μc-Si hybrid layer in tunnel oxide passivated contact n-type silicon solar cells

Solar Energy 144 (2017) 735–739

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Application of a-Si/lc-Si hybrid layer in tunnel oxide passivated contact n-type silicon solar cells Ke Tao a, Qiang Li a, Caixia Hou a, Shuai Jiang a, Jin Wang b, Rui Jia a,⇑, Yun Sun a, Yongtao Li a, Zhi Jin a, Xinyu Liu a a b

Institute of Microelectronics, Chinese Academy of Sciences, 3# Bei-Tu-Cheng West Road, Beijing 100029, China College of Electronic Information and Optical Engineering, Nankai University, #94 Weijin Road, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 30 September 2016 Received in revised form 21 January 2017 Accepted 26 January 2017

Keywords: Tunel oxide passivated contact a-Si lc-Si Passivation Ntype silicon solar cells

a b s t r a c t This paper presents our research on the TOP-Con silicon solar cells, which focuses on the passivated contacts based on a thin tunneling oxide layer and a silicon thin film. The influence of the microstructure of silicon thin film on the passivation property was studied. The experimental results indicated that the tunnel oxide combined with a-Si:H featured a good passivation property compared to the lc-Si:H. whereas, the annealed lc-Si:H led to a high fraction of crystallization, and no blistering was observed. A hybrid structure containing a-Si/lc-Si:H thin films was suggested to improve both the passivation and contact properties. The effective carrier lifetime of 3.2 ms and implied Voc of 716 mV were achieved with symmetric structure on n-type Cz substrate, indicating that our tunnel oxide/n+ hybrid-Si provides excellent passivation. The performance of TOP-Con solar cells with hybrid layer at the rear contact has been drastically improved compared to that of cells with lc-Si layer. Although a relatively low efficiency of 15.09% was achieved due to the poor passivation at the front surface, simulations indicated that the conversion efficiency of solar cells can be easily increased to 21.69% (Jsc = 38.93 mA/cm2, Voc = 0.694 V, FF = 80.29%) by improving the front surface passivation and reducing the front surface reflectivity. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Metals, when directly in contact with Si wafer, introduce very large densities of electronic states near the interface energetically within the bandgap of Si, resulting in a big recombination loss in the high-efficiency solar cells. Two approaches can be applied to minimize the contact recombination: one is to reduce the contact areas and local doping (applied by PERC and PERL cells); another is to use a thin-film that simultaneously passivates the Si surface and separates metals from Si wafer (Zhao et al., 1999). The best example for the second strategy was the heterojunction-based Si solar cells from Parasonic which reported a current record of 25.6% (Masuko et al., 2014). Another example is the named TOPCon (Tunnel Oxide Passivated Contact) solar cell technology, which has been a study focus in the world due to the excellent passivation ability without opening (Feldmann et al., 2014; Stradins et al., 2014; Feldmann et al., 2014; Tao et al., 2016). The TOP-Con structure consists of a doped Si layer and an ultra-thin tunneling SiO2 layer which is sufficiently thin to enable the tunneling of majority ⇑ Corresponding author. E-mail address: [email protected] (R. Jia). http://dx.doi.org/10.1016/j.solener.2017.01.061 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.

carriers while blocking the transport of minority carriers at the same time. The main feature of TOP-Con solar cells is the surface-passivation by tunnel oxide and realized selective contact by highly doped silicon thin films, resulting in a very low recombination rate. Owing to its excellent passivation and junction properties, remarkable cell efficiency up to 25.1% has been reached, which is the world record for both sides-contacted silicon solar cell (www.ise.fraunhofer.de). For the fabrication of TOP-Con structure, after the deposition of Si thin film on the tunneling SiO2 layer, an annealing process is usually needed to increase the crystallization fraction (Feldmann et al., 2014). Highly crystallized Si layer allows more efficient doping, leading to the excellent field passivation and junction property. In this work, the passivation quality of doped Si thin film was investigated by varying the hydrogen dilution rate. It was found that doped a-Si layer exhibited good passivation quality due to the large content of hydrogen atoms in the film. However, with further increasing the hydrogen content in the a-Si layer, blistering was observed on the TOP-Con samples, leading to the degradation of passivation quality. Compared to a-Si layer, lc-Si layer showed a lower passivation quality due to the smaller content of hydrogen. However, a lower thermal budget was required to re-

