Antireflection Coating and Colour

Introduction

All solar cells use Antireflection Coatings (ARCs) to optimise the optical performance, they are a dielectric material which causes destructive interference of the reflected light from the different surfaces. Currently, the standard ARC for silicon solar cells is a thin layer of Silicon Nitride (SiNx) deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). As a single layer antireflection coating, it is typically optimised for minimum reflectance at a wavelength of 600 nm and has larger reflection at other wavelengths. As the visible spectrum of light occurs at wavelengths of ~380 – 740 nm, the visible colours of the AR coated wafer would indicate which wavelengths are being reflected by the coating. For example, wafers with the usual SiNx ARC layer, optimised for 600 nm light, appear blue-ish in appearance.

Learning Objectives

  • Be able to use RAT data to predict the colour of Antireflection Coatings
  • Understand the use of a double-layer antireflection coating
  • Be able to perform a main factor response experiment to determine which layer of the DLARC to optimise
  • Be able to perform a single factor response experiment to optimise the DLARC
  • Understand the resulting colour of a high efficiency solar cell at peak optical performance

Tutorial Exercise

SunSolve can also display colour outputs of the simulated cell and this can be used in conjunction with your own ideas of what the ARC colour would be. However, it is important to note that SunSolve ‘averages’ all the colours of each component that is observed on the Wafer/ Cell/ Module. However, the colour displayed by our monitors will also cause some differences in perceived colour.

In this tutorial, you will be running simulations with a Single Layer and Double Layer ARC solar cells, investigating the minimum wavelength reflectance and optimising the SiO2/SiNx film layers.

Figure 1 – Screenshot of the SunSolve “Si Wafer” template

Reminder

Make sure to save and organise any templates/simulations as you proceed throughout this tutorial; any unsaved progress will be lost if the SunSolve page is closed/changed/refreshed.

Part One – Investigating a Single Layer ARC

Typically, a 75 nm thick SiNx film layer is used to minimise reflection at a wavelength of 600 nm. In this section, you will investigate different single layer antireflection coating (SLARC) film thicknesses and observe the changing ‘minimum reflectance’ wavelength. You will be using the ‘Si Wafer’ template and the responses that you will observe are listed in Table 1 below.

Table 1 - Responses to investigate for an SLARC.
ResponseUnits
Wavelength of light with minimal reflectancenm
Absorbed solar cell bulk current density (JA,L1)mA/cm2

Conducting the Experiment

  1. Open a new simulation using the ‘Si Wafer’ template.
  2. Opening the top textures and interfaces layer, add a SiNx [PECVD 2.09 (Vog15)] film layer. Save this template to be used later.
  3. Using the sweep function, sweep the SiNx layer from 60 nm to 95 nm with 8 steps (5 nm per step).
  4. Under “outputs -> overlay data”, plot front reflectance. Record the wavelength of light with minimal reflectance for each run.
  5. Record the absorbed solar cell bulk current density of each run.
  6. Save the simulation to keep a record of the data collected (different to saving template).
  7. Tabulate your results and graph an X-Y scatter plot for each response (Y axis) vs ARC thickness (X axis).

Questions

  1. What is the relationship between the ARC thickness and the wavelength of light with minimum reflectance?
  2. Is obtaining a minimum reflectance at 600 nm a good definition for “optimal” ARC thickness? If not, what other parameters should be used and why?
  3. What is the significance of recording bulk absorbed current density compared to short circuit current density in this experiment? What was the optimum SiNx thickness?
  4. Record the parasitic absorption current density at the AR layer of the optimised wafer.

Part Two – Investigating a Double Layer ARC

To increase optical performance, the use of double-layer antireflection coatings (DLARC) on higher efficiency solar cells allows for a broader spectrum of reflected incident light to be minimised. There are several different DLARC in use today such as MgF2/ZnS or Al2O3/TiO2 but due to their high costs, their use is typically limited to the laboratory. A possible DLARC is SiNx/ SiO2 [1] as the PECVD method for depositing SiNx is a well known method.

You will be creating a DLAR coated wafer by adding a Silicon Dioxide (SiO2) layer to the SLARC template you saved earlier. By sweeping these layers, you can investigate possible optimum film thicknesses.

For these SunSolve exercises, “-“, “0” and “+” are used to indicate “a lower setting”, “the baseline setting” (or “default”) and “a higher setting” for the factors, respectively. The actual values for these simulation settings are provided in the Table 2 below.

Table 2 - Factor settings for investigating a DLARC
Factor SettingsSiO2 Thickness (nm)SiNx Thickness (nm)
-2020
04040
+5050

The responses you will be observing are listed in Table 3 below.

