PVFactory 8 – Cofiring

Introduction

After aluminium and silver screen printing, cells are fired in an in-line infrared furnace where they experience a peak firing temperature in the range of 750 – 870 °C for 4 to 5 seconds. This firing process results in the formation of both the rear back surface field and aluminium electrode and enables metal contacts to form to the front surface n-type emitter. The firing temperature depends on the type of paste used, how they were screen-printed and the properties of the emitter and the silicon nitride antireflection coating. Typically cells are fired with the screen-printed aluminium in contact with the belt.

Learning Objectives

  • Understand the relationships between p–n junction formation, antireflection coating and metallisation
  • Be able to use characterisation measurements to optimise a co-firing process

Tutorial Exercise

By now you should have optimised all the previous steps in the production line and so it’s going to be much easier for you to simply use your preferred settings for the remaining tutorial exercises. All the experiments in this optimisation should use batches of wafers (at least 10 per batch) with the following properties:

  1. Standard Cz mono-crystalline silicon wafers;
  2. 200 mm thick;
  3. Resistivity of 1 W cm; and
  4. Cut using a standard wire saw.

On creation of a new batch, automatically run all previous steps. When you get to the Cofiring step, enter your settings and complete the batch by running that step. The J-V results for your batch will then be available.

Part 1 – Single Factor Response Experiments

The important factors for the cofiring step are the temperature of zone 2 (firing zone) and the belt speed (which is a measure of how long the wafer experiences the peak firing temperature). Both factors are important and so you must conduct single factor response experiments for both factors on the responses, open circuit voltage (VOC), short circuit current (JSC), fill factor (FF), series resistance (Rs) and average efficiency[1].

[1] Average cell efficiency is defined as the mean cell efficiency × yield (i.e., the parameter that is displayed in the Batch Summary in PV Factory.

To Do:

  1. Process experimental batches (with at least 10 wafers per batch), varying the factor over the range of values allowed by the PV Factory.
  2. Record all responses for each factor setting.
  3. Sketch an X-Y scatter plot for each response (y-axis) versus each factor (x-axis).
  4. Describe the relationships between each factor and each of the responses.
  5. What are your optimal cofiring conditions?

Part 2 – Effect of Silicon Nitride Deposition Conditions

You may recall from PV Factory 6 that the temperature at which you deposit your silicon nitride affects the “hardness” of that layer. This will affect the ability of the frits in the silver paste to fire through the silicon nitride and contact the silicon. Change the temperature at which the silicon nitride is deposited to 400 ºC and re-do one of your single factor experiments. What do you observe?

Part 3 – Understanding the Cofiring Process

Make sure that you understand the process and prepare your answers while completing the above tasks.

General questions:

  1. List the main contributions to series resistance for screen-printed cells.
  2. Why may your series resistance increase if you fire at too high a temperature or at too slow a belt speed?
  3. What change would you expect in your I-V response if you changed your diffusion process such that the sheet resistance of the doped layer was increased?
  4. How would you change your Silver Screen Printing process to accommodate the change you made in Q3?
  5. Would you need to change your cofiring process (for the diffusion change in Q3)?

True/False questions:

  1. It is necessary to dry the pastes to remove volatile components before firing in an inline furnace.
  2. The firing zone(s) of a firing furnace are typically heated using infrared lamps.
  3. Firing furnaces are designed such that the temperature is slowly increased so as to not thermally stress the silicon wafers.
  4. Wafers are fired with the screen-printed aluminium facing up.
  5. The co-firing temperature is normally optimised for the rear aluminium rather than the silver metal contact.
  6. It is desirable to have the silver contact the silicon as close as possible to the junction. 
  7. Preferably cofiring is performed in an inline furnace in an oxygen-free environment.
  8. If you change the silicon nitride deposition to use direct rather than remote plasma chemical vapour deposition it is not necessary to re-optimise the cofiring