Metallisation, More Busbars


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. The metal contact design involves the analysis of several optical and electrical losses. 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 taller/ thicker contacts reduce the series 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 Outcomes

  • Understand the implications of front and rear metal contacts on optical and electrical performance
  • Be able to perform single factor response experiments to find optimum widths in multi-busbar cells
  • Understand why the industry has moved to more busbars
  • Explain the implications of interconnecting ribbons in PV modules

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 front metal grid design and exploring why the industry has moved to more busbars. The relationship between the height and width of the metal contacts were already explored in SunSolve Tutorial 4; here you will optimise the full grid, including the busbars. As manufacturing technology develops, the option for more busbars as a method to further reduce losses is becoming more viable; therefore, you will optimise the busbar width and re-optimise the fingers for a 5 busbar cell (BB5). You will then compare this optimised BB5 cell to your optimised cell from SunSolve Tutorial 4. Make sure to read the relevant lecture slides and SunSolve “about” pages to understand the different settings available and their effects on performance. Refer to multi-busbar pages for further reading.


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 – Optimising 5 Busbars

The width of busbars are a significant source of shading, and the number of busbars also affects the flow of electrons across the fingers of the solar cell. As the screen-printing process and module manufacturing improves, both the number and width of busbars have been further optimised. In this section, you will explore the optimum width for 5 busbars using the c-Si SSP Cell. The range of widths that must be tested can be found in Table 1 below, followed by the respones to be recorded in Table 2.

Table 1 - List of Factor Settings to be used in the optimisation of Busbar widths.
Factor SettingsWidth of 5 Busbars (μm)
Table 2 - List of responses to be observed when optimising busbar widths.
Busbar Series Resistance (RG, Grid) Ω.cm2
Short circuit current density (Jsc) mA/cm2
Max Power (Pmp)W
Fill factor (FF)-
Volume of Silver (V)

Conducting the Experiment

  1. Using the template c-Si SSP Cell, open a new simulation
  2. Set both the front and rear Busbar numbers to 5
  3. Using the sweep function, run a simulation with at least 8 steps starting from the “-“ setting and ending at the “+” setting of the Busbar widths outlined in Table 1
  4. Record the 5 responses as outlined in Table 2. The volume of silver used in the production of the cell is the front metal total
  5. Identify and record the optimum busbar width for the BB5 cell
  6. Create a new template with this optimised value
  7. Make sure you save your completed simulations appropriately (different to ‘create template’)


  1. What is the manufacturing significance of keeping the front and rear busbars the same?
  2. Why do we not consider the rear electrode layer when recording the volume of silver?

Part Two – BB5 Cell Fingers – Main Factor Response

Now that the busbar width of the BB5 cell has been optimised, the number and width of the fingers must be investigated to further optimise the front metal grid design. Using the optimised BB5 template you created in Part One, you will run a main factor response experiment to determine the most important factor between the number of fingers and the width of the fingers. The factor settings are in Table 3 below, followed by the responses to be recorded in Table 4.

Table 3 - Factor settings to be used in the Main Factor Response experiment.
Factor SettingsNumber of fingers per busbar (NF)Finger Width (µm)
Table 4 - List of responses to be observed throughout the Main Factor Response experiment
Short circuit current density (Jsc) mA/cm2
Front metal total series resistance (RS_Grid) Ω.cm2
Max Power (Pmp)W

Conducting the experiment

  1. Create a new simulation for the BB5 cell template with optimised busbar width
  2. Using the sweep function, setup a single simulation to run the main factor experiment. Produce a main factor response graph for each factor
  3. Identify the most important factor to optimise
  4. Make sure to save your simulations; any unsaved data will be lost once SunSolve is closed

Part Three – BB5 Cell Fingers – Single Factor Response

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 (Part Two)

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 chosen factor. SunSolve can create equal intervals automatically 
  3. Record the 3 responses for each run of the simulation
  4. Sketch an X-Y scatter plot for each response (y-axis) versus your factor (x-axis)
  5. Describe the relationships between your factor and each of the responses
  6. Identify and record the optimum setting for your factor. Create a new template with this optimised value  
  7. Make sure you save your completed simulations appropriately (different to creating a template) 


  1. You now have an optimised grid design for a BB5 cell. Investigate the volume of silver used for this optimum case. Compare that to the optimised BB4 cell from SunSolve tutorial 4. What do you notice?
  2. Why do you think industry is moving towards more busbars? List your reasons.

Part Four – Further Understanding of Metallisation

The following questions are used to make sure that you understand the concepts involved. You can prepare your answers while completing the above tasks

  1. You now have 2 optimised cells with differing busbar numbers, BB4 and BB5. Which of these cells has the highest overall performance? Calculate its efficiency.
  2. Does this cell have the best Optical or Electrical performance? Explain why it may not be optimal for both.
  3. List the challenges manufacturers would face when aiming to print more busbars.
  4. How are the cells of a module connected? How might those processes affect the optimised width of busbars?
  5. Investigate the other parameters in the “Electrode” tab of SunSolve, suggest possible advances to the screen-printing process that would further optimise both optical and electrical performance.

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 metal paste used in the metallisation process contain glass fritz, what is their purpose in the manufacturing line?
  • During the cofiring stage, what may occur if the belt speed is too slow or firing temperature too high?


[1] – International Technology Roadmap for Photovoltaics, 10th Edition, 2019. Available: