Surface Modification of Cellulose Films By Dielectric Barrier Discharge for Organic Photovoltaics Encapsulation

Abstract

Organic photovoltaics (OPV) are the third generation of solar cells and are more and more attractive for industrial applications. OPV present numerous advantages, such as flexibility, low cost, light weight, large-area production.[1,2] Nonetheless, OPV are not as efficient as the conventional crystalline silicon photovoltaic (PV) cells. Recently, power conversion efficiency (PCE) has reached 18% for OPV, while conventional PV remains around 30% [3]. In this sense, additional works are necessary to improve their processability. Moreover, components of OPV are highly sensitive to water or oxygen, which strongly affect OPV stability in time when used in harsh conditions. Therefore, development of new barrier coatings are today essential to reduce OPV degradation and increase their shelf life. According to the literature, encapsulation can be performed following three strategies: (i) glass-glass encapsulation, (ii) flexible polymer lamination and (iii) thin film direct deposition. Glass-glass encapsulation provides great protection, with good optical properties but it remains fragile and is not compatible with roll-to-roll processes. On the other hand, although, flexible polymer lamination is suitable for roll-to-roll processes, adhesion between the front and back barriers is still complex to deal with, leading sometimes to partial encapsulation, thus increasing OPV degradation. Finally, direct deposition allows to obtain thin films with excellent optical and oxygen and humidity barrier properties. To date, most of thin layers are synthesized under vacuum [1,2]. This work focus on the development new barrier coatings at atmospheric pressure, compatible with roll-to-roll processes using cellulose nanofibrils with excellent oxygen barrier properties [4]. Being hydrophilic, the surface of cellulose nanofibrils have been modified to improve the water barrier properties. Wettability modification (WCA ~ 140°) was successfully achieved with an atmospheric pressure dielectric barrier discharge to control the fragmentation/reticulation of 2,4,6,8-Tetramethylcyclotetrasiloxane (TMCTS).