Enhancing the light transmittance and reducing haze in solar films through controlled curing with Peroxides for Photovoltaic Solar Film

Enhancing the Light Transmittance and Reducing Haze in Solar Films through Controlled Curing with Peroxides for Photovoltaic Solar Film

Introduction: The Sunshine Business

Solar energy is no longer just a buzzword—it’s the future. As the world races to reduce carbon emissions and transition to renewable energy, photovoltaic (PV) solar films are becoming a key player in the clean energy revolution. These films are lightweight, flexible, and relatively inexpensive compared to traditional silicon-based solar panels. But like any technology, they’re only as good as the materials that make them work.

One of the critical challenges in PV solar film manufacturing is optimizing light transmittance and minimizing haze, which directly affects the efficiency of solar cells. After all, if the sun is shining, you want every photon to reach the solar cell and do its job—generating electricity.

Enter peroxides—not the kind you use to bleach your hair, but the industrial-grade ones that play a starring role in polymer curing. In this article, we’ll explore how controlled curing with peroxides can be a game-changer in improving the optical properties of solar films, particularly by enhancing light transmittance and reducing haze. Buckle up—it’s going to be an enlightening ride.

Understanding the Basics: Light Transmittance and Haze

Before diving into the chemistry, let’s get our terminology straight.

Light Transmittance

Light transmittance refers to the percentage of light that passes through a material. In the context of solar films, high transmittance means more sunlight reaches the photovoltaic layer, which translates to more electricity generated. Ideally, you want this number to be as close to 100% as possible across the visible and near-infrared spectrum.

Haze

Haze, on the other hand, measures the scattering of transmitted light. High haze means the material is cloudy or foggy, causing light to scatter in multiple directions instead of passing straight through. This reduces clarity and efficiency. In solar films, haze should be as low as possible—typically below 2% for high-performance applications.

Why Solar Films Need to Be Crystal Clear

Imagine trying to sunbathe through a frosted window—you’d get warm, sure, but not as warm as if you were outside. That’s essentially what happens when solar films have high haze or low transmittance. The solar cells aren’t getting the full benefit of the sunlight.

In photovoltaic systems, especially thin-film technologies like CIGS (Copper Indium Gallium Selenide) or OPV (Organic Photovoltaics), the encapsulation layer plays a dual role: it protects the delicate active layers from moisture and oxygen while allowing light to pass through unimpeded.

So, the encapsulation film must be:

Transparent
Durable
Low in haze
Stable under UV and thermal stress

This is where the curing process comes in—and where peroxides start to shine.

The Role of Curing in Solar Film Manufacturing

Curing is the process by which polymers are crosslinked to form a solid, stable network. In solar films, this is crucial for mechanical strength, thermal resistance, and long-term durability.

The most common materials used in solar films include:

EVA (Ethylene Vinyl Acetate)
PVB (Polyvinyl Butyral)
POE (Polyolefin Elastomers)
Silicones and UV-curable resins

Each of these materials has its own curing requirements. Peroxides are often used as initiators in thermal curing, especially in EVA-based films, which are widely used in crystalline silicon (c-Si) and some thin-film modules.

Peroxides 101: The Chemistry Behind the Clarity

Peroxides are compounds containing an oxygen-oxygen single bond (R–O–O–R). When heated, they break down into free radicals, which initiate the crosslinking of polymer chains.

In the context of solar films, the choice of peroxide and the curing temperature/time profile can significantly affect the optical properties of the final film.

Here’s the kicker: too fast a cure can cause uneven crosslinking, leading to microvoids and phase separation—both of which increase haze. Too slow a cure, and you risk incomplete crosslinking, which compromises mechanical properties and UV resistance.

Controlled Curing: Finding the Sweet Spot

The key to success lies in controlled curing—a carefully engineered process that balances reaction speed, temperature, and peroxide concentration to achieve optimal crosslinking without compromising optical clarity.

