and transmission spectra with the Tauc–Lorentz dispersion model, whereas XC was eval uated by Raman scattering spectroscopy with 514 nm laser [24]. The optical band gap (E04) was defined by the photon energy for which an optical absorption coefficient of 104 cm 1 was obtained. Solar cells were made from n-type Czochralski silicon (as-cut, 8-inch,
Fig. 1. Cross-sectional view of textured SHJ solar cells in rear emitter configuration. Three different FSF stacks: structure A with conventional n-type a-Si:H; structure B with seed layer (PH3 free) þ n-μc-SiOx:H; structure C with seed layer (PH3 doping 10%) þ n-μc-SiOx:H. 2
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layer changed, other parameters of the FSF layer did not change, espe cially RH2. Fig. 2 shows absorption coefficient spectra of the n-type a-Si:H and nμc-SiOx:H films. The absorption coefficient of n-type a-Si:H is much higher than that of n-μc-SiOx:H. As the amount of PH3 doping increases in the seed layer, the absorption coefficient of n-μc-SiOx:H increases a little and the E04 moves from 2.03 to 2.00 eV. It indicates that the doping content of PH3 does affect the growth of the subsequent n-μc-SiOx:H layer. Raman spectroscopy data of different n-μc-SiOx:H films are reported in Fig. 3. The spectrum is decomposed into three components: the crystalline component peaked at 520 cm 1, the amorphous component peaked at 480 cm 1, and an intermediate component peaked at 494–507 cm 1 which is associated with bond dilation at grain bound aries [26]. Its XC can be calculated as: XC¼(I520þI510)/(I480þI510þI520), the XC is 16.2%/31.4%/33.5%/36.6% respectively. It has a great in fluence on the Xc with or without PH3 in the seed layer, and with the increase of RPH3, the XC increase slightly. This has been attributed to the tetrahedral configuration of phosphorus atoms in the Si matrix, making them act as crystallization centers, therefore, it contributes to the growth of subsequent crystallites [27–29]. In the actual deposition process, we found that the parameters of the seed layer did not form a film on the glass because of its high hydrogen dilution ratio. Therefore, PH3 may act on several atomic layers of the intrinsic a-Si:H layer to form a nucleation layer. The thickness and refractive index (n) of the n-μc-SiOx:H and n-type a-Si:H films are measured by SE and other related performances are summarized in the Table 2. The 43 nm-thick films are used to measure Raman spectra because 13 nm-thick film cannot provide reliable Raman signals due to the low signal/noise ratio.
Fig. 3. Raman spectra of different n-μc-SiOx:H films. Table 2 Optical and electrical properties of n-type a-Si:H and different n-μc-SiOx:H films. Film
Seed layer
Thickness (nm)
E04
n at 632 nm
n-μcSiOx:H n-μcSiOx:H n-μcSiOx:H n-μcSiOx:H n-type aSi:H
PH3 free
43.2
2.03
2.968
doping 5% doping 10% doping 15% -
43.4
2.01
2.944
43.1
2.00
2.943
43.3
2.00
2.941
12
1.83
4.215
σdark (S/
Xc (%)
7.2 � 10 7 3.3 � 10 6 6.4 � 10 6 5.2 � 10 6 -
16.2
cm)
31.4 33.5
3.2. Cell performance
36.6
Fig. 4 shows the Cross-sectional HR-TEM micrograph of the front side cell stack. Fig. 4(a) and Fig.4(b) represent the FSF layer of structure B (seed layer PH3 free) and structure C (seed layer PH3 doping 10%), respectively. Comparing the individual crystals marked by the yellow curve of two figures, it is obvious that the seed layer doped with PH3 contains more microcrystalline phase. Meanwhile, the n-μc-SiOx:H which evolves from the amorphous phase grows in columns and has different orientations. However, the incubation zone and the amorphous passivation layer cannot be distinguished from the micrograph. B and C in Fig. 4 contain both a lattice of crystals (A in Fig. 4) and an amorphous diffraction ring. It indicates that it is not a pure crystal, but a mixed phase of amorphous and microcrystalline. This is because the intro duction of oxygen destroys the Si–Si bond, making it difficult to grow individual crystals. Four kinds of cells are fabricated according to Fig. 1, cell D only
-
3. Results and discussion 3.1. The electrical and optical performances of films To evaluate the electrical and optical properties of n-type a-Si:H and n-μc-SiOx:H films, five different kinds of films were deposited on glass substrates. For simulating the real solar cell structure, a 5 nm intrinsic aSi:H was previously deposited on the glass substrate. n-μc-SiOx:H films had the same structure as structure B in Fig. 1, but the RPH3 in the seed layer were 0%, 5%, 10%, 15%, respectively. When the RPH3 in the seed
Fig. 4. Cross-sectional HR-TEM micrograph of the front side cell stack (ITO/(n) n-μc-SiOx:H /(i) a-Si:H/c-Si) of structure B and C. (a) structure B (seed layer without PH3 doping) (b) structure C (seed layer PH3 doping 10%). The zone axis orientation for the Si substrate is<101> in both micrographs. The small insets are the Fourier transforms calculated in the area indicated by the letters. Yellow lines highlight the boundaries of the passivation layer þ incubation phase and curves individual crystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
