PVFactory 7 – Screen Printing

Introduction

Screen-printing is a way of depositing a material (e.g., paste) on a surface according to a pattern formed in a screen comprising a network of meshed wires or strands. The pattern is formed in a polymer, called an emulsion, which is sensitive to light. When the screen is irradiated according to the pattern, the irradiated emulsion hardens and binds to the screen. The non-irradiated regions can then be washed away by spraying the screen with water leaving openings in the emulsion. The paste can be pushed through the openings in the screen using a squeegee to form a pattern of paste on the cell surface under the screen.

Screen printing is used to form the rear aluminium electrode and the front surface silver grid (busbars and fingers) on the silicon nitride antireflection coating (ARC). When the cell is cofired (in the next production step), the paste etches through the silicon nitride and silver contacts the underlying silicon to form the n-type contacts to the solar cell.

This tutorial focuses on the silver screen printing process as the design of the screens is critical for the way the pattern is used to form the metal grid.

Learning Objectives 

• Understand what is critical for the formation of a back surface field and rear electrode for a screen-printed solar cell
• Understand the process of forming a metal grid on the front surface of a screen-printed solar cell
• Be able to optimise a screen printing process by varying mesh density, strand diameter, emulsion thicknesses and printing parameters
• Be able to use characterisation measurements to help guide the optimisation of a metallisation process

Pre-Work – Aluminium Screen Printing

The rear screen-printed aluminium, when fired, results in a p⁺ layer at the rear surface of the cell. This p⁺ layer is called a back surface field (BSF) and reduces surface recombination by repelling minority carriers from the surface. The thickness and the aluminium concentration of the p⁺ layer depends on how the aluminium is screen-printed and the how the cell is fired. In the pre-work you will identify factors that determine the quality of the BSF that is formed.

Before coming to class you need to perform a multiple factor response experiment for aluminium screen printing which examines the responses of: (i) open circuit voltage (V_OC); and (ii) IQE at 1000 nm as a function of the main factors listed in Table 1. For this experiment maintain the thickness of the emulsion over the screen as 3 mm and the printing parameters (squeegee pressure, viscosity and speed) at their default values. On completion of the main factor experiment, optimise the aluminium screen printing process and save your best recipe so you can use it in the silver screen printing optimisation which you will do in this tutorial.

Table 1 - Suggested main factor settings for Al-BSF screen printing

Factor Settings	Main Factors
	Strand Density (strands/ cm)	Emulsion Thickness Below (μm)	Strand Diameter (μm)
-	50	10	20
0	150	20	50
+	250	30	120

Questions on Aluminium Screen-Printing and BSFs:

1. What is the function of a BSF?
2. Explain why you used the responses of V_OC and IQE at 1000 nm for this experiment?
3. What screen printing parameters result in the highest V_OC?

Tutorial Exercise – Silver Screen Printing

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 Silver Screen Printing step, enter your settings and complete the batch by selecting Run Remaining Steps. The J-V results for your batch will then be available. The results of your simulations this week will depend on the parameters that you are using for Cofiring however the same trends should be evident for everyone.

In the optimisation you will consider the following responses:

1. Silver paste thickness (mm)
2. Finger resistance represented as the voltage drop along a finger (mV);
3. Fill Factor (FF);
4. Short circuit current density (J_SC; mA/cm²); and
5. Mean efficiency (%).

A note on Finger Resistance

Following the cofiring step, the resistance of the silver fingers can be assessed by probing the voltage drop along a silver metal finger as shown in Figure 1. In this test, the cell is placed under the solar simulator and contacted by test probes so as to short-circuit the cell. This causes the maximum photogenerated current to flow within the silver metal lines, thereby maximising the resistive losses in the silver fingers. A multimeter can be used to measure the voltage difference between a busbar and a perpendicular finger at the edge of the wafer as shown on the next page. For most commercially-produced cells that are 15.6 cm × 15.6 cm and have three busbars, the separation between these two measurement points, l, is ~ 2.6 cm.  

Part 1 – Main Factor Response Experiment

The Silver Screen Printing process depends on properties of the screen (mesh density, strand diameter, emulsion thicknesses above and under the screen, finger width and pitch), the paste viscosity as well as the printing process (squeegee pressure and speed). There are many parameters to optimise and so we will make some assumptions in this exercise.

Design a main factor response experiment which explores the effects of the following factors of interest (allowed ranges in PV Factory are in parentheses):

• Mesh density (50 – 250 strands/cm);
• Emulsion thickness below the mesh (0 – 30 mm)
• Strand diameter (20 – 120 mm);
• Finger opening width (50 – 250 mm).

on the responses of silver paste thickness (mm), voltage drop along a finger (mV), fill factor, short circuit current density (mA/cm²) and mean efficiency (%). In your analysis please keep the following parameters constant. In this exercise, you must work out your own factor settings.

Table 2 - Parameters to be kept constant

Parameter	Fixed value
Emulsion above the mesh	3 μm
Finger pitch	3 mm
Squeegee pressure	7 a.u.
Paste viscosity	7 a.u.
Printing speed	7 a.u.

To Do:

1. 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.
2. Process batches for each of your designed experiments and record the results.
3. Produce a main factor response graph for each of the four responses.
4. Explain to your tutor the relationships that you have discovered.
5. Identify at least one trade-off associated with the design of screens for silver paste printing.

Part 2 – Single Factor Response Experiment

Select one factor to investigate in more detail based on your analysis in Part 1.

To do:

1. Use the same wafer parameters as you used for Part 1.
2. Process experimental batches (with at least 10 wafers per batch), varying the factor of interest over the range of values allowed by the PV Factory.
3. Record all responses for each factor setting.
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.

Part 3 – Understanding the Silver Screen Printing Process

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

General questions:

1. List factors which determine the thickness of the silver fingers after cofiring? 
2. List factors which will determine the width of the silver fingers after cofiring?
3. What strategies would you use to increase the aspect ratio of your screen-printed fingers?
4. Why is the resistivity of the silver fingers after firing higher than that of bulk silver (1.6 × 10^-6 Wcm)?

True/False questions:

1. If a small strand diameter is used then the squeegee pressure must be reduced otherwise the screen may be damaged.
2. The final thickness of fired silver fingers can be half that of the screen-printed fingers.
3. A low mesh density will result in finger smearing and therefore negate any advantages of using smaller finger widths.
4. Moving from 3 to 5 busbars for 156 mm monocrystalline cells results in increased silver consumption.
5. It is not necessary to print thick busbars because most of the current collected from the fingers will travel in the interconnect wire when the cell is interconnected in a module.
6. Screens have to be replaced more frequently for print-on-print screen-printing otherwise the accuracy of the alignment can be reduced.