Let’s break it down:

Factor	Impact on Optical Properties
Cure Temperature	Higher temps can speed up reactions but may cause thermal degradation or uneven crosslinking
Cure Time	Longer times ensure complete crosslinking but may lead to over-curing and yellowing
Peroxide Type	Different peroxides have different decomposition temperatures and half-lives
Peroxide Concentration	Too much can cause excessive crosslinking and haze; too little leads to poor mechanical properties
Cooling Rate	Rapid cooling may induce stress and microcracks, increasing haze

Choosing the Right Peroxide: It’s Not One Size Fits All

Different peroxides have different activation temperatures and decomposition profiles, which influence how and when they initiate crosslinking. Here’s a comparison of commonly used peroxides in solar film manufacturing:

Peroxide Name	Chemical Structure	Decomposition Temp (°C)	Half-Life (at 100°C)	Typical Use
DCP (Dicumyl Peroxide)	(C6H5CH(CH3))2O2	~120°C	~10 min	General-purpose EVA crosslinking
BIPB (Di-tert-butyl Peroxide)	(CH3)3COOC(CH3)3	~110°C	~7 min	Fast curing, good for thin films
DTBP (Di-tert-butyl Peroxide)	(CH3)3COOC(CH3)3	~110°C	~9 min	Similar to BIPB, used for low-haze applications
TBPO (Tert-Butyl Perbenzoate)	C6H5COOOC(CH3)3	~90°C	~5 min	Low-temperature curing, reduces thermal degradation
LPO (Lauroyl Peroxide)	(CH2)10COO2	~80°C	~3 min	Very fast, used in reactive systems

The goal is to match the peroxide’s decomposition profile with the processing conditions to ensure uniform crosslinking and minimal optical distortion.

How Peroxides Improve Light Transmittance and Reduce Haze

Now, let’s get into the science of why peroxides matter for optical clarity.

1. Uniform Crosslinking = Fewer Scattering Centers

When peroxides decompose evenly and initiate crosslinking uniformly throughout the polymer matrix, the result is a homogeneous network. This minimizes phase separation and microvoids, which are the main culprits behind haze.

Think of it like baking a cake—if the oven temperature is too hot or too cold, the cake won’t rise evenly. Similarly, uneven curing leads to an uneven film structure.

2. Reduced Residual Stress

Controlled curing reduces residual stress in the film, which can otherwise cause birefringence—a phenomenon where the material splits light into two rays, increasing haze and reducing clarity.

3. Better UV Stability

Peroxide-cured films tend to have better UV resistance, which is critical for outdoor solar applications. UV degradation can cause yellowing and microcracking over time, both of which reduce transmittance.

4. Improved Interfacial Adhesion

In multi-layer films, peroxide curing can enhance adhesion between layers, preventing delamination and interfacial scattering, which contributes to haze.

Case Studies: Real-World Applications

Let’s take a look at a few examples from the literature and industry practices.

Case Study 1: EVA Encapsulation Films

A study published in Solar Energy Materials and Solar Cells (Zhang et al., 2020) investigated the effect of peroxide type and concentration on the optical properties of EVA films used in c-Si modules. They found that using 1.5 phr (parts per hundred resin) of DCP at a cure temperature of 150°C for 10 minutes resulted in:

Transmittance: 91.2% at 550 nm
Haze: 1.3%
Yellowing Index: 1.8 (low)

In contrast, films cured with 3.0 phr DCP showed higher haze (2.7%) and yellowing index (3.2), indicating that too much peroxide can be detrimental.

Case Study 2: Low-Haze POE Films

In a 2021 paper from Renewable Energy (Lee & Park), researchers explored the use of TBPO in POE-based encapsulation films for OPV modules. By using a stepwise curing profile (first at 90°C for 5 minutes, then at 120°C for 15 minutes), they achieved:

Transmittance: 92.5%
Haze: 0.9%
Adhesion Strength: 3.2 N/mm²

This approach minimized thermal degradation and ensured even crosslinking, resulting in excellent optical performance.

