Two types of photovoltaic module structures coexist:

• Glass-polymer film (also called glass-backsheet) type modules. They are made of glass on the front side and polymer film on the rear side. Polymer film, also known as backsheet, is sometimes incorrectly called Tedlar, although this material, developed by Dupont, is only one of the components of polymer film among other options. The thickness of the front glass generally used for this type of structure is 3.2 mm.

• Dual-glass type modules (also called double glass or glass-glass) are made up of two glass surfaces, on the front and on the rear with a thickness of 2.0 mm each. Some manufacturers, in order to reduce the weight of the modules, have opted for a thickness of 1.6 mm. DualSun has chosen to stay with a thickness of 2.0 mm for reasons explained below.

In both configurations, the photovoltaic cells are laminated between the front and rear sides of the module using an encapsulation material. This is melted during the lamination process and helps preserve the integrity and performance of the cells.

There are several types of encapsulation material, specific to each structure:

• EVA (Ethylene Vinyl Acetate), commonly used for glass-backsheet modules

• POE (Polyolefin Elastomer), the preferred solution for dual-glass modules.

As these materials have different chemical compositions, the lamination parameters and expected properties, particularly in terms of moisture penetration and electrical insulation, differ for the two types of structures.

Due to the ease of its manufacturing process, the glass-backsheet type structure was largely dominant during the period 2010-2019. Certain durability problems reportedt from the field  after several years of installation for certain types of polymer films, coupled with the advent of bifacial cells, has led photovoltaic module manufacturers to rethink the design of their products.

It was from 2020 that dual-glass module technology really took off, continually gaining market share, thanks to three technological arguments:

• Hermetic encapsulation: the double glass modules offer a hermetic structure, resistant to aggressive weather conditions, the main one being moisture penetration highlighted during tests so-called Damp Heat, according to standard IEC 61215-2: 2021 (clause MQT13). By testing a wide variety of PV modules in 2023 according to this test, but with a duration extended to 2000 hours, the PVEL laboratory in the United States concluded that glass-glass modules are more resistant to moisture penetration. In fact, the power loss measured after the heat humidity test was 1.0% for the bi-glass modules, compared to 1.9% for the glass-backsheet modules (source: PVEL scorecard 2023)

• Bifaciality: The bifaciality of double glass modules produces a gain of around 10-11% compared to the power measured on the front panel alone, for TOPCon type modules under so-called BNPI (bifacial nameplate irradiance) test conditions. Being given that in the residential sector, the rear face of the modules is at a reduced distance from the roof, the bifaciality of the modules is not a preponderant factor in the choice of modules. However, part of the transmitted light is reflected and saves a few percent of electricity production. Even if bifaciality is only a minor factor in this segment, the choice of double glass for residential applications remains relevant.

> To go further: What is a bi-facial module? What performance gains does it allow?

• Mechanical constraints on cells: the fact that the structure of the double glass modules is symmetrical implies that the cells are located on a so-called neutral line, the upper part of the module being in compression during a downward mechanical load and the lower glass surface being in tension. In this configuration, the cells undergo very little tension-compression stress. This is not the case for glass-backsheet modules which asymmetrical structure places the cells in a tensile zone, increasing the risk of cracking (see details in the sectional views of PV modules below)

Mechanical stresses on a glass-backsheet module during a uniform mechanical load applied to the front face

Mechanical stresses on a dual-glass module during a uniform mechanical load applied to the front face

Furthermore, another point to take into consideration is the PID effect (or potential-induced degradation), mainly due to a migration of sodium ions from the glass towards the cells. Double glass modules, due to the hermeticity of their structure, present less risk of PID. This phenomenon can be avoided by the use of an appropriate encapsulation material and by quality control reinforced by tests in climatic chambers. According to tests carried out at the PVEL laboratory, there is no notable difference in terms of resistance to the PID effect between dual-glass and glass-backsheet modules.

To summarize the advantages cited above, the choice of a double glass structure means that the photovoltaic cells are better protected from external stress, in particular from the penetration of humidity and mechanical stress.

However, dual-glass modules have certain disadvantages that are important to take into consideration during the product design phase.

One of the main disadvantages concerns the hail resistance.

