Polyurethanes as Low Cost and Efficient Encapsulant Materials for Flexible Perovskite Solar Cells
Matteo Bonomo a, Marco Giordano a, Nicole Mariotti a, Babak Taheri b, Thomas M. Brown b, Sergio A. Castro-Hermosa b, Giulia Lucarelli b, Francesca Brunetti b, Claudia Barolo a
a University of Turin, Department of Chemistry and NIS Interdepartmental Center, Via Pietro Giuria, 7, Torino, 10125, Italy
b CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome - Tor Vergata, Via Giacomo Peroni 400/402, Rome, 131, Italy
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Matteo Bonomo, 243
Publication date: 11th February 2019

Long-term stability of Perovskite Solar Cells (PSCs) is the main issue to be solved for a forthcoming commercialization of this technology [1]. The stability of PSCs mainly suffers for water and oxygen infiltration as well as prolonged exposition to UV radiation. Straightforwardly, encapsulation of devices is a mandatory to achieve good long-term stability. Up to now, best encapsulant properties have been reached by the employment of nanometric film of metal oxides deposited by ALD [2]. Nevertheless, the latter approach inadequately fits the requirement of flexibility. Among the feasible matrixes to be used as encapsulant film, we resolved to polyurethanes (PU) that are polymers build up by the condensation between diisocyanate and polyol moieties. PUs have been chosen because they are cheap and environmental-friendly; furthermore, their mechanical, chemical and physical properties could be easily tuned by thoughtful choice of each precursor. Before implementing the PU films on PET-based PSCs, we investigated the physico-chemical properties of both pristine and modified PU: we added to the polymeric matrix some UV absorber molecules as well as home-synthesized down-shifters in order to minimize the effect of UV radiation towards the degradation of the perovskite layer; on the other hand, various fillers were investigated to improve the barrier properties of our encapsulant composites. Polyurethanes were manually deposited onto PET/ITO substrate and let polymerize for 24 hours. The resulting composites were UV (2, 4 and 8 hours under continuous irradiation) and thermally (85 and -50 °C) stressed in order to prove the stability of encapsulant matrixes. Remarkably, both UV-vis and ATR analyses of polymers did not show any significant modification as a consequence of the stressing. This makes us quite confident on the excellent long-term stability of our encapsulants Finally, the polymers were implemented in complete devices: first of all, it is worth mentioning that the addition of PU did not alter the photoelectrochemical performances of the devices as proved by the retention of overall efficiency after the encapsulation. Furthermore, the PEC properties of encapsulated device remain constant up to one month (period so far investigated), whereas unprotected device lost more than 50% of their performance during the same period. In conclusion, we reported here for the first time the implementation of modified polyurethanes as promising encapsulant materials for perovskite-based solar cell: different fillers, UV-absorber and down-shifter will be tested in the forthcoming in order to obtain the better compromise between flexibility, barrier properties and long-term stability. 

This paper has been funded from the Italian Space Agency (ASI) project, PEROSKY-Perovskite and other printable materials for energy application in space (no. 2018-1-R.0). 

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