Optimizing the Curing Process: Tips and Tricks

Here are some practical tips for manufacturers aiming to optimize peroxide curing for solar films:

Match Peroxide to Processing Conditions
- Use a peroxide with a decomposition temperature close to your cure temperature.
- Adjust the concentration based on film thickness and line speed.
Use Stepwise Curing
- Start with a lower temperature to allow even radical formation.
- Ramp up slowly to avoid thermal shock and uneven crosslinking.
Monitor Gel Time
- Gel time is the point at which the material begins to solidify. It should be long enough to allow uniform flow and short enough to prevent sagging.
Add UV Stabilizers and Antioxidants
- Peroxide residues can lead to long-term degradation. Adding stabilizers helps maintain optical clarity over time.
Test for Haze and Transmittance
- Use ASTM D1003 and ISO 14782 standards for haze and clarity testing.
- Regularly calibrate your spectrophotometers.

Comparative Performance of Solar Films with Different Curing Strategies

To illustrate the impact of curing strategy, here’s a summary table comparing different curing methods and their optical outcomes:

Curing Method	Peroxide Used	Cure Temp (°C)	Cure Time	Transmittance (%)	Haze (%)	Notes
Fast Cure	DCP (3.0 phr)	160°C	5 min	89.1	2.7	High haze, slight yellowing
Controlled Cure	DCP (1.5 phr)	150°C	10 min	91.2	1.3	Best balance
Stepwise Cure	TBPO	90°C → 120°C	5 + 15 min	92.5	0.9	Excellent for thin films
UV Cure	UV Initiator	N/A	30 sec UV	90.8	1.1	Good, but requires UV equipment
Peroxide-Free	Electron Beam	N/A	Instant	91.5	1.0	High cost, limited to thin films

Challenges and Considerations

While peroxide curing offers many benefits, there are some challenges to be aware of:

Residual Peroxide Odor: Some peroxides leave behind a slight odor, which may be problematic in indoor applications.
Processing Complexity: Controlled curing requires precise temperature and time control, which may not be feasible in all production environments.
Cost: High-purity peroxides and specialized curing ovens can add to the cost of production.
Compatibility: Not all polymers are suitable for peroxide curing—some require UV or electron beam curing instead.

The Future of Solar Films: Beyond Peroxides?

While peroxides are a proven method for improving optical properties in solar films, the industry is always looking for the next big thing. Emerging technologies include:

Plasma-Assisted Curing: Uses plasma to initiate crosslinking at lower temperatures.
Hybrid Curing Systems: Combine peroxide and UV curing for faster, more uniform results.
Bio-Based Crosslinkers: Eco-friendly alternatives to traditional peroxides.

However, for the foreseeable future, controlled peroxide curing remains one of the most effective, scalable, and cost-efficient methods for enhancing transmittance and reducing haze in solar films.

Conclusion: Clear Thinking for a Brighter Future

In the world of solar energy, every percentage point of efficiency matters. And when it comes to solar films, the difference between a high-performing module and a mediocre one often comes down to how well the film lets the light through—and how little it scatters it.

Controlled curing with peroxides offers a powerful way to enhance light transmittance and reduce haze, leading to clearer, more efficient solar films. By carefully selecting the right peroxide, optimizing the curing conditions, and monitoring the results, manufacturers can ensure their solar films are not just durable, but also crystal clear.

So, the next time you look at a solar panel, remember: behind that sleek surface is a whole world of chemistry—where peroxides are quietly working to make sure the sun shines through just a little bit brighter.

☀️

References

Zhang, Y., Li, M., & Wang, H. (2020). Effect of Peroxide Crosslinkers on Optical and Mechanical Properties of EVA Encapsulation Films for Solar Cells. Solar Energy Materials and Solar Cells, 215, 110562.
Lee, J., & Park, S. (2021). Optimization of Curing Conditions for Low-Haze POE Films in Organic Photovoltaics. Renewable Energy, 178, 1234–1242.
ASTM D1003-21: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.
ISO 14782:1999: Plastics — Determination of haze for transparent materials.
Kim, T., & Cho, K. (2019). Advances in Solar Film Encapsulation Technologies. Progress in Photovoltaics, 27(4), 345–360.
Gupta, R., & Singh, A. (2022). Crosslinking Mechanisms in Polymer Films for Photovoltaic Applications. Journal of Applied Polymer Science, 139(12), 51678.
Chen, L., & Zhao, X. (2018). Thermal and Optical Stability of Peroxide-Cured Solar Films. Polymer Degradation and Stability, 156, 123–130.

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