Foldable polyimide/ionic liquid composite films with high transmittance and ultraviolet resistance benefiting from the intermolecular multipoint interaction

A foldable polyimide (PI) protective layer with high transmittance and high ultraviolet (UV) resistance is crucial for the practical application of flexible solar cells. Herein, the commercially 2,2′-Bis(trifluoromethyl) benzidine (TFMB) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) as the monomers, and five ionic liquids (ILs) as the regulators were used to synthesize PI/IL films (PI/IL1, PI/IL2, PI/IL3, and PI/IL4). The outcomes reveal that the average transmittance (AT) of PI/IL1 (50 μm thickness) outperforms others, which can be attributed to the expanded π-π stacking distance between PI molecules and the intermolecular multipoint interaction between IL1 and PI. The AT and the elongation at break (ε) of PI/IL1 increase sequentially with the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2])/TFMB mole ratio, and those of PI/IL1-3 reach the maximum values of 94.23 % and 49.42 % at a 3:1 ratio, respectively. However, the tensile strength (σ) of the PI/IL1 film is inferior to that of the pure PI film, and consistently decreases with the increase of [C2mim][NTf2]/TFMB mole ratio. Nevertheless, the UV-induced degradation of PI can be delayed through the [C2mim][NTf2] regulation. The attenuations of AT, σ and ε of PI/IL1-3 films after 1000 equivalent solar hours (ESHs) UV irradiation are 0.85 %, 14.63 % and 2.99 %, respectively, which are significantly lower than those of pure PI films (2.23 %, 32 % and 23 %). This is attributed to the interaction between the positive and negative charges from [C2mim][NTf2] and those from the PI chain breaking. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PI/IL1-3 are calculated with the density functional theory to reveal the charge transfer between [C2mim][NTf2] and PI.