Durability of ionomer encapsulants in photovoltaic modules

Highlights

• Ionomer thermoplastics were examined as a viable alternative to EVA for PV module encapsulation.

• Ionomer adhesion to the glass and cell interfaces of PV modules is inferior to EVA.

• Surface preparation and functionalization processes should be investigated to improve adhesion.

Abstract

Ethylene vinyl acetate (EVA) has for decades been the material of choice for photovoltaic (PV) module encapsulation. However, while it is relatively inexpensive and initially adheres well to module components, EVA discolors with age and—as interfacial adhesion degrades—becomes susceptible to delamination, ultimately resulting in reduced module efficiency and shortened service lifetimes. As potential replacements for EVA, ionomer thermoplastic materials cure faster, are more resistant to discoloration and potential induced degradation, and do not evolve corrosive acetic acid, making them compatible with new device materials such as perovskites. Since there is limited information on ionomer durability for PV module applications, a series of field and accelerated laboratory aging studies were conducted to assess ionomer interface stability in the presence of terrestrial environmental stressors. It is shown that adhesion to the glass and cell interfaces of PV modules is inferior to EVA, both before and after aging, rendering ionomers particularly susceptible to delamination after short timeframes. Potential solutions to improve ionomer adhesion are discussed.

Introduction

Ethylene vinyl acetate (EVA) has for decades dominated the photovoltaic (PV) module encapsulation market. Relative to other encapsulant materials available in the early years of module design, EVA was inexpensive, cured quickly, and adhered well to module components (i.e. backsheet, cells, interconnects, and front glass) [1,2]. However, exposure to terrestrial stresses (heat, humidity, irradiance) in the field causes undesirable chemical reactions in EVA, which eventually promote adhesive degradation, interface delamination, and subsequent reduction in cell efficiency [[3], [4], [5], [6], [7], [8], [9], [10]]. EVA discolors as a result of chemical reactions that generate chromophores, which attenuate incident light on the cell thus reducing efficiency [[11], [12], [13], [14]]. Given its large water vapor transmission rate (WVTR) relative to other encapsulants [15,16], moisture diffusion in EVA coupled with exposure to UV light and elevated temperatures leads to hydrolysis—and eventually delamination—at the cell and glass interfaces [5]. Of particular concern given the breadth of new device technologies is the susceptibility of EVA to acetic acid formation [17], which accelerates corrosion at metalized interfaces and degrades materials like perovskites.

As manufacturers seek to extend module service lifetimes beyond 30 years, these chemical limitations of EVA necessitate selection of new encapsulation materials that improve durability and are compatible with evolving device technologies. Examples include several silicones, polyolefins, and ionomer thermoplastic materials [18,19]. Ionomers, which constitute the mechanical toughening layer of safety glass, present a viable alternative and are the focus of this work. Relative to EVA, ionomers possess lower WVTRs, superior optical transmittance, and—due to higher electrical resistivity—are less susceptible to potential induced degradation (PID) [[20], [21], [22], [23]]. Acetic acid does not form as a reaction product in ionomers, rendering them chemically compatible with perovskites. Though more expensive than EVA in bulk, ionomers do not require a crosslinking step, thus resulting in shorter lamination times. With regard to module durability, finite element analyses have found ionomers to be superior to other common encapsulants in minimizing fatigue induced failures in metal interconnect ribbons [24].

The long term suitability of ionomers for terrestrial PV applications, however, has not been well investigated, and the limited field data presently available indicates a potential susceptibility to delamination [25]. We thus present a comprehensive study of encapsulant adhesion for EVA and several types of ionomer through a series of lab and field aging studies of single cell miniature PV modules. It is shown that adhesion of ionomers to the glass and cell interfaces of modules is inferior to EVA, both before and after aging, and are particularly susceptible to delamination after short timeframes. Potential solutions to improve ionomer adhesion and thus leverage their otherwise favorable properties for PV applications are discussed.