Soluble D-A-D Squaraine Derivatives With Aromatic Amine Donor Units:Synthesis,Properties And Applications In Solar Cell

Squaraine derivatives have been synthesized for the first time since 1965,and have been developed for more than 50 years.They attract more and more attention and have been applied to various fields including organic photovoltaic because of their simple synthetic routes,tunable bandgaps,thermal stabilities and excellent photoelectric performances.Squaraine derivatives have high absorption coefficients in the visible-near infrared region,extinction coefficients on the order of 105 L/（mol·cm）due to the interactionan between an electron-decient central fourmembered ring and two electron-donating groups in a donor-acceptor-donor conguration.The performance of the squaraine derivatives is mainly affected by the type of electron-rich nucleophiles,the most common of which is N,N-dialkyl arylamines（R）.In this paper,we choose the alkyl groups R having different solubility,such as isobutyl,hexyl and ethyl as substituents for N,N-dialkyl（R）aryl amines,and a hydroxyl group,a methyl group which can change the intramolecular and intermolecular interactions and thus affecting the absorption characteristics to synthesize a series of N,N-dialkyl（R）aryl amines as donor units for D-A-D squaraine derivatives,which will provide experimental basis for further optimizing the molecular structure.Based on the above experimental results,in order to further improve its absorption and solubility,and then to introduce a stronger electrondonating thiophene group in another substitution position of N,the target product is synthesized,named N-alkyl-N-thienyl aromatic amine as the donor unit D-A-D squaraine derivatives.The structures of the compounds were elucidated by means of NMR,IR spectroscopy and elemental analysis.Its photophysical properties,thermal stabilities,solubility and aggregation of molecules were studied and applied in BHJ-OSC.The specific work is summarized as follows:We choose N,N-disubstituted alkyl groups with different aryl groups R as electron-donating groups and synthesize SQ1,SQ2,SQ3 by condensation reaction with squaric acid（In SQ1,SQ2 and SQ3,R is isobutyl,hexyl and ethyl,respectively）.The solubility of these three squaraine derivatives in chlorobenzene were 4 mg/mL,95 mg/mL and 5 mg/mL,respectively.The effect of improving the solubility of the materials was significant by introducing hexyl groups.The UV-Vis absorption spectra of compounds in solution and in film under different solvents were studied in detail.Compared with the SQ3 thin film,the visible absorption of SQ3 is obviously redshift and broadened,and the maximum absorption wavelength in film can reach 729 nm,which is greater 32 nm than that of SQ1 film.In the SQ3 molecular structure,methyl-modified aromatic amines were substituted for hydroxy-modified aromatic amines,and their absorption spectra showed significant red shift.The J-aggregation morphology of SQ1 and SQ3 films was significant,while the H-aggregation and J-aggregation in SQ2 films were more obvious,the state of aggregation will be significantly affected as the solvent and ratio with PC₆₁ BM change.Based on the experimental results of SQ2 and SQ3,SQ4 a and SQ5 a were synthesized by introducing N-hexyl and methyl-modified arylamine as the parent group and furtherly introducing the thiophene group with stronger electron-donating ability.Moreover,in order to study the effects of methyl groups on the benzene ring the comparative molecules,SQ4 b and SQ5 b were synthesized.The solubility of SQ4 a,SQ4b,SQ5 a,SQ5b in chlorobenzene were 25 mg /mL,18 mg/mL,40 mg/mL and 7 mg/mL,respectively.The UV-Vis absorption spectra of compounds in solution and in film under different solvents were studied in detail.The maximum absorption peaks of SQ4 a and SQ5 a in film closed to 700 nm,which of SQ4 b and SQ5 b were at 640 nm and 670 nm.These four kinds of squaraine derivatives form J-aggregates and H-aggregates in film at the same time.The state of aggregation will be affected as the solvent and ratio with PC₆₁ BM change.As a result,when the electron-donating group CH₃ is introduced into the donor unit,the induction effect of the substituent is obvious,and the absorption spectrum of the squaraine derivative which does not introduce the CH₃ group is obviously redshift.Moreover,when CH₃ is introduced,the coupling between molecules and the hydrogen bonding between molecules are beneficial to the formation of J-aggregation state.J-aggregation behavior is significant both in films formed in high boiling solvents or films formed in low boiling solvents.