PVfactory 2 – Alkaline Texturing

Introduction

Texturing is used to reduce the reflection of light from the front surface of silicon wafers. Alkaline texturing is used to texture mono-crystalline wafers. The texturing performance depends on the sodium hydroxide (NaOH) and isopropanol (IPA) concentration, etching temperature, etching time, and exhaust flow. The objective of texturing is to minimise the front-surface reflectance so that more photons can be absorbed by the solar cell resulting in a larger short-circuit current density, JSC. The percentage of light reflected is wavelength dependent, however, typically we try and minimise reflectance at a wavelength of 600 nm as the photon count from the sun is maximal at this wavelength.

Learning Objectives

  • Explain why wafers need to be textured
  • Understand why the random pyramid texture forms in alkaline solutions
  • Be able to perform a main factor experiment to determine the most important parameter to optimise
  • Be able to perform a single factor experiment to optimise the alkaline texturing process
  • Understand how the saw damage removal step affects texturing

Tutorial Exercise

Alkaline Texturing performance depends on many parameters, including NaOH and IPA concentration, etching temperature, etching time, and exhaust flow. Please review the page, Alkaline Texturing, and the help files in PV Factory before attempting the exercises so that you understand the effect of the different parameters. For this tutorial exercise you will use the ‘Factory Default’ settings for all processes except the Saw Damage Removal process that you optimised last week. All experiments should use batches of wafers (at least 10 per batch) with the following properties:

  1. Standard Cz mono-crystalline silicon wafers
  2. 200 µm thick
  3. Resistivity of 1 Ω cm
  4. Cut using a standard wire saw (this assumes a wire saw that uses a slurry not diamond tips)

You can save these batch settings to your recipe (use the “S” icon on the left-hand screen). By doing this, the latest saved recipe for each step will be stored in the ‘Use my recipe‘ setting, which could be found in ‘Office‘ -> ‘User Settings‘. Hence, saving the optimised step for Saw Damage Etch from the previous exercise. Once you have set the parameters for the Alkaline Texturing, run that step and before you press ‘Complete Batch’ to run the remaining steps, record the average pyramid height, pyramid coverage % and reflection at 600 nm. You will need to go to the Characterisation Lab to do the last measurement. So the procedure you will need to follow this week is:

Create New Batch → Remove Saw Damage → Alkaline Texture (this optimisation) → Return to Alkaline Texturing step and record average pyramid height and pyramid coverage % → Go to Characterisation Lab and measured R600 nm for a selected wafer (non-destructive measurement) → Complete Batch → Record Batch JSC

In the main factor experiment you will need to explore the effects of the following factors of interest (allowed ranges in PV Factory are in parentheses):

  • Bath temperature (50 – 100 ºC)
  • IPA concentration (0 – 10 %)
  • Texturing time (0 – 30 min)
  • Exhaust flow (none to very high)

On the responses of pyramid height (µm), pyramid coverage (%), reflection at 600 nm (%) and JSC (mA/cm2). To simplify this exercise, we have assumed that the NaOH concentration should be maintained at 2 % (w/v). The suggested factory settings are listed on the table below.

Table 1 - Suggested settings for main factor response experiment
Factor Settings
Main Factors for Alkaline Texturing Process
Bath Temp
(oC)		[IPA]
% (v/v)		Texturing Time
(min)		Exhaust Flow
-50
2.515None
0705.020Medium
+907.525Very High

As described in last week’s tutorial, “-“, “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 recipe settings are provided in the table above.


Conduct a Main Factor Response Experiment

  1. Process batches (of at least 10 cells per batch) for each of the following experiments (using your best recipe for saw damage removal) and complete the table below.
  2. Produce a main factor response graph for each of the responses: (i) pyramid height; (ii) pyramid coverage; (iii) reflection at 600 nm; and (iv) JSC (batch mean).
Table 2 - Recorded Factor Responses
Exp. No.Factor SettingsResponse Observations
Bath Temp[IPA]Texture TimeExhaust FlowPyramid Height
(um)		Coverage
(%)		R600nm
(%)		Jsc
(mA/cm2)
10000
2-000
30-00
400-0
5000-
6+000
70+00
800+0
9000+

Questions:

  1. Why did we not vary the NaOH concentration in the main factor experiment?
  2. What is the relationship between R600 nm and JSC?
  3. Do the main factor response curves suggest an optimum value for any of the factors?
  4. Is it clear which parameter to optimise in the single factor response experiment?

Part 2 – Single Factor Response Curve

Identify the factor that you think has the greatest effect on the JSC and run a single factor experiment to determine the optimum value for that factor. We’ll also record the other responses that you considered in Part 1 (i.e. pyramid height and coverage and R600 nm).

Conduct a Single Factor Response Curve Experiment

  1. Use the same wafer parameters that you used for Part 1.
  2. Create and process at least 8 experimental batches (with at least 10 wafers per batch), varying the factor of interest over the range of values allowed by the PV Factory. For example, you might create 10 batches with texturing time varying from 15 min to 30 min, stepping by 2.5 min.
  3. Record all four 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.
  6. Did the amount of silicon removed per side change during your experiment and if so why?
  7. Store your best recipe in PV Factory so you can use it for the following tutorials.

Part 3 – Understanding Alkaline Texturing

General questions:

  1. Why do we minimise reflectance at 600 nm for terrestrial solar cells?
  2. What % coverage should you attempt to achieve?
  3. Why do pyramids form?
  4. How does the age of your saw damage etch bath affect your texturing performance?
  5. What is the effect of exhaust flow rate?
  6. Why is alkaline texturing typically not used for multi-crystalline wafers?

Part 4 – Challenge Exercise

Re-run the single factor experiment, but this time “dump” your saw damage bath before you do the experiment. What happens?