Metallisation

Introduction

In recent years, the front and rear of an industrial silicon solar cell have changed significantly. In this tutorial, you will learn a bit more about the guiding principles the photovoltaic industry is using when optimising the grid. When optimising the grid, you have to take into account both optical and electrical losses, while in addition, you want to reduce the amount of expensive metal (e.g., silver and aluminium) paste used. Optically, the front metal contacts shade the solar cell; hence, you want to keep them as thin as possible. From an electrical point of view, wider contacts have a lower contact resistance, and the cross-section (i.e., width and height) of the finger determines the finger resistance. Similarly, designs for rear metal contacts are just as important. Back Surface Field (BSF) solar cells have dominated the worldwide market up to 2019, however, the ITRPV^[1] predicts that these will be completely phased out to more advanced solar cell architectures such as Passivated Emitter and Rear Contact (PERC) solar cells which also has localised contacts at the rear.

Learning Objectives

• Understand the implications of front and rear metal contacts on optical and electrical performance
• Design an experiment to optimise the number of fingers on the front
• Be able to perform main factor response experiments to determine the most important factor of silver printed fingers to optimise
• Be able to perform a single factor response experiment to optimise the silver screen printed fingers

Tutorial Exercise

In this tutorial, you will be exploring the optical and electrical effects of different features that make up the metal contacts of solar cells, optimising the metal grid design. You will optimise a 4 busbar cell (BB4), analysing several different optical and electrical parameters. Make sure to read the  SunSolve “about” pages to understand the different settings available and their effects on performance. Refer to the PVmanufacturing.org screen-printing pages for further reading.

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 window is closed/changed/refreshed.

Part One – Optimising Number of Fingers

Before optimising the cross section of the front metal fingers, the number of fingers per busbar (N_F) must be optimised. Using the default BB4 cell from the c-Si SSP Cell template, design a single factor response experiment to optimise the number of fingers. You must decide on a range for the number of fingers to sweep across i.e. a “min“ and a “max” setting. Make sure to also record the following responses:

• Short circuit current density (Jsc) mA/cm²  
• Front metal total series resistance (R_{S_Grid}) Ω.cm²
• Max Power (Pmp) W

Save the template with optimised number of fingers to be used in the next 2 parts.

Part Two – Optimising Cross Section  – Main Factor Response Experiment

The silver (Ag) fingers on the front of the wafer affect both the optical and electrical performance of PV cells. There is often a delicate balance between the reduction of shading caused by the metal contacts and limiting resistive losses in them. Working with the template you designed in Part One, you will be optimising the Height and Width of the front metal contact grid i.e., the front metal fingers.

Main Factor Response Experiment

Table 1 - Factor settings to be used in the Main Factor Response experiment.

Factor Settings	Main factors for Finger optimisation
	Finger Height (um)	Finger Width (um)
-	5	35
0	15	45
+	25	55

For SunSolve tutorials, “-“, “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 above. 

The responses you will be investigating are:

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

Response	Units
Front reflected photon current density (JR, Front)	mA/cm2
Front elements, finger series resistance (RS_Grid)	Ω.cm2
Short circuit current density (Jsc)	mA/cm2
Fill factor (FF)	-
Max power (Pmax)	W

Conducting the experiment

1. Open a new c-Si SSP Cell simulation
2. Using the sweep function setup a single simulation to run the main factor experiment. Follow the layout provided in Table 3 below. Use the run summary to check that the sweep is correct before clicking run

3. Produce a main factor response graph for each factor
4. Identify the most important factor to optimise
5. Make sure to save your simulations; any unsaved data will be lost once SunSolve is closed

Questions

1. From the main factor response experiment, can the optimum settings be identified? If so, what are they and which response(s) is this based on?

Part Three – Single Factor Response Experiment

A single factor response experiment is used to find the optimum value for a factor.  Make sure to record the same responses as in the main factor response experiment.

Conducting the experiment

1. Use the default settings (“0” setting) for the other factor(s)
2. Using the sweep function again, run the simulation with at least 8 steps starting from the “-“ setting and ending at the “+” setting for 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 finger width (x-axis) 
5. Describe the relationships between your factor and each of the responses 
6. Identify and record the optimum factor setting. Create a new template with this optimised value. This optimised BB4 cell will be used in SunSolve Tutorial, Metallisation – more busbars
7. Make sure you save your completed simulations appropriately (different to creating a template) 

Questions

1. What is the expected relationship between finger width and front reflected photon current density?
2. What is the relationship between series resistance in the fingers and fill factor of the cell?
3. How would you expect an I-V curve to change with increasing series resistance?
4. Would you expect your optimal cell to obtain better performance with a finger height of 25 µm or more? Why?

Part Four – Further Understanding of Metallisation

General Questions

1. List the 4 sources of parasitic series resistance in silicon solar cells. Describe where they occur in solar cells.
2. Using the plots from Parts Two and Three, does your optimised cell have the best Optical or Electrical performance? Explain why it may not be optimal for both.
3. Increased finger height is optimal for electrical and optical performance. What are some issues that may occur with taller fingers?
4. What aspects of the screen-printing template and silver paste may assist in obtaining increased finger height?
5. What are some manufacturing challenges that result from the trend identified in Q3?

In Class Discussion Points

The following points are related to the above tutorial, discuss these points amongst peers and your tutor to further your understanding of solar cell grid design.

• The tutorial is focussed on the series resistance that occurs in the metal contacts i.e. metal resistance, however, other sources of series resistance are affected by factors such as diffusion. Discuss the effect of diffusion on emitter and contact resistance.

References

[1] – International Technology Roadmap for Photovoltaics, 10th Edition, 2019. Available: https://itrpv.vdma.org/