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crystallize and better crystalline quality was achieved. Simultaneously, no blistering can be observed by lc-Si layer after the thermal annealing. Based on this, new hybrid structure consisting of a thin a-Si layer and a thin lc-Si layer was designed for TOP-Con samples. No blistering was observed while a high passivation quality was reserved. 2. Experimental

iV oc ¼

  kT DnðDn þ ND Þ ; ln q n2i

ð1Þ

wed by a thermal annealing at 800 °C in a N2 ambient. Finally, 1 lm-thick Al electrodes were prepared by e-beam evaporator on both side of substrate. The light J-V characteristics of solar cells were measured under AM1.5 solar simulator at 25 °C. The external quantum efficiency (EQE) was measured via 7-SCSpec solar cell spectral measurement system.

3. Results and discussion It is well known that the atomic hydrogen can improve the interface passivation quality. Therefore, forming gas anneal (Nemeth et al., 2014) and remote plasma hydrogen passivation (Feldmann et al., 2014) are often used after the re-crystallization of a-Si:H thin films. During the deposition of Si thin films, hydrogen dilution rate can also affect the interface passivation quality. A group of samples were fabricated by varying the Hydrogen dilution rate (R = H2/SiH4) from 3 to 209, and the passivation quality was studied. As shown in Fig. 3, the effective minority carrier lifetime was drastically increased, when the hydrogen dilution rate was augmented from 3 to 33. This suggested that increasing hydrogen dilution rate helped to improve the passivation quality. However, with further increasing the hydrogen dilution rate, the effective minority carrier lifetime degraded rapidly. The implied Voc exhibited the similar tendency when changing the hydrogen

4000

740

3500

Effective carrier lifetime (μs)

where Dn is the excess carrier density at one sun, k the Boltzmann constant, T the temperature, q the elementary charge, ND the bulk doping density and ni the intrinsic carrier density. N-type silicon solar cells were fabricated to explore the potential of tunnel oxide/hybrid layer scheme. Fig. 2 shows the schematic of solar cell structure. On the front surface, p+-emitter with a sheet resistance of 60 ± 3 X/sq was formed in a quartz furnace at 930 °C from a BBr3 source. After the diffusion, the boron silicate glass (BSG) on the front surface was removed in a HF solution. Subsequently, SiO2/SiNx stack layers were deposited by PECVD to passivate the p+-emitter. On the rear surface, a single lc-Si and a hybrid layer were deposited on the tunnel oxide, respectively, flo-

Fig. 2. The schematic of solar cell structure with tunnel oxide/n+ poly-crystalline silicon passivated rear contact.

a-Si

3000

μc-Si 720

2500 2000

700

1500

carrier lifetime Implied Voc

1000 500

Implied Voc (mV)

Passivated contacts were fabricated on n type CZ wafers with resistivity of 3–8 X cm on symmetric structures poly-Si(n+)/SiOx/ c-Si(n)/SiOx/poly-Si(n+), as shown in Fig. 1. All the wafers are double-side polished. The wafers were firstly chemically cleaned by a RCA process, followed by a HF-dip to remove the surface oxide. Then the tunnel oxide layer was grown in 70 wt% HNO3 acid at 110 °C for 10 min. The resulted tunnel oxide thickness was about 1.5 nm, which is thinner than the maximum allowed oxide thickness of 2 nm for efficient tunneling (Shewchun et al., 1977). Next, a thin phosphorus-doped Si thin film was deposited on both sides of the wafer using a single-chamber PECVD system. A subsequent anneal at 800 °C crystallized Si thin film into n+ poly-Si. Forming Gas Annealing (5% H2 diluted in N2) at 450 °C was performed to further improve the contact passivation. In order to investigate the microstructure of Si thin films when deposited at different conditions, quartz glass was used as substrate. Film thickness was evaluated by spectroscopic ellipsometry (SE400, Sentech, Germany), and micro-structure was characterized by Raman spectra. The doping profiles were measured by electrochemical capacitance-voltage (ECV) profiling (CVP21, WEP, Germany). The implied open-circuit voltage (iVoc) and effective minority carrier lifetime (seff) were obtained by quasi-steadystate photoconductance (QSSPC) method (WCT-120, Sinton Instruments, USA). Here, iVoc at one sun is calculated according to the following equation:

injection level@5x105cm-3 1

10

680

100

Gas flow ratio between H2 and SiH4

Fig. 1. The cross-sectional structure for passivated contact samples.

