SiO 2 Antireflection Coatings Fabricated by Electron-Beam Evaporation for Black Monocrystalline Silicon Solar Cells

1 School of Electrical Engineering, Guangdong Mechanical & Electrical College, Guangzhou 510515, China 2 Institute for Solar Energy Systems, Guangdong Provincial Key Laboratory of Photovoltaic Technologies, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510006, China 3 Key Laboratory of Automobile Components and Vehicle Technology in Guangxi, Guangxi University of Science and Technology, Liuzhou 545006, China


Introduction
For high-efficiency solar cells, antireflection coating (ARC) is very important for improving the performance of solar cells since it ensures a high photocurrent output by minimizing incident light reflectance on the top surface [1][2][3][4].At present, hydrogen containing silicon nitride (SiN x :H) thin film deposited by plasma enhanced chemical vapour deposition (PECVD) is widely used as ARC and passivation layer for crystalline silicon solar cells [5,6].However, the single-layer antireflection coatings (SARC) used in silicon solar cells still cause considerable optical reflectance loss in a broad range of the solar spectrum.Therefore, doublelayer antireflection coatings (DARC) which consist of heterostructure materials such as MgF 2 /ZnS [3,7], MgF 2 /BN [8], Al 2 O 3 /TiO 2 [9][10][11], and MgF 2 /CeO 2 [12] are considered to be a more effective design in decreasing the reflection in a broad wavelength range for the high efficiency solar cells fabrication.These DARC are not common because of process complexity, which could affect their mass production process.Though SiN x :H/SiN x :H [13] shows unique combination of good electronic and optical properties, it has disadvantages of high absorption in the UV region reducing of the shortcircuit current of the cell.The SiO 2 /SiN x :H DARC are a promising design to improve solar cells efficiency due to its advantages in both surface passivation and antireflection properties.The simulation on the SiO 2 /SiN x :H DARC was carried out by optimizing their refractive index and film thickness [14].Kim et al. [15] have investigated the conversion efficiency improvement of monocrystalline silicon solar cell with double layer antireflection coating consisting of SiO 2 /SiN x :H deposited by PECVD.And the solar cells with DARC showed the better efficiency as 17.57% and 17.76%, compared with 17.45% for single SiN x :H ARC.
In this paper, we present a novel process method that DARC consisting of SiN x :H and SiO 2 films were deposited via PECVD and an electron-beam evaporation technique, respectively.The thickness of SiN x :H films as the bottom layer is kept at 80 nm, which is optimum for SARC.By simply varying the thickness of the SiO 2 layer as the top layer covering the conventional solar cell, monocrystalline silicon solar cells with different SiO 2 /SiN x :H DARC are fabricated.

Experiment
Boron doped monocrystalline wafers, with a thickness of 160 m, a size of 125 mm × 125 mm, and a resistivity in the range of 1∼3 Ωcm, have been used for all experiments.
After standard cleaning and alkaline texturization, a standard POCl 3 emitter diffusion in a quartz tube led to a sheet resistivity of 60 Ω/◻.The wafers were coated with a SiN x :H layer in a PECVD (Centrotherm) system.The refractive index of SiN x :H was adjusted by controlling the NH 3 /SiH 4 gas flow ratio.The thickness of the SiN x :H layer was 80 nm.After a standard front and back side screen printing process, the contact formation was performed by a firing through process.
Then, the solar cells with SiN x :H SARC were performed.
To prepare the SiO 2 /SiN x :H DARC, SiO 2 thin films were deposited on the prepared solar cell with SiN x :H SARC by electron-beam evaporation.Considering of the SiO 2 layer on busbars may lead to contact issue in I-V test, we used steel mask on the top of busbars as shelter during e-beam evaporation.High purity SiO 2 (99.99%) granules were used as the source material for evaporation and the source-tosubstrate distance was 50 cm.The substrates temperature was controlled at 200 ∘ C. High purity oxygen (99.99%) was introduced into the chamber to maintain a pressure of 3.0 × 10 −2 Pa and used as reactive gas during the deposition.The deposition rate was controlled using a quartz crystal sensor placed near the substrate, and set as ∼2 Å/s.The thicknesses of the SiO 2 as top layer were 105 nm and 122 nm, respectively.Finally, the solar cells with different SiO 2 /SiN x :H coatings were obtained.The structure of the solar cell with SiO 2 /SiN x :H DARC is schematically shown in Figure 1.
The Fourier transform infrared spectroscopy (FTIR) measurement for the SiO 2 thin film has been made at 25 ∘ C using a Thermo Nicolet 6700 FTIR spectrometer.The refractive index of the SiN x :H and SiO 2 films were measured by a n&k analyzer 1200.Spectral reflectance and external quantum efficiency (EQE) measurements were performed by a solar cell spectral response measurement system (PV measurement, QEX7).In addition, the I-V characteristics of the solar cells were measured using a Berger I-V tester on a solar cell production line.All measurements were conducted under the standard test conditions (AM1.5G spectrum, 100 mW/cm 2 , 25 ∘ C).Prior to the measurements, the simulator was calibrated with a reference monocrystalline silicon solar cell, which was calibrated by the Fraunhofer ISE.All electrical parameters are presented as the average value of ten cells in the study.

