U S AG E O F P E ROX I D E S

I N S O L A R C E L L C O N S T RU C T I O N

Date: 21st Nov 2016

Mohamed Adam.K(chem.adams@gmail.com)

A device converting solar radiation into electricity

Silicon solar cells are fragile and are especially sensitive to brittleness failurein tension and bending.

The voltage, and thus the power, of a silicon solar cell is temperature dependent.The efficiency decreases with increasing temperature

at the rate of about 0.5% / ᵒC.

Encapsulation design must remove the energy absorbed and convertedinto thermal energy by the cells.

Front Cover : low cost, transparent, elastomeric materials, UV-screening effect, weather-stable material - acrylics, silicones, fluorocarbons, and glass.

Solar Cell - Construction

Encapsulation materials - construction materials (excluding cells and electrical conductors) required in a PV module to provide

mechanical support and environmental isolation.

Earlier encapsulation efforts to identify a single material that could satisfy all of the encapsulation requirements and needs were unsuccessful.

More than one material would have to be assembled in a composite package to fabricate an encapsulated module satisfying all of the requirements.

Significant difference between the thermal-expansion coefficients of polymeric materials and the silicon cells and metallic interconnects;

stresses developed from the thousands of daily thermal cycles can result in fractured cells, broken interconnects or cracks and separations in the pottant material.

Hence the pottant must be a low-modulus, elastomeric material.

In addition, the mechanical properties of many polymeric encapsulation materials aretemperature-dependent.

Solar ultraviolet radiation can be a major cause or initiator fordetrimental changes in properties of normal polymeric materials.

Encapsulation of Solar Cell

Water: Many polymer encapsulation materials absorb some availablemoisture (liquid or vapor) and causes dimensional and mechanical property changes.

Moisture can participate directly in thermochemical and photochemical reactions with polymer encapsulation materials.

An electrolyte of condensed phase moisture and dissolved salts can participatein corrosion of electrical conductors.

Encapsulation Process: Vacuum lamination, Liquid casting, Spraying, Direct extrusion.

Lamination – dry film materials: Ethylene vinyl acetate, Ethylene methyl acrylate.

Casting – liquid material: Poly-n-butyl acrylate, Acrylic and Aliphatic polyether urathane.

EVA advantages : Good melting fluidity, low melting temperature, toughness, excellent adhesion, high electrical resistivity, low processing and cross linking temperature,

very low water absorption ratio, good optical transmission and high transparency.

EVA disadvantages: Poor heat resistance, low cohesive strength, poor creep resistance and expansion & contraction.

Moreover, unsaturated bonds in EVA molecules will be destroyed by ultraviolet light,and crack, degradation and discoloration, and adhesive failure maybe occurred.

EVA resin is compounded with curing agents (i.e., organic peroxides) and other additives such as UV absorber, UV stabilizer, antioxidant and silane coupling agent .

Cross linked during process.

EVA as an Encapsulation Material

UV absorber: 2-Hydroxy-4-octoxybenzophenone

UV stabilizer (HALS):bis(2,2,6,6-tetramethyl-

4-piperidinyl) sebacate

Antioxidant: Tris(Nonylphenyl)phosphate

Coupling agent:3-trimethoxysilyl- propylmethacrylate

Selection of curing agent: • Negligible decomposition at 85 to 90°C range, the extrusion temperature of EVA• Generates only the chemically inert decomposition residues• No UV-sensitivity for itself and the EVA

During the lamination process, gas bubbles formed from the decomposition of peroxides, inhibit the cure-active chemical species and decreases the curing rate.

It can be avoided by laminating at 150ᵒC and 1 atm lamination pressure

Peroxides, which generate free radicals having energy level ≥418.6 kJ/mol, possess high cross linking efficiency due to their ability to abstract secondary hydrogen,

commonly present in most of the cross linkable polymers.

Peroxides as Cross Linking Agents

Preparation of EVA Compounds:EVA resins are mixed with additives at room temperature.

Those additives are dissolved in 20 ml of acetone before being sprayed over the EVA resins. Then, they are compounded by a twin-screw extruder

(L/D ratio: 16.7, screw dia 19.7 mm, die dia 3 mm), screw speed: 20 rpm, barrels temperature: 80°C, and die temperature: 85°C.

Finally, the compounds are sheeted by a two roll mill at 40°C for 5 min.

Cross linking Characteristics Measurement - Monsanto Moving Die Rheometer4 g sample at temperature range 150°C - 170°C with a measuring time of 15 min.

Determination of Cure time and cure rate index (CRI)– Torque vs Time cure curves

2g sample in 100 ml of toluene is heated for 3 hr at 6OᵒC, the residual gel gives the degree of Cure (in %).

The criterion for acceptable cure is the achievement of mechanical-creep resistance of the cured EVA at 90°C, which corresponded to gel content in excess of 65%.

EVA Compounding and Curing Study

The aromatic by-products or chromophores in the curing step are causes of discoloration in the cured EVA. Discoloration of EVA can reduce PV module efficiency

because of decreased light transmittance

Even though DCP can produce strong free radicals, its half-life temperature is significantly high.

Therefore, it takes a long time to decompose at low temperature, or it must be heated up to higher temperature to produce free radicals in a short cure time.

Moreover, high cure temperature and/or long cure time can generate acetophenone, which causes yellowing in EVA.

Dialkyl Peroxide - Performance

Non-aromatic, non-discoloring, and does not produce decomposition by-productsThe optimum cure times (tC90) of these curves are 7.93, 6.63 and 4.52 minutes;

The CRIs are 14.37, 16.85 and 25.74 / min, respectively.

Peroxyester peroxide - Performance

Peroxyketal peroxide - Performance

The peroxyketal peroxide generates a mixture of weak and high energy free radicals

Decomposition products, such as methane, acetone, t-butyl alcohol, and CO2, are non discoloring

Optimum cure times (tC90) of these compounds were 3.15, 2.29 and 1.30 minutes for cure temperatures of 150°C, 155°C and 160°C, respectively.

Comparison of Peroxide Performance

Conclusion

Dialkyl peroxide is not suitable - has a high half-life temperature. By products can discolor the final product.

Peroxyester peroxide is good for curing in the range of 150°C to 160°C.Ultimate cure within 5 to 8 minutes.

Moreover, peroxyketal peroxide has higher performance, optimum cure time is 3 minutes.