Fig. 3. The effective carrier lifetime and implied Voc as a function of gas flow ratio between H2 and SiH4.

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dilution rate. The maximum of effective minority carrier lifetime and implied Voc were 3.6 ms and 719 mV respectively. In fact, the increase in hydrogen dilution rate during the plasma enhanced chemical vapor deposition of a-Si:H results in the increase in the atomic hydrogen content. During the re-crystallization of a-Si:H, atomic hydrogen will in-diffuse from a-Si layer into Si/SiOx interface. A mass of atomic hydrogen can saturates the dangling bonds, thus improve the passivation quality. When the hydrogen dilution rate is further increased, Si thin film will transform from amorphous phase into crystalline phase, leading to a decrease in the hydrogen content in the Si thin films. This will degrade the passivation ability. A post-deposition thermal treatment is beneficial to activate of dopants and improve the micro-structure. High annealing temperature results in a decrease of Si-H bonds corresponding to a reduction of the amorphous matrix in the layer promoting the formation of covalent SiASi bonds (Amor et al., 2016). The crystallization of the Si layer in TOP-Con structure brings advantages. For instance, the lc-Si has higher doping efficiency than a-Si, which means better field passivation and junction property (Bailly et al., 2014). Moreover, a more ordered structure of the Si layer improves the carrier transport property due to the increased crystallization fraction (Hamui et al., 2016). In addition, ITO (Indium Tin Oxide), the most commonly used transparent conductive coating, is n-type doped degenerate semiconductor. A depletion region is formed at the interface of ITO and the TOP-Con emitter. A high doping concentration in the Si layer prevents the complete depletion induced by the ITO, which improves the Voc and the FF of the TOP-Con solar cells.

Although lc-Si thin films exhibit week passivation quality compared with a-Si thin films, the above mentioned advantages of lcSi thin films make it valuable to be applied into solar cells devices. Besides, only a low budget is needed for the lc-Si thin films to crystallize. As shown in Fig. 4(a), the a-Si thin films are still amorphous phase after annealing at 600 °C for 1 h. Whereas, lc-Si thin films already show a big crystalline fraction when annealed at 600 °C for 1 h, as shown in Fig. 4(b). Both a-Si thin films and lcSi thin films exhibit high crystalline fraction when they are annealed at a higher temperature, i.e. P700 °C. However, a higher crystalline fraction and better crystal quality are obtained by lc-Si. In addition, blistering is observed for a-Si thin films after they were annealed at 700 °C, and it is aggravated after annealing at 800 °C, as shown in Fig. 5(a) and (b). Interestingly, lc-si thin films never blister even annealed at 800 °C, as shown in Fig. 5(c). The blistering of Si layer was caused by hydrogen effusion which was provided from the breaking of weakly bonded hydrogen coming from SiH2-bonding and SiH3-bonding, as well as strongly bonded hydrogen of isolated SiAH bonds (Kakalios et al., 1987). The hydrogen can accumulate at the interface between Si substrate and SiO2 layer, leading to the increase of the hydrogen pressure at the interface. When the hydrogen pressure exceeds some critical value, the explosion of the blistering occurs (Hamui et al., 2016; Kroll et al., 1996). The elimination of blistering for lc-Si layer is due to its lower hydrogen incorporation in the silicon network compared with a-Si layer (Acco et al., 1996). The observed blistering can undoubtedly weaken field passivation due to loss of doped Si layer. In addition, the in-diffusion of dopants into Si substrate is accelerated though the blistering area,

B-800oC B-700oC B-600oC B-as dep.

A-800oC A-700oC A-600oC A-as dep.

