New EVA-Based Encapsulants with Enhanced Photoluminescent Properties

As it is well known, the spectral response of commercial solar cells is significantly lower at short wavelengths (λ) compared to longer-λ visible light. A method of increasing the external quantum efficiency (EQE) of photovoltaic devices with poor short-λ response is through the principle of luminescent down-shifting (LDS). LDS involves the conversion of the short-λ photons of the incident light to longer wavelengths, where solar cell has much higher EQE. A large number of luminescent materials have been investigated to be suitable for LDS, the main three are: quantum dots, organic dyes and rare-earth ions/complexes. The introduction of an LDS layer in a PV module has an important overall cost because it creates additional interactions with the light resulting in extra loss mechanisms and increases one or more steps to the manufacturing process [1]. Ethylene-vinyl-acetate (EVA) copolymer has been the dominant material used in PV encapsulation for the last 30 years. A particular opportunity is the possibility of using the existing polymer encapsulation layer of certain PV technologies as a LDS layer as well. This is an approach which does not require any modification in the solar cell s structure or manufacturing process [2]. Taking into account that the known luminescent materials for LDS have several drawbacks (quantum dots and rare-earth complexes remain generally expensive, and organic dyes show low photostability over prolonged periods of UV exposure), recently, a convenient route for the creation of oligoatomic silver clusters within zeolite matrixes has been described by a simple ion-exchange process followed by thermal treatment. These clusters have tunable emission colors. Depending on the zeolite topology and the silver loading, different emissive species are created. These luminescent species are not expensive and exhibit prolonged photostability [3]. In this work, the opportunity to include silver clusters in the pre-existing EVA layer has been explored, in order to achieve LDS with no added production processes or layer to the PV module. Synthesis of the silver-exchanged zeolites has been carried out starting from different zeolite materials: Na-Y zeolite with Si/Al=2.6, K-A zeolite with Si/Al=1 and Na-A zeolite with Si/Al=1 (Figure 1). These luminescent species have been mixed with the copolymer via extrusion and sheets of uniform thickness have been manufactured (Figure 2). Absorption and emission spectra of doped EVA encapsulants have been measured to identify the most promising matrix and find the optimum clusters concentration which is necessary. These spectra have shown an excellent separation between the absorption and emission bands. The luminescent EVA sheets have been applied onto crystalline silicon cells and a vacuum laminator has been used for the encapsulation process, resulting in bubble-free modules. Finally, EQE of modules have been measured under the standard AM1.5 spectrum and an increase of EQE in ultra-violet region has been observed.