To simplify, the hail resistance of a photovoltaic panel is mainly linked to that of its upper layer. In the case of a glass-backsheet module, not only is the upper glass layer thicker (3.2 mm versus 2.0 mm) but also this layer is fully tempered glass, whereas in the case of a thickness of 2.0 mm, the glass is only semi-tempered due to technical limitations of the tempering process. Complete tempering provides better mechanical characteristics, particularly with regard to impact resistance. Glass-backsheet type modules therefore naturally have better resistance to hail than dual-glass modules. Furthermore, dual-glass modules have a specific weakness located in the passage of conductors through the rear face of the glass before being fixed in the junction boxes. In fact at this location the total glass thickness (2.0 mm) of the laminate is lower than at the other locations of the module (where the total glass thickness is 4.0 mm) and constitutes a vulnerable point.

To ensure that its dual-glass panels meet hail resistance standards, DualSun has conducted extensive tests on its entire product range. The size of hailstones necessary to pass IEC standards is 25 mm. All DualSun modules meet this standard, including the doubleglass which pass the RG3 level, corresponding to hailstones with a diameter of 30 mm and 75% higher impact energy than the IEC standard. Glass-backsheet modules (with glass thickness 3.2 mm) such as Shingle technology modules even reach the RG4 level.

> To go further: Does the certification of DualSun panels cover hail risks?

It is also important to take into account the aspect of recyclability products during their design phase.

Current solutions for recycling glass-backsheet type modules are based on a heated blade process scraping the rear face of the modules in order to separate the glass from the other components. Regarding dual-glass modules, certain layer separation solutions by high temperature heating make it possible to separate the 2 layers of glass, but this process is currently not commonly applied and requires more research in order to recover the separated materials in a fair condition. By default, dual-glass modules which have reached the end of their life are currently (early 2024) crushed, which is far from being an ideal recycling solution. Research on the subject of recycling is currently very active and it is hoped that a recycling solution for all PV technologies will be implemented in the years to come.

Finally, the last disadvantage linked to dual-glass modules concerns their generally higher weight compared to glass-backsheet modules, which creates constraints on installation. For example, for a DualSun module with dimensions 1722 mm x 1134 mm, the weight in glass (2.8 mm) - backsheet configuration is 20.0 kg while it is 25.1 kg for bi-glass (2 x 2.0 mm), which corresponds to a weight increase of 25%. In the case of a comparison between a 2 x 2.0 mm dual-glass module and a glass (3.2 mm)-backsheet module, the excess weight of the dual-glass is approximately 20%.

 Summary table comparing a dual-glass structure to a glass-backsheet structure

[1]: Cell technology type: Shingle, 3.2 mm thick glass

[2]: Glass thickness of glass-backsheet module 2.8 mm

Having chosen to base its range of photovoltaic products on TOPCon technology, DualSun favors, among the criteria in the table above, resistance to moisture penetration. Indeed, for TOPCon cells, there is a real risk of corrosion of the front face of the cells in the presence of humidity, due to a chemical reaction between certain types of encapsulation material and the silver paste used for cell metallization. This corrosion does not systematically affect all TOPCon module manufacturers, but DualSun judges that the risk is high enough to take protective measures and therefore naturally moved towards a dual-glass structure, guaranteeing hermetic encapsulation of the photovoltaic cells. Furthermore DualSun ensures that the disadvantages of dual-glass modules are minimized, for example by checking that our modules meet hail resistance criteria that exceed current standards. 

This particular attention to the durability of our products involves the desire to maintain a glass thickness of 2.0 mm on both the front and rear sides of our modules. In this, we stand out from certain manufacturers who have chosen a glass thickness of 1.6 mm to reduce the weight of their products but by accepting an increased risk of accelerated deterioration, particularly under the effect of hail and mechanical load induced by wind and snow.

Furthermore, experience has demonstrated that the two glass-backsheet and dual-glass variants largely satisfy the durability criteria for previous cell technologies, including PERC, which was at the heart of the DualSun offer over the period 2016-2023 and which is notably less sensitive to the effects of humidity than TOPCon.

> To learn more about TOPCon technology, access our article: What are the advantages of DualSun FLASH TOPCON panels?