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coefficient cannot explain the JSC variation from B to C & D since the Jsc increases with the absorption coefficient increase. After analyzing, we think the absorption coefficient cannot be simply related with Jsc because the absorption difference among the n-μc-SiOx:H films is so small. Furthermore, the following two points should be considered. First, the Jsc is a complex result of photo absorption, carrier separation and collection processes. The change in absorption coefficient of a film has an impact indeed, but it is not the only factor. The separation and collection process of the carriers are equally important. Tucci M. et al. reported that the PH3 doping might reduce the defect density at the interface, resulting in the increase of JSC and VOC [30]. This is consistent with our experimental results. Second, the n-μc-SiOx:H film used for the absorption coefficient measurement is grown on smooth glass sub strates, while the n-μc-SiOx:H film in the cell is grown on the textured silicon wafers. The penetration path of photons in the film on glass is obviously different from that on the wafer. For photons, every interface they pass through is important. Therefore, the actual reason for the Jsc variation among Cell B, C, D is not clear and further investigation is needed.
Table 3 The details of cell type. FSF RPH3 of the seed layer
Cell A
Cell B
Cell C
Cell D
n-type a-Si: H -
n-μc-SiOx: H 0%
n-μc-SiOx: H 10%
n-μc-SiOx: H 15%
changes the RPH3 of the seed layer based on the structure C. Each group contains four solar cells of the same batch and is named cell A, B, C, D. More details are shown in Table 3. Fig. 5 illuminates the different performances of the cell. Comparing with the traditional n-type a-Si:H (cell A) FSF, the n-μc-SiOx:H (cell B, C, D) has a large increase in JSC about 0.31 mA/cm2. This may be due to the lower absorption coefficient of the n-μc-SiOx:H than that of n-type a-Si: H, which makes more lights pass through. For cell B, cell C, and cell D, the JSC increases a little bit as the seed layer PH3 doping increases. The median JSC of cell C is 0.025 mA/cm2 higher than that of cell B, and cell D is 0.019 mA/cm2 higher than cell C. However, the absorption
Fig. 5. Cell parameters for SHJ solar cells. The three kinds of cells (A/B/C) adopt structure A /B/C respectively. Cell D uses the same structure as structure C except that the Seed layer is doped with 15% PH3. (a) conversion efficiency, (b) Open circuit voltage, (c) short circuit current density, (d) fill factor.
Fig. 6. Series resistance and pseudo fill factor for solar cells. The type of cells representative has been described in Fig. 5. (a) series resistance, (b)pseudo fill factor. 4
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Fig. 8. Best experimental SHJ solar cell I–V characteristics under AM 1.5G illumination condition.
layer. As shown in Fig. 6, the resistivity and PFF of the seed layer without PH3 doping (cell B) are worse than that of n-type a-Si:H (cell A). This may be because the incorporation of oxygen breaks down the original network structure and the presence of the incubation layer in creases the density of defects. This can also explain the decline of VOC. However, as the seed layer is doped with PH3 (cell C and cell D), the Xc of the microcrystalline layer increases, and both Rs and PFF develop in a good direction. The difference in FF between cell C and cell D is mainly from PFF, which may be related to the contact of microcrystal and ITO. In the end, cell C shows the best performance, reaching 23.68% efficiency. Fig. 7 shows the QE performance of the best cell. Comparing the QE of cell A and cell C1, cell C1 has a higher spectral response in 500–700 nm of EQE due to the lower absorption coefficient of μc-SiOx:H. How ever, due to the introduction of microcrystal, the E04 and refractive index have changed, and the reflection of the cell C1 moves towards the red light. As a result, EQE and IQE are not improved significantly. In the end, we induce the reflection to move towards blue light by reducing the thickness of the ITO, allowing more visible light to pass through. As a result, the EQE of cell C2 increases in the range of 400–650 nm, and also increases in the range of 100–1200 nm due to the decrease of ITO parasitic absorption. Finally, the JSC-EQE reaches 40.98 mA/cm2. The I–V curve of the best cell (242.5 cm2) is shown in Fig. 8. Using the n-μc-SiOx:H as the FSF layer of SHJ cell, through the final optimi zation, its conversion efficiency reached 23.87%, VOC ¼ 739.8 mV, FF ¼ 82.33%, JSC ¼ 39.19 mA/cm2. 4. Conclusion In this paper, we demonstrated the seed layer doped different con tents of PH3 could modify the XC of n-μc-SiOx:H layer. The Jsc of n-μcSiOx:H FSF layer SHJ solar cell was improved about 0.31 mA/cm2 higher than n-type a-Si:H and this attributed to n-μc-SiOx:H had lower absorption coefficient than n-type a-Si:H. However, n-μc-SiOx:H FSF layer of the seed layer without doping PH3 was at a disadvantage in terms of FF. This might be blamed for too low XC. The optical and electrical properties of the n-μc-SiOx:H FSF layer were improved by introducing PH3 in seed layer to promote the crystallization of micro crystalline silicon. The FF of n-μc-SiOx:H FSF layer kept the same level compared to n-type a-Si:H. By decreasing ITO thickness, the cell ob tained good optical matching. Finally, the best cell had a conversion efficiency of 23.87% (242.5 cm2), VOC ¼ 739.8 mV, FF ¼ 82.33%, JSC ¼
Fig. 7. Cell parameters of the best SHJ solar cells. (a) External quantum effi ciency (EQE), the current density in the figure is the integrated current of EQE. (b) reflection (C) IQE calculated from the 1-R and EQE spectra. Cell A repre sents a conventional n-type a-Si:H FSF layer, Cell C1 adopts the structure of the n-μc-SiOx:H FSF layer (structure C), Cell C2 reduces the thickness of front ITO layer by 10% based on Cell C1.
