Solar cell fabrication technologies are still evolving, making room for new materials. The latest product developments and research are summarized.
June 21, 2011 — Solar cell fabrication technologies are still evolving, making room for new materials. Polymers used to make photovoltaic modules include EVA, PVB, silicone, fluoropolymers (such as PVF, PTFE, ECTFE and ETFE), PMMA, thermoplastic elastomers, EPDM, polyamide and PET. Various speakers covered this at AMI’s international conference, Polymers in Photovoltaics 2011, this April in Germany. The trends include higher transparency for maximum efficiency, lighter-weight materials for building-integrated PV (BIPV), and other improvements. Below is a summary of the latest product developments and research.
The solar power industry is growing worldwide — 16,000MW of photovoltaics (PV) systems were installed globally in 2010, and this number will grow in 2011, says Kerry Setterthwaite, senior consultant at Applied Market Information (AMI).
Solar cell efficiency from materials
Dr. Mohan Narayanan, VP, technology, Hanwha SolarOne in China, points out that between 1.4 and 1.8 billion people currently lack access to electricity. By 2020, 750 million people will have the option of solar-powered electricity. Governments, like India’s, where they need an additional 280GW of electricity by 2020, will need to support solar power. The demands on materials will be high: encapsulant would need to be highly transparent for maximum efficiency, water repellent, chemical resistant, have low permeability to water vapor, excellent adhesion, and flexibility, and perform consistently across a wide temperature range.
The durability of a module is linked to the encapsulant, and each material has advantages and disadvantages. PVB is proven in window technology but suffers a small supplier base, some transmission issues, and weakness to moisture; silicone is expensive but has a simple lamination procedure, and another alternative, polyolefin, is widely available but doesn’t offer good adhesion. EVA has a matching refractive index to glass, enabling good transmission and mechanical strength, as well as known processing, but has a poor moisture barrier, outgasses during processing, and sufferes degradation.
Encapsulation materials should be transparent, provide cushioning and impact properties, electrical insulation and a high moisture barrier. EVA is the most commonly used polymer and has been in use for over 30 years. More recently silicone rubber, PVB, ionomers and TPU have all been used as alternatives. There are special considerations for the extrusion of encapsulant polymers, as studied by Davis-Standard. A typical EVA encapsulant has a high VA content (33%) and contains additives, which must be correctly mixed: the extruder should have a corrosion-resistant liner and screw, provide adequate torque for low temperature processing and an L/D size which balances residence time against mixing requirements.
PV module design
Many commercial building roofs in the US cannot handle the weight of solar panel installations. New lightweight crystalline silicon technology is being developed: fluoropolymers can replace the heavy glass front sheet: they are lightweight (as low as 0.1kg per square meter); ETFE and FEP offer excellent light transmittance, and are resistant to chemicals, humidity, light and heat. A rigid reinforcement, such as a rigid back panel, is required in glass-less modules. Saint-Gobain Performance Plastics supplies these front sheets: ETFE is already used as transparent roofing structures and has been subject to extensive testing including hail and cut resistance.
Solvay Solexis has developed solar films with Ajedium Films (a company division). A typical backsheet is a multilayer structure with PET and tie layers. A new transparent grade of PVDF has potential for backsheets and frontsheets. ECTFE has potential in frontsheets, including for UV blocking films.
Toray Films produces PET-based backsheets as an alternative to fluoropolymers, which dominated the market in the 1990s. Backsheets need to offer electrical insulation, mechanical strength, UV and weathering stability: in this case different layers of PET materials provide the different properties required. These Lumirror films have been used in Japan since the 1990s.
The Fraunhofer-Center for Silicon-Photovoltaics (CSP) examines manufacturing processes such as encapsulation, looking at the temperature profile of different components during vacuum lamination. EVA crosslinking is an exothermic reaction and consumes crosslinking agents: for homogeneity of films, the additives must be mixed in thoroughly and gently in the extruder. Ideally, the pressing stage of vacuum lamination should be completed at the gelation point of the EVA. The load-bearing ability of modules is under investigation at Fraunhofer, as the current test, IEC 61215, is carried out at room temperature, whereas snow loading occurs at much lower temperatures. When stress is applied on the front glass, the polymer layer around the cell can influence laminate stiffness, creep and other aspects. The temperature-dependent behaviour of EVA, PVB and Tectosil has been examined.
The Energy research Centre of the Netherlands (ECN) has looked at critical factors in encapsulation of thin film PVs, such as minimizing the water vapor transmission rate (WVTR) and cutting costs. It has accelerated test facilities where exposure conditions can be tailored. The encapsulation is a very significant proportion of the production costs of a module, and the ECN is looking at making more robust silicon components to reduce the encapsulant requirements as well as looking to improve the polymer. It has found that condensated water conditions are a greater stressor compared to humid, so they could be useful for accelerated testing and lifetime prediction.