Results and Discussion
3.1.SiO 2 Thin Film Characterization.XPS was applied to determine the chemical state of the Si and O elements, which can confirm the presence of SiO 2 layer in DARC.XPS analysis for SiO 2 film has been reported in our group [16].
In order to get a qualitative spectra of SiO 2 thin film compositions, we have performed Fourier transform infrared spectroscopy (FTIR) analysis.The samples were prepared on the aluminium thin film with 300 nm thickness on the glass substrate, which deposited by e-beam evaporation.We adopt reflection method to measure the sample.The spectra are presented in Figure 2. The band in the 1040-1150 cm −1 range is assigned to the stretching vibration mode Si-O [17,18].For the supplement of oxygen during the SiO 2 deposition, a clear increase of Si-O intensity peak (1020 cm −1 ) is observed for the SiO 2 layer, which is related to the high oxygen content in this layer.

Optical Property.
The color of the solar cell depends heavily on thickness of its ARC-layer.Figures 3(a) and 3(b) show the photographs of silicon solar cells with single SiN x :H SARC and SiO 2 (105 nm)/SiN x :H (80 nm) DARC, respectively.Two kinds of coatings have good uniformity.It is notable that the front surface color of the solar cells changed from dark blue to black, indicating that there was a lower reflectance loss in the DARC, as shown in Figure 3(b).
The reflectance spectrum was measured to characterize the reflectance loss.Figure 4 depicts the reflectance spectra  Reflectance (%) It is acknowledged that a reduction in light of around 30% resulted from the reflectance at the Si and air interface [20].ARC means an optically thin dielectric layer designed to suppress reflection by interference effects.By using DARC with /4 design, with growing indices from air to silicon, the minimum in reflection is broader in wavelength range.
The measured refractive indices of SiN x :H and SiO 2 were 2.1 and 1.46 at 633 nm wavelength, respectively.Thus, the optimal thickness for each layer in term of their refractive indices can be obtained.EQE data was collected for wavelengths in the range of 300-1100 nm to determine the spectral response of the solar cells, as shown in Figure 5(a); no significant differences in the infrared wavelength range were observed among these cells.On the other hand, the EQE of cells with DARC is higher than that with SiN x :H SARC in the range 300-450 nm wavelength.It was also shown that SiO 2 (105 nm)/SiN x :H (80 nm) stack coatings has the highest improvement in short wavelength.IQE data was collected for wavelengths in the range of 300-1100 nm, as shown in Figure 5  with SiN x :H SARC.The fill factor of each group is nearly the same, while the  oc shows small degradation for solar cells, which probably caused by the surface damages during the ebeam evaporation.
Correspondingly, the highest short-circuit current density ( sc ) was also obtained.It is demonstrated that the conversion efficiency of cells with DARC is dependent on the thickness of SiO 2 coatings, the same as the dependence of reflectance and EQE. Figure 6 shows the J-V characteristic of the solar cell with SiO 2 (105 nm)/SiN x :H (80 nm) DARC.

Conclusions
In this work, SiO 2 /SiN x :H DARC were deposited on monocrystalline silicon solar cells.The results show that the SiO 2 /SiN x :H DARC have a lower reflectance compared with the SiN x :H SARC.Accordingly, solar cells with SiO 2 /SiN x :H DARC exhibit a higher EQE and IQE in the short wavelengths of 300-450 nm.Due to current density improvement, the conversion efficiency of 17.80% was obtained for solar cells with DARC, 0.32% (absolute) higher than that of cells with single SiN x :H coatings.

Figure 5 : 2 ) 2 FFFigure 6 :
Figure 5: EQE and IQE of the single layer ARC and double layer ARC solar cells.
[19]r cells with SiO 2 /SiN x :H DARC and SiN x :H SARC, respectively.Compared with the SiN x :H SARC, SiO 2 /SiN x :H layer stacks show lower reflectance in the range 300-450 nm.The amorphous SiO 2 coating is transparent in the measured wavelength range.It is obvious that the reflectance of the SiN x :H layer stack is dependent on the thickness of the SiO 2 coatings.With the thickness of SiO 2 in the SiO 2 /SiN x :H stack increasing, the reflectance changes correspondingly.A similar simulation trend was also reported by Aguilar et al.[19].In our work, the lowest reflectance was obtained while the thickness of SiO 2 was 105 nm in the SiO 2 /SiN of x :H stack, which is nearly consistent with previous simulation results.The value of calculated weighted reflectance is 1.72%.

Table 1 :
Summary of the average electrical parameters of the different ARC stacks compared with SiN  :H SARC solar cells (AM1.5G, 100 mW/cm 2 , 25 ∘ C).
(b); EQE and IQE curves have the similar trends.3.3.Solar Cell Results.The solar cells fabricated with novel SiO 2 /SiN x :H stacks were tested and compared to conventional solar cells with SiN x :H SARC, as shown in Table 1.All data in Table 1 are the average values of ten samples.With the thickness of SiO 2 thin films varied, the conversion efficiency of the cells changed.Table 1 shows the conversion efficiency of the cells with SiO 2 (105 nm)/SiN x :H (80 nm) DARC reached 17.80%, which was 0.32% (absolute) higher than solar cells