Table 3 - Responses to observe in the DLARC experiment
ResponseUnits
Front reflection current density (JR,Front)mA/cm2
Absorbed solar cell bulk current density (JA,L1)mA/cm2

Conducting the Experiment

  1. Open a new simulation with your SLARC template and add a SiO2 film layer below of the SiNx
  2. Save this new template for future use.
  3. Set the SiNx layer thickness at the “0” setting.
  4. Using the sweep function, sweep the top SiO2 layer from the “-“setting to the “+” setting with 7 equal steps. This should result in 5 nm steps.
  5. Record and tabulate the responses as listed in Table 3.
  6. Graph an X-Y scatter plot for each response (Y-axis) vs SiO2 thickness (X-axis)

Questions

  1. What is the suggested optimum SiO2 thickness for a 40 nm thick SiNx bottom layer? Will this be the optimised thicknesses during module fabrication? Why/ Why not?
  2. Record the parasitic absorption current density at the DLAR layer of the optimised wafer. Compare this value to the SLARC.
  3. Plot the front reflectance graph of the optimised run in excel. Comment on its shape and compare this plot to the front reflectance plots observed in Part One

Part Three – Understanding Colour

It is important to recognise the colour of light that is perceived by the reflected spectrum of visible light. Violet/ Blue light is the colour observed of wavelengths in the range ~380-500 nm where as red light is in the range ~630-740 nm, the human eye cannot respond to wavelengths beyond this range. The perceived colour of a surface is determined by the wavelength(s) of visible light that they are reflecting; which is affected by the thickness and refractive index of the material. Therefore, the perceived colour can act as a rough guide for the thickness of an ARC film layer [2], since slight changes in thickness may cause significant differences in the perceived colour of a wafer.

You will be running a simple experiment to test your understanding of colour by attempting to predict the colour of a wafer through the reflectance data. SunSolve can also display the perceived colour of the wafer by ‘averaging’ the colours; use this feature to cross-check your predictions.

Conducting the Experiment

  1. Open a new simulation using your SLARC template on a bare silicon wafer.
  2. Using the sweep function again, sweep the SiNx ARC film thickness.
  3. Set the sweep for 6 steps from 45 nm to 120 nm. SunSolve can automatically set equal intervals, but make sure the thickness values are 45, 60, 75, 90, 105 and 120 nm.
  4. Under “outputs -> overlay data”, plot the Front Reflected data and observe any large amounts of reflection or peaks that occur. Predict what colour (or mix of colours) the wafer would be perceived.

Questions

  1. What colour (or mix of colours) would you expect the different SiNx ARC layer thicknesses to present:
    • 45 nm thickness?
    • 60 nm thickness?
    • 75 nm thickness?
    • 90 nm thickness?
    • 105 nm thickness?
    • 120 nm thickness?
  2. Compare your guesses with what SunSolve displays in the ‘Colour’ outputs tab, are they what you expected?
  3. Using the reflectance data, explain why some wafers had a strong mix of colours in the SunSolve output.

 

Part Four – Further understanding

  1. In this tutorial, what was the significance of using the Si Wafer template?
  2. What other layers must be considered when optimising reflectance in module fabrication, will these affect the optimum thickness of your SLARC and DLARC?
  3. What other source of loss is significant when optimising these layers?
  4. Do the wavelengths >1100 nm have a significant impact on the colour of the wafer?
  5. What is the merit of reducing lower wavelength reflectance?
  6. What is a disadvantage of collecting photons with energy much higher than the band gap? (Hint: sources of losses)
  7. Consider the overall effect on reflection that metallisation has on the optical performance of silicon solar Therefore, what hue or colour will SunSolve present?
  8. For a solar cell with peak optical performance (0% reflection at all wavelengths), what colour would you expect the solar cell to present? What are some methods in achieving this performance?

 

Part Five – DLARC in Modules

Open a new simulation using the your DLARC wafer template add 2 extra layers on top of the wafer:

  • 45 mm of EVA
  • 2 mm of Glass on top of the EVA layer

Rerun the experiment in Part Two.

Questions

  1. Are there significant differences in front reflection current density occurring between each step? If not, use the leaky bucket analogy to explain why.
  2. Investigate the parasitic absorption in the DLAR layer, compare this value to the value you recorded in Part Two, Q2. Explain any changes using the same analogy.

References

[1] – Kim, J., Park, J., Hong, J.H. et al, “Double antireflection coating layer with silicon nitride and silicon oxide for crystalline silicon solar cell,” Journal of Electroceramics (2013) Vol. 30, p. 41. https://doi.org/10.1007/s10832-012-9710-y

[2] – C. Honsberg and S. Bowden, “PVeducation.org – Double Layer Anti Reflection Coatings,” [Online] Available: https://www.pveducation.org/pvcdrom/design-of-silicon-cells/double-layer-anti-reflection-coatings