Optimisation of Surface Textures

Introduction

Texturing is used to reduce the reflection of light from the front surface and to improve light trapping in a solar cell. The first objective of texturing is to minimise the front-surface reflectance so that more photons remain, which can be absorbed by the solar cell resulting in a larger short-circuit current density, J_sc. The amount 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.

Alkaline texturing is used to texture mono-crystalline wafers resulting in random upright pyramids. Multicrystalline wafers are routinely textured using acidic solutions in an inline process. The resulting surface appears as randomly-arranged concavities (inverted spherical caps). For both texturing processes, their performance can be related to several physical traits such as Periodicity, Angle, Shape and Height.

Learning Objectives

• Explain why wafers need to be textured
• Understand the different types of textures as a result of Alkaline and Acidic process
• Be able to perform a main factor response experiment to determine the most critical parameter to optimise in both Alkaline and Acidic texturing processes
• Be able to perform a single factor response experiment for both Alkaline and Acidic texturing processes to optimise optical performance

Tutorial Exercise

In this tutorial, you will be looking into both Alkaline and Acidic texturing, investigating the ‘potential’ effect of Height and the Angle of the textures on the optical performance of a silicon wafer. You will be using the Si Wafer template, sweeping and editing appropriate ‘morphology’ settings in the top ‘textures and interfaces’ layer of the wafer. Please review the relevant SunSolve about pages and PVmanufacuring.org articles on texturing before attempting this tutorial in order to have a better understanding of the different textures.

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 page is closed/changed/refreshed.

Part One – Main Factor Response Experiment

You will be conducting two main factor response experiments, one for Alkaline and another for Acidic Texturing. For these SunSolve exercises, “-“, “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 below.

Table 1 - Settings used in the main factor response experiment

Factor Settings	Main Factors for Texturing Performance
	Height (um)	Angle (degrees o)
-	2	20
0	5	50
+	8	70

The responses you will be observing are listed in Table 2 below.

Table 2 - List of responses observed throughout the main factor response experiments

Responses	Units
Reflection at 600 nm	%
Front reflection photon current density (JR,Front)	mA/cm2
Front escape photon current density (JE,Front)	mA/cm2
Absorbed solar cell bulk photon current density (JA,Bulk)	mA/cm2

Conducting the Experiment

1. Open a new Si Wafer template from the Wafers and Cells list
2. Apply alkaline texturing to the front of the wafer by altering the morphology of the top textures and interfaces layer. Your wafer texture should have random upright pyramids
3. Save this as a template using the “create template” button. It can then be used in later simulations
4. Using the sweep function, setup a single simulation to run the main factor experiment for an Alkaline Textured cell, it should follow the the layout provided in Table 3 below. Use the run summary to check that the sweep is correct before clicking run
5. In the Outputs -> Photon Currents tab, selecting “Detailed Losses” and unchecking the boxes for “Combine reflection” and “Combine cell components” will allow you to view all the relevant information

6. Produce a main factor response graph for each factor
7. Similarly, open a new Si Wafer template and apply acidic texturing to the front surface. Your wafer texture should have random inverted spherical caps. Save the template for later use
8. Run a separate main factor response simulation for the Acidic Textured cell, following the same layout in Table 3. Produce a main factor response graph for each factor as well
9. Make sure to save your simulations (different to a template); any unsaved data will be lost once SunSolve is closed

Questions

1. What is the relationship between R at 600 nm and J_A,Bulk?
2. Do the main factor experiments suggest any optimum factor settings?
3. Which main factor response graph should be used to identify the most important factor to optimise? Explain.
4. Which factor (Height or Angle) is most important to optimise for Alkaline Texturing?
5. Which factor (Height or Angle) is most important to optimise for Acidic Texturing?
6. Are the key factors in both texturing processes the same? Does the other factor have any major effects on the optical response of the wafer? Explain.

Part Two – Single Factor Response Experiment

After identifying the critical factor to optimise for Alkaline and Acidic texturing in Part One, a single factor response experiment is used to find the optimum value for that factor. You will be conducting two separate single factor responses, optimising the Alkaline and Acidic textured wafers. Make sure to record the same responses as in Part One.

Conducting the Experiment

1. Open a new copy of your Alkaline textured silicon wafer. Make sure to use the default settings (“0” setting) for the other factor
2. Using the sweep function again, run the simulation with at least 8 steps starting from the “-“ and ending at the “+” setting. SunSolve can create equal intervals automatically
3. Record the 4 responses for each run of the simulation
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. Identify and record the optimum value for your Create a new template with this optimised value
7. Repeat Steps 1-6, now with an Acidic textured wafer
8. Make sure you save your completed simulations appropriately (different to creating a template)

Part Three – Understanding the importance of texturing

General Questions

1. Suggest a few reasons for conducting the experiment on a bare Si Wafer.
2. What is the significance in observing front escape current density?
3. Identify 3 properties of light that texturing aims to utilise. Which physical trait (angle/ height) influences these properties the most? Explain. 
4. How does texturing increase absorption of longer wavelength light?
5. In a modern production line, the angle of the alkaline textured cells are typically 52^o, whereas the angle of the acidic textured cells are 65^o. Did your experiments suggest angles higher than these values? If so, what are some possible limitations in manufaturing that does not allow higher angles?
6. There are other methods to texturing silicon wafers such as metal catalysed chemical etching (MCCE) and reactive ion etching (RIE). How do these techniques work and what type of textures can be produced by these methods.

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 wet chemical ething of Si solar cells.

• Describe the etching process, identifying the oxidising and reducing agents of the reaction for both alkaline and acidic etching. What is the purpose of isopropanol?
• Why do pyramids form for alkaline etching compared to the spherical caps in acidic texturing?
• Saw damage etch removal is another wet chemistry step applied to mono-crystalline wafers before alkaline texturing. This is not the case for acidic texturing of multi-crystalline wafers. Why?