Integration of a silk fibroin based film as a luminescent down-shifting layer in ITO-free organic solar cells

We report here a study on the integration of the silk fibroin (SF) protein in organic solar cells. The intrinsic low toxicity, natural availability, biodegradability, water processing, good film forming properties and capability to be doped with functional molecules of SF biopolymer inspired us to integrate it as a transparent and inert or functional bottom layer in organic solar cells. Water stable, optically transparent, smooth and homogeneous SF thin films (thickness ∼400 nm) were successfully prepared on glass and characterized. Then ITO-free bulk heterojunction (BHJ) solar cells employing P3HT:PC61BM as a standard active layer and a highly conductive PEDOT:PSS formulation as a semi-transparent anode were deposited over the SF films. As a result, the power conversion efficiency (PCE) of all silk-integrated BHJ solar cells was comparable to the references on bare glass. The ability of SF to act as a host matrix for functional moieties was exploited to give to the SF layer the functionality of a Luminescent Down-shifting film (LDS), as confirmed by the spectral response measurements, by using a water soluble stilbene derivative (Stb). The photovoltaic performance of all SF-based devices was significantly stable over time, overcoming the problems of the ITO-based reference cells after 70 days. Finally, flexible SF-integrated ITO-free solar cells were successfully fabricated on PET substrates.