(b)

Intensity (a.u.)

Intensity (a.u.)

(a)

100

200

300

400

500

600 -1

Raman shift (cm )

700

800

100

200

300

400

500

600

700

-1

Raman shift (cm )

Fig. 4. Raman spectra of the sample A and B after annealing at 600 °C, 700 °C, and 800 °C. The amorphous Si layer is characterized by a broad peak around 480 cm1 and the lc-Si phase is described by a superposition of the amorphous and crystalline Si peak (520.7 cm1)

Fig. 5. Optical microscope images of Silicon thin films: (a) a-Si annealed at 700 °C, (b) a-Si annealed at 800 °C and (c) lc-Si annealed at 800 °C. Blistering can be observed in (a) and (b).

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as shown in Fig. 6. Both a-Si and lc-Si thin films deposited on c-Si substrate were annealed at 800 °C for 1 h. A abrupt drop in dopant concentration at Si/SiOx interface was observed for lc-Si samples, whereas, an obvious in-diffusion of dopant into c-Si substrate was observed for a-Si samples. The in-diffusion of dopants into the Si substrate also deteriorated the interface quality. In order to combine the advantage of a-Si and lc-Si thin films, a hybrid structure was designed, in which a thin a-Si layer was firstly deposited on tunnel oxide, followed by another lc-Si layer. In fact, the a-Si can work as the incubation layer for lc-Si thin films, thus, promote the crystallization of lc-Si. A series of hybrid-structure samples (including a thin a-Si covered by a thick lc-Si thin film) have been prepared by varying the thickness of lc-Si thin film from 10 nm to 85 nm. The experimental results indicated that no blistering can be observed even if the thickness of lc-Si thin film reached up to 85 nm. For solar cell fabrication, the thickness of aSi and lc-Si layers was 10 nm and 15 nm, respectively. Fig. 7 shows the injection dependent effective minority carrier lifetime for the symmetrically passivated samples with a-Si, lc-Si and hybrid layer. An effective minority lifetime of 3.2 ms and iVoc of 716 mV were achieved, which is comparable to that of a-Si samples, but far better than that of lc-Si samples. No blistering was observed again for the TOP-Con samples when using hybrid layer. Fig. 8(a) exhibits the I-V curves of solar cells featuring passivated rear contact with ls-Si and hybrid silicon films respectively. It can be seen that using a single ls-Si layer as the rear contact resulted in a poor output of device (Jsc = 29.72 mA/cm2, Voc = 0.547 V, FF = 71.8%, Eff = 11.67%), whereas, a hybrid layer containing a-Si/ls-Si stacks drastically improved the performance of solar cells (Jsc = 32.50 mA/cm2, Voc = 0.600 V, FF = 77.4%, Eff = 15.09%). By comparing the EQE curves in Fig. 8(b), a relatively high external quantum efficiency, especially in the long wavelength range, was achieved by solar cells with hybrid layer, indicating the excellent surface passivation of the hybrid layer at the rear side. It should be noted that the Voc of solar cells with hybrid layer was still very low, which was ascribed to the poor passivation at the front side of the substrate. In this work, SiO2/SiNx stacks were applied to passivate the p+-emitter, however, due to the lack of optimization, on the one hand, the carrier recombination rate at the front surface was still high, on the other hand, the reflectivity (@k = 600 nm) of solar cells was larger than 11% (as shown in Fig. 8(b)). Sentaurus Device has been used to simulate the experi-

Concentration (cm-3)

1E22

1E21

Without blistering Blistering

1E20

a-Si:H thin films o

after anneal at 800 C 1E19

1E18

μc-Si:H thin films

1E17

o

after anneal at 800 C

1E16 0.0

0.2

0.4

0.6

0.8

1.0

Depth (μm) Fig. 6. The doping profile of TOP-Con samples passivated by lc-Si and a-Si after annealed at 800 °C for 1 h.

