Optimising Bill of Materials

Introduction

Solar Cells are fragile and can easily be destroyed when exposed to the conditions outside the lab. Therefore, the many processes involved in module manufacturing is almost as important as the solar cells themselves. The ‘bill of materials’ (BoM) of a PV module include the encapsulants, front surface, back sheet (for mono-facial) and cell interconnections. An encapsulant is important for solar cell adhesion to the front and rear surfaces of the module. The front surface must have high transmission (~350-1200 nm wavelength for Silicon) and low reflectance and the back sheet must have low thermal resistance. These layers protect the solar cells from moisture, heat (ambient and cell temperatures) and impact. Module frames are also important for handling and installation but can cause shading, there are also frameless modules.

For most PV modules, Ethyl Vinyl Acetate (EVA) is used as an encapsulant, antireflection coated glass is used for the front surface and glass-foil ‘Tedlar’ is the commonly used backsheet for glass front mono-facial modules. Cell interconnection materials (ribbons) must also be optimised as they may shade the solar cell and create limitations on the thickness of solar cells.

Learning Objectives

• Understand the necessity of a stable encapsulant layer
• Understand the effects of the different layers on optical performance
• Be able to perform a main factor response experiment to determine the most important layer/interface to be optimised 
• Be able to perform a single factor response experiment to optimise the most important layer
• Investigate possible CTM gains with interconnecting ribbons
• Understand the mechanics of the reflective back sheet

Tutorial Exercise

SunSolve provides a range of abilities in simulating the optical and electrical effects of the different BoM layers, you will be working with the Si Wafer template, sweeping the thickness of the layers and observing the effects on performance. In Part Three, you will use the c-Si SSP Module template to investigate cell interconnecting ribbons. Please review the relevant about pages in SunSolve and the relevant PVmanufacturing.org prior to attempting this tutorial in order to have a better understanding of the context of the exercises.

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 – Main Factor Response Experiment

You will be conducting a main factor response experiment in order to determine the most important layer of the BoM to optimise. However, the data you obtain will be in 2 different batches, the first is to observe the optical effects and the second to observe the electrical effects and overall performance. The electrical solver will not consume more rays. 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 Table 1 below.

Table 1 - Settings used in the Main Factor Response experiment. Notice the difference in units.

Factor Settings	Main Factors for Bill of Materials
	Front EVA Thickness (mm)	Front Glass Thickness (mm)	Front Glass ARC Thickness (nm)
-	0.2	1.5	50
0	0.5	3	100
+	0.8	4.5	150

The responses that you will be observing are listed in Table 2 below.

Table 2 - List of responses observed throughout the Main Factor Response experiment.

Response	Units
Front reflection photon current density (JR,Front)	mA/cm2
Parasitic absorption photon current density in ALL module components (JA,non cell)	mA/cm2
Absorbed solar cell bulk photon current density (JA,Bulk)	mA/cm2
Maximum Power Output of a Unit Cell (Pmp)	W
Calculate Efficiency	%

Conducting the Experiment

1. Open a new Si Wafer template. Add a front EVA layer on top of the wafer, a Glass layer on top of the EVA. Finally, add a Glass ARC film to the Glass layer. The types are listed below
   1. EVA – Mcl09A
   2. Glass – Sodalime 0.05 wt%o Fe203 [Vog16b]
   3. Glass ARC – Vog15
2. Convert the template into a unit-cell module in the Input -> Layout tab . The template should look like the one in Figure 2 below. Make sure to save this template for later use

3. Using the sweep function setup a single simulation to run the main factor experiment. It should follow the layout provided in Table 3 below. Use the run summary to check that the sweep is correct before clicking run
4. Under Inputs -> Circuits, enable the electrical solver
5. Produce a main factor response graph for each factor
6. Make sure to save your simulations, any unsaved data will be lost once SunSolve is closed

Questions

1. What is the most important layer to optimise?
2. Does the main factor response experiment indicate any optimal values for any of the factors?

Part Two – Single Factor Response Experiment

After identifying the most important factor to optimise in Part One, a single factor response experiment is used to find the optimum value for that factor. Make sure to record the same responses as in Part One.

Conducting the Experiment

1. Make sure to use the default settings (“0” setting) for other factor(s)
2. Using the sweep function again, run the simulation with at least 8 steps starting from the “-“ and ending at the “+” setting of your factor. SunSolve can create equal intervals automatically
3. Record the 5 responses for each run of the simulation
4. Sketch an X-Y scatter plot for each response (y-axis) versus your factor of interest (x-axis)
5. Describe the relationships between your factor and each of the responses.
6. Identify and record the optimum value for your create a new template with this optimised value
7. Make sure you save your completed simulations appropriately (different to creating a template)

Part Three – Investigating Possible CTM Gains

So far, this tutorial has focused on possible CTM losses that the different BoM layers have on Solar Cell performance. However, an important material that has not been investigated in the module fabrication process is the Interconnecting Ribbons. If poorly designed, they can cause shading on the individual cells but are usually manufactured atop the Busbars, so this is avoided.

You will be investigating the possibility of using different cross sections of interconnecting ribbons to achieve better optical performance. For this, you will be using c-si SSP Module template, which is found under the Unit-cell Modules list. The responses you will be observing are listed in Table 4 below.

Table 4 - Responses to observe in the following experiment

Response	Units
Cross-Sectional Area of the Ribbon	cm2
Short circuit current density (Jsc)	mA/cm2
Total cell series resistance (RS_Grid)	Ω.cm2
Maximum Power Output of a Unit Cell (Pmp)	Wp

Conducting the Experiment

1. Opening a new c-Si SSP Module, investigate the properties of the default interconnecting ribbons.
2. Run 4 different simulations, each with the different options of Cross-Section shape, rectangular, triangular, circular and trapezoidal. Notice the different widths/ radii that the default cross sections provide
3. Record the responses as outlined in Table 4

Questions

1. Which cross-section ribbon has the greatest optical performance?
2. Which cross-section ribbon has the greatest electrical performance? Is it the same as the ribbon type from question 1?
3. Describe the relationship between cross-sectional area and maximum power output.
4. For the ribbon with the greatest optical performance, what property of the interconnecting ribbons could be altered to achieve better electrical performance?
5. Investigate other methods of editing the ribbon optics, what does this suggest about future capabilities of interconnection? 
6. Describe how interconnections could provide CTM gains.

Part Four – Further Understanding of the BoM for PV Modules

1. What was the significance of using a Si Wafer for Parts One and Two?
2. The materials used in the BoM must be chemically stable throughout the lifetime of the module, what is a possible issue with EVA as an encapsulating material?
3. The Glass ARC film layer is exposed to the air, what possible effects on module performance over its lifetime does this placement introduce?
4. What is a major limitation when considering a very thin glass layer for modules?
5. In terms of interconnecting ribbons, how do they affect the number of cells that can fit in a module?
6. Why would interconnections from front to rear not be favourable for thinner solar cells?
7. Investigate the properties of the back sheet in a default c-Si SSP Module, how would a back sheet with peak optical properties introduce CTM gains?