Leveraging the durability of photovoltaic (PV) plants at an acceptable cost is becoming one of the most challenging issue to foster the diffusion of PV. For instance, stringent requirements for the modules life time of at least 30 years are carried out by the US Department of Energy. One of the most important technological factor to enhance the PV modules durability is represented by the availability of suitable coating materials with high barrier properties, flexibility, and affordable cost. These properties are of a special concern for thin film PV cells, which are more sensitive against the degradation effect due to atmospheric agents. A lot of research effort has been devoted to study and to develop novel flexible and low cost coating materials with improved barrier properties against water vapour and oxygen. However, few studies have been carried out to leverage the barrier properties against liquid atmospheric agents of the coating materials for PV cells (e.g., acid and basic rains). As a result, these studies proposed the development of transparent hydrophobic and superhydrophobic surfaces suitable for solar cells applications by means of multi-step processes involving plasma treatment steps of the polymer surfaces as well as high temperature steps (>150°C). Nevertheless, these multi-step processes are likely to turn out to be expensive and difficult to be implemented at industrial level. This work aims at studying and developing highly hydrophobic transparent and flexible coating materials for solar cells by means of a simple, but yet effective, single-step process, which is carried out at room temperature and specifically applied to PV standard coating materials. The current barrier protective films for the PV applications are generally composed of one ore more layers of polymer substrate with an inorganic coating. The inorganic coating is generally consisting of SiOx or AlOx, whereas the polymer substrate can be EVA, ETFE, PET or PEN. This study investigates the chemical deposition of a transparent and hydrophobic nano-coating on a „standard“ bilayer material employed for the solar cells coating, consisting in a PET-SiOx substrate of 12 micron width. The nano-coating has been deposited by applying the self-assembly of monolayers (SAM) of alkylksilanes (OTS: octadecyltrimethoxysilane) and fluoroalkylsilanes (FAS: 1H,1H,2H,2H-per-fluorodecyltrichlorosilane). The hydrophobic properties of the obtained samples have been assessed by means of static contact angle measurement, showing high average values of 130° as a consequence of the FAS deposition with a 105% increase of the contact angle on the SiOx side and a 83% enhancement on the PET side. Therefore, the chemical deposition of the FAS nano-coating successfully changed from hydrophilic to hydrophobic both the PET and SiOx surfaces. The same measures carried out for the OTS modified samples indicated that only the SiOx surface has been coated, with an increase of 36% and an average value of 86°, while a negligible increase of 6% has been relieved on the PET side. A chemical mechanism hypothesis has been proposed to explain the different reactive behaviour of the PET-SiOx samples with FAS and OTS, respectively. Moreover, the light transparency of the nano-coated films has been evaluated. A 90% transmittance value has been measured for the different coated samples, thus resulting adequately transparent for solar cell applications. The water vapour barrier properties (WVTR) have been also assessed in order to understand the effect of the deposition treatment on the gas barrier properties. Furthermore, the morphology of the obtained samples has been studied by AFM. The preliminary results obtained demonstrated that a highly hydrophobic flexible and transparent coating may be successfully obtained by modifying a PV standard coating bilayer with a simple and single-step process carried out at room temperature. These results will be further deepened and investigated in the future research. Therefore, an interesting contribution may be potentially provided from the accomplishment of the present study to the PV sector applications in terms of improved barrier properties of the coating materials against liquid atmospheric agents. This is expected to guarantee a higher durability for the PV modules.

Transparent and hydrophobic nanocoating materials for photovoltaic cells

ROSSI, GABRIELLA;ALTAVILLA, CLAUDIA;SCARFATO, Paola;CIAMBELLI, Paolo;INCARNATO, Loredana
2012

Abstract

Leveraging the durability of photovoltaic (PV) plants at an acceptable cost is becoming one of the most challenging issue to foster the diffusion of PV. For instance, stringent requirements for the modules life time of at least 30 years are carried out by the US Department of Energy. One of the most important technological factor to enhance the PV modules durability is represented by the availability of suitable coating materials with high barrier properties, flexibility, and affordable cost. These properties are of a special concern for thin film PV cells, which are more sensitive against the degradation effect due to atmospheric agents. A lot of research effort has been devoted to study and to develop novel flexible and low cost coating materials with improved barrier properties against water vapour and oxygen. However, few studies have been carried out to leverage the barrier properties against liquid atmospheric agents of the coating materials for PV cells (e.g., acid and basic rains). As a result, these studies proposed the development of transparent hydrophobic and superhydrophobic surfaces suitable for solar cells applications by means of multi-step processes involving plasma treatment steps of the polymer surfaces as well as high temperature steps (>150°C). Nevertheless, these multi-step processes are likely to turn out to be expensive and difficult to be implemented at industrial level. This work aims at studying and developing highly hydrophobic transparent and flexible coating materials for solar cells by means of a simple, but yet effective, single-step process, which is carried out at room temperature and specifically applied to PV standard coating materials. The current barrier protective films for the PV applications are generally composed of one ore more layers of polymer substrate with an inorganic coating. The inorganic coating is generally consisting of SiOx or AlOx, whereas the polymer substrate can be EVA, ETFE, PET or PEN. This study investigates the chemical deposition of a transparent and hydrophobic nano-coating on a „standard“ bilayer material employed for the solar cells coating, consisting in a PET-SiOx substrate of 12 micron width. The nano-coating has been deposited by applying the self-assembly of monolayers (SAM) of alkylksilanes (OTS: octadecyltrimethoxysilane) and fluoroalkylsilanes (FAS: 1H,1H,2H,2H-per-fluorodecyltrichlorosilane). The hydrophobic properties of the obtained samples have been assessed by means of static contact angle measurement, showing high average values of 130° as a consequence of the FAS deposition with a 105% increase of the contact angle on the SiOx side and a 83% enhancement on the PET side. Therefore, the chemical deposition of the FAS nano-coating successfully changed from hydrophilic to hydrophobic both the PET and SiOx surfaces. The same measures carried out for the OTS modified samples indicated that only the SiOx surface has been coated, with an increase of 36% and an average value of 86°, while a negligible increase of 6% has been relieved on the PET side. A chemical mechanism hypothesis has been proposed to explain the different reactive behaviour of the PET-SiOx samples with FAS and OTS, respectively. Moreover, the light transparency of the nano-coated films has been evaluated. A 90% transmittance value has been measured for the different coated samples, thus resulting adequately transparent for solar cell applications. The water vapour barrier properties (WVTR) have been also assessed in order to understand the effect of the deposition treatment on the gas barrier properties. Furthermore, the morphology of the obtained samples has been studied by AFM. The preliminary results obtained demonstrated that a highly hydrophobic flexible and transparent coating may be successfully obtained by modifying a PV standard coating bilayer with a simple and single-step process carried out at room temperature. These results will be further deepened and investigated in the future research. Therefore, an interesting contribution may be potentially provided from the accomplishment of the present study to the PV sector applications in terms of improved barrier properties of the coating materials against liquid atmospheric agents. This is expected to guarantee a higher durability for the PV modules.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11386/3742277
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