0.1

Effective carrier lifetime(sec)

0.1

μc-Si(n) Hybrid α-Si(n)

τeff=960μs τeff=3.2 ms @5x1015cm-3

0.01

0.01

1E-3

1E-3

Hybird Si layer consists of two layers: 1E-4

1E14

1E-4 1E16

1E15 -3

Minority Carrier Density(cm ) Fig. 7. Comparison of injection dependent effective minority carrier lifetime for the symmetrically passivated samples with a-Si, lc-Si and hybrid layer. The figure also depicts the lifetime at injection level of 5  1015 cm3.

35

(a)

(b)

100

100

25 20 15 10 5 0 0.0

hybrid µc-Si

JSC (mA/cm2 VOC (v) 32.5 0.6 29.72 0.547

0.1

0.2

EQE of solar cells: hybrid μc-Si

80

hybrid μc-Si

EQE/Reflectance(%)

Current density (%)

30

0.3

FF (%) 77.4 71.8

0.4

Voltage (V)

60

60

40

40

Reflectance of solar cells: hybrid μc-Si

20

Eff (%)

80

20

15.09 11.67

0.5

0.6

0

0 400

600

800

1000

Wavelength (nm)

Fig. 8. (a) Light I-V curves and (b) external quantum efficiency (EQE) and reflectance of solar cells featuring passivated rear contact with ls-Si and hybrid silicon films respectively.

40

40

30

30

20

20

2

Current density (mA/cm )

K. Tao et al. / Solar Energy 144 (2017) 735–739

10

0 0.0

simulated I-V curve (improved) simulated I-V curve (μc-Si) simulated I-V curve (hybrid layer) measured I-V curve (hybrid layer) measured I-V curve (μc-Si) 0.2

0.3

0.4

0.5

carrier lifetime of 3.2 ms and implied Voc of 716 mV were achieved with symmetric structure on n-type Cz substrate (3–8 O cm, 300 lm), indicating that our tunnel oxide/n+ hybrid-Si provides excellent passivation. The performance of TOP-Con solar cells with hybrid layer at the rear contact has been drastically improved compared to that of cells with lc-Si layer. A relatively low efficiency of 15.09% was achieved due to the poor passivation at the front surface. Simulations indicate that the conversion efficiency of solar cells can be easily increased to 21.69% (Jsc = 38.93 mA/cm2, Voc = 0.694 V, FF = 80.29%) by improving the front surface passivation and reducing the front surface reflectivity.

10

Acknowledgement 0

0.1

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0.6

0.7

Voltage (V) Fig. 9. Simulated and measured I-V curves of solar cells. Arrows in the figure are drawn to guide the eyes.

This work was supported by the Project of Beijing Municipal Science and Technology Commission (Grant No. Z151100003515003) and National Natural Science Foundation of China (Grant Nos. 110751402347, 61274059). References

mental results (Synopsys TCAD, 2013; Li et al., 2016). In Fig. 9, the experimentally determined and simulated results for the solar cell with ls-Si and hybrid silicon films in the rear contact are shown respectively. It can be seen that our model and underlying physics can accurately describe the solar cell featuring passivated rear contact with different silicon layers. It was found that the front surface passivation played an important role in the performance of device. The output of solar cells can be improved drastically if the front surface recombination rate was decreased from 10,000 cm/s to 50 cm/s. Simultaneous, by designing the light-trapping structure, the short circuit current density (Jsc) can be enhanced effectively due to the decrease in reflectivity. Our simulations indicate that the conversion efficiency of solar cells can be easily increased to 21.69% (Jsc = 38.93 mA/cm2, Voc = 0.694 V, FF = 80.29%) by improving the front surface passivation and reducing the front surface reflectivity (from 11.3% to 2% @k = 600 nm). 4. Conclusion In this work, the passivation quality of doped Si thin film was investigated by varying the hydrogen dilution rate. The impact of doped a-Si and lc-Si thin films on the passivation quality has been compared. Blistering was observed for a-Si samples, leading to the degradation of passivation quality. Although lc-Si layer showed a lower passivation quality due to the smaller content of hydrogen, a lower thermal budget was required to re-crystallize and better crystalline quality was achieved. Importantly, no blistering can be observed by lc-Si layer after the thermal annealing. A hybrid structure containing a-Si/lc-Si:H thin films was suggested to improve both the passivation and contact properties. The effective

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