Other new materials
Acrylic materials have a role in photovoltaics too: PMMA from Evonik Röhm is already proven for outdoor applications from automotive glazing to signage under the brand name Plexiglass. The light transmission properties can be adjusted to optimize solar module performance. It has been field tested, for example for more than 12 years in an Amonix concentrated PV (CPV) system, and in protection of CPV lenses for over 17 years. At Intersolar 2010, the use of Plexiglass in frontsheets was demonstrated in a lightweight module measuring 4.5 x 1.5m.
There are new backsheets under development including the polyolefin-based coextruded sheet from Renolit with integrated adhesive. It can also be combined with EVA as an upper encapsulant. Renolit?s waterproof membrane production has given the company extensive experience of weather-exposed polymer materials. Many backsheets are laminates, this one is coextruded with a reactive PE face for adhesion to module components; soft encapsulating PE layers (including functional fillers like flame retardants); a connecting layer; a PP layer with high concentrations of functional fillers such as reflective pigments; a PP layer with improved heat distortion and a surface treatment or primer layer.
3M supplies adhesives to the solar module industry, such as bonding for junction boxes, cell positioning tape and acrylic foam, frame-bonding tape. It also supplies fluoropolymer backsheets, which are UL and IEC certified.
Sealing is an important aspect of module durability: moisture ingress can cause delamination, leakage of current, discoloration and corrosion. SAES Getters provides sealant tape, which can give 3000 hours of damp heat stability in thin film CIGS modules. The breakthrough time is the time required for moisture to break through the barrier sealant. Active barriers based on chemical getters have higher breakthrough times and lower permeation rates.
Specialized Technology Resources Inc. (STR) has been developing a new high light transmission (HLT) range of EVA encapsulants to improve PV efficiency. These have been tested in accelerated ageing conditions, using Xenon Arc Weather-o-meters for example, over 30 weeks, and the second generation EVA has been incorporated in 8 modules from 3 manufacturers in field tests in Tempe, Arizona. There were different results from different module manufacturers due to factors such as failure of other components leading to encapsulant deterioration, for example, corrosion in a junction box. The new EVA is faster curing, which can help speed up module manufacturing. In late 2010 a customer achieved the lamination of 4 modules in 3 minutes 15 seconds at 155C using an XL laminator.
Solutia Solar supplies EVA, TPU and PVB and has opened an EVA plant in Suzhou, China. It has developed a new grade of PVB encapsulant to overcome problems of solar panel discoloration, which occur when PVB is exposed to damp heat and electrical bias while in contact with silver-coated glass. The discoloration is due to a silver compound, not to polymer degradation. The new Saflex PS41 shows much less discoloration and passivates the silver surface, it has no voltage-dependent change in color and actively protects metals, like copper, under test conditions.
Huntsman has introduced several thermosetting materials for PV including a transparent, liquid encapsulant, a liquid dielectric encapsulant/backsheet, and an electrically conductive adhesive. The company is working with Caiten Eco Energy in Austria on experimental modules. The liquid encapsulant plus glass has greater light transmittance than the conventional EVA system, and the material has low temperature processing and cure, as well as excellent adhesive properties. The innovative backsheet is screen printable, which could facilitate manufacturing of cell back contacts using printed circuit board technology.
UV cured polymers are a speciality of Sartomer: a liquid resin is applied to a substrate and UV light is used to initiate polymerisation and ?curing?. The advantage is the low energy use and speed of processing. These materials can be tailored for properties like adhesion and excellent weathering. They are being tested in laminates for PV applications.
The rise in the solar power industry has caused suppliers to divert materials to grab market share. For example, USI Corp. is diverting EVA to the solar industry and Repsol is expanding its EVA production site in Puertollano, Spain. In Q3 2010, DuPont announced increased production of PVF in North Carolina for Tedlar film for backsheet applications.
Machinery suppliers are looking to improve encapsulation, like Meyer Burger Technology (incorporating 3S Modultec).
With the development of any new industry comes regulation and standards. The first IEC standard for crystalline photovoltaics was published in 1993. In 2010 the first standard for back and front sheets was developed alongside the standard IEC 61730-1, which covers module components. Several aspects are tested including the relative thermal endurance index (RTI), the flame spread index and weathering resistance. TÜV Rheinland is involved in the development of standards and performance testing.
Underwriters Laboratories (UL) has set up PV testing laboratories in San Jose, CA; Krefeld, Germany; China; India; and Japan, primarily looking at fire and electrical safety. Approving components prior to module specification can speed up the time to market: each aspect has a different set of requirements from flame spread to hot wire ignition and water immersion. ID scans are carried out on materials to set up references including infrared analysis, thermogravimetry and differential scanning calorimetry.
Upcoming events on PV materials from AMI:
- Plastics in Photovoltaics 2011
September 20-21, 2011
- Polymers in Photovoltaics 2012
April 24-26, 2012