Introduction
A bare silicon wafer is very reflective with more than 30 % of the incident light reflected from the front surface. Texturing can reduce this significantly with alkaline texturing reducing the reflection to as low as ~11%. However, the application of a thin dielectric layer can reduce this further if the thickness of the layer is engineered to be such that light reflected at the silicon-dielectric layer is exactly 180º out of phase with light reflected from the dielectric-air interface. Such a dielectric layer is called an antireflection coating (ARC). Almost all screen-printed solar cells use silicon nitride as the ARC because, in addition to reducing the reflection, these layers can reduce the recombination that occurs at the silicon-dielectric interface and in the bulk of the silicon (i.e. surface and bulk passivation). Typically the silicon nitride is deposited using a plasma-enhanced chemical vapour deposition process.
Learning Objectives
- Explain the reason(s) for depositing a silicon nitride coating on the front surface of the cell
- Be able to determine what factors are critical for optimising the cell’s optical performance
- Understand how the deposition of the silicon nitride can affect the cell metallisation process
- Be able to perform multiple and single factor experiments to optimise the silicon nitride coating process
Tutorial Exercise
Please review the PVmanufacturing page(s) on antireflection coating and the help files in PV Factory before attempting the tutorial so that you understand the effect of the different parameters. For this tutorial exercise, you will use the ‘Factory Default’ settings for all processes following the Silicon Nitride Coating step so leave your UNSW Screen-Print line settings to “Use factory default”. All experiments should use batches of wafers (at least 10 per batch) with the following properties:
- Standard Cz mono-crystalline silicon wafers;
- 200 mm thick;
- Resistivity of 1 W cm; and
- Cut using a standard wire saw.
In this exercise you should use your best recipes that you have established for the previous steps. To load these recipes use the “L” icon on the left-hand screen when you are processing that step for batches. Otherwise, you can set your user settings to “Use my recipe”, provided the steps following silicon nitride coating are PVfactory default. When you get to the Silicon Nitride Coating step, enter your settings, and run that step. Immediately after the step go into the Measurement Lab and view the reflectance for a representative wafer in your batch. It is important that you measure the reflectance before metallisation as the metal fingers will result in some shading and hence the reflectance will increase. To determine the effect of silicon nitride coating process on your cell parameters, select to Run Remaining Steps. This will cause your batch to be processed using the “factory default” parameters for all the remaining steps (because you have not established best recipes for these steps yet). The J-V results for your batch will then be available.
Part 1 – Main Factor Response Experiment
The silicon nitride coating depends on the deposition time and temperature as well as the silane to ammonia gas ratios. The flow rate for each of the gases can be set to 1 to 5 with 1 = low flow rate and 5 = high flow rate. As you may have already anticipated, the final cell efficiency depends on the properties of the silicon nitride coating. Harder and thicker coatings, which result from using a higher deposition temperature and a longer deposition time, are more difficult for the silver paste to penetrate during firing. Also, properties of the coating affect the optical properties of the cell. In this tutorial, you will focus on developing a recipe that optimises the optical properties of your silicon nitride coating. However, in the following tutorials, as you develop your screen-printing processes you will most likely find that you will have to return to this process and re-optimise it with respect to the coating’s hardness.
Pre-Work
Design a main factor response experiment which explores the consequences of the following factors of interest (allowed ranges in PV Factory are in parentheses):
- Deposition time (1 – 30 min);
- Deposition temperature (0 – 400 ºC)
- Silane to ammonia ratio (0.2 – 5.0)
on the responses of the silicon nitride refractive index, silicon nitride thickness (nm), the reflectance at 600 nm and short circuit current density (mA/cm2). You are not required to optimise for cell efficiency this tutorial as the cell efficiency will depend on properties of the silicon nitride coating AND the screen-printing process, which you will optimise in the following tutorials.
The suggested factor settings are listed in the table below but you can also select other settings to explore which factor is most significant.
Table 1 - Suggested settings for main factor response experiment| Factor Settings | Main Factors for Silicon Nitride Coating | ||
| Deposition Time (min) | Deposition Temp (oC) | Silane: Ammonia Ratio | |
| - | 5 | 100 | 0.2 |
| 0 | 10 | 250 | 1.0 |
| + | 15 | 400 | 5.0 |
The silane and ammonia flow rates can be set at 1 to 5 arbitrary flow rate units. It is suggested that you keep the total flow at 6 arbitrary flow units. The table below shows how to achieve the different ratios.
Table 2 - Silane: Ammonia flow rate ratio guide| Silane to Ammonia Ratio | Silane Flow Rate (arbitrary units) | Ammonia Flow Rate (arbitrary units) |
| 0.2 | 1 | 5 |
| 1.0 | 3 | 3 |
| 5.0 | 5 | 1 |
Experiment to do list:
- Before you start this experiment check how many batches you have processed in your saw damage and alkaline texturing baths. Do you need to dump these baths? If so make sure that you run at least 10 batches to stabilise the etching before you start the experiment.
- Process batches for each of your designed experiments and record the results.
- Produce a main factor response graph for each of the four responses.
- Determine which factor is most important to optimise if you want to maximise the short circuit current density.
Determine the best silane to ammonia ratio to use if you want to optimise the optical performance of your cells before encapsulation
Part 2 – Single Factor Response Experiment
Choose the factor that you think will help you optimise the silicon nitride coating process for optical performance and perform a single factor response experiment.
Experiment:
- Use the same wafer parameters as you used for Part 1 and process at least 8 experimental batches (with at least 10 wafers per batch), varying the factor of interest over the range of values allowed by the PV Factory.
- Record all four responses for each factor setting.
- Sketch an X-Y scatter plot for each response (y-axis) versus your factor of interest (x-axis).
- Make sure that you understand the relationships between your factor and each of the responses.
Store your best silicon nitride coating recipe in PV Factory.
Part 3 – Understanding the Silicon Nitride Coating Process
Make sure that you understand the antireflection coating process and prepare your answers while completing the above tasks.
General questions:
- What is the purpose of applying an anti-reflection coating on a solar cell?
- Why do you try to minimise reflectance at 600 nm?
- Would the optimal silicon nitride thickness change if the performance of your cell was to be optimised for its optical performance in a module?
- Why does silicon nitride (deposited by PECVD) passivate n-type silicon surfaces very effectively?
- What deposition parameter has the largest effect on the ability of the screen-printed metal silver fingers to contact the underlying silicon?
True/False questions:
- An antireflection coating should be applied only to cells that are not suitable for texturing.
- A silicon nitride layer is used to reduce a cell’s front surface reflection and minimise recombination at the silicon surface.
- An ARC uses constructive interference to reduce reflection.
- The hydrogen content in silicon nitride deposited by PECVD is beneficial, particularly for multi-crystalline wafers.
- Silicon nitride can also reduce recombination at p-type silicon surfaces however the extent of the reduction in recombination depends on the illumination intensity.