In theory, the FF of solar cells with microcrystalline silicon layer is higher than that of solar cells with a-Si:H layer. However, in order to increase the light transmission, oxygen is incorporated into the micro crystalline silicon layer, which reduces the conductivity of μc-SiOx:H 5
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39.19 mA/cm2.
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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. CRediT authorship contribution statement Chao Lei: Conceptualization, Writing - original draft, Validation. Chen-Wei Peng: Conceptualization. Jun Zhong: Investigation. Hongyu Li: Data curation, Visualization. Miao Yang: Investigation. Kun Zheng: Investigation. Xianlin Qu: Investigation. Lili wu: Visualization, Su pervision. Cao Yu: Supervision. Yuanmin Li: Supervision, Project administration. Xixiang Xu: Supervision, Project administration. Acknowledgment The authors would like to thank the engineers in other groups in Chengdu Zhufeng Yongming Technology Co., Ltd., peculiarly Gangqiang Dong, Shi Yin in PVD group, Guofu Peng in screen printing group. The authors thank Zhiwei Shi (College of Materials Science and Engineering of Beihang University) for Raman measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110439. References [1] K. Masuko, M. Shigematsu, T. Hashiguchi, D. Fujishima, M. Kai, N. Yoshimura, T. Yamaguchi, Y. Ichihashi, T. Mishima, N. Matsubara, T. Yamanishi, T. Takahama, M. Taguchi, E. Maruyama, S. Okamoto, Achievement of more than 25% conversion efficiency with crystalline silicon heterojunction solar cell, IEEE J. Photovoltaics 4 (2014) 1433–1435. [2] F. Haase, R. Peibst, 26.1% Record Efficiency for P-type Crystalline Si Solar Cells, ISFH, Emmerthal/Hannove, 2018. [3] D. Adachi, J.L. Hern� andez, K. Yamamoto, Impact of carrier recombination on fill factor for large area heterojunction crystalline silicon solar cell with 25.1% efficiency, Appl. Phys. Lett. 107 (2015), 233506. [4] L. Mazzarella, A.B. Morales-Vilches, L. Korte, R. Schlatmann, B. Stannowski, Ultrathin nanocrystalline n-type silicon oxide front contact layers for rear-emitter silicon heterojunction solar cells, Sol. Energy Mater. Sol. Cell. 179 (2018) 386–391. [5] A. Kanevce, W.K. Metzger, The role of amorphous silicon and tunneling in heterojunction with intrinsic thin layer (HIT) solar cells, J. Appl. Phys. 105 (2009), 094507. [6] T. Mueller, J. Wong, A.G. Aberle, Heterojunction silicon wafer solar cells using amorphous silicon suboxides for interface passivation, Energy Procedia 15 (2012) 97–106. [7] M. Despeisse, C. Battaglia, M. Boccard, G. Bugnon, M. Charri� ere, P. Cuony, S. H€ anni, L. L€ ofgren, F. Meillaud, G. Parascandolo, Optimization of thin film silicon solar cells on highly textured substrates, Phys. Status Solidi 208 (2011) 1863–1868. [8] M. Taguchi, A. Yano, S. Tohoda, K. Matsuyama, Y. Nakamura, T. Nishiwaki, K. Fujita, E. Maruyama, 24.7% record efficiency HIT solar cell on thin silicon wafer, IEEE J. Photovoltaics 4 (2013) 96–99.
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